Energy 160 (2018) 1091e110 0

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Considering environmental impacts of technologies: A life cycle assessment of power-to-gas business models

* Karin Tschiggerl , Christian Sledz, Milan Topic

Chair of Economics- and Business Management, Montanuniversitaet Leoben, Peter-Tunner-Strasse 25-27, A-8700 Leoben, article info abstract

Article history: The Power-to-Gas technology offers a promising answer to store energy efficiently and in high amounts. Received 1 September 2017 Renewable energy is thereby transformed into gas, which is then transported and stored using the Received in revised form existing infrastructure for . A quite new approach is to store energy from volatile renewable 11 July 2018 sources in the forms of hydrogen or methane in pore spaces of geological formations. Besides its tech- Accepted 16 July 2018 nical and legal feasibility the environmental impacts of an implementation have to be considered before Available online 17 July 2018 large-scale deployment is tackled. In the frame of the demonstration project alternative business models were developed and evaluated regarding their environmental effects using the methodology of Life Cycle Keywords: Renewable energy storage Assessment (LCA). The conducted Life Cycle Impact Assessment clearly shows that, regardless of the Power-to-gas (P2G) implemented business model, the source of energy is the key factor for the environmental performance Environmental impacts of a Power-to-Gas plant. This means that background processes dominate the foreground processes. The Life cycle assessment (LCA) LCA includes sensitivity analyses for relevant parameters and results for different environmental impact Business models indicators. Additionally, further potential to increase the efficiency of Power-to-Gas plants and involved units was uncovered. The outcomes of this innovative approach regarding the storage of renewable energies are from outstanding importance for the strategic development of future energy systems. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction fossil-fuel technology, and promote investment in energy infra- structure and clean energy technology” [3]. In the frame of the Paris In its “Roadmap 2050” the European Council launches a low- Agreement states were appealed for showing their commitment to carbon strategy, which “… will require a revolution in our energy fight climate change. Austria was among the first European coun- system, which must start now” [1]. Within the Europe 2020 targets, tries that ratified the agreement. The common goal is to decrease the objectives regarding climate and energy are defined to reduce the global temperature rise well below 2 C e and if possible to greenhouse gas emissions by 20% compared to 1990 levels, to in- 1.5 C e by 2030 [4]. This target will only be achieved through an crease the share of renewables in final energy consumption to 20%, important restriction in the consumption of fossil energy. By rati- and to increase energy efficiency by 20% [2]. In September 2015 the fying this agreement states have the obligation to set measures to United Nations (UN) introduced the Sustainable Development reduce the utilization of oil and coal. Goals (SDGs) under the title “Transforming our World: the 2030 Fossil fuels and nuclear power will still be of high importance in Agenda for Sustainable Development”. The 17 goals include as well future energy supply, but legislative initiatives and the environ- the goal to “Affordable and Clean Energy” which consists of five mental awareness to reduce greenhouse gas emissions lead to an targets: amongst others the target to “… increase substantially the increasing share of renewable energy. Besides bioenergy, hydro- share of renewable energy in the global energy mix”, and “… power and other renewable energy sources, the contribution of facilitate access to clean energy research and technology, including wind and solar photovoltaic power to the electricity mix will be renewable energy, energy efficiency and advanced and cleaner significantly extended by 2035 [5]. As Dodds and Garvey [6] mention, there are two issues that arise in this context:

Abbreviations: AEC, Alkaline Electrolysis Cell; BM, Business model; LCA, Life (1) The electricity system's stability may be impaired at times of Cycle Assessment; P2G, Power-to-Gas. high demand while renewable generation is low, e.g. at * Corresponding author. seasonal up- and downturns. E-mail address: [email protected] (K. Tschiggerl). https://doi.org/10.1016/j.energy.2018.07.105 0360-5442/© 2018 Elsevier Ltd. All rights reserved. 1092 K. Tschiggerl et al. / Energy 160 (2018) 1091e1100

electrolysis with higher effectiveness is faced by a more impeded storage, distribution and utilization of the product gas. In return, the methanation is connected to further transformation losses and dependency on carbon supply. Thus, the implementation of P2G is strongly constrained by locations and applications and has to be examined specifically [11].

