Renewable and Sustainable Energy Reviews 79 (2017) 1314–1328

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Renewable and Sustainable Energy Reviews

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Progress in concentrated solar power technology with parabolic trough MARK collector system: A comprehensive review ⁎ ⁎ Wang Fuqianga,b, Cheng Ziminga, Tan Jianyua, , Yuan Yuanc, , Shuai Yongc, Liu Linhuab,c a School of Automobile Engineering, Harbin Institute of Technology at Weihai, 2, West Wenhua Road, Weihai 264209, PR China b Department of Physics, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, PR China c School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, PR China

ARTICLE INFO ABSTRACT

Keywords: Advanced solar energy utilization technology requires high-grade energy to achieve the most efficient Concentrated solar power application with compact size and least capital investment recovery period. Concentrated solar power (CSP) Parabolic trough collector technology has the capability to meet thermal energy and electrical demands. Benefits of using CSP technology Tube receiver with parabolic trough collector (PTC) system include promising cost-effective investment, mature technology, Heat transfer fluid and ease of combining with fossil fuels or other renewable energy sources. This review first covered the CSP plant theoretical framework of CSP technology with PTC system. Next, the detailed derivation process of the maximum theoretical concentration ratio of the PTC was initially given. Multiple types of heat transfer fluids in tube receivers were reviewed to present the capability of application. Moreover, recent developments on heat transfer enhancement methods for CSP technology with PTC system were highlighted. As the rupture of glass covers was frequently observed during application, methods of thermal deformation restrain for tube receivers were reviewed as well. Commercial CSP plants worldwide with PTC system were presented, including those that are in operation, under construction, and announced. Finally, possible further developments of CSP plants with PTC system were outlined. Besides, suggestions for future research and application guidance were also illustrated.

1. Introduction The sun releases a tremendous amount of radiation energy to its surroundings: 1.74×1017 W at the upper atmosphere of the earth [28]. 1.1. Why solar energy When sunlight reaches the surface of the earth, it would be multi- attenuated due to the effects of reflection, absorption, and scatter by Energy resources can be divided into three main categories: fossil carbon dioxide, water vapor, and suspensoids in the atmosphere, as fuel, renewable energy, and nuclear energy [1–3]. Fossil fuel is the shown in Fig. 1 [29–31]. The total solar radiation falling on the earth preferred energy because of its competitive price and high-energy accounts approximately 51% of the total incoming solar radiation density [4,5]. Owing the global shortage of fossil fuel supply and which is still of huge amount after multi-attenuation [32–35]. environmental problems, there is an increasing demand of searching Therefore, solar energy is a much more abundant and environmental for renewable energy [6–10]. friendly resource compared to other energy sources and the linchpin of Renewable energy is defined as energy derived from resources sustainable energy development program [36–40]. which can be naturally replenished with close-to-zero emissions of both GHG and pollutions [11–14]. Ordinarily, renewable energy utilizes the 1.2. Why concentrated solar power technology direct forms of sun's energy and its indirect impacts on the earth (falling water, wind, biomass, etc.), tidal energy, and geothermal energy Advanced solar energy technology requires high-grade energy to as the resources from which useful formats of energy are generated produce the most efficient power with compact plant size and minimal [15–20]. These resources have huge energy potential with the char- capital investment recovery period [41,42]. Concentrated solar power acteristics of intermittence, dispersal, and distinct regional variability (CSP) technology has the capability to meet the thermal energy as well [21–25]. These characteristics lead to difficulties in usage, technical as electrical demands [43–45]. and economic challenges [26,27]. For CSP technologies, the incoming sunlight is concentrated on a

⁎ Corresponding authors. E-mail addresses: [email protected] (T. Jianyu), [email protected] (Y. Yuan). http://dx.doi.org/10.1016/j.rser.2017.05.174 Received 14 February 2017; Received in revised form 18 May 2017; Accepted 20 May 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved. W. Fuqiang et al. Renewable and Sustainable Energy Reviews 79 (2017) 1314–1328

y

Glass cover Receiver HTF

PTC x

Fig. 3. Schematic of PTC with tube receiver.

2. Principle of PTC with tube receiver Fig. 1. Earth's energy budget (from NASA) [29].

2.1. Introduction of PTC with tube receiver

A PTC is a line focus solar collector that is straight in one dimension and curved as a parabolic shape in the other two dimensions, lined with high reflectivity mirrors [64–67]. The energy from solar radiation, which enters the PTC parallel to the plane of symmetry, is concentrated along the focal line, where a tube receiver is installed to receive the concentrated solar radiation [68,69]. The governing equation of a PTC shown in Fig. 3 is expressed as

x2 =4fz (1)

A single-axis tracker is employed to orient both the solar concen- trator and tube receiver toward the sun [70,71]. A metal tube coated with spectrum selective layers is placed inside an evacuated glass cover to compose a tube receiver (as shown in Fig. 3). A heat transfer fluid (HTF), such as synthetic thermal oil, molten salt, or water, flows into the tube receiver to absorb concentrated solar radiation [72]. Nickel–cadmium coatings with microstructure are commonly used as spectrum selective coating to achieve maximum solar energy absorption (short wave) and minimum infrared radiation (long wave) emittance [73]. Therefore, the heat losses of tube receiver can be significantly minimized by using spectrum selective coatings and evacuated glass covers [74]. Generally, PTC with tube receiver is arranged on a north-south direction to track the sun as it rotates from east to west to maximize the system optical efficiency. Alternatively, the PTC with tube receiver can also be arranged on an east–west direction, which lessens the system optical efficiency owing to cosine loss. However, this will only require Fig. 2. Typical solar concentrators: parabolic trough concentrator, parabolic dish adjustment of the PTC with tube receiver with variation in seasons, concentrator, linear Fresnel reflector, and heliostat field concentrator [57]. averting the requirement for tracking apparatus [68,69]. This type of tracking mode achieves the maximum efficiency in theory during the spring equinox and autumnal equinox with less precise concentration relatively small target area by mirrors or lens, and thus produces of sunlight at other times of the year. The regular sun tracking across medium to high temperature heat [46–51]. The increase in operating the sky also induces errors, maximum at sunrise and sunset, and temperature and amount of heat collected per unit area produce larger minimum at noon time. Owing to the sources of error, periodically thermodynamic efficiency and smaller absorbing surface area, which adjusted parabolic trough collectors are ordinarily planned with a lower results in significant decrease of convective and conductive heat losses concentration acceptance product [75]. [52–56]. According to the focus geometry and receiving technology, PTC with tube receiver has an unsophisticated structure; however, its solar concentrators can be divided into four classifications, namely concentration ratio is only one-thirdofthemaximumvalueintheoryfor parabolic trough concentrator (PTC), parabolic dish concentrator, the same acceptance angle (107.3), that is, for the same overall tolerances of linear Fresnel reflector, and heliostat field concentrator, as shown in the system to all types of errors, including tracking error, pointing error, Fig. 2 [57–60]. surface error, and alignment error. The maximum value in theory is better Benefits of using CSP technology with parabolic trough collector obtained with more sophisticated concentrators, based on primary–sec- system include promising cost-effective investment, mature technol- ondary designs using non-imaging optics, which could almost double the ogy, abundant operational experience, ease of coupling with fossil fuels concentration ratio of conventional PTCs and is adopted to enhance and other renewable energy sources [61–63]. applications, such as those with fixed receivers [73,76].

