Association on development of the international research and projects in the field of energy «Global Energy»

BREAKTHROUGH IDEAS IN ENERGY FOR THE NEXT

YEARS 2 3 CONTENTS Dear colleagues!

Carbon capture 4 Popularisation and support of research and energy, and biofuels that formalise environmentally developments in the sphere of energy alongside clean solutions, is served by technological burst of with assisting the expansion of energy cooperation new concepts of energy generation, creation, and Smart Grid and Digitalisation of Energy System 9 have always been the crucial to the development of production of new materials, as well as development Russia’s fuel and energy sector. and implementation of digital solutions. These are the materials prepared under Digitalisation of production facilities, problems and High Capacity Hydrogen Carriers 20 the initiative of The Global Energy Association of reducing the volume of harmful emissions into on development of the international research and the atmosphere and climate change as a result projects in the field of energy with the participation of anthropogenic activity of mankind, the Small Modular Reactors 25 of representatives of world expert community, development of alternative energy, fighting energy leading specialists and energy scientists. poverty – these and many other issues are reflected Power-to- 37 Today’s society is actively searching for in this report. adequate solutions to challenges driven by the It is clear that promising planning and rapidly changing and competitive world of high tech. sustainable development projects for Russia’ Technologies for Compact and Efficient Energy Storage 43 Technological development cannot be stopped, fuel and energy complex should be based on the so the efficiency and sustainability of companies’ results of continuous scientific research, advanced business activities depend on the way they are ready technologies, and innovative technical solutions. Waste-to-Energy 53 to react to technological transformations. That is I am sure that the working results of experts, why the work of the Global Energy Association is specialists, and scholars presented in this report essential both from the standpoint of the activity of will find good use in business activity and make Energy recycling 65 its members (PJSC Gazprom, PJSC Surgutneftegaz, their contribution to perspective development PJSC FGC UES, etc.) and from the standpoint of the of Russian energy sector. development of Russia’s energy sector in general. Chemical Fuel of Sunlight 77 The activity aimed at transformation, distribution, and usage of energy resources Artificial Photosynthesis 90 is now undergoing serious change. Thus, the With best regards, traditional technologies of energy production are transforming greatly while the alternative ways of Yury Borisov its generation are applied. The development and Deputy Prime Minister growing popularity of renewable sources, hydrogen of the Russian Federation 4 5

CARBON CAPTURE FIG. 1. Carbon Capture and Stotage (CCS)

Rodney John Allam, Partner, 8Rivers Capital; The Nobel Peace Prize Winner 2007

INTRODUCTION

The whole world has been disrupted by the future way in which we want to live with significant sudden outbreak of the Covid-19 pandemic, which changes to our social, commercial, industrial, seems to have had a disproportionate effect technological and political systems. How will this on Western developed economies. The affect the world’s energy systems? Energy is a basic enforced pause in our whole society has given need for all human activity and development. All the Carbon capture technology applied CC projects in the world, capturing 31.5 Mt of CO us an opportunity to reassess our future. Should world’s population has a basic right to have access 2 to a modern conventional power plant can reduce per year, of which 3.7 is stored geologically2. we continue on the path of extensive growth, much to energy in the form of fuel and electricity. Our CO emissions up to 80–90% compared to a plant Capturing CO is most effective at point of it unsustainable while we continue with the guiding principle must be clean and sustainable 2 2 without carbon capture technology installed. sources, such as large facilities destruction of our planetary environment on land, in energy available to all. The air we breathe The key challenge here is that if used on or industries with major CO emissions, natural the sea and in the air? As the world economy slowly is by far the most fragile part of our environment 2 a power plant capturing and compressing CO , gas processing, synthetic fuel plants. Capturing re-establishes itself we are faced with a very deep and its protection is absolutely imperative regardless 2 other system costs are estimated to increase CO from air is also likely, but due to the significantly world depression, massive unemployment, major of cost. It is up to us to devise reliable low 2 the cost per watt-hour of energy produced by lower concentration of CO in the air compared disruption of global trading and severe strains cost power production and fuel supplies using 2 21–91% for fossil fuel power plants; and applying to sources, this direction has on the international monetary system. Our lives technology, which meets these criteria. the technology to existing plants would be even significant engineering problems. have suddenly changed. We can now decide on the more expensive. As of 2019, there are 17 operating

KEY CHALLENGES TO ADDRESS TECHNOLOGY The use of fossil fuels for energy production commercially viable clean coal systems with near

with carbon capture and storage must be a focus 100% CO2 capture. for immediate demonstration at commercial scale Carbon capture is the technology allowing In general, there are three different methods fuels. In this case, is captured followed by rapid implementation. Coal is the capturing waste carbon dioxide. It can be captured of capture technologies: post-combustion, from flue or other large point sources. This major source of CO2 emissions. Coal will continue directly from the air or from other sources, like pre-combustion and -fuel combustion: technology has been well studied and is currently to be used on a large scale in China, India, and Asia power plants’ flue gas, using different kind being used, although not on a large scale. This in spite of its obvious effects since huge populations of technologies including absorption, adsorption, In post-combustion capture, CO2 is removed method is a most popular one because existing still need access to cheap electricity to lift them out chemical looping, membrane gas separation or gas after burning fossil fuels — this is a scheme power plants can be retrofitted to incorporate CCS 1 of poverty. The only solution is to quickly develop hydrate technologies . suitable for power plants that burn fossil technology into their configuration3.

1 — Bui et al., 2018 2 — IPCC special report, 2005 3 — Sumida et al., 2012 6 7

Pre-combustion technology is widely used There is room for significant improvement in the production of fertilisers, chemicals, and to existing systems, especially the oxygen-fuel COSTS gaseous fuels. In these cases, the fossil fuel system combined with the use of new energy cycles

is oxidised. The CO from the resulting synthetic of the CO2 working fluid. What can be done now is to combine international efforts to demonstrate gas (CO and H2) reacts with the added steam new technical solutions in the field of clean energy It is believed that there are a number of reasons per ton of injected CO2, plus an additional US $ 0.10– (H2O) and converts to CO2 and H2. The resulting using coal fuel. New clean power units using coal why carbon capture and storage is expected US $ 0.30 for monitoring costs. However, if storage CO2 is captured from the already clean exhaust can only be installed as the existing power units to cause a rise in prices when used in gas-fired power is combined with enhanced reservoir recovery gas stream. The resulting H2 can be used as fuel; carbon dioxide is removed before become obsolete and must be replaced. This plants. First, the increased energy costs of capturing to extract additional oil from an oil field, storage can combustion begins. This method is suitable for means that coal will remain a constant, though and compressing CO2 significantly increase the bring a net benefit of US $ 10–16 per ton of injected new power plants. decreasing, source of pollution for the next 40–50 operating costs of CCS-equipped power plants, not CO2 (based on 2003 oil prices). This would probably years. Electricity production using natural gas to mention investment and capital costs. negate some of the carbon capture effect when oil

In oxygen-fuel combustion, fuel is burned reduces CO2 emissions per kWh by more than 50% If we talk about chemical plants, most of the was burned as a fuel.If CO2 capture was part of a fuel in oxygen instead of air. Flue gases consist compared to coal. construction of CCS blocks is capital-intensive. cycle then the CO2 would have value rather than be mainly of carbon dioxide and water vapour, Pre-commercial demonstration projects are likely a cost. According to UK government estimates made the latter of which condenses when cooled. to be more expensive than mature technologies; in the late 2010s, carbon capture will add 7 pounds The result is an almost pure stream of the total additional cost of an early large-scale per MWh by 2025 to the cost of electricity from carbon dioxide that can be transported and demonstration project is estimated at 0.5-1.1 billion a modern gas-fired power plant: however, most stored. Such cycles are called zero emission euros5. of the CO2 will need to be stored, so the total The cost of CCS depends on the method used. increase in the cost of gas or electricity produced cycles. A small fraction of the CO2 produced by combustion will enter condensed water. Geological storage in saline formations or depleted from biomass will be about 50% 6. Thus, in order to get zero emission, the water oil or gas fields usually costs US $ 0.50–US $ 8.00 is treated or disposed of accordingly4.

TACKLING CLIMATE CHANGE GLOBAL VISION CCS is a critical solution for achieving net tonnes of CO2 annually by 2050 if it is to reach A newly developed power cycle currently being problem but generates new ones, principally the cost zero emissions by 2050. It can significantly reduce net zero7. demonstrated in the US uses combustion of natural of hydrogen generation, safety issues surrounding emissions in energy-intensive industries (such According to the International Energy gas with pure oxygen at high pressure to heat hydrogen, and the need to set up a universal hydrogen as metallurgy, cement, and oil refining), heat, and Agency, globally more than 30 million tons of

a circulating CO2 working fluid, which is expanded infrastructure for fuel delivery to the vehicles. The transport. Combined with natural gas, CCS has CO2 is captured from large scale carbon capture,

in a turbine producing power with 100% CO2 capture. latest hydrogen production units with 100% CO2 the ability to produce large volumes of low carbon utilisation, and storage facilities every year. Over Liquid oil products are used for virtually 100% of road capture using natural gas in Western Europe would hydrogen -which is now an attractive solution for 70 percent of this is done in North America. and shipping transport worldwide. The emission produce hydrogen at $1/Kg compared to about $5/ decarbonising some countries power systems. However, industrial facilities are capturing less

of CO2 from road vehicles in the US exceeds CO2 Kg using electrolysis of water. There needs to be The climate change Committee, in its 2019 than one percent of the CO2 that is required to emitted from all power generation. Replacing a transition from battery power to H2 fuel-cell power report «Net Zero: The UK’s contribution to ending meet the Paris agreement targets for 2040, says

hydrocarbon fuel with battery power or hydrogen fuel if we are to achieve zero net CO2 emission by 2050. global warming», concluded that the UK will need a 2018 report compiled by the Global CCS Institute. cells will also eliminate current gross pollution in our A hydrogen infrastructure must be developed, which to capture and store between 75–175 million major cities caused by oxides, hydrocarbon, could be based on the replacement of natural gas and particulate emissions at street level. Battery with hydrogen in the lower pressure pipeline networks electric storage for vehicle propulsion requires the that serve domestic, commercial, and industrial users. power for recharging provided by the electricity grid. Natural gas would be retained in the high pressure CONCLUSION A massive use of battery power would require a very pipeline grid. This change would not only eliminate the

large increase in power generation from fossil fuels, CO2 emission from heating systems but also provide hopefully from new technology power stations with the means to supply H to vehicle fuelling points. 2 The good news is that the technology is inject and store carbon dioxide. As of today, five CO capture. Using off-peak renewable energy is only A fuel cell in a private car could also supply domestic 2 developing every year and it is hoped that the problems more carbon capture and storage facilities are being going to be possible while the fraction of vehicles electricity, when not in use, at high efficiency. To make associated with the use of this technology on a large constructed and another 20 are in “various stages of using batteries is low, probably below 10%. There these necessary changes possible the recovered CO 2 scale, such as cost and so on, will be resolved soon development” globally. will also be large investment requirements for much must be permanently disposed of in deep geological and there will be no technical barriers to effective Finally, we must all give the highest priority larger capacity electrical distribution systems because storage zones that need to be quickly developed and permanent capture and storage of CO2 on a large immediately to provide power to the approximately of the demands for rapid domestic recharging licensed by governments by 2050. scale. Experts say that if used more widely, it could 2 billion people of low income in the world who capability. Using hydrogen fuel cells avoids this come much closer to achieving the ambitious climate have no access to a permanent power supply. To goals set in the Paris agreement. meet their short-term needs we must be prepared to Based on data collected over the last several build conventional polluting fossil fuel capacity, which decades, there is a wide consensus among experts, we can plan to replace with fully developed clean engineers, and geologists that it is safe to permanently systems at a later date.

4 — Sweet et al., 2008 5 — Keating, 2019 6 — BBC Briefi ng: Energy, 2019 7 — The Committee on Climate Change, 2019 8 9 REFERENCES SMART GRID AND DIGITALISATION

1. Bui, M; Adjiman, CS; Bardow, A; Anthony, EJ; Boston, A; Brown, S; Fennell, PS; Fuss, S; Galindo, A; OF ENERGY SYSTEM Hackett, LA; Hallett, «Carbon capture and storage (CCS): the way forward» JP (2018).

2. Metz, B., O. Davidson, H. C. de Coninck, M. Loos, and L.A. Meyer (eds.). IPCC special report on Carbon Dioxide Capture and Storage. Pre-pared by working group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA (2005).

3. Sumida, Kenji; Rogow, David L.; Mason, Jarad A.; McDonald, Thomas M.; Bloch, Eric D.; Herm, Zoey R.; Bae, Tae-Hyun; Long, Jeffrey R. «Carbon Dioxide Capture in Metal–Organic Frameworks» (2012).

4. Sweet, William. «Winner: Clean Coal - Restoring Coal’s Sheen». IEEE Spectrum (2008)

5. Keating, Dave. We need this dinosaur’: EU lifts veil on gas decarbonisation strategy (18 September 2019)

6. BBC Briefing: Energy (9 December 2019) https://news.files.bbci.co.uk/include/newsspec/pdfs/bbc-briefing-energy-newsspec-25305-v1.pdf Sauro Pasini, 7. The Committee on Climate Change, 2019 report «Net Zero: The UK’s contribution to stopping Energy & Information Technology global warming». Consultant, President of International Flame Research Foundation

Even if from the outside it may seem The world needs energy systems more resilient that the electrical network always resembles and flexible and the grid is supposed to play the one familiar to Thomas Edison, in reality an important role in this game. In order to be able we are in the presence of disruptive transformations, to do this, digitalisation can be an enabler to unlock due to mainly external forces: the true potential of the grid, holding the capabilities to build new architectures of interconnected energy Government and regulatory actions to address systems, including breaking down traditional climate change; boundaries between demand and supply. Digital, a world that seems to be everywhere, Increasing empowered and demanding today, was first used by Latins to indicate a finger, and customers; was a fairly unimportant world until the early 20th century, when it became significant and widespread Proliferation of distributed generation and thanks to the development of modern computers. growth of electric vehicles; Today it means the conversion of information into a digital format that can be processed by a computer: Digitalisation of the grid, with increasing it is a process for collecting all the characteristics integration of information technology and of a system (image, sound, signal …) and making operational technology; them available in a numeric format for any future analysis. Cyber-attack risks; The digitalisation of electrical power distribution systems is evolving rapidly and Energy market reform, opening to completely is investing the whole business sector, enabling new forms of competition. operations, markets and services non feasible 10 11

otherwise. It is also becoming a trend, cluttering “smart” involves two-way electric and information FIG. 1. NIST conceptual model up newspapers, technical presentations, articles flow across generation, transmission, distribution and reviews: scope of this chapter is to favour the and utilisation systems, to improve their efficiency, understanding of this digital transformation, trying sustainability, reliability and resilience compared to identify which technology will make the biggest to traditional ones. “Smart” reflects the layer of impact in the future in this sector. intelligence added to the power system that is able Before going into detail, it is worth investigating to “sense” power system’s conditions, “interact” the expected evolution of the network for years with producer and users and “react” to any to come. The penetration of renewables, storage unexpected condition. Existing power systems are and the growth of electric vehicles will start to have evolving to decentralised and self-healing systems, a big impact on the grid management, due to its composed by cooperative and self-organising greater variability. There will be the need to run the energy resources [1] grid in a more responsive fashion, more responsive This evolution is raising the need of digital to customers, to the environment and to the new tools to enable seamless integration of grid layers electrified technologies. Data will be needed and edge devices and enable new technologies to understand how the grid is performing at any and players to take an active role in supporting grid given time, so that to manage variations, possibly operations: the greatest potential for digitalisation autonomously and automatically, as parameters is its ability to break down boundaries between change, and to understand customer needs and energy sectors, increasing flexibility and enabling their interaction with grid services. To collect data, integration across entire systems. Following distributed sensors are needed; once the data ENTSO-E definition [2]: has been collected, it needs to reach, through the “Digitalisation: the increased use of information network, a processing center to be interpreted and and communication technologies and data to trigger a response. In some cases operational to deliver value to the customers and stakeholders, needs require decisions to be made immediately, at maintain and improve the security of supply, utilise say, substation level, at the edge, how it is called. the grid in a cost efficient manner, facilitate new Customers are starting participating to the game, and existing markets, and contribute to energy they can be both consumers and producers at the sector transformation”, the digital grid will be a new same time, depending on market condition or on space in which interoperability criteria predominate, specific request od the operator to reduce demand together with transparency, to facilitate the (Demand Management). exchange of data, and optimisation through The grid needs to be prepared for being the integration of information from different areas responsive and interactive with this variable of interest, from market operators, to customers, Figure 1 outlines one of the most widely Bulk Centralised Generation Domain produces sources of demand and load, and this process will to service providers. accepted smart grid’s conceptual model, developed a high percentage of total required power and offers depend fundamentally on digitalisation. The grid by the National Institute of Standards and Technology the ancillary services to maintain stability and security (NIST). It distinguishes seven different functional of the whole power system: it is connected to market, areas in smart grids, called domains. Each domain operation and transmission domains, although it is is characterised by a set of actors and applications electrically coupled only to the transmission network. that perform actions on energy and data inside the This domain is shrinking in terms of electricity domain and allow the exchange of power and infor- contribution and its modifications towards smart mation between domains, by means of interfaces grid have seen the deployment of large batteries called domain gateways. to support conventional plant’s ancillary services. 12 13

The Transmission Domain has seen Market Domain, this is the place where, FIG. 2. VPP = real-time control of many units and data the development of several innovative technologies currently, electricity markets are changing to fully to improve power transmission’s reliability and exploit the possibilities introduced by smart grids. efficiency, grouped into four clusters [3]: Market domain is connected to every other domain: it receives information with low latency regarding Passive equipment, mainly associated with system’s state and constraints by the operators and high voltage AC (HVAC) transmission devices, service providers and proceed to correctly operate including conductors made with advanced the system. Customers are going to play a more materials to improve their performance active role in satisfying market needs, thanks to (XLPE/polyethylene, Gas Insulated Lines, High the integration of demand-side management or Temperature Low Sag conductors); the aggregation of various distributed generation and loads in the so-called virtual power plants, Active equipment, high power electronics, to The future will also see the integration of electric improve control while increasing transmission vehicles, thanks to the evolution of the existing system capacity and stability, like Phase vehicle-to-grid (V2G) infrastructure. Shifting Trans-formers, FACTS (Flexible A Virtual Power Plant (VPP) consists Alternating Current Transmission System) and in coordinating generating units with different Fault Current Limiters (FCLs); characteristics, renewable, dispatchable ones, storage units, and managing them in such a way Real Time monitoring equipment, to sense that the market sees them as a single, flexible, loading and the limits of individual system “dispatchable” power output toward the grid. components in real time, as well as the overall The introduction of VPP leads to less risky and state of the system, while also monitoring more efficient bidding in electricity market (bad region-wide dynamic loadability constraints prediction of renewable production can be adjusted (WAMS, wide area monitoring systems and by dispatchable generators and energy storage), RTTR, real time thermal rating). and, from the point of view of network operators, VPP simplifies system’s management, since Equipment impacting on TSOs’ operations unbalances are locally addressed by the aggregator such as smart metering devices and electricity managing VPP. storage technologies (CAES, Compressed Air Demand Response efers to the active participation Energy Storage, FES, Flywheel Energy Storage, of customers to power system’s balancing by changes Expanding these services and favouring of rules and regulations. For example, recognition PHS, Pumped Hydro Storage,…). in price of electricity or incentive payments. The smart consumer participation require that market and of the importance of aggregation is still lagging, grid is an essential element for the development of regulatory environments allow it to compete governments and regulators should study the What above is integrated by the fact that demand response systems, since it requires advanced with other forms of flexibility. The success of feasibility of using ICT platforms and smart transmission and distribution players will interact monitoring, communication and control systems, to be more sophisticated forms of flexibility, including contracts to favour consumers involvement. more and more with each other, to face the correctly implemented. Although demand response VPPs, demand response aggregation and locally Big data analytics, monitoring and control, will increase in decentralised stochastic generation has the potential to provide a range of flexibility provided flexibility depends on the evolution increasingly favour this process. and the change in operation rules and procedures services, in most countries around the world demand- to improve load control. A concern related to side flexibility still exists only as an interruptibility the application of the above technologies is the service when a large industrial company provides load FIG. 3. Demand response schematisation and advantages increase in complexity of the transmission system reduction services for reserves or other short term that has to be managed. market. 14 15

The vast majority of demand-side response to aggregate with a software that whole-home load FIG. 5. Home Energy Management System potential lies in large industrial processes, flexibility and put it into service for California’s grid thermal comfort in buildings, EV charging, and operator, CAISO. It can do this without having a direct Behind-the-Meter (BTM) storage and generation. relationship with utilities, under some of the state’s The instantaneous load from EVs can be an order first-ever pilot programs that allow non-utility actors of magnitude higher than the average household to participate directly in the same systems that load and its management could favour call upon large generators and demand response the integration of higher shares of EVs, and enhance portfolios to help CAISO manage its real-time grid- overall system integration, thus enabling greater balancing needs. This is a new window open in the renewable electricity generation. Demand response hidden potential of smart grids. has several benefits related to different domains: A similar approach, combining VPP, customers participating have direct economical demand management, mobile storage systems, rewards, market performance is improved, due to is represented by Citizen Energy Communities. This the lower market power of producers, and operators is a way to increase the involvement of consumers have another flexible tool to address load balancing. accelerating, at the same time, the integration Energy-smart homes вmainly pay themselves of RES into the distributed network, thanks off in energy saved. They can also enlist in a demand- to the implementation of mobile storage systems response program and get paid a set amount for and smart substations, demand-side management turning down power use via direct utility control, or in schemes and low voltage grid digitalisation. A pilot response to set price signals. But with the ongoing for testing these innovative technical solutions developments in smart grid technology, energy- and business models is being carried out within responsive homes will be playing the grid markets. the EU supported projects IElectrix and WiseGRID. Now, an ongoing pilot project in California is seeking

FIG. 4. Home Energy Management System

Operation Domain is the part of the system for but, despite these benefits, AMIs must be very the reliable, safe and efficient operation of power carefully designed in order to ensure the protection systems, challenged by the increasingly diffusion against cyber- attacks. of Distributed Energy Resources (DER) causing With the rise in successful IoT deployments, many challenges like backward flows or intermittent paradigms associated with digital transformation renewable generation. used for concrete business outcomes are emerging. Smart Meters, which have facilitated applications Among these, an idea gaining rapid popularity is the such as dynamic load management, time-of-the-day notion of a “Digital Twin”, after Gartner naming it as rates, and net metering, to name a few, is a two way a top trend in 2017. Digital Twins are virtual models communication network linking a huge number of assets that can be used to gain both real time and of these measuring devices. They have also fueled predictive insight on performance. As a platform, a great deal of behind-the-meter applications, such they can leave in the cloud, and significantly reduce as customer owned renewables, HEMS, electric costs and risks associated with construction, vehicle and more. AMI is not a single technology, maintenance, and performance optimisation it is an incorporation of several technologies which strategy. They can enable saving through process provides an intelligent connection between utilities improvements and for utilities open the door both and consumers. Trough AMIs, utilities can improve for managing complex assets operations, and their analytic capabilities and operate systems create new business opportunities integrating DER, in a more efficient, economic and reliable way customers, … 16 17

A Digital twin provides a model of an asset strategies for microgrid inverters, to maintain FIG. 6. Microgrid schematisation and main features based on its design information, it is than correct frequency and voltage, to ensure stable layered with all relevant data gathered from operations for the power systems connected the system (SCADA, sensors, meters, and any with dynamically variable loads. In real time, the other IoT data that might have an impact): operating conditions of microgrid are variable the more data the digital twin has, the because of intermittent distributed sources more accurately performance and potential and dynamic electrical load demand. This leads outcomes can be modelled. The complexity to change in network topology frequently to aim multiplies as more data streams are integrated, to minimise loss, economic load dispatch, and as in the case of the electric grid. Figure 5 shows proper unit commitment with satisfying all the the data structure of an electrical digital twin for constraints [6]. synchronised planning and operations model. The evolution of microgrid is able to provide A utility can quickly find itself in possession solution to problem of integrating huge amount of of hundreds, or even thousands of digital twins, micro-energy sources without affecting the main each being fed data from just as many systems power supply from the power utility provider. and IoT devices: a digital twin scenario will With the intelligent controllers, microgrid require exponentially more data from the edge works effectively with the existing power system do be brought home to fed the model. Digital distribution services to match the variable load twin technologies will be immensely valuable demand. With the support of protection systems to utilities, especially in an increasingly distributed is able to disconnect from the disturbance energy future, but they will necessitate important if there is any fault in main grid, and microgrid growth through the development of “smart” We have already described innovative concepts for investments in data management capabilities: is able to operate in a standalone mode, which services made possible by the evolving Smart the customers of the future, like demand response, it is worth mentioning Enel’s Urban Futurability improves the reliability and quality of the power Grid. home energy management systems and vehicle to project in San Paolo, with the development to the consumer. Microgrid is a decentralised Services may be performed by the utilities, grid (V2G) and we will now expand this last one, of a Network Digital Twin of Vila Olimpia district, grid with improved efficiency, located near by existing third parties or by new participants which is going to play an important role in future one of the main financial centers of Brazil. to the load, and capable of extracting power from drawn by the new business models. The major smart grid management. Distribution Domain is electrically connected distributed renewable energy sources, whose challenge in the utility domain is to develop the Bidirectional charging (V2G technology) is likely to the transmission domain and to the customers main features are shown in Fig. 6. key interfaces and standards that will enable to become an important feature to assist the adoption domain, it is where most of the distributed Service Providers Domain is where new and a dynamic market-driven ecosystem while of electric vehicles, and already, we are seeing the generation has been installed during the years innovative services to producers, distributors, protecting the critical power infrastructure. formation of partnerships between auto makers, and in the past it was the least smart domain and and customers are managed. This domain These interfaces must be capable to operate power suppliers and platform providers, to provide the source of most of the power outages. Smart is connected through communication flows over a variety of networking technologies while the benefits of bidirectional charging to fleets and grid transition started from this domain with to customer, operation, and market domains. maintaining consistent messaging semantics. consumers. There are encouraging signs globally a large deployment of smart systems coupled Actors in the service provider domain perform Customer domain is what defines the goal at examples of early adoption of bidirectional EV with two way communication links. services to support the business processes of a smart grid. It is electrically connected to charging, and there are major projects taking place One of the best examples of smart distribution of power producers, distributors and consumers. the distribution domain. With the introduction across the globe to study the various use-cases grid is the microgrid: «a group of interconnected These business processes range from of distributed energy sources, the customer of this technology. Over half of these projects loads and distributed energy resources within conventional utility services such as billing and is evolving to a prosumer, i.e., it both produces are in Europe where there is greater access to an clearly defined electrical boundaries, which act consumer account management to enhanced and consumes energy, and it has an active role environment, which enables easier adoption with as a single controllable entity with respect to the consumer services such as management in power systems. Most of energy efficiency policies like consolidated, country-level regulations grid. A microgrid can connect and disconnect of energy use and home energy generation. policies are addressed to this domain, and and experience with energy aggregation markets from the grid to enable it to operate in both The service providers must not compromise automation is playing a big role in reshaping it. where energy can be bought and sold openly. grid-connected and islanded-modes»[5]. In the the stability, reliability, integrity, cyber security islanded mode microgrids are required to ensure and safety of the electrical power network reliable operation even at fault conditions, power when delivering existing or emerging services. system stability during disturbances and power Communications with the operations domain quality. The grid-connected microgrid is needed are vital for situational awareness and system to be maintained synchronism at any situation. control, communications with the consumer and Microgrids need to have advanced control markets domains are vital for enabling economic 18 19

FIG. 7. V2G schematisation REFERENCES

1. ELicwPML. Office of the National Coordinator for Smart Grid Interoperability and I. T. Laboratory, Nist Framework and Roadmap for Smart Grid Interoperability Standards, release 2.0.

2. ENTSO-E The Cyber Physical System for the Energy Transition RDIC/ENTSO-E/POYRY, 2019.

3. L’Abbate A., Migliavacca G., Pagano T., Vafeas A.; Advanced transmission technologies in Europe: a roadmap towards the smart grid evolution, IEEE Xplore Conference: PowerTech, Trondheim 2011.