1.1.2. Hydrogen production using P2G The production of hydrogen e by using the power-to-gas e Fig. 1. The P2G system, adapted from Ref. [8]. concept varies with the alternating availability of renewable energy. Thus, the demand for accurate storage options is at hand. There are different kinds of storage options, like metal hydrides, (2) Electricity that is generated at times of low demand while pressurized and cryogenic tanks, and underground caverns, which renewable generation is high, will be lost if it's not stored. can be considered to provide a constant supply of hydrogen. An accurate storage depends on several parameters, such as the amount of hydrogen to be stored, space constraints, the cycling rate, the transport distance, infrastructural requirements and safety 1.1. Energy storage of renewables aspects [12]. Table 1 gives an overview of underground reservoirs suitable for hydrogen storage. The fluctuating nature of wind and photovoltaic power provi- Due to that crucial research question, the project “Underground sion, which is difficult to predict, has led to different options of Sun.Storage” (USS) was initiated, which was supported financially energy storage to cover the demand for peak load electricity supply. by the Austrian Climate and Energy Fund. The idea behind the With the increased share of fluctuating wind and solar photovoltaic project is to synthesize a renewable energy product and to store power, also the demand for adequate energy storage options arises. energy as hydrogen or methane in the pore space of a geological Therefore (future) research is challenged by finding solutions to formation to utilize excess renewable energy. The pilot and store high amounts of electricity, especially for long time periods demonstration project is a power-to-gas plant in Pilsbach/Austria, [7]. At current state, energy storage technologies are still under- where hydrogen is produced by an electrolysis unit. The storage researched compared to other low-carbon technologies, caused by facility for the synthesis product is a small depleted gas reservoir a high complexity due to technological, legal, economical, social, [14]. and ecological factors or even barriers, and resulting in unclearness The following flow chart in Fig. 2 gives an overview of the USS regarding their integration in an low-carbon energy system [6]. project, whereby the research activities include several work packages conducted by a broad consortium of universities and 1.1.1. Power-to-gas: a promising technology for energy storage companies. It has to be mentioned, that CH4 from local production One of the key technologies to store renewable energies over a concerns a “neutral” through post, which can be replaced by longer period in an efficient way (technical and economical) is the methane from P2G if renewables increase [12]. power-to-gas (P2G) technology. It allows to transform power from The conducted research activities are constructed around the renewables into hydrogen (or methane) with an electrolysis, to built P2G pilot plant, with the aim to demonstrate the decoupling of store and transport it in existing gas infrastructures, and to provide power generation and consumption. Surplus renewable energy will it again tailored to suit market needs [7]. be transformed to hydrogen and stored in the existing natural gas In the underlying investigations the P2G system, as shown in infrastructure. Fig. 1, is based on an electrolyzer that uses electricity to separate The research topics included various investigations and analyses water into hydrogen (H2) and oxygen (O2). Besides this proceeding, regarding (1) Geochemistry and Reactive Transport Modelling, (2) the P2G technology also enables the synthesis of H2 and carbon Microbial Processes in Hydrogen Exposed Reservoirs, (3) Demixing dioxide (CO2) to methane (CH4). This methanation process is a so- of Natural Gas and Hydrogen, (4) Materials and Corrosion, (5) called Sabatier reaction. As there exists a maximum allowance of Hydrogen Separation, (6) Design and Construction of a Testbed, (7) feed-in of H2 into the gas distribution grid e which is different in Testbed Operation, (8) Risk Assessment and Life Cycle Assessment, each country (i.e. 5% vol. in Germany, and 4% vol. in Austria) e CH4 and (9) Economic and Legal Analysis [14]. Results from the various might be advantageous as its injection is not limited [9]. An investigations and analyses, and interdependencies within the economical useful long term concentration is approximately 10% system have to be considered when an environmental evaluation is vol. Particularly critical aspects in Austria are resulting from the intended. shortage in R&D, technical design and legal interpretation of gas The understanding and awareness regarding potential envi- turbines, compressors for transport and storage, storage in cavern ronmental impacts of new energy technologies and comparisons and pore storage (as illustrated option), process gas chromato- with alternatives are essential to guarantee the societal acceptance graphs and vehicle tanks [10]. However, and thus the “break-through” towards clean energy systems. The every conversion step is linked to losses, which is reflected by the objective of this study is to conduct a life cycle assessment of P2G effectiveness of a process. A short process chain including only the business models, where as an advancement to former studies the

Table 1 Qualitative comparison of geological hydrogen storage options [13].

Capacity Cycling rate Costs Risk Reactions with H2 Cushion gas Salt caverns Medium High High Low Low Low Depleted O&G fields High Medium Low Medium High High Aquifers High Low Medium High High High K. Tschiggerl et al. / Energy 160 (2018) 1091e1100 1093

way a car does. To comprehensively capture and use the function- alities, their components and their interactions have to be under- stood in detail. Furthermore they define the business model as the “reflection of the firm's realized strategy”. In their analogy the car itself would be the business model, whereas the design and the construction represent the strategy with the driving behavior as a tactic. Schallmo [25] concludes that despite all definitions, a busi- ness model involves the five dimensions: clients, benefits, value creation, partners, and finances.

Fig. 2. Flow-chart of the Underground Sun.Storage project. 2.2. Identification of power-to-gas business models

Tichler et al. [26] identified in a study regarding the efficiency option to store a renewable energy product as hydrogen or and systems of power-to-gas concepts, 26 business models that are methane in a pore space is given, and to assess related potential based on the different intentions of a market actor to build and environmental impacts. The results are integral part of an overall operate a P2G plant. This definition thus focuses on a concrete and risk assessment of the demonstration project, which has high specific benefit that can be derived from the P2G system. However, importance regarding a future large scale deployment of this regarding the best operating mode of P2G plants in short and technological solution. Furthermore, findings from the LCA, by middle time, a combination of different business models will be making the system transparent regarding the relevant processes necessary to guarantee competitive compatibility with alternative and energy and material flows, can be used for further de- products and systems. Thematically the following benefits e or velopments and optimizations before bringing the technology into values created in business language e within these defined busi- industrial application. ness models can be distinguished [26]:

2. Methodology - Storage (of electrical energy) - Substitution of power lines The challenge of any new clean technology and thus for energy - Release of the load management of power grids storage solutions is the combined consideration of technological - Substitution of alternative storage systems with high topo- feasibility, economic usability, ecological necessity and social graphic interventions acceptance. As stated by Parra et al. [15] there are no comprehen- - Increased energy generation from volatile renewable energy sive methodologies and studies of P2G systems that consider both sources techno-economic and environmental aspects in a consistent way. - Production of an additional renewable product “ ” Besides rather hard facts , the challenge to establish and to - Recycling of the “resource”/the by-product carbon dioxide market new technologies strongly depends on their social accep- - Increased capacity utilization of the gas infrastructure tance. As P2G and hydrogen as an energy carrier are relatively novel - Reduction of CO2 certificates concepts, education and the provision of information regarding - Optimizing the volume of purchased electricity safety and environmental favorability may be important drivers to - Development of new remote areas with a high potential for a broad commercialization [13,16]. power production - Provision of self-sustaining energy systems 2.1. The business model concept - Filtration of different gases (biogas, coal gas)

