A thorough review of the existing concentrated solar power technologies and various performance enhancing techniques

Abstract

Solar thermal power plants today are the most viable alternative to replace conventional thermal power plants to successfully combat climate change and global warming. In this paper, the reasons behind this imminent and inevitable transition and the advantages of solar thermal energy over other renewable sources including solar PV have been discussed. The current literature on different types of solar thermal power plants and their performance optimisation techniques is quite scattered. Efforts have been made in this paper to bring the scattered information together in one thorough review so that it helps researchers across the spectrum undertaking studies on concentrated solar power technologies. This paper has established a brief background of these technologies. Furthermore, it has put forth a comprehensive review of different concentrated solar power technologies implemented throughout the world. The review in its latter part has highlighted the current trends of various hybrid, performance enhancing techniques being employed with these technologies. A brief review of the importance of economic analysis of these technologies has also been done. The future scope and course of action adopted to keep this technology growing are also discussed.

Introduction

The energy crisis and renewables as the alternative energy source

The rapidly exhausting fossil fuel resources will gain enormous importance quite soon. Conventional energy resources such as crude oil, coal, and natural gas have been accumulating under the earth’s surface for more than 500 million years. Going by the current rates of demand for electrical and mechanical power, it is estimated that crude oil will be depleted in less than a hundred years and coal in the next few hundred years. However, the energy consumption of the world since the 1950s has increased rapidly, and it is expected that it will continue this growth until the 2070s. There were two causes for this humungous increase in global energy consumption in the past: the relatively low cost of fossil fuel and rapid industrialisation in North America, Europe, and Japan. The additional complicating factors currently include China’s and India’s rapid increase in energy consumption as they represent approximately one-third of the world’s population, the expected depletion of oil resources before long, and the effect of human activities (industrial, municipal, etc.) on global climate change [1].

Renewable energy sources such as solar, hydro, wind, and biomass are finally showing technological maturity and cost competitiveness. The oil price shocks of the early and late 1970s, climate change, and other complicating factors mentioned before developed a new interest in renewable energy. According to the IEA, renewable resources account for roughly 13% of the primary energy supply of the world [2]. India made the largest contribution to the growth of demand for the primary energy supply of almost 30%. India’s share of global energy use is expected to rise to 11% by 2040 [3]. Hence, rapid adoption of renewable energy systems in India is absolutely necessary not only for the country, but for the world.

Also, conventional thermal power plants (the most common source of electrical power throughout the world) have been susceptible to various energy crises throughout history (especially in the 1970s and 1990s). The price of fossil fuel can be influenced by frequent changes in the political stability or geopolitics of the OPEC countries. Large-scale deployment of renewable energy power plants, especially CSPs, would increase the energy security of different countries, which would help accelerate government action through policy formulation and implementation on climate change and global warming. Also, the recent trend of high fossil fuel prices and the energy emergencies in China and India in 2021 due to low coal supply are an urgent reminder for an alternative to conventional thermal power plants (Fig. 1).

Fig. 1

Primary energy consumption of the world for 70 years, 2017 Energy Outlook [3]

Need for solar energy, especially Concentrated Solar Power

“The only energy earnings or income we have is the direct radiative energy from the sun.” The sun radiates solar energy uniformly in all directions [1]. This solar energy is essential to sustain life on our planet. It is a clean, inexhaustible, abundant, and universally available energy source. It is obvious that mass deployment of solar power is the backbone of a successful transition to a renewable-heavy economy. While other renewable energy sources (e.g. wind, biomass, hydroelectricity, etc.) are derivatives of solar energy, tapping directly into solar radiation presents the opportunity to increase solar-to-electricity efficiency by a higher order of magnitude [4]. Solar irradiation at a location varies daily, and the daily irradiation varies throughout the year. Yet, solar energy is still more predictable than wind energy. It is important to note that peak insolation almost always coincides with peak energy demand during the day, so it can be efficiently utilised to match commercial power needs. The sun has an energy output of 2.8 10²³ kW, out of which 1.5 10¹⁸ kWh of energy reaches the earth per year [5]. Hence, our yearly energy demand is more than met by the daily insolation of the earth.

Solar energy can be converted to electricity by two methods: photovoltaic conversion (PV) and concentrated solar power (CSP). Solar power provides the following socio-economic benefits: environmental protection; economic growth; job creation; diversification of energy sources; rapid deployment; and potential for technology transfer and innovation throughout the world.

Solar thermal power plants had been hardly considered as a viable alternative to conventional thermal power plants (oil-fired, gas-fired, or coal-fired) by the wider public or the scientific and engineering community until a few years ago. This was quite unfortunate and very surprising, as solar thermal power has several distinct advantages over other large-scale energy technologies, detailed as follows [5]:

• 1. They offer relatively low power costs for mass production.

• 2. They have a simple structure consisting of conventional and straightforward components such as mirrors and pipes. These components can be produced fairly quickly and in a very short time.

• 3. Available industrial facilities and technologies can manufacture the individual components, unlike wind, tidal, and other sources of renewable energy.

• 4. They are the quickest way to replace today’s coal-fired and gas-fired thermal power plants for the base load power supply.

