ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 CONCENTRATED SOLAR THERMAL POWER TECHNOLOGIES: A REVIEW Abdullahi Bello Uma1, Mukesh Kumar Gupta2, Dharam Buddhi3

1Centre of Excellence-Renewable and Sustainable Energy Studies, Suresh Gyan Vihar University, Jaipur India 2Department of Physics, Kebbi State University of Science and Technology, Aliero, Nigeria 3School of Mechanical Engineering, Lively Professional University Phagwara, India Email: [email protected]

Received: 14 April 2020 Revised and Accepted: 8 August 2020 ABSTRACT: Supply of energy in sufficient quantity and quality while sustaining the environment is one of the key strategies of human development adopted by most countries and the United Nations at large. This makes solar energy a strong candidate in achieving the aim of sustainable development goals and thus many countries launched several researches towards harnessing and utilisation of solar energy in different forms. Electricity generation using photovoltaic systems have over the years becomes the most successful applications of solar energy and solar thermal systems are also gaining grounds in many countries especially Spain and United States. With their variable designs with at least a design suiting any environment at variable cost, the potential for concentrated solar power development globally is high especially in areas with high solar insolation. Similarly, solar energy storage was reviewed visa- vis the challenges associated with it giving much emphasis on the thermal storage component. This paper also reviews the developments in the field of solar energy technology applications with reference to some case studies of some plants. Finally, the various policies employed through international bodies such as United Nations and some adopted by individual countries were highlighted. It was concluded that concentrated solar power is one of the promising renewable energy technologies that will meet the needs of man.

KEYWORDS: CSP, Solar Energy, thermal storage, efficiency and energy demands I. INTRODUCTION Due to increasing population and fast industrialization in many developing countries, the present world energy demand is expected to increase by about 60% in the year 2030, this will pose a threat in the form of energy supply imbalance to so many applications [1]. Globally, the twin issues of climate change and global warming have been of concern not only to individual countries but also to the body of a whole – the United Nations. Similarly, the geometrical increase in population especially in the developing and underdeveloped countries pushed up the demand for freshwater and energy which is not in phase with the current available supply rate makes it incontrovertible for humanity to turn to renewables [2]. It has been observed that utilisation of renewables is the cheapest and safest means of mitigating the above problems. This prompt many countries to drift towards utilisation of the various components of renewable energy in order to meet both their energy demands as well as safeguard the environment. Solar energy is the most abundant form of energy in terms of availability, though it variesin supply according to geographical location. It is much in tropical countries such as North African countries of Egypt, Libya, Tunisia, Morocco and Algeria, South Asian countries such as India, Pakistan and Nepal are also in this category. In America and Europe, USA, Mexico and Germany are extensively utilising solar energy in both the photovoltaic and thermal components [3]. Solar thermal technology is gaining traction with the advancement of technology in renewable harnessing and utilisation. In recent years, attention has been shifted towards the various solar thermal energy technologies and systems due to their advantages especially in high thermal application for generation of electrical energy which is the highest demanding form of energy. According to IEA [4] statistics, industries consumes a large chunk of the energy generated for process heat and much of it fall within a temperature range of 0-300 0C. Ibrahim and Aggrey [5] had posited that the fact that it is easier to store heat than electricity in large scale makes many researchers, governments and industries to give more attention towards the various concentrated solar thermal technologies. Abbott [6] had explained (Table 1 below) that the current world power consumption of 15 TW is by far a less amount compared to the 85 PW available in the terrestrial region from the sun after absorption by clouds which constitutes about 19% and reflection back to the space which engulfs about 30% of the original amount from the sun. This indicates that the solar energy alone if harnessed efficiently and effectively, can provide far more than the energy needs of man and can substitute much of the contributions of other non-renewables within the energy mix in order to safeguard the environment, drives economy and ensure synergy security. Other renewable sources combined don’t supply more than 1% of the amount supplied by solar, but the unequal distribution of

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 solar energy as well as the effect of other climatic variables such as topography and wind makes them (other renewables) good candidates for stand-alone systems, back-up systems and low power temporary supply systems in case of natural disasters of special needs. Table 1: Global Available Power from Renewable Sources as of 2009 [6].

S/no Energy Source Maximum Power (TW) 1. Total Surface Solar 85,000 2 Desert Solar 7650 3 Ocean Thermal 100 4 Wind 72 5 Geothermal 44 6 River Hydroelectric 7 7 Biomass 7 8 Open Ocean Wave 7 9 Tidal Wave 4 10 Coastal Wave 3

POTENTIALS OF POWER Energy production plays an important role in the industrial development of all nations. All development variables indicates a strong correlation between energy resources and economic growth and development [7]. Solar energy is the most abundant form of energy and plays a role of mother of all other renewables and non- renewables (excluding nuclear and geothermal energies) by influencing their production directly or indirectly. Zhao and Ma [8] had explained that globally, solar radiation is distributed into four (4) major belts as follows: 1. Most Favourable Belt: These region falls between 15° and 35° and it consist of most of the arid regions with about 3000 hours of sunshine per year and the rainfall is very low only around 250mm per year. The region also has very low cloud cover which makes most of the radiation to reach the Earth directly with very little or no absorption or reflection by clouds. Countries in this region include India, China, Sri Lank, Pakistan and most of North Africa. 2. Moderately Favourable Belt: This region lies between latitudes 0° (Equator) and latitude 15°. It has higher rainfall that the first region and also has higher humidity and about 2500 hours of sunshine. The seasonal variation is quite low thus making the average radiation over the area to be relatively constant, though most of it is scattered due to higher cloud cover than the first region. Countries in this category include many South America, Australia and Indonesia. 3. Less Favourable Belt: This region ranges from latitude 35° and latitude 45° immediately following the second region. It is characterized by clear seasonal variations and daily sunshine hours. Solar radiation intensity is higher in summer and less in winter. In this category, we find countries such as Southern Europe and some parts of South Africa. 4. Least Favourable Belt: This region starts from latitude 45° and continues to the pole in both the northern and the southern hemisphere. This region is characterized by higher cloud cover which results in diffusion of much of the radiation. Thus more radiation is there in winter than in summer due to the diffusion. Most parts of USSR, northern Europe and North America falls into this group. These are all indicated in the figure 1 below:

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Most Favourable Moderately Favourable Less Favourable Least Favourable Fig.1 Global Distribution of Solar Energy Resource [8] SOLAR ENERGY SYSTEMS Solar energy from the Sun can basically be harnessed in two forms: 1. Photovoltaic Technology 2. Thermal Technology Photovoltaic energy utilises the light segment of the radiation and it converts the radiation from the sun directly to electricity using Photoelectric effect. It is already an advanced technology. However, it faces a drawback in its efficiency which is still below 30% and it starts to drop as soon as the temperature rises above 25°. For energy storage, the most common and mature technology used is the battery system Thermal Technology on the other hand, has many applications ranging from low, medium to high temperature applications [9]. For the purpose of electricity generation, only medium and high temperature applications are required. They utilised the concentration of solar radiation on a focal point in order to generate high temperatures which will be used to heat a fluid and ultimately feed the turbine for generation of electricity. They include parabolic trough collector, parabolic dish, linear Fresnel reflector, solar tower and solar chimney.