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n =2a /f (8) The relationship between the maximum theoretical geometric W concentration ratio, CRmax, and the relative aperture n for a PTC can f be expressed as

Ac nf W R α CR == =2 tan AfocW f (9) R In the equation above, the symbol A represents the aperture area of α c the PTC, and the symbol A denotes the area of the solar image on the b foc focal plane of the PTC. f ψ The expression of the parameter height (b)inFig. 4 is expressed in terms of the relative aperture (n) and focal length (f): a ⎛ ⎞ a2 (/2)nf 2 n2 bf=− =−f =1−f ⎜ ⎟ 4f 4f ⎝ 16 ⎠ (10) y Therefore, the relationship among the parameters R, ψ, f, and n follow the following expressions: Fig. 4. Schematic diagram of solar ray imaging on focal plane of parabolic concentrator ⎛ ⎞2 ⎛ ⎞ system. nf2 n2 Rab=+=−22 ⎜ f ⎟ +(nf /2)2 = f ⎜ 1+ ⎟ ⎝ 16 ⎠ ⎝ 16 ⎠ (11) 2.2. Theoretical maximum concentration ratio of PTC ⎛ 22⎞ ⎛ ⎞ ⎜ nn⎟ ⎜ ⎟ Theoretically, any solar image generated by concentrating systems cosψbR = / = 1 − /1+ ⎝ 16 ⎠ ⎝ 16 ⎠ (12) has a particular size, which depends on the geometry of the concen- trating system and the perspective of solar energy [77]. In this By substituting Eqs. (11) and (12) into Eq. (9), the relationship research, the detailed derivations for the values of relative aperture between the theoretical maximum concentration ratio (CRmax) of a PTC (n), rim angle (ψ), and the maximum geometrical concentrating ratio and the relative aperture (n) is expressed as in theory are given when the parabolic trough solar collector has the ⎛ 2 ⎞ maximum geometrical concentrating ratio. 107.3n⎜ 1 − n ⎟ nf nψcos (1 + cos ψ ) ⎝ 16 ⎠ Fig. 4 presents the schematic diagram of an image focused by a CR == = W 4tanα ⎛ ⎞2 parabolic trough/dish solar concentrator with a focal length f.As f n2 ⎜1+ ⎟ ⎝ 16 ⎠ observed in the figure, the solar image size (Wf) on the focal plane, (13) which is concentrated by a parabolic concentrator, varies with the ⎛ ⎞ radius (R) and focal distance (f), and the relations can be expressed as Based on the differentiation formula, ⎜ ux()⎟′= uxvxuxvx′( ) ( )− ( ) ′( ), the ⎝ vx()⎠ vx2() follows [78]: solution to the differential equation is W 2tanRα W = = dCR 107.3(nn42 − 96 + 256) f cos ψ cos ψ (2) = ⎛ ⎞3 dn n2 α 256⎜ 1 + ⎟ where is the non-parallelism of sunlight with the value of 16´. ⎝ 16 ⎠ (14) Based on the expression for the PTC (Eq. (1)) and using trigono- dCR At =0, the theoretical maximum concentration ratio (CRmax)ofa metric function, the parameters, width (a) and height (b), vary with the dn radius (R) and rim angle (ψ)as PTC is 107.3, where the value of the rim angle ψ is 44.9° and the value of the relative aperture (n) is 1.6569 [78]. Similarly, the theoretical aR=sin ψ maximum concentration ratio (CRmax) of a parabolic dish concentrator bR=cos ψ (3) is 11513(107.32) with the relative aperture (n) is 1.6569 and the rim Ψ Combining Eqs. (1) and (2) yields angle is 44.9°.

2 (sin)=4(−cosRψ ffRψ) (4) 3. Heat transfer fluid for PTC with tube receiver Substituting the expression sin22ψ = 1 − cos ψ into Eq. (4),we 3.1. Synthetic thermal oil obtain:

(1−cos22ψR ) +4 f cos ψR −4 f2 =0 (5) HTF is a crucial factor in a solar thermal power plant as it directly influences the tube receiver efficiency, determines the type of thermo- fi The nal solution R of the quadratic equation above is therefore dynamic cycle and the performance it can acquire, as well as the 2fψ (− cos + 1) 2f thermal energy storage technology that must be adopted. Till now, the R = = 1−cos2ψ 1+ cosψ (6) operating temperature of a solar thermal power plant is mainly limited by the thermal stability of the HTF flowed in the tube receiver. By substituting the solution R into Eq. (2), the solar image size (Wf) Synthetic thermal oil is commonly used as an HTF in the tube on the focal plane can be rewritten as receiver of a CSP plant with PTC system, most widely known under the 4tanfα brand names Therminol VP-1, Therminol D-12 and Dowtherm A Wf = [79,80]. Synthetic thermal oil is a proven HTF used in SEGS plants cosψψ (1 + cos ) (7) in California, since two decades without any major incidents. However, The aperture area of the PTC is a key factor in determining the hydrogen build up in the vacuum annulus of the glass cover, which had overall absorbing energy, which is usually expressed by the relative significantly increased the heat losses of the tube receivers, was aperture n defined as discovered at numerous absorber tubes of the SEGS plants. The