4. Goebl S.J. Integrating RES into European Energy Markets: it rocks the boat, VGB Meeting Essen, 2015.

5. US Department of Energy Microgrid Available online: http://www.energy.gov.

6. Kannan N. Microgrid Intech Open, 2019.

Bidirectional charging is now within reach the schematisation in domains; let’s now address and with advances in the vehicles, regulations, some challenging open problems, which could and chargers, it is part of an industry that is hinder their deployment in existing power grids. poised to take off. For consumers, this shift will Still work is needed to develop decentral-ised give people an incredible amount of control and architectures, to enable harmonious operation flexibility over which energy they use for their of small-scale electricity supply systems with home, car, or even allow them to be compensated the total system. Communication infrastructure for sending energy back onto the grid. For needs special attention, to allow the operation utilities, this change means having another and trade of potentially millions parties in a resource available to support the grid, to balance single market. Enabling all consumers to play an electricity demand, and to smooth consumption active role in the operation of the system, with spikes with the help of electric car batteries. For or without their own generation, is still a task automakers, it represents a new opportunity to that needs future developments, like managing move beyond transportation and into the energy the huge amount of complexity at the grid edge, management business. that is coming now with increasing intermittent Bidirectional charging is the wave of the renewables and other sources of demand and future and it will see another big jump with supply. connected autonomous electric vehicle (CAEV), Probably the biggest challenge smart playing a vital role in emerging revolution in grids have to face is the protection of their IoT sustainable low-carbon mobility. Despite the based architectures against cyber-attacks. As potential revenue streams and numerous digitisation, DER integration and cloud solutions nation-al and regional schemes to incentivise the are rolled out across the network, the vulnerability uptake of EVs, there are still some challenges of the network is increasing exponentially. facing the technology, like the fact that regulation Associated cybersecurity risk becomes a bigger and ener-gy markets prohibit EVs participation priority with more access points for malicious in the provision of balancing services, some threat actors. There is a variety of approaches technological challenges, and the cyber security to comprehensively safeguard smart grid risk (EVs being connected to smart grids would systems, including encryption and role-based be a new entry point for viruses/malware). access control as well as established IT security In the previous pages we have given an technology such as VPNs and firewalls or even overview of smart grid technologies, following quantum cryptography. 20 21

HYDROGEN AND HIGH CAPACITY GREEN HYDROGEN AND HYDROGEN-BASED FUELS HYDROGEN CARRIERS HYDROGEN FUELS

The need for decarbonised fuels – that we could to make green hydrogen (H2) and oxygen (O2) also name green or greener fuels – is undoubted. to replace conventional fossil fuels. However, this not When coupling renewable electricity with a pro-cess the only viable option. Hydrogen can also be produced, that converts water to hydrogen (and often pure starting from biomass via several thermochemical oxygen as co-product), e.g., an electrolyser, green and biological routes. Besides, there are growing hydrogen is produced, which will never emit carbon oppor-tunities to convert solar radiation directly to neither during the fuel production phase nor when hydrogen via photo electrocatalytic and photocatalytic it will be used. Hence, hydrogen is inherently an option processes2. for decarbonisation1. Hydrogen is also a building block of many Hydrogen has many renewable production other high energy density fuels, including synthetic pathways and diverse opportunities to enter the market hydrocarbons. There are several cat-alytic routes and reach end-uses (Figure 1). On the production side, to convert hydrogen to synfuels. For instance, large amounts of green electricity at a competitive the well-known Fischer-Tropsch catalytic process can price could feed electrolysers. So the industry can convert an H2-rich syngas to even long-chain synthetic use electrolysis (i.e., power-to-gas technology) hydrocarbons3.

Andrea Lanzini Associate Professor, Polytechnic of Turin

FIG. 1. The many production pathways and ends of hydrogen fuel4

ABSTRACT

There is no silver bullet to massive and durable. I explore in this chapter the many decarbonisation. Hydrogen might likely be one pillar applications hydrogen can have in several sectors,

of the energy transition and the green economy. virtually any CO2 emitting segment. Hydrogen is At the beginning of this century, an influential indeed a versatile option to decarbonise the energy economist indeed prophetised the myth of hydrogen sector, large industrial emitters such as cement and economy1. Today, the hydrogen economy did not iron-steel making plants, and the mobility sector. be-come a reality yet, and electrification is more Through green hydrogen injection in the existing trendy. Nonetheless, hydrogen technology – both gas infrastructure, an even more comprehen-sive for its production and storage – has much advanced and cross-sectorial penetration of renewable in the last twenty years, becoming more reliable electricity will be attained.

1 — Rifkin, 2002 3 — Marchese et al., 2020 2 — Calise et al., 2019 4 — Rozzi et al., 2020 22 23

APPLICATIONS KEY ISSUES TO BE ADDRESSED Hydrogen production from renewable power A proper electric market design will be required INDUSTRIAL USE did not reach good economics yet. So additional to allow, and favour, the diffusion of such hybrid plants effort is required over the next decade to make in which fluctuating renewable electricity can be either Cement plants are responsible for about 8% and rotating kiln of a cement plant) to natural hy-drogen more affordable, thus enabling sold to the grid or used for producing hydrogen via decarbonisation pathways in which this energy power-to-gas plants. of global CO2 emissions, while iron-steel making gas and finally hydrogen could deliver a high rate accounts for another 4-7%. A fuel switch from of decarbonisation of heavy industry sectors carrier will play a role. Actually, recent estimates Besides lowering down the production cost on the eco-nomics of converting fluctuating of hydrogen, a significant challenge remains in the coal or petroleum coke (both generally used that are among the top concentrated-source CO2 in blast furnaces of steel plants, and in the calciner emitters. renewable electricity to hydrogen via power- infrastructure. Hydrogen, if not con-verted to other to-gas show that a hydrogen production cost carriers/forms, is a different clearly a different matter at around 2.5 €/kg could be attained within a than electricity or natural gas. New infrastructure will decade provided learning curves keep the same be required to distribute and store it. There will be high CHEMICALS AND HIGH-CAPACITY ENERGY FUELS trend as in the recent past6. So affordable green financial risks for investing in green hydrogen unless hydrogen to decarbonise industry and energy clear policy directions are taken to support to some Green hydrogen can be further processed a circular economy paradigm, where the use of sectors might become soon a viable option. extent a hydrogen-based economy. to produce a wide range of chemicals and high- fossil material is recycled in favour of recycled

capacity energy carriers (green synfuels). For carbon and sustainable H2. Besides FT-fuels,

instance, H2 and CO2 can be blended in a catalytic there are course many other catalytic pathways

reactor like to produce high-carbon number to explore. Methanol production from H2 and

molecules. The so-called syncrude coming CO2 is another widely investigated option. If an out a Fischer-Tropsch (FT) plant can deliver a overly optimistic, or maybe utopic, economist CONCLUSION broad mixture of synthetic hydrocarbons that (Jeremy Rifkin) postulated the benefits of could replace oil extracted from the ground. ‘hydrogen economy’, a few years later a Hydrogen has enormous potential the path toward a heavily decarbonised society. Syncrude typically includes hydrocarbons Nobel laureate in Chemistry (George A. Olah)5 as it effectively presents a platform for a range The benefits of green synfuels is that they are ranging from carbon number C to C , in the advocated a ‘methanol economy5. Methanol 1 80+ of applications including fuel for transportation, generally more practical fuels than hydro-gen form of n-paraffins, α-olefins, with a lower might indeed be a more practical fuel than feedstock in chemical and processing industries, simply because they better resemble (physically content of alcohols and aromatic compounds. hydrogen in that would be more energy dense or bulk energy storage for heat and power and chemically) the fossil fuels that would Hence, there is an enormous potential of the FT (at ambient conditions) and would require less generation. While several processes are available displace. Higher energy density and (generally) process, and other similar catalytic H -to-fuel infrastructural changes to be delivered through 2 for renewable hy-drogen production, the power- liquid or solid form can make synfuels more easily routes to displace hydrocarbons of fossil origin. the market. Just to mention on business case, to-gas way bears the advantage of complementing adaptable to our current infrastructure and also Several sector could benefit from synthetic the oil company Eni S.p.A. has recently invested the intermittent power supply from wind and more compatible with end-use technologies than hydrogen-based fuels and chemicals. The on methanol fuel as a possible substitute solar power genera-tors and thus seems the best hydrogen. transportation sector uses gasoline, diesel and of petroleum fuel on internal combustion short-term opportunity for having large amounts Nonetheless, synfuels contain CO2 that in the jet fuels, and the chemical industry needs long- engines claiming that methanol burns in a more of hydrogen available for deploying decarbonisation. long term would be ‘neutral’ only if captured from chain hydrocarbons as a feedstock for chemical efficient way than gasoline, it also emits less Besides hydrogen, high-capacity hydrogen air. And capturing CO2 from air is doable but high products. Synfuels from green hydrogen and and other pollutants, including carriers are well positioned to enter the market in energy and capital intensive. Hence, a bright future recovered CO could be thus inserted into particulate matter. 2 place of fossil-based counterparts to accelerate for entirely hydrogen only applications is expected.

ENERGY STORAGE

Bulk energy storage is likely to be more and Chemical energy storage can provide the required more one of those crucial assets future power flexibility. Besides, hydrogen could be directly systems must have in order to balance intermittent inserted in the gas grid to make hydrogen-methane production from reneable energy soruces such blends thus using directly the transmission gas grid as wind and solar generators and demand. as a storage infrastructure to some extent

MOBILITY

Green mobility is not just fully electric. Fuel cell economics: hydrogen production costs vs. electricity electric cars with on-board hydrogen fuels can deliver costs, and the fuel cell vs. battery cost. Nonethless, a carbon-free and pollutants-free mobility. Fuel cell the hydrogen pathway is inherently less energy technology has much evolved in the last two decades, efficient since electricity is first converted to hydrogen and reliable technology for moving car, trucks and and hydro-gen is then converted to electricity to drive even trains. In the context of sustanble mobility, the the vehicle motor. Round-trip efficiency of a battery fate of hydrogen mobility is now much depending on system is roughly double than a battery system.

5 — Olah et al., 2009 6 — Glenk et al., 2019 24 25

REFERENCES SMALL MODULAR REACTORS

1. Rifkin J. The Hydrogen Economy: The Creation of the Worldwide Energy Web and the Redistribution of Power on Earth. Polity; 2002.

2. Calise F, D’Accadia MD, Santarelli M, Lanzini A, Ferrero D. Solar Hydrogen Production: Processes, Systems and Technologies. Elsevier 2019. doi:10.1016/C2017-0-02289-9.

3. Marchese M, Giglio E, Santarelli M, Lanzini A. Energy performance of Power-to-Liquid applications integrating biogas upgrading, reverse water gas shift, solid oxide electrolysis and Fischer-Tropsch technologies. Energy Convers Manag X. 2020;6:100041. doi:10.1016/j.ecmx.2020.100041.

4. Rozzi E, Minuto FD, Lanzini A, Leone P. Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses. Energies. 2020;13(2):420. doi:10.3390/en13020420.

5. Olah GA, Goeppert A, Prakash GKS. Beyond Oil and Gas: The Methanol Economy: Second Edition Wiley-VCH; 2009. doi:10.1002/9783527627806.

6. Glenk G, Reichelstein S. Economics of converting renewable power to hydrogen. Nat Energy. 2019;4(3):216-222. doi:10.1038/s41560-019-0326-1. Anthony Roulstone, Business consultant in the nuclear sector, Lecturer in Nuclear Energy, University of Cambridge

Both smaller units and lower costs – to combat Climate Change?

INTRODUCTION

Nuclear power is facing critical and deep issues SMRs also have the potential for much shorter of competitiveness, impeding its ability to play build schedules through the application of modular a significant part in combating Climate Change. construction. Building larger numbers of standardised Small Modular Reactors, with powers less than SMRs would enable further reductions in cost through 500 MWe, are being considered because they can production learning, with the potential to achieve energy be more readily funded and can be delivered more costs as low as $75/MWh. Major and wholesale changes quickly using modular construction and deployment to practices in the nuclear industry will be required to methods. change its approach and enable deep standardisation. SMRs are thought by many to be uneconomic. Also, it will require new strategic suppliers and a change New SMR analyses have shown that the radical use of mind-set from one-off projects to sequential product- of production engineering methods - making use of like delivery systems both for nuclear components and standardisation and modularisation — can provide for whole power stations. a route to offsetting this cost disadvantage. These These changes could make nuclear much more means are widely used by other industries and would competitive allowing it to expand rapidly through the enable capital costs of SMRs to improve upon the delivery of large numbers of SMRs. The question is: economics of large reactors. Whether such SMR projects will be funded and pursued? 26 27

The problems of these projects were analysed Too big to fund – $7-12bn (£5-9bn) per reactor; WHY SMRS? before, the main ones being cost and schedule overruns, leading to bankruptcy and cancelling. Also, Too slow to construct to meet power market t the turn of the decade in 2010, there was a Some of these designs were developments there were similar experiences of higher and uncertain needs – 8-10 years; swell of enthusiasm for Small Modular Reactors of light water reactor (LWR) technology such as costs with US projects, funded by Westinghouse in the (SMRs) and many designs were proposed. New Westinghouse IRIS and B&W mPower, both with 1980s. At the time, there was more success with the Energy that is too expensive to be competitive large reactors had been designed for improved integral reactor designs in which the main vessel French programme of 54 series-built reactors derived - $100-150 (£70-105)/MWh. safety, such as the EPR and AP1000, and these contained both the core and the steam generators. from a common Westinghouse design. They had were starting to be built. They were very big and NuScale proposed an integral mini-reactor (50 a very much better cost and schedule performance3. To become competitive, nuclear needs expensive, being developments and enhancement MWe) that could provide power levels between 200 However, with the desire in the West for private to change and change quickly. If SMRs are the of designs from the 1980s. There was a view that MWe and 600 MWe by adding more modules. Also, funding of nuclear and for a project-by-project answer we need first to understand: Why the SMR the industry should be more innovative in solving there were some smaller (50-100 MWe) designs, approach, we can say that the scale of large reactors enthusiasm of decade ago was dented and whether its historical problems, to make nuclear both safe often offshoots of military reactor technology make them: there is another way of making them competitive? and economic. The aim was also to bring an air proposed for remote locations such as the Russian of modernity to the industry and to better reflect barge-mounted reactor (KLT-40S) and the Chinese the idealism of the battle against Climate Change. ACP100. In principle, all these designs could be Smaller reactors would provide the opportunity deployed within about ten years because they did for both experimentation in design and speed in not need extensive fuel and materials testing, or a implementation. prototype reactor. SMR ECONOMICS Many of these new SMRs were proposed by Other designs are more ambitious and in some small start-up companies and funded by private cases have more technical risk. To distinguish equity. Over fifty SMR concepts were proposed1, them from SMRs, they are called Advanced Nuclear economics are largely driven by economic. In the 2000s, it was proposed that with power outputs broadly in the range 100-500 Modular Reactors (AMRs). They would take longer construction costs and by build timescales. More SMRs could be made competitive in a number MWe2. They could be viewed in two groups. to develop and require more funding prior to being complex designs take longer to build and cost of ways: Firstly, there are designs that proposed radical deployed. Programmes for some of these AMRs more, impacting the competitiveness of nuclear change by either taking up the ideas of Gen IV are being pursued in China, the US, and Canada, but energy. For more than forty years the response Simplification of design resulting from reactors and sustainability as a design aim. They they will take about twenty years to commercialise. to increasing costs has been to make reactors the inherent safety of integral LWRs; proposed systems that either recycled fuel burning There are more immediate reasons for bigger — following the ‘economies of scale’ transuranic elements (and/or thorium), or they considering SMRs now. The Nuclear Renaissance argument that capital costs of larger units do not Co-siting of reactors; offered better thermal efficiency from the use of has stumbled because the large reactor projects grow or scale in proportion to their output. higher temperatures. Secondly, there are designs launched about 2010 have run over budget and As a result, new reactor designs have grown Replication and learning and improved that sought both lower cost and improved safety by run over time (see Table 1) that shows planned in size, with outputs increasing from 300-400 financing. the use of existing reactor technology with integral and actual or current estimate of costs and build MWe in the early 1970s, to 1,750 MWe now. The 4 design and passive cooling. Most of the reactor duration for some recent Western projects. idea of the economies of scale is hardwired A substantial and detailed design study system was housed in a single large vessel and into the thinking of nuclear vendors everywhere. of Westinghouse 335 MWe IRIS was compared able to provide cooling for many hours, or in some Whether larger reactors result in lower cost (per with AP1000, investigating a case where four SMRs cases days. unit power output) is much less certain. It was were built on a single site instead of one AP1000. seen that larger reactors in the US in the 1980s It failed to demonstrate cost parity. The economies took longer to build and resulted in higher costs of scale outweighed the benefits of the IRIS design. due to the issues of managing sites and supply This was a serious blow to the cause of SMRs. TABLE 1. RECENT WESTERN LARGE REACTOR PROJECTS COST & SCHEDULE PERFORMANCE chains and constructing these complex reactor A further US comparative study5 using multiple expert Project Plan Actial / Est Plan Actial / Est designs, which were often unique. Even in the judgements of cost of SMRs and large reactors, French program, where there was standardisation either in small numbers or with larger programmes, EPR-OLK3 €3 bn €11 bn 5 years 15 years of design and practice, larger units showed little provided a wide range of estimates. It gave little or EPR-FL3 €3.3 bn €12.4 bn 4.5 years 15 years evidence of the economies of scale. no support to the idea that the cost of SMRs would Nevertheless, SMRs need to be able to show be lower than larger reactors, but it did support AP1000 Vogtle 3 & 4 $14 bn $25 bn 7 years each 12 years each they can offset the economies of scale to be the view that they could be built more quickly.

1 — OECD-NEA (2011) 3 — Cour de Comptes (2012) Costs of nuclear power 2 — Carelli (2014) SMR Handbook 4 — Carelli (2010) 5 — Abdullah (2013) 28 29

In their studies, Lyons and Lloyd’s approach Cost modelling first applied scaling (see SQUARING THE CIRCLE OF SMR COSTS was to accept the industry-standard methods cost scaling box) to these large reactor costs of modelling cost11 applied to detailed cost centres in the same way and using the EEDB indices13 Because SMRs are smaller in scale and on SMR capital costs and hence energy from the EEDB, which provides the most detailed similar to those used by Carrelli14 but applied require more units to supply an amount of power, costs. If the purpose of SMRs is to provide and the best available breakdown of construction in a more detailed way to cost centres. This the ‘economies of multiples’ argument could be economic nuclear power, these studies cost12. EEDB data were collected in a consistent provided a consistent baseline for the increased useful in offsetting higher costs. Two high level show that innova-tion will be required more manner over many US nuclear projects for specific cost of SMRs built in the stick-built studies6 7 looked at the economies of multiples in the means of production rather than a period of 10 years. This database provides manner, which is typical for large reactors. for SMRs, making use of the scope for production in the reactor technology. LWR reactor technology an analysis of about 200 cost centres including learning from one reactor system to another. is well proven with more than 15,000 reactor years the cost of: compo-nents; site labour, bulk They showed that SMRs could be competitive of global operating experience. Nuclear safety materials and overheads. with large reactors for high rates of production will always be a top priority and the high standards and large build-ing program. of safety in modern reactors can be maintained More recent detailed studies8 9 have confirmed if SMRs employ the same design approaches this view and have sought to define the effect and methods. SMRs will, however, require of key processes and significant varia-bles a completely different approach to con-struction.

FIG. 1. Breakdown of reactor construction cost categories – OECD 7195. THERE ARE THREE STRATEGIES THAT CAN BE DEPLOYED TO ADDRESS THE DIS-ECONOMIES OF SCALE OF SMRS:

1 STANDARDISATION 2 MODULARISATION 3 PRODUCTION LEARNING

The breakdown of costs by category in Figure or related overheads. In fact, indirect labour costs 110 below shows that only a minority of overall costs are higher than direct labour costs. These costs STANDARDISATION relates to bought-out systems: reactor, turbine, reflect the difficulty of construction and the quality control equipment and fuel. The majority of costs standards of nuclear con-struction. are site-related — for civil mechanical construction We know from the French nuclear power for the other production engineering strategies. programme and similar more recent projects in the Standardisation is not often employed in the East15 that standardisation is important. Design nuclear industry because of frequent changes and safety work do not have to be repeated for each in reactor design and the project-by-project new project. More importantly, standardisation approach of the industry, driven by the funding of detailed design and construction enable the requirements of these very large investments. use of the same supply chain and the same A prime example of a lack of standardisation construction teams, which consequently reduces is the US nuclear programme that constructed 100 Cost Power Scaling: For each category of cost, or cost centre (C), costs are proportional cost. There are many different opportunities for cost power reactors with almost none the same. Even to the ratio of power output (P) raised to the index (n): reduction through standardisation. These occur when a similar design was employed, different at all stages in construction and commissioning, contracting teams, different detailed designs and C/C =((P)⁄(P ))n and subsequently in operation. Standardisation is different construction systems were used. Cost 0 0 a prime method of reducing cost and construction were high and extremely variable. duration in other industries and is a prerequisite

6 — Rosner & Goldberg (2011) 9 — Lloyd (2019) 11 — EMWG (2007) 14 — Carrelli (2010) 7— EY (2016) 10 — OECD (2015) 12 — EEDB (1988) 15 — ETI Cost drivers (2018) 8 — Lyons (2019) 13 — EMWG (2007) 30 31

Reviews of the energy industry learning curves19 in factories for a stick-built reactor, this factor share MODULARISATION show that production learning is present in all sectors can be as high as 60% or more for SMRs. Also, other than nuclear. Production learning rates (cost the larger numbers associated with their small size Design for modular construction and assembly Modularisation has the scope to both improve reduction for a doubling of volume) are normally encourages production learning through both higher is not new. It is widely used in shipbuilding, direct labour productivity and to reduce build 15-20%. production volumes and also importantly, increased construction, and the oil & gas industries. It aims to schedule duration. Its success depends on the This absence of cost learning is at the root of the rate of production. transfer work from sites where productivity is low, standardisation of the design, the supply chain, and long term lack of economic competitiveness of nuclear Lyons20 also showed the importance of supply either because of congestion, poor working conditions the construction process. The extent of the benefit power. Once again, the causes are the lack of design chain configuration. Production learning effects were or the lack of tooling and systems, to factories where of modularisation depends on the ability to break the standardisation, the small volumes of production, strongest when the supply chain was stable, well better conditions, tooling and support systems can be design and its systems down into modules that can and a one-off site-based project approach, with long aligned, and with incentives both for continuous cost provided. The scale of nuclear construction together be fabricated, transported and assembled in-situ. In periods between projects. Standard SMRs that are improvement and sharing of this between suppliers with its project-by-project approach means that a study16 of modularisation and SMRs, the focus was designed for modularisation and hence for high and reactor vendors. The results provide evidence modularisation is either difficult, or it fails to pay-back on factory modules, hence the importance of studying degrees of factory build, are better able to establish of the scale and rate of build required for a successful the investment required within a single project. transportation as a constraint. We have seen from and access savings from production learning. Rather SMR programme. They are similar to those of the Hitachi Group’s establishment of a long-term AP1000 construction projects that the size of large than perhaps 30% of the overall cost being made earlier EPIC study of SMR economics21. plan called Environmental Vision 2025 to contribute reactors makes off-site modularisation more difficult.

to an annual reduction of 100 million tons of CO2 SMRs have better potential for modularisation emissions in 2025 compared with 2005 has shown because of their smaller size. that on-site modularisation of their large ABWR design Transport constraints also affect the schedule with design control and standardisation pays-off. They reduction from modularisation. In this study and demonstrated shorter construction schedules and a similar US study of a Westinghouse SMR17 a detailed much reduced site construction labour costs over linked construction schedule was used as the baseline. a number of projects. Conversely, Westinghouse’s Where areas of construction were modularised, ECONOMIC COMPARISONS experience of modularisation with AP1000 has the construction schedule was shortened, taking been less successful, perhaps because of their lack into account the remaining preparation and Recent work has shown that SMRs can be of standardisation, modularisation, and production of experience in the use of these design and assembly time and the critical path. Taken together competitive with large reactors, but can they learning reduce cost to match the energy costs construction techniques and because they did not the productivity improvements from off-site go further and be competitive with other forms of coal, gas or renewables? have several consecutive projects to learn lessons manufacture and the economic effects of reducing of zero-carbon energy? Can the techniques from the first which can be applied as improvements the length of build schedules both have a significant on later ones. effect on construction cost.

THIS QUESTION IS IN TWO PARTS:

PRODUCTION LEARNING 1 How low could SMR total capital costs be reduced?

Reductions in unit cost as production numbers sometimes called ‘learning by doing’, results 2 How do these compare with projected future costs of renewables including increase – ‘production learning’ — were first from improvements in tooling and manufacturing observed for aircraft manufacture18, but are now the processes and practices, and is driven their system costs? norm in almost all industries. Production learning, by an economic need to reduce cost.

Production Learning: For each cost centre, average costs (C) are reduced for each doubling of the number of units (d) with a learning rate of (x):

d C/C0=(1-x)

Where the initial cost is C_0 and: • - Factory learning rate 15%- • - Site learning rate 2%

16 — Lloyd (2019) 19 — MacDonald & Schrattenholzer (2001) 17 — Maronati (2016) 20 — Lyons (2020) 18 — Wright (1936) 21 — Rosner & Goldberg (2011) 32 33

FIG. 2. Sources and distribution of capital costs savings for 250 MW SMR. FROM ONE-OFF PROJECTS TO A PRODUCTION SYSTEM APPROACH

The radical application of the principles What is new? of standardisation and modularisation provide the opportunity for LWR-based SMRs to be Number of customers required to build a viable competitive on capital costs with large reactors for programme (10GWe); First-in-a-Series. Off-site modularisation improves productivity, cuts build time and lowers risk, with Scale of funding ($1-2bn per unit) which the modu-larisation effectiveness being dependent is capable of being provided by private capital on both commodity type and the plant size/output once the build schedule risk is understood; of the reactor system. Large reactors are unable to access many of these modularisation benefits Reactor vendors responsible for the whole because of their design, their size, and the transport power plant design, controlling design constraints. standardisation; SMR can have shorter build schedules. The use of radical modularisation would make Suppliers that are more specialised and build schedules of 3-4 years possible. Production focused on continuous cost reduction over learning for a large SMR programme with a high a period of years and a larger number of units. build rate could reduce capital costs to $4,200/ kWe — equivalent to energy costs of $75/MWh. SMR programmes will involve radical change This would be competitive with both large reactors for an industry that is both ill-prepared and has been and other forms of zero-carbon energy, such weakened by low levels of funding and low levels as renewables. of activity for many years. Leadership will need to The SMR concept is more about building come from new places and take the industry in a new industry than about new reactor technology. new directions. Are there signs that these ideas are It uses LWR technology for which there is extensive being taken up? experience and a good record of safety.

A breakdown of TCIC (Total cost of Such an SMR programme would have Construction including Interest during substantial development and first-of-a-kind Construction) is given in Figure 2 below, for a 250 costs. Lyons estimated these safety and devel- MWe SMR. It shows on the left-hand-side that a opment costs to be over $3bn before production stick-built SMR, constructed as one-off project, starts. Sharing these costs over a 10 GW build would cost over $14,000/kWe compared with an programme would increase the unit capital costs equivalent figure for a large reactor of just over by about 8%. $8,000/kWe. Whole system costs for renewables are Applying the principles of standardisation contentious when they become a significant and modularisation with the linked schedule gains part of the energy system (>30% of supply). would reduce total capital costs to a level where While prime future energy costs of renewables they are competitive with a large reactor on a first- are thought to be in the range $50-90/MWh22 the of-a-series basis. Production learning for a 10 GW additional system costs are in the range $30-40/ programme produced would fur-ther reduce capital MWh23. Nuclear would be competitive with costs to below $4,000/kWe which is equivalent to renewables if it could generate at energy costs an LCOE of $62/MWh (at 7% rate of return). below $80/MWh.

22 — IEA WEO (2019) 23 — OECD 7299 (2019) 34 35

FIG. 3. GE Hitachi BWRX-300 SMR FIG. 4. RR Consortium 400 MWe SMR

REALISATION OF THE SMR CONCEPT

Economically competitive nuclear using SMRs The deployment of what will be completely different Many design studies of SMRs are proposed but four SMR projects are currently showing progress appears to be feasible. They have the potential construction philosophies and production systems towards realisation: for nuclear to make a significant contribution will be a huge challenge for the industry. Though in ad-dressing the challenge of climate change. some reactor vendors are making progress the The questions are less about economics and are willingness to pursue the new production systems Hitachi and GE have designed BWRX-300, a 300 Rolls-Royce is leading a consortium of more about the establishment of programmes of - on which SMR economic success is crucially MWe natural circulation reactor with passive engineering companies to design a 3-loop 440 build and about change in the nuclear industry. dependent - looks less than certain. safety features. These safety features allow MWe PWR, which is conventional in layout and the design to be simplified and hence reduces tech-nology. They understand the importance of its cost. Hitachi has substantial experience standardisation, and of production programmes in design for modularisation. This experience and supply chains for many customers. can be ap-plied to reduce both costs and build This pro-ject has some support from the UK schedule. The design has started the NRC Government but is at an early stage. There is a licensing process in the US and is being heavily need for replacement nuclear capacity in the UK marketed in the US and in Europe. within 10 years and the consortium is building a potential export order book to justify the large NuScale in the US is concluding its safety investment that will be required. licensing process with the NRC and is planning the first demonstration unit to be built in Idaho South Korea and Saudi Arabia have committed on DoE land. Their design is of a very small to build a SMART system, which is a 100 MWe 50-60MWe passively-cooled reactor module, integral PWR designed by KAERI. It is probably several of which are assembled into a larger too small to be economic at 100 MWe but power plant with the modules in a very large could be grown in size to 200 MWe, similar earthquake-proof pool for shielding and cooling. to B&W’s mPower and Westinghouse’s SMR It is more innovative that some other SMRs, designs. They have access to South Korea’s particular-ly with its refuelling system. It is highly capable nuclear industry with its record probably economic only with many modules. of innovation in nuclear construction, and which This could make some of the features of an SMR has demonstrated world-leading levels of build production system difficult to deploy. and cost performan 36 37

REFERENCES POWER-TO-GAS 1. Abdullah (2013) Expert assessments of the cost of light water small modular reactors. 9686–9691 PNAS June 11, 2013 vol. 110 no. 24.

2. Cantor & Hewlett (1988) The economics of nuclear power: Further evidence on learning, economies of scale, and regulatory effects. Resources & Energy. Vol 10. 4 pp 315-335 Dec 1988.