• 5. Production of carbon neutral hydrogen and carbon neutral synthetic fuels using various cycles.

By 2032, the primary energy requirement in the country would be 3–4 times the present level. Hence, CSP is crucial for the rapid and sustainable development of a nation.

Current installed capacity of concentrated solar power

The graph shown in Fig. 2 indicates the cumulative growth in installed capacity of CSPs in the past ten years. It has been observed that the installed capacity has grown four times in the last decade because of the push by climate conservation organisations and positive investment in large parts of the developing world. Unfortunately, as it has been highlighted later in this study, that most of these installations have taken place in the G7 countries. China is the only country in the developing world seriously accelerating the installation of CSP plants.

Fig. 2

solar thermal installed capacity from 2010 to 20, IRENA [6]

Introduction to Concentrated Solar Power

Solar thermal power plants are not an innovation of the last few years. Records of their use date as far back as 1878, when a small solar power plant made up of a parabolic dish concentrator connected to an engine was exhibited at the World's Fair in Paris [7]. In 1913, the first parabolic trough solar thermal power plant was implemented in Egypt. After the energy crisis of the 1970s, nine parabolic trough power plants were installed during 1984–1991.

The SEGS PTC plant in the USA became the first commercially deployed CSP in 1984. All stages of the plant were completed in 1990. Unfortunately, from 1991 to 2005, no commercial CSPs were deployed throughout the world. However, from 2005 to this day, the world has rapidly embraced CSP technology for multiple reasons [8].

Concentrated solar power (CSP) harvests solar energy by concentrating the insolation onto a small receiver area by means of mirrors, lenses, and other optical devices. The heat from the concentrated solar radiation is transferred to a heat transfer fluid (HTF) through an absorber, which operates a thermodynamic system based on a thermodynamic cycle to generate electrical power. Concentrated solar thermal power is a global-scale technology that has the capacity to satisfy the energy and development needs of the world without destroying it. The desert regions of India are one of the few places in the world with a high amount of ‘Direct solar radiation’, perfect for solar thermal power plants [9].

CSP generation is expected to increase by 34% in 2019. Though this figure is exceptionally high, it (CSP) still does not agree with the sustainable development scenario (SDS) [10] (Fig. 3).

Fig. 3

IEA, Concentrating solar power generation in the Sustainable Development Scenario, 2000–2030, IEA, Paris [11]

A CSP plant can be roughly divided into three major units:

• 1. Solar energy collection: this consists of the concentrators, the receiver, tracking mechanism, piping systems, etc.,

• 2. Thermal energy storage,

• 3. Thermal power generation unit: this includes the generator, the turbine/heat engine, controls of the cycle, etc.

The capacity of a CSP plant is dependent upon the capacity of the generator unit. The capacity is determined by the designers considering the energy demand of the demand source (which can be an industrial facility, residential area, factory, etc.) and factors that influence the energy demand (such as energy fluctuation, peak load magnitude, and duration). The decision to determine the capacity considers the following factors relating to the amount of solar energy available (such as daily insolation, peak insolation, and duration of sunlight) and other environmental and meteorological conditions secondary. For this reason, different CSP plants of the same type and capacity have varying sizes and configurations of solar fields. A CSP plant can be complemented by coal, natural gas, and oil-fired plants as sources of auxiliary power [9] (Fig. 4).

Fig. 4

Thermal energy collection techniques of solar thermal plants, wind and solar power systems design, analysis, and operation [12]

Classification of concentrated solar power based on type of concentrators

Central Solar tower receiver

From the early 1980s to late 1990s, many research activities in the field of solar tower technology took place in North America and Europe. In 1982 in Southern California, world’s first CSP tower plant ‘Solar One’ was established. It had a capacity of 10 MW and was functional from 1982 to 1986 [13]. Since then, a lot of solar power towers have been built throughout the world for research as well as for commercial purposes. Spiros et al. [14] believe that this was a result of the experience gained from the research activities conducted initially. Germany and Spain in Europe are the pioneers of this technology.

Solar tower power generation is a type of CSP that concentrates insolation onto a receiver mounted at a certain height on a tower (also called as the solar tower). The solar irradiation is concentrated by means of a heliostat field that surrounds it. The receiver heats up a heat transfer fluid/ working fluid, which operates a turbine/heat engine to generate electrical power. The solar tower CSP mainly includes the following:

• a. Heliostat field: Several small mirror panels are integrated to form a single concave surface, supported on a steel structure. The focal length of the heliostats is equal to the distance between each of them and the receiver. The heliostats in the concentrator field reflect the insolation onto the receiver mounted on the solar tower, through double-axis tracking [1].

• b. Receiver tower: The receivers are designed to absorb the heat from the reflected solar flux and transfer that heat to an HTF at a very high temperature. The HTF can be heated directly or indirectly. Receivers can be classified as directly (external and cavity receivers) and indirectly irradiated (volumetric) receivers [1].

• c. Thermal energy storage.

• d. Generator unit: They generate power directly or indirectly through a thermal cycle.