1316 W. Fuqiang et al. Renewable and Sustainable Energy Reviews 79 (2017) 1314–1328 hydrogen was generated from the degradation of HTF [81]. with parabolic trough collector system, an experimental facility had It should be noted that adopting synthetic thermal oil as HTF limits been set up in Italy [93]. Their operation experiences indicated that it the upper temperature of the thermodynamic cycle to 400 °C, and thus was practical to use molten salt as HTF for a CSP plant with parabolic the achievable efficiency is limited to approximately 38%. When trough collector system. However, very sophisticated operations of the synthetic thermal oil being operated above 400 °C, the hydrocarbons parabolic trough collector system need to be performed to prevent breakdown quickly and generate hydrogen, which would reduce the molten salts from freezing. In addition, Abengoa Solar is also devel- overall HTF lifetime and induce buildup of sludge or other byproducts oping a test loop to evaluate the adoption of certain molten salts as that reduce the system heat transfer efficiency and increase main- HTF, which allows the CSP plant with PTC system to operate at 500 °C tenance cost [82]. [85]. Molten salts can exhibit superior thermophysical properties at high 3.2. Water/steam temperatures: high density, large specific heat capacity, high thermal stability, and very low vapor pressure [94]. Furthermore, molten salts When a higher operating temperature is required, the state-of-the- also exhibit superior price advantage and environmental impact than art HTF needs to be substituted. In addition to the higher operating synthetic thermal oils: abundant resources with low cost, no pollution, temperature, there are three key challenges to be addressed for the next and nonflammable [95]. The main disadvantage of molten salts is the generation of HTFs for CSP plants with PTC systems: high melting point which results in operation and maintenance costs for freeze protection. Commercially used synthetic thermal oils in CSP (1) Allowing for simple and safety operation, plants with parabolic trough collector system freezes at approximately (2) Allowing for simple storage concept, 15 °C, whereas ternary and binary molten salts freeze in the tempera- (3) Low cost without toxins. ture range of 100–230 °C [96]. During operation and particularly non- operation hours, operators need to ensure that no molten salt freezing As steam is used in the Rankine cycle for power plants, it is obvious phenomenon occurs in a CSP plant with parabolic trough collector that water is used as the HTF in the tube receiver of solar parabolic system. The molten salt freezing phenomenon can severely damage the trough collector systems, in which process water is directly evaporated valves, ball-joints, and pumps [97]. in the tube receivers [83]. This so called direct steam generation (DSG) has been researched in detail in Europe in the past two decades [84]. 3.4. Pressurized gases Abengoa Solar had built the largest parabolic trough CSP plant with

DSG technology, which opened in the spring of 2009 at the Solucar Pressurized gases, such as air, He, CO2, and H2, have been proposed Platform [85]. DSG technology in CSP plants with parabolic trough to be used as HTFs for CSP plants with PTC system in recent years [98– collector system eliminates the demand for an intermediate HTF. 100]. The main advantages of using pressurized gases are the wide Liquid water flows through the tube receiver and absorbs the incoming range of operating temperature, abundance of resources with low cost, concentrated solar energy, and then the water undergoes a phase and environment-friendly properties [101]. change from liquid to saturated steam and finally to superheated To reduce the pressure loss of a PTC field, Rubbia [102] proposed to steam. subdivide the collector field into several subfields with a thermal The main benefit of DSG technology compared with existing capacity of 7 MW to 10 MW. Each subfield transfers the heat via a commercial plants which adopt synthetic thermal oil as HTF is that it heat exchanger to the molten salt loop. The final collection, transport, can get rid of maximum temperature limitations, thus increasing the and storage of thermal energy are conducted with this salt loop. Thus, CSP plant efficiency [86]. Furthermore, the experiences obtained at the the two-circuit system for a PTC with tube receiver is increased to a DISS test facility at the Plataforma Solar de Almería on the operation three-circuit system (pressurized gases→molten salt→water/steam) using DSG technology have presented that DSG is feasible in CSP with corresponding increase in complexity. Moreover, the two required plants with PTC system [87]. However, the operation using DSG heat exchangers would also induce additional energy losses and cost technology needs more sophisticated control strategies and concepts. increase. The exergetic and the energetic performance analyses of parabolic 3.3. Molten salt trough collector system conducted by Evangelos et al. [102] indicated that pressurized gases are the only solutions for temperatures greater One of the principal goals of the next generation HTF is to reduce than 1100 K. However, using pressurized gases as HTF for CSP plants the cost of CSP plants with PTC system. Researchers worldwide are with PTC system needs to overcome major challenges, such as low heat conducting several studies on the development of advanced fluids that transfer coefficient and high pumping power consumption due to the work at higher operating temperatures, thereby increasing the cycle high pressure required. efficiency without sacrificing other important parameters that would increase the cost or the plants’ own energy consumption [88]. Molten 4. Heat transfer enhancement of PTC with tube receiver salts are salt mixtures, mainly nitrates, which can be used as HTF in solar thermal applications owing to their chemical characteristics. 4.1. Nanofluids Sodium–potassium nitrate has been widely used in CSP technology with salt composition expressed in mass fractions as 60 wt% NaNO3 + The conventional HTF for CSP plant with PTC system, such as 40 wt% KNO3 [89]. However, the heat transfer characteristics of synthetic thermal oil, water, pressurized air, even molten salt, have molten salts depend on the molten salt composition [90]. relatively low thermal conductivity and thus cannot achieve high Since 1996, the Solar Two tower power plant was operated in the thermal performance in tube receiver and heat exchanger. An effective US while molten salt was adopted as HTF. Even at the increased method to overcome this barrier is using nano-sized particles (1– operating temperature of the tower receiver and the relatively compact 100 nm) suspended in HTF to improve their effective thermal con- size of the system, the operation of the molten salt system was still a ductivity [103]. The commonly used nanofluids in applications contain challenge for the operation personnel [91]. The challenge of operating the following nanoparticles: Al2O3, ZnO, Cu, Au, TiO2, Al, Fe2O3, CuO the molten salt system will increase significantly for CSP plants with and SiO2 [104–106]. PTC system because of the lower operating temperature and increased Coccia et al. [107–109] had performed numerical analyses on the dimension of the collector field [92]. With the aim of determining the evaluation of the annual yield of a low-enthalpy PTC with tube receiver. feasibility of using molten salt as HTF in tube receiver of a CSP plant With the aim of enhancing the thermal performance of a PTC with tube

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Table 1 Maximum outlet fluid temperature and corresponding thermal efficiency for the cases 40, 50, 60 and 70 °C at 0.5 kg/s [107–109].