3. Carelli & Ingersoll (2014) Handbook of Small Modular Nuclear Reactors. Woodhead

4. Carelli (2010) Economic features of integral, modular, small-to-medium size reactors. Progress in Nuclear Energy. 52 (2010);

5. Cour de Comptes (2012) The costs of the nuclear power sector. Jan 2012. www.cccomptes.fr

6. Economic Modelling Working Group (2007) Cost estimating guidelines for Generation IV nuclear energy systems. GIF/EMWG/2007/004. Sep 2007.

7. EEDB (1988) Nuclear Energy Cost Data Base. A Reference Data Base for Nuclear and Coal-fired Power plant. Power Generation Cost Analysis. DOE NE-0095 Sep 1988.

8. EY et al. (2016) Can building nuclear reactors become more economic? ММР TEA (2016) Study for BEIS. Project 5-7 Final. Andrea Lanzini Associate Professor, Polytechnic of Turin 9. IEA (2019) World Energy Outlook 2019. Appx B. IEA/OECD

10. Kajiyama (2009) Hitachi’s Involvement in Nuclear Power Plant Construction in Japan. Hitachi Review 58 May 2009.

11. Lloyd C. (2019) Modular Manufacture and Construction of Small Nuclear Power Generation Systems. PhD Dissertation U of Cambridge.

12. https://doi.org/10.17863/CAM.49641 OPPORTUNITIES FOR THE NEXT DECADE 13. Lyons, R. E. (2020) The Effect of Supply Chain Configuration on Small Modular Reactor Economics (Doctoral thesis). CAM.49463Maronati & Petrovic (2016). Total capital investment cost evaluation of ММР modular construction designs. ICAPP (2016). https://doi.org/10.17863/ INTRODUCTION 14. McDonald & Schrattenholzer (2001) Learning rates for energy technologies. Hydrogen and power-to-gas have the potential coupling — which essentially means transferring Energy Policy 29 255-261. to become a leading technology for the next electricity to non-electrified final uses. The decade. Despite the mega-trend of electrification gas infrastructure supplied with greener gas 15. OECD (2015) Nuclear New Build: Insights into Financing and Project Management. NEA 7195. of pow-er sources and final uses, a balanced and could help to maintain the security of supply by reliable power system will likely need the easiness addressing the mismatch between electricity 16. OECD (2019) The cost of decarbonisation. System Costs with high shares of nuclear of transmission and storage of gas — possibly peak generation (more and more by intermittent and renewables. OECD-NEA 7299. a decar-bonised gas. Hence, there is a vast renewable power sources such as wind and solar potential of power-to-gas to enable sector plants) and demand. 17. OECD-NEA (2011) Current status, technical feasibility and economics of ММРs. June 2011.

18. Rosner & Goldberg (2011) Small Modular Reactors – Key to Future Nuclear Power Generation in the US. EPIC U of Chicago Nov 2011.

19. Roulstone (2018) Nuclear at the Cross-Roads. Nuclear Futures J. NF632 May-June 2018.

20. Wright (1936) Factors Affecting the Cost of Airplanes. Journal of Aeronautical Sciences, 3(4) (1936): 122–128 38 39

THE ROLE OF POWER-TO-GAS FIG. 1. Power-to-gas pathways IN THE FUTURE ENERGY SYSTEM

THE ROLE IN THE ENERGY SYSTEM

Power-to-gas can play a pivotal role widely used chemicals such as ammonia in the decarbonisation of energy systems and, or methanol, specialty chemicals, conventional consequently, in the energy transition of our fuels such as substitute natural gas or gasoline, societies. After several ‘start-and-stops’, the and so forth2 . International Energy Agency (IEA) underlined, Moreover, power-to-gas could enable in a recent report dedicated to hydrogen, how the seasonal storage of surplus electricity in conditions are now fa-vourable for this energy addition to pumped hydro-power, and with a larger carrier and chemical feedstock to reappear once energy capacity provided that apt storage sites again on the scene of the global energy system will be identified. Seasonal storage is nec-essary, world and establish a market1 . as several studies have shown that a high level Hydrogen becomes thus a platform of renewable generation based intermittent molecule that could be used as-is – i.e., renewables, such as solar and wind generation, as an energy vector and a fuel – or to synthesise will require seasonal storage systems.

MAIN CONCEPT AND TECHNOLOGY

Power-to-gas (often abbreviated P2G or PtG) to methane, also known as synthetic natural describes the process of converting renewable gas (SNG), by reaction with carbon dioxide electricity to a gas such as hydrogen or methane in a catalytic reactor. via water or steam electrolysis – mainly alkaline Commercial low-temperature electrolysers electrolysis, proton exchange membrane (PEM) (PEM and alkaline ones) generally deliver electrolysis, and solid oxide electrolysis cells hydrogen at high pressure (around 30 bars), (SOECs) – see Figure 1 for a schematic od the and the conversion efficiency is within the different pathways that could be developed range of 60-65%. High-temperature electrolysis starting from the electrolytic hydrogen. (based on SOECs) results in higher power- The electrolyser is thus the key to-hydrogen efficiency up to 80%, even more, electrochemical process component in if by-product steam is available on-site, according a power-to-gas plant, which essentially to the roadmap of Green Industrial Hydrogen splits water into hy-drogen and oxygen. 2.0 project. Unfortunately, the SOEC technology Produced hydrogen is either used as is (i.e., is still not fully mature, and cannot operate as a chemical feedstock or fuel without further above atmospheric pressure without significant chemical modification), or can be upgraded degradation.

1 — The Future of Hydrogen report by IEA, June 2019 2 — Marchese et al., 2020 40 41

TECHNOLOGICAL OPTIONS KEY ISSUES TO BE ADDRESSED Low-priced electricity and/or high price volatility would be that of direct-air-capture instead of using POWER-TO-HYDROGEN OR POWER-TO-METHANE? is key to reach H2 production costs from power-to-gas the same energy to produce methanol to substitute plants that are competitive. The Internation-al Energy gasoline. Power-to-hydrogen: green hydrogen can Power-to-methane: synthetic methane is Agency (IEA) estimated for hydrogen from renewables Power-to-power solutions, in which produced be used in a range of pathways that help produced by reacting electrolytic hydrogen with carbon (i.e., power-to-gas) a cost range of 3.0-7.5 USD/kg, hydrogen is converted back to electricity with a fuel to decouple renewable generation from electricity dioxide in a catalytic reactor. The produced gas can be which is still at least double the cost of fossil hydrogen cell generator, are also attractive, as they would provide demand, thus helping to avoid curtailment. Green used as a direct replacement for fossil natural gas in (that also includes the cost of carbon capture). a means to store electricity. However, the overall round- hydrogen indeed can be used for energy storage, gas networks. CO2 is generally sourced from locally The potential of power-to-methane or power-to- trip efficiency of such systems is lower than 40% when sustainable chemicals, mobility, blending into available waste sources such as biogas upgrading methanol seems limited and controversial compared relying on low-temperature electrolysers/fuel cells. the natural gas grid, and more. plants or captured from industrial emitters located in to the power-to-hydrogen op-tion. The idea of recycling Higher round-trip efficiency could be attained with high the proximity of the power-togas plant. captured or waste CO2 and re-using it via power-to-gas temperature solid oxide cells (SOCs), and reversi-ble

for producing a fuel (that soon will turn again into CO2 concepts are being developed in which the same cells emis-sions) has been recently criticised by the scientific are operated in both electrolysis and fuel cell modes. community . Authors argued that a better way of using The reversible SOC could be an ex-citing opportunity surplus renewable electricity to assess climate change to reduce capital costs. POWER-TO-GAS PLANTS

Recently, large-scale hydrogen projects have for hydrogen and power-to-gas – and the widest been deployed, and manufacturers can deliver opportunity for the deployment of related technologies MW-scale plants. For instance, the well-known and infrastructure – is the coupling with large-scale industry group thyssenkrupp now delivers 10 to 20 intermittent power sources such as offshore wind. MW power-to-H2 modules. As reported by ENTSO-G For instance, in Europe, to be in line with the Paris CONCLUSION (the European Network of Transmission System Agreement, there is the potential for the North Sea Operators for Gas), there are hundreds of MW of to become a hub for the production of renewable The fate of power-to-gas within the next decade, Besides power-to-H2, possible alternative already planned installations for power-to-gas electricity by offshore wind turbines by means is strictly linked to the role that decarbonisation will routes are viable. Produced hydrogen can be plants across Europe within the next two years. of North Sea Wind Power Hub programme. Power-to- take in the energy and climate agenda of Na-tions. easily upgraded to other fuels or chemicals. Besides providing decarbonised fuel hydrogen would enable sector coupling and maintain As a strong push towards decarbonisation is Especially interesting are those pathways that or feedstock to industrial sectors, which are the security of supply by addressing the mismatch expected, this will inevitably increase the share of foresee the hydrogenation of waste or captured currently using hydrogen, the longer-term focus between electricity peak generation and demand. intermittent power sources, especially wind and CO2 to make durable chemicals. For instance, solar power. Power-to-hydrogen solutions can the EU-funded project ICO2CHEM plans the

provide the required flexibility to the overall energy conversion of industrial waste CO2 and H2 into system to accommodate such variable re-newable marketable chemicals such as white oils and power sources. heavy wax. FUTURE OPPORTUNITIES

Decarbonisation and electrification are such as refineries and steel-making plants. Hence, paradigms of the ongoing energy transition. the hydrogen demand is expected to increase in the They often are the same thing as renewable coming decades, thanks to the decarbonisation of (i.e., carbon-free) electricity could be abundantly the industry sector. Then, the mobility sector could produced by solar and wind farms at low or soon boost hydrogen demand, starting from the competitive levelised-cost-of-electricity (LCOE) public transportation segment with hydrogen buses in many regions worldwide. Nonetheless, gas replacing conventional buses. Then, the blending could maintain a leading as an energy carrier of hydrogen into the NG grid is another new and to provide flexibility and storage to the overall concrete perspective. Several trials are going to

energy system. In addition, a bright and emerging test the grid reliability in accepting H2-NG blends. direction is that the gas will be more and more For instance, the Italian transmission gas operator hydrogen or non-fossil synthetic methane. (Snam S.p.A.) has recently conducted a successful

Hydrogen is gradually replacing fossil gas (from trial with 5% and 10% H2 blends transported in the either coal or natural gas) in the industry sector, NG gas grid.

3 — Daggash et al., April 2018 4 — Fasihi et al., July 2019 42 43

REFERENCES TECHNOLOGIES FOR 1. The Future of Hydrogen – Analysis IEA. Accessed May 3, 2020. COMPACT AND EFFICIENT https://www.iea.org/reports/the-future-of-hydrogen.

2. Marchese M, Giglio E, Santarelli M, Lanzini A. Energy performance of Power-to-Liquid applications ENERGY STORAGE integrating biogas upgrading, reverse water gas shift, solid oxide electrolysis and Fischer-Tropsch technologies. Energy Convers Manag X. 2020;6:100041. doi:10.1016/j.ecmx.2020.100041

3. Daggash HA, Patzschke CF, Heuberger CF, et al. Closing the carbon cycle to maximise climate change mitigation: Power-to-methanol: vs. power-to-direct air capture. Sustain Energy Fuels. 2018;2(6):1153-1169. doi:10.1039/c8se00061a

4. Fasihi M, Efimova O, Breyer C. Techno-economic assessment of CO 2 direct air capture plants. J Clean Prod. 2019;224:957-980. doi:10.1016/j.jclepro.2019.03.086

Aliasghar Ensafi, Professor, Department of Chemistry, Isfahan University of Technology

INTRODUCTION

Breakthroughs in the electrochemical energy Meanwhile, supercapacitors play a pivotal role storage technologies like lithium (or sodium) ion that bridges the gap between traditional capacitors batteries and supercapacitors, for mobile electronics and rechargeable batteries, which occupy a prominent (small sizes), transportation (medium sizes), electric position in the development of energy storage devices. grid storage (large sizes), and portable and stationary Despite all the extensive researches that have been applications, have paved the road toward an emerging done on energy storage mechanisms, lithium battery market with unlimited potential1 . Electrochemical technologies are still lagging behind the increasingly energy storage plays an important role in the punitive performance defined by industries3 . To effective utilisation of solar and wind as renewable overcome existing weaknesses, supercapacitors with energy sources to achieve a cleaner world. Today, unique features such as unrivaled ability to deliver high the production, development, and penetration of power density, ultra-high charge and discharge rate, a new generation of high-performance, affordable excellent stability, long cycling life, and safe operation, cost electrochemical energy storage systems with have shown an amazing and promising perspective on improved safety in the major new markets, require the commercialisation process 4. These energy storage understanding, controlling, and predicting newly devices are widely used as portable devices, industrial developed structures with novel synthesis approaches power, and energy applications to create long-term and enhanced properties2 . change towards sustainable generation, management,

1 — Yang et al., 2019 3 — Sarno, 2019 2 — Du et al., 2020 4 — Satpathy et al., 2020 44 45

and energy consumption without explicit reliance on a long cycle of life in the best possible way 7. area, low electrical resistance, and high chemical Thanks to the double-layer mechanism, renewable resources. The combination of specific and Supercapacitors can cycle hundreds of thousands stability12. As shown in Fig. 1b, by applying the amount of charge stored at each unit high energy with high power density requires special of times with minimal change in performance. The the appropriate voltage across the device, the voltage (i.e. the capacitance) is proportional energy storage mechanisms in which supercapacitors lifetime span of supercapacitors is 10 to 20 years, positive and negative ionic charges within the to the interfacial specific surface area are the best in terms of performance and delivery of so that their capacity may be reduced to only 80% of electrolyte utilise the high surface area of porous of the electrodes. By increasing the specific energy consumption on a small scale to industrial their initial value after 10 or so years. Due to the low electrodes and accumulate at the surface surface area of the electrodes used in the design scale 5. equivalent series resistance, high power density, and of them13. This is a successful ending to the story of supercapacitors, the efficiency of these energy In general, supercapacitors have a much longer high load currents, supercapacitors have the potential of conventional capacitors. storage devices is significantly improved. Using lifespan in terms of the charge cycle’s number to achieve almost instantaneous charge in seconds. In supercapacitors, each of the charged very porous materials that increase the specific compared to ion batteries6 . Besides, supercapacitors Thanks to the unique features of supercapacitors, electrodes; continuously attracts the oppositely surface area to more than 1000 m2 (approximately discharge their stored energy faster. Researches to their temperature performance is very strong, and they charged species from the electrolyte solution, one-fifth the size of a football field) per gram are date have proven the high efficiency and advantages deliver energy at temperatures as low as -40°C 8. which balances the charge of the electrode very suitable and fabulous options18. of supercapacitors. In this way, they can maintain (Fig. 1b). The potential profile of the whole cell In pseudo-supercapacitors, charge storage is given by equation 1: is performed using faradic or electrochemical processes following the across interface19. U = EP - EN (1) By applying a suitable voltage to the system, the polarised ions in the electrolyte move towards In this equation, U is the voltage, and EP the opposite polarised electrode. The interface and EN respectively, are related to the positive created between the surfaces of the electrodes and negative electrodes. As shown in Figure and the adjacent electrolyte leads to the formation 2, during the charge phase, the surface of the of an electric double layer. The movement of one SUPERCAPACITOR PERFORMANCE electrodes attracts the opposing charged ions layer of ions at the surface of the electrode20 in the electrolyte solution.This separation and the second layer of adjacent solvated ions The structure of supercapacitors is very and characteristics affect the overall performance of charges in the electrode-electrolyte interface to the polarised electrode creates a static electric similar to conventional capacitors and batteries, of supercapacitors9. is known as the “electric double-layer effect” and field, which forms to double-layer capacitance. consists of electrode and electrolyte material, The storage energy in supercapacitors follows is how electrical energy is stored. Species that are Along with the electric double layer created, some current collector, binder, and separators (Fig. 1b). two main mechanisms, which include electric absorbed directly on the surface of the electrode of the dissolved electrolyte ions act as electron In the design of supercapacitors, like capacitors, double layer charge storage at the interface forms a layer called the «inner Helmholtz plane». donors, penetrating the separating solvent layer a pair of parallel porous electrodes are used, except between the electrolyte and the electrode, and Besides, there is a layer called the “outer Helmholtz and absorbed on the surface of the electrode. Then, that the space between them is filled with an electrolyte pseudo-supercapacitors, which includes reversible plane” which is composed of ions solvated by the atoms on the electrode’s surface by delivering solution instead of a solid dielectric. Usually, fast faradic redox reactions at the surface opposite charge to the electrode. the absorbed charges to the electrode, lead the electrodes used in the structure of supercapacitors of the electrode10. In supercapacitors, the charge Because the potential drop is largely limited to the formation of a faradic current. This process are identical. To prevent the electrodes from is electrostatically stored (non-faradic) using a double to this region, which varies from 0.1 to 10 nm, of faradic charge transfer is followed by a sequence touching each other and creating a short circuit, layer (Helmholtz layer) and thus accumulates at the the corresponding electric field strength of rapidly reversible redox reactions, electrosorption, an ion-permeable separator is used between the two electrode/electrolyte interfaces following natural is in the range of one thousand kV/mm, allowing or an intercalation process between the electrode electrodes. Dielectric strength, chemical immobility, attraction. The voltage applied to the electrodes supercapacitors to be 10,000 times more charge surface and electrolytes. porosity (ion permeability), and shallow thickness controls the performance of supercapacitors11. per unit mass than electrolytic capacitors17. are the most important requirements for their The electrodes used in the construction effective performance. These three key elements of supercapacitors should be selected in such in supercapacitors, their unique properties, a way that they have the highest specific surface

FIG. 1A. Structure of supercapacitors7 FIG. 1B. Charged and discharged states of an electric double-layer capacitor14

5 — Burke et al., 2014 8 — Wang et al., 2019 11 — Chen, 2017 12 — Wu et al., 2017 15— Samantara, 2018 18— Kim et al., 2015 6 — Musolino et al., 2010 9 — Yao et al., 2020 13 — Najib et al., 2019 16 — Sharma et al., 2010 19 — Bakker et al., 2012 7 — Samantara, 2018 10 — Vangari et al., 2013 14 — Khanna, 2019 17 — Kim et al., 2015 46 47

FIG. 2 FIG. 3. Schematic illustration of different heterogeneous materials based on structural complexity26.

In both methods, the storage mechanisms charged layers (double layer) with an extremely between the electrode and the electrolyte material small distance between them21. The value are synergistic. In general, when voltage is applied of capacitor C is proportional to the surface (A), and to supercapacitors, the ions in the electrolyte the distance (d) between the two layers and relative solution diffuse into the opposite charged dielectric constant (εr) is shown in equation 2: porous electrodes. The accumulation of charge on the surface of the electrodes then creates two C/A = ε0εr/d, (2)

supercapacitors have found a special place in short- angle of the incident between the incoming sunlight SUPERCAPACITOR FUTURE ASPECTS term energy storage applications and applications and a photovoltaic panel, and significantly improve that require high-energy intermittent energy pulses. electricity generation. Therefore, supercapacitors Supercapacitors can lead to a huge revolution as the storage elements can help the solar panel The most important approach of researchers which has a strong and packed hexagonal in the automotive industry. Since auto batteries suffer provide the energy needed to move according is to use materials, compounds, and methods honeycomb lattice structure with high strength25. from lifespan, power delivery, and environmental to the direction of the sun. As the supercapacitor that can significantly increase the efficiency and Graphene continuously stores high-energy stage limitations, supercapacitors are good is a maintenance-free product, it can be used performance of supercapacitors so that they can capacitive and is very cost-effective and highly alternatives to lead-acid and lithium-ion batteries29. in most weather conditions. Supercapacitors have maintain and improve their special position22. efficient, accelerating the kinetic response One of the most attractive uses of supercapacitors shown a wide range of applications and tremendous Undoubtedly, attention to nanotechnology can of the diffusion process as well as achieving good is to use them as solar trackers30. Today, solar performance for use in small gadgets31. Thus, due provide tremendous prospects for energy storage cycling stability27. Figure 3, designed by Liu et al., power, as a source of clean, green, and free energy to providing sustainable and sufficient power, applications23. Among all the new materials uses nanostructured materials as high-energy has become a promising source of renewable without sacrificing their performance and proposed in energy storage applications, graphene electrodes with high-rate capabilities, which can be energy in the industrial world. Due to the ever- reliability, they are very important. They charge has shown special potential. Graphene has proven dramatically improved if the optimal nanostructured increasing progress, solar panels have been able the device in just a few minutes. The long lifespan that it can be the mother of all possible graphitic materials are used28. Examining the efficiency and to become an integral part of electricity generation. of supercapacitors extends the lifespan forms24. In this way, it can be wrapped into 0D performance of graphene nanostructures with Supercapacitors can store electrical energy of electronics without losing storage space with buck balls, a roll of 1D nanotubes, and stacked 3D different morphologies can significantly increase by connecting to solar panels. Recently, using new age. Besides, compared to batteries that contain graphitic forms. These compounds have structural the storage capabilities of energy storage devices. and advanced technologies, energy storage devices toxic chemical substances in their structure, they advantages. Graphene consists of a thin layer The use of supercapacitors has opened with the ability to track solar energy have been significantly reduce the dangers of disposing of pure carbon with unique conductivity, up tremendous prospects for improving the introduced. Thanks to this technology, trackers can of the device and are environmentally friendly. environmental compatibility, and high stability, quality of life and the environment. As a result, orient a payload toward the sun by minimising the

20 — Khanna, 2019 23— Cao et al., 2018 26 — Panda et al., 2020 29 — Liu et al., 2011 21 — Jiang et al., 2019 24 — Zhao et al., 2011 27 — Lai et al., 2019 30 — Moftah et al., 2019 22 — Najib et al., 2019 25 — Yusuf et al., 2019 28 — Khanna, 2019 31 — Hamdan et al., 2019 48 49

33 FIG. 4 Figure 4a shows the use of supercapacitors as shown in Figure 4 (d) . This application as a buffer in front of rechargeable batteries. In this has attracted many fields of research, such case, the battery life is extended, and fast energy as bioengineering, drug delivery, tissue 4А. Application of graphene-based 4B. Graphene-based supercapacitors recycling is possible. Supercapacitors can also engineering, biotechnology, and bioinformatics supercapacitors in various sectors or miniaturised bioelectronics be used as flexible and wearable electronics (Figure (Figure 4 (e))34. Another prominent example is the 4b). Other key aspects of graphene-based biological antibacterial activity of graphene oxides, which supercapacitors may improve pacemakers have potential effects on treatment methods and and implantable medical devices using ions derived disease diagnosis (Figure 4 (f)). that can lead to long-lasting cardiac pacemakers,

4C. Properties affecting the 4D. Improved pacemakers 4E. Graphene-based synthesis of graphenebased and implantable medical devices supercapacitors for supercapacitors varying from 2D tissue engineering grapheneto 3D curved graphene REFERENCES 1. Yang, D.; Liu, C.; Rui, X.; Yan, Q. Embracing High Performance Potassium-Ion Batteries with Phosphorus-Based Electrodes: A Review. Nanoscale 2019, 11 (33), 15402–15417. https://doi.org/10.1039/C9NR05588F.

2. Poonam; Sharma, K.; Arora, A.; Tripathi, S. K. Review of Supercapacitors: Materials and Devices. J. Energy Storage 2019, 21, 801–825. https://doi.org/10.1016/j.est.2019.01.010.

3. Wei, L.; Wu, M.; Yan, M.; Liu, S.; Cao, Q.; Wang, H. A Review on Electrothermal Modeling of Supercapacitors for Energy Storage Applications. IEEE J. Emerg. Sel. Top. Power Electron. 2019, 7 (3), 1677–1690. https://doi.org/10.1109/JESTPE.2019.2925336.

4. Du, M.; Li, Q.; Zhao, Y.; Liu, C.-S.; Pang, H. A Review of Electrochemical Energy Storage Behaviors Based on Pristine Metal–Organic Frameworks and Their Composites. Coord. Chem. Rev. 2020, 416, 213341. https://doi.org/10.1016/j.ccr.2020.213341.

5. Agudosi, E. S.; Abdullah, E. C.; Numan, A.; Mubarak, N. M.; Khalid, M.; Omar, N. A Review of the Graphene Synthesis Routes and Its Applications in Electrochemical Energy Storage. Crit. Rev. Solid State Mater. Sci. 2019, 1–39. https://doi.org/10.1080/10408436.2019.1632793. 4F. Graphene-based 4G. Antibacterial activity of graphene supercapacitors for drug-delivery with altered biological properties 32 6. Sun, H.; Zhu, J.; Baumann, D.; Peng, L.; Xu, Y.; Shakir, I.; Huang, Y.; Duan, X. Hierarchical 3D Electrodes for Electrochemical Energy Storage. Nat. Rev. Mater. 2019, 4 (1), 45–60. https://doi.org/10.1038/s41578-018-0069-9.

7. Sarno, M. Nanotechnology in Energy Storage: The Supercapacitors; 2019; pp 431–458. https://doi.org/10.1016/B978-0-444-64337-7.00022-7.

8. Satpathy, S.; Das, S.; Bhattacharyya, B. K. How and Where to Use Super-Capacitors Effectively, an Integration of Review of Past and New Characterization Works on Super-Capacitors. J. Energy Storage 2020, 27, 101044. https://doi.org/10.1016/j.est.2019.101044.

9. Gautham Prasad, G.; Shetty, N.; Thakur, S.; Rakshitha; Bommegowda, K. B. Supercapacitor Technology and Its Applications: A Review. IOP Conf. Ser. Mater. Sci. Eng. 2019, 561, 012105. https://doi.org/10.1088/1757-899X/561/1/012105.

32 — Panda et al., 2020 33 — Inamuddin et al., 2019 34 — Mosa et al., 2017 35 — Verma et al., 2018 50 51

10. Burke, A.; Liu, Z.; Zhao, H. Present and Future Applications of Supercapacitors in Electric and Capacitors. Phys. Chem. Chem. Phys. 2014, 16 (14), 6519. https://doi.org/10.1039/c3cp55186e. Hybrid Vehicles. In 2014 IEEE International Electric Vehicle Conference (IEVC); IEEE, 2014; pp 1–8. (27) Kim, B. K.; Sy, S.; Yu, A.; Zhang, J. Electrochemical Supercapacitors for Energy Storage and https://doi.org/10.1109/IEVC.2014.7056094. Conversion. In Handbook of Clean Energy Systems; John Wiley & Sons, Ltd: Chichester, UK, 2015; pp 1–25. https://doi.org/10.1002/9781118991978.hces112. 11. Musolino, V.; Tironi, E.; di Milano, P. A Comparison of Supercapacitor and High-Power Lithium Batteries. In Electrical Systems for Aircraft, Railway and Ship Propulsion; IEEE, 2010; pp 1–6. 27. Ban, S.; Zhang, J.; Zhang, L.; Tsay, K.; Song, D.; Zou, X. Charging and Discharging Electrochemical https://doi.org/10.1109/ESARS.2010.5665263. Supercapacitors in the Presence of Both Parallel Leakage Process and Electrochemical Decomposition of Solvent. Electrochim. Acta 2013, 90, 542–549. 12. 1González, A.; Goikolea, E.; Barrena, J. A.; Mysyk, R. Review on Supercapacitors: Technologies and https://doi.org/10.1016/j.electacta.2012.12.056. Materials. Renew. Sustain. Energy Rev. 2016, 58, 1189–1206. https://doi.org/10.1016/j.rser.2015.12.249. 28. Wang, C.-M.; Wen, C.-Y.; Chen, Y.-C.; Chang, J.-Y.; Ho, C.-W.; Kao, K.-S.; Shih, W.-C.; Chiu, C.-M.; Shen, Y.-A. The Influence of Specific Surface Area on the Capacitance of the Carbon Electrodes 13. Wang, L.; Xie, X.; Dinh, K. N.; Yan, Q.; Ma, J. Synthesis, Characterizations, and Utilization of Oxygen- Supercapacitor. In The Proceedings of the 2nd International Conference on Industrial Application Deficient Metal Oxides for Lithium/Sodium-Ion Batteries and Supercapacitors. Coord. Chem. Rev. Engineering 2015; The Institute of Industrial Applications Engineers, 2015; pp 439–442. 2019, 397, 138–167. https://doi.org/10.1016/j.ccr.2019.06.015. https://doi.org/10.12792/iciae2015.077.

14. Wang, R.; Yao, M.; Niu, Z. Smart Supercapacitors from Materials to Devices. InfoMat 2020, 2 (1), 29. Vondrak, J.; Sedlarikova, M.; Dvorak, P. Review on Electrodes with Extended Surface Area for 113–125. https://doi.org/10.1002/inf2.12037. Supercapacitors; 2012; pp 75–84. https://doi.org/10.1149/1.4729089.