The higher temperatures attainable in solar towers can help push for solar hydrogen generation to manufacture ‘solar fuels’, a green alternative to transportation fuels [4]. In fact, the Hydrosol plant project in Germany has succeeded in direct hydrogen production from solar radiation. The multi-channelled honeycombed monolithic solar reactors have been installed on the solar tower facility of Plataforma Solar de Almeria (PSA) in Spain, showing that hydrogen production is possible on a solar tower under realistic conditions and demonstrating the high potential of the particular thermochemical cycle technology for further scale‐up. Currently, solar towers are a commercial-scale technology with multiple projects of over 100 MW capacity being undertaken. Also, the LCOE of solar towers is rapidly reaching the levels of fossil fuel plants. All operational commercial solar thermal power plants are based on the use of Rankine cycles, which are limited in the efficiencies they can achieve by the relatively low temperatures at the receiver. However, recent developments in the field of high-temperature receivers [15] along with rock-based packed-bed storage systems [16] have opened up an interesting possibility. High-temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage which permits the consideration of solar-only operation.

Initially, solar power towers used water as the working fluid. However, solar power towers in USA nowadays use nitrate salts as the working fluid. These salts are non-flammable and non-toxic and have better thermal storage capabilities than water. Pavlovic (2012) et al. [13] further state that European solar power towers use air as the working fluid. The authors also make the point that solar power towers are quite profitable in the 50–100 MW range. Likewise, the authors also make the case that of all the CSPs, solar power towers require the largest land area and quantity of water per unit electricity generated.

Currently, there are 10 active commercial solar power towers in the world: five in China, two in the USA, and one each in Israel, Chile, and South Africa (Fig. 5).

Fig. 5

A schematic for a typical solar tower power plant with molten salt storage [17]

Parabolic trough concentrators

In PTC solar power technology, solar irradiation is focused on the focal line of the paraboloid mirrors. A metal tube housed in an evacuated glass tube runs along the length of the focal line of the paraboloid mirrors. This metal tube is coated with an absorber material so that the maximum amount of heat reflected onto it is absorbed by the HTF flowing within. Problems with PTC are as follows: low concentration ratio (70–80), complex piping arrangement and high frictional losses, difficulty in raising the temperature of HTF, and restrained system efficiency.

Even with all of these problems, PTC-based plants are the readiest for commercial use of all the solar thermal power systems [1, 9, 12]. According to Pavlovic et al. [13], PTC systems provide the best land use factor among all CSP systems. It makes it easier to research and commercialise co-generation, hydrogen production, desalination, and other technologies. Bamisile et al. [18] conducted a performance analysis of a multi-generation PTC system which is capable of electricity generation, cold or hot water, and hydrogen production. The study involved analysing two Rankine cycles with regeneration and reheating. The system used a double-effect and a single-effect absorption cycle for cooling. Also, a proton exchange membrane was employed to produce hydrogen. The results of the study showed reduced energy and energy efficiencies and a reduction in GHG emissions. This study gave direction for further research in the allied applications of CSP plants to improve their overall efficiency. Such a commercial plant would be quite economical for the developing and underdeveloped worlds as it might solve the issues of load-shedding, freshwater shortages, and less electricity coverage. The hydrogen that is made could be used, especially in the developed world, to charge EV batteries or make electricity [18].

Many researchers have carried out studies in the past decade that have led to several advancements being reported and implemented for various components and sub-systems of PTC plants. These advancements have resulted in improved performance of the PTC plant, reduced operating and maintenance costs, or both. These advancements include desalination, improved parabolic troughs, different and efficient HTFs, hydrogen production, etc. One such experimentation is done by Alguacil et al. [19], where direct steam generation has been satisfactorily attempted in the trough absorber tubes. This experiment would help solve the toxicity problem of conventional HTFs and also reduce energy losses in pipes and heat exchangers. Certain conscious attempts have also been made to increase the aperture area of the troughs to improve their thermal and optical performance. Wang et al. [20], in their thermo-economic assessment of PTCs, have introduced a radiation shield to reduce the operation temperatures in order to improve the solar to thermal conversion efficiencies of PTCs. In their study, Aseri et al. [21] looked at how well two PTC systems worked in a city in India to figure out how much money they could save. In the first case, a larger aperture area parabolic trough was used with an advanced HTF to reach higher temperatures. In the second case, the same parabolic trough was used with molten salt as an HTF and storage medium. The results showed a 12.3% capital cost reduction in case one and a 29.9% reduction in case two. According to El Gharbi et al. [22], an important cost factor in PTC plants is the manufacturing of the glass in parabolic form. These studies will help to reduce the cost of initial investment, an issue hindering the progress of not just CSP but most renewable energy power generation techniques. The developing world will most benefit from these cheaper plants, as they account for a majority of the pollution caused by conventional thermal power plants. Table 7 explains similar studies done by Linrui Ma et al. [23], Wang et al. [20], Ceckici et al. [24], and Moharram et al. [25] (Fig. 6).