As the underlying system is based on a technology that repre- In the frame of the underlying project Underground Sun.Storage sents one step towards sustainability of future energy supply sce- and based on earlier studies of P2G systems analyses [10,27,44], a narios, it becomes obvious that it will be subject to environmental number of potential business models were identified [28], whereas evaluations. In this sense, the business model concept offers a two of them were selected for being evaluated regarding their useful framework for setting the boundaries of the system, as it environmental impacts: focuses on the value proposition. When applying this definition, the effects of the value creation logic become visible [17], and therefore (1) Renewable energy storage in an underground reservoir manageable. As a consequence, also the potential environmental a. Provision of a service to store energy as hydrogen in the effects of a business model should be examined. pore space of a geological formation Over the last decade the business model (BM) became the usual b. Gas from a renewable energy source stored in the pore tactic to describe the creation, the (dis-)functioning, and the space of a geological formation transformation of businesses, which may be a product, an organi- (2) Synthesis of a renewable source of energy zation, or even a whole industry [18e20]. In general, every purpose a. Pure hydrogen as raw material for the chemical industry or business idea may generate various alternative business models. b. A mixture of hydrogen and methane as fuel in transport To create value and a competitive advantage, the value proposition, the profit scheme and the key competencies of an organization The following Table 2 characterizes the selected business have to be combined in a unique way [21,22]. Although there exists models regarding the dimensions proposed by Schallmo [25] and no clear definition nor a constructive perspective on the construct, based on previous analyses by Reiter et al. [28]. both practitioners and scientists use the term “business model” to With reference to the underlying topic the dimensions integrate describe how a business creates, delivers, and captures value the following elements [25,28]: [18,23]. A simple description is offered by Casadesus-Masanell and Ricart [24] who conduct an analogy between business models and - “Clients” describes the targeted customers, and the channels cars. According to the authors, a business model works in the same and relationships that are used to reach them. In the case of the 1094 K. Tschiggerl et al. / Energy 160 (2018) 1091e1100

Table 2 Description of selected P2G business models, based on [25,28].

BM 1a BM 1b BM 2a BM 2b

Clients New market (creation of demand necessary) New market (creation of Chemical industry Transport - Mobility demand necessary) Benefit Service: provision of a storage option Product: renewable storage Renewable product for mobility Renewable product for

for renewable H2 in the pore space and product from the pore space (H2 or CH4) industrial application (H2) Product: renewable CH4 from the pore space Value creation - Injection into the gas grid, - Direct storage in the pore - Utilization on site - Utilization on site incl. filling storage/withdrawal from the space and following - AEC electrolyzer station pore space injection into the gas grid - AEC electrolyzer - AEC electrolyzer - AEC electrolyzer

Partners Providers of renewable H2 Provider of renewable energy Provider of renewable energy Provider of renewable energy (, PV, hydropower, (wind power, PV, hydropower, (wind power, PV, hydropower, biomass) biomass) biomass)

Finances - Sale of heat and O2 - Sale of heat and O2 - Sale of heat and O2 - Sale of heat and O2 - P2G investment costs - P2G investment costs - P2G investment costs - P2G investment costs

- Storage costs - Storage costs - Electricity, water and CO2 - Filling station costs - Electricity, water and CO2 costs - Electricity, water and CO2 costs - Electricity, water and CO2 - Taxes and grid tariffs for storage/withdrawal costs - Taxes and grid tariffs costs - Operation and maintenance costs - Taxes and grid usage fee for - Operation and maintenance - Taxes and grid tariffs injecting biogenic gases costs - Operation and maintenance - Operation and maintenance costs costs