15. Berrueta, A.; Ursua, A.; Martin, I. S.; Eftekhari, A.; Sanchis, P. Supercapacitors: Electrical 30. Taer, E.; Agustino, A.; Farma, R.; Taslim, R.; Awitdrus; Paiszal, M.; Ira, A.; Yardi, S. D.; Sari, Y. Characteristics, Modeling, Applications, and Future Trends. IEEE Access 2019, 7, 50869–50896. P.; Yusra, H.; et al. The Relationship of Surface Area to Cell Capacitance for Monolith Carbon https://doi.org/10.1109/ACCESS.2019.2908558. Electrode from Biomass Materials for Supercapacitor Aplication. J. Phys. Conf. Ser. 2018, 1116, 032040. https://doi.org/10.1088/1742-6596/1116/3/032040. 16. Vangari, M.; Pryor, T.; Jiang, L. Supercapacitors: Review of Materials and Fabrication Methods. J. Energy Eng. 2013, 139 (2), 72–79. https://doi.org/10.1061/(ASCE)EY.1943-7897.0000102. 31. Bakker, M. G.; Frazier, R. M.; Burkett, S.; Bara, J. E.; Chopra, N.; Spear, S.; Pan, S.; Xu, C. Perspectives on Supercapacitors, Pseudocapacitors and Batteries. Nanomater. Energy 2012, 1 (3), 136–158. 17. Li, X.; Wei, B. Supercapacitors Based on Nanostructured Carbon. Nano Energy 2013, 2 (2), https://doi.org/10.1680/nme.11.00007. 159–173. https://doi.org/10.1016/j.nanoen.2012.09.008. 32. Wang, Y.; Song, Y.; Xia, Y. Electrochemical Capacitors: Mechanism, Materials, Systems, 18. Samantara, A. K.; Ratha, S. Components of Supercapacitor; 2018; pp 11–39. https://doi. Characterization and Applications. Chem. Soc. Rev. 2016, 45 (21), 5925–5950. https://doi. org/10.1007/978-981-10-7263-5_3. org/10.1039/C5CS00580A.

19. Chen, G. Z. Supercapacitor and Supercapattery as Emerging Electrochemical Energy Stores. Int. 33. Jiang, Y.; Liu, J. Definitions of Pseudocapacitive Materials: A Brief Review. ENERGY Environ. Mater. Mater. Rev. 2017, 62 (4), 173–202. https://doi.org/10.1080/09506608.2016.1240914. 2019, 2 (1), 30–37. https://doi.org/10.1002/eem2.12028.

20. Gautham Prasad, G.; Shetty, N.; Thakur, S.; Rakshitha; Bommegowda, K. B. Supercapacitor 34. Viswanathan, B. Supercapacitors. In Energy Sources; Elsevier, 2017; pp 315–328. Technology and Its Applications: A Review. IOP Conf. Ser. Mater. Sci. Eng. 2019, 561 (1). https://doi.org/10.1016/B978-0-444-56353-8.00013-7. https://doi.org/10.1088/1757-899X/561/1/012105. 35. Huang, S.; Zhu, X.; Sarkar, S.; Zhao, Y. Challenges and Opportunities for Supercapacitors. APL 21. Salanne, M.; Rotenberg, B.; Naoi, K.; Kaneko, K.; Taberna, P.-L.; Grey, C. P.; Dunn, B.; Simon, P. Mater. 2019, 7 (10), 100901. https://doi.org/10.1063/1.5116146. Efficient Storage Mechanisms for Building Better Supercapacitors. Nat. Energy 2016, 1 (6), 16070. https://doi.org/10.1038/nenergy.2016.70. 36. Wu, Y.; Cao, C. The Way to Improve the Energy Density of Supercapacitors: Progress and Perspective. Sci. China Mater. 2018, 61 (12), 1517–1526. 22. Wang, F.; Wu, X.; Yuan, X.; Liu, Z.; Zhang, Y.; Fu, L.; Zhu, Y.; Zhou, Q.; Wu, Y.; Huang, W. Latest https://doi.org/10.1007/s40843-018-9290-y. Advances in Supercapacitors: From New Electrode Materials to Novel Device Designs. Chem. Soc. Rev. 2017, 46 (22), 6816–6854. https://doi.org/10.1039/C7CS00205J. 37. Zhao, X.; Sánchez, B. M.; Dobson, P. J.; Grant, P. S. The Role of Nanomaterials in Redox-Based Supercapacitors for next Generation Energy Storage Devices. Nanoscale 2011, 3 (3), 839. 23. Najib, S.; Erdem, E. Current Progress Achieved in Novel Materials for Supercapacitor Electrodes: https://doi.org/10.1039/c0nr00594k. Mini Review. Nanoscale Adv. 2019, 1 (8), 2817–2827. https://doi.org/10.1039/C9NA00345B. 38. Yusuf, M.; Kumar, M.; Khan, M. A.; Sillanpää, M.; Arafat, H. A Review on Exfoliation, 24. Khanna, V. K. Supercapacitors. In Flexible Electronics, Volume 3; IOP Publishing, 2019. Characterization, Environmental and Energy Applications of Graphene and Graphene-Based https://doi.org/10.1088/2053-2563/ab0d19ch1. Composites. Adv. Colloid Interface Sci. 2019, 273, 102036. https://doi.org/10.1016/j.cis.2019.102036. 25. Sharma, P.; Bhatti, T. S. A Review on Electrochemical Double-Layer Capacitors. Energy Convers. Manag. 2010, 51 (12), 2901–2912. https://doi.org/10.1016/j.enconman.2010.06.031. 39. Fang, B.; Chang, D.; Xu, Z.; Gao, C. A Review on Graphene Fibers: Expectations, Advances, and Prospects. Adv. Mater. 2020, 32 (5), 1902664. https://doi.org/10.1002/adma.201902664. 26. Burt, R.; Birkett, G.; Zhao, X. S. A Review of Molecular Modelling of Electric Double Layer 52 53

40. Wei, Y.; Yang, R. Nanomechanics of Graphene. Natl. Sci. Rev. 2019, 6 (2), 324–348. https://doi. org/10.1093/nsr/nwy067. WASTE-TO-ENERGY

41. Lv, W.; Tang, D.-M.; He, Y.-B.; You, C.-H.; Shi, Z.-Q.; Chen, X.-C.; Chen, C.-M.; Hou, P.-X.; Liu, C.; Yang, Q.-H. Low-Temperature Exfoliated Graphenes: Vacuum-Promoted Exfoliation and Electrochemical Energy Storage. ACS Nano 2009, 3 (11), 3730–3736. https://doi.org/10.1021/nn900933u.

42. Panda, P. K.; Grigoriev, A.; Mishra, Y. K.; Ahuja, R. Progress in Supercapacitors: Roles of Two Dimensional Nanotubular Materials. Nanoscale Adv. 2020, 2 (1), 70–108. https://doi.org/10.1039/C9NA00307J.

43. Dong, X.; Wang, X.; Wang, L.; Song, H.; Zhang, H.; Huang, W.; Chen, P. 3D Graphene Foam as a Monolithic and Macroporous Carbon Electrode for Electrochemical Sensing. ACS Appl. Mater. Interfaces 2012, 4 (6), 3129–3133. https://doi.org/10.1021/am300459m.

44. Lai, E.; Yue, X.; Ning, W.; Huang, J.; Ling, X.; Lin, H. Three-Dimensional Graphene-Based Composite Hydrogel Materials for Flexible Supercapacitor Electrodes. Front. Chem. 2019, 7. https://doi. org/10.3389/fchem.2019.00660.

45. Ho, K.-C.; Lin, L.-Y. A Review of Electrode Materials Based on Core–Shell Nanostructures for Electrochemical Supercapacitors. J. Mater. Chem. A 2019, 7 (8), 3516–3530. https://doi.org/10.1039/C8TA11599K.

46. Lokhande, P. E.; Chavan, U. S.; Pandey, A. Materials and Fabrication Methods for Electrochemical Supercapacitors: Overview. Electrochem. Energy Rev. 2020, 3 (1), 155–186. https://doi.org/10.1007/s41918-019-00057-z. Sergey Alekseenko, RAS Academician, Head of Heat and Mass Transfer Laboratory of Institute of Thermophysics of the RAS Siberian Branch 47. Liu, R.; Duay, J.; Lee, S. B. Heterogeneous Nanostructured Electrode Materials for Electrochemical Energy Storage. Chem. Commun. 2011, 47 (5), 1384–1404. https://doi.org/10.1039/C0CC03158E.

48. Moftah, A.; Shetiti, A. Al. Review of Supercapacitor Technology; 2019.

49. Hamdan, I.; Maghraby, A.; Noureldeen, O. Stability Improvement and Control of Grid-Connected Photovoltaic System during Faults Using Supercapacitor. SN Appl. Sci. 2019, 1 (12), 1687. https://doi.org/10.1007/s42452-019-1743-2. INTRODUCTION

50. Jayananda, D.; Kularatna, N.; Steyn-Ross, D. A. A Validity of MPPT Technique Using Supercapacitors as Energy Storage Devices: Example of the SCALED Converter Technique. In The global problem of humanity is associated concentrated, which are caused by the most rapid IECON 2019 - 45th Annual Conference of the IEEE Industrial Electronics Society; IEEE, 2019; pp with the rapid growth of waste. Surveys show growth in their volumes, uncertainty of composition, 2301–2306. https://doi.org/10.1109/IECON.2019.8926996. that waste, along with global warming and a lack release of the dangerous ingredients, pollution of of clean water, is the greatest concern for the soil, groundwater and atmosphere, accumulation population and professionals. In total, the world of huge amounts at landfills and illegal dumps, as 51. Wu, S.-L.; Li, S.-S.; Gu, F.-C.; Chen, P.-H.; Chen, H.-C. Application of Supercapacitors in Photovoltaic generates about 25 billion tons of waste per year. well as regular landfill fire. Recently, the problem Power Generation System. Sensors Mater. 2019, 31 (11), 3583. The leader in specific waste production is Canada, of spreading infectious diseases through rodents, https://doi.org/10.18494/SAM.2019.2502. which generates 36 tons per person per year (but birds, and insects that habituate at landfills has most of them are industrial waste from oil refining, become particularly acute. On the other hand, MSW 52. Supercapacitor Technology: Materials, Processes and Architectures; Inamuddin, Rajender metal processing, and the chemical industry). This contains many products and substances that can Boddula, Mohd Imran Ahamed, A. M. A., Ed.; Materials Research Forum LLC, 2019. is followed by Bulgaria (26.7 tons) and the USA be disposed of for reuse. This is especially relevant (26 tons). A huge amount of waste is generated for the extraction of energy, which is contained in 53. Mosa, I. M.; Pattammattel, A.; Kadimisetty, K.; Pande, P.; El-Kady, M. F.; Bishop, G. W.; Novak, in Russia amounting to about 7 billion tons/year. the significant amounts in the combustible part M.; Kaner, R. B.; Basu, A. K.; Kumar, C. V.; et al. Ultrathin Graphene-Protein Supercapacitors for At that, over the past ten years, there has been a of the waste. This is exactly the key problem of Miniaturised Bioelectronics. Adv. Energy Mater. 2017, 7 (17), 1700358. twofold growth. As in other countries, the vast the topic under consideration. Note that in many https://doi.org/10.1002/aenm.201700358. majority of waste is related to the industry, namely, countries, MSW and other combustible waste are 91% relate to mining. classified as renewable energy sources (RES), or 54. Verma, S. K.; Jha, E.; Panda, P. K.; Das, J. K.; Thirumurugan, A.; Suar, M.; Parashar, S. Molecular Municipal solid waste (MSW) takes a small defining more strictly, as secondary RES. Aspects of Core-Shell Intrinsic Defect Induced Enhanced Antibacterial Activity of ZnO share – just about 5% in the total amount of waste. In 2016, 1.3 billion tons of MSW were produced Nanocrystals. Nanomedicine 2018, 13 (1), 43–68. https://doi.org/10.2217/nnm-2017-0237. But they should be given the most attention because worldwide or 1.2 kg of MSW/person per day, which this is where the greatest number of problems are is almost twice more than six years ago (0.64 kg of 54 55

MSW/person per day in 2010). At that, 1.42 kg is no single technology capable of processing FIG. 1. WAYS TO MANAGE MSW IN EUROPEAN COUNTRIES of MSW/person per day or two billion tons of MSW/ waste under acceptable conditions. The average year is predicted in 2025. For the largest countries, morphological composition of MSW is represented the indicators for MSW are as follows. Russia by the following components: paper and produces about 70 million tons/year or 490 kg/year cardboard (33–40%); food waste (27–33%); wood per person. The USA is the largest global producer (1.5–5%); ferrous metals (2.5–3.6%); non-ferrous of the MSW (18% of global waste) and generates metals (0.4–0.6%); bones (0.5–0.9%); leather 200 million tons/year or 590 kg/year per person and rubber (0.8–1.3%); textiles (4.6–6.5%); glass (according to other sources – 262 million tons/ (2.7–4.3%); polymer materials (4.6–6.0%), etc. year, these figures strongly depend on the waste Waste in the USA comprises mainly of paper (26%), calculating method). Another related problem, as food waste (15%), garden waste (13%), wood (6%), already noted, is the huge accumulation of waste metals (9%), leather, textiles and rubber (9%), glass in landfills. (4%), plastic (13%), other (4%). In any case, the To choose the best methods of waste above data shows that the vast majority of MSW management and determine the degree of their comprises of combustible material. Therefore, impact on the environment, it is necessary to know for a long time, garbage was burned in dumps the waste composition. Although the composition or simply stored with subsequent spontaneous of MSW in large cities is approximately the same, combustion. It is only in the last decades that the ratio of waste components may vary quite the waste management system has taken on a widely. Over the years, there are also serious civilised character for the reasons mentioned changes, whose striking example is various plastic above, namely, rapid growth in the volume of waste materials. Besides, toxic substances, such as and a concomitant sharp increase in environmental mercury, batteries, expired medicines, etc. appear problems. Without going into the history of the more and more. The extremely complicated issue, we will present the latest results on waste chemical composition of MSW, as well as their management technologies and the global trends huge diversity in size and phase state of fractions, followed by them. allows stating a fundamental postulate: there primarily in developed countries. This approach combusting, the volume of MSW is reduced by is used with particular success in the separate 90%, and the weigh is decreased by 75% while waste collection (usually sorting out the glass, providing biological neutralisation and the ability plastic, paper, and food waste), as well as at large to generate heat and electricity. Energy recovery waste processing plants, where automated and from waste has become widespread in the world even robotic sorting lines with the most advanced and is a worldwide trend called Waste-to-Energy. technical means are used. Recycling makes it The percentage of thermal processing in Denmark possible to earn income from the sale of secondary reaches 55%, Norway – 54%, Sweden – 50%, and raw materials. A high percentage of recycling is Germany – 35%. This approach is possible due to implemented in Germany (47%), the USA (35%), and the high carbon content of MSW (and several other Korea (58%). industrial wastes) and high heating value. The lower WASTE MANAGEMENT METHODS Compost is very demanded in agriculture, heating value LHV of MSW ranges within the limits of although there is no confidence in the 4.2–12.6 MJ/kg with averaged value of 8.4 MJ/kg. appropriateness of using compost from urban To compare, this indicator for brown coal varies Figure 1 shows data on MSW management the soil, dioxin is absorbed by plants (especially by waste. The biochemical transformation process of within the range of 6.3–17 MJ/kg, that is, it has a methods in several European countries [1]. their underground part), soil fauna, through which the biomass contained in MSW is called composting comparable value, so that MSW can be considered There are four main methods: landfill disposals, it is transmitted along the food chain to birds similarly to the aerobiosis or methanation – in the as a low-grade fuel. However, after processing recycling, composting, and thermal neutralisation and other animals. Besides, dioxin, carried out case of anaerobiosis. When producing compost, waste, cleaning, or selecting individual fractions, (mainly incineration). The most common (and from the soil by air and water flows to the water gaseous waste products are released into the it is possible to get a high-calorie fuel with high ancient) method in countries with underdeveloped bodies, also gets to birds and mammals through atmosphere, namely, methane, CO , H S, and other environmental characteristics. For example, the infrastructure for waste management is disposal zooplankton, crustaceans, and fish. In other words, 2 2 gases. Methanation is carried out in a closed heating value of the organic part of individual in landfills. In Romania, for example, the share of within vegetables, meat, and especially dairy and volume, and in the course of this process part of components of MSW are given below in terms of landfills accounts for 98%, and only 2% of total fish products got from the infected area, dioxin will the organic matter is converted into biogas, which, dry ash-free basis [2] in MJ/kg: for the paper, it is waste is processed; in Russia, these figures are 95% one way or another end up on a human table. The as a combustible gas, can be used for local heat 16.9; wood – 20.3; textiles – 22.6; leather, rubber – and 5%, respectively. Contemporary landfills are high stability of this poison favours its repeated and electricity production (but this gas can be 31.1; plastics – 27.4; composite materials (Tetrapak technically complicated and expensive structures. circulation through the food chain. In developed toxic or cause equipment corrosion). The share of packaging, etc.) – 26.4; food waste – 18.2; fine However, a significant part of the waste (in some countries, only processed (neutral) waste is subject composting in developed EU countries is 15–20%, waste materials – 20.1. As is seen, for rubber and countries – completely) is sent simply to landfills, to burial. At that, the admitted amount of waste to while in Austria it reaches 35%. plastic, these values reach 31.1 and 27.4 MJ/kg, that often unauthorised. Russian landfills emit over be buried is constantly minimised aiming at zero Thermal processing is the most radical means is, two to four times higher than the corresponding 1.5 million tons of methane and 21.5 million tons (Switzerland – 0%, Germany, Sweden, Denmark – of decontamination and waste disposal. Today, values for brown coal. So rubber and plastics are of CO per year. Dioxins and furans, which are 1% each). 2 incineration at waste incineration plants is one of considered the most suitable for energy generation, formed both in landfills and during uncontrolled Processing into secondary raw materials or the main methods of thermal processing. When especially when applying gasification methods. combustion, are particularly dangerous. Once in recycling has been particularly developed lately, 56 57

The role of MSW in energy generation is most quite clear. When uncontrolled combustion within WASTE-TO-ENERGY clearly shown in Fig. 2, which presents the production the temperature range of 800–850°C, which is and share of electricity and heat produced from typical for most existing waste incineration plants The International Energy Agency calls 130 TWh of energy from waste (for comparison, MSW in different countries in 2015 [3]. For example, (and when burning landfills), the entire spectrum of energy waste management with controlled high- in Russia, all sources produce about 1,000 TWh in Denmark, almost 6% of electricity is generated harmful substances are formed, released into the temperature combustion and pollution control of electricity). In 2015, 485 enterprises for thermal from waste. A very high proportion of waste in the atmosphere through the chimney, as well as with technology the best alternative to MSW landfills. processing of MSW were operating in 22 European heat supply is noted in several countries, namely, liquid waste and ash. These are sulfur and nitrogen According to Ecoprog’s Waste-to-Energy report countries. In Japan, which is one of the leading 60% in Switzerland, 50% in Norway, 24% in Sweden. oxides, hydrogen chloride and fluoride, carbon 2019/2020, in 2016, there were over 2,200 countries in waste processing, there are about As for the thermal processing methods, currently, monoxide CO, toxic metals (mercury, lead, bismuth, enterprises (4,150 installations) in the world 1,900 installations for the thermal processing of there are more than 2.5 thousand units in the world antimony, etc.), polyaromatic hydrocarbons (PAH), operating based on thermal processing of MSW MSW, which use 75 % of the country’s MSW. At that burn solid municipal waste on mechanical such as benzopyrene, fluoranthene, etc. The with a total capacity of 300 million tons of waste that, 69% of MSW is combusted to produce energy grates, about 200 furnaces for thermal processing peculiarity of metals is their accumulation, which per year. In the period from 2011 to 2015, over in the amount of 6.6 TWh at an installed capacity of waste in the fluidised bed, about 20 rotary kilns, is dangerous at any even negligible concentrations. 280 such enterprises were put into operation with of 1.5 GW (2015). In Tokyo, within the city limits, as well as single pyrolysis and gasification plants, Mercury has a unique ability to migrate. PAHs are a total capacity of about 80 million tons of MSW there are 21(!) waste incineration plants. In China, including those using plasma. A popular method extremely dangerous carcinogenic substances. per year. It is expected that 600 new plants with at the beginning of 2019, there were 339 plants of using MSW for energy generation represents But in terms of toxicity, a special place is occupied a capacity of about 170 million tons/year will be with a total capacity of about 100 million tons per pre-production of the so-called refused derived fuel by dioxins and furans, which are formed of waste built by 2025. About half of the waste is disposed year. By 2020, China plans to commission 30 GW of (RDF), which consists of pressed pellets of 2–3 cm containing chlorinated derivatives (polyvinyl of producing at the same time energy, both thermal capacity based on MSW and biomass. in size produced from about 1/3 of MSW remained chloride, cardboard, newspapers, etc.). They have and electrical. In total, the world produces about after sorting. In terms of calorific value, 1.7 kg of the highest maximum allowable concentration RDF corresponds to 1 m3 of natural gas. Due to requirements in flue gases – 0.1 ng/m3 (EU the significant content of harmful substances, standard), while in the atmosphere it equals 0.5 pg/ their use in the municipal sector is considered m3. But some environmentalists consider even strict FIG. 2. PRODUCTION AND SHARE OF ELECTRICITY AND HEAT PRODUCED FROM MSW IN unacceptable. High-quality combustion requires restrictions unacceptable since these substances SOME COUNTRIES IN 2015 elevated temperatures within the range of 1,500– are strongly accumulated. And, certainly, CO2 as 2,000°C, therefore RDF is mainly used in cement a greenhouse gas remains among the harmful and metallurgical plants. For example, in Germany, substances. Although the hazard-intensity of RDF covers 90% of the fuel needs in cement plants. modern waste incineration plants that meet the While talking about the energy utilisation of MSW, it main environmental indicators is incomparably less is impossible to ignore the combustible industrial in comparison with landfills containing untreated waste, which is much over MSW, and which is also waste, nevertheless further steps are required to offered for co-incineration. They comprise of waste improve conventional combustion technologies from coal enrichment and oil refining (120 million and find fundamentally new approaches to waste tons of coal waste per year are produced in the USA thermal processing. alone); biomass, including wood; animal waste; A lot of experience has been gained in hazardous waste, including biomedical waste; combustion technologies since the end of the 19th sewage sludge; and automobile tires. century, and in most cases, they are based on the The following conclusions can be drawn method of combustion on mechanical grates [2]. based on the analysis of existing world practices. The main requirements are strict compliance with There is no single universal technology for waste the regulations for combustion and purification disposal. A recognised approach consists in of gaseous, liquid, and solid emissions, and creating an integrated waste management system, provision of the control of the content of harmful which includes a set of measures — from reducing substances. Thus, the proven approach to reduce potential waste at the stage of production of goods the formation of dioxins is providing of zones with to the disposal of completely neutralised residues a high temperature of over 1,200°C with a residence from waste processing. Thermal processing is a time of at least two seconds, when the dioxins are mandatory element of any waste management completely destroyed, and following rapid cooling system. The release of energy from waste during or catalytic afterburning to avoid a new process thermal processing has become widespread of dioxin formation. However, all this leads to an globally and has become a worldwide trend called essential increase in capital expenditures and Waste-to-Energy. The economic feasibility and operating costs and is not always fulfilled. This efficiency of implementing a waste management results in long payback periods (for example, system strongly depend on the composition of the over 25–30 years in China), and irreconcilable processed waste and the region. opposition of the public and environmentalists Despite the significant contribution of MSW to against building waste incineration plants (the the energy sector and the fact that many countries famous Not In My Back-Yard (NIMBY) syndrome) declare plans for further use of waste for energy up to lawsuits. Moreover, this technology requires purposes, there are many environmental and mandatory government support in terms of energy economic concerns that make the situation not tariffs and waste costs. 58 59

DEEP PROCESSING METHODS PLASMA GASIFICATION OF WASTE

Plasma methods have been successfully recycling fee of 110 US$/ton, the plant suffers a loss A fundamentally new trend in the thermal generation (integrated gasification combined cycle used in the industry for several decades. There of 190 US$/ton. However, a more powerful plant with processing of combustible waste is the use of IGCC). A wide variety of hydrocarbon raw materials, are not many examples of plasma gasification on a capacity of 100 TPD will have reduced operating deep processing methods, namely, pyrolysis including solid and liquid waste, can be used for a commercial scale, but there are a huge number costs of 101 US$/ton, and with the sale of electricity, and gasification, of which plasma gasification gasification. However, coal plays a dominant role of laboratory studies and pilot installations, whose a profit may reach 86 US$/ton. Companies, such as should be considered the most promising. Deep among the raw materials used for gasification. authors are unanimous in their opinion about the Westinghouse, Europlasma, Phoenix, PyroGenesis, processing methods are the most promising for Currently, it’s proportion is 75.5%. Further growth is prospects of plasma gasification of combustible and Air Products can be noted among the reputable the coal industry. And here the alliance between the expected in the future, up to 83% in 2020. Several waste and the uniqueness of plasma technologies developers. A significant contribution to the industries is quite possible because there is a huge commercial gasification technologies have been [6, 11-18]. Thus, in the city of Cheongsong (South development of plasma technologies was made experience in deep processing of coal, while the proposed. According to the type of gasification Korea) with a population of 30,000 inhabitants and by Russian scientists, namely, Academicians M.F. technologies are essentially similar. Besides, there technology, there are gas generators with a mobile the production of MSW in the amount of 15 tons Zhukov and F.G. Rutberg. The latter was awarded are proposals for joint (more efficient) processing of layer or moving-bed gasifiers (with a syngas per day (TPD), a plant with a plasma gasifier with a the International Global Energy Prize precisely MSW and coal. Methods of deep processing of coal temperature at the output of 350–800°C), fluidised capacity of 10 TPD is successfully operating [11]. for the development of technologies for plasma include pyrolysis, gasification, direct liquefaction bed gasifiers (800– 1,000°C), and entrained-flow With an operating cost of 300 US$/ton and MSW gasification of waste. (hydrogenation), and dissolution in supercritical gasifiers (1,350–1,700°C, molten slag removal). The water. Pyrolysis can be defined as the thermal latter are the most common in the world. Among decomposition of carbon-containing materials with the specific technologies, we should mention the heat supply without oxygen to produce synthesis technology of General Electric (entrained-flow gas (syngas). Syngas is a mixture of hydrogen gasifier, water-coal mixtures), whose share in the and carbon monoxide. In production processes, world amounted to 25% in 2014, Shell technology the resulting combustible gas has a much more (entrained-flow gasifier, dry coal supply) – 25%, THE MAIN ADVANTAGES OF PLASMA GASIFICATION complicated composition including in particular and Lurgi technology (moving-bed gasifier, dry coal

CO2, CH4 and others. So it is called generator supply) – 16%. There is a steady increase in the OF MSW INCLUDE: gas, which is cleaned to the required condition proportion of waste- and biomass gasifiers. deep decomposition of waste into simple the ability to quickly adjust the process depending on the purpose. Gasification is a thermal Gasification technologies have not yet become compounds that greatly simplifies their by changing the flow rate of the oxidiser process in which carbon-containing materials are widespread since they cannot compete with cleaning against harmful impurities; (air, steam or other plasma-forming gas) converted to synthesis gas at a limited amount traditional energy technologies in economic terms. and the power of the plasma torches; of air or oxygen. Normal gasification conditions But taking into account the unique environmental formation of reactive species such as atomic correspond to a wide temperature range of characteristics (up to the creation of waste- oxygen and hydrogen or hydroxyl radicals; the ability to obtain more calorific and 350–1,700°C. Water steam can be additionally free technologies) and with further increase in more clean syngas from the organic part injected into the gasification reactor to stimulate efficiency, we can expect real breakthroughs in the the possibility of joint processing of various of the waste, which is not contaminated the production of CO and H . About 70–85% of the application of gasification methods for processing 2 types of waste without pre-sorting, which is with by-products typical for conventional calorific value of the initial coal is preserved in the organic fuels, as well as various wastes, including especially important for the processing of gasification of MSW (this is especially resulting synthesis gas. MSW, waste from coal enrichment and refining, biomedical and other toxic waste that is not true for tar); Syngas can be efficiently burned in gas sludge, medical waste, and automobile tires. In this subject to sorting; turbines (or combined-cycle plants), or used as regard, the most promising method for processing production of vitrified slag that can a valuable chemical raw material to produce both solid and liquid combustible waste is plasma the possibility of processing difficult wastes be used as a building material; hydrogen, methanol, synthetic fuel, and many gasification, which refers to the gasification such as tires, carpets, and sludge; other products. Gasification is the main method process in the presence of an external heat source vitrification of incinerator ash; of deep processing. About 170 million tons of in the form of a plasma torch with a temperature a significant reduction in the volume of flue coal per year is gasified in the world at present of 4000–6,000°C. Another deep processing gas and, consequently, the load on the gas production of value-added products [8]. This corresponds to about half of the annual technology is the conversion of hydrocarbon raw cleaning system; (metals) from slag; coal production in Russia. According to forecasts, materials in supercritical water SCW (at pressure global capacity is expected to double from 147 over 22 MPa, temperature over 374°C). At that, the less entrainment of dispersed particles; ability possibility to recycle waste at the GW of thermal capacity in 2015 to about 359 GW SCW is an active solvent of organic substances existing landfill and eliminate the old landfill; in 2020. Accordingly, the number of gas generators and oxygen [9, 10]. This technology can be applied high performance with small dimensions will grow from 833 to 2559 (together with the in the conversion of organic substances to liquid of the equipment; gasification with incomplete combustion projected ones). Currently, 65% of the produced hydrocarbon fuel, as well as the combustion of at high temperatures and rapid cooling synthesis gas is processed into chemical products organic substances to produce high-enthalpy the ability to create a desirable gas of the syngas allows avoiding the formation (methanol, olefins, oxo-compounds, ammonia, products for power plants. The already studied atmosphere; of dioxins and furans, which are the most urea, hydrogen, etc.), 18% — into liquid fuel (diesel, organic substances are coal, oil residues, biological dangerous toxicants. kerosene, gasoline, etc.), 10% — into gaseous silts, and sewage. the ability to get the final product in a stable fuel (synthetic natural gas), and 7% — to electric form; 60 61

FIG. 3. SCHEMATIC DIAGRAM OF PLASMA GASIFICATION OF MSW the most dangerous toxicants such as dioxins Depending on the purpose of the plasma torch, air or and furans. Besides, slag and metals are melted. other gases (, N2, , H2, air, CH4, CO, CO2, Further, metals may separate, and the solidification propane, oxygen) are used as a plasma-forming of the slag during cooling leads to the formation gas. A typical powerful plasma arc torch has the of so-called vitrified slag, which is completely following parameters: power – 2 MW, arc current harmless, and can be used as a building material, – 600– 1000 A, voltage – 2–3 kV, and gas flow and sent to a safe burial. In the gasification zone, rate – 0.2 kg/s. The main disadvantage of plasma the temperature varies within a wide range (1,200– technologies is considered to be the low service 1,800°C) depending on the required composition life of the electrodes – from 100 to 300 hours. of the reaction products, the type of raw material, Although, for example, replacement electrodes and the type of oxidiser. An important feature of produced by Westinghouse Plasma Corporation the plasma gasifier is the ability to easily change can operate over 1,000 hours on average, and the temperature mode by changing the current in there are no problems in replacing them (within 30 the plasma arc. The syngas is removed at the top minutes without stopping the process). Another of the reactor, cooled (often by water injection), option of the plasma torch is its design where an purified, and then used either to generate energy in electric arc is formed directly in the working space the Integrated Gasification Combined Cycle (IGCC) of the reactor between the anode in the body of the or to produce synthetic fuels or valuable chemical plasma torch and the bottom cathode electrode, products. which is a slag melt with conducting properties [14] The main element of a plasma gas generator (Fig. 4). In this case, the efficiency of the plasma is a plasma torch. The DC electric arc plasma torch is significantly increased and direct contact torches with a power of several kW to 2 MW are of waste with a high-temperature electric arc is considered the most proven and reliable, although implemented. The original device is a plasma arc there are plasma torches with a power of over 10 torch with molten electrodes, which was specially MW. Plasma is formed by heating and ionisation designed for waste processing [4]. In such a plasma of a plasma-forming gas due to energy release in torch, there is no problem with the resource of the an electric arc between two or more electrodes. electrodes at all, while its power is not limited.