HTF 40 °C 50 °C 60 °C 70 °C

Tfu,max (°C) η (Tfu,max)(%) Tfu,max (°C) η (Tfu,max)(%) Tfu,max (°C) η (Tfu,max)(%) Tfu,max (°C) η (Tfu,max)(%)

H20 47.79 63.14 57.73 62.73 67.67 62.30 77.61 61.85

TiO2 1 wt% 47.85 63.14 57.80 62.74 67.74 62.30 77.67 61.85

TiO2 10 wt% 48.49 63.12 58.43 62.71 68.36 62.28 78.29 61.83

TiO2 20 wt% 49.33 63.07 59.26 62.67 69.19 62.24 79.11 61.79

TiO2 35 wt% 50.94 62.96 60.87 62.55 70.78 62.13 80.69 61.68

SiO2 1 wt% 47.85 63.14 57.79 62.73 67.73 62.29 77.66 61.84

SiO2 5 wt% 48.09 63.13 58.04 62.72 67.97 62.29 77.90 61.83

SiO2 25 wt% 49.60 63.03 59.53 62.61 69.45 62.18 79.37 61.73

Fe2O3 5 wt% 48.13 63.12 58.07 62.71 68.01 62.28 77.94 61.83

Fe2O3 10 wt% 48.50 63.09 58.44 62.68 68.38 62.25 78.30 61.80

Fe2O3 20 wt% 49.36 63.02 59.30 62.62 69.22 62.19 79.14 61.74 ZnO 1 wt% 47.86 63.14 57.80 62.73 67.74 62.30 77.67 61.85 ZnO 5 wt% 48.14 63.13 58.8 62.73 68.02 62.29 77.95 61.84 ZnO 10 wt% 48.53 63.12 58.47 62.71 68.40 62.28 78.33 61.83

Al2O3 0.1 wt% 47.79 63.14 57.74 62.73 67.68 62.30 77.61 61.85

Al2O3 1 wt% 47.85 63.14 57.79 62.73 67.73 62.30 77.67 61.85

Al2O3 2 wt% 47.91 63.13 57.85 62.73 67.79 62.30 77.73 61.85 Au 0.1 wt% 47.79 63.15 57.73 62.74 67.67 62.31 77.61 61.86

receiver, six types of water-based nanofluids at different weight performances of a PTC with tube receiver using Al2O3/synthetic concentrations were numerically and experimentally studied—Fe2O3 thermal oil nanofluids as HTF with non-uniform heat flux distribu- (5, 10, and 20 wt%), SiO2 (1, 5, and 25 wt%), TiO2 (1, 10, 20, and 35 wt tions. The numerical results indicated that the temperature gradient %), ZnO (1, 5, and 10 wt%), Al2O3 (0.1, 1, and 2 wt%), and Au (0.01 wt and deformation of the tube receiver decreased with the nano-particle %). Two prototypes of PTC with tube receiver were set up on the roof of concentrations, and the deformation of the tube receiver decreased DIISM (Department of Industrial Engineering and Mathematical from 2.11 mm to 0.54 mm when the volume fraction of nanoparticle Sciences) located in the city of Ancona (central Italy) [107–109]. The increased from 0 to 0.05. convective heat transfer coefficient of nanofluids in the tube receiver Taylor et al. [114] had evaluated the economic performance of was gauged through a dedicated apparatus. As shown in Table 1, the using nanofluids as HTF for a solar thermal power plant by theoretical investigations conducted by Coccia et al. [107–109] indicated that only method, and declared a $3.5 million increase in annual revenue for a

Au, TiO2, ZnO, and Al2O3 nanofluids at lower concentrations exhibit 100 MW CSP plant when nanofluids were used. Alibakhsh et al. [115] very small overall thermal performance improvements, while increas- regarded that utilizing nanofluids in solar systems can result in many ing the concentration of nanoparticles had very small heat transfer environmental and economic benefits such as CO2 emission reduction performance improvement. through enhancing the thermal efficiency of tube receiver. Khullar et al. [110] had adopted Al nanoparticles with 0.05% by It should be pointed out that utilizing nano-fluid as HTF in tube volume suspended in Therminol VP-1 as HTF for the tube receiver of a receiver for solar PTC systems would increase the quality requirement PTC. They had numerically researched the heat transfer performance of for valves and pumps as well as aggravate abrasion of tube receiver and nanofluid-based PTC with tube receiver and found that the thermal other components. Therefore, the literature survey indicated that efficiency can be improved by approximately 5–10% by adopting nanofluids had not been used for application in CSP plants [116]. nanofluids. This is based on the comparison of experimental results with those of conventional concentrating parabolic solar collectors 4.2. Inserts for tube receiver operating under similar conditions. A 3D, fully developed, and turbulent mixed convection heat transfer In recent years, studies on tube receiver inserts for parabolic trough of Al2O3/synthetic thermal oil nanofluid in a PTC with tube receiver solar collector systems, such as metal foams, plates, and porous discs, was numerically investigated by Sokhansefat et al. [111], using a non- on the thermal efficiency enhancement were extensively conducted. uniform heat flux as boundary condition. The numerical results The mechanism can be explained from the following three aspects: (1) indicated that the effect of nanofluid concentrations on the average disturbing of boundary layer to decrease the thermal resistance, (2) heat transfer coefficient at the operating temperature of 400 K in- increasing of turbulence intensity by fluid mixing augmentation, and creased by 8.6% with the addition of 5% by volume of Al2O3 (3) increasing the effective thermal conductivity of fluid by the high nanoparticles. area density and thermal conductivity of the inserted material. A forced convection heat transfer turbulent fluid flow inside the A 3D numerical model of a PTC using a tube receiver by inserting with tube receiver of a parabolic trough solar collector was numerically metalfoaminsertwassetupbyWangetal.,asshowninFig. 5 [117].A researched by Seyed et al. [112], using CuO–water and Al2O3–water realistic non-uniform heat flux distribution and experimentally measured nanofluids as HTF. The effects of nanoparticle volume fraction on the physical properties of three different porous medium were used to precisely thermal performance of the parabolic trough solar collector with tube represent the heat transfer characteristics in the superheated section of receiver were investigated. The numerical results indicated that using DSG in the tube receiver of a parabolic trough solar collector system. The nanofluids as HTF can enhance the thermal efficiency of a parabolic numerical investigations indicated that the Nusselt number of the tube trough solar collector with tube receiver effectively compared with receiver with metal foam insert was 400–700 times that of a conventional using pure water as HTF. The heat transfer coefficient of the tube tubereceiverandtheperformanceevaluationcriteriaforthetubereceiver receiver was found to increase up to 28% and 35% by using Al2O3– of the parabolic trough solar collector system ranged from 1.1 to 1.5. water and CuO–water nanofluids at 3% by volume, respectively [112]. Moreover, the maximum circumferential temperature difference on the A multi-field coupling simulation based on finite element method outer periphery of the tube receiver decreased by up to 45%, which can was developed by Wang et al. [113] to study the thermal and structural greatly reduce the thermal deformation.

