Solar Energy Materials & Solar Cells 95 (2011) 2703–2725

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

journal homepage: www.elsevier.com/locate/solmat

Review Innovation in concentrated solar power

David Barlev a,c, Ruxandra Vidu b,c, Pieter Stroeve a,b,c,n a Department of Electrical and Computer Engineering, University of California Davis, Davis, CA 95616, USA b Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA 95616, USA c California Solar Energy Collaborative (CSEC), University of California Davis, Davis, CA 95616, USA article info abstract

Article history: This work focuses on innovation in CSP technologies over the last decade. A multitude of advancements Received 30 October 2010 has been developed during this period, as the topic of concentrated solar power is becoming more Accepted 12 May 2011 mainstream. Improvements have been made in reflector and collector design and materials, heat absorption and transport, power production and thermal storage. Many applications that can be Keywords: integrated with CSP regimes to conserve (and sometimes produce) electricity have been suggested and Concentrated solar power (CSP) implemented, keeping in mind the environmental benefits granted by limited fossil fuel usage. Design & 2011 Elsevier B.V. All rights reserved. Materials Heat absorption Transport Thermal storage

Contents

1. Introduction ...... 2703 2. Concentrating solar collectors ...... 2704 3. Parabolic trough collectors (PTC) ...... 2705 4. Heliostat field collectors (HFC) ...... 2707 5. Linear Fresnel reflectors (LFR) ...... 2711 6. Parabolic dish collectors (PDC) ...... 2712 7. Concentrated photovoltaics ...... 2714 8. Concentrated solar thermoelectrics...... 2716 9. Thermal energy storage ...... 2717 10. Energy cycles ...... 2719 11. Applications ...... 2720 12. Discussion ...... 2722 13. Conclusion ...... 2723 References ...... 2723

1. Introduction an entire year. Despite of this, solar electricity currently provides only a fraction of a percent of the world’s power consumption. As the world’s supply of fossil fuels shrinks, there is a great A great deal of research is put into the harvest and storage of solar need for clean and affordable renewable energy sources in order energy for power generation. There are two mainstream cate- to meet growing energy demands. Achieving sufficient supplies of gories of devices utilized for this purpose—photovoltaics and clean energy for the future is a great societal challenge. Sunlight, concentrated solar power (CSP). The former involves the use of the largest available carbon-neutral energy source, provides the solar cells to generate electricity directly via the photoelectric Earth with more energy in 1 h than is consumed on the planet in effect. The latter employs different methods of capturing solar thermal energy for use in power-producing heat processes. Concentrated solar power has been under investigation for

n several decades, and is based on a simple general scheme: using Corresponding author at: Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA 95616, USA. mirrors, sunlight can be redirected, focused and collected as heat, E-mail address: [email protected] (P. Stroeve). which can in turn be used to power a turbine or a heat engine to

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.05.020 2704 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725

Table 1 Description and specifications of the four main CSP technologies. Data compiled from [1,2].

Collector Description Rel. thermodynamic Operating temp. Relative Concentration Technology Tracking type efficiency range (1C) cost ratio (sun) maturity

PTC – Parabolic sheet of reflective material Low 50–400 Low 15–45 Very mature One-axis (aluminum, acrylic) – Linear receiver (metal pipe with heat transfer fluid)

Linear – Linear Fresnel mirror array focused on tower Low 50–300 Very low 10–40 Mature One-axis Fresnel or high-mounted pipe as receiver

Solar tower – Large heliostat field with tall tower in High 300–2000 High 150–1500 Most recent Two-axis its center – Receiver: water/HTC boiler at top – Can be used for continuous thermal storage

Dish-Stirling – Large reflective parabolic dish with Stirling High 150–1500 Very high 100–1000 Recent Two-axis engine receiver at focal point – Can be used with/out HTC, if heat engine produces electricity directly from reflected thermal energy (in this case, thermal storage cannot be achieved by the system)

generate electricity. Despite being relatively uncomplicated, this per unit area at the earth’s surface. Though more costly, concentrating method involves several steps that can each be implemented in a collectors have numerous advantages over stationary collectors, and plethora of different ways. The chosen execution method of every are generally associated with higher operation temperatures and stage in solar thermal power production must be optimally matched greater efficiencies. The addition of an optical device to the conven- to various technical, economic and environmental factors that may tional solar collector (receiver) has proved useful in several regards; favor one approach over another. Extensive explorations of various various concentration schemes can achieve a wide range of concen- solar collector types, materials and structures have been carried out, tration ratios, from unity to over 10,000 sun [2]. This increases the and a multitude of heat transport, storage and electricity conversion operation temperature as well as the amount of heat collected in a systems has been tested. The progress made in every aspect of CSP, given area, and yields higher thermodynamic efficiencies. Radiation especially in the last decade, was geared towards expanding the focusing allows the usage of receivers with very small relative surface efficiency of solar-to-electric power production, while making it areas, which leads to significant reductions in heat loss by convection. affordable in comparison with near-future fossil fuel derived power. Despite the added capital investment necessary for manufacturing This work describes the four main types of concentrating solar the optical elements of the apparatus, the materials used for these collectors (Tables 1 and 2) [1,2] and discusses innovation in each mirrors/lenses are generally inexpensive compared with thermal over the last decade. Progress in the related fields of concentrated collector materials, which are needed in much smaller amounts in photovoltaics and thermoelectrics will also be presented, along a concentrator scheme. The reduction in receiver size and material with advances made in thermal energy storage methods, energy amounts makes expensive receiver conditioning (vacuum insulation, conversion cycles and CSP applications. surface treatments, etc.) for higher efficiency and heat loss minimiza- tion economically sensible. Finally, the ability to control the concen- tration ratio of a system allows delicate manipulation of its operation 2. Concentrating solar collectors temperature, which can be thermodynamically matched to specific applications as needed to avoid wasted heat. It is important to note A solar energy collector is a heat-exchanging device that trans- that reflective materials used in CSP technologies must meet certain forms solar radiation into thermal energy that can be utilized for reflectivity and lifetime requirements to be cost-effective. A study of power generation. The basic function of a solar collector is to absorb the optical durability of solar reflectors was presented by Kennedy incident solar radiation and convert it into heat, which is then carried and Terwilliger [4] and an investigation specific to aluminum first- away by a heat transfer fluid (HTF) flowing through the collector. The surface mirrors was carried out by Almanza et al. [5]. heat transfer fluid links the solar collectors to the power generation Tyagi et al. [6] investigated the effects of HTF mass flow rates system, carrying thermal energy from each collector to a central and collector concentration ratios on various system parameters. steam generator or thermal storage system as it circulates. Results showed that exergy output (available work from a process There are two general categories of solar collectors. The first that brings a system to thermal equilibrium), exergetic and includes stationary, non-concentrating collectors, in which the thermal efficiencies and inlet temperature increased with solar same area is used for both interception and absorption of incident intensity, as expected. Exergetic and thermal efficiencies and radiation. The second category consists of sun-tracking, concen- exergy output were found to increase with mass flow rate as trating solar collectors, which utilize optical elements to focus well. Optimal inlet temperature and exergetic efficiency at high large amounts of radiation onto a small receiving area and follow solar intensity were both found to be the decreasing functions the sun throughout its daily course to maintain the maximum of the concentration level. At low intensity values, however, solar flux at their focus. A comprehensive review of sun-tracking efficiency first increases and then decreases with increase in methods and principles was published by Mousazadeh et al. [3]. concentration. This behavior results from increased radiative Light concentration ratios can be expressed in suns, with a single losses associated with high concentration ratios. Both concentra- sun (1000 W/m2) being a measurement of average incident light flux tion ratios of solar collectors and the mass flow rates at which D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2705

Table 2 Schematic diagrams of each CSP technology listed in Table 1. Figures from [2].

Collector type Schematic diagram

Parabolic trough collector

Linear Fresnel reflector

Heliostat field collector

Parabolic dish reflector

they operate must be meticulously chosen to achieve optimal proportional and strictly dependent on the operation temperature. performance. In practice, however, the materials chosen for light concentration The four main types of concentrating solar collectors are and absorption, heat transfer and storage, as well as the power conversion cycles used are the true deciding factors [7].The (1) Parabolic trough collectors; following sections will describe the aforementioned collector (2) heliostat field collectors; schemes in detail, and present technological advancements that (3) linear Fresnel reflectors; and have been made in each over the last 10 years. (4) parabolic dish collectors. 3. Parabolic trough collectors (PTC) Concentrating collectors can achieve different concentration ratios and thus operate at various temperatures. From a theoretical Parabolic trough technology is the most mature concentrated standpoint, the efficiency of power producing heat processes is both solar power design. It is currently utilized by multiple operational 2706 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 large-scale CSP farms around the world. Solar Electric Generating Systems (SEGS) is a collection of fully operational PTC systems located in the California desert with a total capacity of 354 MW. SEGS is at present the largest PTC power plant in the world. Another PTC plant with a 280 MW capacity is being built in Arizona and is scheduled to become operational in 2011. PTCs effectively produce heat at temperatures ranging from 50 to 400 1C. These temperatures are generally high enough for most industrial heating processes and applications, the great majority of which run below 300 1C. The parabolic trough collector design features light structures and relatively high efficiency. A PTC system is composed of a sheet of reflective material, usually silvered acrylic, which is bent into a parabolic shape. Many such sheets are put together in Fig. 2. Schematic flow diagram of Recirculation mode of operation of direct steam series to form long troughs. These modules are supported from generation. Figure reproduced with permission from ref. 9, &2005 Elsevier. the ground by simple pedestals at both ends. The long, parabolic shaped modules have a linear focus (focal line) along which a receiver is mounted. The receiver is generally a black metal pipe, with preheated water. This process guarantees good wetting of encased in a glass pipe to limit heat loss by convection. The metal absorber tubes and prevents stratification. Steam is separated from tube’s surface is often covered with a selective coating that water and fed into the inlet of a superheating section. The Recircula- features high solar absorbance and low thermal emittance. The tion regime is more easily controlled than the Once-through regime, glass tube itself is typically coated with antireflective coating to but has an increased parasitic load due to the additional process enhance transmissivity. A vacuum can be applied in the space steps. Usage of water as a HTF inflicts more stress on the absorber between the glass and the metal pipes to further minimize heat tubes than other heat transfer media, due to water’s relatively high loss and thus boost the system’s efficiency. volatility. A simulation of thermohydraulic phenomena under the The heat transfer fluid (HTF) flows through the receiver, DSG process was carried out by Eck and Steinmann [10].Sufficient collecting and transporting thermal energy to electricity genera- cooling of the absorber tubes and a moderate pressure drop between tion systems (usually boiler and turbine generator) or to storage inlet and outlet can help moderate the stress, reduce corrosion and facilities. The HTF in PTC systems is usually water or oil, where oil promote tube lifetime. is generally preferred due to its higher boiling point and relatively Knowledge of short-time dynamics of flow and feed systems low volatility. Several water boiler designs have been suggested in a DSG regime is crucial for successful design and operation. by Thomas [8]. The preferred boiling system implements direct A transient non-linear simulation tool was developed to study steam generation (DSG), where water is the heat transfer fluid. dynamic behaviors of the aforementioned PTC system designs, for It is partially boiled in the collector and circulated through a which several feed control systems were suggested [11].Itis steam drum where steam is separated from the water. important to mention that for DSG systems, the temperature The DISS (Direct Solar Steam) project PTC plant in Tabernas, difference registered between the hottest and the coldest points Spain, is a leading DSG test facility, where two successful DSG over the external wall of the pipe will increase if feed flow is too operational modes and control systems were developed and high [12]. This is a result of non-constant heat transference from tested [9]. Both methods utilize pressure control in addition to the receiver to the HTF, and can potentially affect the quality of temperature control of circulating water. This approach is done to produced steam. A test facility for a solid sensible heat storage achieve a constant output of steam at a monitored temperature system was developed for the DSG parabolic trough collector throughout most hours of the day (9 am–6 pm). A pressure level design discussed. A performance analysis of the storage system of 100 bar and temperatures of up to 400 1C have been demon- integrated with the power plant was implemented by Steinmann strated. The Once-Through mode (Fig. 1) features a preheated water et al. [13]. Integration of thermo-chemical storage through feed into the inlet. As water circulates through the collectors, it is ammonia de-synthesis was theoretically investigated as well, evaporated and converted into superheated steam that is used to and efficiencies of up to 53% were reported [14]. power a turbine. In the more water-conservative Recirculation mode In contrast with the DSG scheme, which employs water as the (Fig. 2), a water–steam separator is placed at the end of the collector HTF, recent innovation also promotes the use of ionic liquids loop. More water is fed to the evaporator than can be evaporated in (molten salts) for heat transfer media [15], as they are more one circulation cycle. Excess water is re-circulated through the heat-resilient than oil, and thus corrode the receiver pipes less. intermediate separator to the collector loop inlet, where it is mixed Ionic liquids are, however, very costly, and such an investment would have to be weighed against the incurring costs of receiver maintenance and replacement to determine their cost-effectiveness. PTCs are mounted on a single-axis sun-tracking system that keeps incident light rays parallel to their reflective surface and focused on the receiver throughout the day. Both east–west and north–south tracking orientations have been implemented, with the former collecting more thermal energy annually, and the latter collecting more energy in the summer months when energy consumption is generally the highest [2]. The east–west orientation has been reported to be generally superior [16]. The tracking mechanism must have parabolic collectors for tracing the sun’s path very accurately in order to achieve efficient heating of the receiver tube. However, trough collectors are generally exposed to wind Fig. 1. Schematic flow diagram of Once-Trough mode of operation of direct steam generation. drag, and must thus be robust enough to account for wind loads and Figure reproduced with permission from ref. 9, &2005 Elsevier. prevent deviations from normal insolation incidence. D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2707