Gasification of 1 kg of MSW requires electricity (0.2–0.5 kWh/kg of MSW). As a result, 1 kg consumption which varies within the range of of waste can generate from 0.5 to 2.3 kWh of 0.2–0.5 kWh depending on the waste composition electricity supplied to the grid. The lower estimate and gasification mode. For unsorted waste with is closer to reality due to a lot of unaccounted high mineral content, this figure can reach 1 kWh. heat losses. But even in the worst-case scenario The basic scheme of plasma gasification of The main part of the plant is the plasma From 1 kg of waste, one can get 0.6–1.2 kg of of unsorted waste, a positive balance is possible. MSW is as follows [16] (Fig. 3). The MSW entering gasification reactor (plasma gasifier). There synthesis gas with a heating value of 3200–3500 These are rough estimates, but they demonstrate the plant is sorted. Due to the «omnivorous» are various designs, but usually, the gasifier is a kcal/kg (13–15 MJ/kg). According to [13], high- the energy value of waste during plasma nature of plasma methods, sorting can be minimal vertical shaft furnace, often with an extension at calorie syngas with a concentration of 82.4% gasification. Here are other data [6], where different

(selection of metal and large-sized fractions). But the bottom to collect waste and molten slag. Waste (CO – 31.7%, H2 – 50.7%) can be produced with technologies are compared by the net energy to in this case, there will be more requirements for is fed to the upper part of the reactor (from the air-plasma gasification of MSW, and for case of grid: Incineration – 0.544 kWh/kg, MSW Pyrolysis – cleaning the generator gas. A contemporary (even side or through the top), from where waste lowers steam-plasma gasification – with a concentration 0.571 kWh/kg, MSW conventional gasification

revolutionary) approach is to use automated waste to the bottom of the reactor, where there is a bath of 94.5% (CO – 33.6%, H2 – 60.9%). Comparison of – 0.685 kWh/kg, MSW Plasma Arc Gasification sorting employing intelligent robotic systems of molten slag (and metals). The plasma torch plasma-air and plasma-steam gasification of waste – 0.816 kWh/kg MSW. It follows that plasma-arc developed based on deep learning artificial neural from the plasma arc torch is formed in the lower shows that within in-air gasification, the specific gasification can be considered the most efficient networks. Such systems can recognise hundreds part of the reactor in the waste accumulation zone, yield of combustible gas from 1 kg of waste is process of thermal processing combined with of thousands of objects, surpassing human which is located above the molten slag layer. At the by 26% higher than that in steam gasification. electricity generation. The commercial project capabilities. It is even more profitable when the bottom there is a flyer for the release of molten slag However, the energy value of the resulting Plasma Gasification of MSW in Utashinai, Japan plant will receive already sorted waste due to the and metals. The gasification zone is located above combustible gas in plasma-steam gasification demonstrates good performance. With gasification implementation of a system of separate collection the plasma torches. This zone is supplied with air or is 30% higher. In terms of energy released per 1 of approximately 300 TPD of MSW, the plant from the population and organisations. Then most oxygen, as well as water vapor in the case of steam kg, MSW gives 7.8–18 MJ which are contained produces up to 7.9 MW of electricity with ~ 4.3 MW of the waste will be sent directly to secondary use gasification. The temperature in the plasma torch in syngas. In the case of using waste-to-energy to the grid [17]. (recycling), bringing income through sales. While the is 4,000–6,000°C. The waste comes in contact with technology, the resulting syngas can be combusted It is quite difficult to make economic remaining part (relatively small) is fed to the reactor the plasma-forming gas, which is blown through using an Integrated Gasification Combined Cycle estimates. While the economy is running high, it is for thermal processing. As a result, energy costs the plasma torch channel into the reactor volume, (IGCC) with an efficiency of up to 59% by gas and difficult to expect great economic benefits for the for plasma generators are significantly reduced, or directly with the arc if it is formed in the bottom 46.2% by waste (wood in [18]) which is much higher plasma gasification of waste, although there are and the resulting generator gas (syngas) is of much of the reactor. High temperature makes it possible as compared to 18–22% in the steam turbine undeniable advantages of an environmental nature. higher quality compared to the case of unsorted to convert high-molecular organic substances to cycle used at conventional combustion. Then it is Nevertheless, the following conclusions are made waste. At that, the situation where the received the simplest ones, which is especially important possible to generate electricity in the amount of in [6]. Incineration shows a negative net annual sorted waste is of homogeneous composition, for when processing dangerous medical and biological 3.9–9.0 MJ or 1 –2.5 kWh per 1 kg of MSW, which income (before tax), while pyrolysis, conventional example, plain plastic is especially favourable. waste and leads to the complete destruction of is obviously higher than the cost of gasification gasification, and plasma arc gasification indicate 62 63

FIG. 4. SCHEMATIC DIAGRAM OF THE GAS GENERATOR TO FORM AN CONCLUSION ELECTRIC ARC IN THE WORKING SPACE OF THE REACTOR While drawing general conclusions about the the final conclusion can be formulated as follows. role of waste in generating energy using plasma The most promising approach for the near future gasification, we can give the following impressive is an integrated approach, which consists of estimates concerning the potential of different developing an integrated waste management types of waste [12]: MSW — world production is system for each region. It includes minimisation of 3.6 mln tons/day, energy potential equals 178 waste production at the stage of material products GW; hazardous waste — 1.2 mln tons/day with the manufacturing; separate garbage collection and energy potential of 43 GW; waste biomass — 14 mln automated sorting of waste using intelligent tons/day, 685 GW; waste tires — 28,000 tons/day, robotic systems; recycling; composting of the 1.4 GW. In total one gets 907.4 GW of power, which organic component of waste; mandatory thermal is comparable to the entire installed capacity of processing with the predominant use of plasma the USA amounting to 1100 GW. Another example: technologies (gasification of waste, vitrification plasma processing of MSW has the potential to of slag) including that in combination with other supply ~ 5% of U.S. electricity needs [17]. types of high-quality fuel and industrial waste; To sum up, we can conclude the exceptional and disposal of only neutral residues from waste possibilities of plasma gasification of waste (both processing, striving to decrease this share to zero. municipal and any other combustible waste) for the The implementation of this approach will allow production of energy, synthetic fuels, and valuable getting both the maximum environmental effect chemical products. However, the maximum and economic benefit, which has always been effect can be achieved by combining various problematic in the waste management sector. technologies, approaches, and activities. Therefore,

a positive net annual income (before tax). The plants, plasma ignition systems (instead of using plasma gasification process brings the highest net natural gas or heavy oil) have long proved to annual revenue. Besides, it should be noted that the be commercially viable and have been used in plasma-arc gasification process produces vitrified more than 800 boilers [20]. And plasma methods slag, which is an environmentally acceptable of processing medical waste (also with power REFERENCES by-product with revenue as road material. We can generation, but most effectively as part of MSW add two more examples. When burning low-grade incineration plants) are certainly the best [13-14]. 1. Titov B. Systems of household waste management in different countries: Recipes for Russia. coal in pulverised coal boilers of thermal power Stolypin Institute for the Economy of Growth.

2. Tugov A.N. Energy utilisation of municipal solid waste at the thermal power plant. VTI, Moscow, 2017. 178 p.

3. Energy Bulletin. Analytical center under the government of the Russian Federation. 2017, № 48.

4. Alekseenko S.V. Efficient Production and Use of Energy // Chapter 3 in Book: Sustainable Energy Technologies, ed. K. Hanjalic, R. Van de Krol, A. Lekic. Springer, 2008. P. 51-74.

5. Higman C., Burgt M. Gasification. Amsterdam, Gulf Professional; 2003.

6. Young G.C. Municipal solid waste to energy conversion processes: Economic, technical, and renewable comparisons, Copyright 2010 by John Wiley &Sons, Inc.

7. Litvinenko V. and Meyer B. Syngas Production: Status and Potential for Implementation in Russian Industry, Springer International Publishing AG, 2018. P. 75 – 161. https://doi.org/10.1007/978-3- 319-70963-5.

8. Higman C. State of the gasification industry: worldwide gasification database 2015 update. Colorado Springs; 2015. 64 65

1. Kamler J. and Soria J. A. Supercritical Water Gasification of Municipal Sludge: A Novel Approach to Waste Treatment and Energy Recovery, Chapter 6, 131 – 182, © 2012 Kamlerand Soria, licensee ENERGY InTech. (http://creativecommons.org/licenses/by/3.0). RECYCLING 2. Vostrikov A.A., Shishkin A.V., Sokol M.Ya., Dubov D.Yu., Fedyaeva O.N., Conversion of brown coal continuously supplied into the reactor as coal-water slurry in a supercritical water and water- oxygen fluid // J. Supercritical Fluids, 2016, v. 107, p. 707-714.

3. Byun Y., Cho M., Hwang S.-M. and Chung J. Thermal Plasma Gasification of Municipal Solid Waste (MSW). Chapter 7, 183 – 210. © 2012 Chung et al, licensee InTech. (http://creativecommons.org/ licenses/by/3.0).

4. Alter NRG Plasma Gasification: The Next Generation of Waste-To-Energy Solutions. Deep Dive Workshop on Waste-to-Energy, 2016. Asia Clean Energy Forum, 7 June 2016.

5. Messerle V.E., Mosse A.L., Ustimenko A.B.Processing of biomedical waste in plasma gasifier. Waste Management 79 (2018) 791–799.

6. Anshakov A.S., Aliferov A.I., and Domarov P.V. Features of plasma gasification of organic waste. IOP Conf. Series: Materials Science and Engineering 560 (2019) 012057. Doi:10.1088/1757- 899X/560/1/012057 2019.

7. Sesotyo P.A., Nur M., Suseno J. E. Plasma Gasification With Municipal Solid Waste As A Method Of Energy Self Sustained For Better Urban Built Environment: Modeling and Simulation. The 2nd International Conference on Smart City Innovation. IOP Conf. Series: Earth and Environmental Science 396 (2019) 012002. IOP Publishing. Sergey Elistratov, 8. Westinghouse Plasma Corporation, Madison, PA, www.westinghouse-plasma.com Chair of Thermal Stations Department, Novosibirsk State Technical University 9. Circeo L.J. Plasma Arc Gasification of Municipal Solid Waste, Georgia Tech, Research Institute.

10. Rutberg Ph.G., Bratsev A N., Kuznetsov V A., Popov V E., Ufimtsev A A., Shtengel’ S V. On efficiency of plasma gasification of wood residues. Biomass and Bioenergy 35, 495-504 (2011).

11. Fabry F., Rehmet Ch., Rohani V., Fulcheri L. Waste gasification by thermal plasma: A review// Waste and Biomass Valourisation. 2013; 4:421-39. Doi : 10.1007/s12649-013-9201-7.

12. Messerle V.E., Karpenko E.I., Ustimenko A.B. Plasma assisted power coal combustion in the furnace of utility boiler: Numerical modelling and full-scale test // Fuel. – 2014. – Vol. 126. – P. INTRODUCTION 294–300. The global problem of humanity is the into the natural environment, and the pollution of formation of a large amount of industrial and the atmosphere, soil, and groundwater. Scientific household waste — about 25 billion tons per year. and technological progress leads to the formation Of these, 91% relates to mining and 5% to municipal of a large amount of waste that is most harmful solid waste (MSW). On average, for every inhabitant to the environment, which it is not able to process of the planet, according to the Organisation for (toxic chemicals, mercury, radioactive waste, etc.). Economic Cooperation and Development, 525 kg Of particular danger are dioxins and furans that are of MSW per year. Only New Zealanders (781 kg), formed both in landfills and during uncontrolled Danes (771 kg) and Norwegians (736 kg) filled burning. Once in the soil, dioxins are absorbed by their garbage cans more than the Swiss (705 kg), plants and transferred to birds and other animals which were close to their full processing. Colombia along the food chain. With vegetables, meat, dairy produces the least amount of waste per capita — and fish products obtained from the infected 240 kg per person per year. In Russia, this figure is territory, they get to the table to the person. The at the level of 400 kg. most powerful carcinogen is 2,3,7,8 TCDD (tetra- The danger of MSW lies in the rapid growth chloro-dibenzodioxin). It can get into MSW with of their global volumes, the uncertainty of their household waste products, as well as synthesised composition, the release of hazardous ingredients from chlorine-containing aromatic products during 66 67

rotting, decay or direct burning of MSW at countries and continents. The average indicators of TO MEET ITS GROWING NEEDS, HUMANITY MUST CHOOSE landfills and especially at unauthorised landfills. their composition for Russia: paper and cardboard ONE OF TWO AREAS OF WASTE MANAGEMENT: Methane and carbon dioxide are the main (33–40%), food waste (27–33%), wood (1.5–5%), waste products. Russian landfills annually emit ferrous metals (2.5–3.6%), non-ferrous metals 1. Continue the path of increasing the rate of extraction of natural resources for the production 1.5 million tons of methane and 21.5 million tons (0.4–0.6%), bones (0.5–0.9%), leather and rubber technologies of various kinds of socially useful products with the formation of a progressive volume of CO into the atmosphere. In 2015, there were (0.8–1.3%), textiles (4.6–6.5%), glass (2.7–4.3% ), 2 of industrial and household waste in landfills. 13.9 thousand active landfills in Russia. Hazardous polymers (4.6–6.0%), etc. To a large extent, they landfills include hundreds of diverse volatile contain combustible components, which allows 2. To efficiently process the resulting waste with the return to production (recycling) of their material organic substances. British scientists [1] also found them to produce heat and electric energy, recycling and energy components with the disposal of only ecologically neutral waste and/or quickly about 140 different substances in the landfill gas. the energy spent on their production. processed to environmental indicators. Among them are alkanes, aromatic hydrocarbons, But the world has not developed a cycloalkanes, terpenes, alcohols and ketones, single universal technology for the complete chlorine compounds, including organochlorine processing of such complex in size and chemical compounds such as chloroethylene. During decay composition of MSW under acceptable economic The development of new technologies for the and household waste from human activities. and burning of landfills, thermal pollution of the conditions. This leads to their progressive processing of waste as a special business, as well Initially present in the socially useful product, the environment also occurs. accumulation and alienation of large areas of as the organisation of non-waste production should intellectual component of useful products in waste In Russia, up to 94% of garbage falls into cities for the organisation of landfills. In Russia, be carried out taking into account the specifics of is no longer present, and material and energy can landfills, of which only 4% is recycled and 2% is 100 billion tons of MSW have already been waste as a special product of material production. become a raw material base for organising waste incinerated. For comparison: in the EU, 45% of accumulated in landfills with an annual level of In the process of implementing technologies in the recycling technologies and generating energy from garbage is recycled, 28% goes to landfills, and their production of 60-70 million tons. According to sphere of material production, natural resources waste. 27% is burnt. Numerous landfills and unauthorised expert estimates, the world leader in the production are removed from the environment and production For an objective perception of the problem, dumps have become a haven for rodents, birds and of MSW is the United States, where 262 million tons and consumption wastes are generated. The final the following is important. From the environment insects, which are carriers of infectious diseases. of MSW are produced per year (18% of the global product of any technology [2] includes material, we extract not energy, but substances with a high The morphological composition of MSW volume). energy and intellectual interconnected components content of exergy (workable energy). It is necessary is extremely diverse and varies greatly across (Fig. 1). The peculiarity of the energy component is to save fuel as the main source of exergy and that it is not explicitly contained in the product and electricity as an exergy stream. Based on the exergy is consumed for the production of the product. approach, the concept of low-waste environmentally Material and energy resources borrowed from friendly technologies for the production of socially the environment for the production of socially useful useful products will be formed in the future. products are ultimately transformed into industrial

FIG. 1. THE RELATIONSHIP OF MATERIAL, ENERGY AND INTELLECTUAL PRODUCT

WASTE MANAGEMENT METHODS

Fig. 2 presents the current world hierarchy of approach is especially successfully implemented waste management. In European countries [3], four in the separate collection of garbage with the main methods of waste management are practiced separation of glass, plastic, paper, food waste, (Fig. 3) — landfill, recycling into secondary raw as well as in large waste processing plants that materials (recycling), composting and thermal use automated and robotic waste collection lines disposal (incineration). The most common way in with the most modern technical means. Recycling countries with underdeveloped waste management makes it possible to earn income from the sale of infrastructure is landfill. In Romania, the share of recycled materials. A high proportion of recycling landfills is 98%, only 2% goes for processing, and in takes place in Germany — 47%, in the USA — 35%, Russia, respectively, 95% and 5%. Disposal is at the in Korea –58%. lowest level of the European waste hierarchy. This The State Duma of Russia has simplified the means that only that which cannot be processed waste disposal procedure by law. The law adopted or used for energy recovery, i.e. inert or mineral in 2019, the Law on the Utilisation of MSW relates fractions, should be buried. to the utilisation of “the use of MSW as a renewable Processing into secondary raw materials energy source after extracting useful components or recycling has received special development from them at processing facilities”. According to recently in economically developed countries. This the legislation of Russia, only recycling can be 68 69

FIG. 2. GENERAL HIERARCHY OF WASTE MANAGEMENT FIG. 3. WAYS OF MANAGING MSW IN EUROPEAN COUNTRIES

attributed to recycling — the reuse of waste fractions from them. IMPORTANT ADVANTAGES OF MODERN METHODS for the production of goods (for example, delivery Composting is a waste processing technology OF THERMAL PROCESSING ARE: of rubber tires), regeneration — the return of based on its natural biodegradation. For this reason, goods to the production cycle after preparation composting is widely used for the processing 1. effective waste disposal (complete destruction of pathogenic microflora); (glass delivery), recovery — the recovery of useful of organic waste. Today, there are composting 2. reduction of waste up to 10 times; components for reuse (for example , extraction technologies for both food waste and an undivided of steel cord from tires and its use in concrete solid waste stream. When compost is produced, 3. use of the energy potential of organic waste. slabs). Before burning the waste, useful fractions gaseous waste products are emitted into the

must be extracted from them, they must be atmosphere: methane, CO2, H2S and other gases. processed and only the remaining so-called «tails» Methanisation is carried out in a closed volume, Thermal processing is the most radical means can be subjected to energy recovery. Following and during this process, part of the organic matter (tetrapack packaging, etc.) — 26.4; food waste — of disinfecting and disposing of waste. Today, 18.2; small waste — 20.1. Rubber and plastics are this principle, four plants in the Moscow Region is converted into biogas, which, like combustible incinerator incineration (MRZ) is one of the main will process 2.8 million tons of waste per year to gas, can be used for local production of heat considered the most suitable for re-use of energy, methods of thermal processing. The extraction of especially using gasification methods. produce 280 MW of electric energy, and a plant and electricity. The share of composting in energy from waste has become widespread in the near Kazan will produce 550 thousand tons of developed EU countries is 15-20%, and in Austria The factories of the project “Energy from world and is a worldwide trend called Waste-to- Waste” (Fig. 5) perform an important task — the waste (55 MW). Great importance is given to the it reaches 35%. Energy (Fig. 4). Today around the world there are qualitative sorting of incoming waste. After the MSW contain organic fractions and thermal involvement in the secondary circulation of about 2450 waste heat treatment plants. In 2018 waste that is not suitable for classical recycling. automatic sorting of materials, “tails” remain which methods are often used for their processing. alone, more than 60 new plants were built. Plants cannot be processed. They are pressed and sent to Thermal processing is a set of processes of They differ from the classic MRZs not only in with a capacity of more than 530 million tons of their environmental friendliness, but also in their landfills or to the plants where they are burned. The thermal impact on waste necessary to reduce their waste per year should be put into operation by 2028. sorting task is the smallest receipt of the “tailings” volume and mass, neutralisation, and to obtain approach to waste processing. Waste is considered The share of thermal processing in Denmark a renewable source of energy that can be compared of solid waste. The adopted amendments to energy and inert materials with the possibility of reaches 55%, Norway — 54%, Sweden — 50%, Federal Law-89 “On Production and Consumption further disposal. with the energy of the sun or wind. “Tails” arrive Germany — 35%. This approach is possible due at the plants — only the waste that remains after Wastes” prohibit the incineration of waste without to the high carbon content in MSW and a number preliminary processing and extraction of useful sorting and is unsuitable for recycling. Garbage of other industrial wastes. The lower heat of trucks at the plant undergo mandatory radiation combustion of MSW is in the range of 4.2 ... 12.6 MJ monitoring, weighing and metering procedures, / kg, as a result of which MSW can be considered after which the waste is discharged into a receiving as low-quality fuel. The heat of combustion of the hopper. Here, waste can accumulate and then enter organic part of the individual components of MSW the boiler, where two heat treatment zones pass. in terms of dry ashless mass [4] is in MJ / kg: In the first, the waste is thermally processed at a paper — 16.9; wood — 20.3; textiles — 22.6; leather, temperature of 1,260°C to destroy toxic dioxins and rubber — 31.1; plastic — 27.4; composite materials 70 71

FIG. 4. CONSTRUCTION OF FACTORIES FOR THE PRODUCTION OF ENERGY FROM WASTE FIG.5. THE SCHEME OF «ENERGY FROM WASTE» PLANT BY HITACHI ZOSEN INOV

other harmful elements. The second zone is the After incineration, waste is reduced by 90% in gas afterburning chamber. Flue gases generated volume. After thermal processing of garbage, ash during combustion at temperatures above 850°C and slag remain, representing “neutral” waste. come here. A special urea solution is injected In developed countries, only specially treated oxides, hydrogen chloride and fluoride, carbon (Fig. 6), or to support combustion by using additional into the afterburner to completely remove organic neutral wastes should be disposed of, moreover, monoxide CO, toxic metals (mercury , lead, bismuth, fuel having a higher calorific value, or heated air compounds and neutralise flue gases. Then the with minimisation of their amount (Switzerland — antimony, etc.), polyaromatic hydrocarbons — blast, or blast enriched with oxygen. The use of flue gases and slag enter the reactor, where they about 0%, Germany, Sweden, Denmark — 1% each). PAHs (benzapyrene, fluorantene and many others). oxygen reduces the total volume of gases (air + are treated with activated carbon and ammonia, The authoritative publication Waste Management The low-temperature incineration of MSW at MSF, oxygen) used in the incineration of waste, thereby chemical elements are added for additional and Research published a large editorial in 2020 where the carcinogen tetra-chloro-dibenzodioxin, allowing to increase the temperature during the neutralisation. Already cleaned from the reactor [5], in which three leading European scientists, which is converted into a liquid or gaseous state, incineration of MSW and at the same time reduce flue gases, they enter the bag filters for cleaning who have been dealing with the problem of waste is completely restored upon cooling, is the main the volume of gases and the size of gas treatment microparticles. Due to the heat of the flue gases, recycling for a long time, explain why, with all the source of dioxins in the environment. facilities. Coal-processing waste, when co-fired, steam is produced that enters the turbogenerator ideality of the Zero High-quality combustion requires elevated can provide the necessary combustion conditions to generate electricity. Up to 10% of the energy Waste concept, it is unattainable. temperatures in the range of 1,500–2,000°C. within the Tanner fuel triangle. The temperature produced is spent on the plant’s own needs, and the A proven approach to reduce the formation of in the waste incinerator using oxygen will not be rest goes to the electric network. dioxins is to form a zone of high temperatures lower than 1,400°С. However, the use of oxygen of more than 1,200°C with a residence time of at in recycling will increase the cost of the process. least 2 seconds, when the dioxins are completely However, the reduction in the size of gas treatment destroyed, followed by rapid cooling or catalytic facilities due to the use of oxygen and the decrease afterburning to avoid a new process of formation in the concentration of harmful substances in the of dioxins. The International Energy Agency exhaust gases at temperatures above 1,300°C calls energy waste management with such high- compensate for this drawback. FEATURES OF THE WASTE BURNING PROCESS temperature incineration and environmental Studies have shown that the composition of the pollution control technology the best alternative to gas phase waste generated during incineration is MSW landfills. safe if the process temperature is at least 1,300°C. he main trend in the development of waste A lot of experience has been gained in The low calorific value of solid waste is not It is this temperature that is taken as the minimum incineration is the transition from direct burning combustion technologies since the end of the a serious obstacle to their burning. If we consider combustion temperature when designing the MWF of solid waste to optimised burning of the hot 19th century, and in the vast majority of cases they waste as a solid fuel having a certain ash, moisture abroad. In gas treatment plants, lime is used to (fuel) fraction extracted from solid waste and are based on the method of burning on grates. and combustible part, then it can either be used remove chlorine and sulfur; complex compounds the transition from burning as a process for the The main requirements are strict adherence to to achieve the conditions of the Tanner triangle are removed using activated carbon. disposal of solid waste to burning as a process the regulations of the combustion process and that ensures, along with the disposal of waste, heat the subsequent implementation of the cleaning and electric energy. processes for gaseous, liquid and solid emissions Currently, the level of incineration of solid with the control of the content of harmful waste in the world is different. In Austria, Italy, substances. During uncontrolled combustion in France and Germany it makes up 20–40%, in the temperature range 800-850°С, which is typical Belgium and Sweden — 48–50%, Denmark and for most existing waste incineration plants (MSZ) Switzerland — 80%, England and the USA — 10%. and when burning in landfills, a whole spectrum of In Russia, about 2% of household waste is burned. harmful substances is formed: sulfur and nitrogen 72 73

FIG. 6. TANNER TRIANGLE method will be characterised by high investments of the waste and require preliminary sorting or in dust and gas cleaning to reduce emissions of averaging of the composition. The last two methods harmful substances into the atmosphere. have the best indicators: plasma gasification and Pyrolysis and conventional gasification have waste processing in slag melt furnaces. They are approximately the same indicators, which can be characterised by high (over 1,300°C) processing explained by the close processing temperatures. temperatures, form liquid slags, which is important They have higher performance compared to for their further processing. incineration, but are sensitive to moisture content

TABLE. COMPARATIVE ANALYSIS OF TECHNOLOGIES FOR THE THERMAL PROCESSING OF SOLID WASTE [6].