Fig. 6

A schematic for solar parabolic trough concentrator [26]

Solar dish concentrators

A solar parabolic dish is a point-focusing type of CSP. These systems employ double-axis tracking to concentrate insolation onto a thermal receiver mounted at the focal point of the parabolic dish. The parabolic dish has several small mirror panels that concentrate the solar energy.

The receiver has thin tubes within, which circulate the working fluid (helium, hydrogen, or air). This working fluid absorbs heat from the concentrated beam, heats up, and expands. This expansion drives a piston operating on a thermodynamic cycle (a Stirling cycle in almost all cases) producing work. The rotary motion obtained drives a generator, producing electricity. While designing the parabolic dish, one needs to keep in mind that the dish has to point directly at the sun to prevent the formation of shadows on the collector surface. This ensures optimum heat collection [5] (Fig. 7).

Fig. 7

A solar dish concentrator power generation system [26]

For this system, the concentration ratio ranges from 600 to 3000, the operational temperature is up to 800 C, and the solar dish-Stirling net efficiency is 30%. The dish-Stirling system can generate power within a range of 10–100 kW. These systems are best suited for modular, off-grid, and low-power applications [5, 9].

The heat engine of this system can employ both Rankine and Stirling power cycles. However, Stirling cycles are the most effective and widely used. Since the Stirling engine is more compact than a Rankine power system, the latter is favoured. However, Stirling engines are not a completely mature technology yet, so microturbines (for Rankine) are being considered. Also, for systems generating more than 50 kW, Stirling systems are difficult to build as the heat sink of the cold side becomes unwieldy. These systems are an excellent renewable energy alternative to diesel engines [1, 27].

Linear Fresnel Lens-type concentrators

The Fresnel lens was invented in 1822 by Augustine Jean Fresnel, a French mathematician and physicist.

The LFR CSP plant is similar to a PTC plant, where the linear Fresnel reflectors replace the cylindrical parabolic troughs. The reflectors focus the incoming solar irradiation onto the focal line of the Fresnel mirrors. An absorber tube, with the HTF flowing within, runs along the focal line of the mirrors. This absorber tube is located at a fixed height. The mirrors track the sun by rotating about their axes to reflect the insolation onto the absorber tube. High temperatures are reached in the HTF, which runs a thermodynamic engine to make power [9] (Fig. 8).

Fig. 8

A schematic of a Fresnel lens concentrator used in concentrated solar power systems [28]

The need to manufacture the glass of PTC CSP reflectors in parabolic form inflates the initial investment. To reduce this cost, El Gharbi et al. [22] make the point that many researchers are working on Fresnel reflectors. They argue that the parabolic shape is roughly approximated by a succession of flat mirrors whose slopes are regulated according to the position of the sun. While they may reduce investment costs, they also, unfortunately, reduce efficiency. Hence, engineers might have to make a trade-off between cost and efficiency depending on the requirements.

The Linear Fresnel plant is the ‘youngest’ of all the CSP technologies and quite nascent; hence, further research must be done for this technology to mature more [5]. Vinod Kumar et al. [29] argue that Fresnel plants have the ability to attain higher temperatures, compact size, higher thermal efficiency, and lower investment costs. This makes them suitable for hybridisation applications, combined industrial steam power generation, hydrogen production, space power applications, and DSG-type thermal power generation. The authors conclude by saying that development of techno-economic viability of supply chain management of Fresnel CSP components can help commercialise the technology, which will lower the LCOE for CSP plants.

El Gharbi et al. [22] conducted a comparative study of PTC and Fresnel CSP, using data from a site in the Hassi Rmel region of Algeria. A numerical analysis was performed to compare their performance and efficiencies. The study found that Fresnel CSP is optically analogous to PTCs. The optical quality and thermal efficiency of Fresnel CSP were found to be lower than those of PTC. This, they reasoned, was due to the higher influence of the angle of incidence and the cosine factor.

Common accessories and technologies used in concentrated solar power

Layout of the solar field for a typical concentrated solar power plant

There are three types of layouts of a solar field for a CSP plant: direct return layout, inverse return layout, and central feed and return layout. The first two types are used for small industrial processes and heat applications, while the latter is used for large applications.

The central feed and return layout are classified into two types:

• 1. H-field layout: In this type of layout, the field is divided into two header pairs, two on each side of the power block. These four header pairs are arranged in the East–West direction.

• 2. I-field layout: In this type of layout, the field is again divided into two header pairs. The power block is situated at the centre of the field. The headers are arranged in an east–west direction. [1]

The optimisation of solar field is a rather complex problem. Besides the complications of working the energy on an annual basis, the non-exhaustive list of variables to optimise requires multiple software tools to improve the performance to obtain maximum efficiency. Yiyi Zhou et al. [30] and Anass Zaaoumi et al. [31] have in their respective studies applied different optimisation techniques to improve the performance of the solar field. Reflector or mirrors used in CSPs account for 50% of the installation’s costs. Vinod Kumar et al. [29] mention that due to difficult operating conditions and environmental impacts, they degrade easily leading to a substantial drop in efficiency and reduced life cycle. Hence, they also account for high maintenance costs.

Therefore, current studies focus on optimising the design of the solar field to trap maximum amount of insolation in the cheapest way possible. This reduces both the size and cost of the solar field and consequently the CSP plant.