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Fig. 7. Schematic diagram of tube receiver with perforated plate insert developed by Fig. 5. Diagram of parabolic trough collector using tube receiver with metal foam insert Mwesigye et al. [120,121]. [117]. types of tube receiver were obtained in the range of 63.9–66.66% under 2 Experimental investigation of a 15 m parabolic trough solar the ASHRAE standard experiment condition. The experimental results collector with porous disc-enhanced tube receiver was conducted by showed that the overall thermal performance of the parabolic trough Reddy et al. [118] to improve the thermal performance. Two conven- solar collector with porous disc-enhanced tube receivers was consider- tional tube receivers and four porous disc-enhanced tube receivers (as ably higher than that of conventional tube receivers: the efficiency of shown in Fig. 6) were developed to compare their thermal perfor- BPDR, IBPDR, and APDR increased from 66.18% to 66.80%, 67.62– mance. The obtained thermal efficiencies of the PTC with six different 68.13%, and 67.96–68.38%, respectively, for a flux increase from

Fig. 6. Photographs of porous discs developed by Reddy et al. [118]: (a) BPDR, (b) UBPDR, (c) IBPDR, and (d) APDR.

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500 W/m2 to 900 W/m2 [118]. Ghasemi et al. [119] also had numeri- cally investigated the inserting of porous disc on overall thermal performance of parabolic trough solar collector system. Their numer- ical results presented that the heat transfer characteristic increases by increasing the inner diameter of the porous discs and by decreasing the distance between porous rings. A numerical research of the thermal and thermodynamic perfor- mance of a PTC using a tube receiver with perforated plate insert (as shown in Fig. 7) was conducted by Mwesigye et al. [120,121]. The analyses were performed for different perforated plate geometrical parameters including dimensionless plate orientation angle, dimen- sionless plate spacing, and dimensionless plate diameter. The numer- ical analyses indicated that the thermal efficiency of the parabolic trough solar collector with tube receiver can be increased up to 8% by inserting a perforated plate in the tube receiver. With the aim of enhancing heat transfer and reducing the size of tube receiver, parabolic trough solar collectors were researched experi- mentally and numerically by Hacı et al. [122]. Their investigated results indicated that the heat transfer enhancements of a parabolic trough solar collector with tube receivers inserted with coiled wire turbulators were 2.28, 2.07, and 1.95 times higher compared to those of conventional smooth tube receiver for pitch distances of 15, 30, and 45 mm, respectively. It should be noted that the usage of inserts for tube receiver to fi Fig. 8. Diagram of unilateral milt-longitudinal vortex-enhanced parabolic trough solar enhance the heat transfer performance of PTR would sacri ce the receiver [124]. pressure drop of tube receiver. Therefore, it is necessary to use overall heat transfer performance factor which can consider both the heat increase the overall heat transfer performance (as shown in Fig. 9). A transfer enhancement and pressure drop to conduct a comprehensive multi-field sequential coupled method was put first forward by the heat transfer enhancement evaluation of the inserts for tube receiver. authors to study the heat transfer performance and thermal deforma- tion of a tube receiver for a parabolic trough solar collector system. The 4.3. Geometry structural improvement for tube receiver variations of the overall heat transfer performance for SCPTR and ACPTR with increase of Reynolds number at different critical geometry Currently, numerous geometry structural improvement techniques parameters were calculated by the authors and the results are for tube receiver have been developed with the aim of increasing the presented in Fig. 10. As shown in this figure, the introduction of overall heat transfer performance. Huang et al. [123] had introduced a symmetric and asymmetric outward convex corrugated metal tube as dimpled tube as the metal tube of tube receiver for a PTC system to the metal tube of tube receiver for a parabolic trough solar collector produce substantial heat transfer surface augmentations with relatively system can enhance the overall heat transfer performance effectively small pressure drop penalties. They had numerically studied the and the maximum enhancement of the overall heat transfer perfor- thermal performance characteristics of the parabolic trough solar mance factors were 135% and 148%, respectively. 4 collector with dimpled tube receiver at a Reynolds number of 2 × 10 The authors of this study had also proposed using a metal tube with ff 10 and di erent Grashof numbers ranging from 0 to 3.2 × 10 . Compared pin fin arrays as the absorber tube for a parabolic trough receiver to fl with forced convection under non-uniform heat ux distribution, the increase the overall heat transfer performance of the tube receiver. friction factor and Nusselt number of mixed convection in a deeply Sketches of the parabolic trough solar collector with pin fin arrays 9 – 10 dimpled tube receiver (d = 7 mm) at Gr = 10 3.2 × 10 increased by inserted in the tube receiver (PFAI-PTR), proposed by the authors, are – – 1 34% and 1.0 21%, respectively, while those in the tube receiver with shown in Fig. 11. The numerical results indicated that using a metal – – shallow dimples (d=1 mm) increased by 1 28% and 1.0 18%, respec- tube with pin fin arrays as the absorber tube for a parabolic trough tively [123]. receiver can effectively increase the heat transfer performance: the Cheng et al. [124] had proposed a novel parabolic trough solar tube average Nusselt number can be increased up to 9.0% and the overall receiver, named unilateral milt-longitudinal vortex-enhanced parabolic heat transfer performance factor can be increased up to 12.0% (as trough solar receiver, where longitudinal vortex generators were only shown in Fig. 12) [127]. located on the side of the tube receiver with concentrated solar Essentially, the usage of geometry structural improvement techni- radiation (as shown in Fig. 8). The unilateral milt-longitudinal ques for tube receiver to enhance the heat transfer performance of PTR vortex-enhanced parabolic trough solar tube receiver and the corre- would also sacrifice the pressure drop of tube receiver. Therefore, sponding parabolic trough receiver with smooth tube receiver were overall heat transfer performance factor is more reasonable to evaluate fi numerically investigated by coupling nite volume method and Monte which type of geometry structural improvement technique should be fi Carlo ray tracing method for comparison and veri cation from the used for application. viewpoint of field synergy principle. The numerical results indicated that both the averages of wall temperature and thermal loss decrease with the increase of the Reynolds number. The thermal loss of the 5. Thermal deformation restrain of tube receiver unilateral milt-longitudinal vortex-enhanced parabolic trough solar receiver reduced by 1.35–12.10% compared with that of the corre- The PTC with tube receiver is one of the mature solar technologies sponding parabolic trough receiver with smooth tube receiver within for thermal power generation. During application, the parabolic trough the range studied [124]. collectors concentrate the incoming sunrays on the bottom periphery of Symmetric and asymmetric outward convex corrugated tubes were the tube receiver, while the top periphery is subjected to solar introduced by Wang et al. [125,126] as the metal tube of tube receiver irradiation with low energy density. The PTC with tube receiver needs for a parabolic trough solar collector system (SCPTR and ACPTR) to to run under extremely non-uniform heat flux, cyclic weather condi-

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Fig. 9. Partial sectional view of structured hexahedral meshes for SCPTR and ACPTR put forward by the authors [125,126].