A study of turbulent flow around a PTC of the 250 kW solar [25,26]. Hot water and steam from geothermal wells can be plants in Shiraz, Iran, was conducted by Naeeni and Yaghoubi directly fed into an absorber pipe going through a PTC field. The [17]. The study investigates stress applied to the collector, taking combination of both thermal energy sources increases the volume into account varying collector angles, wind velocities and air flow and the quality of (directly) generated steam for power produc- distribution with respect to height from the ground. A second tion. Several hybrid designs have been suggested by the authors. study by the authors models the effects of the same parameters PTCs can also be integrated with solar cells in concentrated on heat transfer from the PTC receiver tube [18]. photovoltaics (CPV) modules. Heat-resistant, high-efficiency In order to make the PTC structure more resilient to external photovoltaic cells can be mounted along the bottom of the forces, it is possible to reinforce collector surfaces with a thin receiver tube to absorb the concentrated solar flux. The perfor- fiberglass layer. A smooth, 901 rim angle reinforced trough was mance of a CPV parabolic trough system with a 37 sun concen- built by a hand lay-up method [19]. The fiberglass layer is added tration ratio was characterized by Coventry [27] at Australian underneath the reflective coating (on the inner surface) of the National University in 2003. Monocrystalline silicon solar cells parabolic trough. The reflector’s total thickness is 7 mm, and can were used, along with the thermal PTC apparatus. Measured withstand a force applied by a 34 m/s wind with minimal electrical and thermal efficiencies were 11% and 58%, respectively, deviation; deflection at the center of the parabola vertex was producing a total efficiency of 69%. It is important to note that only 0.95 mm, well within acceptable limits. uneven illumination of the solar cell modules causes a direct Receiver design considerations are crucial for efficient heat decrease in the cells’ performance, and thus optical considera- transfer to the HTF and heat loss management. Radiative heat tions must be weighed carefully. losses from receiver tubes play an important role in collector The mature field of parabolic trough collectors provides an performance. Thermal loss due to the temperature gradient efficient, relatively inexpensive power production scheme. Multiple between the receiver and the ambient has a significant impact advances in reflector and receiver design have been made in the last on a system’s thermal efficiency. PTCs operating at high tempera- decade to enhance efficiency and reduce losses. Heat collection and tures (around 390 1C) can experience up to 10% radiative losses transfer methods have been modeled and tested repeatedly in order annually. At this temperature range, thermal loss from receiver tube to achieve optimal power output throughout the day. The PTC reaches 300 W/m of the receiver pipe [20]. A loss of 220 W/m was scheme also lends itself to easy storage schemes, as well as to reported for an operational temperature of 180 1C, with collector simple integration with both fossil fuels and other renewable efficiency ranging from 40% to 60% [21]. In both of the aforemen- energy sources. tioned studies, synthetic oil was used as the heat transfer fluid. The high temperature difference between the receiver tube’s interior and the ambient also induces a thermal stress, which can 4. Heliostat field collectors (HFC) cause bending of the pipes. Thermal analysis of an energy-efficient PTC receiver was presented by Reddy et al. [22], and a numerical The most recent CSP technology to emerge into commercial model to evaluate its heat transfer characteristics was proposed. The utility was the heliostat field collector design. This expensive, new receiver design features porous inclusions inside the tube, powerful design has so far been incorporated in relatively few which increase the total heat transfer area of the receiver, along locations around the world. The 10 MW Solar One (1981) and with its thermal conductivity and the turbulence of the circulating Solar Two (1995) were the first HFC plants to be demonstrated, HTF (synthetic oil). Heat transfer for this scheme was enhanced by built in the Mojave Desert of California. They have since been 17.5% compared with regular (no inclusions) design, but the system decommissioned. Other plants, such as the 11 MW PS10 and 20 MW suffered a pressure decrease of about 2 kPa. PS20 in Spain, and the 5 MW Sierra SunTower in California, were The use of a heat pipe as a linear receiver for PTCs was proposed recently completed. by Dongdong et al. [23]. The heat pipe can keep an essentially The heliostat field collector design features a large array of flat uniform circumferential temperature, despite the uneven illumina- mirrors distributed around a central receiver mounted on a (solar) tion provided by trough collectors. Since heat does not flow from the tower. Each heliostat sits on a two-axis tracking mount, and has a HTF to the heat pipe, smaller heat losses occur during hours of low surface area ranging from 50 to 150 m2. Using slightly concave insolation. PTC systems featuring a heat pipe as the receiver have mirror segments on heliostats can increase the solar flux they 65% thermal efficiency at 380 1C. They are also cheaper to manu- reflect, though this elevates manufacturing costs. Every heliostat facture because the bellows system generally incorporated into is individually oriented to reflect incident light directly onto the conventional receiver tubes is not necessary. Lifetime testing of central receiving unit. Mounting the receiver on a tall tower the heat pipe receiver with respect to various operation tempera- decreases the distance mirrors must be placed from one another tures is still being investigated, but meets the general requirements to avoid shading. Solar towers typically stand about 75–150 m (12–15 years) under operation below 380 1C. height. A fluid circulating in a closed-loop system passes through Parabolic trough collector systems generally operate in unsteady the central receiver, absorbing thermal energy for power produc- state. For this reason, a dynamic model is essential for effective tion and storage. An advantage of HFCs is the large amount of design and performance prediction of a PTC system. A dynamic radiation focused on a single receiver (200–1000 kW/m2), which simulation of PTC was conducted by Ji et al. [24], modeling a south minimizes heat losses and simplifies heat transport and storage facing, one-axis tracking parabolic trough collector. The simulation requirements. Power production is often implemented by steam and calculated variations in incidence angle of solar beam to collector turbine generators. The single-receiver scheme provides for uncom- aperture, as well as the distribution of concentrated solar radiation plicated integration with fossil-fuel power generators (hybrid along the focal line. Effects of HTF mass flow rate and receiver tube plants) [2]. length on outlet temperature and system efficiency were investi- HFC plants are typically large (10 MW and above), as the benefit gated. An increase in tube length augments outlet temperatures and from an economy of scale is required to offset the high costs efficiency, as expected due to greater total insulation. A decrease in associated with this technology. They can incorporate a very large mass flow rate increases outlet temperature and slightly decreases number of heliostats surrounding a single tower. The immense solar system efficiency. flux reflected towards the receiver yields very high concentration The integration of a parabolic trough collector field with ratios (300–1500 suns). HFC plants can thus operate at very high geothermal sources has been suggested by Lentz and Almanza temperatures (over 1500 1C), which positively impacts collection 2708 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 and power conversion efficiencies by enabling the use of higher- energy cycles. A reflective solar tower design has been suggested, in which a secondary reflector is mounted on the tower, and the central receiver is grounded (Fig. 3). A review of the optics of the reflector tower was presented by Segal and Epstein [28]. Since HFCs operate at such high temperatures, the greatest losses are incurred convectively at the receiver’s surface. Aside from the convenience associated with having the recei- ver situated at ground level, the optics of the design increase the concentration ratio, allowing the collector to be smaller and diminish losses. Transport losses can also be lowered by situating the turbine generator in close proximity with the receiver. Never- theless, Segal and Epstein [29] reported that the reflector tower scheme is not more efficient than the solar tower regime, and that superiority of either technology is subject mainly to economic factors. The integration of a solar reformer with a heliostat field array Fig. 4. Solar ground reformer integrated with a reflector tower HFC system. was proposed in 2002. Solar reforming of methane with steam or Figure reproduced with permission from ref. 30, &2003 Elsevier.

CO2 is an efficient chemical heat storage method. The syngas produced can be converted into electricity using a gas turbine or combined cycle. The suggested reformer rests on the ground, and has a collector mounted above it (Fig. 4). A solar reflector tower is used to concentrate solar flux from heliostats onto the ground reformer. In this fashion, the power producing unit can be separated from the concentrator field entirely. Landfill gas and biogas can be used to supplement gas pro- duced by the reformer. The design and operation of a large-scale reformer are discusses by Segal and Epstein [30]. The synthesis gas produced by this technology can also be utilized for the production of methanol. An optimization study of an HFC system’s main parameters was conducted by Segal and Epstein [7]. The effects of operation temperature, heliostat field density and the use of a secondary reflector (reflector tower regime) on power conversion were tested across different energy cycles (Fig. 5). The investigation concluded that maximum overall efficiency of an HFC system is reached at 1600 K, with an average field density of 35%. The authors emphasize that differences between large and small HFC Fig. 5. Brayton cycle and combined cycle efficiencies as a function of the plants with regards to these values are negligible. temperature and gas turbine pressure ratio. The solar tower reflector can also be integrated with concen- Figure reproduced with permission from ref. 7, &2003 Elsevier. trated photovoltaics (CPV). The principle behind this design is to split the solar spectrum into PV-used and thermal-used portions. For rest of the light can be used for electricity generation using Rankine– example, monocrystalline silicon solar cells operate at efficiencies Brayton cycles, or otherwise be stored for later use. Discussion of ranging between 55% and 60% at wavelengths of 600–900 nm. The spectrum splitting optics and HFC–CPV hybrid design is given by Segal et al. [31]. The study’s results show that a heat input of 55.6 MW yields 6.5 MW from the solar cells array and 11.1 MW from a combined energy cycle. This was done under concentration ratios of 200–800 sun. The concept of a dual receiver for solar towers was suggested by Buck et al. [32]. The proposed receiver is made of an open volumetric air heater with a tubular evaporator section (Figs. 6 and 7). In this design, the receiver has both a water heating section and an air heating section. Water (HTF) is circulated through, evaporated in the tubular evaporator, and is then superheated by hot air. Feed water is also preheated using the hot air. This concept essentially combines direct steam generation with regular water HTF operations. The results (Table 3) of the new design demonstrate numerous benefits, which include a higher receiver thermal efficiency, lower receiver temperature and lower parasitic losses. A 27% gain in annual output is facilitated by these improvements, compared with the solar air heating system. Separation of evaporation and superheating sections also alleviates thermo-mechanical stress on the receiver to some degree. Planning the layout of a heliostat field presents a great optimiza- Fig. 3. Schematic diagram of solar reflector tower in an HFC system. tion challenge. A novel methodology for layout generation based on Figure reproduced with permission from ref. 7, &2003 Elsevier. yearly normalized energy surfaces (YNES) was presented by Sanchez D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2709 and Romero [33]. This ‘Heliostat Growth Method’ (HGM) uses the same system. System configurations were assessed for technical YNES program to evaluate the usable solar energy flux at each point performance and cost. in a solar field year-round, given a specific solar tower height. Using Forsberg et al. [35] suggested the use of liquid fluoride salt as this data, the method splits impacting factors such as shadowing, an HTF in order to raise the heat-to-electricity conversion effi- blocking, atmospheric attenuation and others into two categories: ciency of HFCs to about 50%. The molten salt operates at those associated with spatial position of the solar tower and those temperatures between 700 and 850 1C, delivering heat to a closed affected by the geometry of the heliostat. This provides greater multi-reheat Brayton cycle using N2 or He as the working fluid. insight and flexibility to the field layout process and its optimization. Due to such high operation temperatures, thermal energy storage A clever design for small-scale ‘tri-generation’ solar power as sensible heat in graphite is suggested. A schematic diagram of assisted plant was brought forth by Buck and Friedmann [34]. The such an HFC plant is shown (Fig. 8). Graphite, a low-cost solid design puts together a solar–gas turbine hybrid system, which featuring a high heat capacity, is compatible with the fluoride salt incorporates a small heliostat field, a receiver mounted on a solar at high temperatures. The efficiency boost reported by the tower, a micro-turbine and an absorption chiller. In this regime, authors can greatly reduce electricity costs. electric power, heating and cooling can all be produced by the The combination of a single central receiver with molten salts as the HTF generally allows the highest operation temperatures of any CSP regime and produces electricity with the highest effi- ciencies. High-efficiency heat storage with molten salts enables solar collection to be decoupled from electricity generation in a simpler manner than water/steam systems permit [36]. The design and performance of a novel high-temperature air receiver was presented by Koll et al. [37]. The receiver suggested is a porous absorber module consisting of extruded parallel channel structures of silicon carbide ceramics. The inner surface area of the channel exceeds that of the aperture by a factor of 50. This allows the usage of air as the exclusive HTF, despite its low heat transfer coefficient. The receiver design is modular and promotes easy scaling. The hot air is delivered at 700 1Ctoa water boiler system for steam generation. Steam can be produced consistently at 485 1C and 27 bar, but these parameters vary according to the system’s capacity. Using air as a heat transfer Fig. 6. Scheme of dual receiver unit from top view (left) and side view (right). medium greatly reduces capital investment as it is free and Figures reproduced with permission from ref. 32, &2006 Elsevier. readily available anywhere.

Fig. 7. Schematic plant incorporating dual receiver, outlining three heat transfer stages (preheating, evaporation and superheating). Figures reproduced with permission from ref. 32, &2006 Elsevier.

Table 3 Comparison of dual receiver CSP plant performance with a control [32].

Reference plant Dual receiver plant

Design conditions Receiver outlet temp. (1C) 700 500 Receiver efficiency (including recirculation losses) (%) 79 87 Air temp at blower (1C) 110 80 Air mass flow (kg/s) 56 40 Annual performance Annual receiver efficiency (including recirculation losses) (%) 66.7 79.4 Capacity factor (%) 14.3 18.2 Net annual electric energy (GWh) 12.5 15.9 2710 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725

Fig. 8. Solar power tower with liquid-salt heat transport system, graphite heat storage and Brayton power cycle. Figure reproduced with permission from ref. 35, &2007 ASME.

Fig. 9. Schematic diagram of torque tube heliostats (TTH). Figure reproduced with permission from ref. 41, &2008 ASME.