USS project the customers can be aggregated into specific 2.3. Life cycle assessment of P2G business models relevant markets. - “Benefit” consists of the outputs and their value for the target During the 20th century different methods were developed to group. For the selected business models the benefit lies mainly determine and assess environmental impacts by considering flows in the provision of a renewable product, whereby additional of material and energy. One of the most comprehensive approaches value can be generated for example through a reduced CO2 to evaluate and assess environmental impacts of products, product footprint and thus reduced compensation cost, or substitution systems, processes and services is Life Cycle Assessment (LCA). The of investment costs by using existing infrastructure, like the International Organization for Standardization published two public gas grid. relevant international standards, which are ISO 14040:2006 [29], - The dimension “value creation” includes the resources, capa- defining principles and framework, and ISO14044:2006 [30], bilities and processes that are necessary to create the benefit. addressing the requirements and guidelines for a LCA. This aspects can be described by the required infrastructure and As stated by Decourt et al. [13], hydrogen solutions which are the applied electrolyzer technology. based on renewables face few environmental challenges as they in - “Partners” lists the partners, partner channels and relationships. general result in lower emissions than other energy-storage tech- In this context the providers of inputs are given as the most nologies. Nonetheless, the effects on the environment have to be relevant partners. considered and can be crucial in the production of renewable - The “finances” dimension confronts turnovers with costs. As for sourced energy carriers, with special interest in controlling the underlying system the analyses are limited to the “gate” (see greenhouse-gas emissions. The authors further indicate that the flow-chart in Fig. 2), only these positions are considered in the environmental impact strongly depends on the technology which table from the viewpoint of the P2G operator. Costs are domi- generates the source, whereby PV is environmen- nated mainly by infrastructure to be built and the production tally more problematic than wind, because of the manufacturing of costs of inputs. PV cells. However, as environmental impact assessment is very system specific and influenced by various factors, it is recom- The environmental assessment and its results will be illustrated mendable to integrate environmental evaluations frequently into in detail for business model 1b, as it includes both of the other the planning and design phase of energy storage infrastructures. selected business models e service to store energy as hydrogen in Bhandari et al. analyzed several studies on LCA of hydrogen the pore space, and synthesis of a renewable source of energy. production technologies, concluding that the share of electrolysis While in BM 2 (a, b) the focus is entirely on the electrolysis and in global H2 production is very small with about 4% [31]. Another methanation process without a storage option, in BM 1a the storage comprehensive review of life cycle assessments of hydrogen energy facility is provided for inputs originating from an external natural systems is given by Valente et al. [32]. They summarize that most of gas or hydrogen source. Summing up, BM 1b incorporates all units the hydrogen energy systems apply cradle/gate-to-gate boundaries, that are required to synthesize inputs for the storage and retrieve where the functional unit is usually mass- or energy-based. They them for the generation of a renewable product. Therefore it allows further indicate that the data sources for background and fore- the most comprehensive identification of potential environmental ground processes are mainly scientific literature and life cycle da- impacts. tabases, like Eco-invent or GaBi. However, as a life cycle assessment In comparison to BM 1b and to understand the potential impacts addresses always a specific research context, the choice of methods from the underground gas storage, also the results for 1a will be applies to the individual objective and system. discussed. Further BM 2 will be analyzed regarding the synthesis of Based on the standard ISO 14040:2006 [29] and ISO 14044:2006 1 MJ of hydrogen or methane. In both cases a sensitivity for the [30] respectively, two selected P2G business models were investi- energy source will be applied as it was identified as the key aspect gated regarding their environmental impacts. The steps to conduct for the environmental performance of the business models. a life cycle assessment can be summarized in four phases [29]: K. Tschiggerl et al. / Energy 160 (2018) 1091e1100 1095

1) Goal and scope definition with the purpose to define the level of breath, depth and detail based on the intention and the target audience of the LCA 2) Inventory analysis (LCI) 3) Impact assessment (LCIA) 4) Interpretation

The major part of the computation and visualization was per- formed with Umberto NXT Universal [33] supported by a GaBi as well as converted data from ecoinvent database for background LCI data. To incorporate future opportunities, the LCA targets a hydrogen concentration of 10% vol., in comparison the current law fl in Austria and Germany only allows 4e5% vol. [9]. Fig. 3. AEC electrolysis ow-chart [12].

2.3.1. System boundaries Table 3 The system boundaries rely on the included process units of a AEC electrolysis process data [12]. specific business model, with integration of the energy sources. Input/Output Unit Quantity Thus the LCA can be defined as a cradle-to-gate assessment, which means that the phases manufacturing, maintenance and end-of-life Deionized water Input kg/MJ H2 1.39E-1 electrical current (EC) Input MJ/MJ H 1.80 treatment for energy/process/storage units are not included. In fact, 2 Hydrogen Output MJ/MJ H2 1 the focus of the study is on the value-added process of the gener- Waste heat Output MJ/MJ H2 7.16E-1 ated products or services. The software modelling [12]was Oxygen Output kg/MJ H2 7.09E-2 designed therefore with only one reference flow, assuming that by- Waste water Output kg/MJ H2 2.95E-2 products of foreground and background processes were not further Transformer and conversion losses Output MJ/MJ H2 8.58E-2 utilized.

2.3.2. Function and functional unit capacity of the electrolyze in the selected business models [12]. Functions are related to the process modules of the pilot plant from the Underground Sun.Storage project (Fig. 2). As the outputs 2.3.4. Life Cycle Impact Assessment (LCIA) of the investigated business models are hydrogen, synthetic The impact study was performed using the CML method. This methane, natural gas, or mixtures of these, the energy content per method, which was developed by the Institute of Environmental unit of MJ was defined as the functional unit. This ensures the Sciences at Leiden University, is a mid-point method (partly consideration of different heat values and densities of the relevant aggregating), which means that it is impact oriented without substances. weighting: The “interventions” as a result of the inventory analysis are allocated to different impact categories, like climate change or 2.3.3. Life cycle inventory (LCI) acidification, etc. [35]. Concluding from previous LCA studies of For the inventory analysis, the necessary modules for the busi- hydrogen production technologies, other impacts besides climate ness models (see Fig. 2) were analyzed regarding their inputs and change are often not investigated [31]. As for the underlying case outputs, where the data were gathered from other work packages the LCA supported a comprehensive risk assessment, an exhaustive of the project (data sheets, predefined and/or measured data), from consideration of potential environmental impacts had to be con- databases (GaBi, converted data from ecoinvent) or using concep- ducted. Therefore the LCIA includes the following impact categories tual designs e the usage of physical/chemical law added by as- [36]: sumptions to approximate realistic data where the data situation was insufficient. Usually, the inventory analysis is the most inten- - Acidification sive part when conducting a LCA and includes e as in the under- - Depletion of abiotic resources (ADP fossil) lying case e as well foreground and background processes. This - Eutrophication means, that foreground processes rely to newly generated data, - Freshwater ecotoxicity while background data are consulted from existing datasets [34]. - Global warming potential (GWP 100a) Based on each unit's characteristics, a specific flow-chart was - Human toxicity designed and process data gathered from the previously mentioned - Photo-oxidant formation (Photochemical) sources. Represented by the major process units, which are the AEC - Resource depletion electrolysis unit with H as reference flow, and the underground 2 - Stratospheric ozone depletion (Ozone depletion) gas storage (pore space) this proceeding was conducted in an iterative manner for all other units, and is the basis for future impact assessments of the business models. The AEC, which is in- Table 4 fi tegrated in business model 1b and 2, is illustrated simpli ed in AEC electrolysis process data [12]. Fig. 3 and Table 3. For the storage of gas, the pore space of a depleted gas field in a Value Unit geological well explored area in Austria was intended. The reservoir Depth 1070 m properties can be obtained from Table 4. Thickness 1.5 m fl Approx. Porosity 22 % The ow-chart and the process data for the reservoir are given Permeability (OHT) 400e700 mD in Fig. 4 and Table 5. Initial reservoir pressure 107 bara All measured data from the demonstration plant were extrap- Reservoir temperature 42 C olated linearly to compensate the difference in electrical nominal Initial water saturation 35 % Storage volume 6 106 Nm3 capacity of the pilot AEC electrolysis unit and a targeted 5 MWel 1096 K. Tschiggerl et al. / Energy 160 (2018) 1091e1100