1.16

1.12

1.08

1.04

η 1.00 PTR 0.96 PFAI-PTR, δ /L=0.1000 PFAI-PTR, δ /L=0.0625 PFAI-PTR, δ /L=0.0500 0.92 PFAI-PTR, δ /L=0.040 PFAI-PTR, δ /L=0.040 0.88 1500 3000 4500 6000 7500 9000 10500 12000 Re

Fig. 12. Variation of overall heat transfer performance factor (η) with increase of Fig. 10. Variation of overall heat transfer performance for ACPTR (p/D = 5.8, H/D = Reynolds number (Re) for PFAI-PTR at different δ/L values [127]. 0.06) with increase of Reynolds number at rl/D values [125,126].

tions, and cloud transient cycle conditions, which can lead to high temperature gradients [128,129]. The large thermal strain, induced by large temperature gradients, can cause thermal deformations of the absorber tube and glass envelope. Wang et al. [123,124,130] was the pioneer to develop the multi- field sequential coupled method to study the heat transfer performance and thermal deformation of a tube receiver for a parabolic trough solar collector system. Their investigations indicated that the dimensionless von-Mises thermal strain on the outer surface of the metal tube was approximately twelve times of that on the outer surface of the glass cover [125,126,131,132]. Owing to the large differences of the thermo- physical and structural properties between metal and glass, the thermal deformation differences between the absorber tube and glass cover can induce rupture of the glass cover, which will lead to the increase of heat loss. Fig. 13 shows the rupture of a glass cover due to large thermal deformation of tube receiver of a solar PTC system [133]. According to the reported data, 30–40% failures of parabolic trough tube receiver occurred at SEGS VI–IX for 9–11 years of operation, which were mainly caused by the extremely non-uniform heat flux on the periphery of the tube receiver [134–136]. Furthermore, in the solar power plant of the National University of Mexico, the stainless steel tube receiver of solar PTC system had experienced frequent deflection and glass envelope rupture during the experimental test and application [137– 139]. Currently, the methods of thermal deformation restrain of tube receiver for solar PTC systems can be classified into four categories: (1) heat transfer enhancement, (2) optical design for uniform heat flux

Fig. 11. Sketches of absorber tube with pin fin arrays inserted (PFAI-PTR) put forward distribution, (3) structural improvement of metal tube, and (4) by the authors [127]. adopting advanced materials.

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z

b

Tube receiver Glass cover

-a o a x

Fig. 13. Typical picture of a ruptured glass cover due to thermal deformation of tube receiver [133].

Heat transfer enhancement can achieve uniform temperature distribution of the metal tube and in turn reduce the thermal deformation of the tube receiver. The methods of heat transfer -b enhancement for parabolic trough solar collectors with tube receiver Concentrated sunlight had been described in the previous section and will not be explained further in this section. Fig. 15. Schematic diagram of glass cover with elliptic-circular cross section put forward by the authors [142].

5.1. Optical design for uniform heat flux distribution in Fig. 15. The numerical results indicated that adopting a glass cover with elliptic-circular cross section for tube receiver can reduce the heat A new type of secondary reflector was designed by He et al. [140] as flux gradient effectively, and the peak heat flux reduction was up to a homogenizing reflector for a parabolic trough solar collector with 32.3%. tube receiver to homogenize the heat flux distribution, which in turn The essence of advanced optical designs for PTR is to homogenize can improve the reliability of the tube receiver. The schematic of the the heat flux distribution on the tube receiver, which in turn to homogenizing reflector cross section designed by He et al. [140] is minimize the temperature gradient and reduce the thermal strain shown in Fig. 14. Sunlight transmission and concentration problems and deformation. However, the homogenizing reflectors and glass were solved using the Monte Carlo ray tracing method. Then, the covers with elliptic-circular cross section are hard to manufacture. coupled heat transfer process within the parabolic trough tube receiver These odd-shaped optical components can significantly increase the was simulated by treating the calculated heat flux distribution as cost of PTR during application process, and have not been applied yet. boundary condition for the finite volume method model. Diogo et al. [141] had used a secondary reflector as a homogenizing reflector for the PTC with the aim to homogenize the solar flux 5.2. Geometry structural improvement for tube receiver distribution, which in turn can decrease the thermal stress on the tube receiver of a parabolic trough solar collector system. The stainless steel tube receiver with solar parabolic trough As already known, the PTC reflects the radiant energy from the sun, collectors had experienced frequent deflection and glass envelope concentrates it on the bottom periphery of the glass cover (or glass rupture during the experimental test and application at the solar power envelope), and transmits the concentrated solar energy to the tube plant of the National University of Mexico. Therefore, Almanza and – receiver. Owing to the vital function of the glass cover, Wang et al. Flores [137 139] had proposed a compound copper-steel type tube [142] had adopted the Monte Carlo ray tracing method to analyze the receiver, which was composed of two parts, as shown in Fig. 16. The fi effects of the glass cover on the heat flux distribution. Moreover, a glass internal tube strati ed was made of copper to achieve a superb thermal cover with an elliptic–circular cross section was put forward by Wang performance and decrease temperature gradients, and the external fi et al. [142] with the aim of minimizing the heat flux gradient, which in tube strati ed was made of steel to strengthen the tube receiver. The turn can decrease the thermal stress of the tube receiver. The schematic compound wall copper-steel tube receivers had been applied in the diagram of the glass cover with elliptic-circular cross section is shown

Fig. 16. Compound copper-steel type tube receiver proposed by Almanza and Flores Fig. 14. Schematic of homogenizing reflector cross section designed by He et al. [140]. [137–139] for the solar power plant of the National University of Mexico.

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Fig. 18. Relationship between eccentricity variation and peak thermal stress values [131].