Way

Index Pyrolysis and thermal Waste treatment in slag Burning decomposition of solid Plain gasification Air Plasma Gasification furnaces * waste without oxygen

The destruction of the 70% destruction 90% destruction ( 90% destruction Complete destruction Complete destruction organic part, furans, (650…1050 °C) 450…900 °C) (800…1150 °C) (2000 °C) (1300…1650 °С) dioxins

Resin and Furan For- There are pitches and There are pitches and Many resins and furans No resins and furans No resins and furans mation furans furans

INTEGRATED DISPOSAL TECHNOLOGIES Ash formation 30% toxic resin 10% ash 10% ash No ash 0.15% ash per turnover

Incineration is a commonly used method for Plasma gasification — the processing of In addition to certain types In addition to certain types In addition to certain types Recyclable waste types Any kind of waste Any kind of waste MSW disposal. The final combustion products are waste in an air plasma jet at temperatures up to of inorganic waste of inorganic waste of inorganic waste ash, as well as significant amounts of benzapyrenes 2,000°C. The development of this technology and dioxins that are released into the environment. was carried out in Russia, Israel, Japan. The Requires a uniform With this in mind, effective environmentally disadvantages include the need for a lining in The need for pre-sorting Waste sorting required composition of MSW Waste sorting required No waste sorting required No waste sorting required waste friendly disposal should not be based on simple the combustion area of the plasma, which must throughout the year incineration, but on deep processing with withstand high temperatures. Besides, large intermediate neutralisation of components. investments are required. The project, designed for

Pyrolysis is the decomposition of organic a capacity of 110 tons of solid waste per day, was Large amount of waste up Waste volume in pyrolysis Waste volume up The volume of waste is up The volume of waste is up Recyclable volume matter into less heavy molecules under the influence carried out by the Japanese company Eco Valley to 500t/day plants up to 30t/day to 250 t/day to 110 t/day to 330 t/day (project) of increasing temperature without oxygen. Raw in Utasinai on the island of Hokkaido. The final materials for pyrolysis can be municipal, industrial products are electricity, heat, synthesis gas. and agricultural waste, coal, etc. The disadvantage MSW gasification in slag melt furnaces. The The level of gas emis- For comparison, there are Flue gas of this method is the production of a solid product technology was developed in Russia. In the 1980s sions with a conditional High fue gas emissions Gas emissions — no installations for this emissions — No data capacity of 120 thousand up to 60 thousand nm3/h 30 thousand nm3/h that requires additional processing. End products in Ryazan, at a pilot plant in a furnace of slag melt, performance 50 thousand nm3/h are synthetic fuel, synthesis gas, heat, electricity. pilot industrial technological tests were carried tons of solid waste/year Gasification — the conversion of the organic out, which gave a positive result. part of biomass into combustible gases during Comparative characteristics of various Humidity of the waste is high-temperature heating with an oxidising technologies for the thermal processing of solid Waste humidity up to Sensitivity to waste about 20% when removing Sensitive to humidity 50% with a low level of Insensitive to humidity Insensitive to humidity agent (air and water vapor), with the production waste are given in the table. An analysis of the data moisture the inorganic part up inorganic part of a gaseous energy carrier — synthesis gas. shows that the least profitable is the combustion to 40% The gasification process involves pyrolysis as a technology — although it is the cheapest, but it stage of the process, therefore, the generator gas creates a large amount of secondary waste that The quality of the result- mixture consists of pyrolysis and generator gases. requires additional processing or disposal, and Generator gas (technical) Ballasted synthesis gas Generator gas (technical) High quality synthesis gas Generator gas (technical) ing synthesis gas For this reason, many use a two-stage combustion emissions of toxic substances; the technology scheme, that is, pyrolysis at low temperature and requires preliminary sorting of waste and reducing Syngas, liquid fuels, Synthesis gas, liquid fuels, Synthesis gas, electricity, Outlet products Heat, electricity Heat, electricity high temperature afterburning of the resulting its moisture content. The indicated disadvantages electricity, heat electricity, heat heat, fused slag are a consequence of the low combustion gases. At the same time, they receive electricity, * According to the GINSVETMET Institute heat, and slags that can be used in construction. temperature of 650°C. The implementation of this 74 75

Ecotechnological parks developed all over the of domestic and surface wastewater for further use Thus, based on the exergy method it is account the composition, place of occurrence of world are aimed at integrated waste processing, in the production process. The introduction of slag possible to analyse all the material and energy the waste and the technologies for its processing. which combines the production of electricity and melt into this furnace chain will make it possible to transformations of energy recycling, taking into heat from waste (which makes it possible for the widely use it both at the stage of generating heat remaining links of the ecotechnological park to and electricity, and at the stage of processing slag function), marketable products and the treatment to produce building materials.

NEUTRAL WASTE

Chemical exergy of waste can be an indicator emissions can also be represented in units of EXERGET ANALYSIS of determining the “neutrality” of waste during their operability with respect to molecular gas.

its disposal and answer the question about the So for nitrogen oxides NO, NO2, NO3 and N2O, possibility of practical implementation of the Zero it will accordingly be equal to 2,888.5; 1,114.7; The term «exergy» was introduced in 1956 and processing without additional external attraction of Waste concept. The final destination of the products 1,873.6 and 2,369 kJ/kg, and for sulfur dioxide — comes from the Greek word «ergo» n — work and natural resources. of thermal processing of the “tailings” of waste, 3,474.9 kJ/kg. Crystalline solar silicon (Si), as the prefixes «ex», which means a high degree. It is Chemical exergy of traditional fuels practically as well as other waste that must be buried, is the a waste of solar silicon solar cells, will have an important that exergy is a physical, not an economic coincides with their calorific value during environment, the total exergetic potential of which is excess exergy of 30,615.9 kJ/kg. Chemical the criterion, and determines the independence of combustion [10]. However, this does not apply equal to or close to zero for all constituent types of exergy of gaseous methane (CH ), which is the this parameter from market fluctuations in prces. 4 to waste having a complex composition, where exergy. In the scientific literature [11], environmental main component of landfill gas, is 51,101.2 kJ/ The universality of the exergy analysis method as non-combustible substances are present. According substances with zero chemical exergy are given. kg. For complex organic compounds, data on the applied to various basic processes and technical to the existing procedure for processing waste, only These include water in liquid state (H O), in gaseous values of chemical exergy are also available in the systems is described in detail in the literature 2 “tailings” from waste processing resulting from the oxygen (O ), nitrogen (N ) and carbon dioxide literature. [7,8]. Unlike energy, exergy quantifies the ability to 2 2 allocation of all valuable components for recycling (CO ), crystalline silicon oxide (SiO ), crystalline Thus, knowing the elemental composition do work. Concepts such as “Energy Saving” and 2 2 into the sphere of material production should be sodium chloride (NaCl), crystalline iron oxides of the waste, we can assess the extent to which “Energy Saving” actually mean saving exergy. Exergy incinerated. For an objective assessment The (Fe O ), radioactive uranium oxide (U O ) and other they are far from the parameters of the natural is a single measure of the efficiency of energy 2 3 3 8 efficiency of energy recycling with the generation substances of inorganic origin. They constitute the environment. Ideally, substances with only zero resources and its application allows you to give an of electricity and heat in the process of burning natural background of the environment in relation exergy should be buried. Estimates of the exergy objective assessment of energy resources of any “tailings” requires an assessment of the initial to which the chemical exergy of waste of inorganic of the tailings of waste and combustion products kind, including secondary ones. The use of exergy of energy and exergy potential of these substances origin can be determined. So, the chemical exergy from the standpoint of the exergy methodology production waste not only reduces the consumption with different compositions of conglomerates. of aluminum (Al), from which cans for drinks are provide an objective picture of the completeness of of the corresponding energy raw materials, but also The exergy losses in any technological process made, is, respectively, in a gaseous, liquid and energy recycling processes. leads to a reduction in investment in the extraction can be estimated in two ways: by the balance solid state 41,156.3; 31,289.9 and 31,083.0 kJ/kg, and processing of this raw material. In some leading of exergy in the process and by the exergy of respectively. The exergetic potential of harmful European countries and the United States, exergy production waste. The first way involves a detailed analysis has been introduced as a mandatory analysis of the exergy fluxes at the input and output component of projects under development, as well of the technological process, the second involves as plans for modernisation of production. It is also taking into account all types of exergy losses used in assessing natural resources. For example, with heat and material waste. When analysing the US Geological Committee takes into account the technogenic load on the environment, it is geothermal resources for their exergy. advisable to separately consider the energy (heat) CONCLUSION In accordance with the law of conservation losses and exergy losses associated with material of matter, the total mass of all substances and emissions. All energy losses are realised in the The main trend in the development of waste substances with a high content of exergy (workable compounds on the planet, including those contained form of heat dissipated into the environment, and incineration is the transition from direct burning energy). It is necessary to save fuel as the main in industrial and household waste, remains this can occur by various physical mechanisms: of solid waste to optimised burning of the hot source of exergy and electricity, as an exergy constant. But the transfer of waste to its original through heat transfer and / or thermal radiation (fuel) fraction extracted from solid waste and the stream. natural state requires significant exergy costs. and with the help of material carriers — transition from burning as a process for the disposal The specific values of exergy and enthalpy When assessing the costs of waste processing emissions. Heat losses are one of the types of of solid waste to burning as a process that ensures, can be determined for almost all elements of the technologies, it is necessary to take into account technogenic pressure on the natural environment, along with the disposal of waste, heat and electrical periodic table if zero substances are selected for the principle of unequal exergy losses [9]. Waste, creating «thermal pollution» of natural ecosystems. energy. Thermal disposal is an essential element their calculation. Such can be substances of the as a rule, is formed at the end of technological The economic damage associated with heat loss is of any waste management system. Extraction of natural environment: water, carbon dioxide, air, cycles of production of socially useful products obvious. In general, exergy losses associated with energy from waste has become widespread in the metal oxides, sulfur, and other elements of natural and therefore the cost of working energy for their material emissions (tailings) from industrial and world and is a global trend of Waste-to-Energy. origin. This allows you to evaluate the exergetic processing will increase the relative selection of other sources have three components: temperature, Energy from Waste plants are an obvious part value of the waste by its known composition, natural resources at the beginning of the processes chemical and mechanical — associated with of the energy recycling solution. To further optimise including for recycling processes. Using the concept at resource-mining enterprises. In this regard, the excess pressure of emissions. Each type of exergy waste recycling processes with the generation of of exergy allows you to create the foundation for issues of efficient heat and power generation from can be calculated by thermodynamic formulas heat and electricity, to reduce the harmful effects the development of new, more advanced low-waste waste are of paramount importance for the energy or determined by the corresponding nomograms on the environment, it is necessary to use an (environmentally friendly) technological processes. supply of the implementation of all stages of their [7–9]. exergy approach. We do not produce energy, but 76 77

REFERENCES CHEMICAL FUEL 1. Trace Organic Compounds in Landfill Gas at Seven U.K. Waste disposal sites Matthew R. Allen, OF SUNLIGHT Alan Braithwaite, and Chris C. Hills Environmental Science & Technology 1997 31 (4), 1054-1061 DOI: 10.1021 / es96056

2. Raizberg B.A., Lozovsky L.Sh., Starodubtseva E.B. “The Modern Economic Dictionary. - 6th ed., Revised. And ext. - M.” (INFRA-M, 2011). – 512p.

3. Titov B. Household waste management systems in different countries: Recipes for Russia. Institute for Growth Economics Stolypina P.A.

4. Energy Bulletin. Analytical center under the government of the Russian Federation. 2017, No. 48.

5. The Zero Waste utopia and the role of waste-to-energy / Stefano Consonni, Peter Quicker, Mario Grosso // Waste Management & Research, 2020, Vol. 38 (5) 481–484

6. Vlasov O.A., Mechev V.V. Analysis of the operation of waste incinerators / Municipal solid waste. - 2017- No. 8. - p. 38-41.

7. Brodyansky VM, Fratsher V., Mikhalek K. Exergetic method and its applications. - M .: Energoatomizdat, 1988.288 s.

8. Szargut J., Morris D.R., Steward F.R. Exergy analysis of thermal, chemical and metallurgical Suleyman Allakhverdiev, processes, 1st ed. N.Y .: Hemisphere Pubs, 1988.332 p. Head of Controlled Photobiosynthesis Laboratory, Institute of Plant Physiology, Russian Academy of Sciences 9. Brodyansky V.M., Verkhivker G.P., Karchev Y.Ya. and other Exergy calculations of technical systems: a reference guide. - Kiev: Naukova Dumka, 1991 - 360s.

10. Stepanov V.S., Stepanova T.B. Calculation of chemical energy and exergy of industrial fuels // Bulletin of the Russian Academy of Sciences. Energy - 1994. - No. 4. S. 106-115.

11. Stepanov V.S. Chemical energy and exergy of substances. - 2nd ed., Revised. and add. — Novosibirsk: Science. Sib. Department, 1990. — 163 s ABSTRACT

Humankind needs energy for electricity in ponds and bioreactors. The main disadvantage generation, heat, industry, transport. The energy of the biofuel industry is the low efficiency of demand will grow in future. Fossil fuels are solar-to-chemical conversion. This shortcoming the primary global source of energy. This is an can be avoided by the artificial or semiartificial exhaustible energy source and its exploitation is systems imitating the primary reactions of the accompanied by emission of greenhouse gases oxygenic photosynthesis. This more sophisticated and pollutants. Exploitation of sunlight for the and at the same time, interesting field is called synthesis of high-energy organic species from artificial photosynthesis. There are different low-energy inorganic precursors is the good role kinds of solar fuels. Most perspective of them is

model that nature shows us. The easiest and molecular hydrogen (H2). It is absolutely carbon- commercially available way to obtain chemical zero, eco-friendly gaseous fuel, which is believed fuel due to sunlight — is biofuel production from to be the fuel of future. This fuel can be obtained the biomass of plants. However, the biomass both through the photosynthetic metabolism of industry for the biofuel production competes with microalgae and artificial photosynthesis. The main food industry for lands. In addition to this, biofuel methods of hydrogen production at present are is not exactly carbon-zero fuel. CO2 emission at the the high-temperature, ineffective, environmentally biofuel combustion is much more intensive than harmful treatments of the fossil fuels. Solar CO2 fixation at plant growth. Promising alternative production of hydrogen is the promising area for of the plant biomass is the algal biomass, cultivated research and developments. 78 79

INTRODUCTION FIG. 2.

The amount of the global energy demand (Fig. 1B) [5,6]. In 2017, almost 80% of consumed is correlated with on the Earth population and energy all over the world came from fossil fuels the global quality of life. An increase in the world (Fig. 2A) [7]. Of this part, about 58% is accounted for population is projected. According to a moderate the transportation sector [8]. Fossil fuel (oil, natural forecast, the population will continue to grow gas, and coal) is concentrated organic materials. until at least 2100 (Fig. 1A) [1]. In 2050, the urban It is formed from remnants of plants and animals population is expected to be approximately 20% that lived millions of years ago [9]. Hydrocarbons higher than today [2]. Both of these factors cause are the main components of the fossil fuels. These the increase of the global energy consumption. high-energy compounds can be synthesised from The increasing use of energy-consuming devices carbon dioxide and ubiquitous water. This synthesis and the growing demand for energy in developing requires external energy. Oxygenic photosynthesis countries contribute to the growth of global apparatus uses sunlight as energy source for this energy consumption too [3,4]. It is predicted, that reaction. A vast amount of solar energy is stored in the increase of energy consumption will continue chemical bonds of hydrocarbon via photosynthesis Estimated renewable share of total final Contribution to environmental pollution much longer than that of the world population since over 3 billion years [10]. energy consumption, 2017 [7] from the consumption of various types of fossil fuels in 2017 [14]

FIG. 1.

nuclear power, hydropower, traditional biomass, chemical fuel. This solar fuel can be used instead as well as more eco-friendly and less developed of fossil fuels in engines. renewable energy sources (Fig. 2A). Several of them Solar fuels include photohydrogen, biofuel and have ecological problem like traditional biomass hydrocarbon fuel obtained via artificial systems. and hydropower. The building of hydroelectric Biofuels like biodiesel or bioethanol can be used power plants destroys entire biocenoses. Nuclear without or with a little bit of postprocessing in the power engineering is connected with considerable contemporary engines. Biofuel can serve either as risks of ecological and humanitarian disasters [15]. an addition to the traditional engine fuels or main In least developed countries traditional biomass is fuel in motors [19]. However, burning of the biofuel

the main fuel. Main component of the traditional leads to CO2 emission, similarly to that of fossil fuel.

biomass is the wood. Intensive use of traditional This is not large problem if the CO2 assimilation biomass causes great damage to forest resources in the processes of biofuel production is very

[17]. Presently, the available sources of renewable intensive and comparable with the CO2 emission energy could generate only approximately 7% of the rate. Actually, the rate of biofuel production is much energy used (Fig. 2A) [7]. They include solar power, lower than the rate of its combustion. So it does not wind, ocean power, geothermal heat, traditional suggest very good ecological solution. biomass, biofuel and hydropower [18]. Hydrogen is not compatible with conventional These charts show estimates and probabilistic projections of the total Global energy consumption (Exa Joule) over time based on world population. The population projections are based on the probabilistic moderate population growth and moderate energy consumption The fact that photosynthetic organisms can engines. However, it is ideally carbon-zero fuel. Only projections of total fertility and life expectancy at birth. The figures display per capita. Traditional biomass energy (pink)and modern use inexhaustible solar energy to synthesise high- water is the product of the hydrogen combustion. the probabilistic median, and the 80 and 95 per cent prediction intervals of biomass energy (blue) are indicated separately [11]. energy molecules, attract scientific attention to At the same time, hydrogen has high calorific the probabilistic population projections, as well as the (deterministic) high and low variant (+/- 0.5 child) [1] the sunlight energy exploitation for the synthesis content. Its eco-friendliness stimulates mankind to of chemical fuels. Sunlight has many perspectives development of an appropriate infrastructure. So in the alternative energy development, both in far, the main source of hydrogen is fossil fuels. solar-to-electricity and solar-to-chemical devices. In this chapter, I will briefly describe the Sun provides vast amount of energy to the Earth. main aspects of biofuel production, methods of The rate of the fossil fuel production is much 2017. The coal and oil are the main contributor to the Unfortunately, solar energy does not radiate obtaining hydrogen by the activity of microalgae continuously and it depends on the cloudiness. and principles of artificial photosynthesis, aimed at lower than the rate of its combustion. Fossil fuels CO2 emission by fossil fuel (Fig. 2B) [14]. Significant is exhaustible resource. It is expected, that the amount of other harmful gases, carcinogens and Due to uncertainty with solar condition, solar-to- the production of chemical fuels. The main focus storages of oil and natural gas will be depleted poisons are released into the atmosphere besides electricity system should be connected to the will be on hydrogen, because it is more promising storage system for efficient conversion. Solar-to- for the future than fossil fuels. within about 50–150 years [12]. The situation of CO2 [15]. Another problem is the uneven distribution coal reserves is a little better [13]. Exhaustibility is of fossil fuels on the Earth. This can cause political chemical conversion allows the accumulation of not the only problem of the fossil fuels. The main conflicts [16]. The disadvantages of the fossil fuels solar energy in the chemical bonds of the resulting disadvantage of this energy source is significant stimulate the development of the energy sources,

CO2 emission to the environment. This is the cause which must be renewable, sustainable, easily of climate change. Global carbon dioxide emissions accessible, inexpensive and eco-friendly. There from fuel combustion reached 32.8 gigatons in are many alternatives to fossil fuels. They include, 80 81

and sugarcane, which contain sugar in their stems. is the main product of gasification. Bio-oil is the BIOFUEL Secondly, this biofuel industry compete with food main product of liquification and pyrolysis. It is crop industry for the cultivated lands [15]. the mixture, which contains more than 350 various The second generation of biofuels includes components of low molecular weight [15]. Syngas Biofuel has many perspective, most particularly heating. Biofuels, derived from biomass via special the production of bioethanol and biodiesel from and bio-oil may be used to make various types of in the transport sector [19]. Biofuels have been treatment procedures called secondary biofuels. several nontraditional species of plants such as fuel. Chemical conversions include hydrolysis and proposed as an energy source which would These include biodiesel, bioalcohols, biogas and jatropha, cassava, miscanthus, straw, grass, and solvent extraction [21]. Biological treatment of significantly reduce the environmental pollution bio-oil. Three main components of biomass are wood. The technical innovation of the second biomass is more complex than the thermochemical [20]. It is produced from biomass. Exploitation of the molecular precursors of secondary biofuel: generation of biofuel is the possibility to use one. However, the production of some compounds the biomass of photosynthetic organisms — plants vegetable oils or animal fats (triglycerides), starch lignocellulose-enriched biomass as the fuel is possible only via chemical or biochemical and microalgae — is of special interest. These and oligomeric sugars, lignocellulose are the feedstock. Lignocellulose is the main component methods. Second generation of biofuel offers organisms do not need any organic substrate for main precursors of the biofuel. Fats are used for of the cell wall. Cell walls make up the majority of more efficient land use. This has less impact on the growth. Proteins, lipids and carbohydrates are biodiesel synthesis. Fermentation of starch and the plant biomass. However, extraction of biofuel the food sector due to the use of non-food crops. the main components of dry biomass. Composition sugars is a way for bioalcohol and biohydrogen from lignocellulose is quite difficult. It requires However, the land competition persists. In addition and ratio of these molecules depends on the synthesis [15]. There are three generations of complicated (bio)chemical or high temperature to this, difficulty of the lignocellulose-enriched growth condition and the type of organism. This secondary biofuels, distinguished by the biomass processes. Thermochemical methods include biomass treatment hinders competitiveness of this composition in turn impact on the calorific value of type used and treatment techniques: 1st, 2nd and gasification, liquification and pyrolysis. The mixture biofuel [15]. the biofuel. In the case of exploitation of biomass 3rd generation biofuels (Table 1). The first two of CO, CO2, H2, CH4, and N2 gases, called syngas as fuel without any treatment, such biomass is generations are based on the plant biomass and called primary biofuel. Primary biofuels include the third perspective generation is based on the wood, wood chips, animal fats, residues of forest algal biomass. and agricultural crops [15]. These are used often for

TABLE 1. CLASSIFICATION OF BIOFUELS [19]

Secondary biofuel

First generation Second generation Third generation ALGAL BIOFUEL Bioethanol or butanol by fermentation Bioethanol and biodiesel produced via Biodiesel from microalgae; of starch (from wheat, barley, corn, potato) or conventional technologies but based on novel bioethanol from microalgae sugars (from sugarcane and sugar beet); starch, oil and sugar crops such as Jatropha, and seaweeds; biodiesel by transesterification of oil crops cassava or Miscanthus; bioethanol, biobutanol, hydrogen from green microalgae Third-generation biofuels are produced from grow microalgae of one particular species without (rapeseed, soybeans, sunfower, palm, coconut, produced from lignocellulosic materials (e.g. and microbes. algal biomass. Algae, in comparison with higher used cooking oil, and animal fats) straw, wood and grass). any impurities. However, they need external energy plants, do not require arable land. Microalgae and are harder to maintain than open-air systems can grow under conditions that are not suitable [15]. for plant growth: saline soils, sewage. The use Algal biomass can accumulate considerably of algae as a source of biofuel avoids the impact high amounts of lipids in comparison with biomass on the food sector. There are different methods of oil plants. Some algae species can produce more of growing algae, allowing to adjust the ratio of biodiesel from one kilogram of biomass compared lipid/carbohydrate/protein and the composition to higher plants [15]. Due to this, it may be very PLANT BIOFUEL of the algal biomass. Careful management of effective to obtain biodiesel from microalgae. these methods can improve the production of The first step of the production of any of algal the necessary biofuels. On the other hand, the biofuels is the cultivation of algal biomass under Plant biofuels include first and second the exploitation of recombinant enzymes is an cultivation of microalgae requires reliable control well-defined conditions. In the case of biodiesel generations. The first generation of biofuels is effective alternative to saccharification by yeast of growth conditions and special growth systems. production, these conditions should provide high the production of ethanol from starch- or sugar- [22]. Plants, enriched by oils, like soybean and While plants need arable land, microalgae grow lipid content in the resulted biomass. The next stage enriched food crops like wheat, barley, corn, potato, rapeseed, can be used as source of biodiesel. in special bioreactors or in open ponds. The includes extraction of the lipids and its preliminary sugarcane, and also biodiesel production from Biodiesel is the fatty acid methyl ester mixture that advantages of open-air systems are their simplicity, treatment. This may be esterification by methanol soybean, sunflower and animal fat. Sugar crops is produced from vegetable oils through the reaction low cost of their design and maintenance, and in the presence of acid catalyst. This is followed by like sugarcane and sugar beet should be grinding of transesterification [21]. It is catalytic reaction of long duration of use. The disadvantages of these the transesterification reaction in the basic medium and fermenting by microorganisms for bioethanol hydrolysis of ester bonds between glycerol and fatty systems are their sensitivity to weather changes, [23]. In some techniques, preliminary treatment is production. This is the oldest techniques for biofuel acid chain followed by esterification with methanol the inability to carefully control temperature, avoided [19]. production. Starch crops, like corn and wheat [15]. First generation of biofuels has two main lighting and pollution with toxic chemicals and Also, algae are perspective raw materials for need additional stage of saccharification of starch disadvantages. Firstly, only an insignificant part of other organisms and the lack of opportunities for bioethanol, biomethanol, syngas and other biofuel before fermentation. At this stage, starch breaks grown biomass can be used for biofuel production. monocultural cultivation. The photobioreactor is a products. Depending on the algae species and down to glucose. Previously, saccharification was It decreases the productivity of cultivated lands. The closed system, which allows carefully controlling cultivation conditions, one can increase the sugar carried out by means of an enzymatic reaction source of starch and sugar biofuel is only grains, the temperature, illumination and composition of content in the algae biomass for the bioalcohol provided by special microorganisms [21]. Currently, fruits and root crops. The exceptions are sorghum the medium. Photobioreactors make it possible to production. 82 83

BIOLOGICAL HYDROGEN PRODUCTION DARK FERMENTATION

Fossil fuels are currently the main source of chemical compounds. However, they have very Hydrogen production can be performed in comparison with photofermentation and it

hydrogen. The production of H2 from fossil fuels is low efficiency. Another biological method for H2 via the fermentation of carbohydrate-enriched has shown the highest H2 evolution rates. Dark accompanied by the release of CO2. CO2 emission production is dark fermentation. This process compounds. The process occurs in facultative or fermentation can be catalysed by single organisms rate depends on feedstock and conversion is performed by heterotrophic organisms. Dark obligate anaerobic bacteria that consume organic or consortia. Waste biomass can be used as efficiency [24]. Two main pathways for hydrogen fermentation is less energetically costly than substance under anaerobic conditions and in the substrate for this process [28]. Nevertheless, it is photoproduction exist: biohydrogen synthesis photofermentation. absence of light. Fermentative microorganisms important to take into account several factors such by the microorganisms and photocatalysis via Hydrogenases are the enzymes required for use multiple hydrogenases: Escherichia coli utilises as pH of medium, and concentration of nutrients. artificial devices. All these processes require the biohydrogen production via all above mentioned [NiFe]-hydrogenase, Clostridium species utilise Fermentation allows to produce many different significant improvements in efficiencies, reduced methods. Some bacteria can produce hydrogen with [FeFe]-hydrogenases [15]. Dark fermentation has chemicals in addition to hydrogen [19]. capital costs, and enhanced reliability and operating the help of the nitrogenase enzyme. Hydrogenases been considered to be the less expensive approach flexibility [25]. Using sunlight for the water splitting are one of the components of hydrogen and hydrogen synthesis is the promising alternative metabolism. They can reduce protons to molecular of the natural gas services reaction (NGS) that is hydrogen under special conditions. Another part

industrial process for H2 synthesis from natural of the metabolism must generate these protons. gas. Three types of hydrogenases are known, distinct Biohydrogen synthesis processes include by their catalytic center. They are the [FeFe]-, [NiFe]- direct biophotolysis, photofermentation and and [Fe]-hydrogenases. The nitrogenase system dark fermentation. The first two processes are of cyanobacteria and [FeFe]-hydrogenase of green ARTIFICIAL PHOTOSYNTHESIS

performed by photosynthetic organisms: green algae are extremely sensitive to O2 [20,26]. [NiFe]-

algae, cyanobacteria and other phototrophic hydrogenases include O2-sensitive and O2-tolerant The above-mentioned methods for chemical mentioned above, molecular hydrogen is produced. bacteria. H2 production by microalgae is a very forms. [Fe]-hydrogenases are found exclusively fuel production at the expense of solar light share The membrane electron transport conjugates promising approach [19]. These processes do not in methanogenic archaea and are not very well one common disadvantage – low efficiency. with proton transfer across the photosynthetic require high temperature or rare and expensive characterised [27]. Efficiency of the photosynthetic solar-to-biomass membrane. The resulting transmembrane proton conversion is less than 1% [29]. Only small part of gradient drives the synthesis of ATP. NADPH the absorbed light energy is converted to biofuel. and ATP take part in the carbohydrate synthesis

The phototrophic organisms and heterotrophic from CO2 (Fig. 3A). This two-photon scheme is DIRECT BIOPHOTOLYSIS OF WATER organisms need energy for sustaining their life. the blueprint for artificial systems that produces Significant part of the absorbed energy is used for chemical fuel due to sunlight [30,31]. One of the In the case of direct biophotolysis, hydrogenase enzyme instead of the ferredoxin: vital processes in their cells. main ideas of artificial photosynthesis, inspired photosynthetic electron transport chain with NADP oxidoreductase system. In the case of At the same time, primary reactions of by oxygen photosynthesis, is the separation of oxygen evolving catalyst play this role. This chain dark-adapted microalgae, the rate of hydrogen photosynthesis has high quantum yield [25]. a photocatalyst oxidising water and a (photo) receives electrons and protons for hydrogen evolution is very high. However, hydrogenases are Development of the artificial or hybrid systems, catalyst synthesising molecular fuel (Fig. 3B). The production from water. Reduced ferredoxin, progressively inactivated by oxygen evolved by which mimic these reactions and utilise the solar most popular materials for the artificial devices are the last electron acceptor of photosystem I photosystem II (PS II). energy only for the production of chemical fuels, semiconductors. However, semiconductor without (PS I), under certain conditions interacts with the is a promising area for research. The processes modification is either unstable or ineffective. of synthesis of chemical fuel, driven by sunlight, in Stable semiconductors like TiO2 require an external systems built on the principle of a photosynthetic photocatalyst for water oxidation and/or fuel apparatus, are called artificial photosynthesis. synthesis for efficient solar-to-chemical conversion Natural processes of photosynthesis are the [25]. One of the emerging directions in this area is INDIRECT BIOPHOTOLYSIS OF WATER blueprints for the solar energy converters. mimicking the natural catalytic centers for water Comprehensive knowledge of these processes oxidation and proton reduction for designing stable (PHOTOFERMENTATION) is necessary for development effective, stable, photocatalysts [12]. Oxygen evolving complex low-cost and eco-friendly biohybrid solar cells. The is the native blueprint for the water oxidation Microalgae can produce hydrogen from stored fixation via a nitrogenase system. Under nitrogen primary interest oxygenic photosynthesis. This is catalyst and hydrogenase is the blueprint for the glycogen and starch. This process proceeds in two starvation, nitrogenase catalyses molecular because oxygenic photosynthesis uses water as hydrogen evolving catalyst. Another pathway is stages. Photosynthesis of these carbohydrates is hydrogen synthesis. In these cyanobacteria, an essentially inexhaustible source of electrons associated with the usage of the native protein the first step. The second step is the fermentation whole chain photosynthetic electron transport is and protons. Oxygenic photosynthesis uses two complexes (PSI, PSII, hydrogenase) with some of carbohydrate; molecular hydrogen is the carried out in the vegetative cells. Carbohydrates photons for the electron transfer from water to modifications allowed to connect with inorganic result of this fermentation. Microalgae, carrying from the vegetative cells are transferred into the NADP+. One of the photons activates photosystem substrate, increase their efficiency and durability. out indirect biophotolysis, should separate heterocysts, where they undergo fermentation. II, which is responsible for the oxidation of water. Systems consisting of both an inorganic substrate Another photon activates photosystem I that and reconstituted or native enzymes are called the evolved O2 and H2 because the system of Other photosynthetic nitrogen-fixers accumulate reduces ferredoxin. Ferredoxin is a hydrophilic semiartificial systems. H2 evolution highly sensitive to oxygen. The carbohydrates via photosynthesis during heterocystous cyanobacteria, oxidise water and daytime, and assimilate nitrogen at night. Like electron mediator with a redox potential, suitable Vast majority of the solar-to-fuel systems evolve hydrogen in different cells. They possess heterocystous cyanobacteria, these species for electron donation to either ferredoxin: NADP- produce fuel indirectly through generation of special cells, called heterocysts, which contain reduce protons to hydrogen in the absence oxidoreductase or hydrogenase. In the first photocurrent. These are the photoelectrochemical only photosystem I; these cells perform nitrogen of nitrogen. case, NADP+ is reduced. In the second case, as cells (PEC) [32]. The exception is systems like 84 85

FIG. 3. SCHEMATIC ILLUSTRATION OF THE MULTIPLE CHARGE TRANSFERS FIG. 4.

