Innovation in concentrated solar power

Innovation in concentrated solar power This work focuses on innovation in CSP technologies over the last decade. A multitude of advancements has been developed during this period, as the topic of concentrated solar power is becoming more 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 integrated with CSP regimes to conserve (and sometimes produce) electricity have been suggested and implemented, keeping in mind the environmental benefits granted by limited fossil fuel usage. 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,

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 b c Department of Electrical and Computer Engineering, University of California Davis, Davis, CA 95616, USA Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA 95616, USA California Solar Energy Collaborative (CSEC), University of California Davis, Davis, CA 95616, USA a r t i c l e i n f o abstract Article history: Received 30 October 2010 Accepted 12 May 2011 This work focuses on innovation in CSP technologies over the last decade A multitude of advancements has been developed during this period, as the topic of concentrated solar power is becoming more 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 integrated with CSP regimes to conserve (and sometimes produce) electricity have been suggested and implemented, keeping in mind the environmental benefits granted by limited fossil fuel usage & 2011 Elsevier B.V All rights reserved Keywords: Concentrated solar power (CSP) Design Materials Heat absorption Transport Thermal storage Contents 10 11 12 13 Introduction Concentrating solar collectors Parabolic trough collectors (PTC) Heliostat field collectors (HFC) Linear Fresnel reflectors (LFR) Parabolic dish collectors (PDC) Concentrated photovoltaics Concentrated solar thermoelectrics Thermal energy storage Energy cycles Applications Discussion Conclusion References Introduction As the world’s supply of fossil fuels shrinks, there is a great need for clean and affordable renewable energy sources in order to meet growing energy demands Achieving sufficient supplies of clean energy for the future is a great societal challenge Sunlight, the largest available carbon-neutral energy source, provides the Earth with more energy in h than is consumed on the planet in n Corresponding author at: Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA 95616, USA E-mail address: pstroeve@ucdavis.edu (P Stroeve) 0927-0248/$ - see front matter & 2011 Elsevier B.V All rights reserved doi:10.1016/j.solmat.2011.05.020 2703 2704 2705 2707 2711 2712 2714 2716 2717 2719 2720 2722 2723 2723 an entire year Despite of this, solar electricity currently provides only a fraction of a percent of the world’s power consumption A great deal of research is put into the harvest and storage of solar energy for power generation There are two mainstream categories of devices utilized for this purpose—photovoltaics and concentrated solar power (CSP) The former involves the use of solar cells to generate electricity directly via the photoelectric 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 several decades, and is based on a simple general scheme: using mirrors, sunlight can be redirected, focused and collected as heat, which can in turn be used to power a turbine or a heat engine to 2704 D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 Table Description and specifications of the four main CSP technologies Data compiled from [1,2] Collector type Description Rel thermodynamic efficiency Operating temp range (1C) Relative cost Concentration Technology ratio (sun) maturity Tracking PTC – Parabolic sheet of reflective material (aluminum, acrylic) – Linear receiver (metal pipe with heat transfer fluid) Low 50–400 Low 15–45 Very mature One-axis Linear Fresnel – Linear Fresnel mirror array focused on tower Low or high-mounted pipe as receiver 50–300 Very low 10–40 Mature One-axis Solar tower High – Large heliostat field with tall tower in its center – Receiver: water/HTC boiler at top – Can be used for continuous thermal storage 300–2000 High 150–1500 Most recent Two-axis Dish-Stirling – Large reflective parabolic dish with Stirling High 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) 150–1500 Very high 100–1000 Recent Two-axis generate electricity Despite being relatively uncomplicated, this method involves several steps that can each be implemented in a plethora of different ways The chosen execution method of every stage in solar thermal power production must be optimally matched to various technical, economic and environmental factors that may favor one approach over another Extensive explorations of various solar collector types, materials and structures have been carried out, and a multitude of heat transport, storage and electricity conversion systems has been tested The progress made in every aspect of CSP, especially in the last decade, was geared towards expanding the efficiency of solar-to-electric power production, while making it affordable in comparison with near-future fossil fuel derived power This work describes the four main types of concentrating solar collectors (Tables and 2) [1,2] and discusses innovation in each over the last decade Progress in the related fields of concentrated photovoltaics and thermoelectrics will also be presented, along with advances made in thermal energy storage methods, energy conversion cycles and CSP applications Concentrating solar collectors A solar energy collector is a heat-exchanging device that transforms solar radiation into thermal energy that can be utilized for power generation The basic function of a solar collector is to absorb incident solar radiation and convert it into heat, which is then carried away by a heat transfer fluid (HTF) flowing through the collector The heat transfer fluid links the solar collectors to the power generation system, carrying thermal energy from each collector to a central steam generator or thermal storage system as it circulates There are two general categories of solar collectors The first includes stationary, non-concentrating collectors, in which the same area is used for both interception and absorption of incident radiation The second category consists of sun-tracking, concentrating solar collectors, which utilize optical elements to focus large amounts of radiation onto a small receiving area and follow the sun throughout its daily course to maintain the maximum solar flux at their focus A comprehensive review of sun-tracking methods and principles was published by Mousazadeh et al [3] Light concentration ratios can be expressed in suns, with a single sun (1000 W/m2) being a measurement of average incident light flux per unit area at the earth’s surface Though more costly, concentrating collectors have numerous advantages over stationary collectors, and are generally associated with higher operation temperatures and greater efficiencies The addition of an optical device to the conventional solar collector (receiver) has proved useful in several regards; various concentration schemes can achieve a wide range of concentration ratios, from unity to over 10,000 sun [2] This increases the operation temperature as well as the amount of heat collected in a given area, and yields higher thermodynamic efficiencies Radiation focusing allows the usage of receivers with very small relative surface areas, which leads to significant reductions in heat loss by convection Despite the added capital investment necessary for manufacturing the optical elements of the apparatus, the materials used for these mirrors/lenses are generally inexpensive compared with thermal collector materials, which are needed in much smaller amounts in a concentrator scheme The reduction in receiver size and material amounts makes expensive receiver conditioning (vacuum insulation, surface treatments, etc.) for