Figure 2: Solar energy Power System SOLAR THERMAL TECHNOLOGY FOR POWER GENERATION Solar thermal technology for power generation is also referred to as the solar thermal power plant. It involves tracking, concentrating and conversion of solar radiation from the sun unto a focal or receiving point. The temperature ranges from medium of 300-400 0C in the case of parabolic trough to the high temperature of 800- 1200 0C in the case of heliostats [10]. This heat is use to heat a Heat Transfer Fluid (HTF) which in turn is used to generate steam for driving turbine in order to generate electricity. Four major systems are in existence which have reached commercial applications with two referred to as line focus systems (Parabolic trough collector and linear Fresnel reflector) and point focusing systems (Parabolic dish and heliostat fields or central receivers). However, of latest interest is the solar chimney which is a solar-wind hybrid form of technology. Over the years, there are a lot of developments in the field of CSP and though at variable rates among the various technologies, the growth and development has been steady. Gunther et al. [11] had itemized the state of the various technologies with respect to their individual share within the total CSP market. With a total of 14,200 MW form all the systems in operation, under construction and under planning, PTC leads with a lion share of 56%, followed by SPT which has 27%, SPD has 26% and LFR has the lowest share of 1%. These ae presented in the figure below:

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SPT 27%

PTC LFR

PTC SPD 56% SPT SPD 16% LFR 1% Figure 3: CSP Plant in operation, under construction and Planned for Construction [12] - 1.3.1 SOLAR PARABOLIC TROUGH Parabolic trough us the oldest CSP technology dating back to 1880. It was first commercially put into operation in 1983 with the construction of plants called Solar Electric Generating System (SEGS) in Mojave desert of California USA [13]. From 2007 to date, PTC technology witnesses a wide expansion in its development with the establishment of many projects. Parabolic trough collector is the most matured technology among the concentrated solar thermal collectors due to its various and distinct features that helps to gives it an edge over other high temperature thermal collectors. The major components of the PTC include the collector with its tracking system which receives, concentrates and tracks the radiation from the sun, the receiver which receives the radiation and converts it to hear energy, the HTF which carries the thermal energy from the receiver and transport it to the turbine and/or the storage system, the power block which converts the steam generated to electricity [14]. The performance of PTC system depends solely on the performance of the individual component mentioned above which makes the complete system. The optical, thermal, geometrical as well as the mechanical properties of the materials utilised in the system construction determines the performance of the PTC system. There are two ways that a parabolic trough can be utilised for power generation: indirect and direct (DSG). In the indirect generation which is the most common involved two sets of heat transfers. First, the heat is collected from the receiver by the HTF and then transferred to the working fluid usually water where steam is produced and feed the Rankine cycle. In the second case involving the direct steam generation, the working fluid which is the water serves both the function of the HTF as well as the working fluid and the heat so generated in the receiver is used to generate steam directly which is fed into the Rankine cycle directly [15]. Both has its merits and demerits. In the case of indirect generation, its merits lies in many factors such as more control of temperature and pressures compared with DSG as well as usage of variety of HTFs. On the shortcomings, limitations of temperatures to 400 0C, cost and degradation of HTF are matters of concern. Though much research is on towards developing low cost, high effective and durable HTFs. If these researches succeeded, it means even the PTC temperature may exceed the 400 0C benchmark, thus the production will increase. Thermal oil which is usually the most common HTF used in the parabolic trough is limited to 400 0C and beyond this temperature, it experiences thermal cracking which eventually destroys the oil. Although, experiments are on to determine the suitability of using molten salts such as nitrate [16], carbonate and sodium salts as HTF, but that can only be successful if the capacity of the power block is optimised to work beyond the traditional steam temperature of 370 0C. As for the DSG, its merits is in the achievement of higher temperatures and pressures up to 550 0C and 120 bars respectively [17]. Heat exchangers are avoided in DSG because no HTF is used and by implication the cost is therefore reduced. This also is accompanied by the avoidance of thermal losses due to heat transfer between HTF, exchanger and working fluid, thus efficiency is improved. Lastly, the environment is protected as the working fluid (water) has no any negative environmental impact as compared to the various HTFs. The drawbacks to DSG include the high pressure associated with the absorber tubes which is essentially transmitted to the power block all through as live steam [18]. In PTC systems, due to tracking of solar radiation, receivers are movable and has connections that are flexible, this will not be suitable under such high pressure of DSG and therefore shows the wisdom why DSG is not suitable and not yet employed as standard in PTC systems [19]. Furthermore, incorporating a storage system in DSG systems appears to be a challenging task, this is because a large storage system using sensible heat that will last for a longer period is desired for preheating and superheating which is not available

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 currently and LHS appears not a suitable substitute. LHS is more suitable for evaporation rather than preheating or superheating. However, with all these, if a cascaded system may be employed, it may serve as a suitable replacement for the SHS as different stages of the CLHS will have different temperatures and pressures [20]. In the loop control dimension, German Aerospace Centre (DLR) had chosen the recirculation mode in loop operation as the most appropriate method for separation of the evaporated and the superheated steams and thus recommended it as the best method for DSG.

- - Fig. 4 Parabolic Trough Collector [21] 1.3.2 LINEAR FRESNEL REFLECTOR The focus of line-focusing solar collector systems was largely on the PTC systems due to its advanced level of development in terms of high achievable solar thermal concentration and higher efficiency. However, that has not negated the need for further research towards overcoming some of the shortcomings of the PTC, this gives birth to the LFR. Since the solar field is the most expensive section of a CSP plant, the major objective of the LFR is achieving a less cost solar field than that of the PTC. In physics, the role of Fresnel lens is to refract light rays and focus them on a particular single focal point [22]. In conventional solar thermal systems, mirrors are arranged either in flat form in the form of an array which functions to focus reflected radiation unto a linear receiver. The main difference between LFR and PTC is the flat shape of the system which utilises the relatively cheap mirrors as compared with PTC’s, requires less heavy steel materials for support structure and it is assumed that installation of LFRs is easier and faster than PTC’s. It operates within temperature of 250-390 0C but also possible up to 560 0C [23] within a capacity of 100-200 MWe. As compared with all other CSP systems, land requirement is less since it allows higher land use efficiency due to its simplicity in construction and operation. However, its thermal efficiency is not well verified practically though it was reported by [24] to be about 37.5% via numerical simulation. Bellos [25] examined the use of nanofluids and internal fins for enhancement of efficiency in LFR. The result shows that the efficiency can be improved significantly with internal fins as compared with the smooth absorber system and same applies to the case of using nanofluids as compared with pure oil. However, the combination of LFR system as the results shows 7% increase, though Jaramillo [26] has also achieved 10% efficiency increase with water as HTF. The lower efficiency is also due to geometric configurations which are not as effective as that of the PTC since the tracking of the LFR will not be as effective as the PTC. This can be overcomed by the use of trapezoidal cavities as well optimisation of secondary reflectors and both have been investigated by several researchers [27-29] with Qiu et al [30] able to achieve 75% optical efficiency improvement which is close to the performance of PTC. The major HTF used by LFRs is usually pressurized water/steam which are cheaper in cost, environmentally friendly and can be operated as a DSG that engenders the requirement of few number of heat exchangers than in the case of power plants utilising thermal oil as HTF. The development of this system is still at mostly relatively at experimental stage with few commercial plants currently operating such as Dacheng Dunhuang and eLLO solar thermal plants in China and France respectively. Also, the Spanish company NOVATEC is currently building a 30 MW power plant for commercial purpose. The merit of the Fresnel reflector lies in its simplicity in technology. The simplicity in the technology gives it much advantages in terms of flexibility for changes aimed at improvement either in design or in operation, suitability for installation in different environments and cost. On the economic front, LFRs are cheaper in terms of installation as compared to PTCs though they suffer from lower efficiency when compared with PTCs. Thus, the low efficiency is complemented by the lower cost. Purohit and Purohit [31] had reported that all areas with 1800 kWhr/m2 are the best for CSP systems installations. The fact that the study considered all types of CSP systems signifies that LFR is also feasible economically.