3.1. Business model “renewable energy storage in an underground reservoir”

The underlying business model is built up on a scenario, where the energy used for the P2G plant is from the renewable sources photovoltaics and wind turbines (volatile renewable energy). The synthesized products from BM 1 are stored in a pore space of a geological formation to overcome weaknesses of renewable pro- cesses, like fluctuating energy yield. The term “fluctuating energy” in this context means a temporary and spatial variation of energy [28]. In business model 1b, shown in Fig. 5, the owner of the storage

Fig. 4. Underground gas storage flow-chart [12]. facility acts as a deliverer of methane enriched with hydrogen. To achieve the requirements the plant is modified by a methane synthesis unit [28]. Table 5 As highly concentrated hydrogen in a pore space of geological Reservoir process data [12]. formations might cause several impacts with different risk poten- Input/Output Unit Quantity tial one of the work packages focused on investigating the hydrogen compatibility of geological formations. A risk reduction CH4, synth. þ fossil Input kg/MJ OPG 1.935E-2 can be achieved, if the hydrogen is blended with other gases to H2 Input kg/MJ OPG 2.692E-4 Electrical current Input e Cut off decrease the partial pressure [37]. To optimize the energy storage Corrosion inhibitor Input e 0 and reduce risks, the pore space is used in combination with either e Bactericide Input 0 gas from a fossil source (1a) or gas which is produced by the CH , synth. þ fossil Output kg/MJ OPG 1.935E-2 4 extension of P2G with a methane synthesis unit (1b). On the other H2 Output kg/MJ OPG 2.692E-4 H2S Output e Cut off hand, the intention is to increase the share of renewables, therefore Methanol Output kg/MJ OPG 3.38E-05 the whole infrastructure is subject to an evaluation of its hydrogen

OPG … Output Gas 1/9 H2/CH4. compatibility. Risks associated with high concentrations of hydrogen in the reservoir could be the gas integrity of the cement, corrosion of the - Terrestrial ecotoxicity borehole completion, development of hydrates and microbiological growth, resulting in a source of hydrogen sulfide. Hydrogen was already stored successfully under controlled conditions in a cavern 2.3.5. Creation of scenarios storage up to a concentration of 55% as town gas in Germany, and To analyze the impact of different options on the environmental further hydrogen underground storages exist in France and former performance of a specific P2G business model, individual parame- Czechoslovakia [10]. Regarding bacterial influence, the main risks ters with important influences on the results were variegated. The concern methanogenesis, homoacetogenesis and sulfate reduction. sensitivity analyses include. According to Pichler [38] and Schritter et al. [39] underground hydrogen storage would only be influenced by microbial activities if (1) scenarios for the electricity supply for the AEC electrolysis: there exist electron acceptors such as CO2 and sulfate. As analyses wind power, photovoltaics, hydropower, electricity grid mix and results from related work packages showed, the potential for Austria, and electricity grid mix for EU-27; occurence of the microbiological activities is absolutely given, but (2) scenarios for the H2/CH4 mixing ratio: 1H2/9CH4; 0,6H2/ in the context of the underlying field test and comparable set ups 9,4CH4; 0,4H2/9,6CH4 (legally allowed value for H2 at insignificant (marginal disposability of CO2 and sulfur) and there- present); fore not included in the life cycle assessment. But it has definitely to (3) combined scenarios for the implementation of the process be stated that the potential stimulation to develop H2S has to be þ units AEC methanation versus only AEC based on different considered in other subsurface storage environments, if higher H2 energy sources; concentrations into gas are targeted [10,40]. (4) scenarios for different operation modes (injection or pro- duction phase) of the P2G plant and its units, including 3.2. Results and sensitivity analysis for business model 1b variations regarding the efficiency, specifically of the elec- trolysis unit (depending on mode, pressure, H2 purity etc.). The results for the impact assessment were achieved by using the software Umberto NXT [33]. As the electricity production units The sensitivity analyses were primarily focused to analyze var- would dominate the impact assessment, a “dummy” power plant e iations in the environmental performance including the different impact categories as well as the primary energy demand. As there was a specific interest in evaluating the energy efficiency of specific process units, aiming at the optimization of future design and implementation of P2G plants with underground storage, several scenarios were investigated to that effect.