higher efficiency and heat loss minimization economically sensible Finally, the ability to control the concentration ratio of a system allows delicate manipulation of its operation temperature, which can be thermodynamically matched to specific applications as needed to avoid wasted heat It is important to note that reflective materials used in CSP technologies must meet certain reflectivity and lifetime requirements to be cost-effective A study of the optical durability of solar reflectors was presented by Kennedy and Terwilliger [4] and an investigation specific to aluminum firstsurface mirrors was carried out by Almanza et al [5] Tyagi et al [6] investigated the effects of HTF mass flow rates and collector concentration ratios on various system parameters Results showed that exergy output (available work from a process that brings a system to thermal equilibrium), exergetic and thermal efficiencies and inlet temperature increased with solar intensity, as expected Exergetic and thermal efficiencies and exergy output were found to increase with mass flow rate as well Optimal inlet temperature and exergetic efficiency at high solar intensity were both found to be the decreasing functions of the concentration level At low intensity values, however, efficiency first increases and then decreases with increase in concentration This behavior results from increased radiative losses associated with high concentration ratios Both concentration 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 Schematic diagrams of each CSP technology listed in Table 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 performance The four main types of concentrating solar collectors are (1) (2) (3) (4) Parabolic trough collectors; heliostat field collectors; linear Fresnel reflectors; and parabolic dish collectors proportional and strictly dependent on the operation temperature In practice, however, the materials chosen for light concentration and absorption, heat transfer and storage, as well as the power conversion cycles used are the true deciding factors [7] The following sections will describe the aforementioned collector schemes in detail, and present technological advancements that have been made in each over the last 10 years Parabolic trough collectors (PTC) Concentrating collectors can achieve different concentration ratios and thus operate at various temperatures From a theoretical standpoint, the efficiency of power producing heat processes is both Parabolic trough technology is the most mature concentrated 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 series to form long troughs These modules are supported from 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, encased in a glass pipe to limit heat loss by convection The metal tube’s surface is often covered with a selective coating that features high solar absorbance and low thermal emittance The glass tube itself is typically coated with antireflective coating to enhance transmissivity A vacuum can be applied in the space between the glass and the metal pipes to further minimize heat loss and thus boost the system’s efficiency The heat transfer fluid (HTF) flows through the receiver, collecting and transporting thermal energy to electricity generation systems (usually boiler and turbine generator) or to storage facilities The HTF in PTC systems is usually water or oil, where oil is generally preferred due to its higher boiling point and relatively low volatility Several water boiler designs have been suggested by Thomas [8] The preferred boiling system implements direct steam generation (DSG), where water is the heat transfer fluid It is partially boiled in the collector and circulated through a steam drum where steam is separated from the water The DISS (Direct Solar Steam) project PTC plant in Tabernas, Spain, is a leading DSG test facility, where two successful DSG operational modes and control systems were developed and tested [9] Both methods utilize pressure control in addition to temperature control of circulating water This approach is done to achieve a constant output of steam at a monitored temperature throughout most hours of the day (9 am–6 pm) A pressure level of 100 bar and temperatures of up to 400 1C have been demonstrated The Once-Through mode (Fig 1) features a preheated water feed into the inlet As water circulates through the collectors, it is evaporated and converted into superheated steam that is used to power a turbine In the more water-conservative Recirculation mode (Fig 2), a water–steam separator is placed at the end of the collector loop More water is fed to the evaporator than can be evaporated in one circulation cycle Excess water is re-circulated through the intermediate separator to the collector loop inlet, where it is mixed Fig Schematic flow diagram of Once-Trough mode of operation of direct steam generation Figure reproduced with permission from ref 9, &2005 Elsevier Fig Schematic flow diagram of Recirculation mode of operation of direct steam generation Figure reproduced with permission from ref 9, &2005 Elsevier with preheated water This process guarantees good wetting of absorber tubes and prevents stratification Steam is separated from water and fed into the inlet of a superheating section The Recirculation regime is more easily controlled than the Once-through regime, but has an increased parasitic load due to the additional process steps Usage of water as a HTF inflicts more stress on the absorber tubes than other heat transfer media, due to water’s relatively high volatility A simulation of thermohydraulic phenomena under the DSG process was carried out by Eck and Steinmann [10] Sufficient cooling of the absorber tubes and a moderate pressure drop between inlet and outlet can help moderate the stress, reduce corrosion and promote tube lifetime Knowledge of short-time dynamics of flow and feed systems in a DSG regime is crucial for successful design and operation A transient non-linear simulation tool was developed to study dynamic behaviors of the aforementioned PTC system designs, for which several feed control systems were suggested [11] It is important to mention that for DSG systems, the temperature difference registered between the hottest and the coldest points over the external wall of the pipe will increase if feed flow is too high [12] This is a result of non-constant heat transference from the receiver to the HTF, and can potentially affect the quality of produced steam A test facility for a solid sensible heat storage system was developed for the DSG parabolic trough collector design discussed A performance analysis of the storage system integrated with the power plant was implemented by Steinmann et al [13] Integration of thermo-chemical storage through ammonia de-synthesis was theoretically investigated as well, and efficiencies of up to 53% were reported [14] In contrast with the DSG scheme, which employs water as the HTF, recent innovation also promotes the use of ionic liquids (molten salts) for heat transfer media [15], as they are more heat-resilient than oil, and thus corrode the receiver pipes less 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 drag, and must thus be robust enough to account for wind loads and prevent deviations from normal insolation incidence D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 A study of turbulent flow around a PTC of the 250 kW solar plants in Shiraz, Iran, was conducted by Naeeni and Yaghoubi [17] The study investigates stress applied to the collector, taking into account varying collector angles, wind velocities and air flow distribution with respect to height from the ground A second study by the authors models the effects of the same parameters on heat transfer from the PTC receiver tube [18] In order to make the PTC structure more resilient to external forces, it is possible to reinforce collector