Fig. 17. Schematic diagram of physical domain and coordinate system for the eccentric tube receiver put forward by the authors [131]. 5.3. Advanced materials for tube receiver

In the solar power plant of the National University of Mexico, the stainless steel tube receiver for the PTC deflected as a wave during the solar power plant of the National University of Mexico [137–139]. The experimental tests that examined the receiver behavior. When a copper experimental test results presented that, when operated at low tube receiver was used instead of a steel one, no appreciable bending pressure, a large thermal shock was generated, which induced thermal was observed during application [137–139]. Low cycle fatigue tests of deformation in the steel receiver causing a deflection of over 50 mm, materials for the tube receiver were performed at different tempera- whereas the maximum deflection measured was only 10 mm in the tures by Lata et al. [143]. The experimental results indicated that high compound copper-steel type tube receiver. nickel alloys exhibited superb thermo-mechanical properties compared An eccentric tube receiver was proposed by Wang et al. with the aim with austenitic stainless steels. of reducing the thermal stresses [131]. Fig. 17 illustrates the diagram Thermal stress analyses of tube receiver for solar PTC system under of the eccentric tube receiver for a solar PTC system. The eccentric tube concentrated solar irradiation condition with four different materials receiver was put forward based on the concentric tube receiver. As were numerically studied by Wang et al. [132]. The numerical results observed in this figure, the center of the internal cylinder surface of the indicated that the temperature gradients and effective stresses of concentric tube receiver was moved upward (or at other directions) at a stainless steel and silicon carbide (SiC) materials were much higher different location from the center of the external cylinder surface. than those of aluminum and copper materials. The stress failure ratio Therefore, the wall thickness of the bottom half section of tube receiver was introduced by the authors to assess the thermal stress level for would increase without adding any mass to the entire tube receiver. each material. As shown in Fig. 19, stainless steel has the highest stress The increase of wall thickness would not only strengthen the tube to failure ratio whereas copper has the lowest value. The numerical results increase the resistance to thermal stress, but also enlarge the thermal obtained by Wang et al. [132] were consistent with the experimental capacity, which in turn will reduce the non-uniform temperature results tested in the solar power plant of the National University of distribution condition. Ray-thermal-structural sequential coupled nu- Mexico. merical analyses were the pioneer to develop by Wang et al. [131] to In the future, inexpensive metal materials with superb thermo- obtain the concentrated heat flux distributions, temperature distribu- tions, and thermal stress fields of both the eccentric and concentric Aluminum tube receivers. As shown in Fig. 18, the numerical results indicated that 90 16 SiC adopting the eccentric tube as the metal tube of the PTC system can 120 60 Copper reduce the effective thermal stress up to 41.1%. 12 Stainless Steel The symmetric and asymmetric outward convex corrugated tubes proposed by Wang et al. [125,126] as the metal tube of tube receiver for 150 30 8 a parabolic trough solar collector system can increase not only the overall heat transfer performance, but also decrease the thermal 4 deformation of the tube receiver. The optical-thermal-structural se- quential coupled analysis indicated that the maximum von-Mises 0 180 0 thermal strain of the metal tube of SCPTR and ACPTR were smaller than that of the metal tube of conventional tube receiver at all Reynolds 4 number conditions. Using symmetric and asymmetric outward convex corrugated metal tube to replace the smooth tube as tube receiver for Stress Failure Ratio (%) 8 210 330 solar parabolic trough system can decrease the von-Mises thermal strain up to 26.8% [125,126]. 12 Although the compound copper-steel type tube receiver had been 240 300 tested in the solar power plant of the National University of Mexico, no 16 literature survey showed that the concept of geometry structural 270 improvement for tube receiver had not been widely used for CSP Fig. 19. Stress failure ratio profiles of different materials across the circumference on plants in operation yet. the inner surface of tube receiver at the outlet section [132].

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Raumfahrt (DLR) are presented as follows: DNI of economic potential based on measured values should be larger than1800 kW h/m2 per annum, and DNI of technology potential based on measured values should be larger than 2000 kW h/m2 per annum. The most favorable global horizontal irradiation for power plants with CSP technology are found between 151°N to 40°S and occasion- ally at higher latitudes as depicted in Fig. 21 [153]. The regions covered major areas in North Africa, southern Africa, Middle East, western India, the southwestern United States, Mexico, places in South America, and Australia. Other suitable areas are in the extreme south of Europe and Turkey, central Asian countries, and western China. Owing to the cheapest option for utility scale solar electricity production, the use of parabolic trough collectors for electricity production is the most developed in CSP technology. At present, Fig. 20. Working principle of a concentrated solar thermal power plant [145]. several CSP plants with PTC technology are in operation, for example, the Solar Energy Generating Systems (SEGS) plants in California, mechanical properties would be the major research fields for thermal which is the world's first commercial parabolic trough plants; Acciona's deformation restrain of tube receiver for solar PTC systems. Nevada Solar One near Boulder City, Nevada, and Andasol, which is Europe's first commercial parabolic trough plant, along with Plataforma Solar de Almería's SSPS-DCS test facilities in Spain [154]. 6. CSP plant with PTC system Furthermore, several CSP plants with PTC technology are under development in Spain, USA, Egypt, Morocco, Mexico, China, Algeria, For a concentrated solar thermal power plant, electricity is pro- and Iran [155]. Commercial CSP plants worldwide using PTC system duced when the concentrated solar energy is converted into heat, which are listed in Table 2, which include plants that are operational, under drives a heat engine (usually a steam turbine) connected to an electric construction, and announced [156]. power generator. For large-scale concentrated solar thermal power It is worthwhile to note that most commercial CSP plants with PTC plant, a thermal storage unit is generally provided to allow electricity system are hybrids; fossil fuels are combusted at night hours; however, production at night and on overcast days [144]. The working principle the amount of fossil fuel combustion is limited to a maximum of 27% of a concentrated solar thermal power plant is illustrated in Fig. 20 electricity generation [157]. Moreover, parabolic trough solar collectors [145]. with tube receiver are also widely utilized for other applications, such Currently, establishment of power plants using CSP technology is as heating, cooling, and desalination [158,159]. growing faster than any other renewable energy technology owing to its capability to meet the electrical and thermal energy demands [146– 148]. Statistics indicate that a 1 MW of installed power plant with CSP 7. Conclusions and future directions technology can decrease the emission by 688 t CO2 compared to a combined cycle system and 1360 t CO2 compared to a power plant with CSP technologies can offer many advantages over conventional coal/steam cycle. A one square mirror of the solar field generates plate collectors, and are associated with higher operating temperatures,

400 kW h electrical energy per year, decreases 12 t CO2 emissions, and greater system efficiencies and least capital investment recovery period. contributes to a 2.5 t savings of fossil fuels during its 25-year operation The benefits of using CSP technology with PTC systems include lifetime [149]. promising cost-effective investment, mature technology, and ease of Unlike solar photovoltaic systems that can take advantage of direct combining with fossil fuels or other renewable energy sources. The as well as diffuse components of solar radiation, CSP technology can present research provides a comprehensive review of CSP technology only make use of the direct components with the need for high direct with a PTC system. The review covers the theoretical framework of the normal irradiance (DNI) for efficient functioning [150–152]. The CSP plant with PTC technology, detailed derivation process of the location shaving high annual DNI availability is best suited for CSP maximum theoretical concentration ratio of a PTC, commonly used installations. Threshold values of annual DNI for power plants with HTFs in the tube receiver, and recent progress of heat transfer CSP technology as suggested by Deutsches Zentrum für Luftund enhancement methods. The progress in thermal deformation restrain

Fig. 21. World map of global solar horizontal irradiation [153].