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Fig. 5 Linear Fresnel Reflector [47] - 1.3.3 SOLAR PARABOLIC DISH (SPD) Solar parabolic-dish concentrators are two-axis solar radiation tracking systems that concentrate solar radiations toward a thermal receiver located at the focal point of the dish collector [33]. This class of receiver has a satellite-like point-focus concentrator which functions to reflect solar radiation to a central focal point. The surface of the dish is made shiny in order to increase its reflectivity [34]. This can easily be achieved with aluminum or silver which is coated on a plastic or glass. But glass coated with silver at 1μm gives far higher output than plastic or other coatings. The glass usually used is combined with iron and such combination has proved to increase the reflectance by about 90-94%. The key advantage with this is that the electrical generator system is placed at the focal point so that as heat is generated it is used in the same point, this makes it more efficient in terms of power conversion than the parabolic trough collector. Usually, the concentrators are assembled with a double-axis tracking system that follows the position of the sun vertically and horizontally and therefore works throughout the daytime. They are so far recognized to have the highest heat-electricity conversion efficiency among the various CSP systems with an efficiency of about 30% [35]. The engine type determines the size of the concentrator. On average, the diameter of parabolic dish collectors varies from 0.5 m-10.0 m at a nominal maximum direct solar irradiation of 1000 W/m2 and the surface area is usually between 40- 120 m2 [36]. It usually employs dish/Stirling system up to a temperature of 650 0C [23, 37]. However, in some places, Brayton cycle has been experimented and it worked excellently. In such cases, Helium, air or other gases are compressed, then heated and then fed into a turbine. This system can be used as a stand-alone or grouped together to form a small grid. However, currently storage is not feasible in this system and like in many other similar systems, the energy generated has to be used immediately. A single parabolic dish system is capable of producing up to a maximum of 0.5 MW under optimum conditions of the atmosphere. Parabolic dish is almost twice more costly than parabolic trough in terms of initial investment cost [38].

Figure 6 A sample Parabolic Dish collector system [39]

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1.3.4 SOLAR POWER TOWER (SPT) This is a system that utilises fields of two-axes tracking mirrors to track and reflect the rays of the sun towards a solar receiver mounted at the top of a centrally positioned tower [40]. In this system, two axis tracking mirrors (also known as heliostats) ensured a combined energy of many suns is concentrated on the receiving tower, thus generating a very high temperature. It operates between 250 0C and 650 0C [41]. This high temperature operation requires materials that can remain highly stable at elevated temperatures and such materials include ceramics and metals. The working fluid is contained in the receiver and as it is heated, steam is generated which will subsequently drive the turbine to generate electricity. Working fluids are usually molten salts or water and in some plants with higher capacity of around 100-200 MW, liquid sodium or air can be used also in addition to other molten salts and water/steam [23]. The land requirement for solar tower is averagely medium as it is mostly for the positioning of the mirrors and the shape of the plant is usually in rectangular form. This all helps to give it the ability to perform with a thermal efficiency of 30-40% and a corresponding plant peak efficiency ranging between 23-35% [42]. For the cost of the system, it is reported by Ummadisingu and CEC [43-44] that capital cost is higher for solar tower at around 476 US$/m2 as compared to parabolic trough with 424 US$/m2 and Linear Fresnel receiver with 234 US$/m2. For the purpose of supplying smaller communities, Multi-Tower Solar Arrays (MTSA) are been developed. The array consist of several solar mounted receivers that are closely located in an arranged form in such a way that there is an overlap of heliostat fields [45]. The target is to ensure that all unutilized radiation that will otherwise be lost in the case of conventional towers is put to use, reduce construction area size and increase ground area efficiency. This increases the amount of radiation utilisation. Incorporating molten salt storage system has been experimented and found to be feasible. Currently, it is available commercially and has been installed in Terrasol Gemasolar Seville in Spain.

Figure 7 A sample Solar Power Tower system [46] 1.3.5 SOLAR CHIMNEY Solar operates based on the principle of air expansion due to rise in temperature wherein hot air rises. It is a form of tower but differs with solar tower in the sense that the collectors are spread- out on the ground and they transmit the heat absorbed into the system, thus acting like a greenhouse together with ground instead of reflecting if to a receiver like in the case of solar tower. The first prototype was implemented in Manazares, Spain in 1981 [47]. Solar chimney is worked based on three (3) main parts: solar collector, chimney (tower) and wind generator unit. The fact that the collected energy is solar while the generating unit is wind depicts solar chimney power system as a form of multi-energy sources integrated system. Heat is absorbed by the collector and forms a greenhouse where air is heated and it rises through the column of the chimney where it enters the wind generator unit to drive the turbine. Air is the HTF for the system and with differential pressure generated by the warm condition in the chimney [48], which makes the air to expand and rise up through the chimney to the generator. The performance of solar chimney is determined by the height of the chimney, the solar collector area and intensity of radiation to in the environment. As the height increase, conversion efficiency increases, so also collector area and radiation. The efficiency of the system is very low just about 2% [49] which is not unconnected with various forms of energy conversions at different stages of the system. Such low efficiency can be improved by increasing the solar collector area and height of the chimney which will increase the mass flow rate upwards and therefore increase the conversion rate. SCPP has a relatively longer life span than other forms of CSP and wind systems and requires no combustible fuel since it uses air which is both available and free [50]. It also requires less maintenance efforts and pose no hazard to the environment since it doesn’t emit greenhouse

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 gases. The cost of installation is quite low and the technology is simple [51], these two factors makes Solar Chimney Power Plant (SCPP) to be feasible for all environments with sufficient solar energy supply and low economic development. The focus for improvement should be on the open solar–air collector which takes about 50% of the total investment cost but correspondingly responsible for about 30% total energy losses of the whole system [52], this is despite the fact that performance investigations on chimney system are highly underreported. Incidentally, most of the researches have been focused on the design of the chimney structure with some few numerical researches focusing on the chimney shape.

Figure 8 Solar Chimney [49] 1.1 Comparisons between Different Systems Similarities and differences exist between different systems which gives each system its peculiarity in terms of merits and demerits. In this, line-focus systems (PTC and LFR) have more in common just as point focus systems (SPT and SPD) have more in common also. When we consider the line-focus systems, we will see that they are similar in the sense that they are both line-focus and both have single- axis tracking. However, materials such as receivers may be similar in shape but different in properties especially in terms of resistance to pressure, simplicity and cost [53]. This is because, whereas PTC has some level of control on the pressure which is generally lower due to indirect system employed, LFR work on high pressure due to DSG system employed, as such the receivers used must be able to withstand the high pressure that will be generated in the absorber tubes [54]. Similarly, PTC system requires two HTFs to be used as working fluids while LFR usually requires only 1- fluid for operation. Thus the cost of PTC will be higher in this regard especially if the HTF is costly. For water consumption, LFR consumes more water than PTC because it is using the water as the only working fluid, so much of it is been converted to steam and some of it is also lost during thermal processes. In terms of land requirement, PTC requires more land than LFR. This may not be unconnected with the movable nature of the system due to shape and tracking [55]. However, this gives it higher performance (efficiency) than LFR. For power generation system, while PTC requires superheated system to run, LFR requires saturated steam to operate. This reveals more why PTC works more with indirect steam generation in contrast with LFR that performs better with DSG [56]. The PTC also requires only single (primary) reflector in contrast with LFR that needs also a secondary reflector because while PTC employs curved mirrors that reflects almost all radiation towards the receiver, LFR system that utilises flat mirrors will eventually lose much of the reflected radiation if not for the secondary mirror which is placed to capture and redirect the reflected radiation. In both PTC and LFR, a flat topography is more desired as it makes the tracking simpler since a non-flat surface will add more difficulty in determining the angle of tilt and setting the PTC and LFR to focus on the Sun correctly at all time horizontal E- W with N-S tracking as the Sun moves approximates to an ideal collector in summer with great reduction in effectiveness in winter [57]. Though, both mechanical and electrical-electronic systems of tracking exist, electrical-electronic systems are more efficient in terms of both accuracy and reliability, however, they are more expensive. Also, PTC that has two-dimensional tracking becomes more difficult to control as compared with LFR but its efficiency is higher. The absorber tubes in PTC are more complex than that of the LFR because of the two stage process involved in heat transfer using two HTFs of different properties in PTC in contrast with LFR that needs only 1 HTF and it is usually water. This also gives LFR a little more advantage as its HTF is more environmentally-friendly than the HTFs used in PTC systems [58]. Attention must be given to both the receiver and the absorber tube because almost all the three heat transfer processes are taking place in them and their quality determines the thermal efficiency of the system. In the economic front, PTC has more LCOE with corresponding higher energy production as compared with LFR that has lower LCOE with a disadvantage of lower solar-electric efficiency [55,59]. Another factor is the dependence on solar radiation availability, PTC can work better under variable solar radiation supply than the LFR due to its two stage heat transfer process and its lower pressure operation, LFR performs poorly under such condition and that makes it more feasible and easy to