The solar tower technology can be used for desalination of water in co-generation systems. The heliostat field can be exploited in multiple ways for this purpose. Likewise, Spiros et al. [14] make the case that out of the large number of desalination technologies available, only few have reached a semi-commercial state that can be effectively combined with CSP operation. Currently, multi-effect desalination and reverse osmosis systems are used. The former needs electricity and heat, whereas the latter needs only electricity (Fig. 9).

Fig. 9

I-field (above) and H-field layout (below) of solar field [1]

Importance of thermal storage systems for concentrated solar power systems and the review of recent technological trends

A solar thermal power plant can operate only when there is a sufficient amount of direct solar radiation available. Solar thermal power is not dispatchable, which means that it is unable to produce and supply power on demand at the behest of power grid operators or market demands. However, solar thermal power can be made dispatchable by employing thermal storage. This way, the plant can operate independent of the solar resource, producing power even at night or during cloudy days. The extraction of heat from solar energy for generating power has the added advantage that thermal storage is possible. Solar PV plants have to employ chemical storage through batteries or fuel cells. Here, thermal storage in a solar thermal power plant is relatively cheaper than chemical storage employed in solar PV due to high investment costs and a high loss rate of 20–50%. Due to the intermittent supply of renewable energy sources, energy storage is a necessary precondition for them to seriously compete with conventional energy sources like fossil fuels [5]. The future of CSP technologies depends on coming up with a cost-effective way to store heat [32].

For TES analysis and review, Ulgo Pelay et al. [32] surveyed CSP plants and divided them into five groups depending on their time of commissioning. They have then performed a statistical analysis of the purpose of the plant (research, commercial power generation, both, etc.), the type of CSP technology, hybridisation (co-generation, gas-fired, etc.), and the type of thermal storage system incorporated. The authors have found that 47% of the current CSP plants are integrated with TES, 74% of the under construction CSP plants, and 77% of the planned CSP plants are to be integrated with TES. However, the majority of TES use sensible heat storage technology due to its dependability, low cost, and ease of implementation.

The authors have thus concluded that to make future CSP plants more economically competitive and dispatchable, TES system integration plays a crucial role. Also, latent heat and thermochemical storage systems, because of their high energy density, are more suitable for large-scale CSP plants than the current sensible heat storage systems. For this to be possible, some technological barriers such as high thermal conductivity of PCMs and thermochemical materials and removal of solid deposits from heat exchanger surfaces are necessary [32].

Alnaimat et al. [33] make a valid point as to why thermal energy storage or energy storage is almost inevitable for the sustenance of CSP technologies. According to them, conventional systems are used to supply excess power during over-demand, which causes an increase in carbon footprint. Due to the temporal intermittency of solar energy, the production side is highly non-deterministic. Similarly, the utility patterns too are non-uniform, with peaks and troughs in energy consumption by the users. The argument put forth, which is quite valid, is that an energy storage system will effectively dampen this mismatch by storing surplus energy during off-peak hours. The stored energy can then be released during the non-availability of solar energy. For CSPs, energy stored in the form of heat is the most economical and environmentally friendly.

Gil et al. [34] have summarised the following major requirements of thermal energy storage systems for CSP: energy capacity, charge and discharge heat rates, safety and environmental impact, max and min temperatures, thermal and chemical stability for thousands of cycles in contact with different materials, heat losses, and costs. Here, one can argue that small-scale CSP plants are not economically compatible with a TES. Hence, the above-mentioned requirements need to be fulfilled for a CSP to have a TES system (Table 1).

Table 1 A summary of advantages and disadvantages of different sensible heat storage thermal energy storage systems

Therminol VP-1 is the most common HTF for PTC plants. The oil, Therminol 11 VP-1, is a eutectic mixture that is capable of being sufficiently stable up to 400. However, large-scale storage of the oil in oil tanks is not economically feasible as it is quite expensive. A major problem with heat storage with PTC and oil circuit is the limited temperature range between the oil input and output temperatures (the maximum being 100 K). For molten salt-solar tower heat storage, the temperature difference is nearly 300 K. To store the same amount of heat, a molten salt-PTC system would require nearly three times the quantity of salt. Hence, an interest in low-cost heat storage systems for PTC plants is understandable. Heat storage with concrete for PTC is also being researched. Potassium nitrate, sodium nitrate, and calcium nitrate are the salts used for molten salt heat storage [5].

The use of phase change materials (PCM) in the form of latent heat storage (LHS) is a very promising technology for thermal energy storage in CSP systems. They have the capability to absorb and release large amounts of heat. Unfortunately, LHS has found restricted commercial applications due to the thermal stability of the storage material and the limited size of storage containers. Lebedev et al. [35] describe how the study of the influence of thermal cycling on the physical properties of PCMs is important for their commercialisation. Lukas Heller’s report [36] mentions that high energy density and constant temperature heat transfer are the advantages of LHS systems. The PCMs can be implemented favourably into a saturated or superheated steam power cycle, in which large amounts of heat are required at a constant temperature for evaporation of the working fluid. Hence, solar towers and PTC plants seem quite favourable for the application of PCMs.