The heliostat material selection is a crucial aspect of HFC power plant design. These large mirrors make up about 50% of the total system’s cost and must feature high reflectivity and stiffness, be light-weight, easily cleaned and corrosion resistant. Xiaobin et al. [38] suggested the use of PVC composite plastic steel for heliostat fabrication. This polymer material has similar properties Fig. 10. Schematic of mini-mirror array design featuring ‘ball-in-socket joint’ tracking mechanism. to metal–aluminum alloys conventionally used, but is not as Figures reproduced with permission from ref. 42, &2010 ASME. heavy, and has a significantly longer lifetime. Its stiffness is high relative to its weight and it is reported by the authors to be cheaper. One significant issue with this material is its low heat regular HFC system of a slightly smaller area. Although the TTH resilience, a problem which must be contended with in order to system indeed experienced smaller wind torques, it suffered an ensure heliostat operation temperatures can be accommodated. annual energy output reduction of 3%. Furthermore, the high Several heliostat cleaning methods are proposed by Xiliang et al. number of moving elements and the more involved control make [39], such as using highly pressurized air/water depending on this system hardly advantageous compared with the conventional various environmental conditions. design. Conventional heliostat design dictates that cost reduction is Another novel design to help avoid heavy mirror tracking in implemented by increasing the area of the mirrors. Doing this the face of wind loads was suggested by Gottsche¨ et al. [42]. This reduces specific drive cost while increasing the torques heliostats regime utilizes mini-mirror arrays (10 10 cm) made of high experienced by wind loads. A study by Ying-ge et al. [40] quality materials. Each mirror is mounted on a ball-in-socket joint demonstrates the distribution and characteristics of heliostats’ driven by a step motor (Fig. 10). The mirrors are encased in a clear mean and fluctuating wind pressure while wind direction angle is box that shields them from the wind. The purpose of this design is varied from 01 to 1801 and vertical angle is varied from 01 to 901. to avoid wind loads and save on stiff materials (mainly steel) that Moreover, a finite element model was constructed to perform are necessary to make large heliostats resilient to wind torques. calculations of wind-induced dynamic responses. Increased wind Unfortunately, the low-cost achieved by the group was countered torques result in higher specific weight and drive power. The by a 40% drop in optical performance compared with conven- usage of torque tube heliostats (TTH) (Fig. 9) is suggested by tional HFC systems. Amsbeck et al. [41]. TTH systems incorporate arrays of long, For initial planning of an HFC power plant, a general efficiency narrow mirrors mounted on turning tubes that control their evaluation tool can be quite useful. Collado [43] presented a elevation. An optical performance and a weight estimation of a quick, non-specific evaluation method for annual heliostat field TTH system were carried out by the authors, and compared with a efficiency evaluation. The model is a combination of an analytical D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2711 assessment of the flux density produced by a heliostat from increased receiver tower height, which augments the cost. Zaragoza University, an optimized mirror density distribution A novel solution to the shading problem is discussed by Mills developed by University of Houston for the Solar One project, and Morrison [46] at Sydney University, Australia. Their design of and molten salt receiver efficiencies measured during the Solar the compact linear Fresnel reflector (CLFR) scheme features Two project. This model does not take into account many adjacent mirrors oriented towards two separate receivers in impacting factors specific to a particular HFC system and is opposite directions (Fig. 11). The use of multiple receivers allows limited in its accuracy. a more compact reflector distribution, avoiding shading and Similarly, a new method for approximating geometrical para- utilizing a portion of solar flux that otherwise goes to waste. meters and sizing of the tower reflector regime was developed by Reflectors near the base of a receiver are always oriented towards Segal and Epstein [44]. The method utilizes edge rays originating it. Yet, when reaching a nearly equidistant point between two from the heliostat field boundaries and is particularly useful for separate receivers, the mirrors from each will reverse their geometrical assessment of very large arrays of heliostats. The orientation, allowing them to come very close together without method’s results were compared with real field calculations and blocking one another. For commercial power production (greater found to be a good first approximation regime. than 1 MW scale), it is very reasonable to have multiple receivers, A simulation using the same ‘edge ray’ principle method was and thus the CLFR design is very useful without incurring extra developed by Xiudong et al. [45]. Its purpose was to promote costs, especially in areas where land is limited. more efficient placement of heliostats and obtain a faster gen- A useful addition to the CLFR design is the incorporation of an erating response of the design and optimization. A novel module inverted cavity receiver attached to a planar array of boiling tubes for the analysis of non-spherical heliostat arrangements has been (Fig. 12). This structure allows plant operation in a direct steam incorporated into the simulation. A toroidal heliostat field was generation (DSG) regime. Mills and Morrison [47] indicate that designed and analyzed by the authors and proved significantly this receiver design bypasses receiver thermal uniformity chal- less efficient that conventional HFC arrangements. A method for lenges with parabolic trough DSG system. Design considerations calculating the annual solar flux distribution of a given area is an of the inverted cavity receiver are presented by Singh et al. [48]. added feature, with the purpose of evaluating feasibility of crop This work compares thermal performance of circular and rectan- growth around heliostat fields. gular absorber tubes, as well as black nickel and black paint Heliostat field collector technology has greatly improved over coated tubes. Circular absorbers in the receiver are reported to have the last few decades, and continues to draw much attention as a a higher thermal efficiency by 8% compared with a rectangular suitable scheme for large solar thermal plants. The exceedingly absorber. Nickel selective surface coating performed 10% better high temperatures at which they operates it grant HFC plants than ordinary black paint. A heat loss study of the same variables is excellent efficiencies, while allowing them to be coupled to a also performed by the authors. Nickel selective-coated absorbers variety of applications. The high capital investment necessary for experience a 20–30% heat loss coefficient reduction. Additionally, the construction of HFC systems is an obstacle, however, and a double glass absorber cover is compared with a single glass cover, further technological advancements in efficiency must be accom- and is found to reduce the heat loss coefficient by 10–15% [49]. panied by low cost materials and storage schemes for this CSP An innovative design to further limit wasted solar radiation in method to become more economical. a CLFR regime was presented by Chavez and Collares-Pereira [50].

5. Linear Fresnel reflectors (LFR)

Concentrated solar power production using linear Fresnel reflectors is quite similar to the parabolic trough collector scheme. The two share common principles in both arrangement and operation. In March 2009, the German company Novatec Biosol constructed a LFR solar power plant known as PE 1 that has an electrical capacity of 1.4 MW. The success of this project inspired the design of PE 2, a 30 MW plant based on the LFR technology, to be constructed in Spain. The 5 MW Kimberlina Solar Thermal Energy plant has been recently completed in Bakersfield, California. Fig. 11. Schematic diagram of the CLFR design. Figure reproduced with permission from ref. 2, &2004 Elsevier. Linear Fresnel reflectors incorporate long arrays of flat mirrors that concentrate light onto a linear receiver. The receiver is mounted on a tower (usually 10–15 m tall), suspended above and along reflector arrays. The mirrors can be mounted on one or two-axis tracking devices. The flat, elastic nature of the mirrors used makes the LFR design significantly cheaper than PTC. Additionally, central receiver units save on receiver material costs, which are generally higher than reflector costs. Several Fresnel reflectors can be used to approximate a parabolic trough collector shape, with the advantage that the receiver is a separate unit, and does not need to be supported by the tracking device. This makes tracking simpler, accurate and more efficient. A heat transfer fluid circulates through the receiver, collecting and trans- porting thermal energy to power production and storage units. A significant challenge with LFR systems is light blocking between adjacent reflectors. Solving this issue requires either Fig. 12. Schematic diagram of inverted air cavity receiver. increased spacing between mirrors, which takes up more land, or Source: Wikipedia. 2712 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725

New geometries for reflector fields are explored in this study, with the purpose of limiting blocking/shading while maximizing the field layout density. The authors propose reformation of the platform on which reflectors are resting (ground) into a wave- shaped one (Fig. 13). Individual reflectors’ size/shape adjustments based on their position in the heliostat field are also suggested. A concentration increase of up to 85% of theoretical maximum is reported under this design. Dey [51] describes several receiver design considerations for the CLFR concept. The absorber is a basic inverted air cavity with a glass encasing that encloses a selective surface. The central design goals analyzed are (1) maximization of heat transfer between the absorb- ing surface and the steam pipes, and (2) ensuring uniform absorber surface temperature to avoid degradation of the selective surface. Heat calculations are presented for absorber temperature distribution, and satisfactory absorber pipe separations and sizes are shown to alleviate temperature differences between the fluid surface and the absorbing surface. Similar work using finite element calculations was done by Eck et al. [52] for three separate parts of a LFR system–the evaporator, pre-heater and superheater (Table 4). Thermal loads for each section were modeled and maximum temperatures were investigated. In the case of the superheater, the maximum temperature derived was 570 1C, exceeding the temperature limit of the absorber coating. A novel step-by-step heat flux reduction method is thus required for safe and successful operation. Such a control system would adjust reflectors to an off-focus position one by one to prevent over- heating while operating at the highest allowed temperature. This kind of sensitive, intelligent system would surely increase power plant costs. AstudybyHoshietal.[53] investigated the suitability of high melting point phase change materials (PCMs) for storage use in large-scale CLFR plants (Fig. 14a–c). Several candidates for latent heat storage materials are discussed, and mathematical models of charging and discharging heat storage from each are presented.

NaNO2 is emphasized as a particularly suitable contender for large- scale latent heat storage due to its high melting point and low cost. LFR technology offers many of the advantages of PTC systems while incurring smaller reflector costs. It too can be easily coupled to direct steam generation as well as molten salts for thermal energy transport. The central receiver regime it incorporates shrinks costs further, but tags on the challenge of maximizing the amount solar radiation that can be collected. Innovation in receiver design and reflector organization has made LFR relatively

Fig. 13. Wave platform structure for a CLFR system allows maximization of solar radiation collected from a given area. Figure reproduced with permission from ref. 50, &2010 Elsevier. Fig. 14. (a–c) Heat storage materials and their properties. (a) Heat capacity of high melting point phase change materials. (b) Heat capacity of molten salts. (c) Media costs of high melting point phase change materials. Figures reproduced with permission from ref. 53, &2005 Elsevier. Table 4 FEM analysis results of thermal loads for three LFR system sections. Data compiled from [52]. inexpensive in comparison with other CSP technologies. It readily couples to thermal storage methods and numerous applications. Pre-heater Evaporator Superheater

Heat transfer coefficient (W/m2 K) 1700 860 500 Average fluid temp. (1C) 140 275 440 6. Parabolic dish collectors (PDC) Max tube temp. (1C) 189 325 569 Min tube temp. (1C) 142 281 455 Parabolic dish reflectors are point-focus collectors. As such, Temperature drop (K) 47 44 114 they can achieve very high light concentration ratios, reaching up D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2713 to 1000 sun. At temperatures exceeding 1500 1C, they can pro- At such high operation temperatures, heat losses become duce power efficiently by utilizing high energy conversion cycles. extremely significant, and must be contended with to achieve The collector type features a large parabolic-shaped dish, which high efficiencies. A detailed two-dimensional simulation of heat must track the sun on a two-axis tracking system to maintain transfer in a modified cavity receiver of PDC system is presented light convergence at its focal point. A receiver is mounted at the by Reddy and Kumar [58]. Combined heat losses due to both focus, collecting solar radiation as heat. Two general schemes are laminar convection and surface radiation from the receiver are possible for power conversion; the less popular has a heat transfer calculated by this model. The modified cavity receiver (Fig. 15a fluid system connecting the receivers of several dishes, conducting and b) has a semi-circle shape that features a small aperture at thermal energy towards a central electricity generation system. This the dish’s focal point. The receiver is essentially hollow (air design is less convenient as it requires a piping and pumping cavity) and its inner surface is laid with absorber tubes. The system resilient to very high temperatures, and suffers from encasing of the tubes is made of insulating material. transport thermal losses. The more prevalent system mandates Reddy and Kumar published another numerical analysis in a heat engine be mounted near/at the focal points of individual 2009 [59], in which a three-dimensional model is used to dishes. The heat engine absorbs thermal energy from the receiver, estimate receiver heat losses at different dish inclination angles and uses it to produce mechanical work, which an attached and various operating temperatures. The model evaluates heat alternator then converts into electricity. A heat-waste exhaust loss reductions realized through secondary concentrator integra- system must be incorporated to release excess heat from the tion. A cone collector, compound parabolic collector (CPC) and system. Finally, a control system is necessary to ensure matching trumpet reflector were compared as second stage concentrators of the heat engine’s operation to the incoming solar flux. (Fig. 16a–c), and yielded natural convection heat loss reductions An advantage of this design is that the reflector, collector and of 29.23%, 19.81% and 19.16%, respectively. engine can operate as separate units, making fossil-fuel hybridi- Another thermal analysis of a PDC system was done by Nepveuat zation a relatively simple task. It is important to note however, al. [60]. The authors constructed a thermal energy conversion model that this PDC system does not lend itself to thermal storage of the 10 kW Eurodish/Stirling unit erected at the CNRSPROMES methods. laboratory in Odeillo. The model analyzes spillage and radiation The Stirling engine is often used for this application, although (reflection and IR-emission) losses of the reflector, and calculates gas turbines can also be employed in the Brayton or Rankine/ conduction, convection, reflection and thermal radiation losses Brayton combined cycles. Stirling engine performance is better in through the receiver cavity (Fig. 17). A thermodynamic analysis of temperatures below 950 1C, whereas at higher temperatures, a SOLO Stirling 161 engine is also presented. The model was combined cycle gas turbines can achieve higher efficiencies [54]. compared to experimental results of the solar power system and The operations and specifications of a 10 kW single dish-Stirling was determined a good fit. system were described in detail by Jin-Soo et al. [55]. Aninnovativesolarthermalpowerapproachwasformulatedby Due to its high concentration ratios, the parabolic dish collector Shuang-Ying et al. [61]. This design features a dish concentrator is an excellent candidate for concentrated photovoltaics. The usage of state-of-the-art, high-cost high-performance photovoltaic cells is justified when they are utilized at concentrations exceeding 100 sun; a large solar flux focused in a small region of cells can produce enough power to offset the high capital investment required. GaAs and multi-junction PV cells are very expensive to fabricate. Yet, operational module efficiencies exceeding 30% have been demonstrated by multiple manufacturers and verified by the National Renewable Energy Lab (NREL). Moreover, these PV tech- nologies are very heat-resistant, and perform better under high concentration ratios. Incorporating such modules into the parabolic dish collector apparatus is fairly simple, and can yield results that are comparable to or better than heat engine systems, potentially with a longer lifetime. Further discussion of concentrated photo- voltaics is developed in a later section. A numerical simulation of a heat-pipe receiver for the para- bolic dish collector was performed by Hui et al. [56]. Using this type of receiver between the dish and the Stirling engine is reported to provide power uniformly and nearly isothermally to the engine heater. This results in improved engine performance. Heat-pipe utilization also limits convective heat loss from the receiver. Parabolic dish collectors are high-cost devices: they are very large mirrors that must feature nearly perfect concavity to effectively concentrate solar radiation. They are also very heavy, and their tracking system must thus be very sensitive and finely tuned. A novel suggestion by Kussul et al. [57] to moderate the high collector cost is to manufacture an approximated parabolic dish using many small, flat mirrors. A prototype was constructed by the group, which contains 24 mirrors in the shape of equi- lateral triangles, each with a side length of 5 cm special nuts are used to maintain required positions of nodes in the connection Fig. 15. (a) Light collection and (b) general schematics of air cavity receiver in a points of mirror apexes. These small mirror arrangements approx- dish/Stirling system. imate a parabolic collector in a relatively inexpensive way. Figures reproduced with permission from ref. 58, &2008 Elsevier. 2714 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725

relying on pre-existing technologies. The mini-dish scheme was also suggested for integration with high concentration photovol- taics [64]. Innovation in parabolic dish reflector technology has promoted this highly efficient yet expensive technology towards the goal of being reasonably affordable. Novel improvements in reflector structure and collector design continue to boost the thermal efficiency of this concentrated solar power scheme. The use of a Stirling engine at a PDC’s focus helps alleviate the losses and costs associated with heat transport. However, this regime does not comply with thermal storage in a simple manner, a significant issue in the scope of year-round power production.