3. Results and discussion

Before the results of the conducted LCA are discussed, the selected business model 1b, where energy from renewable sources is stored in a depleted gas reservoir, will be explained. Fig. 5. Business model 1b e Energy from a renewable source stored in a pore space. K. Tschiggerl et al. / Energy 160 (2018) 1091e1100 1097 referred to as “reference” in the results e was created to replace the electricity generation by a source without environmental impact. The energy demand of the plant is identical to the primary energy demand. The resulting model supports a better representation of the results and is used as a reference model e which excludes environmental impacts of power plants e to represent the envi- ronmental profile of the analyzed P2G business models. Thus, its results will be explicitly shown. As the results in Table 6 and Fig. 6 display, the main focus regarding environmental aspects is on the energy source, all other analyzed input and output flows related to the different process units, like synthesis and storage, are of minor importance. The “reference” results indicate the contribution of the P2G plant to the total environmental burden and the energy demand. Regarding climate change as well as other impact categories, all energy sources are preferable against energy from the Austrian and European power grid mix. The impacts in the category human toxicity and resource depletion are dominated by the use electricity Fig. 6. Environmental profile for business model 1b. from photovoltaics (which also shows relatively higher values in other categories). This can be explained by the use of non- impact from the storage is marginal and the gas source dominates. renewable resources and the appearance of heavy metal emis- The respective contributions of storage, gas and energy source to sions in the production of single PV system components, which for the environmental burdens are given in Fig. 7. example stem from non-European production [41,42]. Considering the scaling of the dependent variable, the potential impacts for 1 MJ of energy from a renewable source stored in a pore space seem preferable if hydropower, wind power or wind power þ natural gas are applied. 3.4. Results and sensitivity analysis for business model 2 Presented by the reference scenario, the major environmental burden originates from the waste water treatment, whereby the Business model 2 has its focus on the main parts of the P2G fi data were gathered from the databank GaBi as speci c data for the plant, which are the AEC electrolyzer and the methanation process. plant waste water could not be obtained. Minor burdens are caused As a consequence, a comparison between the synthesis of methane by the dehydration and the other effective units. and hydrogen can be applied, which is represented in Table 8. Further process units incorporated in this option are the water well, 3.3. Results and sensitivity analysis for business model 1a the deionization unit, a compressor for hydrogen bottling, and a glycol dehydration unit. In contrast to business model 1b, which incorporates the other The main consumption of electricity is at the AEC unit which selected options (1a and 2), business model 1a is focused on the results in the main contribution to the total environmental per- storage process and the necessary energy. Relevant process units formance. Further, relevant impacts can be identified at the include a pressurization cascade containing four compressors, the methanation unit. This implies, that the specificefficiency of the storage (pore space reservoir), the free water knock-out, and a electrolysis units and thus the source of energy is an important glycol dehydration. factor regarding the environmental aspects. Table 7 shows the aggregated results for BM 1a including energy As a main conclusion from the environmental profile, the GWP and gas source, and the storage process. For the scenario building, results indicate, that the synthesis of methane would compensate also the non-fluctuating renewable sources hydropower and the carbon dioxide emission during the energy generation and biomass were targeted, which are likely due to high capacities in contribute to a reduction of carbon dioxide emission of a plant Austria. providing carbon dioxide, which is illustrated by the reference case. The evaluation of only the storage process shows, that the en- Contrary to the synthesis of hydrogen, the reference model shows a ergy source is the key factor for environmental burdens. In contrast, general stronger environmental impact. In total, the synthesis of a sensitivity analysis considering methane produced from a natural methane has a stronger pronounced environmental impact. Fig. 8 source and hydrogen extracted from a fossil source shows that the shows the results for four selected impact categories.

Table 6

Environmental profile for 1 MJ 1/9 H2/CH4 (business model 1b).

Impact Category Reference Photovoltaics Hydropower Wind power Wind power þ natural gas Grid Mix AT Grid Mix EU

Primary energy (MJ) 2.067234 16.19400 3.34141 5.27400 1.361665 4.97390 5.901759

GWP 100a (kg CO2-Eq) 0.053256 0.029752 0.005202 0.005139 0.014370 0.218142 0.207130 Human toxicity (kg 1,4-DCB-Eq) 1.08E-05 0.044000 0.000126 0.00054 0.000126 0.008688 0.015990 Res. Depletation (MJ) 1.04E-10 1.99E-06 1.29E-07 6.37E-08 7.19E-09 7.57E-08 4.62E-08

Eutrophication (kg PO4-Eq) 1.58E-06 1.24E-05 2.47E-06 3.09E-06 3.24E-06 5.32E-05 7.28E-05 Acidification (kg SO2-Eg) 8.19E-07 0.000134 7.09E-06 1.42E-05 2.37E-05 0.000600 0.00129 Freshwater ecotox. (kg 1,4-DCB-Eq) 2.15E-06 0.000312 1.21E-05 2.40E-05 2.68E-05 0.000338 0.000636 Terrestrial ecotox (kg 1,4-DCB-Eq) 1.02E-06 0.000169 4.15E-06 1.75E-05 3.02E-06 0.000277 0.000345 Photochemical (kg ethylene-Eq) 8.34E-07 1.93E-05 1.22E-06 1.94E-06 1.03E-05 4.52E-05 7.65E-05 Ozone depletion (kg CFC-11-Eq) 7.41E-15 7.48E-12 5.36E-14 3.05E-13 1.75E-14 7.02E-12 1.94E-10 ADP fossil (MJ) 0.00055 0.374855 0.018987 0.054254 1.06803 2.387484 2.91003 1098 K. Tschiggerl et al. / Energy 160 (2018) 1091e1100

Table 7

Environmental profile for 1 MJ 1/9 H2/CH4 including storage, gas and energy source (business model 1a).