Recent developments in receiver designs to optimise concentrated solar power plant performance

The receivers are also called ‘heat-collecting elements’. They absorb the heat of the concentrated solar radiation focused on them by the concentrators. The heat is then transferred to the HTF that flows within the receivers. The physical orientation of the receivers varies depending upon the type of concentrators used. Line-focusing collectors like the parabolic trough and the LFR make use of metal tubes (usually stainless steel) that run along the length of the collectors. Point-focusing collectors used in solar towers and parabolic dishes make use of receiver panels that cover the entire receiver area. The receiver tubes and panels are coated with an absorber material that absorbs the solar irradiation. The PTC and LFR receivers are present in an evacuated glass tube to prevent heat losses. In a solar tower, a heat loss caused by convection is stopped by an internal cavity receiver [10].

Many researchers are conducting studies to improve the performance of the receivers and absorbers in order to improve the performance of CSP systems. In one such study, Gonzalez-Portillo et al. [37] have done techno-economic optimisation of CSP systems with free-falling particle receivers. Their model provides upgrades to previous similar models, which help in the improvement in their fidelity, accuracy, and representativeness of actual systems. The results of the study conducted on a 250-m-high solar tower with 16 h of thermal storage showed a reduction of LCOE to 0.056 USD/kWh. This is a positive development as it brings researchers closer to the goal of reducing the LCOE to 0.05 USD/kWh. Conroy et al. [38] compiled literature that engaged in steady-state thermal and mathematical modelling of tubular receivers. In their study, the authors adopted contrasting approaches and indicated a general trend towards semi-empirical techniques over simplified analytical models. The semi-empirical techniques were found to be more flexible and accurate with fewer computations, allowing testing and simulation of various geometries, configurations, HTFs, materials, and operational conditions at the design stage of the receiver.

Mahdi et al. [39] in their review have summarised the analysis and findings of different research in the field of central receiver tower design in CSPs to enhance the amount of insolation absorbed. Among many studies, most dealt with solar power tower life extension, material selection, heat loss reduction, maximising thermal and optical efficiencies, and study of the effects of different variables. The summary indicated that most authors focused on the high capacity of hot HTF production to be sufficient for power conversion cycles (Figs. 10 and 11).

Fig. 10

A typical Absorber tube for a Parabolic trough concentrator [40]

Fig. 11

The receiver of the Solar One central receiver facility at Barstow, California. This is an external type receiver. [1]

Brief overview of heat transfer fluids and their applications in concentrated solar power technology

The heat transfer fluids are used in line-focusing CSP plants. They absorb heat from the solar field and transfer it to the power generation unit and the thermal storage system. Following are the probable HTFs for CSPs: air, water, molten salts, glycol-based, glycerol-based, and synthetic oils. The use of air and water as HTF in CSP is impractical. Air has very low thermal conductivity, and on top of that, it has a high specific volume. This means air will expand a lot at high temperatures, which would require large absorber tubes, consequently increasing the cost of the equipment. Water, despite its availability, non-toxicity, and impressive thermal conductivity, will react with the equipment material, leading to corrosion. Molten salts have a tendency to solidify at higher temperatures, so they are limited to use in thermal storage systems. Although many researchers, including Juan Ignacio et al. [41], have experimented with molten salt HTFs and have observed positive results. Hence, the viable options left are glycols, alcohols, and synthetic oils. Glycols and alcohols oxidise at high temperatures, leading to the formation of acids. These acids are corrosive in nature. Various corrosion inhibitors can be added to them to avoid the corrosive effect. Glycols and alcohols are slightly toxic, and alcohols are highly flammable. These disadvantages are combated while designing CSP plants [42].

Glycols are limited to CSP applications up to 175 and synthetic oils up to 450. The following are important points that highlight the selection criteria for the HTF (Tables 2 and 3):

• a. High operating temperatures,

• b. Stability at high temperatures,

• c. Low maintenance costs,

• d. Non-corrosive,

• e. low freezing point and low vapour pressure,

• f. Low viscosity.

Table 2 Physical properties of various HTFs

Table 3 Summary and short comparison of the different types of concentrated solar power systems

A review of various studies conducted on CSP technology and a discussion of various enhancements, system modification strategies adopted for better performance

Design review for CSP, PV-T plants

Table 4.

Table 4 Literature review of PTC systems

Review of studies on performance improvement strategies for solar tower power plant

Table 5.

Table 5 Literature review of solar tower systems

Review of studies on performance improvement strategies for Fresnel lens concentrator power plants

Table 6.

Table 6 Literature review of LFR systems

Review of studies on performance improvement strategies for parabolic trough concentrator power plants

Table 7.

Table 7 Literature review of PTC systems

Review of studies on performance improvement strategies for PV/T systems

Table 8.

Table 8 Literature review of PV/T systems

Gaps and objectives of the study:

Objectives of the study

• 1. To explain with factful reasoning the need for imminent energy transition to renewable energy resources.

• 2. To use reliable data to help understand why concentrated solar power is the best alternative to replace conventional thermal power plants.