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 connect PTC to the grid than LFR. Even when storage is considered, PTC has more established and proved storage systems than LFR and the duration for the storage is quite longer for PTC which runs to several hours as compared with that of the LFR which runs for only few minutes [60]. This can only be overcomed in LFR through hybridization with other systems. For thermal efficiency, it has been observed that low turbine efficiency is one of the drawbacks of PTC system with similar share though to a lesser degree in LFR system. This is partly due to the heat loss in between the point of absorption by HTF and the point of delivery to the generation system and also partly due to transfer of heat energy in the heat exchanger. Montes et al [61] and Islam et al. [23] had conducted similar researches on how to overcome energy loss and improve efficiency focusing on solar multiple and improvement in the efficiency of the turbine system. Both inclined towards DSG system. DSG system has been tested under numerical simulations by researchers at Plataforma solar de Almeria and the result shows that it is feasible as the desired results can be obtained if all other conditions are maintained with high sensitivity. The drawback to this system is that even small variations in steam pressure and or absorber pipe diameter can result into large variations in the steam temperature outlet which will eventually affect the performance of the turbine system [62]. Such is the problem with LFR. Lastly but not the least, PTC is a more matured technology with proved performance and data available as compared with LFR, this placed a heavy burden on LFR projects as they suffer shortage of funding and sometimes outright rejection due to scanty information and perception for high risk in such investment. For the point-focus systems, SPT and SPD have higher concentration ratios than line focusing systems. This is because their mirrors track the sun in two dimensions (double axis tracking). The concentration ration is usually high in point-focusing systems usually greater than 2000 for SPDs and 500-800 for SPTs [63]. This justifies their higher solar-electric efficiencies as compared with line-focus concentrators. The receiver for the SPDs is mobile and can follow the direction of the sun while that of SPTs is fixed and only the mirrors facing the sun at any point can effectively reflect. However, this movement also increases the cost of maintenance of the SPD over SPT and gives an upper hand to SPT in this regard. The receiver system also is connected with the reflector in the SPD while it is separate from the collector system in the SPT [64]. Thus, in case of any accident, chances are more that the SPD may be damaged more than the SPT. In the design, both requires optimisation in the size in order to check the shadow that the receiver may cast on the reflector. The piping also should be minimized as much as possible in order to avoid excessive shadowing and unnecessary load increase [65]. For the SPTs, also need special optimisation in terms of closeness between the reflectors and the receiver in order to minimize discontinuity in the reflective area or aperture while also allowing sufficient space for maintenance. SPDs supports no storage so far with chemical storage system under development while SPTs utilises the direct 2-tank molten salt storage system. Furthermore, SPDs mostly operate a dish Stirling or Brayton cycle while SPTs operate superheated Rankine cycle at operating temperatures of 800 0C and 250-650 0C. Both the SPD and SPT were reported to have a thermal efficiency of between 30-40% and an annual solar to electric efficiency of 25- 30% and 20-35% respectively [35, 42]. The efficiency of solar dish can be enhanced by more than 3% [66] if optimisation can be done on heater temperature design, pump operating speeds and the aperture diameter. This will have a multiplier effect on the annual energy production by about 19%. The grid stability is low in SPD and high in SPT hence the reason why SPD is mostly restricted to stand alone and mini-grid systems while SPT can smoothly be connected to main grid without any problem. For the water consumption, SPD has an advantage over SPT in that it requires water only for cleaning of the mirror surfaces and it also don’t require HTF as either it uses hydrogen or helium as working fluid depending on the type either kinematic or free- piston respectively [65, 67]. This helps in avoiding heat losses associated with various heat transfer processes involving HTF and also reduce the cost. On the other hand, SPT consumes an average of 3000 liters of water per each MWh of production and may require HTF as well. For cooling, SPD mostly utilises the dry cooling in most constructions, this enables its usage in arid regions in contrast to the SPT. Similarly it has high adaptation to slopes and mountainous regions, this gave it advantage over almost all CSP systems, though its LCOE is still higher. The comparisons are summarized in Table 2 below.

Table 2 Performance data for various concentrating solar power (CSP) technologies [27, 35 & 38] S/No Features of Parabolic Parabolic Dish Solar Tower Linear Solar the Troug FresneChimney System h Collector l Reflector 1 Operating 290-550 800 250-650 250-390, up to ----- Temperature (0C) 560 2 Thermal Efficiency30 -40 30-40 30-40 37.5 (%) (Simulation) 3 Capital Cost (O0.012 -0.02 0.21 0.34 Low Low and M) ($/kWhe)

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 4 Grid Stability Medium-High Low High Medium ---- 5 Land Use6 -8 8-12 8-12 4-6 High (m2MWh- 1y-1) 6 Most CommonWater/Steam and Air HTF Oil 7 Capacity 10-200 0.01-0.04 10-200 10-200 --- 8 Annual Solar-to-15 25-30 20-35 8-10 >2 Electric Efficiency (%) 9 Storage Possibility Possible but notDepends on Plant Possible Possible Possible with Configuration DSG 10 Storage withCommercially Possible but Commercially Possible but not Molten Salt available not Available proven proven 11 Storage System Direct 2-tank moltenCurrently none, butDirect 2- Pressurized Ground and salt (550 0C) orChemical storagetank Steam Storage.Phase Change Indirect 2 tanksystem molten Short term ofMaterials molten salt (380 une salless than r development t storage (550 10 0C). Up to 14 0 minutes hrs Storage time C). 12 Power BlockSuperheated Stirling/Brayton Superheated Saturated steamPressure System Stea steam Rankine Rankine staged m Rankine hig h volume flow turbine(s) 13 Absorber/Receiver Directly connectedDirectly connectedSeparated Separate Connected Connection to each other to each other 14 Hybrid Possibility Possible Possible withPossible Possible Possible limitation 15 Typical shape ofRectangular Rectangular Circular/Rectang Rectangular Circular Plant ular 16 Levelized Cost of0.22 -0.34 (With 0.20-0.29 (6-7.50.17 -0.37 (6 h Electricity TES) h TES) (USD/kWh) 0.26-0.37 TES) and (Witho 0.17- ut TES) 0.24 (12-15 h TES)

II. THERMAL STORAGE In solar thermal power plants, due to variation in the amount of solar radiation with time and season coupled with other environmental factors such as cloud cover, storage system option stands a good alternative in order to ensure continuity of supply and stability of the system [68]. This can easily be done especially in the period when so much radiation from the Sun is available in such a way that not all the energy absorbed is being used to generate electricity either due to limitation of the system or deliberately done in order to ensure continuous operation of the system. Therefore, part of the energy is stored to be used in the period of low or no supply. The possibility of incorporating such storage system varies between systems due to the variation in geometry and system demand [69]. For parabolic trough collector system it is possible to incorporate a storage either using 2- tank molten salt storage or other PCMs and the performance has been feasible even in commercial power plants. Like in the case of 50 MW Andasol-1 in Spain where 2-tank indirect storage system is installed which can keep the system running for up to 7.5 hours. So many others also utilised same system and in some cases the direct method is also used like in the case of Italy’s Archimede 5 MW thermal power plant [70]. However, it is not possible to integrate storage system to a DSG system due to some peculiarities associated with that system. In DSG system, water serves as the HTF which is heated to produce steam and subsequently utilised to drive the turbine. So in this case, same HTF cannot be used to transfer thermal energy to the phase change material or molten salt. This is because the heat capacity of steam is quite low compared to other HTFs used like oils and