Impact Category Reference Photovoltaics Hydropower Wind power Grid Mix AT Grid Mix EU Biomass

Primary energy (MJ) 1.25Eþ00 1.34Eþ00 1.25Eþ00 1.27Eþ00 1.27Eþ00 1.28Eþ00 1.28Eþ00

GWP 100a (kg CO2-Eq) 1.86E-02 1.87E-02 1.86E-02 1.86E-02 2.01E-02 2.03E-02 1.88E-02 Human toxicity (kg 1,4-DCB-Eq) 1.81E-04 4.71E-04 1.80E-04 1.85E-04 2.41E-04 2.91E-04 4.51E-04 Res. Depletation (MJ) 5.30E-09 1.83E-08 5.96E-09 5.72E-09 5.82E-09 5.61E-09 5.33E-09

Eutrophication (kg PO4-Eq) 3.40E-06 3.47E-06 3.40E-06 3.41E-06 3.76E-06 3.87E-06 4.80E-06 Acidification (kg SO2-Eg) 2.63E-05 2.71E-05 2.63E-05 2.63E-05 3.04E-05 3.49E-05 3.35E-05 Freshwater ecotox. (kg 1,4-DCB-Eq) 4.31E-05 4.52E-05 4.32E-05 4.33E-05 4.54E-05 4.73E-05 5.27E-05 Terrestrial ecotox (kg 1,4-DCB-Eq) 7.02E-06 8.12E-06 7.03E-06 7.13E-06 8.92E-06 9.32E-06 1.90E-05 Photochemical (kg ethylene-Eq) 1.07E-05 1.08E-05 1.07E-05 1.07E-05 1.10E-05 1.12E-05 1.13E-05 Ozone depletion (kg CFC-11-Eq) 4.60E-14 9.60E-14 4.63E-14 4.80E-14 9.40E-14 1.35E-12 5.54E-14 ADP fossil (MJ) 1.86E-02 1.87E-02 1.86E-02 1.86E-02 2.01E-02 2.03E-02 1.88E-02

Fig. 7. Environmental profile for business model 1a. Fig. 8. Environmental profile for business model 2eselected impact categories.

3.4.1. Specific results to consider energy efficiency aspects methanation process, the incurring waste heat has a higher po- Within the business models that include the relevant unit, the tential for further internal or external recovery due to its higher main consumption of energy arises from the electrolysis unit and temperature level, even with a lower amount. Fig. 9 represents the the applied AEC process, as well as from the methanation unit. As energy flows of business model 2, where no storage option is the temperature level for AEC is rather low, the application of required. thermal energy for other operations is limited. Pertaining to the

Table 8

Comparison between the synthesis of 1 MJ of H2 or CH4 (business model 2).

Impact Category Source Reference Photo-voltaics Hydro-power Wind power Grid Mix AT Grid Mix EU

Primary energy (MJ) Hydrogen (H2) 1.74 11.80 0.40 2.67 2.42 3.19 Methane (CH4) 2.07 14.13 0.50 3.21 2.91 3.83 GWP 100a (kg CO2-Eq) Hydrogen (H2) 0.00Eþ00 2.40E-02 2.90E-03 3.80E-03 1.80E-01 2.20E-01 Methane (CH4) 6.10E-04 2.90E-02 3.50E-03 4.50E-03 2.20E-01 2.60E-01 Human toxicity (kg 1,4-DCB-Eq) Hydrogen (H2) 0.00Eþ00 3.70E-02 8.80E-05 4.40E-04 7.30E-03 1.30E-02 Methane (CH4) 1.10E-05 4.40E-02 1.10E-04 5.30E-04 8.70E-03 1.60E-02 Res. Depletation (MJ) Hydrogen (H2) 1.00E-12 1.70E-06 8.30E-08 5.30E-08 6.30E-08 3.90E-08 Methane (CH4) 1.00E-10 2.00E-06 9.90E-08 6.40E-08 7.60E-08 4.60E-08 Eutrophication (kg PO4-Eq) Hydrogen (H2) 1.30E-09 9.00E-06 5.70E-07 1.30E-06 4.30E-05 6.00E-05 Methane (CH4) 1.60E-06 1.10E-05 6.80E-07 1.50E-06 5.20E-05 7.10E-05 Acidification (kg SO2-Eg) Hydrogen (H2) 1.00E-12 1.10E-04 4.00E-06 1.10E-05 5.00E-04 1.10E-03 Methane (CH4) 8.20E-07 1.30E-04 4.80E-06 1.30E-05 6.00E-04 1.30E-03 Freshwater ecotox. (kg 1,4-DCB-Eq) Hydrogen (H2) 1.00E-12 2.60E-04 6.40E-06 1.80E-05 2.80E-04 5.30E-04 Methane (CH4) 2.20E-06 3.10E-04 7.70E-06 2.20E-05 3.40E-04 6.30E-04 Terrestrial ecotox (kg 1,4-DCB-Eq) Hydrogen (H2) 0.00Eþ00 1.40E-04 2.00E-06 1.40E-05 2.30E-04 2.90E-04 Methane (CH4) 1.00E-06 1.70E-04 2.40E-06 1.60E-05 2.80E-04 3.40E-04 Photochemical (kg ethylene-Eq) Hydrogen (H2) 0.0Eþ0 1.5E-5 2.5E-7 9.2E-7 3.7E-5 6.3E-5 Methane (CH4) 1.00E-06 1.70E-04 2.40E-06 1.60E-05 2.80E-04 3.40E-04 Ozone depletion (kg CFC-11-Eq) Hydrogen (H2) 0.00Eþ00 6.30E-12 3.00E-14 2.50E-13 5.90E-12 1.60E-10 Methane (CH4) 7.40E-15 7.50E-12 3.50E-14 3.00E-13 7.00E-12 1.90E-10 ADP fossil (MJ) Hydrogen (H2) 0.00Eþ00 2.40E-02 2.90E-03 3.80E-03 1.80E-01 2.20E-01 Methane (CH4) 6.10E-04 2.90E-02 3.50E-03 4.50E-03 2.20E-01 2.60E-01 K. Tschiggerl et al. / Energy 160 (2018) 1091e1100 1099