surfaces with a thin fiberglass layer A smooth, 901 rim angle reinforced trough was built by a hand lay-up method [19] The fiberglass layer is added underneath the reflective coating (on the inner surface) of the parabolic trough The reflector’s total thickness is mm, and can withstand a force applied by a 34 m/s wind with minimal deviation; deflection at the center of the parabola vertex was only 0.95 mm, well within acceptable limits Receiver design considerations are crucial for efficient heat transfer to the HTF and heat loss management Radiative heat losses from receiver tubes play an important role in collector performance Thermal loss due to the temperature gradient between the receiver and the ambient has a significant impact on a system’s thermal efficiency PTCs operating at high temperatures (around 390 1C) can experience up to 10% radiative losses annually At this temperature range, thermal loss from receiver tube reaches 300 W/m of the receiver pipe [20] A loss of 220 W/m was reported for an operational temperature of 180 1C, with collector efficiency ranging from 40% to 60% [21] In both of the aforementioned 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 cause bending of the pipes Thermal analysis of an energy-efficient PTC receiver was presented by Reddy et al [22], and a numerical model to evaluate its heat transfer characteristics was proposed The new receiver design features porous inclusions inside the tube, which increase the total heat transfer area of the receiver, along with its thermal conductivity and the turbulence of the circulating HTF (synthetic oil) Heat transfer for this scheme was enhanced by 17.5% compared with regular (no inclusions) design, but the system suffered a pressure decrease of about kPa The use of a heat pipe as a linear receiver for PTCs was proposed by Dongdong et al [23] The heat pipe can keep an essentially uniform circumferential temperature, despite the uneven illumination provided by trough collectors Since heat does not flow from the HTF to the heat pipe, smaller heat losses occur during hours of low insolation PTC systems featuring a heat pipe as the receiver have 65% thermal efficiency at 380 1C They are also cheaper to manufacture because the bellows system generally incorporated into conventional receiver tubes is not necessary Lifetime testing of the heat pipe receiver with respect to various operation temperatures is still being investigated, but meets the general requirements (12–15 years) under operation below 380 1C Parabolic trough collector systems generally operate in unsteady state For this reason, a dynamic model is essential for effective design and performance prediction of a PTC system A dynamic simulation of PTC was conducted by Ji et al [24], modeling a south facing, one-axis tracking parabolic trough collector The simulation calculated variations in incidence angle of solar beam to collector aperture, as well as the distribution of concentrated solar radiation along the focal line Effects of HTF mass flow rate and receiver tube length on outlet temperature and system efficiency were investigated An increase in tube length augments outlet temperatures and efficiency, as expected due to greater total insulation A decrease in mass flow rate increases outlet temperature and slightly decreases system efficiency The integration of a parabolic trough collector field with geothermal sources has been suggested by Lentz and Almanza 2707 [25,26] Hot water and steam from geothermal wells can be directly fed into an absorber pipe going through a PTC field The combination of both thermal energy sources increases the volume and the quality of (directly) generated steam for power production Several hybrid designs have been suggested by the authors PTCs can also be integrated with solar cells in concentrated photovoltaics (CPV) modules Heat-resistant, high-efficiency photovoltaic cells can be mounted along the bottom of the receiver tube to absorb the concentrated solar flux The performance of a CPV parabolic trough system with a 37 sun concentration ratio was characterized by Coventry [27] at Australian National University in 2003 Monocrystalline silicon solar cells were used, along with the thermal PTC apparatus Measured electrical and thermal efficiencies were 11% and 58%, respectively, producing a total efficiency of 69% It is important to note that uneven illumination of the solar cell modules causes a direct decrease in the cells’ performance, and thus optical considerations must be weighed carefully The mature field of parabolic trough collectors provides an efficient, relatively inexpensive power production scheme Multiple advances in reflector and receiver design have been made in the last decade to enhance efficiency and reduce losses Heat collection and transfer methods have been modeled and tested repeatedly in order to achieve optimal power output throughout the day The PTC scheme also lends itself to easy storage schemes, as well as to simple integration with both fossil fuels and other renewable energy sources Heliostat field collectors (HFC) The most recent CSP technology to emerge into commercial utility was the heliostat field collector design This expensive, powerful design has so far been incorporated in relatively few locations around the world The 10 MW Solar One (1981) and Solar Two (1995) were the first HFC plants to be demonstrated, built in the Mojave Desert of California They have since been decommissioned Other plants, such as the 11 MW PS10 and 20 MW PS20 in Spain, and the MW Sierra SunTower in California, were recently completed The heliostat field collector design features a large array of flat mirrors distributed around a central receiver mounted on a (solar) tower Each heliostat sits on a two-axis tracking mount, and has a surface area ranging from 50 to 150 m2 Using slightly concave mirror segments on heliostats can increase the solar flux they reflect, though this elevates manufacturing costs Every heliostat is individually oriented to reflect incident light directly onto the central receiving unit Mounting the receiver on a tall tower decreases the distance mirrors must be placed from one another to avoid shading Solar towers typically stand about 75–150 m height A fluid circulating in a closed-loop system passes through the central receiver, absorbing thermal energy for power production and storage An advantage of HFCs is the large amount of radiation focused on a single receiver (200–1000 kW/m2), which minimizes heat losses and simplifies heat transport and storage requirements Power production is often implemented by steam and turbine generators The single-receiver scheme provides for uncomplicated integration with fossil-fuel power generators (hybrid plants) [2] HFC plants are typically large (10 MW and above), as the benefit from an economy of scale is required to offset the high costs associated with this technology They can incorporate a very large number of heliostats surrounding a single tower The immense solar flux reflected towards the receiver yields very high concentration ratios (300–1500 suns) HFC plants can thus operate at very high 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 higherenergy 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 receiver 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 Nevertheless, 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 was proposed in 2002 Solar reforming of methane with steam or 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 produced 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 plants with regards to these values are negligible The solar tower reflector can also be integrated with concentrated photovoltaics (CPV) The principle behind this design is to split the solar spectrum into PV-used and thermal-used portions For example, monocrystalline silicon solar cells operate at efficiencies