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 salts. Also in DSG, as the name implies, as the heat is absorbed, it is converted to electricity which makes it able to generate maximum electrical energy in the time of sunshine hours but produces nothing when solar energy supply is very low of off. For the LFRs, it has been proposed in several researches [43] since it has so many similarities with parabolic troughs. It has been experimented in Dacheng Dunhuang 50 MW LFR plant in China. A 2- tank direct molten salt system is installed and it can has a storage capacity up to 13 hours. For the point focus systems, solar tower has more chances of incorporating a storage system as compared with parabolic dish. Parabolic dish usually suffers from plant configuration. The most common system of storage employed in solar tower is the direct 2-tank molten salt storage like in the case of Aurora solar energy project power tower in Australia, Atacama-1 in Chile and Gemasolar Thermosolar plant in Spain which had storage of 8, 17.5 and 15 hours respectively. Research is still ongoing on chemical storage system for parabolic dish [23]. Two currently non-operational power dish plants that have not used any storage system are the Toole army depot and the Maricopa Solar Project both in the US. For a better performance is solar thermal plants, proper understanding of thermophysical properties of storage materials is important especially such properties as: melting point, conductivity, latent heat of fusion, specific heat capacity (SHC), subcooling and solidification, repeatability, low or non-corrosive nature, density, stability, compatibility with container materials and cost [71-73]. In solar thermal power plants, due to high temperatures, most of the materials used for storage have higher melting points, higher conductivity and others in the same manner [74]. Sensible heat storage (SHS) materials behave differently from their latent heat storage (LHS) counterparts. Some of the most common SHS systems include rock bed storage, two-tank direct system, two- tank indirect system and single tank thermocline system. For the latent heat storage, systems such as thermochemical storage, organic phase change materials such as paraffins, inorganic PCMs such as molten salts, salt hydrates, metals and their alloys and eutectics exist and been applied in various systems [75]. In rock bed storage, air is used as HTF to transfer heat to the rocks [74]. Though rock bed storage system has been in existence for a long time, its application to solar plants storage has been limited. This is partly due to high temperature involved. In a research carried out by Rachid et al.[76] had investigated the different types of rocks used in Morocco based on thermophysical properties and thermal cycling, the result shows that while some rocks are stable up to 650 0C, others experience degradation and low resistance to thermal stress. Consequently, based on the required characteristics for SHS, gabbro rock was chosen as the best storage material for SHS in CSP plants. The advantages of rocks over other materials include high energy density and high thermal capacity. Also they exhibit high thermal conductivity up to 1.9 W/m.K, this is backed up by higher values of hardness which helps to guarantee its stability against cracking and grinding at high temperatures [77-80]. All these suggested that rock can be used as s good thermal storage materials in CSP plants. In single tank thermocline system, some of the most common storage materials is the silica sand and mineral oil [81]. In both cases, steam is used as HTF. The main feature of this system is the thermal gradient (thermocline) that exist between the two regions such that at any point in time a section of the medium is at high temperature while the other is at low temperature. Another advantage of this system is its less cost as only a single container is used and water (steam) is the HTF. Such system has been experimented at solar one power tower. For the two- tank direct system, two different tanks are used with one tank at high temperature while the other is at low temperature. The same material used as HTF is also the storage material. Some of the materials used for this system include mineral oil and molten salts [82-83]. This system is more costly than single-tank thermocline or rock bed. It has been experimented at solar two power tower in and SEGS plant in California. The main disadvantage of this system is cost as the two different materials used as HTF and also as storage materials are averagely costly. However, its efficiency is very high. Lastly in SHS, the two-tank indirect system, it is the same as two-tank direct system in principle of operation, only that the HTF and the storage material are two different fluids. Many parabolic trough plants in Spain and United States have proposed to use this system. The merit of this system is that cost can be reduced in terms of fluid choices as HTF and storage fluid as well as give room for finding more alternative properties in fluid choice [84]. Latent heat storage stores far more energy than sensible heat storage usually in the range of 5-14 times. For the LHS, the most common materials used in CSP plants storage systems are the inorganic materials including salts, hydrated salts, metals, alloys and eutectics. Organic materials such as paraffins and non paraffins are generally not used due to low thermal conductivity, toxicity, instability at high temperatures and inflammability even though they have high heat of fusion [85]. All these falls under the class of PCMs. Salt hydrates are preferred due to their low cost, high volumetric energy density and availability which is a serious consideration in any project. However, they suffer from supercooling, corrosive tendencies, phase segregation at elevated temperatures and a lack of thermal stability also at higher temperatures [86]. Phase separation can be checked with surfactants but the cost will be increased with this. Such compounds include some chlorides and hydroxides. Pure anhydrous salts popularly called molten salts on the other hand, have high energy density, stable at high temperatures, less tendency to corrosion and performs better at high temperatures without undergoing phase separation. Though some of them such as solar salt experiences low specific heat capacity, such can be enhanced using nanoparticles [87-88]. Whether molten salts or salt hydrates can either be used as single salts or in combined forms as eutectics

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 and the performance of eutectics is usually higher than that of single PCMs. Many nitrates and carbonates fall into this group. When used in a mixture as eutectics they undergo both melting and solidification at same temperature without segregating and are highly stable at elevated temperatures [89]. Metallic PCMs usually considered are those with low melting point such as Gallium and metal eutectics. Not much has been explored in this area due to their weight. Conversely, they have high conductivity values, high latent heat of fusion and less voluminous. Water and most organic salt hydrates are generally not considered due to their low temperature, low specific heat capacity and low heat of fusion [90]. Consequently, most solar thermal energy storage systems utilise inorganic salts due to their high operating temperatures in terms of repeatability, stability, conductivity, high specific heat capacity and less chances of phase separation and supercooling. Inorganic salts generally meets the high temperature demands of CSP plants such as solar power towers. They are also applicable both as heat transfer fluids (HTFs) and/or thermal storage media in CSP plants [91-92]. Nitrate salts are the most commonly used storage materials in CSP plants especially in two- tank storage systems. However, due to their explosive nature, their application is limited to 600 0C temperature. For PTC system, this temperature is adequate but same temperature cannot be sufficient for SPT system which are operating up to 950 0C. In such condition, salt nitrates cannot be the best candidates. The fact that they use gases such as hydrogen and helium as working fluids which can be conveniently heated to high temperature ranging from 800-1100 0C, with conveniently low vapour pressure around 801 0C (except Nickel (II) Chloride) also shows that nitrates cannot be used in this case. Instead, chloride salts are largely recommended due to their non-explosive nature as against nitrates as they remain stable even at such high temperatures. Another advantage of chlorides is that they are both easily available and cheap in terms of cost. Thus if utilised for TES, they will make the project easier in terms of material availability and lower cost. The various PCMs can be used individually in a TES but for greater exergetic efficiency, they can also be used in a cascade [93]. Details of various salts used in TES for CSP plants is shown in the table 3 below.

Table 3 Some HTFs used in PTC Systems and their properties [94-98] Name Melting Stability Heat Capacity KJThermal Corrosion Corrosion Cost Point (0C) Limit (0C) kg-1 K-1 Conductivity Rate Temperature ($/kg) (Wm-1 K-1) (μm/year) (0C) Air - - 1.12 (at 600 0C) 0.06 (at 600 0C) 1100 0 Water/Steam 0 - 2.42 (at 600 0C) 0.08 (at 600 0C) 1.7-3.5 300 ~0 Synthetic oil -20 350 NA ~0.1 NA NA 3 Xceltherm 600 NA 315 2.436 (at 300 0C) ~0.1 NA NA Mineral Oil -20 300 NA ~0.1 NA NA 0.3 Silicone Oil -20 400 NA ~0.1 NA NA 5 Solar Salt 220 600 1.1 (at 600 0C) 0.55 (at 400 0C) 5 316 0.5 Hitec 142 535 1.56 (at 300 0C) ~0.2 (at 300 0C) 2 570 0.93 Hitec XL 120 500 1.45 (at 300 0C) 0.52 (at 300 0C) 6-10 570 1.1 Sandia Mix <95 500 1.16-1.44 (at 300 0.005-0.007 (at NA NA 0.62- 0C) 300 0C) 0.81 Halotechnics 65 500 1.22 (at 150 0C) NA NA NA NA SS-500 Halotechnics 257 700 0.79 (at 300 0C) 0.35-0.4 165 700 0.75 SS-700 Na-K-Li 99 550 1.66 (at 500 0C) 0.5 NA NA 0.6- Nitrates/Nitr 0.8 ites Biphenyl/Di 12 393 1.93 (at 300 0C) ~0.01 (at 300 NA NA 100 phenyl oxide 0C) Na-K-Zn 204 850 0.81 (at 300 -600 0.325 (at 300 110-200 800 <1 Chlorides 0C) 0C) Pb-Bi 125 1533 0.15 (at 600 0C) 12.8 (at 600 0C) 25-250 800 13