• 3. To provide a brief overview and short comparison of the types of CSP along with the description of the basic accessories used in a CSP.

• 4. To conduct a thorough literature review of the different studies undertaken in the field of CSP till date.

Gaps in the study

• 1. A further detailed study of literature on co-generation systems involving CSP in combination with different renewable sources such as wind, ocean thermal, and bio-gas can also be done.

Economic overview of concentrated solar power technologies

Economics is the discipline that helps individuals and organisations make rational decisions about the allocation of scarce resources. This is done by reducing the parameters of design and the options available in monetary terms. Then, certain economic tools are employed to compare and select the best option.

While designing a powerplant, in addition to technical (capacity, peak load, etc.) and meteorological factors (seismic activity, wind speed, solar DNI, etc.), many economic factors are also considered. In the context of CSP, the initial and operating cost of the four different types of plants, the different types of receivers, layouts, etc., are the economic factors that are involved in the decision-making. In the world of renewable power generation technologies, solar thermal power generation faces stiff competition from solar PV and wind energy systems. The latter two systems are not just more technologically mature, but also cheaper than the former. Hence, economic analysis of various power generation technologies is done to determine the most economically rewarding technology for given conditions. The risk assessment and cost analysis help determine the places where cost reduction can be done to make a certain technology cheaper, hence more economically competitive [14]. CSP will grow only if it becomes accessible to the developing and underdeveloped world. For this to happen, it has to be cheaper than its conventional and renewable peer technologies. Determining the LCOE of a plant is the best way to compare different technologies to find the most economically viable alternative.

According to the US Energy Information Administration (EIA), LCOE ‘represents the average revenue per unit of electricity generated that would be needed to cover the costs of building and running a generating plant over an assumed financial life and duty cycle’.

This includes factors such as:

• a. Upfront costs,

• b. Maintenance,

• c. Degradation over time,

• d. Financing,

• e. Market shifts.

For large-scale energy developers, LCOE also acts as an effective way to compare competing sources of energy. They use it to compare solar power with wind, coal, and other energy sources.

$$ {\text{LCOE}} = \frac{{I - \mathop \sum \nolimits_{t = 1}^{n} \frac{{{\text{Dep}} \times {\text{TR}}}}{{\left( {1 + r} \right)^{t} }} + \mathop \sum \nolimits_{t = 1}^{n} \frac{{{\text{LP}}}}{{\left( {1 + r} \right)^{t} }} - \mathop \sum \nolimits_{t = 1}^{n} \frac{{{\text{Int}} \times {\text{TR}}}}{{\left( {1 + r} \right)^{t} }} + \mathop \sum \nolimits_{t = 1}^{n} \frac{{{\text{AO}}}}{{\left( {1 + r} \right)^{t} }} - \frac{{{\text{SV}}}}{{\left( {1 + r} \right)^{n} }} }}{{\mathop \sum \nolimits_{t = 1}^{n} \frac{{{\text{Initial}}\;{\text{kWh}} \times \left( {1 - {\text{system}}\;{\text{degradation}}} \right)}}{{\left( {1 + r} \right)^{t} }}}} $$

(1)

where I = initial investment, Dep = depreciation, TR = tax rate, LP = loan payment, Int = interest on loan payment, AO = annual operations cost, SV = salvage value, and r = discount rate.

Thermal energy storage makes CSP dispatchable and eases its integration with the power grid.

Other metrics for economic analysis of power plants are as follows:

• a. Energy return on investment in energy (EROEI),

• b. Payback period,

• c. Return on investment,

• d. Net present value (NPV),

• e. Internal rate of return (IRR).

The DEWA project in Dubai holds the record for the lowest LCOE at USD 73/MWh in 2017. It is a combined PTC-Tower project, with 600 MW PTC and 100 MW tower CSPs. According to a survey conducted by an independent media site, the LCOE for coal power plants in the same year ranged from USD 60 to 143/MWh. This project is a great example that, with technological innovation, CSPs are more than capable of replacing conventional thermal power plants.

Ismael et al. [7] performed an exergetic analysis and life cycle assessment on 50 MW PTC plants. The study used the eco-indicator 99 method, the cumulative energy demand method, and the specific exergy costing (SPECO) method in order to conduct a thermo-economic analysis of the systems. It was found that the solar field contributed 79% to environmental impact, the most. Steel, molten salt, and synthetic oil had the most impact of all the construction materials. The LCA reports showed that the human health damage category had the most impact with 69%, followed by the resource category with 24% and the ecosystem quality damage category with 7%.

The results also showed that the CSP technology trumped the oil-fired and natural gas-fired technologies in all the categories. The analysis also showed that within the 50 MW PTC plants, the solar field followed by the storage system had the greatest contribution impact because of the high amount of steel, molten salt, and synthetic oil used in these components. Also, the high-cost rate due to the huge initial investment in the solar field can be overcome by increasing the operation time of the CSP plant. Hence, in order to economise CSPs, optimisation for cost reduction of the solar field is of utmost importance.