ranging between 55% and 60% at wavelengths of 600–900 nm The Fig Schematic diagram of solar reflector tower in an HFC system Figure reproduced with permission from ref 7, &2003 Elsevier Fig Solar ground reformer integrated with a reflector tower HFC system Figure reproduced with permission from ref 30, &2003 Elsevier Fig Brayton cycle and combined cycle efficiencies as a function of the temperature and gas turbine pressure ratio Figure reproduced with permission from ref 7, &2003 Elsevier rest of the light can be used for electricity generation using Rankine– Brayton cycles, or otherwise be stored for later use Discussion of 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 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 optimization challenge A novel methodology for layout generation based on yearly normalized energy surfaces (YNES) was presented by Sanchez D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 and Romero [33] This ‘Heliostat Growth Method’ (HGM) uses the YNES program to evaluate the usable solar energy flux at each point in a solar field year-round, given a specific solar tower height Using this data, the method splits impacting factors such as shadowing, blocking, atmospheric attenuation and others into two categories: those associated with spatial position of the solar tower and those affected by the geometry of the heliostat This provides greater insight and flexibility to the field layout process and its optimization A clever design for small-scale ‘tri-generation’ solar power assisted plant was brought forth by Buck and Friedmann [34] The design puts together a solar–gas turbine hybrid system, which incorporates a small heliostat field, a receiver mounted on a solar tower, a micro-turbine and an absorption chiller In this regime, electric power, heating and cooling can all be produced by the Fig Scheme of dual receiver unit from top view (left) and side view (right) Figures reproduced with permission from ref 32, &2006 Elsevier 2709 same system System configurations were assessed for technical performance and cost Forsberg et al [35] suggested the use of liquid fluoride salt as an HTF in order to raise the heat-to-electricity conversion efficiency of HFCs to about 50% The molten salt operates at temperatures between 700 and 850 1C, delivering heat to a closed multi-reheat Brayton cycle using N2 or He as the working fluid Due to such high operation temperatures, thermal energy storage as sensible heat in graphite is suggested A schematic diagram of such an HFC plant is shown (Fig 8) Graphite, a low-cost solid featuring a high heat capacity, is compatible with the fluoride salt at high temperatures The efficiency boost reported by the authors can greatly reduce electricity costs 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 efficiencies 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 1C to a 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 medium greatly reduces capital investment as it is free and readily available anywhere Fig Schematic plant incorporating dual receiver, outlining three heat transfer stages (preheating, evaporation and superheating) Figures reproduced with permission from ref 32, &2006 Elsevier Table Comparison of dual receiver CSP plant performance with a control [32] Design conditions Annual performance Receiver outlet temp (1C) Receiver efficiency (including recirculation losses) (%) Air temp at blower (1C) Air mass flow (kg/s) Annual receiver efficiency (including recirculation losses) (%) Capacity factor (%) Net annual electric energy (GWh) Reference plant Dual receiver plant 700 79 110 56 66.7 14.3 12.5 500 87 80 40 79.4 18.2 15.9 2710 D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 Fig 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 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 to metal–aluminum alloys conventionally used, but is not as 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 resilience, a problem which must be contended with in order to ensure heliostat operation temperatures can be accommodated Several heliostat cleaning methods are proposed by Xiliang et al [39], such as using highly pressurized air/water depending on various environmental conditions Conventional heliostat design dictates that cost reduction is implemented by increasing the area of the mirrors Doing this reduces specific drive cost while increasing the torques heliostats experienced by wind loads A study by Ying-ge et al [40] demonstrates the distribution and characteristics of heliostats’ mean and fluctuating wind pressure while wind direction angle is varied from 01 to 1801 and vertical angle is varied from 01 to 901 Moreover, a finite element model was constructed to perform calculations of wind-induced dynamic responses Increased wind torques result in higher specific weight and drive power The usage of torque tube heliostats (TTH) (Fig 9) is suggested by Amsbeck et al [41] TTH systems incorporate arrays of long, narrow mirrors mounted on turning tubes that control their elevation An optical performance and a weight estimation of a TTH system were carried out by the authors, and compared with a Fig 10 Schematic of mini-mirror array design featuring ‘ball-in-socket joint’ tracking mechanism Figures reproduced with permission from ref 42, &2010 ASME regular HFC system of a slightly smaller area Although the TTH system indeed experienced smaller wind torques, it suffered an annual energy output reduction of 3% Furthermore, the high number of moving elements and the more involved control make this system hardly advantageous compared with the conventional design Another novel design to help avoid heavy mirror tracking in ă the face of wind loads was suggested by Gottsche et al [42] This regime utilizes mini-mirror arrays (10 Â 10 cm) made of high quality materials Each mirror is mounted on a ball-in-socket joint driven by a step motor (Fig 10) The mirrors are encased in a clear box that shields them from the wind The purpose of this design is to avoid wind loads and save on stiff materials (mainly steel) that are necessary to make large heliostats resilient to wind torques Unfortunately, the low-cost achieved by the group was countered by a 40% drop in optical performance compared with conventional HFC systems For initial planning of an HFC power plant, a general efficiency evaluation tool can be quite useful Collado [43] presented a quick, non-specific evaluation method for annual heliostat field efficiency evaluation The model is a combination of an analytical D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 assessment of the flux density produced by a heliostat from Zaragoza University, an optimized mirror density distribution developed by University of Houston for the Solar One project, and molten salt receiver efficiencies measured during the Solar Two project This model does not take into account many impacting factors specific to a particular HFC system and is limited in its accuracy Similarly, a new method for approximating geometrical parameters and sizing of the tower reflector regime was developed by Segal and Epstein [44] The method utilizes edge rays originating from the heliostat field boundaries and is particularly useful for geometrical assessment of very large arrays of heliostats The method’s results were compared with real field calculations and found to be a good first approximation regime A simulation using the same ‘edge ray’ principle method was developed by Xiudong et al [45] Its purpose was to promote more efficient placement of heliostats and obtain a faster generating response of the design and optimization A novel module for the analysis of non-spherical heliostat arrangements has been incorporated into the simulation A toroidal heliostat field was designed and analyzed by the authors and proved significantly less efficient that conventional HFC arrangements