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Table 2 Commercial CSP plants with parabolic trough collector technique in the world [156].

Capacity Name Country Location Status Notes and references (MW)

1 280 Mojave Solar Project USA Barstow, California In operation Completed December 2014. Gross capacity of 280 MW corresponds to net capacity of 250 MW 2 280 Solana Generating Station USA Gila Bend, Arizona In operation Completed in October 2013, with 6 h thermal energy storage 3 250 Genesis Solar Energy Project USA Blythe, California In operation Online April 24, 2014 4 200 Solaben Solar Power Station Spain Logrosán In operation Solaben 3 completed June 2012 Solaben 2 completed October 2012 Solaben 1 and 6 completed September 2013 5 160 Noor I Morocco Ghassate, Ouarzazate In operation with 3 h heat storage Province 6 150 Solnova Solar Power Station Spain Sanlúcar la Mayor In operation Solnova 1 completed May 2010 Solnova 3 completed May 2010 Solnova 4 completed August 2010 7 150 Andasol solar power station Spain Guadix In operation Completed: Andasol 1 (2008), Andasol 2 (2009), Andasol 3 (2011). Each equipped with a 7.5-h thermal energy storage. 8 150 Extresol Solar Power Station Spain Torre de Miguel Sesmero In operation Completed: Extresol 1 and 2 (2010), Extresol 3 (2012). Each equipped with a 7.5-h thermal energy storage. 9 100 KaXu Solar One South Africa Pofadder, Northern Cape In operation with 2.5 h heat storage 10 100 GUZMAN Spain Palma del Río In operation Palma del Rio 2 completed December 2010 Palma del Rio 1 completed July 2011 11 100 Manchasol Power Station Spain Alcázar de San Juan In operation Manchasol 1 and 2 completed in 2011, each with 7.5 h heat storage 12 100 Valle Solar Power Station Spain San José del Valle In operation Completed December 2011, with 7.5 h heat storage 13 100 Helioenergy Solar Power Spain Écija In operation Helioenergy 1 completed September 2011 Station Helioenergy 2 completed January 2012 14 100 Aste Solar Power Station Spain Alcázar de San Juan In operation Aste 1 A Completed January 2012, with 8 h heat storage Aste 1B Completed January 2012, with 8 h heat storage 15 100 Solacor Solar Power Station Spain El Carpio In operation Solacor 1 completed February 2012 Solacor 2 completed March 2012 16 100 Helios Solar Power Station Spain Puerto Lápice In operation Helios 1 completed May 2012 Helios 2 completed August 2012 17 100 Shams solar power station UAE Abu Dhabi In operation Shams 1 completed March 2013 MadinatZayed 18 100 Termosol Solar Power Station Spain Navalvillar de Pela In operation Both Termosol 1 and 2 completed in 2013 19 100 Palma del Río I & II Spain Palma del Río In operation Completed July 2012 20 64 Nevada Solar One USA Boulder City, Nevada In operation In operation since 2007 21 50 Bokpoort South Africa Groblershoop In operation with 9 h heat storage 22 50 Puertollano Solar Thermal Spain Puertollano, Ciudad Real In operation Completed May 2009 Power Plant 23 50 Alvarado I Spain Badajoz In operation Completed July 2009 24 50 La Florida Spain Alvarado (Badajoz) In operation Completed July 2010 25 50 Majadas de Tiétar Spain Caceres In operation Completed August 2010 26 50 La Dehesa Spain La Garrovilla (Badajoz) In operation Completed November 2010 27 50 Lebrija-1 Spain Lebrija In operation Completed July 2011 28 50 Astexol 2 Spain Badajoz In operation Completed November 2011, with 7.5 h thermal energy storage 29 50 Morón Spain Morón de la Frontera In operation Completed May 2012 30 50 La Africana Spain Posada In operation Completed July 2012, with 7.5 h thermal energy storage 31 50 Olivenza 1 Spain Olivenza In operation Completed July 2012 32 50 Orellana Spain Orellana la Vieja In operation Completed August 2012 33 50 Godawari Green Energy India Naukh In operation 2013 Limited 34 50 EnerstarVillena Power Plant Spain Villena In operation Completed 2013 35 5 Thai Solar Energy (TSE) 1 Thailand HuaiKrachao In operation Completed November 2011 36 2 Keahole Solar Power USA Hawaii In operation 37 1 Saguaro Solar Power Station USA Red Rock, Arizona In operation 38 0.5 Shiraz solar power plant Iran Shiraz In operation Iran's first solar power plant 39 200 Noor II Morocco Ghassate (Ouarzazate Under province) construction 40 100 Xina Solar One South Africa Northern Cape Under with 5 h heat storage construction 41 100 Kathu Solar Park South Africa Northern Cape Under with 4.5 h heat storage construction 42 100 El Reboso 2+3 Spain El Puebla del Rio Under (Seville) construction 43 100 Diwakar India Askandra Under with 3 h heat storage construction 44 100 KVK Energy Solar Project India Askandra Under with 4 h heat storage construction 45 50 Erdos Solar Power Plant China Hanggin Banner Under construction 46 50 Megha Solar Plant India Anantapur Under (continued on next page)

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Table 2 (continued)

Capacity Name Country Location Status Notes and references (MW)

construction 47 50 CGNSED power plant China Delingha Under construction 48 25 Gujarat Solar One India Kutch Under with 9 h heat storage construction 49 17 Stillwater USA Nevada Under construction 50 3 Airlight Energy Ait Baha Morocco Ait Baha Under with 12 h heat storage Plant construction 51 280 Al-Abdaliya Kuwait Announced 52 120 Shneur Solar Power Station Israel Tze'elim, Israel Announced

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