Future prospects in concentrated solar power technology

Combining technological solutions with investment profitability is a critical aspect in designing both traditional and innovative renewable power plants. Often, the introduction of new advanced-design solutions, although technically interesting, does not generate adequate revenue to justify their utilisation [56].

Solar energy is an easily replenishable power source that has the ability to provide energy security and energy independence to everyone and everything. Such a proclivity is tremendously important for the socio-economic prosperity of all individuals, companies, and industries and for the conservation of the planet. In CSP, direct solar radiation is the primary source of energy. Hence, solar thermal plants would flourish at locations within the sunbelt of the Earth. The major markets are areas with a DNI of greater than 2000 kWh/m². Among the most promising areas of the world are: the South-Western USA; Central and South America; Africa; the Middle East; the Mediterranean countries of Europe; Iran, Pakistan, and the desert regions of India; the former Soviet Union; China; and Australia. The solar DNI has a direct impact on the LCOE.

In order to make CSP economically competitive, a lot of research has been undertaken to create new trends. Every aspect of the CSP plants has been thoroughly researched to devise new techniques, which have been listed below:

• 1. The improvement in solar energy collection capacity and efficiency has been the aim of many researchers. For a given solar DNI, higher collector efficiency not only increases the capacity but also reduces the solar field area. For this, anti-soil coatings (to remove dust from the concentrator surface), high reflectivity coatings (e.g. silver polymers), and stainless steel parabolic troughs are being tested.

• 2. The performance of absorber tubes and receivers is also being investigated by the researchers with the motive of increasing the absorptivity and decreasing convection losses. Borosilicate glass covers and AR coatings are employed to reduce reflection and convection losses. Hydrogen in the vacuum annulus makes a huge difference in how much heat is lost.

• 3. The use of HTF also has a huge impact on the plant. Therminol VP-1 is the most commonly used HTF. However, direct steam generation, use of molten salts and other synthetic oils are also being investigated. Molten salts are not suitable for PTC and LFR plants. DSG is ideal for solar tower and parabolic dish collectors.

• 4. Thermal storage is vital for CSP plants to make them economic. Recently, high-temperature latent heat storage has gained popularity. However, the low thermal conductivity and thermal stability of the phase change materials is a big challenge.

• 5. Various thermodynamic cycles are being experimented with to suit different operating conditions, HTFs, applications, etc. The thermodynamic cycles that can be employed are:

• a. The Brayton cycle (air and helium),

• b. Supercritical CO_2,

• c. The organic Rankine cycles,

• d. Kalina cycle.

Alfredo et al. [57] have studied the application performance by summarising and analysing the main degradation mechanisms in a CSP plant. Detection, prevention, and mitigation of degradation mechanisms such as absorber tube deformations and cracks; corrosion in the absorber tubes and the TES tanks; mirror soiling and dust accumulation; and mirror erosion and ageing are discussed by qualitative and quantitative analysis of literature in this field. The authors concluded by summarising the above-mentioned degradation mechanisms and ending by discussing the current trends in their prevention and mitigation. This study shows that CSP is getting more mature and that current industries are able to adapt to it.

Conclusions

Solar thermal power plants have the ability to increase the pace of the energy transition from conventional sources to renewables. They can quickly replace the conventional thermal power plants of the developing world, reducing carbon emissions and consequently avoiding climate change. CSP has gained prominence in recent years. However, a lot of research and policy amendments need to be done to familiarise it with the world. This is evident from the current scenario where PTC (which is less efficient than its point-focusing counterparts) dominates the market and almost 22 out of the 58 operational CSPs in the world are in Spain alone. The conclusions of this study are summarised as follows:

From the reviews, it is clear that power generation is not the only purpose of CSP. CSP can be used in co-generation and conventional plants during peak loads. In fact, its use in co-generation plants can help reduce the carbon footprint of conventional thermal power plants. CSP can also be utilised for reheating and regeneration, reducing fuel consumption and carbon emissions while also increasing the plant capacity and thermal efficiency.

The current study also shows that molten salts are the best way for CSPs to store energy for use both during the day and at night.

The concentrator field accounts for almost 40–50% of the plant's cost. Hence, its optimisation to collect as much thermal energy as possible has been the research objective of a lot of papers. This can be achieved by optimising the layout of concentrators, enhancing the performance of absorber tubes, etc.

CSP technologies that can attain higher temperatures like PTC and solar tower can be very effectively used for power generation and desalination simultaneously. This technology can help combat the water shortage crisis and conserve groundwater levels in many drought-hit areas throughout the world.

CSP can also be used with PV-T technology to extract the excess heat from the PV module for heating purposes.

This paper has tried to cover all topics relating to CSP technologies, from technical to economic. It has also conducted a comparative study of the current scenario to throw light on the probable future trends. Further research can be done in a detailed review of the energy storage technologies for CSPs, especially hydrogen fuel cells and carbon neutral synthetic fuels. All the papers reviewed in the literature survey and the review have focused on individual topics in the context of CSP technology. Those aspects could be the progress, the economics, the design, the optimisation of individual components to improve performance, etc. Keeping this in mind, an effort has been made in this paper to bring all these topics together to present a concise review of concentrated solar power technology.