A method for calculating the annual solar flux distribution of a given area is an added feature, with the purpose of evaluating feasibility of crop growth around heliostat fields Heliostat field collector technology has greatly improved over the last few decades, and continues to draw much attention as a suitable scheme for large solar thermal plants The exceedingly high temperatures at which they operates it grant HFC plants excellent efficiencies, while allowing them to be coupled to a variety of applications The high capital investment necessary for the construction of HFC systems is an obstacle, however, and further technological advancements in efficiency must be accompanied by low cost materials and storage schemes for this CSP method to become more economical increased receiver tower height, which augments the cost A novel solution to the shading problem is discussed by Mills and Morrison [46] at Sydney University, Australia Their design of the compact linear Fresnel reflector (CLFR) scheme features adjacent mirrors oriented towards two separate receivers in opposite directions (Fig 11) The use of multiple receivers allows a more compact reflector distribution, avoiding shading and utilizing a portion of solar flux that otherwise goes to waste Reflectors near the base of a receiver are always oriented towards it Yet, when reaching a nearly equidistant point between two separate receivers, the mirrors from each will reverse their orientation, allowing them to come very close together without blocking one another For commercial power production (greater than MW scale), it is very reasonable to have multiple receivers, and thus the CLFR design is very useful without incurring extra costs, especially in areas where land is limited A useful addition to the CLFR design is the incorporation of an inverted cavity receiver attached to a planar array of boiling tubes (Fig 12) This structure allows plant operation in a direct steam generation (DSG) regime Mills and Morrison [47] indicate that this receiver design bypasses receiver thermal uniformity challenges with parabolic trough DSG system Design considerations of the inverted cavity receiver are presented by Singh et al [48] This work compares thermal performance of circular and rectangular absorber tubes, as well as black nickel and black paint coated tubes Circular absorbers in the receiver are reported to have a higher thermal efficiency by 8% compared with a rectangular absorber Nickel selective surface coating performed 10% better than ordinary black paint A heat loss study of the same variables is also performed by the authors Nickel selective-coated absorbers experience a 20–30% heat loss coefficient reduction Additionally, a double glass absorber cover is compared with a single glass cover, and is found to reduce the heat loss coefficient by 10–15% [49] An innovative design to further limit wasted solar radiation in a CLFR regime was presented by Chavez and Collares-Pereira [50] 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 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 MW Kimberlina Solar Thermal Energy plant has been recently completed in Bakersfield, California 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 transporting 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 increased spacing between mirrors, which takes up more land, or 2711 Fig 11 Schematic diagram of the CLFR design Figure reproduced with permission from ref 2, &2004 Elsevier Fig 12 Schematic diagram of inverted air cavity receiver 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 waveshaped 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 absorbing 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 overheating while operating at the highest allowed temperature This kind of sensitive, intelligent system would surely increase power plant costs A study by Hoshi et al [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 largescale 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 Table FEM analysis results of thermal loads for three LFR system sections Data compiled from [52] Heat transfer coefficient (W/m2 K) Average fluid temp (1C) Max tube temp (1C) Min tube temp (1C) Temperature drop (K) Pre-heater Evaporator Superheater 1700 140 189 142 47 860 275 325 281 44 500 440 569 455 114 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 inexpensive in comparison with other CSP technologies It readily couples to thermal storage methods and numerous applications Parabolic dish collectors (PDC) Parabolic dish reflectors are point-focus collectors As such, they can achieve very high light concentration ratios, reaching up D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 to 1000 sun At temperatures exceeding 1500 1C, they can produce power efficiently by utilizing high energy conversion cycles The collector type features a large parabolic-shaped dish, which must track the sun on a two-axis tracking system to maintain light convergence at its focal point A receiver is mounted at the focus, collecting solar radiation as heat Two general schemes are possible for power conversion; the less popular has a heat transfer fluid system connecting the receivers of several dishes, conducting thermal energy towards a central electricity generation system This design is less convenient as it requires a piping and pumping system resilient to very high temperatures, and suffers from transport thermal losses The more prevalent system mandates a heat engine be mounted near/at the focal points of individual dishes The heat engine absorbs thermal energy from the receiver, and uses it to produce mechanical work, which an attached alternator then converts into electricity A heat-waste exhaust system must be incorporated to release excess heat from the system Finally, a control system is necessary to ensure matching of the heat engine’s operation to the incoming solar flux An advantage of this design is that the reflector, collector and engine can operate as separate units, making fossil-fuel hybridization a relatively simple task It is important to note however, that this PDC system does not lend itself to thermal storage methods The Stirling engine is often used for this application, although gas turbines can also be employed in the Brayton or Rankine/ Brayton combined cycles Stirling engine performance is better in temperatures below 950 1C, whereas at higher temperatures, combined cycle gas turbines can achieve higher efficiencies [54] The operations and specifications of a 10 kW single dish-Stirling system were described in detail by Jin-Soo et al [55] Due to its high concentration ratios, the parabolic dish collector 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 technologies 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 photovoltaics is developed in a later section A numerical simulation of a heat-pipe receiver for the parabolic 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 equilateral triangles, each with a side length of cm special nuts are used to maintain required positions of nodes in the connection points of mirror apexes These small mirror arrangements approximate a parabolic collector in a relatively inexpensive way 2713 At such high operation temperatures, heat losses become extremely significant, and must be contended with to achieve high efficiencies A detailed two-dimensional simulation of heat transfer in a modified cavity receiver of PDC system is presented by Reddy and Kumar [58] Combined heat losses due to both laminar convection and surface radiation from the receiver are calculated by this model The modified cavity receiver (Fig 15a and b) has a semi-circle shape that features a small aperture at the dish’s focal point The receiver is essentially hollow (air cavity) and its inner surface is laid with absorber tubes The encasing of the tubes is made of insulating material Reddy