showing the environmental advantages when H2 and CH4 from P2G have a renewable electricity source. They further analyze the application of these products as a fuel in transportation and compared to fossil fuels, with the result that renewable energy sources lead to a lower GWP. Future research regarding environmental impacts of P2G busi- ness models should be based on data gathered over a longer time period, as LCAs should follow an iterative approach. Furthermore it would be interesting to analyze potential synergies of P2G plants with plants demanding oxygen and waste heat. The integration of Fig. 9. Energy flow-chart of business model 2. P2G into a broader system could show further environmental improvement potential and advantages for the development of this technology. 4. Conclusions and outlook Overall, power-to-gas offers systemic benefits as it links a vol- atile regenerative power system to a flexible gas storage system. For The focus of the conducted life cycle assessment was to analyze this reason it can provide renewable electricity via gas and tailored the value-added process of a renewable product including the to market needs: for heating and electricity supply, as well as for storage in a pore space of a geological formation, regarding its mobility or as chemical binder. As several investigations showed, environmental impacts. Due to this fact the impacts of the usage the existing gas infrastructures reveal compatibility of hydrogen and disposal phase were not evaluated as this would require a tolerance up to 10% vol. Therefore the assessment was based on this different view on the whole system, and amongst others a change optimistic scenario. of analyzed functions and functionalities. In this context, the study fi can be classi ed as a cradle-to-gate LCA. The results of the impact 4.1. Economic assessment of P2G alternatives assessment indicate that the source of energy is the key aspect for the environmental performance of the investigated power-to-gas Techno-economic investigations showed, how the contribution plant and the presented P2G business models, and that improve- of P2G to stabilize the electricity system can be evaluated if no ments can be achieved by the utilization of hydro and wind power. other power storage is available, and how provided balancing po- As a consequence of the impact assessment results, background wer should be priced. Under current legal regulations P2G business processes are dominating the foreground processes. Within the models are unviable if examined separately [43]. The challenge is to business models the AEC unit was identified as the major source of facilitate the market entry of P2G through an effective combination fi fi energy consumption and the speci cef ciency of the electrolysis of different business models, which is one research question within unit as the most critical parameter. The methanation has in relation the USS project. P2G plants are currently far from being competitive fi a higher ef ciency, and therefore this process steps shows minor and the technology is still under development. Therefore it has to environmental impacts than the electrolysis. The highest potential be supported from the public sector to make use of the overall environmental impact refers to the waste water treatment, positive system benefits. With the further development, learning whereby it has to be mentioned, that this facility was implemented curve effects and economies of scale will probably reduce its costs to capture potential demands. The (underground) storage process and make it a competitive technology with greatest value [44]. itself has a minor importance regarding impacts. The choice and However, the underlying project shows that the utilization of fl impacts of an energy source are strongly in uenced by system exhausted gas reservoirs to store energy e which was transformed conditions, like the local availability as for instance solar power. In to synthetic natural gas e has economic advantages against the the underlying case, the sensitivity analyses showed that hydro and development and creation of new, expensive and technical con- wind power had a favorable environmental compatibility in all spicuous storage alternatives. Nonetheless, the modelling for H2 selected business models. Strong environmental disadvantages storage solutions has to be conducted at a very high level of detail to would result using energy from the Austrian and European grid. consider the inherent system and application specifics of each Considering the system boundaries, the results yield a preference of venture which remains a challenge at present [13]. the renewable sources, although it has to be pointed out, that the different environmental indicators are not comparable among one 4.2. A statement regarding future energy systems another as they are related to diverging impacts. Due to the limited analyzes conducted during the demonstration project, where the It's the solution regarding the storage of renewable energy to hydrogen compatibility of underground gas storages was investi- reach a high level within the energy mix and to reduce CO and fi fl 2 gated for the rst time, the in uence of maintenance in the pro- other greenhouse gas emissions substantially. Thus, such research duction phase could not be considered. activities have an outstanding relevance for industry, political de- As stated by Parra et al. [15], the number of studies that cision makers and authorities for the strategic development of addressed the environmental performance of P2G systems is future energy systems. To that end, also the environmental burdens limited, and the reporting of environmental impacts was mainly of “greener” alternatives have to be discovered in the sense of a focused on climate change and neglecting other impact categories. comprehensive sustainable development. Not least, and as pro- For this study, 10 different impact categories were considered, and posed by several authors [13,16,45,46], education is essential to as well the primary energy demand was analyzed. In a cradle-to- attain social acceptance for P2G solutions and hydrogen applica- gate LCA study of P2G systems from Ref. [9] with H and CH as 2 4 tions, stressing arguments regarding safety and environmental products, but without considering the storing facility, the author aspects. conclude that the ecological performance depends on the elec- tricity generation source. As a result, H2 and CH4 production Acknowledgment through P2G with electricity from renewables has a promising potential to reduce the global warming potential and the primary The Life Cycle Assessment was carried out in the frame of the energy demand. Steinmüller et al. [27] get the same results, research project Underground Sun.Storage from 2013 to 2017. We 110 0 K. Tschiggerl et al. / Energy 160 (2018) 1091e1100

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