7. Concentrated photovoltaics Fig. 16. (a–c). Secondary reflectors for a parabolic dish reflector system. Figure reproduced with permission from ref. 59, &2009 Springer. The concept of concentrated photovoltaics is rapidly becoming a dominant player in the solar power production market. In March 2010, the 330 kW ‘OPEL Solar’ (Spain) became the first operational utility-grade CPV power plant. CPV systems employ various light concentration schemes to focus large amounts of solar radiation onto small solar cell modules. Very small units of high-cost high- efficiency solar cells are used to absorb the high incoming flux, which makes the CPV model economically competitive. Mainstream concentrator technologies utilized are parabolic dish collectors and Fresnel lenses. Designs using PTC [27] and HFC [31] systems (discussed in previous sections) have been reported as well. The type of solar cell technology used in a CPV system is chosen according to the desired concentration level. While the performance of most PV technologies increases with solar con- centration ratios, excessive heating can be detrimental to the efficiency and lifetime of solar cells. Organic and amorphous silicon cells are generally too heat-sensitive to be used with concentrators. Conventional monocrystalline silicon cells can operate efficiently at lower concentrations (1–100 sun) without Fig. 17. Eurodish receiver heat flow and heat loss diagram. needing active cooling mechanisms. Low concentration systems Figure reproduced with permission from ref. 60, &2009 Elsevier. generally feature wider acceptance angles, and in some cases do not need to track the sun, reducing their cost. Two-axis tracking systems are required in high concentration cascaded with an alkali metal thermal-to-electric converter (AMTEC) systems. Gallium arsenide and multi-junction cells are better used through a coupling heat exchanger. The proposed system employs in medium–high concentration systems (100–300 sun, 300 sun and a heat-pipe receiver for isothermal energy transfer from the above). These cells are very expensive to manufacture, but have dish to the AMTEC unit. Theoretical investigation of this system’s exhibited record conversion efficiencies and operate well under high performance predicts a 20.6% peak thermal-to-electric conversion temperature. Still, heat sinks are often integrated with high-con- efficiency. Effects of various parameters on the overall conversion centration CPV modules in order to alleviate high temperature efficiency of the parabolic dish/AMTEC system are discussed in detail. effects and prolong cell lifetime. Examples of cooling mechanisms Increasing the geometric concentration and tilting angles of the dish include direct water cooling and thermal conduction by heat pipes, both result in efficiency enhancement. The authors report that this discussed by Farahat [65]. design has a potential to become a leading low-cost renewable Due to very high material and manufacturing costs, multi- energy source because of its passive nature. junction cells are significantly more expensive than silicon cells A paradigm shift in PDC design suggested by Feuermann and per unit area. Yet, multi-junction cell efficiency can be up to 15% Gordon [62] utilizes arrays of mini-dishes coupled with fiber greater than that of silicon cells, which can make a big difference optics that carry solar radiation to a central receiver. Each mirror in performance at high solar concentrations. Furthermore, the is about 20 cm in diameter and has a small flat mirror at its focal small PV receivers account for only a fraction of the total CPV point, to which a single optical fiber is attached. The fiber system cost, hence system economics may very well favor the use transports collected light to a central receiving unit, where it of multi-junction cells. can be converted into heat. Low attenuation fibers of high Another recently explored concept is the Concentrating Photo- numerical aperture, coupled with mass produced highly accurate voltaics and Thermal (CPVT) design. This scheme produces both parabolic dishes, can operate at 80% efficiency and yield incred- electricity and heat simultaneously in a single system. The heat ibly high concentration ratios of up to 30,000 sun. Experimental can be used for industrial heat processes, heating and cooling of realization and field experience of this proposed system were buildings, or simply to increase electricity output. A parabolic carried out by Feuermann et al. [63]. One mm diameter optical trough CPVT design was introduced by Coventry [27], and a solar fibers repeatedly transported solar flux levels of 11–12 ksun to a tower design was suggested by Segal et al. [31]. Small CPVT target as far as 20 m away. The prototype proved impervious to systems can be installed in private homes, and can feature a total dust penetration and condensation, and was reportedly con- energy output of over 50% compared with 10–20% of the basic PV structed solely from off-the-shelf parts and customized items panels. D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2715

A novel design for a miniature parabolic dish collector CPVT (polymethylmethacrylate, PMMA). They are flat on one side and system for residential use was presented by Kribus et al. [66]. ridged on the other. The Fresnel lens structure is composed of Analysis of the electric and thermal performance, heat transport many concentric rings, which are thinner towards the center. system, manufacturing cost and resulting cost of energy for Each ring is slightly angled to concentrate incident light onto the domestic water heating is carried out. The reflector is made of a focal point of the lens (Fig. 18). single thermally bent glass sheet coated with silver to produce the Linear Fresnel lenses operate in a similar manner, but feature a reflective surface. An external protective coating prevents exposure focal line instead of a focal point. A linear Fresnel collector can of the silver to the environment. A 32% conversion efficiency multi- include an array of these lenses positioned side-by-side. The array junction module is mounted at the focal point, over a cooling plate is mounted on a sun-tracking device. Every lens is mounted on a that removes the surplus heat from the cells to a coolant fluid small axis through the center of its length, which can orient it to (typically water). The heated coolant is directed to a heat exchanger follow the sun. The entire collector unit can track the sun along where the transported thermal energy may be used as an additional the second dimension, providing the system with a two-axis energy product. Performance testing of a 0.95 m2 dish area under tracking regime (Fig. 19). An optical and thermal performance direct insolation of 900 W/m2 yielded an electrical output of 172 W simulation for this type of system was done by Mallick and Eames and a thermal output of 530 W, exceeding 60% of the input energy. [68]. The effects of varied spacing between linear lenses within an Miniature dish collectors can be used to achieve very high array on the efficiency are presented. Linear Fresnel lenses also concentration CPV systems. Investigation of this type of system can be coupled with small secondary concentrators to minimize operating at a concentration ratio of 1000 sun was presented by the PV receiver area needed [69]. Feuermann and Gordon [64]. The system features high-efficiency Optimization of concentration level, cell technology, receiver heterojunction cells as the PV receiver and utilizes optical fibers size and shape and heating/cooling management is necessary to for heat conduction towards a passive heat sink. Arrays of these achieve high performance systems. A study of the energetic and small systems can be mounted together on large, two-axis thermal characteristics of a small CPV system was conducted by tracking systems. The merits and identified problems of a similar Mirzabaev et al. [70]. The module was based on a Fresnel lens and design were discussed by Anton et al. [67]. an AlGaAs–GaAs PV receiver, and compared several receiver sizes Fresnel lenses used in CPV systems are small and very thin and contact shapes (tetragonal and circular). Analysis of the (3–5 mm), and are generally made of glass, plastic or acrylic resin Fresnel lens solar collector thermal efficiency was done by Zhai et al. [71], and was found to be about 50% when using an evacuated tube receiver on a clear day. One problem with the use of conventional Fresnel lenses for concentrated photovoltaic is uneven illumination of the solar cell receiver. Non-uniform intensity distributions can result in local heating and ohmic drops in CPV systems, preventing maximum power extraction. Several innovative designs to overcome this issue have been presented in the last 5 years. Ryu et al. [72] devised a new concept of a modular array of Fresnel lenses for low-medium concentration CPV systems, which is based on the concept of superposition. A two-dimensional array of lenses is constructed. Each lens is slightly larger than the PV receiver itself. Individual lens facets are angled to direct normal incident light Fig. 18. Schematic side-view of a Fresnel lens (left) compared with a circular lens (right). onto specific regions of the solar cell module (Fig. 20a and b). Source: Wikipedia. Proper determination of the facet angle for each lens in the array

Fig. 19. Photovoltaic cell arrays encased with Fresnel lenses, mounted on a two-axis sun-tracker. Figure reproduced with permission from ref. 68, &2007 Springer. 2716 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725

Fig. 20. (a) Modular Fresnel lenses concept for concentrated photovoltaics. (b) Cross-sectional view of modular Fresnel lenses array. Figures reproduced with permission from ref. 72, &2006 Elsevier.

electricity is produced directly by solar cells, which removes the need for complicated heat transport and large boiler/turbine systems. On the other hand, the efficiencies associated with CSP alone are generally higher, and collected solar energy can be stored thermally, a benefit solar cells do not enjoy. Combining state-of- the-art solar cells with high-concentration reflectors allows a great amount solar flux to be converted to electric power at high efficiency, while keeping solar cell expenses to a minimum (as only a small photovoltaic cell area is needed). The combined CPVT scheme yields very high conversion efficiencies, but is inevitably more complicated and thus more costly to execute. Still, further progress in solar cell and reflector designs will reduce these expenses, making this type of power production scheme more affordable.

8. Concentrated solar thermoelectrics Fig. 21. Wide acceptance angle design for cylindrical Fresnel lens. Figure reproduced with permission from ref. 73, &2009 SPIE. Conversion of solar energy into electricity directly can also be achieved using the concept of thermoelectrics. Recent developments must be implemented, and can vary across different systems in thermoelectric applications have been exploring ways to utilize (according to size, output, etc.). Mathematical evaluations of the CSP to generate electricity. Solar thermoelectric devices can convert performance and concentration efficiency are presented, along a solar thermal energy (typically waste heat) induced temperature with illustrations of the new concept. gradient into electricity. They can also be modified to perform When investing in high-quality solar cells, it is desirable to cooling or heating. One advantage of thermoelectric methods integrate them with systems that achieve very high concentrations. (compared with heat engines) is their increased reliability, as such At such conditions, however, Fresnel lenses have a very narrow devices could work 10–30 years with little technical problems [75]. acceptance angle range (on the order of 711), and the system must Moreover, thermoelectric generators are a flexible source of clean include very fine tracking mechanisms for efficient absorption to energy capable of meeting a wide range of requirements. occur. The design of a cylindrically symmetric Fresnel lens was Hybrid systems that combine thermoelectric and photovoltaic explored by Yu-Ting and Guo-Dung [73]. A simulation of a CPV are under development. This type of system allows harvesting of system incorporating this technology was carried out at high solar radiation in both the ultraviolet and infrared ranges of the concentration (300–400 sun). A couple of system designs was spectrum. Such a hybrid can also reduce wasted thermal energy, presented. The most successful design (Fig. 21) incorporated the since it ‘functionalizes’ a wide temperature range for power cylindrical Fresnel lens, two reflective surfaces, a biconic lens and a production. While most silicon solar cell performance begins to light pipe. This structure, though fairly complicated, expanded the degrade at temperatures approaching 100 1C, thermoelectric acceptance angle to 7101. Theoretical discussion of the optical devices actually perform better at temperatures over 200 1C. capabilities of a cylindrical lens was presented by Gonzalez [74]. A solar thermoelectric power generator typically consists of a Both a concentration level of 70% of the theoretical maximum and a thermal collector and a thermoelectric generator. Heat is 100% geometrical optical efficiency were reported. The lens also absorbed by the thermal collector, then concentrated and con- featured very uniform illumination of the receiver, an important ducted over the thermoelectric generator by a fluid pipe. The attribute for concentrated photovoltaic systems. thermal resistance of the generator creates a temperature differ- The integration of solar cells with CSP technologies requires a ence between the absorber plate and the fluid, which is propor- cautious balancing of the advantages and issues of each. On one hand, tional to the incoming heat flux. The current produced by the D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2717 thermoelectric generator is in turn proportional to the tempera- promote great interest in its exploration and motivate continued ture difference. research of design and materials. To increase the efficiency of current solar thermoelectric devices, two main things must be accomplished: (i) improved thermal transmission of the solar collector and (ii) higher con- 9. Thermal energy storage centration of the solar radiation onto the hot side of the thermo- electric device. Since thermoelectrics made of high quality A significant complication with the utilization of solar thermal materials are relatively expensive, a key design consideration power as a primary source of energy is the variable supply of solar for these solar generators is minimal use of thermoelectric flux throughout the day, as well as throughout the year. Although materials. Naturally, amounts used must be adjusted in accor- there is a reasonable match between the hours of the day in which dance with desired power requirements. both available solar energy and electricity consumption peak, night- The use of solar concentrating elements can augment the time energy usage must be taken into consideration. Additionally, magnitude of the heat flux absorbed by a thermoelectric device, seasonal and weather changes greatly influence the amount of solar contributing to a higher temperature gradient across it. Among thermal energy that can be harvested. An affordable, reliable energy the single line focusing parabolic trough collector, the compound storage method is thus a crucial element in a successful year-round parabolic concentrator and the two-stage concentrator, the latter operation of a thermal solar power plant. uses a secondary receiver to further concentrate the incident solar The cyclical availability of solar energy determines two types radiation. A design for a two-stage solar concentrator has been of thermal storage are necessary for maintaining a constant proposed [76], which is well-suited to commercially available supply of solar thermal power driven electricity. The first is thermoelectric devices for small scale power generation. The two- short-term storage, where excess energy harvested daily is stored stage solar concentrators comprise of a primary, one-axis PTC, for nighttime usage. The second is long-term storage in which with a secondary, symmetrical CPC mounted at its focus. excess energy is stored during spring and summer months in Several designs have been suggested to further increase the order to complement the smaller energy flux available in winter. hot side temperature of the generator. Solar concentration must Thermal energy storage can be divided into three main be greater than 20 sun to effectively irradiate a thermoelectric categories: sensible heat storage, latent heat storage and chemical device [76,77]. Schematics of two solar thermoelectric regimes storage. Sensible heat storage involves heating a solid or liquid that incorporate concentrators are shown (Fig. 22a and b). Both and insulating it form the environment until the stored thermal schematics are based on the two-stage concentrator design, energy is ready to be used. Latent heat storage involves the phase where the second concentrator also acts as a receiver and can change (generally solid–liquid) of the storage material. The heat- generate a larger temperature difference across the thermoelec- induced phase change stores a great deal of thermal energy while tric device. The receiver can combine a thermionic converter (TIC) maintaining a constant temperature, and can be easily utilized for with a thermoelectric converter (TEC) to use thermal energy more nighttime energy storage if kept under proper isolation. A plot efficiently (Fig. 22a). The TIC is a cylindrical cavity-type solar demonstrating sensible and latent heat storage is given (Fig. 23). receiver made of graphite, which is heated in a vacuum by the Chemical storage is implemented using harvested thermal energy solar concentrator. Once the TIC emitter is uniformly heated up to in reversible synthesis/de-synthesis endothermic reactions. The heat 1800 K, a hot side generator temperature of 1800 K can be ‘invested’ in producing/dissociating a certain material (ammonia, achieved [78]. The thermoelectric device can also be attached methane, etc.) can be easily stored indefinitely. The reverse, exother- directly to the absorber plate of the receiver (Fig. 22b). mic reaction will release the heat with minimal losses for electricity The field of solar thermoelectric power generation, its coupling generation at a later time. Chemical storage is thus most suitable for with two-stage solar concentrators in particular, is a very recent long-term or seasonal storage. innovation in the scope of CSP. Many solar thermoelectric designs Sensible heat storage can employ a large variety of solid and are not fully developed or are still in their initial stage. However, liquid materials. It can be put into practice in a direct or indirect the usefulness and diversity of applications this concept offers manner. For storage in solids such as reinforced concrete, solid NaCl and silica fire bricks, an indirect storage method must be implemented. This type of system uses a heat transfer fluid to circulate through absorbers, collect heat and transport it to the storage tank. The HTF is then put in thermal contact with the storage solids, allowing them to absorb the heat convectively. Sensible heat storage in liquids can be achieved in a direct fashion, i.e. the heat storage liquids themselves are used as heat transfer fluids, and are transported to an insulating storage tank after circulating through the solar absorbers. Mineral oil, synthetic oil, silicone oil, nitrate, nitrite and carbonate salts,