and Kumar published another numerical analysis in 2009 [59], in which a three-dimensional model is used to estimate receiver heat losses at different dish inclination angles and various operating temperatures The model evaluates heat loss reductions realized through secondary concentrator integration A cone collector, compound parabolic collector (CPC) and trumpet reflector were compared as second stage concentrators (Fig 16a–c), and yielded natural convection heat loss reductions of 29.23%, 19.81% and 19.16%, respectively Another thermal analysis of a PDC system was done by Nepveuat al [60] The authors constructed a thermal energy conversion model of the 10 kW Eurodish/Stirling unit erected at the CNRSPROMES laboratory in Odeillo The model analyzes spillage and radiation (reflection and IR-emission) losses of the reflector, and calculates conduction, convection, reflection and thermal radiation losses through the receiver cavity (Fig 17) A thermodynamic analysis of a SOLO Stirling 161 engine is also presented The model was compared to experimental results of the solar power system and was determined a good fit An innovative solar thermal power approach was formulated by Shuang-Ying et al [61] This design features a dish concentrator Fig 15 (a) Light collection and (b) general schematics of air cavity receiver in a dish/Stirling system 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 photovoltaics [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 Concentrated photovoltaics Fig 16 (a–c) Secondary reflectors for a parabolic dish reflector system Figure reproduced with permission from ref 59, &2009 Springer Fig 17 Eurodish receiver heat flow and heat loss diagram Figure reproduced with permission from ref 60, &2009 Elsevier cascaded with an alkali metal thermal-to-electric converter (AMTEC) through a coupling heat exchanger The proposed system employs a heat-pipe receiver for isothermal energy transfer from the dish to the AMTEC unit Theoretical investigation of this system’s performance predicts a 20.6% peak thermal-to-electric conversion efficiency Effects of various parameters on the overall conversion efficiency of the parabolic dish/AMTEC system are discussed in detail Increasing the geometric concentration and tilting angles of the dish both result in efficiency enhancement The authors report that this design has a potential to become a leading low-cost renewable energy source because of its passive nature A paradigm shift in PDC design suggested by Feuermann and Gordon [62] utilizes arrays of mini-dishes coupled with fiber optics that carry solar radiation to a central receiver Each mirror is about 20 cm in diameter and has a small flat mirror at its focal point, to which a single optical fiber is attached The fiber transports collected light to a central receiving unit, where it can be converted into heat Low attenuation fibers of high numerical aperture, coupled with mass produced highly accurate parabolic dishes, can operate at 80% efficiency and yield incredibly high concentration ratios of up to 30,000 sun Experimental realization and field experience of this proposed system were carried out by Feuermann et al [63] One mm diameter optical fibers repeatedly transported solar flux levels of 11–12 ksun to a target as far as 20 m away The prototype proved impervious to dust penetration and condensation, and was reportedly constructed solely from off-the-shelf parts and customized items 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 highefficiency 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 concentration 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 needing active cooling mechanisms Low concentration systems generally feature wider acceptance angles, and in some cases not need to track the sun, reducing their cost Two-axis tracking systems are required in high concentration systems Gallium arsenide and multi-junction cells are better used in medium–high concentration systems (100–300 sun, 300 sun and above) These cells are very expensive to manufacture, but have exhibited record conversion efficiencies and operate well under high temperature Still, heat sinks are often integrated with high-concentration CPV modules in order to alleviate high temperature effects and prolong cell lifetime Examples of cooling mechanisms include direct water cooling and thermal conduction by heat pipes, discussed by Farahat [65] Due to very high material and manufacturing costs, multijunction cells are significantly more expensive than silicon cells per unit area Yet, multi-junction cell efficiency can be up to 15% greater than that of silicon cells, which can make a big difference in performance at high solar concentrations Furthermore, the small PV receivers account for only a fraction of the total CPV system cost, hence system economics may very well favor the use of multi-junction cells Another recently explored concept is the Concentrating Photovoltaics and Thermal (CPVT) design This scheme produces both electricity and heat simultaneously in a single system The heat can be used for industrial heat processes, heating and cooling of buildings, or simply to increase electricity output A parabolic trough CPVT design was introduced by Coventry [27], and a solar tower design was suggested by Segal et al [31] Small CPVT systems can be installed in private homes, and can feature a total energy output of over 50% compared with 10–20% of the basic PV panels D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 A novel design for a miniature parabolic dish collector CPVT system for residential use was presented by Kribus et al [66] Analysis of the electric and thermal performance, heat transport system, manufacturing cost and resulting cost of energy for domestic water heating is carried out The reflector is made of a single thermally bent glass sheet coated with silver to produce the reflective surface An external protective coating prevents exposure of the silver to the environment A 32% conversion efficiency multijunction module is mounted at the focal point, over a cooling plate that removes the surplus heat from the cells to a coolant fluid (typically water) The heated coolant is directed to a heat exchanger where the transported thermal energy may be used as an additional energy product Performance testing of a 0.95 m2 dish area under direct insolation of 900 W/m2 yielded an electrical output of 172 W and a thermal output of 530 W, exceeding 60% of the input energy Miniature dish collectors can be used to achieve very high concentration CPV systems Investigation of this type of system operating at a concentration ratio of 1000 sun was presented by Feuermann and Gordon [64] The system features high-efficiency heterojunction cells as the PV receiver and utilizes optical fibers for heat conduction towards a passive heat sink Arrays of these small systems can be mounted together on large, two-axis tracking systems The merits and identified problems of a similar design were discussed by Anton et al [67] Fresnel lenses used in CPV systems are small and very thin (3–5 mm), and are generally made of glass, plastic or acrylic resin Fig 18 Schematic side-view of a Fresnel lens (left) compared with a circular lens (right) Source: Wikipedia 2715 (polymethylmethacrylate, PMMA) They are flat on one side and ridged on the other The Fresnel lens structure is composed of many concentric rings, which are thinner towards the center Each ring is slightly angled to concentrate incident light onto the focal point of the lens (Fig 18) Linear Fresnel lenses operate in a similar manner, but feature a focal line instead of a focal point A linear Fresnel collector can include an array of these lenses positioned side-by-side The array is mounted on a sun-tracking device Every lens is mounted on a small axis through the center of its length, which can orient it to follow the sun