PSII-modified gold electrodes prepared by the deposition of PSII reconstituted with platinum nanoparticles on Au electrodes. This electrode served as working electrode in the three-electrode electrochemical cell. This cell can generate photocurrent in the presence of sacrificial reagent, 1,5-diphenylcarbazide (DPC) [33].

artificial leaves. In artificial leaves, charge generating a photocurrent in a PEC system is often transfer between the catalysts is carried out through studied separately from the possibility of producing semiconductor or/and liquid electrolyte [31,32]. In fuel [4]. Below, we give examples of semiartificial the PECs, catalysts are connected with each other and completely artificial systems. through wire as well as electrolyte. The possibility of

SEMIARTIFICIAL PHOTOELECTROCHEMICAL CELLS FIG. 5.

It was reported in many works that the are two different ways to immobilise. The first of photosynthetic components can be isolated from these is immobilisation due to physical adsorption cyanobacteria or algae, and their photoinduced without special nanowires. Another way is to electron-transfer activity can be maintained even reconstitute the native photosystem attaching a after purification [33–36]. Laccases are native special linker molecule to the native protein globule. enzymes that are often used in biohybrid devices. It This special linker allows the photosystem to easily is multicopper oxidases found in plants, fungi, and connect to the substrate. In some cases, these bacteria. They can be used in photoelectrochemical linkers serve as nanowires. Photoelectrons migrate cells for catalysis of H+ reduction [37]. The use through this linker to the electrode (Fig. 4). To do of native hydrogenases in artificial systems is this, the linker must replace the native cofactor difficult because of their high sensitivity to oxygen. involved in electron transfer. Another problem is the However, a number of successes were achieved low absorption cross section of the monolayer of in the development of photoelectrochemical photosystems. This can be solved by the use of a devices based on tandems of photosystems nanostructured electrode [38–40]. Another method

and hydrogenases. These systems can produce is to use PSII multilayer complexes obtained Electron transfer mechanism of a hydrogen-evolving dye-sensitised Schematic picture of Z-scheme water photocatalyst. C.B.: conduction band, V.B.: valence band, HOMO: highest splitting using Ru dye-sensitised Al O / molecular hydrogen from water [38]. by crosslinking. For crosslinking, linkers with 2 3 occupied molecular orbital, LUMO: lowest unoccupied molecular orbital, D: Pt/HCa2Nb3O10 nanosheets and PtOx/

The main problem in the construction of a organically functionalised amphiphilic platinum electron donor, D+: oxidised electron donor, A: electron acceptor, A–: reduced HCs-WO3 [41]. electron acceptor. Solid and broken arrows represent forward and back electron bio-hybrid electrode is the fixation of pigment- nanoparticles can be used [33]. transfers, respectively. protein complexes on an inorganic substrate. There 86 87

ARTIFICIAL PHOTOELECTROCHEMICAL CELLS REFERENCES

The main problem of the semiartificial system In a recent report, Oshima et al. (REF), suggests 1. UN Department of Economics and Social Affairs. 2019 World Population Prospects 2019. 9. See is the small half-life of the isolated enzymes or effective and stable construction of the system of https://population.un.org/wpp/Graphs/Probabilistic/POP/TOT/900 (accessed on 11 June 2020). pigment-protein complexes. From this point of hydrogen evolution that do not need any sacrificial

view, development of the artificial stable analogues donor. It is owing to the WO3-based photocatalyst 2. Landi M, Zivcak M, Sytar O, Brestic M, Allakhverdiev SI. 2020 Plasticity of photosynthetic of the native photosynthetic apparatus is a better that obtains electrons from water. Electron back processes and the accumulation of secondary metabolites in plants in response to

perspective. As mentioned above, most popular transfer is suppressed by the Al2O3 loaded on the monochromatic light environments: A review. Biochimica et Biophysica Acta - Bioenergetics1861,

material for artificial system is the wide band gap Pt/HCa2Nb3O10 nanosheets. Pt nanoparticles serve 148131. (doi:10.1016/j.bbabio.2019.148131) semiconductor. Such semiconductor is stable, but as hydrogen producing catalysts. Photoelectrons it cannot use visible light for the photocatalytic are generated by Ru(II) tris-diimine dye complexes 3. Grätzel M. 2009 Recent Advances ins sensitized Mesoscopic Solar Cells. Accounts of Chemical activity. It should be sensitised by special dye. immobilised on the semiconductor nanosheets. Research42, 1788–1798. (doi:10.1021/ar900141y) In a dye-sensitised system, water splitting can Redox pair I3–/I– is the redox mediator (Fig. be initiated by electron injection from an excited 5B). The turnover number and frequency for H 2 4. Musazade E et al. 2018 Biohybrid solar cells: Fundamentals, progress, and challenges. Journal state dye molecule into the conduction band of a evolution reached 4580 and 1960 h–1, respectively of Photochemistry and Photobiology C: Photochemistry Reviews35, 134–156. (doi:10.1016/j. semiconductor (Fig. 5A) [41]. Then, photoelectrons [41]. The main problem of artificial systems is jphotochemrev.2018.04.001) from the conduction band can participate in the the utilisation of rare and expensive components

evolution of H2 at special catalytic centers, such [4]. The development of systems based on easily as Pt nanoparticles. Unfortunately, undesirable accessible and widespread elements can solve this 5. Pandey AK, Tyagi V V., Selvaraj JA, Rahim NA, Tyagi SK. 2016 Recent advances in solar electron transfer reverse reactions also occur. In problem [12]. photovoltaic systems for emerging trends and advanced applications. Renewable and Sustainable addition, such systems need a sacrificial electron Energy Reviews. 53, 859–884. (doi:10.1016/j.rser.2015.09.043) donor. 6. International Energy Agency. 2015 Key World Energy Statistics 2009. Statistics , 82. (doi:10.1787/9789264039537-en)

7. REN21. 2019 RENEWABLES 2019 GLOBAL STATUS REPORT. REN21 Secretariat.

CONCLUSION 8. Surriya O, Saleem SS, Waqar K, Gul Kazi A, Öztürk M. 2015 Bio-fuels: A Blessing in Disguise. In Phytoremediation for Green Energy, pp. 11–54. Springer Netherlands. (doi:10.1007/978-94-007- 7887-0_2) Sunlight offers many different pathways to to be the fuel of the future. This is due to the fact produce chemical fuel. These pathways include that it is a carbon-free, energy-enriched chemical 9. Schobert H. 2010 Chemistry of fossil fuels and biofuels. Cambridge University Press. biofuel production from biomass; hydrogen compound. Photohydrogen can be obtained (doi:10.1017/CBO9780511844188) production through the vital activity of the microalgae; either by the activity of the microorganisms or biofuel production via photocatalysis performed due to photocatalysis. The first method is simpler; by artificial devices. Each of these pathways have however, it is less effective. Inexpensive, stable, 10. Hou HJM, Allakhverdiev SI, Najafpour MM, Govindjee. 2014 Current challenges in photosynthesis: their own benefits and disadvantages. The benefits effective, eco-friendly artificial or semiartificial from natural to artificial. Frontiers in Plant Science5. (doi:10.3389/fpls.2014.00232) allow me to think that some of these solar fuels can systems inspired by natural photosynthesis for replace fossil fuels in the future. Further research hydrogen production from water are the most 11. Weijermars R, Taylor P, Bahn O, Das SR, Wei Y-M. 2012 Review of models and actors in energy in all these sectors is necessary for the successful attractive subjects for R&D on solar energy. Current mix optimization – can leader visions and decisions align with optimum model strategies for our development of these methods and to overcome advances in the development of these devices are future energy systems? Energy Strategy Reviews1, 5–18. (doi:10.1016/j.esr.2011.10.001) existing shortcomings. Biofuels from plant significant, but many obstacles prevent them from biomass are currently the most popular solar fuel. being more widely used technology. 12. Najafpour MM et al. 2016 Manganese Compounds as Water-Oxidizing Catalysts: From the Natural At the same time, molecular hydrogen is believed Water-Oxidizing Complex to Nanosized Manganese Oxide Structures. Chemical Reviews116, 2886–2936. (doi:10.1021/acs.chemrev.5b00340)

13. Kostic MM. 2007 Energy : Global and Historical Background. In Encyclopedia of Energy Engeneering (ed BL Capehart), pp. 1–15. Taylor & Francis/Marcel Dekker. (doi:10.1081/ E-EEE-120042341)

14. IEA. 2019 CO2 Emissions from Fuel Combustion. 2019th edn. IEA. See www.iea.org/t&c/.

15. Voloshin RA, Rodionova MV, Zharmukhamedov SK, Nejat Veziroglu T, Allakhverdiev SI. 2016 Review: Biofuel production from plant and algal biomass. International Journal of Hydrogen Energy41, 17257–17273. (doi:10.1016/j.ijhydene.2016.07.084) 88 89

1. Hamann TW, Jensen R a., Martinson ABF, Van Ryswyk H, Hupp JT. 2008 Advancing beyond current 16. Park H, Kim H, Moon G, Choi W. 2016 Photoinduced charge transfer processes in solar generation dye-sensitized solar cells. Energ. Environ. Sci.1, 66. (doi:10.1039/b809672d) photocatalysis based on modified TiO 2. Energy Environ. Sci.9, 411–433. (doi:10.1039/ C5EE02575C) 2. Bull G. 2018 Forests and Energy. 3rd edn. United Nations Forum on Forests. See https://www. un.org/esa/forests/wp-content/uploads/2018/04/UNFF13_BkgdStudy_ForestsWater.pdf. 17. Reece SY, Hamel JA, Sung K, Jarvi TD, Esswein AJ, Pijpers JJH, Nocera DG. 2011 Wireless solar water splitting using silicon-based semiconductors and earth-abundant catalysts. Science334, 3. Voloshin RA, Kreslavski VD, Zharmukhamedov SK, Bedbenov VS, Ramakrishna S, Allakhverdiev SI. 645–648. (doi:10.1126/science.1209816) 2015 Photoelectrochemical cells based on photosynthetic systems: A review. Biofuel Research Journal. 2, 227–235. (doi:10.18331/BRJ2015.2.2.4) 18. Miyachi M, Ikehira S, Nishiori D, Yamanoi Y, Yamada M, Iwai M, Tomo T, Allakhverdiev SI, Nishihara H. 2017 Photocurrent Generation of Reconstituted Photosystem II on a Self-Assembled Gold Film. 4. Rodionova MV et al. 2017 Biofuel production: Challenges and opportunities. International Journal Langmuir33, 1351–1358. (doi:10.1021/acs.langmuir.6b03499) of Hydrogen Energy42, 8450–8461. (doi:10.1016/j.ijhydene.2016.11.125) 19. Terasaki N, Yamamoto N, Hiraga T, Sato I, Inoue Y, Yamada S. 2006 Fabrication of novel 5. Nath K et al. 2015 Photobiological hydrogen production and artificial photosynthesis for clean photosystem I-gold nanoparticle hybrids and their photocurrent enhancement. Thin Solid energy: from bio to nanotechnologies. Photosynthesis research126, 237–47. (doi:10.1007/ Films499, 153–156. (doi:10.1016/j.tsf.2005.07.050) s11120-015-0139-4) 20. Voloshin RA et al. 2019 Influence of osmolytes on the stability of thylakoid-based dye-sensitized 6. Naik SN, Goud V V., Rout PK, Dalai AK. 2010 Production of first and second generation biofuels: A solar cells. International Journal of Energy Research43, 8878–8889. (doi:10.1002/er.4866) comprehensive review. Renewable and Sustainable Energy Reviews. 14, 578–597. (doi:10.1016/j. rser.2009.10.003) 21. Kavadiya S, Chadha TS, Liu H, Shah VB, Blankenship RE, Biswas P. 2016 Directed assembly of the thylakoid membrane on nanostructured TiO2 for a photo-electrochemical cell. Nanoscale8, 7. Celińska E, Borkowska M, Białas W. 2016 Evaluation of a recombinant insect-derived amylase 1868–1872. (doi:10.1039/C5NR08178E) performance in simultaneous saccharification and fermentation process with industrial yeasts. Applied Microbiology and Biotechnology100, 2693–2707. (doi:10.1007/s00253-015-7098-8) 22. Calkins JO, Umasankar Y, O’Neill H, Ramasamy RP. 2013 High photo-electrochemical activity of thylakoid-carbon nanotube composites for photosynthetic energy conversion. Energy & 8. Rahman MA, Aziz MA, Al-khulaidi RA, Sakib N, Islam M. 2017 Biodiesel production from Environmental Science6, 1891–1900. (doi:10.1039/C3EE40634B) microalgae S pirulina maxima by two step process: Optimization of process variable. Journal of Radiation Research and Applied Sciences10, 140–147. (doi:10.1016/j.jrras.2017.02.004) 23. Sekar N, Ramasamy RP. 2014 Recent advances in photosynthetic energy conversion. Journal of Photochemistry and Photobiology C: Photochemistry Reviews22, 19–33. (doi:10.1016/j. 9. Staffell I, Scamman D, Velazquez Abad A, Balcombe P, Dodds PE, Ekins P, Shah N, Ward KR. jphotochemrev.2014.09.004) 2019 The role of hydrogen and fuel cells in the global energy system. Energy & Environmental Science12, 463–491. (doi:10.1039/C8EE01157E) 24. Voloshin RA, Bedbenov VS, Gabrielyan DA, Brady NG, Kreslavski VD, Zharmukhamedov SK, Rodionova M V., Bruce BD, Allakhverdiev SI. 2017 Optimization and characterization of TiO2- 10. Allakhverdiev SI, Kreslavski VD, Thavasi V, Zharmukhamedov SK, Klimov V V., Nagata T, Nishihara based solar cell design using diverse plant pigments. International Journal of Hydrogen Energy42, H, Ramakrishna S. 2009 Hydrogen photoproduction by use of photosynthetic organisms and 8576–8585. (doi:10.1016/j.ijhydene.2016.11.148) biomimetic systems. Photochemical and Photobiological Sciences8, 148–156. (doi:10.1039/ b814932a) 25. Mershin A, Matsumoto K, Kaiser L, Yu D, Vaughn M, Nazeeruddin MK, Bruce BD, Gratzel M, Zhang S. 2012 Self-assembled photosystem-I biophotovoltaics on nanostructured TiO(2 )and ZnO. 11. Allakhverdiev SI et al. 2010 Photosynthetic Energy Conversion: Hydrogen Photoproduction by Scientific reports2, 234. (doi:10.1038/srep00234) Natural and Biomimetic Means. In Biomimetics Learning from Nature (ed Amitava Mukherjee), pp. 49–75. InTech. 26. Oshima T et al. 2020 An Artificial Z-Scheme Constructed from Dye-Sensitized Metal Oxide Nanosheets for Visible Light-Driven Overall Water Splitting. Journal of the American Chemical 12. Sickerman NS, Hu Y. 2019 Hydrogenases. In Methods in Molecular Biology, pp. 65–88. Society142, 8412–8420. (doi:10.1021/jacs.0c02053) (doi:10.1007/978-1-4939-8864-8_5)

13. Schuchmann K, Chowdhury NP, Müller V. 2018 Complex multimeric [FeFe] hydrogenases: Biochemistry, physiology and new opportunities for the hydrogen economy. Frontiers in Microbiology9, 1–22. (doi:10.3389/fmicb.2018.02911)

14. Barber J, Tran PD. 2013 From natural to artificial photosynthesis. Journal of The Royal Society Interface10, 20120984–20120984. (doi:10.1098/rsif.2012.0984)

15. Purchase RL, De Groot HJM. 2015 Biosolar cells: Global artificial photosynthesis needs responsive matrices with quantum coherent kinetic control for high yield. Interface Focus5, 1–16. (doi:10.1098/rsfs.2015.0014) 90 91

ARTIFICIAL INTRODUCTION PHOTOSYNTHESIS According to various forecasts, global energy fuels or learn to synthesise hydrocarbons from

consumption will grow in the foreseeable future atmospheric CO2. The contemporary carbon budget 1 (Fig.1A) . This growth is accompanied by an is unbalanced; this is because CO2 emission increasing demand of fossil fuels that is the main exceeds its uptake for biomass production4. The source of energy. It is the biomass processed, in primary goal of our ecological policy is to try to the past, by high pressure for millions of years2. maintain the CO2 concentration in air at a constant We, the people of this Earth, obtain energy by level5. combustion of this mix of organic molecules. This If we could synthesise carbohydrates from

is done by burning high energy carbohydrates in atmospheric CO2 at a rate comparable to the

the power plant boilers and in machine engines. anthropogenic CO2 production, this would partially The ultimate product of this combustion is carbon solve the ecological problem. Carbon dioxide

dioxide (CO2) as in the respiratory process. Indeed, is low-energy molecule; obtaining energy-rich its atmospheric concentration has been increasing hydrocarbons from it requires energy. Nature 3 (Fig.1B) . CO2 is the greenhouse gas! Thus, the fossil suggests us to use solar energy for CO2 reduction. fuel combustion is one of the reasons of the global Organisms performing oxygenic photosynthesis

warming. The worldwide population is increasingly acquire CO2 from the atmosphere and combine paying attention to the ecological problems it with hydrogen (electrons and protons) from associated with this trend. Finding a carbon-free ubiquitous water by using solar energy.The main energy source has become one of the important means to obtain hydrocarbons (carbohydrates)

global challenges. In this point of view, we should from CO2 by sunlight for growing of fuel crop is either replace hydrocarbon fuels by carbonless through photosynthesis8.

Suleyman Allakhverdiev, Head of Controlled Photobiosynthesis Laboratory, Institute of Plant Physiology, Russian Academy of Sciences

FIG. 1.

ABSTRACT

Fossil fuels have been the main resource made by nature in this area. To obtain a stable, for our energy needs. The problems associated functioning artificial copy, we need to use the with this resource are now becoming more and latest achievements in the field of nanomaterial more obvious; they make us look for alternatives. science, organic and inorganic synthesis. This World energy consumption in three cases calculated distinctly for Annual mean daily emissions in the period 1970–2019 and Molecular hydrogen is one such alternative. review describes the recent advances in the field OECD (Organisation for Economic Co-operation and Development) the daily emissions up to end of April 2020 (drastic decrease Nature tells us that the most economical way to of artificial photosynthesis. Emphasis is placed on member countries and for other countries6. due to the COVID-19 pandemic)7. obtain molecular hydrogen from water is by using artificial catalysts for the oxidation of water and the sunlight. We must copy the best achievements reduction of protons to hydrogen.

1 — IEA 2019 4 — Alakhverdiev et al., 2009 7 — Le Quere et al., 2020 2 — Kiang, 2018 5 — Barber et al., 2013 8 — Blankenship, 2002 3 — Le Quere et al., 2020 6 — IEA 2019 92 93

(light) CO2+H2O CHOH+O2 OXYGENIC PHOTOSYNTHESIS The disadvantage of this method is competition replacement of carbohydrates. H has a high 2 AND HYDROGEN METABOLISM for area with agriculture crops. Another problem calorific value of 122 kJ g9, which is 2.75 times is the fact that this method is not carbon-free: greater than the hydrocarbons10. The ultimate product of H combustion is water. Thus, hydrogen the synthesis of hydrocarbons from CO22 is much 2 The term photosynthesis usually refers to turn activates another transmembrane protein, is really an ecological fuel. It will be a carbon-free slower than the CO2 production in the power plants the metabolic processes of the photoactivated ATP-synthase, and the synthesis of ATP is initiated. and engines. fuel if the production of it will not need hydrocarbon production of redox equivalents and ATP molecules The linear electron transfer is outlined above. Another way is to use carbonless compounds processing. The answer is, again, the molecular and the subsequent synthesis of high-energy Another way for charge transport is a cyclic electron with high calorific value. On energy basis, hydrogen hydrogen produced from water, with the use of organic compounds from low molecular weight transfer. This kind of electron transfer leads to the sunlight. (H2) is a highly promising compound for the precursors using reducing power in the firm of photogeneration of transmembrane proton gradient NADPH (or NADH) and ATP molecules. Synthesis but does not produce redox equivalents. (light) 2H2O 2H2+O2 of carbohydrates from CO2 is the main process There are two types of reaction centers. utilising the photoproduced NADPH and ATP18. The two types use different electron acceptors: We should keep in mind that hydrogen is a gas gradient, which, in turn, is formed at the expense Primary photoreactions leading to the formation of quinone-type (in Photosystem II) and iron-sulfur and it is very explosive when it comes in contact of light energy. Investigation of this mechanism NADPH and ATP molecules include the absorption RC (in Photosystem I). They use light quanta of with atmospheric oxygen! Yes, explosiveness is is necessary for the development of an efficient, of light by antenna pigments and the transfer of different wavelengths. The type of RC, in part, a disadvantage of this gaseous hydrogen fuel11. cost-effective and ecofriendly solar light convertor. excitation energy to the reaction centers; charge may determine whether the transport is linear or However, this can be taken care of. This is because the natural metabolic system has separation in the reaction centers; and, transfer cyclic. The photosynthetic organisms have been Sunlight is a very attractive energy source due already successfully satisfied these requirements. of electrons and protons. They take place in the present about 3.4 billion years ago. The earliest to its abundance. Taking into account the entire Indeed, nature uses widespread chemical elements special photosynthetic membranes, the thylakoid photosynthesis was anoxygenic. Anoxygenic solar spectrum and the entire surface area of the and available energy sources with maximum membranes. Light quanta are absorbed by the organisms used hydrogen sulfide, hydrogen, Earth subjected to irradiation, we can calculate efficiency and minimal destructive energy leaks. pigments localised in the special pigment-protein and elemental sulfur, as primary donor21. These that the Sun provides our planet just in one hour Unfortunately, we cannot use photosynthetic complexes. The energy of these quanta, if it is not compounds have less positive redox-potential than the energy equivalent to all that is used by the apparatus as it is to meet our demands. This is dissipated as heat or light (fluorescence), activates water and they need less energy for oxidation and mankind time in one year12. At the same time, because the “goal” of the plants and those of the the transfer of electrons from the primary donor electron transfer to NAD+. However, they are not solar energy has several disadvantages. Firstly, humans for sunlight conversion do not match. The to the redox equivalent. The primary donor (H O) as widespread as water. Oxygenic photosynthesis it is diffuse; thus, to meet the contemporary photosynthetic apparatus is designed to maintain 2 has a much more positive redox-potential than began more than 2.4 billion years ago. Unlike global power demands by conventional solar cells its viability under certain environmental conditions. NADP/NADPH couple, and this electron transfer anoxygenic photosynthetic bacteria that use only (with an efficiency of about 10%) an huge area Photosynthetic apparatus is arranged for the is thermodynamically unfavourable. This process one type of RC, oxygenic photosynthesis uses RCs would be required13. Secondly, solar irradiation continuous interaction with other systems of the becomes possible only after photo-induced charge of both types simultaneously22. The use of two is intermittent. Sunlight is, obviously, not present living cell. It constantly renovates its components separation in the two photosystems (I and II). The different RCs allows photosynthetic organisms to at night, and is essentially absent under dense due to interaction with its gene expression system. electron-transfer chain (ETC) contains the pigment- utilise water as source of electrons and protons. For clouds. Solar convertors must work in tandem with Also, it changes its efficiency in response to protein complexes, the so-called reaction centers a perspective of the Z-scheme, used to run oxygenic storage systems to become the primary energy alteration in its environment. However, our goal (RCs), which perform the primary photoinduced photosynthesis, see Govindjee et al. (2017)23. It systems14. This last thesis applies primarily to is to obtain as much usable energy from sunlight, redox-reactions. Light energy induces charge was indeed an evolutionary breakthrough. Redox the solar-to-electricity convertors. In the solar-to- as possible by making a robust system. The low separation in the two reaction centers: special potential of water is strongly positive and energy chemical fuel convertors, the final products can robustness of the isolated components of the pigment dimers after excitation with light reduce of one visible quantum is clearly insufficient for store the energy in their chemical bonds. Another photosynthetic apparatus makes it inappropriate primary acceptors located in a certain way in transfer of electrons from water to NADP+. Working problem with solar energy is the construction of for artificial solar convertor. I believe that it would be relation to the special pairs at the RCs. After the in tandem, two distinct RCs allow the use of two the conversion systems. These systems must be unlikely that we would use the native photosystem, primarily photochemistry and several of the early photons for electron transfer from water to NADP. efficient, durable, eco-friendly and cost effective15. per se, to obtain energy. However, it is more likely secondary electron transfer reactions, one part Oxygenic photosynthesis is an attractive prototype It is only when all the four requirements are fulfilled, that we would imitate the native process and put it in of the RC became sufficiently redox-positive to for artificial photosynthesis, because in this case, these systems will be able to compete with fossil a more stable and appropriate man-made system. oxidise the primary donor and another co-factor ubiquitous water is used as a source of protons fuel on the energy market or, at least, with fossil Such artificial devices should have the benefits became sufficiently redox-negative to reduce the and electrons for chemical fuel. In this section we fuel combustion systems on equal terms in of natural photosynthesis and be effective and redox equivalent. RC pigments, the chlorophylls outline the basic steps of oxygenic photosynthesis complex energy grids. Until now, no solar system sustainable. The phrase “artificial photosynthesis” usually do not absorb light quanta themselves; (Fig. 2). has been designed that has satisfied all the four is a common name for all processes, mimicking their absorption cross-section is too small (they Oxygenic photosynthesis is carried out by requirements simultaneously. the natural photosynthesis, aimed to use sunlight are few in number) for efficient light absorption. plants and cyanobacteria24. RCs in oxygenic As I have mentioned above, photoinduced to make high-energy chemicals, but with far higher Photosynthetic organisms use special light organisms are associated with light-harvesting and fixation of atmospheric CO by photosynthetic efficiencies and simplicity of design for scale-up 2 harvesting pigment-protein antenna complexes to accessory proteins in their photosystems. The three organisms and biomass production occurs with and large-scale production. Unresolved problems absorb light and funnel the excitation energy to the main complexes that are involved in oxygenic linear an insufficient rate to meet the carbon budget that prevent systems of artificial photosynthesis RCs. Its design increases the number of photons electron transfer are: PS II containing water oxidation imbalance. In addition to this, photosynthetic from entering into the energy global market do and the range of photon energies that can be used or, in other words, the oxygen evolving complex efficiency of the conversion of solar energy to the not obscure the great successes that have been by a RC for charge separation19. As electrons are (OEC), & the quinone-type RC and, PS I containing chemical bond energy is just too low16. However, achieved in this direction. In this chapter, I will transported through the electron-transport chain, FeS-type RC, and the cytochrome b6/f complex there is a very efficient and finely adjusted consider artificial photosynthesis in relation to its protons (H+) outside the thylakoid are carried to the (Cyt b6/f)25. Each of the photosystems, PS I and mechanism at the heart of photosynthesis. It natural prototype and outline different approaches inner thylakoid space20. It generates photoinduced PS II, couples with its external light-harvesting synthesises ATP, using transmembrane proton that have been developed thus far. transmembrane proton gradient. This gradient in complex, LHCI and LHCII respectively. PS II RC has

9 — IEA 2019 12 — Concepcion et al., 2012 15 — Purchase et al., 2015 17 — Concepcion et al., 2012 20 — Allakhverdiev et al., 2010 23 — Govindjee et al., 2017 10 — Babu et al., 2012 13 — Concepcion et al., 2012 16 —Barber et al., 2013 18 — Shevela et al., 2018 21 — Brune et al., 1995 24 — Najafpour et al., 2013 11 — Purchase et al., 2015 14 — Concepcion et al., 2012 19 — McConnell et al., 2010 22 — Blankenship et al., 1998 25 — Allakhverdiev et al., 2009 94 95