Fig. 22. Schematic of two-stage concentrator design featuring a (a) thermionic Fig. 23. Sensible vs. latent heat storage. converter and (b) thermoelectric device (only). Figure reproduced with permission from ref. 79, &2010 Elsevier. 2718 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 as well as liquid sodium, can all be used for sensible heat storage. conductivity, which results in slow charge–discharge rates. One Desired characteristics of ‘sensible-heat-storage-friendly’ molten suggested initiative for alleviating this problem involves the fabrica- salts include high density, low vapor pressure, moderate specific tion of PCM composite materials; mixing pure PCMs with graphite, heat, low chemical reactivity and low cost. One big disadvantage for example, can boost thermal conductivity and promote faster of molten salts is that they are usually quite pricey. Detailed energy storing and releasing. characteristics of storage materials (Table 5a and b) are given by Since sensible and latent thermal energy storage schemes can Gil et al. [79]. only retain their energy efficiently for so long, the need for long- Latent heat storage in the solid–liquid phase transition of term, cross-seasonal storage is made possible by thermo-chemical materials is considered a good alternative for sensible heat storage. storage processes. Thermal energy storage in heat intensive From an energy perspective, storage using phase change materials endothermic reactions has the possibility to realize higher energy (PCM) can operate in relatively narrow temperature ranges between efficient processes the thermal storage regimes. Potentially high charging and discharging thermal energy. Additionally, PCM materi- energy densities can be stored using chemical storage. als generally feature higher densities than sensible heat storage Reformation of methane and CO2 [30], metal–oxide/metal materials. The interest in PCM latent heat storage systems is conversions [80] and ammonia synthesis/dissociation [14,81] increasing, mainly due to potential improvements in energy effi- are just a few examples of heat-assisted chemical reactions ciency and nearly isothermal energy storage and release. In addition that can store solar thermal energy in their endothermic reac- to the few commercially available PCMs today, many organic and tion products and release it at a later time/place by the reverse inorganic compounds are being investigated for latent heat storage process. Numerous heat-storing chemical reactions are listed purposes (Table 5c–e). A disadvantage of PCMs is their low thermal (Table 5f).

Table 5 a–f. Various thermal storage materials and their properties. Data compiled from [79].

(a) Sensible heat storage liquid materials and their properties

Storage medium HIETC Mineral Synthetic Silicone Nitrite Nitrate Carbonate Liquid solar salt oil oil oil salts salts salts sodium

Temp. (cold) (1C) 120 200 250 300 250 265 450 270 Temp. (hot) (1C) 133 300 350 400 450 565 850 530 Avg. density (kg/m3) n/a 770 900 900 1825 1870 2100 850 Avg. thermal conductivity (W/m K) n/a 0.12 0.11 0.10 0.57 0.52 2.0 71.0 Avg. heat capacity (kJ/kg K) n/a 2.6 2.3 2.1 1.5 1.6 1.8 1.3 Volume specific heat capacity (kWht/m3) n/a 55 57 52 152 250 430 80 Cost per kWh (US$/kWh) n/a 4.2 43.0 80.0 12.0 3.7 11.0 21.0

(b) Sensible heat storage solid materials and their properties

Storage Medium Sand-rock Reinforced NaCl Cast Iron Cast Silica Magnesia Mineral Oil Concrete (Solid) Steel Fire Bricks Fire Bricks

Temp. (cold) (1C) 200 200 200 200 200 200 200 Temp. (hot) (1C) 300 400 500 400 700 700 1200 Avg. density (kg/m3) 1700 2200 2160 7200 7800 1820 3000 Avg. thermal conductivity (W/m K) 1.0 1.5 7.0 37.0 40.0 1.5 5.0 Avg. heat capacity (kJ/kg K) 1.30 0.85 0.85 0.56 0.60 1.00 1.15 Volume specific heat capacity (kWh/m3) 60 100 150 160 450 150 600 Cost per kWh (US$/kWh) 4.2 1.0 1.5 32.0 60.0 7.0 6.0

(c) Commercial phase change materials (PCMs) and their properties

Name Type Phase change Density Specific heat Thermal Latent heat temp. (1C) (kg/m3) (kJ/kg K) conductivity (kJ/kg) (W/m K)

RT110 Paraffin 112 n/a n/a n/a 213 E117 Inorganic 117 1450 2.61 0.70 169 A164 Organic 164 1500 n/a n/a 306

(d) Inorganic substances with potential use as phase change materials

Compound Phase change Density Specific heat Thermal conductivity Latent heat temp. (1C) (kg/m3) (kJ/kg K) (W/m K) (kJ/kg)

MgCl2-6H2O 115–117 1450 (liquid, 120 1C) n/a 0.570 (liquid, 120 1C) 165 1570 (solid, 20 1C) 0.598 (liquid, 140 1C) 0.694 (solid, 90 1C) 0.704 (solid, 110 1C)

Hitec: KNO3–NaNO2–NaNO3 120 n/a n/a n/a n/a

Hitec XL: 48%Ca(NO3)2–45%KNO3–7%NaNO3 130 n/a n/a n/a n/a

Mg(NO3)–2H2O 130 n/a n/a n/a n/a

KNO3–NaNO2–NaNO3 132 n/a n/a n/a 275

68% KNO3–32% LiNO3 133 n/a n/a n/a n/a

KNO3–NaNO2–NaNO3 141 n/a n/a n/a 75 Isomalt 147 n/a n/a n/a 275

LiNO3–NaNO3 195 n/a n/a n/a 252 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2719

Table 5 (continued )

(d) Inorganic substances with potential use as phase change materials

Compound Phase change Density Specific heat Thermal conductivity Latent heat temp. (1C) (kg/m3) (kJ/kg K) (W/m K) (kJ/kg)

40%KNO3–60%NaNO3 220 n/a n/a n/a n/a

54%KNO3–46%NaNO3 220 n/a n/a n/a n/a

NaNO3 307 2260 n/a 0.5 174

KNO3/KCl 320 2100 1.21 0.5 74

KNO3 333–336 2.11 n/a 0.5 266 KOH 380 2.044 n/a 0.5 149.7

MgCl2/KCl/NaCl 380 1800 0.96 n/a 400

AlSi12 576 2700 1.038 160 560

AlSi20 585 n/a n/a n/a 460

MgCl2 714 2140 n/a n/a 452

80.5% LiF–19.5% CaF2 eutetic 767 2100 1.97 1.7 790 NaCl 800–802 2160 n/a 5.0 492

NaCO3–BaCO3/MgO 500–850 2600 n/a 5.0 n/a LiF 850 n/a n/a n/a 1800 (MJ/m3)

Na2CO3 854 2533 n/a 2.0 275.7 KF 857 2370 n/a n/a 452

K2CO3 897 2290 n/a 2.0 235.8

KNO3/NaNO3 eutetic n/a n/a n/a 0.8 94.25

(e) Organic substances with potential use as phase change materials

Compound Phase change Latent heat Latent heat temp. (1C) (kJ/kg) (kJ/L)

Isomalt: ((C12H24O11–2H2O)þ(C12H24O11)) 147 275 n/a Adipic acid 152 247 n/a Dimethylol propionic acid 153 275 n/a Pentaerythritol 187 255 n/a

AMPL ((NH2)(CH3)C(CH2OH)2) 112 28.5 2991.4

TRIS ((NH2)C(CH2OH)3) 172 27.6 3340 (kJ/kmol)

NPG ((CH3)2C(CH2OH)2) 126 44.3 4602.4 (kJ/kmol)

PE (C(CH2OH)4) 260 36.9 5020 (kJ/kmol)

(f) Chemical storage materials and reactions

Compound Material energy density Reaction temp. (1C) Chemical reaction

Ammonia 67 kJ/mol 400–500 NH3þDH’-1/2N2þ3/2H2

Methane/water n/a 500–1000 CH4þH2O’-COþ3H2 3 Hydroxides 3.0 GJ/m 500 Ca(OH2)’-CaOþH2O 3 Calcium carbonate 4.4 GJ/m 800–900 CaCO3’-CaOþCO2 3 Iron carbonate 2.6 GJ/m 180 FeCO3’-FeOþCO2 3 Metal hydrides 4.0 GJ/m 200–300 Metal xH2’-metal yH2 þ(xy)H2

Metal oxides (Zn and Fe) n/a 2000–2500 2-step water splitting: Fe3O4/FeO redox system Aluminum ore alumina n/a 2100–2300 n/a

Methanolation–demethanolation n/a 200–250 CH3OH’-COþ2H2 3 Magnesium oxide 3.3 GJ/m 250–400 MgOþH2O’-Mg(OH)2