The entire collector unit can track the sun along the second dimension, providing the system with a two-axis tracking regime (Fig 19) An optical and thermal performance simulation for this type of system was done by Mallick and Eames [68] The effects of varied spacing between linear lenses within an array on the efficiency are presented Linear Fresnel lenses also can be coupled with small secondary concentrators to minimize the PV receiver area needed [69] Optimization of concentration level, cell technology, receiver size and shape and heating/cooling management is necessary to achieve high performance systems A study of the energetic and thermal characteristics of a small CPV system was conducted by Mirzabaev et al [70] The module was based on a Fresnel lens and an AlGaAs–GaAs PV receiver, and compared several receiver sizes and contact shapes (tetragonal and circular) Analysis of the 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 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 onto specific regions of the solar cell module (Fig 20a and b) 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 not enjoy Combining state-ofthe-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 Concentrated solar thermoelectrics Fig 21 Wide acceptance angle design for cylindrical Fresnel lens Figure reproduced with permission from ref 73, &2009 SPIE must be implemented, and can vary across different systems (according to size, output, etc.) Mathematical evaluations of the performance and concentration efficiency are presented, along with illustrations of the new concept When investing in high-quality solar cells, it is desirable to integrate them with systems that achieve very high concentrations At such conditions, however, Fresnel lenses have a very narrow acceptance angle range (on the order of 711), and the system must include very fine tracking mechanisms for efficient absorption to occur The design of a cylindrically symmetric Fresnel lens was explored by Yu-Ting and Guo-Dung [73] A simulation of a CPV system incorporating this technology was carried out at high concentration (300–400 sun) A couple of system designs was presented The most successful design (Fig 21) incorporated the cylindrical Fresnel lens, two reflective surfaces, a biconic lens and a light pipe This structure, though fairly complicated, expanded the acceptance angle to 7101 Theoretical discussion of the optical capabilities of a cylindrical lens was presented by Gonzalez [74] Both a concentration level of 70% of the theoretical maximum and a 100% geometrical optical efficiency were reported The lens also featured very uniform illumination of the receiver, an important attribute for concentrated photovoltaic systems The integration of solar cells with CSP technologies requires a cautious balancing of the advantages and issues of each On one hand, Conversion of solar energy into electricity directly can also be achieved using the concept of thermoelectrics Recent developments in thermoelectric applications have been exploring ways to utilize CSP to generate electricity Solar thermoelectric devices can convert a solar thermal energy (typically waste heat) induced temperature gradient into electricity They can also be modified to perform cooling or heating One advantage of thermoelectric methods (compared with heat engines) is their increased reliability, as such devices could work 10–30 years with little technical problems [75] Moreover, thermoelectric generators are a flexible source of clean energy capable of meeting a wide range of requirements Hybrid systems that combine thermoelectric and photovoltaic are under development This type of system allows harvesting of solar radiation in both the ultraviolet and infrared ranges of the spectrum Such a hybrid can also reduce wasted thermal energy, since it ‘functionalizes’ a wide temperature range for power production While most silicon solar cell performance begins to degrade at temperatures approaching 100 1C, thermoelectric devices actually perform better at temperatures over 200 1C A solar thermoelectric power generator typically consists of a thermal collector and a thermoelectric generator Heat is absorbed by the thermal collector, then concentrated and conducted over the thermoelectric generator by a fluid pipe The thermal resistance of the generator creates a temperature difference between the absorber plate and the fluid, which is proportional to the incoming heat flux The current produced by the D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 thermoelectric generator is in turn proportional to the temperature difference 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 concentration of the solar radiation onto the hot side of the thermoelectric device Since thermoelectrics made of high quality materials are relatively expensive, a key design consideration for these solar generators is minimal use of thermoelectric materials Naturally, amounts used must be adjusted in accordance with desired power requirements The use of solar concentrating elements can augment the magnitude of the heat flux absorbed by a thermoelectric device, contributing to a higher temperature gradient across it Among the single line focusing parabolic trough collector, the compound parabolic concentrator and the two-stage concentrator, the latter uses a secondary receiver to further concentrate the incident solar radiation A design for a two-stage solar concentrator has been proposed [76], which is well-suited to commercially available thermoelectric devices for small scale power generation The twostage solar concentrators comprise of a primary, one-axis PTC, with a secondary, symmetrical CPC mounted at its focus Several designs have been suggested to further increase the hot side temperature of the generator Solar concentration must be greater than 20 sun to effectively irradiate a thermoelectric device [76,77] Schematics of two solar thermoelectric regimes that incorporate concentrators are shown (Fig 22a and b) Both schematics are based on the two-stage concentrator design, where the second concentrator also acts as a receiver and can generate a larger temperature difference across the thermoelectric device The receiver can combine a thermionic converter (TIC) with a thermoelectric converter (TEC) to use thermal energy more efficiently (Fig 22a) The TIC is a cylindrical cavity-type solar receiver made of graphite, which is heated in a vacuum by the solar concentrator Once the TIC emitter is uniformly heated up to 1800 K, a hot side generator temperature of 1800 K can be achieved [78] The thermoelectric device can also be attached directly to the absorber plate of the receiver (Fig 22b) The field of solar thermoelectric power generation, its coupling with two-stage solar concentrators in particular, is a very recent innovation in the scope of CSP Many solar thermoelectric designs are not fully developed or are still in their initial stage However, the usefulness and diversity of applications this concept offers Fig 22 Schematic of two-stage concentrator design featuring a (a) thermionic converter and (b) thermoelectric device (only) 2717 promote great interest in its exploration and motivate continued research of design and materials Thermal energy storage A significant complication with the utilization of solar thermal power as a primary source of energy is the variable supply of solar flux throughout the day, as well as throughout the year Although there is a reasonable match between the hours of the day in which both available solar energy and electricity consumption peak, nighttime energy usage must be taken into consideration Additionally, seasonal and weather changes greatly influence the amount of solar thermal energy that can be harvested An affordable, reliable energy storage method is thus a crucial element in a successful year-round operation of a