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 III. CASE STUDIES 1. Parabolic Trough Plant Case Study (Noor I, Morocco) The Noor I power plant in Morocco utilises the parabolic trough technology for power generation. It has an gross capacity of 160 MW which is part of the Moroccan Agency for Solar Energy (MASEN) mission of accomplishing 2000 MW of solar power by 2020. The plant is located in Imin’Tagoulflt which has many positive factors with respect to solar power plant siting. Such qualities include: proximity to water sources, wind behaviour, solar radiation resource availability, land availability and assessment of power grid system [99]. All these were reported in detail by [100-103]. The power plant has a thermal energy storage system. It employs the two-tank molten salt storage system capable of maintaining the system in operation for at least 3 hours after the daytime when the supply of solar energy is minimal or zero. The solar field consist of about 500,000 mirrors merged in about 400 loops with a HTF flow of 7.3 kg/s. The HTF in use is Dowtherm with a freezing point of 12 0C, boiling point of 257.1 0C, fire point C.O.C of 118 0C, Flash point (SETA) 113 0C, critical temperature of 497 0C, molecular weight of 166.0, critical pressure of 31.34 bar and critical volume of 3.17 l/kg respectively [104]. The reason for the choice of this HTF is its longer lifespan and better thermal stability as compared to mineral fluids which are commonly used. To achieve both the objective of steam generation and storage simultaneously, the HTF pipe is divided into two (2) with each of the pipes supplying the turbine system and the storage system respectively. The HTF after been utilised in both the two (2) sections is transferred back to the loops for reheating again and continuation of the cycle. The plant system operates under Rankine cycle with reheater and bleeding system. Two economizers serves as heat exchanger while the demineralized water used as cold source enters the steam generation system where it is converted into steam. Heat loss is reduced through high pressure pre-heaters. As for the storage system, the system employs a phase change material (PCM) of molten salt comprising sodium and potassium nitrates. This combination is good as it works as a eutectic, thus saving the PCM from negative effects such as phase separation, low thermal conductivity and repeatability. The storage system comprises three (3) heat exchangers that absorbs heat from the HTF to the salts and vice versa at the reverse stage. During charging, the pumps circulates the salt through the exchangers from the cold to the hot tanks and through the three (3) exchangers and absorb the heat energy of HTF coming from the solar field and transfer it to the salt for storage. In the discharge process, the reverse is the case, as the HTF will be sent to the heat exchangers that will now functions to absorb the heat from the salt to the HTF fluid and when the required temperature is reached, the HTF will be transmitted to the steam generation system where steam is generated for subsequently production of electricity. Data from the Noor I solar power plant is shows that its capacity is thrice (3 times) higher than the worldwide average capacity of solar plants which is approximately 55.21 MW with highest capacity plant is USA tagged at 280 MW while that of Spain that is leading in CSP produces 50 MW [102]. Conversely, the universal average storage time is 6.9 hours. When we compared Noor I storage time of 3 hours with the world average, it translates to only about 43%. This shows that its storage time is short as compared with storage system time of many existing solar thermal power plants. Therefore, the storage time can be increased in order to increase the operating time and make it more economically feasible and technologically possible as well. In the economic point of view, though the plant annual average generation is higher than the average world efficiency (2564.85 MW) by 21.08%, other variables are not available to determine the LCOE of the plant, though the positive performance of several indicators points to a bright prospect for the project. 2. Linear Fresnel Reflector Case Study (Puerto Errado 1 & 2) Puerto Errado 1 (PE 1) is a LFR technology based solar thermal power plant. It is located in the Calasparra region of Spain. It is a 1.4 megawatts (MW) capacity plant with a corresponding annual production of 2000 MWh/yr and happens to be the first LFR based solar power plant to be connected to the grid sometimes in March 2009. It occupied a land area of 5 hectares located in the Calasparra mountainous region. The second plant, Puerto Errado 2 (PE 2) was added in 2012 to add 30 MW to the system with a corresponding 49,000 MWh/yr. Located in the same place with Puerto Errado 1, it occupies an area of 70 hectares. Solar resource availability for the region ranges from 2095 t0 2100 kWh/m2/yr [105-106]. Details about the mirror surface area of Puerto Errado 1 is sketchy but nonetheless it is far smaller than that of Puerto Errado 2 which has a mirror surface of 302,000 m2 and consequently becomes the world's largest LFR based power plant currently in operation. Both plants (PE 1 and PE 2) have a solar field inlet temperature of 140 0C with a corresponding solar field outlet temperature of 270 0C. This is with a corresponding power cycle pressure of 55 bar, employing dry cooling involving air cooled condensers and using water as HTF in both PE 1 and PE 2. The high pressure signifies that the two plants both employs DSG as the high pressure of 55 bar is only possible in DSG [53]. The receivers used are also special grade as normal receivers like the used in PTC can only operate at maximum

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 pressure of 40 bars which is far less than the working pressure in PE 1 and PE 2. In the storage perspective, both utilises single tank thermocline system using Ruths tank which is pressure-variable accumulator containing both hot water and steam and it is most suitable for DSG operation. While the data for storage time for plant 1 is not available, that of plant 2 is 0.5 hour. On the economic front, [107] had indicated that the technology employed by NOVATEC (The owners of both PE 1 and PE 2) is a novel one as similar technology was employed to study the performance of SPP1 plant in Algeria . The results obtained indicates that the plant was able to produce 25MWh within nine (9) months from March to October with minimal water consumption when compared with conventional thermal power plants that consumes up to 36.106 l/ year. This is because of the innovative cleaning system adopted by the NOVATEC system. Secondly, in NOVATEC technology, effort is made to use the minimum amount of material possible especially in the solar field (which takes the highest percentage of the cost of the plant), this in conjunction with the first advantage mentioned above eventually results in lower LCOE for the plant as compared with other conventional plants such as PTC. 3. Solar Parabolic Dish (SPD) Case Study (Maricopa Solar Project and Toole Army Depot) Although a lot feasibility and simulation studies have been carried out on SPD, no such plant is in existence currently. Two plants have been initiated in the past in the US and both are currently not operating [108]. These are the Maricopa Solar Project and Tooele Army Depot. In their designs, both plants were designed with a turbine capacity of 1.5 MW. Since both are currently non-operational, not much data is available on them as they have been decommissioned shortly after they started production in 2011 and 2013 respectively. Also, while Maricopa is a demonstration type project, that of Tooele was a commercial type. 4. Solar Power Tower (SPT) Case Study (Ivanpah Solar Power Facility) Ivanpah solar power facility also called Ivanpah solar electricity generating system located in Mojave desert, California, USA is the world largest solar thermal electricity generation plant [109]. It comprises of 3 separate central receivers with a combined capacity of 392 MW divided into 126, 133 and 133 MW for Ivanpah 1, 2 and 3 respectively. Started in the year 2014, the plant has a solar field comprises of 173,500 heliostats. Occupying a landmass of 3500 acres with solar resource availability to the tune of 2717 kWh/m2/yr, it is expected to generate 1.079,232 MWh/yr of electricity annually. Water is the HTF used in this station and receiver inlet temperature is pegged at 249 0C with a corresponding outlet temperature of 566 0C and a receiver temperature of 299 0C [110]. This is fed into a Siemens SST-900 Rankine cycle turbine system working at a pressure of 160 bar. The system employs dry cooling involving air as cooling fluid, this helps reduce the water consumption by about 90% as the steam is converted back to water for re-use again. Because the plant has no storage system integrated into it, the heat so generated is used to produce steam in the receivers that will be used directly for electricity generation. The average annual gross solar-electricity efficiency is at 29%, though the plant also utilises natural gas as back- up. In 2014, there have been a lot of complaints on the low performance of this station with a claim that it is given out only about 50% of its expected annual output, however, the low performance was attributed to cloud cover, jet contrails and weather [111]. However, by 2017, the plant was able to meet the expected contract requirements due to improvements. In terms of environmental impacts, the plant is helping the environment by neutralizing CO2 emissions of up to 500, 000 tons a year [112]. However, reports suggest that CO2 emissions from its natural gas section jumps by 48.4% to reach 68,676 metric tons. Similarly there were concerns over the fate of birds and desert tortoise that are been killed annually. Others are: groundwater pollution and depletion, mega disturbance to hydro and geological systems and desertification/drought [113]. 5. Solar Chimney Power Plant (SCPP) Case Study (Manzanares SCPP) Professor Schlaich restated this idea which has been stated long ago by other researchers in some conferences [113] in 1970s and went ahead to build prototype SCPP in Manzanares, south of Madrid, Spain in 1982 [114- 118]. This system composed of a solar collector with a radius of 122 m and a chimney with a diameter and height of 10.8m and 194.6 m respectively. The shape of the canopy of the collector sloped from the inlet towards the center with increasing height ascending from 2 to 6 m linearly. The solar collector has a height 1.85 m and the power plant was originally designed to produce 50 kW of electrical power output though the actual average power output was 36 kW. The plant operated for 7 years and during the 7 years of continuous operation, the average chimney exit air velocity was 15 m/s under no-load condition. However, under full load condition, average speed was 9 m/s with a corresponding low operation cost. For the 9 years of its existence, the design was been improved continuously by researchers. In the 7-year service time, the running time of the power plant exceeded the expected 95% [119-120], thus justifying the investment. However, a problem occurred, the tower’s guy-wires were left unprotected against corrosion and they subsequently failed due to rust and storm winds. Due