Every storage method mentioned can play an important role in to a closed loop system, which usually uses water as its working several concentrated solar power designs. The chosen storage fluid. Cycle operation is outlined in several repeating steps. scheme must, however, be carefully matched to the size (total Working fluid is pumped from low to high pressure. This requires power output) and operational procedures associated with a specific little input energy for the pump if the fluid is a liquid. This is one plant, as well as to its governing environmental and economic advantage of the Rankine cycle. High pressure liquid is heated in a factors. Luckily, the developments made to date in all three thermal boiler at a constant pressure to become saturated vapor. The storage methods offer a great diversity of materials from which one vapor is then allowed to expand through a turbine generator to can choose in order to meet varying necessary parameters. produce electricity. Next, it is condensed at a constant pressure to become a saturated liquid, and is transferred back into the pump’s reservoir. The working fluid is constantly re-used in this thermo- 10. Energy cycles dynamic loop. If vapor temperature is not very high (wet vapor), condensation can occur during release through the turbine, and The conversion of solar thermal energy into electricity generally fast moving water droplets damage the turbine and reduce its requires the use of a thermodynamic cycle. lifetime and efficiency. Rankine operations at high temperatures Several types of cycles are the mainstream options for heat produce ‘dryer vapor’, and can thus considerably increase system conversion into work. They can vary in design and process performance. Solar powered Rankine cycles using low cost efficiency, but all cycles use heat harvested from solar collectors collectors for clean water and power generation are reviewed to power a generator for electricity production. by Garcı´a-Rodrı´guez and Blanco-Ga´lvez [82]. The most common thermodynamic cycle used is the Rankine The ‘organic’ Rankine cycle utilizes organic fluid such as cycle. In this regime, heat is supplied externally (from collectors) toluene or n-pentane for working fluids. The cycle operation 2720 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 process is identical, but can operate at lower temperatures utilizes is a magneto-hydrodynamic (MHD) cycle. This cycle (70–90 1C). These lower temperatures result in a lower thermo- operates at very high temperatures, upwards of 2000 1C. It passes dynamic efficiency, but this may be counter-balanced by the hot ionized gas through a magnetic field, resulting in electric lower heat inputs required to drive the system. Organic fluids that current generation. The great amount of heat is exhausted into a have boiling points above water can be used, and this may have Brayton and Rankine bottoming cycles connected in series. The thermodynamic benefits. A comparison of several working fluids triple cycle needs to be integrated with an HFC design in order to for organic Rankine cycle operation of PTCs was carried out by meeting the high temperature requirement. The overall peak Delgado-Torres and Garcia-Rodriguez [83]. conversion efficiency of the solar triple cycle is shown to be The Brayton cycle has also been adapted for CSP electricity significantly higher than the solar combined cycle scheme. The generation. This cycle uses a gas compressor, a combustion sensitivity of this result to several system parameters and chamber and an expansion turbine. General operations of the the technological feasibility of the triple cycle are examined by Brayton cycle begin with ambient air being drawn into a com- the authors. pressor to be pressurized. It is then directed into a combustion The improvement of well-understood energy cycles and the chamber, where it is heated at a constant pressure. Convention- development of new ones greatly extend the potential of all nearly ally, this heating is done by burning fossil fuels, but thermal all concentrated solar power production regimes. The contributions energy harvested from solar collectors performs this task in a CSP of advanced/high energy cycles to the overall thermal-to-electric power plant. The heated air is allowed to expand through a gas power conversion efficiency can be very significant, and help bring turbine (or a series of turbines) to produce electricity. The CSP closer to the realm of grid-parity. It is important to note that compressor can be powered by the turbine generators. Excess relative costs associated with this step become quite considerable heat is exhausted into the atmosphere. In 2002 a hybrid open with increased levels of sophistication, a fact that must be weighed solar Brayton cycle was operated for the first time consistently against the benefits such clever designs provide. and effectively in the frame of the EU SOLGATE program. Air was heated from 570 K to over 1000 K in the combustor chamber. One clear advantage of the Brayton cycle is that air is cheap and 11. Applications available everywhere. A regeneration mechanism can be incorpo- rated to improve Brayton cycle efficiency. Still-warm air that has In addition to the main objective of electricity production, already passed through the turbine can be circulated back concentrated solar power technologies offer a large variety of towards the compressor intake and pre-heat air before it enters applications for which solar thermal energy can be harnessed. the combustion chamber. Less heat is exhausted out of the Industrial heat processes, chemical production, salt-water desali- system, and less power is consumed by the chamber’s heating nation, heating and cooling are just a few examples of the mechanism. The Brayton cycle generally operates at significantly plethora of available applications that can be implemented using higher temperatures than the Rankine cycle. Despite this fact, the CSP technologies. It is important to note that some applications overall efficiencies of large-scale steam generators and gas tur- are CSP technology selective – they require integration with a bines seem to be similar. specific CSP design – while others can be coupled to several of the The combined cycle utilizes a hybrid of the Rankine and regimes discussed in this article. Brayton cycles, and can achieve higher efficiencies than either. The use of solar thermal power for water desalination and The combined cycle uses the Rankine cycle as a bottoming cycle; purification has been discussed repeatedly in the literature. The heated air is first used to power turbines in the Brayton regime. fact that regions of the world where clean drinking water is scarce Excess heat, which would otherwise be exhausted into the also have an abundance of solar radiation, which makes this CSP atmosphere, is instead employed as a heating mechanism for a application very worthwhile. Desalination is generally done by Rankine (steam) cycle. Though it is more efficient, this design is evaporating salt-water to leave salt behind, then condensing salt more bulky and expensive to implement. A cost-efficiency analysis free vapor back into its liquid state. The process of heating large must be carried out for a given plant size and output in order to amounts of water for drinking and agricultural purposes requires evaluate economic viability. A discussion of a combined cycle CSP immense amount of energy. Concentrating solar radiation and design using solar tower reflector technology is presented by Kribus converting it to heat is an efficient method by which this process et al. [84]. Both hybrid and solar-only power plants are investigated. can be achieved using emission-free, renewable energy. In addi- An efficiency study of the combined cycle was done by Donatini tion to boiling the water, thermal power could be used to power et al. [85]. The project examined the combined cycle integrated in a absorption chillers, thus using the same power source both for parabolic trough collector regime using molten salts as the heat evaporation and condensation of water. Several plant designs for transfer fluid. solar powered desalination, detoxification and disinfection Decreasing the cost and improving the efficiency of power of water are presented by Blanco et al. [88]. Designs for both production cycles can greatly influence the market penetration of large-scale and small-scale operations are discussed. Solar water- concentrated solar power technologies. A few innovative energy detoxification schematics are presented, which are based on the cycles have been discussed in the literature, which use multi- concept of using near-ultraviolet visible spectrum bands to component working fluids or employ additional cycle steps to promote oxidizers generation. Solar water disinfection utilizes improve efficiency and limit power consumption. the same method, but incorporates a supported photocatalyst to A multi-component working fluid features variable boiling generate powerful oxidizers to control and destroy pathogenic temperatures according to its composition. This process can yield water organisms. A different desalination design by Alrobaei [89] a better thermodynamic match with different sensible heat serves the same purpose using parabolic trough collectors sources than can be achieved with a single-component fluid. coupled to a gas turbine operating in the combined Rankine/ The advantages of using an ammonia/water mixture as a working Brayton cycle. fluid are reviewed by Goswami et al. [86]. The mixture is utilized A novel application of CSP was presented by Perez-de-los- in the bottoming Rankine cycle of a combined cycle operated Reyes et al. et al. [90], where an array of six parabolic trough plant design. collectors were used to harvest thermal energy for disinfestation An innovative addition to the combined cycle was suggested of greenhouse soils. The system was able to bring soil tempera- by Kribus [87]. A solar triple cycle is proposed, the first of which ture up to 60 1C, and was reported effective by the authors. D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2721

CSP technologies can supply electricity and heat for chemical discussed is used for forming of sheet metals. Utilization of CSP production processes. The production of hydrogen using concen- for this process is reported to be efficient and cost-effective. trated solar power is discussed by Glatzmaier and Blake [91]. The Thermal treatment of crude oil using a parabolic trough collector authors compare two separate processes involving concentrated system was suggested by Mammadov et al. [95]. solar power and the electrolysis of water. In one regime, CSP is Concentrated solar energy can also be used for driving the used to produce alternating current electricity, which is then endothermic reaction that produces lime (calcination reaction). supplied to an electrolyzer operating in ambient temperature. The Running this reaction at above 1300 K is reported to reduce other method utilizes high thermal electrolysis of steam. This emissions of the process by 20–40%, depending on the manufactur- regime was operated at about 1273 K and, thermodynamically, ing plant [96]. An economic assessment for a large-scale (25 MW) required less energy than ambient temperature electrolysis. plant based on this process found estimates lime cost to be roughly A solar collector can provide both AC electricity and thermal twice the current price of conventionally produced lime. This energy to the system in this design. process produces very high purity lime, and its prices might be Heat conversion into electricity followed by the electrolysis of competitive with fossil-fuel based calcination processes for chemical water is a process that involves several lossy steps and thus has a and pharmaceutical sectors requiring unadulterated lime. low overall efficiency. Kolb et al. [92] suggested utilizing solar Solar power can be utilized for temperature control of build- towers for large scale production of hydrogen. The authors ings, providing both heating and cooling mechanisms. A high proposed an alternative design, by which hydrogen is produced efficiency solar cooling process is outlined by Gordon and Choon using a thermo-chemical process. This regime features an HFC, Ng [97]. A cascade of mini-dish collector and gas micro-turbine a solid-particle receiver, a particle thermal energy storage system produces electricity that drives a mechanical chiller, with turbine and a sulfuric acid cycle. Such a thermo-chemical plant is said to heat rejection running absorption chiller. A special feature of this produce hydrogen at a much lower cost than solar-electrolyzer system is that energy can be stored compactly as ice. The plants of similar size. Hydrogen production is an effective chemi- compactness of the solar mini-dish system is conducive for cal storage medium for thermal energy, and can be used for many small-scale ultra-high-performance solar cooling systems. industrial processes as well. The utilization of Fresnel lenses was also suggested for lighting The production of zinc can also be achieved using CSP and temperature control of buildings [98]. A collection system using a technology. A 300 kW solar chemical pilot plant was demon- Fresnel lens concentrator and a solar receiver generally absorbs strated in the framework of the EU-project SOLZINC [80]. Produc- between 60% and 80% of incoming radiation. The remaining solar tion was implemented using a carbothermic reduction process of flux can be distributed in the interior space for illumination and zinc oxide. This process makes zinc production possible at heating needs. On days when solar radiation is high, this provides temperatures of 1300–1500 K, compared with the ZnO dissocia- coolingofinteriorspacesaswellasbrightnesscontrol.Duringlow tion process, which requires temperatures exceeding 2000 K. solar intensity periods, the absorber can be shifted off-focus to permit A ‘beam-down’ HFC regime was used to concentrate solar radia- 100% of light to be distributed around the interior (Fig. 24a–c). The tion onto a dual-cavity solar chemical reactor. The top cavity is a receivercanbeofPVtype,thermaltypeorahybridofthetwo,and solar absorber, and the bottom one is a reaction chamber contain- will collect solar energy for heat and/or electricity generation. ing a ZnO/C packed bed. Demonstration of the plant yielded A parabolic trough collector system was constructed at the 50 kg/h of 95% purity zinc. The measured conversion efficiency Carnegie Mellon University to study the potential of this CSP was 30%. Zinc can be used in batteries and fuel cells, and can be regime in solar heating and cooling [99]. The collective area of the reacted with water to produce high purity hydrogen gas. This is mirrors was 52 m2. The collector system was coupled to a 16 kW an exothermic reaction, and can itself be used for power genera- double effect, water–lithium bromide (LiBr) absorption chiller tion, making zinc a possible thermo-chemical storage candidate. and a heat recovery heat exchanger. Generation of hot and chilled The product of this reaction is in turn ZnO, which can then be water was available depending on the season. Under optimal used again for zinc production. design, the system was able to achieve 39% of cooling and 20% of A process for carbon dioxide recycling was reviewed by heating energy for the interior space off the building it was