FIG. 2. HYDROGEN OR HYDROCARBON FUEL

Liquid carbon-based fuel is attractive to fuel, and it produces only water after combustion. power our existing energy infrastructure35. H2 can be produced from methane by natural gas Nature produces carbon-based fuels, included services reaction, gasification and renewable in fossil fuels and biomass. Unfortunately, the liquid reforming fossil fuels or biomass. Also,

photoconversion efficiency of CO2 and water to molecular hydrogen can also be produced by the carbohydrates is very low36. Artificial systems splitting water molecules by high-temperature

for carbon fuel photoproduction from CO2 are thermochemical water splitting and electrolytic also being developed37. However, the processes water splitting systems. All these methods need are much more challenging than the production carbon-based source and either high temperature of hydrogen, because they involve more complex or high voltage. Both temperature and voltage are multi-electron chemistry. In addition, reaction generated during fossil fuel combustion. So, the of carbohydrate synthesis from CO and H O 40 2 2 resulting H2 is not s carbon-free fuel . Sunlight requires more energy than the H2 production. utilisation may address this issue. Hydrogen is Another obstacle for such system realisation is a natural choice of fuel when water is the raw 38 that the atmospheric concentration of CO2 is low . material. To make hydrogen, the protons from Carbon-based fuel produces greenhouse gases water need to be reduced. This reaction needs less after combustion. Its synthesis is expected to energy than carbohydrate synthesis from H2O and have a rate comparable to the rate of CO emission 2 CO2. It is because of this fact, we can expect the in order to maintain CO concentration in the 34 2 efficiency of H2 photoproduction to be higher than The scheme of the oxygenic photosynthesis and solar-powered H2 production due to electrons from photosynthetic ETC . All atmosphere at a constant level. 41 designations are deciphered in the main text. carbohydrate photosynthesis . As of now, many Overwhelming majority of current fuel different photocatalysts are being developed for infrastructure is set up for liquid fuels. water-to-hydrogen conversion. Based on these Nevertheless, gaseous hydrogen fuel offers many facts, we can predict that hydrogen may play a key advantages that some global manufacturers have role in the future renewable energy technology. released limited series of cars with hydrogen The following text describes the systems for the a special form of Chl a, P680 (special pair). The or nitrogenase (Fig. 2). These enzymes catalyse engines39. Hydrogen has higher energy content per production of hydrogen. electron from excited P680 is transferred through the reversible oxidation of molecular hydrogen30. unit mass than alternative fuels. It is carbon-zero

a number of carriers to the cytochrome complex In cyanobacteria, hydrogenase can synthesise H2 (for a complete background on PS II, please see under anaerobic conditions using the electrons and Wydrzynski and Satoh (2005)26, on PS I, see Golbeck protons, ultimately produced by water oxidation (200627, and for Cyt b6/f complex, see Cramer and and redirected at the level of ferredoxin/NADPH 28 31 Kallas (2016) ). The participants of this transfer into hydrogenase . This “photosynthesis” of H2 include bound co-factors pheophytin and QA (a is an efficient pathway in cyanobacteria, but not PRINCIPLES OF ARTIFICIAL PHOTOSYSTEMS one-electron acceptor) fixed in protein scaffold in algae. In addition to this, another metabolic of PS II and mobile carrier plastoquinone (PQ). pathway for hydrogen production exists. It is photo- Electrons derived from water are transferred to the fermentation that is efficiently provided by green Hypothetical artificial photosynthetic systems Reduction catalyst for hydrogen production, oxidised P68029. Electrons, available on reduced algae with more oxygen-sensitive hydrogenases. should include several components (Fig. 3), each such as hydrogenase. PQ pass first to the cytochrome b6/f complex, and The first stage of photo-fermentation is the aerobic having prototype in natural photosynthesis. The list then from there to the PS I via plastocyanin (PC). carbohydrate photosynthesis by the Calvin-Benson follows43. Oxidation catalysts for water oxidation, such In PS I, they go from the excited PS I RC special cycle. The second stage is anaerobic oxidation of as the OEC. pair (P700) to ferredoxin (Fd). The oxidised P700 stored reductants and NADPH-mediated electron Photosensitiser excited by energy from the is reduced by accepting electrons from the reduced transfer to the PQ pool32. Through PQ, b6/f photons, similar to that in the special pair in Electron carriers to provide electron transfer PC. Normally, Fd transfers electrons to the enzyme complex and photoactivated PS I, electrons pass the RC. from the oxidation catalyst to the electron donor, similar to the plastocyanin for PS I. ferredoxin–NADP+–oxidoreductase (FNR). Oxygen into hydrogenase, where H2 is reduced. Another An antenna system that absorbs photons, and evolving complex, plastoquinone and cytochrome pathway for H2 synthesis in phototrophic organisms b6/f complex participate in the generation of the involves another enzyme called nitrogenase. This funnels energy to the reaction center, and can Electron carriers to help electron transfer proton gradient across the thylakoid membrane, pathway is much less effective in comparison with also act to protect the system by dissipation from the electron acceptor to the reduction which in turn is used by ATP-synthase to make ATP other pathways and hence, makes it economically of excess light energy44. catalyst, similar to the ferredoxin for PS I and (Fig. 2). ATP and NADPH molecules are necessary impractica33. hydrogenase. Donor-acceptor system that, in conjunction for the enzymatic Calvin-Benson cycle of CO2 Photosynthesis uses energy from the sunlight assimilation and carbohydrate production. for the synthesis of chemical species with high with a photosensitiser, can generate charge- Photosynthesis is a key part of the calorific value (carbohydrates and hydrogen) using separated state under the light, similar to complicated system of interconnected metabolic electrons and protons from ubiquitous water. This the donor and acceptor parts of the natural pathways. In cyanobacteria and many microalgae, fact makes this natural metabolic process very photosystems. it is connected with hydrogen metabolism appealing for artificial photosynthesis. provided by the special enzymes, hydrogenase

26 — Wydrzynski et al., 2005 29 — Nath et al., 2015 32 — Allakhverdiev et al., 2010 35 — Purchase et al., 2015 38 — Purchase et al., 2015 41 — Purchase et al., 2015 27 — Golbeck, 2006 30 — Schuchmann et al., 2018 33 — Allakhverdiev et al., 2010 36 — Barber et al., 2013 39 — Purchase et al., 2015 43 — McConnell et al., 2010 28 — Cramer et al., 2016 31 — Allakhverdiev et al., 2009 34 — Allakhverdiev et al., 2010 37 — Concepcion et al., 2012 40 — Nath et al., 2015 44 — Allakhverdiev et al., 2010 96 97

FIG. 3. Native enzymes are very susceptible to the is a charge transfer system: holes pass to oxidise impact of the environment such as temperature, the oxidation (photo) catalyst, and electrons — high light, and some active chemical compounds55. go to reduce the reduction (photo) catalyst through However, we must study these systems in order the semiconductor. Besides, semiconductor is to know more about the natural approach to a scaffold and a stabiliser for fragile organic Основные компоненты photosynthesis. components. These systems need robust, efficient, The investigation of organic or, particularly, low-cost photosensitiser-catalyst complexes. The Антенна organic-inorganic system, seems to have a better mimicking of the catalytic center of the native future. This research field began in 1974 with enzymes and chromophores in more stable shell is the experimental demonstration that the metal- a future research possibility. Below, we will briefly organic complex, tris(bipyridine)ruthenium(II) outline the achievements in the field of artificial ion [Ru(bpy)32+], can play a role similar to a reduction and oxidation catalysts that can be used reaction center special pair in photosynthesis56. with semiconductor substrate. In the organic-inorganic system, semiconductor

The schematic view of the artificial photosynthetic system for the production of hydrogen. P, photosensitiser; A, electron acceptor; D, electron donor; C, electron carrier; Catox, catalyst for oxidation of water; Catred, catalyst for reduction of H+ 42. CATALYSTS FOR ARTIFICIAL PHOTOSYNTHESIS

OXYGEN EVOLVING COMPLEXES AND ITS ARTIFICIAL ANALOGUES

Artificial photosynthesis must combine on the design of artificial OEC composed of readily In this model, each component performs its own of materials used, such as organic, inorganic, single-electron transfer process with multi- available elements such as Mn, Co, Fe. Artificial function. This is a big simplification. Also, the model organic-inorganic and hybrid. Organic systems electron catalyst process. These catalysts should catalysts based on Mn are of particular interest. describes only one photosensitiser interacting comprise of organic or metal-organic complexes accumulate redox equivalents similar to that in the This is because Mn is abundant on Earth, and is both with oxidation and reduction catalysts. This mimicking the analogous elements of natural native OEC. Natural photosynthesis uses transition used by the natural photosystem. The CaMn O is different from oxygenic photosynthesis, which photosynthesis apparatus. Inorganic systems are 4 5 metal cluster to catalyse multi-electron reactions. cluster in PS II can be considered as a cluster of deals with two types of RCs working in tandem. the semiconductor or semiconductor-conductor The active site for water oxidation is a CaMn O Mn oxides in a protein environment61. Recent Actual convertors may vary significantly from this devices. However, organic-inorganic systems 4 5 cluster (Fig. 4A). reports have shown that nanosize Mn oxides may rough model. Firstly, different functions may be combine the best properties of organic and The OEC oxidises water with a low overpotential be considered as model OEC62. A large number of performed by the same components. For example, inorganic materials and are represented by the and a high turnover number. Ca and Mn utilised such catalysts are being developed that differ in the photosensitiser, primary donor and the semiconductor or conductor sensitised by organic oxidation catalyst may be the same compound, a in this catalytic center are the earth-abundant their catalytic center structure and their organic catalysts and photosensitisers. In hybrid systems, 63 45 metals. Four oxygen ions, three manganese ions shell construction . Organic shell determines photocatalyst . Secondly, artificial photosynthesis the native or modified enzymes and photosynthesis and one Ca ions form the asymmetric structure57. the oxidation state of the catalytic center. There in some cases may work with two photosensitisers, apparatus components are used in conjugation 46 In accordance to an earlier theory, water oxidation are many publications (see below) that deal like in native photosynthesis . In many developing with inorganic substrates49. proceeds in five stages, by the so-called S-state with the impact of different ligands attached to devices, each catalyst is kept bound to its own In the early 1970s, Fujishima and Honda50 cycle. S-state transitions, S0 › S1, S1 › S2, S2 › the central Mn ions. They include (OH )(terpy) photosensitiser. Currently, studies of the individual reported on the results of their pioneered 2 S3, and S3 › S4, are known to be induced by the Mn(μ-O) Mn(terpy)(OH )] and their derivatives components is more common than research on the experimental work in artificial photosynthesis 2 2 3+ 47 photochemical oxidation of P680. It is known that (Fig. 5A), Mn-phthalocyanine (Fig. 5B), whole artificial photosynthesis devices . field. They were able to oxidise water by TiO 2 the S4 › S0 transition is light-independent. During Mn-porphyrin, Mn−oxo cube complexes64. Organic We should keep in mind that all intermediate photocatalysts51. In the photoelectrochemical cell the first four stages, four oxidising equivalents shell is a susceptible part of the catalyst. To address stages of solar-to-chemical conversion, including, for with TiO anode and Pt cathode, O generation at 2 2 accumulate sequentially at the CaMn O cluster these issues, we should encourage development of example, electron transfer from the photosensitiser the anode under illumination is accompanied by 4 5 and an oxygen molecule is evolved in the final step. all-inorganic catalysts. Co-Oxo cube complexes, to the reduction catalyst, take away part of photon hydrogen production at the cathode. This work laid However, the more recent extended S-state cycle coordinated and stabilised by polyoxometalate energy and decrease energy conversion efficiency. the foundation for inorganic artificial photosynthesis predicts nine different intermediate steps in the ligands, are the successful examples of these All the energy losses must be accounted for in the devices52. We note that the catalytic efficiency of water-oxidation cycle (Fig. 4B)58 . catalysts (Fig. 5C)65. Another important issue is so-called energy budget. This budget calculates semiconductor is not really high53. Wide band gap A variety of artificial oxygen evolving catalysts the ability to self-assemble and to self-repair the the balance between the input energy and all the semiconductors are stable, but they absorb only UV energies used in different pathways including the have been developed, and these are based on artificial enzyme in question. This property has light. Narrow band gap semiconductors can absorb 60 48 ruthenium and iridium metals . Since these are rare been shown to be true for a cobalt-oxide phosphate losses and desirable consumption . visible light, but they are susceptible to corrosion54. metals, the catalysts based on them are expected to catalyst and for several Mn-catalysts66. One can distinguish four approaches to Due to these reasons, inorganic devises are not yet be expensive. Thus, attention is now being focused artificial photosynthesis depending on the nature promising.

55 — Voloshin et al., 2015 60 — Barber et al., 2013 64 — Young et al., 2013 42 — Nath et al., 2015 47 — Nath et al., 2015 50 — Babu et al., 2012 53 — Barber et al., 2013 56 — Nath et al., 2015 61 — Najafpour et al., 2016 65 — Yin et al., 2010 45 — Hashimoto et al., 2005 48 — Purchase et al., 2015 51 — Concepcion et al., 2012 54 — Grätzel, 2001 57 — Najafpour et al., 2016 62 — Mousazade et al., 2019 66 — Reece et al., 2011 46 — Barber et al., 2013 49 — Purchase et al., 2015 52 — Hashimoto et al., 2005 58 — Najafpour et al., 2020 63 — Najafpour, 2012 98 99

FIG. 4. FIG. 6. THE ACTIVE CENTERS OF [FEFE] HYDROGENASE (LEFT) AND [NIFE] HYDROGENASE (RIGHT) 73

The radiation-damage-free structure of the Mn4CaO5 Extended reaction cycle of photosynthetic water oxidation cluster in PS II from Thermosynechoccocus vulcanus (outer cycle) compared to the classical S-state model (inner in the S1 state at a resolution of 1.95 Å. cycle). The extended cycle includes not only the electron

transfer from Mn4CaO5 complex to YZ, but also the proton removal from the complex or its ligand environment. The subscripts indicate the number of oxidation equivalents accumulated at the Mn-complex and the superscripts indicate the charge relative to the dark-stable S1 state (+, positive; n, neutral). The pK values, indicated in the above diagram are from published measurements 59. HYDROGENASES AND THEIR ARTIFICIAL ANALOGUES

Natural hydrogenases are divided into The first experimental metal-organic catalyst FIG. 5. three main classes: [NiFe]- and [FeFe]- and for hydrogen production contained rhodium (Rh) [Fe]-hydrogenases70. [Fe]-hydrogenases have been or iridium (Ir) metal. Both Rh and Ir are even less found only in archaea and has not been well studied71. abundant than Pt in the earth crust75. As in the Catalytic center of bimetallic hydrogenases is case with oxygen evolving catalysts, the less represented by two metal ions, NiFe and FeFe expensive transition metals, iron, cobalt and nickel,

respectively (Fig.6). Residual cysteine, dithiolate have shown promising results as H2 production 76 and CO and CN2 ligands coordinate the metal ion catalysts . Cobalt is the popular metal for the in tandem. Protons pass to the catalytic center construction of catalytic devices77. In the Co-based through the specific proton transfer pathway. Here, catalysts for proton reduction, Co(III)-hydride the transfer of molecular hydrogen is facilitated complexes are generally accepted to be a crucial by gas channels. Further, [FeS] clusters ensure the intermediates (Fig. 7A)78. transfer of electrons72. Fe and Ni are of particular interest since they The most popular inorganic catalyst of are used by native hydrogenase. The efficiency hydrogen production is platinum. It is very effective, and stability are the main issues for the artificial but it is a noble metal. This is the reason for the catalysts based on these base metals. Ogo et al. high cost of the Pt-based devices. Hydrogenase reported a catalyst, which was based on Ni-Ru uses metals that are abundant in the Earth’s crust bimetallic catalytic center79. Replacement of iron and the frequency of action of these enzymes is by more robust ruthenium allows the catalyst to also very high. Hydrogenase is as electrochemically be more stable (Fig. 7B). Artificial catalysts do not active as a platinum nanoparticle5. However, necessarily have to contain a bimetallic center. hydrogenase is very oxygen sensitive74. Thus, High efficiency catalyst has been obtained by Helm Co O core stabilised Mn complexes used by the research The structure of the 4 16 exploitation of hydrogenases as industrial catalysts et al., which was based on the complex [Ni(P(Ph) group of Brudvig67; Mn-phthalocyanine68; within [PW9O34] ligand is not practical. Nevertheless, native hydrogenase (2)N(Ph))(2)](BF(4))(2), where (P(Ph)(2)N(Ph)) = synthesised by Hill and 80 co-workers69. is blueprint for artificial catalysts of hydrogen 1,3,6-triphenyl-1-aza-3,6-diphosphacycloheptane . production. This complex contains only single nickel metal (Fig. 7C).

59 — Najafpour et al., 2020 68 — Mousazade et al., 2019 70 — Barber et al., 2013 74 — Allakhverdiev et al., 2010 78 — Hu et al., 2005 67 — Young et al., 2013 69 — Yin et al., 2010 71 — Schuchmann et al., 2018 75 — Allakhverdiev et al., 2010 79 — Ogo et al., 2007 72 — Barber et al., 2013 76 — Barber et al., 2013 80 — Helm et al., 2011 73 — Allakhverdiev et al., 2010 77 — Allakhverdiev et al., 2010 100 101

FIG. 7. ARTIFICIAL LEAF Semiconductors can serve simultaneously and co-authors90. They compared the traditional as both light absorbers and charge separators. photoelectrochemical cell (PEC) with an “artificial However wide band gap semiconductors can leaf” system (Fig. 8). In the PEC system, the anode absorb only a small UV fraction of sunlight and and the cathode were connected by a wire. Here, narrow band gap semiconductors are inappropriate the Si anode was functionalised by an oxygen for the solar-to-hydrogen devices due to their evolving cobalt-based catalyst; further, the cathode corrosivity. At the same time, wide band gap was the Ni mesh functionalised by NiMoZn proton semiconductors can serve as electron and hole reduction catalyst. The wireless “artificial leaf” transfer systems and provide the contact between contains only one silicone sheets. On its one side, the oxygen evolving and the hydrogen production this sheet is functionalised by an oxygen evolved catalysts. This is, indeed, the basis of the so-called catalyst, and on another side, it is functionalised by “artificial leaf”. A successful example of the a proton reduced catalyst. The efficiency of such an artificial leaf has been demonstrated by Reece attached system was shown to be 8%91.

FIG. 8. A – WIRED PHOTOELECTROCHEMICAL CELL SCHEME; B – ARTIFICIAL LEAF SCHEME (a) – The cobalt complex that produces hydrogen with small overpotential81; (b) – [NiRu] models designed by Ogo and co-workers82; (c) – bioinspired model designed by DuBois and co-workers83.

OTHER ARTIFICIAL COMPONENTS

We focus here only on the development are the photosensitisers, the antennas, and the of artificial catalysts. Important elements of electronic converters. artificial photosynthesis along with catalysts

PHOTOSENSITISERS The most successful photosensitisers in These complexes involve the rare and expensive artificial photosynthetic systems are ruthenium ruthenium. This makes them commercially less polypyridyl complexes. They have been successfully promising. However, phthalocyanine based used in the hydrogen production system, as well organic photosensitiser is a promising inexpensive as in the photocurrent generated solar cell84. substitute for Ru-based sensitiser85.

ANTENNA As for the antenna, the Ru-based conditions, or in the case of a photosensitiser with photosensitisers have wide absorption spectra a narrower absorption spectrum. Also, antenna can and are often used without any artificial antenna photoprotect the system from high light87. Artificial complex, as in a dye-sensitised solar cell86. At the antennas include organic dendritic molecules and same time, it is desirable to use antenna for low light inorganic quantum dots88.

DONOR, ACCEPTOR, CHARGE CARRIER At first, we should note that metal-organic carrier, like methyl viologen and EDTA, and a solid photosensitisers perform metal-to-ligand charge semiconductor, carrying electrons and holes. transfer and, in this way, they combine the Motion of soluble mediators is often the bottleneck function of a photosensitiser and a donor. Charge for the efficiency of this device89. transfer system may include soluble electron

81 — Hu et al., 2005 84 — Grätzel, 2006 87 — McConnell et al., 2010 90 — Reece et al., 2011 82 — Ogo et al., 2007 85 — Ragoussi et al., 2013 88 — Musazade et al., 2018 91 — Barberet al., 2013 83 — Helm et al., 2011 86 — Karmakar et al., 2011 89 — Su’Ait et al., 2014 92 — Blankenship, 2002 102 103

CONCLUSION REFERENCES

Success of the “artificial leaf” system allows devices. The intellectual power and knowledge of 1. U. S. Energy Information Administration. International Energy Outlook 2019 with projections to us to believe in the promising future of the artificial scientists in various fields (e.g., biology, chemistry, 2050. (2019). photosynthesis field. Nature shows us very efficient biochemistry, physics, biophysics, chemical molecular processes that we must imitate to meet engineering, mechanical & electrical engineering, 2. Kiang, Y.-H. Basic properties of fuels, biomass, refuse derived fuels, wastes, biosludge, and our needs. Their advantages are not only in their high and computer science) should be combined to biocarbons. in Fuel Property Estimation and Combustion Process Characterisation 41–65 efficiency, but that they use chemical elements that achieve a successful result in this area. Finally, (Elsevier, 2018). doi:10.1016/b978-0-12-813473-3.00003-9 are widely distributed on the Earth, and are found we must remind ourselves to get a complete in their metabolic pathways. If we do the same, we background in the basics of photosynthesis92. 3. Le Quéré, C. et al. Temporary reduction in daily global CO2 emissions during the COVID-19 forced will get inexpensive and environmentally friendly confinement. Nature Climate Change 1–7 (2020). doi:10.1038/s41558-020-0797-x

4. Allakhverdiev, S. I., Casal, J. J. & Nagata, T. Photosynthesis from molecular perspectives: Towards future energy production. Photochemical and Photobiological Sciences 8, 137–138 (2009).

5. Barber, J. & Tran, P. D. From natural to artificial photosynthesis. Journal of The Royal Society Interface 10, 20120984–20120984 (2013).

6. Blankenship, R. E. Molecular Mechanisms of Photosynthesis. (Blackwell Science Ltd, 2002). doi:10.1002/9780470758472

7. Shevela, D., Björn, L. O. & Govindjee. Photosynthesis. (WORLD SCIENTIFIC, 2018). doi:10.1142/10522

8. Babu, V. J., Kumar, M. K., Nair, A. S., Kheng, T. L. & Ramakrishna, S. Visible light photocatalytic water splitting for hydrogen production from N-TiO2 rice grain shaped electrospun nanostructures. International Journal of Hydrogen Energy 37, 8897–8904 (2012).

9. Purchase, R. L. & De Groot, H. J. M. Biosolar cells: Global artificial photosynthesis needs responsive matrices with quantum coherent kinetic control for high yield. Interface Focus 5, 1–16 (2015).

10. Concepcion, J. J., House, R. L., Papanikolas, J. M. & Meyer, T. J. Chemical approaches to artificial photosynthesis. Proceedings of the National Academy of Sciences 109, 15560–15564 (2012).

11. McConnell, I., Li, G. & Brudvig, G. W. Energy conversion in natural and artificial photosynthesis. Chemistry and Biology 17, 434–447 (2010).

12. Allakhverdiev, S. I. et al. Photosynthetic hydrogen production. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11, 101–113 (2010).

13. Brune, D. C. & Govindjee. Sulfur compounds as photosynthetic electron donors. Anoxygenic Photosynthetic Bacteria 2, 847–870 (1995).

14. Blankenship, R. E. & Hartman, H. The origin and evolution of oxygenic photosynthesis. Trends in Biochemical Sciences 23, 94–97 (1998).

15. Govindjee, Shevela, D. & Björn, L. O. Evolution of the Z-scheme of photosynthesis: a perspective. Photosynthesis Research 133, 5–15 (2017).

16. Najafpour, M. M., Amouzadeh Tabrizi, M., Haghighi, B. & Govindjee. A 2-(2-hydroxyphenyl)-1H- benzimidazole–manganese oxide hybrid as a promising structural model for the tyrosine 161/ histidine 190-manganese cluster in photosystem II. Dalton Transactions 42, 879 (2013). 104 105

1. Shevela, D., Björn, L. O. & Govindjee. Oxygenic Photosynthesis. in Natural and Artificial 19. Reece, S. Y. et al. Wireless solar water splitting using silicon-based semiconductors and earth- Photosynthesis 13–63 (John Wiley & Sons Inc., 2013). doi:10.1002/9781118659892.ch2 abundant catalysts. Science 334, 645–648 (2011).

2. Wydrzynski, T. J. & Satoh, K. Photosystem II. 22, (Springer Netherlands, 2005). 20. Hu, X., Cossairt, B. M., Brunschwig, B. S., Lewis, N. S. & Peters, J. C. Electrocatalytic hydrogen evolution by cobalt difluoroboryl-diglyoximate complexes. Chemical Communications 4723–4725 3. Golbeck, J. H. Photosystem I. 24, (Springer Netherlands, 2006). (2005). doi:10.1039/b509188h

4. Cramer, W. A. & Kallas, T. Cytochrome Complexes: Evolution, Structures, Energy Transduction, and 21. Ogo, S. et al. A dinuclear Ni(μ-H)Ru complex derived from H2. Science 316, 585–587 (2007). Signaling. 41, (Springer Netherlands, 2016). 22. Helm, M. L., Stewart, M. P., Bullock, R. M., DuBois, M. R. & DuBois, D. L. A synthetic nickel 5. Nath, K. et al. Photobiological hydrogen production and artificial photosynthesis for clean energy: electrocatalyst with a turnover frequency above 100,000 s-1 for H2 production. Science 333, from bio to nanotechnologies. Photosynthesis research 126, 237–47 (2015). 863–866 (2011).

6. Schuchmann, K., Chowdhury, N. P. & Müller, V. Complex multimeric [FeFe] hydrogenases: 23. Grätzel, M. The advent of mesoscopic injection solar cells. Progress in Photovoltaics: Research Biochemistry, physiology and new opportunities for the hydrogen economy. Frontiers in and Applications 14, 429–442 (2006). Microbiology 9, 1–22 (2018). 24. Ragoussi, M. E., Ince, M. & Torres, T. Recent advances in phthalocyanine-based sensitisers for 7. Allakhverdiev, S. I. et al. Hydrogen photoproduction by use of photosynthetic organisms and dye-sensitised solar cells. European Journal of Organic Chemistry 2013, 6475–6489 (2013). biomimetic systems. Photochemical and Photobiological Sciences 8, 148–156 (2009). 25. Karmakar, A. S. & Ruparelia, J. P. A Critical Review on Dye Sensitised Solar Cells. INTERNATIONAL 8. Allakhverdiev, S. I. et al. Photosynthetic Energy Conversion: Hydrogen Photoproduction by Natural CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY (INSTITUTE OF TECHNOLOGY, NIRMA and Biomimetic Means. in Biomimetics Learning from Nature (ed. Amitava Mukherjee) 49–75 UNIVERSITY, AHMEDABAD, 2011). (InTech, 2010). 26. Musazade, E. et al. Biohybrid solar cells: Fundamentals, progress, and challenges. Journal of 9. Hashimoto, K., Irie, H. & Fujishima, A. TiO 2 Photocatalysis: A Historical Overview and Future Photochemistry and Photobiology C: Photochemistry Reviews 35, 134–156 (2018). Prospects. Japanese Journal of Applied Physics 44, 8269–8285 (2005). 27. Su’Ait, M. S. et al. The potential of polyurethane bio-based solid polymer electrolyte for 10. Grätzel, M. Photoelectrochemical cells. NATURE 414, 338–344 (2001). photoelectrochemical cell application. International Journal of Hydrogen Energy 39, 3005–3017 (2014). 11. Voloshin, R. A. et al. Photoelectrochemical cells based on photosynthetic systems: A review. Biofuel Research Journal 2, 227–235 (2015).

12. Najafpour, M. M. et al. Water-oxidising complex in Photosystem II: Its structure and relation to manganese-oxide based catalysts. Coordination Chemistry Reviews 409, 213183 (2020).

13. Najafpour, M. M. et al. Manganese Compounds as Water-Oxidising Catalysts: From the Natural Water-Oxidising Complex to Nanosized Manganese Oxide Structures. Chemical Reviews 116, 2886–2936 (2016).

14. Mousazade, Y. et al. A manganese(II) phthalocyanine under water-oxidation reaction: New findings. Dalton Transactions 48, 12147–12158 (2019).

15. Najafpour, M. M. Manganese compounds as water oxidising catalysts for hydrogen production via water splitting: From manganese complexes to nano-sized manganese oxides. International Journal of Hydrogen Energy 37, 8753–8764 (2012).

16. Najafpour, M. M., Carpentier, R. & Allakhverdiev, S. I. Artificial photosynthesis. Journal of Photochemistry and Photobiology B: Biology 152, 1–3 (2015).

17. Young, K. J., Takase, M. K. & Brudvig, G. W. An anionic N-donor ligand promotes manganese- catalysed water oxidation. Inorganic Chemistry 52, 7615–7622 (2013).

18. Yin, Q. et al. A fast soluble carbon-free molecular water oxidation catalyst based on abundant metals. Science 328, 342–345 (2010).