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 to heavy windstorms, the tower was blown away and the system collapsed thus necessitating the decommissioning of the plant in 1989 [121]. Subsequently, learning from its successes and failures, attention of researchers worldwide was focused more on SCPP. Below is the list some of the various CSP systems and Solar Chimneys developed over the years for electricity generation and their features.

Table 4: Some CSP Plants and Solar Chimney plants developed for electricity generation (122- 126) S/ Name/Status Year ofCountry Installed CSP Solar FieldSolar FieldAnnual Power Heat No Installation Capacity Tech. Outlet Area (m2) Output System Transfer (MW) Temp. (0C) GWh Type Fluid 1 SEGS (I-VIII) 1984 USA 354 PTC 307-390 2,314,978 539 SRC Synthetic oil 2 Nevada Solar2007 USA 75 PTC 393 357,200 134 ST Dowtherm A One Plant 3 Solnova 1 2009 Spain 50 PTC 393 300.000 113.52 SRC Thermal oil 4 Shiraz 2008 Iran 0.25 PTC 265 Thermal Oil 5 Andasol Solar2009 -2010 Spain 150 PTC 393 809,371 491 SRC Dowtherm A Power Station & Thermal 1, 2 & 3 Oil 6 Gujurat Solar2014 India 28 PTC 393 326,800 130 SRC Diphyl One 7 Alvarado I 2009 50 PTC 393 352,854 105, - Biphenyl/Di 200 phenyl oxide 8 Megha Solar2014 India 50 PTC 393 366,240 110 SRC Xceltherm Plant MK1 9 Saguaro Solar2006 USA 1.35 PTC 300 10,341 2 ORC Xceltherm Power Station 600 10 Noor 1 2015 Morocco 146 PTC 393 - - SRC DOW 11 Andasol-1 2008 Spain 50 PTC 393 510,120 158 SRC Dowtherm A 12 Dacheng 2019 China 50 LFR - - 214 SRC Molten Salt Dunhuang/ Operational 13 Puerto Errado2009 Spain 1.4 LFR 270 - 2 - Water 1/ Operational 14 Puerto Errado2012 Spain 30 LFR 270 302,000 49 - Water 1/ Operational 15 eLLO/ 2019 France 9 LFR 285 153,000 20.2 Water Operational 16 ACME/ 2011 India 2.5 SPT 440 14,280 SRC Water/Steam Operational 17 Ashalim Plot2019 Israel 121 SPT - 1,052,480 320 SRC Water/Steam B/ Operational 18 Dahan/ 2012 China 1 SPT 400 10,000 1.95 SRC Water/Steam Operational 19 Planta Solar2007 Spain 11 SPT 250-300 75,000 23.4 SRC Water 10/ Operational 20 Ivanpah Solar 2014 USA 392 SPT 550 14,163,99 718 - Water Power Facility 7 21 Atacama-1 2018 Chile 110 SPT 550 1,484,000 - SRC Molten Salt 22 Terrasol 2011 Spain 19.9 SPT - 1,942,491 80 - - Gemasolar 23 Maricopa/ 2010 USA 1.5 SPD - 1.394 119.459 Stirling - Currently No Operational 24 Tooele Army2013 USA 1.5 SPD - 15,015 - Stirling Helium Depot/ Currently Non Operational

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 25 Manzanares/ 1982 Spain 0.05 Hybrid of- - - Stirling Air Currently Non Solar and Operational Wind

IV. POLICIES One of the key factors that influence the promotion of renewable energy harnessing and utilisation apart from research and development is effective policies by governments both at National and International levels. At the international level, efforts had been made through the United Nations Framework Convention on Climate Change (UNFCCC) by making the developing countries to benefits from the payments on CO2 reductions made by the developed countries [127]. These efforts are achieved through the World Bank’s Global Environmental Facility (GEF), the German Kreditanstaltfur Wiederaufbau and the European Investment Bank [43]. Pablo et al [128] identified the drivers as well as the barriers towards the development of concentrated power technology in Europe. These drivers and barriers are tied around three key factors viz: Environmental sustainability, energy supply security and economic sustainability. About 187 countries that are responsible for the 97% of the pollution and by implication the climate itself had made individual commitments on the steps they will take towards mitigating the effects of climate change ranging from production cuts in order to build a low-carbon economy, promotion of renewable of energy in order to mitigate CO2 production and enhance the environment. One of the key challenges of designing energy and environmental policies is ensuring a balance trade-off among the main factors, hence variations in the steps adopted by various countries. Like in the case of the European Union, the Energy Union needs to conform with Paris Agreement and at the same time needs to be flexible so as to be accommodated by the various sections of the economies within the Union [129]. According to report by the Natural Resources Defense Council (NRDC) as of 15th December 2015, China alone releases 23% of greenhouse gases which is almost the combined emissions of a number of countries that have submitted Internationally Determined Contributions to mitigate climate change and also double the amount of release from the United States of America (USA) as in Fig 9 below.