Hartvigsen et al. [93]. Co-electrolysis of CO2 and steam can be connected to (Pittsburgh, PA). applied to produce synthesis gas in a large-scale fashion. This The design of a solar absorption refrigeration system directly process not only reduces CO2 emissions into the atmosphere, but powered by a LFR concentrator has also been suggested [100]. can utilize syngas for further clean energy production. Carbon Evaluation of the technical feasibility of LFR integrated solar-GAX dioxide can be recovered from concentrated sources, such as fossil power plants. Using high concentration CSP technologies for endothermic electrolysis reactions can employ both thermal and electrical inputs such that the conversion efficiency within the solid oxide electrolysis cell is 100%. Large-scale implementation of synthetic fuel production from CO2 enables greater use of intermittent renewable energy sources. The large amount of thermal energy that can be harvested using solar concentrators makes them a lucrative option for integration with industrial heat processes. A substantial fraction of these pro- cesses run below 300 1C, an operational temperature achievable by most solar concentrator regimes. An article discussing heat process integration of parabolic trough systems in Cyprus was presented by Kalogirou [16]. CSP can be integrated with existing fossil fuel power plants, and provide thermal energy to aid their operation. An example is presented by Mills et al. [47], in which a linear Fresnel reflector Fig. 24. a–c. Receiver shifting from focus of linear Fresnel lens can be used to plant supplies heat to a coal-fired power station. manage the amount of solar radiation introduced into a building, providing The usage of solar thermal power for superplastic forming temperature control. Dashed lines represent diffuse sunlight. (a) Receiver is in processes is suggested by Lytvynenko and Schur [94]. The process focus, blocking light. (b) Receiver is out of focus. (c) Ventilation mode [98]. 2722 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 cycle is carried out. A parametric study for several design configura- directly converted into steam for power production, and must thus tions is performed in order to obtain optimal operation conditions. be constantly replenished. Water transport costs are thus another The study validates this technology as more than satisfactory; the issue that requires attention. Using the Recirculation DSG mode in numerical simulation demonstrated that this scheme answers both PTC operation will aid water conservation to some degree. The quantity and quality of the advanced cooling system’s energy choice to use synthetic oils may be the best option in a PTC site demands. Furthermore, the operation conditions obtain higher global where water is not abundant. While ionic liquids can be used as system efficiencies than previously used technologies. For example, heat transfer media, they are very expensive to manufacture and the LFR system experienced a 17.9% efficiency increase compared may thus be better suited for higher temperature operations, such with single effect water–lithium bromide cycle coupled in an indirect as those of heliostat field collectors. The operational temperatures form with a PTC system. of PTCs can exceed 400 1C, high enough for a plethora of industrial A great deal of work has also been done to develop small-scale, heat processes, yet too low for the more efficient, high energy solar powered food (fruit, vegetables and nuts) dryers that can be conversion cycles available for power production. Despite the built with local materials [101–107]. However, the existing dryer drawbacks mentioned, it should be noted that the maturity and designs are suited to cloudless, dry environments and they dry successful experience to date with PTC technology put it at the too slowly in hazy situations, typical of many tropical developing forefront of CSP regimes. While other CSP methods may exceed PTC countries. Excessively slow drying allows product degradation efficiencies or be better geared towards storage and applications, caused by microbial decay, insects and naturally occurring the fact that large (upwards of 100 MW) power stations based on enzymes. Some existing designs are also expensive and relatively the PTC scheme have been operational for several years and inefficient, and have low capacity (o50 kg/day). continue to be built proves this technology both successful and Adding a solar concentrating surface increases the heat output economical. of solar devices operating in cloudy or hazy conditions [108]. An interesting comparison can be made between the concepts With indirect solar dryers this can be accomplished by adding of linear Fresnel reflectors and parabolic trough collectors. LFR glazed concentrated solar panels to the system. Concentrating systems prove the cheapest of all CSP regimes, utilizing flat solar panels can be used to inexpensively increase the heat output mirrors instead of concave ones, and having incorporating cen- for indirect dryers. Additionally, they can be used to focus a tralized receiver systems that save on receiver material. Though greater light flux onto the drying zone in direct dryers, allowing they reach an operational temperature of only about 300 1C, they them to operate in low-insolation environments. The reflective can still be used in a variety of applications. The use of DSG works surfaces can be as sophisticated as precision-machined, polished well with LFRs, and the reasoning needed to select a particular surfaces or as simple as cardboard covered in aluminum foil. type of HTF for this type of method is very similar to that of PTCs. The development of a multitude of CSP applications is bene- A multitude of phase change materials have been proposed for ficial in many regards; such applications help turn many carbon use in LFR latent heat storage systems. Although these substances emitting industrial processes into ‘clean’ ones, conserve large are costly, they can preserve thermal energy effectively for over- amounts of electricity that would otherwise be used up and night usage. The shading issue that accompanies LFR systems is a promote a general environmentally friendly approach to energy maximization problem to which many solutions have been consumption for both industries and individuals. Furthermore, suggested. The compact LFR regime greatly reduces shading the growing number of these applications aids CSP technologies between neighboring reflectors, and allows significantly greater in taking root, increasing the demand for solar thermal power and collection of available sunlight. The formation of a wave-shaped advancing it into world markets. platform further enhances solar radiation collection. The inverted air cavity receiver is reported to have substantial mitigating effects over heat loss in LFR during LFR operation, an important 12. Discussion feature that can help boost thermal efficiency. The coating of absorber tubes with Nickel also aids the heat loss issue, and the The variety of available CSP technologies and the advance- two could be used in tandem for maximum heat loss reduction. ments made in each can bring a sense of uncertainty as to which The linear Fresnel reflector method is suited for lower efficiencies technology works best. This is a complicated issue because of the than the rest of its CSP counterparts, but it does so with the many factors that need to be considered in selecting a particular benefits of a significantly more affordable technology. CSP design. Every regime features advantages and disadvantages The relatively young but very powerful CSP concept of helio- that must be accounted for in accordance with the size, location, stat field collectors has come leaps and bounds over the last few purpose and budget of the specific CSP plant one wishes to build. decades. The immense flux a large collection of heliostats can Advantages of the parabolic trough collector CSP regime direct towards the central receiving unit generates very high include relatively low costs, mature and well-tested technologies temperatures (up to 2000 1C), and can thus operate very effi- and easy coupling to fossil fuel/geothermal energy sources. PTC ciently using complex energy conversion cycles, such as the systems are becoming more efficient with the incorporation of combined cycle and the magneto-hydrodynamic (MHD) cycle. novel receiver designs such as the heat pipe receiver, which High operation temperatures may boost electricity production significantly limit convective heat losses while reducing receiver efficiencies, but are accompanied with both a higher thermal cost. The reinforcement of PTCs with a light fiberglass structures stress on many components of the HFC system and a challenging grants them great stability against wind loads, which further boosts convection heat loss problem. The usage of air as a heat transfer the efficiency as it provides for more accurate sun-tracking. The medium becomes available at these high temperatures, which incorporation of direct steam generation into PTC systems is helps relieve some of the stress heated liquids would exert on generally a very positive scheme to produce high quality steam at system components and significantly reduces HTF costs. This a constant rate throughout daylight hours, and the usage of water hybridized air–water heating system can produce steam at very as a heat transfer fluid is generally cheaper than synthetic oils or high temperatures (485 1C) at a constant rate. The special design ionic liquids. That being said, water is a more volatile substance suggested for a receiving unit that has a very large inner surface than other HTFs and will exert more stress on PTC absorber pipes, area compared with its aperture is an excellent solution to help which may increase maintenance costs. It also needs to be readily minimize convective losses, but will increase costs due to its available at the site, since, unlike oils and molten salts, it is being complicated structure. The dual receiver concept for solar towers D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2723 is another novel design that can help boost the total power output than CSP, but the latter comes with much higher initial capital by a significant portion. The quest for cheap materials for heliostat investments. fabrication is a crucial one, as the large mirrors can make up close to The integration of Fresnel lenses with solar cells is thus a great half the cost of an HFC plant. The use of PVC composite plastic steel venture, since the lenses are relatively cheap to manufacture and offers a light yet stiff structure, which helps ease stress on mirror can concentrate light very well. Uniform illumination issues were trackers while increasing their accuracy (high stiffness materials are considered by several researchers, to which the answer of cylindri- more wind resistant). The torque tube heliostat (TTH) scheme cally symmetrical Fresnel lenses proved a formidable solution. suggested for wind load reduction is not very effective; it increases Fresnel lens CPV systems that can track the sun have been developed, the cost and decreases the energy output of the system without to further enhance radiation collection and boost power output significantly diminishing wind stress. The suggested use of mini- throughout the day. The concentrated photovoltaic thermal regime mirror arrays resulted in similar results. Experimentation and is also of interest, as it permits power harvesting of both regimes modeling for non-spherical arrangements of heliostat fields presents simultaneously and can result in extremely high conversion efficien- some potential to increase the amount of solar flux collected from a cies. The mounting of solar cells along the absorber tube of PTC given area, but the great height of the solar tower makes heliostat systems, or at a portion of the focal region of parabolic dishes (and shading a non-vital issue. The design incorporating a reflector tower mini-dishes), has been shown to be quite successful. The installment and a ground receiver is helpful in reducing transport losses, and of PV cells on an HFC receiver for high energy photon absorption makes good organizational sense. The HFC scheme can easily couple made significant contributions to the overall system efficiency. to all three thermal storage methods discussed, giving it a big Unlike CPV systems, CPVTs can store a large portion of collected advantage over other CSP regimes. It is, however, very costly, and energy for later use, but the trade-off from this advantage is based large amounts of power must be produced at high conversion in the HTF costs, which CPV systems do not have. The field of efficiencies to make HFCs a more economically viable technology. concentrated solar thermoelectrics seems to draw much attention as The parabolic dish collector system operates somewhat differ- well, but is currently in its infancy developmental stages and is far ently compared with the aforementioned CSP regimes, as very from commercial power generation capabilities in any scale. large dish is a power generating system within itself. The mounting The great variety of application that can be incorporated into of a Stirling engine (or a Brayton/combined cycle engine) at a dish’s concentrated solar power provides further incentive to invest in focus allows it to operate at very high temperatures throughout the it. Industrial processes can utilize thermal energy directly to save day (usually up to 1000 1C). PDCs are heavy and expensive on the costs of fossil fuels while maintaining an environmentally structures that must track the sun very accurately to fulfill their conscientious image. Desalination of water could be done cheaply maximum potential. The structural design to incorporate many (in the long run), and temperature control of homes could begin small mirrors to form the large dish can help mitigate some of the producing power instead of consuming it. In agriculture, CSP can required costs. The use of an intermediate heat pipe receiver as a be used for food drying, roasting of beans and nuts and cooking. link between the reflective dish and the heat engine can be quite Furthermore, concentrated solar power can be used for steriliza- positive, as it promotes uniform and nearly isothermal power tion of surgical tools in remote areas. The CSP applications delivery to the heat engine, boosting its efficiency. The heat pipe mentioned in this work are all novel ideas that are potentially receiver also helps mitigate convective heat losses. The suggested very useful, but each of them (like the CSP technologies that fuel modified air cavity receiver can serve a similar purpose. The use of them) must stand the test of economics in order to penetrate heat engines and high energy conversion cycles makes PDC power world markets and become universal. production highly efficient. PDC systems do not require the use of heat transfer media, which helps decrease their cost. The flip side of this coin is the fact that PDCs cannot be easily coupled to 13. Conclusion thermal storage methods, a very serious disadvantage in the scope of large power production plants. The use of thermoelectric Over the past few decades, great progress has been made in materials with parabolic dish collectors is an interesting and fresh every facet of concentrated solar power technology. Striving idea, but current efficiencies of this scheme are quite low and towards a sustainable, ‘clean’ energy based culture has instilled further investigation of thermoelectric materials and their integra- many with the drive to help rid society of its dependence on fossil tion with CSP technologies must be carried out. The mini-dish fuels. With the sun being an obvious and overabundant form of concept for CSP is reported to yield record efficiencies and renewable energy, it is no wonder that it has been the subject of fantastically high concentration ratios, while maintaining fairly so much attention, especially at the turn of the 20th century. The low system costs. The development of this concept in the coming variety of technologies with which we can harness the sun’s years may be proved the best execution of the PDC concept. energy continues to grow, and improvements in every element of The up-and-coming field of concentrated photovoltaics pre- each concentrated solar power production regime are constantly sents a medium between CSP and photovoltaics that shows great added onto form more efficient, robust, economical and environ- promise. State-of-the-art solar cells can be coupled to any of the mentally safe facilities. four main CSP regimes in order to absorb very high solar concentrations that can be directly converted into current. High References quality silicon cells can be used at concentration of up to 100 sun without exhibiting degradation in efficiency. For higher solar [1] D. Barlev, R. Vidu, P. Stroeve, Innovation in concentrated solar power. concentrations, multi-junction cells can be utilized. The cost- University of California Davis Solar Energy Collaborative Workshop, Davis, CA, May 11, 2010. benefit analysis of CPV systems takes into account the price of [2] S.A. Kalogirou, Solar thermal collectors and applications, Progress in Energy both the CSP method used and the solar cells chosen for particular and Combustion Science 30 (3) (2004) 231–295. systems. The costs of the latter are generally quite high and must [3] H. Mousazadeh, et al., A review of principle and sun-tracking methods for maximizing solar systems output, Renewable and Sustainable Energy be offset by high conversion efficiencies to make economic sense. Reviews 13 (8) (2009) 1800–1818. Silicon cells also require a cooling mechanism at higher concen- [4] C.E. Kennedy, K. Terwilliger, Optical durability of candidate solar reflectors, trations, which may result in their ‘out-the-door’ cost to become Journal of Solar Energy Engineering 127 (2) (2005) 262–269. [5] R. Almanza, et al., Development and mean life of aluminum first-surface similar to that of the very expensive multi-junction cells. It seems mirrors for solar energy applications, Solar Energy Materials and Solar Cells that on the whole, photovoltaic power production is less efficient 93 (9) (2009) 1647–1651. 2724 D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725

[6] S.K. Tyagi, et al., Exergy analysis and parametric study of concentrating type [39] Z. Xiliang, L. Xiaobin, W. Zhifeng, Research of the heliostat cleaning method, solar collectors, International Journal of Thermal Sciences 46 (12) (2007) in: ISES Solar World Congress 2007, Solar Energy and Human Settlement, 1304–1310. Springer-Verlag, Beijing, China, 2007. [7] A. Segal, M. Epstein, Optimized working temperatures of a solar central [40] W. Ying-ge, et al., Wind pressure and wind-induced vibration of heliostat, receiver, Solar Energy 75 (6) (2003) 503–510. Key Engineering Materials 400–402 (2009) 935–940. [8] A. Thomas, Solar steam generating systems using parabolic trough concen- [41] L. Amsbeck, et al., Optical performance and weight estimation of a heliostat trators, Energy Conversion and Management 37 (2) (1996) 215–245. with ganged facets, Journal of Solar Energy Engineering—Transactions of [9] L. Valenzuela, et al., Control concepts for direct steam generation in the ASME 130 (2008) 1. parabolic troughs, Solar Energy 78 (2) (2005) 301–311. [42] J. Gottsche, et al., Solar concentrating systems using small mirror arrays, [10] M. Eck, W.D. Steinmann, Modelling and design of direct solar steam Journal of Solar Energy Engineering 132 (1) 011003 (4 pp). generating collector fields, Journal of Solar Energy Engineering—Transactions [43] F.J. Collado, Quick evaluation of the annual heliostat field efficiency, Solar of the ASME 127 (3) (2005) 371–380. Energy 82 (4) (2008) 379–384. [11] M. Eck, T. Hirsch, Dynamics and control of parabolic trough collector loops [44] A. Segal, M. Epstein, Practical considerations in designing large scale Beam with direct steam generation, Solar Energy 81 (2) (2007) 268–279. Down optical systems, Journal of Solar Energy Engineering—Transactions of [12] I. Martinez, R. Almanza, Experimental and theoretical analysis of annular the ASME 130 (2008) 1. two-phase flow regimen in direct steam generation for a low-power [45] W. Xiudong, et al., A new code for the design and analysis of the heliostat system, Solar Energy 81 (2) (2007) 216–226. field layout for power tower system, Solar Energy 84 (4) (2010) 685–690. [13] W.D. Steinmann, M. Eck, D. Laing, Solarthermal parabolic trough power [46] D.R. Mills, G.L. Morrison, Compact linear Fresnel reflector solar thermal plants with integrated storage capacity, International Journal of Energy powerplants, Solar Energy 68 (3) (2000) 263–283. Technology and Policy 3 (1–2) (2005) 123–136. [47] D.R. Mills, et al., Multi-tower line focus Fresnel array project, Transactions [14] K. Lovegrove, et al., Developing ammonia based thermochemical energy of the ASME. Journal of Solar Energy Engineering 128 (1) (2006) 118–120. storage for dish power plants, Solar Energy 76 (1–3) (2004) 331–337. [48] P.L. Singh, R.M. Sarviya, J.L. Bhagoria, Thermal performance of linear Fresnel [15] M.E. Van Valkenburg, et al., Thermochemistry of ionic liquid heat-transfer reflecting solar concentrator with trapezoidal cavity absorbers, Applied fluids, Thermochimica Acta 425 (1–2) (2005) 181–188. Energy 87 (2) (2010) 541–550. [16] S.A. Kalogirou, Parabolic trough collectors for industrial process heat in [49] P.L. Singh, R.M. Sarviya, J.L. Bhagoria, Heat loss study of trapezoidal cavity Cyprus, Energy 27 (9) (2002) 813–830. absorbers for linear solar concentrating collector, Energy Conversion and [17] N. Naeeni, M. Yaghoubi, Analysis of wind flow around a parabolic collector Management 51 (2) (2010) 329–337. (1) fluid flow, Renewable Energy 32 (11) (2007) 1898–1916. [50] J. Chaves, M. Collares-Pereira, Etendue-matched two-stage concentrators [18] N. Naeeni, M. Yaghoubi, Analysis of wind flow around a parabolic collector with multiple receivers, Solar Energy 84 (2) (2010) 196–207. (2) heat transfer from receiver tube, Renewable Energy 32 (8) (2007) [51] C.J. Dey, Heat transfer aspects of an elevated linear absorber, Solar Energy 1259–1272. 76 (1–3) (2004) 243–249. [19] A.V. Arasu, T. Sornakumar, Design, manufacture and testing of fiberglass [52] M. Eck, et al., Thermal load of direct steam-generating absorber tubes with reinforced parabola trough for parabolic trough solar collectors, Solar large diameter in horizontal linear Fresnel collectors, Heat Transfer Engi- Energy 81 (10) (2007) 1273–1279. neering 28 (1) (2007) 42–48. [20] E. Lupfert, et al., Experimental analysis of overall thermal properties of [53] A. Hoshi, et al., Screening of high melting point phase change materials parabolic trough receivers, Journal of Solar Energy Engineering—Transactions (PCM) in solar thermal concentrating technology based on CLFR, Solar of the ASME 130 (2008) 2. Energy 79 (3) (2005) 332–339. [21] Q.B. Liu, et al., Experimental investigation on a parabolic trough solar [54] H. Karabulut, et al., Construction and testing of a dish/Stirling solar energy collector for thermal power generation, Science China—Technological unit, Journal of the Energy Institute 82 (4) (2009) 228–232. Sciences 53 (1) (2010) 52–56. [55] K. Jin-Soo, et al., Operation results of dish-Stirling solar power system, in: [22] K.S. Reddy, K.R. Kumar, G.V. Satyanarayana, Numerical investigation of ISES Solar World Congress 2007. Solar Energy and Human Settlement, energy-efficient receiver for solar parabolic trough concentrator, Heat Springer-Verlag, Beijing, China, 2007. Transfer Engineering 29 (11) (2008) 961–972. [56] X. Hui, Z. Hong, Z. Jun, Numerical study of natural convection heat loss of [23] Z. Dongdong, et al., Investigation on medium temperature heat pipe heat pipe receiver for dish/Stirling system, in: ISES Solar World Congress receiver used in parabolic trough solar collector, in: ISES Solar World 2007, Solar Energy and Human Settlement, Springer-Verlag, Beijing, China, Congress 2007. Solar Energy and Human Settlement, Springer-Verlag, Beijing, 2007. China, 2007. [57] E. Kussul, et al., Flat facet parabolic solar concentrator with support cell for [24] J. Ji, et al., Dynamic performance of parabolic trough solar collector, in: ISES one and more mirrors, WSEAS Transactions on Power Systems 3 (8) (2008) Solar World Congress 2007, Solar Energy and Human Settlement, Springer- 577–586. Verlag, Beijing, China, 2007. [58] K.S. Reddy, N.S. Kumar, Combined laminar natural convection and surface [25] A. Lentz, R. Almanza, Solar–geothermal hybrid system, Applied Thermal radiation heat transfer in a modified cavity receiver of solar parabolic dish, Engineering 26 (14–15) (2006) 1537–1544. International Journal of Thermal Sciences 47 (12) (2008) 1647–1657. [26] A. Lentz, R. Almanza, Parabolic troughs to increase the geothermal wells [59] K. Reddy, N. Sendhil Kumar, Convection and surface radiation heat losses flow enthalpy, Solar Energy 80 (10) (2006) 1290–1295. from modified cavity receiver of solar parabolic dish collector with two- [27] J.S. Coventry, Performance of a concentrating photovoltaic/thermal solar stage concentration, Heat and Mass Transfer 45 (3) (2009) 363–373. collector, Solar Energy 78 (2) (2005) 211–222. [60] F. Nepveu, A. Ferriere, F. Bataille, Thermal model of a dish/Stirling systems, [28] A. Segal, M. Epstein, The optics of the solar tower reflector, Solar Energy 69 Solar Energy 83 (1) (2009) 81–89. (Supplement 6) (2000) 229–241. [61] W. Shuang-Ying, et al., A parabolic dish/AMTEC solar thermal power system [29] A. Segal, M. Epstein, Comparative performances of ‘tower-top’ and ‘tower- and its performance evaluation, Applied Energy 87 (2) (2010) 452–462. reflector’ central solar receivers, Solar Energy 65 (4) (1999) 207–226. [62] D. Feuermann J.M. Gordon, High-concentration collection and remote [30] A. Segal, M. Epstein, Solar ground reformer, Solar Energy 75 (6) (2003) delivery of sunlight with fiber-optic mini-dishes, in: Proceedings of the 479–490. SPIE—The International Society for Optical Engineering, vol. 3781, 1999, [31] A. Segal, M. Epstein, A. Yogev, Hybrid concentrated photovoltaic and pp. 47–57. thermal power conversion at different spectral bands, Solar Energy 76 (5) [63] D. Feuermann, J.M. Gordon, M. Huleihil, Solar fiber-optic mini-dish (2004) 591–601. concentrators: first experimental results and field experience, Solar Energy [32] R. Buck, et al., Dual-receiver concept for solar towers, Solar Energy 80 (10) 72 (6) (2002) 459–472. (2006) 1249–1254. [64] D. Feuermann, J.M. Gordon, High-concentration photovoltaic designs based [33] M. Sanchez, M. Romero, Methodology for generation of heliostat field layout on miniature parabolic dishes, Solar Energy 70 (5) (2001) 423–430. in central receiver systems based on yearly normalized energy surfaces, [65] M.A. Farahat, Improvement in the thermal electric performance of a Solar Energy 80 (7) (2006) 861–874. photovoltaic cells by cooling and concentration techniques, in: 39th [34] R. Buck, S. Friedmann, Solar-assisted small solar tower trigeneration International Universities Power Engineering Conference, IEEE, Bristol, UK, systems, Journal of Solar Energy Engineering—Transactions of the ASME 2004. 129 (4) (2007) 349–354. [66] A. Kribus, et al., A miniature concentrating photovoltaic and thermal [35] C.W. Forsberg, P.F. Peterson, H.H. Zhao, High-temperature liquid-fluoride- system, Energy Conversion and Management 47 (20) (2006) 3582–3590. salt closed-Brayton-cycle solar power towers, Journal of Solar Energy [67] I. Anton, et al., The PV-FIBRE concentrator: a system for indoor operation of Engineering—Transactions of the ASME 129 (2) (2007) 141–146. 1000X MJ solar cells, Progress in Photovoltaics 15 (5) (2007) 431–447. [36] J.I. Ortega, J.I. Burgaleta, F.M. Tellez, Central receiver system solar power [68] T.K. Mallick, P.C. Eames, Optical and thermal performance predictions for a plant using molten salt as heat transfer fluid, Journal of Solar Energy high concentration point focus photovoltaic system, in: ISES Solar World Engineering—Transactions of the ASME 130 (2008) 2. Congress 2007, Solar Energy and Human Settlement, Springer-Verlag, [37] G. Koll, et al., The solar power tower Julich: a solar thermal power plant for Beijing, China, 2007. test and demonstration of air receiver technology, in: ISES Solar World [69] D. Chemisana, M. Ibanez, J. Barrau, Comparison of Fresnel concentrators for Congress 2007, Solar Energy and Human Settlement, Springer-Verlag, building integrated photovoltaics, Energy Conversion and Management 50 Beijing, China, 2007. (4) (2009) 1079–1084. [38] L. Xiaobin, et al., A study on non-metallic structure of heliostat, in: ISES [70] M. Mirzabaev, et al., Investigations of the energetic and thermal characteristics Solar World Congress 2007, Solar Energy and Human Settlement, Springer- of a module based on a Fresnel lens and a hetero-photo-converter in the Verlag, Beijing, China, 2007. AlGaAs–GaAs system, Applied Solar Energy 43 (3) (2007) 133–135. D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2725

[71] H. Zhai, et al., Experimental investigation and analysis on a concentrating International Solar Energy Conference, Solar Engineering 1998, 1998, pp. solar collector using linear Fresnel lens, Energy Conversion and Manage- 277–859ixþ502. ment 51 (1) (2010) 48–55. [92] G.J. Kolb, R.B. Diver, N. Siegel, Central-station solar hydrogen power plant, [72] K. Ryu, et al., Concept and design of modular Fresnel lenses for concentra- Journal of Solar Energy Engineering—Transactions of the ASME 129 (2) tion solar PV system, Solar Energy 80 (12) (2006) 1580–1587. (2007) 179–183. [73] H. Yu-Ting, S. Guo-Dung, Cylindrically symmetric Fresnel lens for high [93] J. Hartvigsen, et al., Carbon dioxide recycling by high temperature concentration photovoltaic, in: Proceedings of the SPIE—The International co-electrolysis and hydrocarbon synthesis, in: Carbon Dioxide Reduction Society for Optical Engineering, vol. 7423, 2009, p. 74230W. Metallurgy, held during TMS 2008 Annual Meeting & Exhibition, Minerals, [74] J.C. Gonzalez, Design and analysis of a curved cylindrical Fresnel lens that Metals & Materials Society, New Orleans, LA, 2008. produces high irradiance uniformity on the solar cell, Applied Optics 48 [94] Y.M. Lytvynenko, D.V. Schur, Utilization the concentrated solar energy for (11) (2009) 2127–2132. process of deformation of sheet metal, Renewable Energy 16 (1–4) (1999) [75] M. Rowe, CRC Handbook of Thermoelectrics, in: R.D.M. (Ed.), CRC Press, 753–756. Boca Raton, FL, 1995, pp. 1–3. [95] F. Mammadov, U. Samadova, O. Salamov, Experimental results of using a [76] S.A. Omer, D.G. Infield, Design and thermal analysis of a two stage solar parabolic trough solar collector for thermal treatment of crude oil, Journal concentrator for combined heat and thermoelectric power generation, of Energy in Southern Africa 19 (1) (2008) 70–76. Energy Conversion and Management 41 (7) (2000) 737–756. [96] A. Meier, N. Gremaud, A. Steinfeld, Economic evaluation of the industrial [77] S.A. Omer, D.G. Infield, Design optimization of thermoelectric devices for solar power generation, Solar Energy Materials and Solar Cells 53 (1–2) solar production of lime, Energy Conversion and Management 46 (6) (2005) (1998) 67–82. 905–926. [78] H. Naito, et al., Development of a solar receiver for a high-efficiency [97] J.M. Gordon, K. Choon Ng, High-efficiency solar cooling, Solar Energy 68 (1) thermionic/thermoelectric conversion system, Solar Energy 58 (4–6) (1996) (2000) 23–31. 191–195. [98] Y. Tripanagnostopoulos, C. Siabekou, J.K. Tonui, The Fresnel lens concept for [79] A. Gil, et al., State of the art on high temperature thermal energy storage for solar control of buildings, Solar Energy 81 (5) (2007) 661–675. power generation. Part 1—concepts, materials and modellization, Renew- [99] M. Qu, H.X. Yin, D.H. Archer, A solar thermal cooling and heating system for able and Sustainable Energy Reviews 14 (1) (2010) 31–55. a building: experimental and model based performance analysis and [80] C. Wieckert, et al., A 300 kW solar chemical pilot plant for the carbothermic design, Solar Energy 84 (2) (2010) 166–182. production of zinc, Transactions of the ASME. Journal of Solar Energy [100] N. Velazquez, et al., Numerical simulation of a linear Fresnel reflector Engineering 129 (2) (2007) 190–196. concentrator used as direct generator in a Solar-GAX cycle, Energy Conver- [81] Q. Ma, et al., A review on transportation of heat energy over long distance: sion and Management 51 (3) (2010) 434–445. Exploratory development. Renewable and Sustainable Energy Reviews. [101] R.L. Adams, J.F. Thomson, Improving drying uniformity in concurrent flow 13(6–7): p. 1532–1540. tunnel dehydrators, Transactions of the ASAE (American Society of Agri- [82] L. Garcia-Rodriguez, J. Blanco-Galvez, Solar-heated Rankine cycles for water cultural Engineers) 28 (3) (1985) 890–892. and electricity production: POWERSOL project, Desalination 212 (1–3) [102] D.M. Barrett, Processing of horticultural crops, in: I.N. Adel, A. Kader (Eds.), (2007) 311–318. Postharvest Technology of Horticultural Crops, Publication 3311, University [83] A.M. Delgado-Torres, L. Garcia-Rodriguez, Preliminary assessment of solar of California Division of Agriculture and Natural Resources Press, 2002, organic Rankine cycles for driving a desalination system, Desalination 216 pp. 465–480. (1–3) (2007) 252–275. [103] B. Mjawa, The status of horticulture industry in Tanzania, in: Presentation to [84] A. Kribus, et al., A solar-driven combined cycle power plant, Solar Energy 62 identification of appropriate post harvest technologies for improving (2) (1998) 121–129. market access and incomes for small scale horticultural farmers in sub [85] F. Donatini, et al., High efficiency integration of thermodynamic solar plant Saharan Africa and South Asia workshop at the University of California with natural gas combined cycle, in: IEEE International Conference on Clean Davis, Davis, CA, 2009. Electrical Power, ICCEP’07, IEEE, Capri, Itlay, 2007. [104] J.F. Thompson, M.S. Chinnan, M.W. Miller, G.D. Knutson, Energy conserva- [86] D.Y. Goswami, et al., New and emerging developments in solar energy, Solar tion in drying of fruits in tunnel dehydrators, Journal of Food Process Energy 76 (1–3) (2004) 33–43. [87] A. Kribus, A high-efficiency triple cycle for solar power generation, Solar Engineering 4 (1980) 155–169. Energy 72 (1) (2002) 1–11. [105] J.F. Thompson, Methods of prune dehydration, Prune Orchard Management [88] J. Blanco, et al., Review of feasible solar energy applications to water Special Publication 3269 (1981) 150–152. processes, Renewable and Sustainable Energy Reviews 13 (6–7) (2009) [106] J.F. Thompson, H.E. Studer, Sun drying of prunes on continuous trays. 1437–1445. American Society of Agricultural Engineers Paper No. 81-3051, 1981. [89] H. Alrobaei, Novel integrated gas turbine solar cogeneration power plant, [107] J.F. Thompson, in: L.P. Christesen (Ed.), Tunnel dehydration. Raisin Produc- Desalination 220 (1–3) (2008) 574–587. tion Manual, vol. 3393, Division of Agriculture and Natural Resources [90] C. Perez-de-los-Reyes, A. Porras-Soriano, M.L. Soriano, Use of flat plate solar Publication, University of California, 2000, pp. 224–227. collectors and parabolic trough concentrators for greenhouse soil disin- [108] D.M. Barrett, P. Stroeve, J. Thompson, B. Mjawa, D. Schmidt, S. Li, and festation, Spanish Journal of Agricultural Research 7 (2) (2009) 315–321. C. Flint, Concentrated solar drying for mangoes and tomatoes, in: University [91] G.C. Glatzmaier, D.M. Blake, Analysis of hydrogen production methods of California Davis Solar Energy Collaborative Workshop, Davis, CA, May 11, using electrolysis and concentrated solar energy, in: Proceedings of the 2010.