% Emission 30 25 20 15 10 5 0

Figure 9 Share of GHG Emissions by Countries With Climate Targets

Source: Natural Resources Defense Council (15th December 2015) Table 5: The steps adopted by the various countries are shown below [130]:

S/No Country Steps Adopted 1. China 1. Increasing of Non-Carbon fuels to 20% of the total energy mix 2. Cut of peak carbon emissions latest by 2030 3. Reduction of carbon emission per unit of GDP to between 60-65% by 2030 as compared with the 2005 levels. 2. United States 1. Cut off in industrial carbon emissions by 26-28% below its 2005 levels by 2025 3. Mexico 1. Reduction of greenhouse gas by 22% targeting 2030 2. Cut off of greenhouse gas and short-lived climate pollutants by 25% 3. Reduction in black carbon by 51% by 2030 4. India 1. Reduction of carbon emission by 33-36% from 2005 levels by 2030

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 2. Increase in installation capacity of electricity from renewable sources to 40% by 2030 3. Creation of extra carbon isolation of 2.5 to 3 billion tons of carbon dioxide equivalent by 2030. 5. European Union 1. Reduction of emission by 40% and above by 2030 through domestic energy usage measures 6. Australia 1. Cut off in industrial carbon emissions by 26-28% below its 2005 levels by 2030 7. Japan 1. Reduction of greenhouse gas by 26% targeting 2030 as compared to 2013 levels. 8. Brazil 1. Cut off in industrial carbon emissions by 37% below its 2005 levels by 2025. 2. Increase in renewable resources contribution to 45% of the total energy mix by 2030. 3. Increasing the contribution of non-hydropower renewables in the electricity mix to 23% by 2030. 9. South Korea 1. Cut off in greenhouse emissions by 37% from Business As Usual (BAU) levels by 2030 2. Such cut-off is across all economic sectors. 10. Indonesia 1. Cut off in carbon emissions by 29% below its 2013 levels by 2030. However, a look into the efforts by the various countries especially in the promotion of non-carbon fuels (Renewable energy sources) show that in 2017, the United States’ renewable energy promotion increased by 19.7% with solar electricity capacity increasing by 26%, implying 56% of the total renewable electricity installed in 2017 [131]. At the global scale, renewable energy installation capacity rose by 8.9% with hydropower taking the lead, followed by wind, solar PV, CSP, Biomass and Geothermal respectively. The details are shown in Table 6 below: Table 6: Global Statistical increase in renewable installation as of 2017

S/No Energy Resource Percentage Leading Country Increase (%) 1. Hydropower 50.7 China 2 Wind 24.5 China 3 Solar (PV and CSP) 18.5 China and Spain Respectively 4 Biomass 5.6 United States 5 Geothermal 0.6 United States

China is leading other countries in renewable electricity installed capacity relying majorly on wind, hydropower and grid-connected solar PV. This may not be unconnected with the multidimensional approach adopted by that country in checkmating the release of greenhouse gases, reducing CO2 and mitigation of climate change. Conversely, as expected, many countries with smaller population and low industrial development especially in Africa and Asia contributes very less in terms of GHGs, but nonetheless have agreed to promote the objective of the Paris agreement and subsequently the sustainable development goals (SDGs) of the United Nations.

V. CONCLUSION So far solar energy has been seen to exist and function as the mother of all energies as it has influence the formation, transformation and existence of other energy sources directly or indirectly. This creates a lot of opportunities especially considering the negative effects posed to the environment by non- renewable sources of energy. Therefore, from the foregoing, we can see that: 1. Solar energy can be harnessed in either thermal or photovoltaic forms and has the potential of replacing a large chunk of the energy mix especially in this era of dwindling energy sources and resources coupled with large scale environmental impacts occasioned by the use of fossil fuels and other non-renewable resources. 2. Solar thermal technology is gaining traction especially in Europe, America and many parts of Asia as a result of its variety of its merits and availability. This creates a more enabling environment for its utilisation considering the fact that its applications are diverse in accordance with the needed temperature for particular applications. Thus, due to this reason, a window is there for having many design for various equipments and systems that will harness it at various temperatures and applications. This is unlike photovoltaic

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ISSN- 2394-5125 VOL 7, ISSUE 19, 2020 system that can only be used for electricity generation. 3. Solar thermal electricity generation is one of the most promising systems with high output while also significantly preserving the environment. With its various technologies that can be adopted in various environments with variable costs, such systems can be harnessed in order to meet the energy needs of man, preserve the environment and promote economic growth and development through industrialization. 4. Solar thermal storage has proved to be effective in ensuring a sustained system for many applications especially at low temperatures. At high temperatures also much progress has been made and currently research is still ongoing to develop a highly reliable systems especially on phase change materials such as single molten salts, metals and eutectics. This has positively increased the merits of solar thermal especially with respect to electricity generation during off- peak hours. 5. Most countries irrespective of economic, technological and political strength, have resolved to mitigate the production of GHGs directly or indirectly and this can be seen from the way renewable energy sources especially solar thermal are daily getting acceptance across different regions of the world, though at different phase levels. VI. REFERENCES [1] https://www.iea.org/reports/world-energy-outlook-2017 [2] United Nations Environment Programme (2019). Emissions Gap Report 2019. UNEP, Nairobi. http://www.unenvironment.org/emissionsgap [3] https://www.irena.org/newsroom/pressreleases/2019/Apr/Renewable-Energy-Now-Accounts-for- a-Third- of-Global-Power-Capacity. Accessed last on 02/04/2019 [4] IEA. Technology roadmap – concentrating solar power. Paris: International Energy Agency; 2010, http://www.iea.org/papers/2010/csp roadmap.pdf [accessed 21.09.10]. [5] Aggrey Mwesigye and İbrahim Halil Yılmaza (2018), Modeling, simulation and performance analysis of parabolic trough solar collectors: A comprehensive review, Applied Energy 225 (2018) 135–174 [6] Derek Abbott, Keeping the Energy Debate Clean: How Do We Supply the World’s Energy Needs? Proceedings of the IEEE|Vol.98,No.1, January 2010, Digital Object Identifier: 10.1109/JPROC.2009.2035162 [7] Jaber J.O, Badran O.O, Abu-Shikhah N. Sustainable energy and environmental impact: role of renewable as clean and secure source of energy for the 21st century in Jordan. Clean Technologies and Environmental Policy 2004;6:174–86. [8] Zao Xudong and Ma Xiaoli, Advanced Energy Efficiency Technologies for Solar Heating, Cooling and Power Generation, Green Energy and Technology, ISBN 978-3-030-17282-4 https://doi.org/10.1007/978- 3-030-17283-1 [9] Sambo, A. S. (2001). Renewable energy technologies for national development: Status, prospects and policy directions. The Nigerian Engineer, 39(1), 23-31. [10] John A. Duffie and William A. Beckman Solar Engineering of Thermal Processes. Fourth Edition, [11] ISBN 978-1-118-41541-2, John Wiley & Sons, Inc., Hoboken, New Jersey [12] Günther, M. Solar Radiation (enerMENA CSP Teaching Materials). DLR Website. http://www.dlr.de/sf/Portaldata/73/Resources/dokumente/... (14 March 2020) [13] Matthias Günther, Michael Joemann , Simon Csambor1, Advanced CSP Teaching Materials, Parabolic Trough Technology, enerMENA, Deutsches Zentrum fur Luft-und Raumfahrt e.V. in der Helmholtz- Gemeinschaft [14] Cable, R. (2001): ―Solar Trough Generation – The California Experience‖. http://www.nrel. gov [15] /csp/troughnet/pdfs/cable_frier_calexpr.pdf [June 2011] [16] Flagsol (2010): ―Advanced High Temperature Trough Collector Development, CSP Program Review‖, 2010, www1.eere.energy.gov/solar/pdfs/csp_prm2010_solar_millennium.pdf [June 2011] [17] Eck, M., Zarza, E. (2006): ―Saturated steam process with direct steam generating parabolic troughs‖. Solar Energy 80 (2006), 1424-1433 [18] Kearney, D. et al. (2002): ―Assessment of a molten salt heat transfer fluid in a parabolic trough solar field‖ Journal of Solar Energy Engineering, http://pointfocus.com/images /pdfs/saltw- troughs.pdf [19] Eickhoff, /Zarza, E. (2007): „Solare Direktverdampfung in der Praxis― DLR http://www.dlr. de/sf/Portaldata/73/Resources/dokumente/Soko/Soko2007/Vortraege/Direktverdampfung_Pra xis Eickhoff.pdf [June 2011] [20] Feldhoff, J.F., Benitez, D., Eck, M., Riffelmann, K.-J. (2009): ―Economic Potential of Solar Thermal Power Plants with Direct Steam Generation Compared to HTF Plants‖. Proceedings of the ASME 2009 3rd International Conference of Energy Sustainability, San Francisco 2009 [21] Soteris A. Kalogirou, A detailed thermal model of a parabolic trough collector receiver, Energy 48 (2012) 298-306, http://dx.doi.org/10.1016/j.energy.2012.06.023 [22] H. Michels and R. Pitz-Paal, "Cascaded latent heat storage for parabolic trough solar power plants," Solar

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