thermal solar power plant The cyclical availability of solar energy determines two types of thermal storage are necessary for maintaining a constant supply of solar thermal power driven electricity The first is short-term storage, where excess energy harvested daily is stored for nighttime usage The second is long-term storage in which excess energy is stored during spring and summer months in order to complement the smaller energy flux available in winter Thermal energy storage can be divided into three main categories: sensible heat storage, latent heat storage and chemical storage Sensible heat storage involves heating a solid or liquid and insulating it form the environment until the stored thermal energy is ready to be used Latent heat storage involves the phase change (generally solid–liquid) of the storage material The heatinduced phase change stores a great deal of thermal energy while maintaining a constant temperature, and can be easily utilized for nighttime energy storage if kept under proper isolation A plot demonstrating sensible and latent heat storage is given (Fig 23) Chemical storage is implemented using harvested thermal energy in reversible synthesis/de-synthesis endothermic reactions The heat ‘invested’ in producing/dissociating a certain material (ammonia, methane, etc.) can be easily stored indefinitely The reverse, exothermic reaction will release the heat with minimal losses for electricity generation at a later time Chemical storage is thus most suitable for long-term or seasonal storage Sensible heat storage can employ a large variety of solid and liquid materials It can be put into practice in a direct or indirect 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 23 Sensible vs latent heat storage 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 Desired characteristics of ‘sensible-heat-storage-friendly’ molten salts include high density, low vapor pressure, moderate specific heat, low chemical reactivity and low cost One big disadvantage of molten salts is that they are usually quite pricey Detailed characteristics of storage materials (Table 5a and b) are given by Gil et al [79] Latent heat storage in the solid–liquid phase transition of materials is considered a good alternative for sensible heat storage From an energy perspective, storage using phase change materials (PCM) can operate in relatively narrow temperature ranges between charging and discharging thermal energy Additionally, PCM materials generally feature higher densities than sensible heat storage materials The interest in PCM latent heat storage systems is increasing, mainly due to potential improvements in energy efficiency and nearly isothermal energy storage and release In addition to the few commercially available PCMs today, many organic and inorganic compounds are being investigated for latent heat storage purposes (Table 5c–e) A disadvantage of PCMs is their low thermal conductivity, which results in slow charge–discharge rates One suggested initiative for alleviating this problem involves the fabrication of PCM composite materials; mixing pure PCMs with graphite, for example, can boost thermal conductivity and promote faster energy storing and releasing Since sensible and latent thermal energy storage schemes can only retain their energy efficiently for so long, the need for longterm, cross-seasonal storage is made possible by thermo-chemical storage processes Thermal energy storage in heat intensive endothermic reactions has the possibility to realize higher energy efficient processes the thermal storage regimes Potentially high energy densities can be stored using chemical storage Reformation of methane and CO2 [30], metal–oxide/metal conversions [80] and ammonia synthesis/dissociation [14,81] are just a few examples of heat-assisted chemical reactions that can store solar thermal energy in their endothermic reaction products and release it at a later time/place by the reverse process Numerous heat-storing chemical reactions are listed (Table 5f) Table 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 solar salt Mineral oil Synthetic oil Silicone oil Nitrite salts Nitrate salts Carbonate salts Liquid sodium Temp (cold) (1C) Temp (hot) (1C) Avg density (kg/m3) Avg thermal conductivity (W/m K) Avg heat capacity (kJ/kg K) Volume specific heat capacity (kWht/m3) Cost per kWh (US$/kWh) 120 133 n/a n/a n/a n/a n/a 200 300 770 0.12 2.6 55 4.2 250 350 900 0.11 2.3 57 43.0 300 400 900 0.10 2.1 52 80.0 250 450 1825 0.57 1.5 152 12.0 265 565 1870 0.52 1.6 250 3.7 450 850 2100 2.0 1.8 430 11.0 270 530 850 71.0 1.3 80 21.0 (b) Sensible heat storage solid materials and their properties Storage Medium Sand-rock Mineral Oil Reinforced Concrete NaCl (Solid) Cast Iron Cast Steel Silica Fire Bricks Magnesia Fire Bricks Temp (cold) (1C) Temp (hot) (1C) Avg density (kg/m3) Avg thermal conductivity (W/m K) Avg heat capacity (kJ/kg K) Volume specific heat capacity (kWh/m3) Cost per kWh (US$/kWh) 200 300 1700 1.0 1.30 60 4.2 200 400 2200 1.5 0.85 100 1.0 200 500 2160 7.0 0.85 150 1.5 200 400 7200 37.0 0.56 160 32.0 200 700 7800 40.0 0.60 450 60.0 200 700 1820 1.5 1.00 150 7.0 200 1200 3000 5.0 1.15 600 6.0 (c) Commercial phase change materials (PCMs) and their properties Name Type Phase change temp (1C) Density (kg/m3) Specific heat (kJ/kg K) Thermal conductivity (W/m K) Latent heat (kJ/kg) RT110 E117 A164 Paraffin Inorganic Organic 112 117 164 n/a 1450 1500 n/a 2.61 n/a n/a 0.70 n/a 213 169 306 (d) Inorganic substances with potential use as phase change materials Compound Phase change temp (1C) Density (kg/m3) Specific heat (kJ/kg K) Thermal conductivity (W/m K) Latent heat (kJ/kg) MgCl2-6H2O 115–117 1450 (liquid, 120 1C) 1570 (solid, 20 1C) n/a 165 Hitec: KNO3–NaNO2–NaNO3 Hitec XL: 48%Ca(NO3)2–45%KNO3–7%NaNO3 Mg(NO3)–2H2O KNO3–NaNO2–NaNO3 68% KNO3–32% LiNO3 KNO3–NaNO2–NaNO3 Isomalt LiNO3–NaNO3 120 130 130 132 133 141 147 195 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 0.570 (liquid, 120 1C) 0.598 (liquid, 140 1C) 0.694 (solid, 90 1C) 0.704 (solid, 110 1C) n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 275 n/a 75 275 252 D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2719 Table (continued ) (d) Inorganic substances with potential use as phase change materials Compound Phase change temp (1C) Density (kg/m3) Specific heat (kJ/kg K) Thermal conductivity (W/m K) Latent heat (kJ/kg) 40%KNO3–60%NaNO3 54%KNO3–46%NaNO3 NaNO3 KNO3/KCl KNO3 KOH MgCl2/KCl/NaCl AlSi12 AlSi20 MgCl2 80.5% LiF–19.5% CaF2 eutetic NaCl NaCO3–BaCO3/MgO LiF Na2CO3 KF K2CO3 KNO3/NaNO3 eutetic 220 220 307 320 333–336 380 380 576 585 714 767 800–802 500–850 850 854 857 897 n/a n/a n/a 2260 2100 2.11 2.044 1800 2700 n/a 2140 2100 2160 2600 n/a 2533 2370 2290 n/a n/a n/a n/a 1.21 n/a n/a 0.96 1.038 n/a n/a 1.97 n/a n/a n/a n/a n/a n/a n/a n/a n/a 0.5 0.5 0.5 0.5 n/a 160 n/a n/a 1.7 5.0 5.0 n/a 2.0 n/a 2.0 0.8 n/a n/a 174 74 266 149.7 400 560 460 452 790 492 n/a 1800 (MJ/m3) 275.7 452 235.8 94.25 (e) Organic substances with potential use as phase change materials Compound Phase change temp (1C) Latent heat (kJ/kg) Latent heat (kJ/L) Isomalt: ((C12H24O112H2O) ỵ(C12H24O11)) Adipic acid Dimethylol propionic acid Pentaerythritol AMPL ((NH2)(CH3)C(CH2OH)2) TRIS ((NH2)C(CH2OH)3) NPG ((CH3)2C(CH2OH)2) PE (C(CH2OH)4) 147 152 153 187 112 172 126 260 275 247 275 255 28.5 27.6 44.3 36.9 n/a n/a n/a n/a 2991.4 3340 (kJ/kmol) 4602.4 (kJ/kmol) 5020 (kJ/kmol) (f) Chemical storage materials and reactions Compound Material energy density Reaction temp (1C) Chemical reaction Ammonia Methane/water Hydroxides Calcium carbonate Iron carbonate Metal hydrides Metal oxides (Zn and Fe) Aluminum ore alumina Methanolation–demethanolation Magnesium oxide 67 kJ/mol n/a 3.0 GJ/m3 4.4 GJ/m3 2.6 GJ/m3 4.0 GJ/m3 n/a n/a n/a 3.3 GJ/m3 400–500 500–1000 500 800–900 180 200300 20002500 21002300 200250 250400 NH3 ỵ DH-1/2N2 ỵ 3/2H2 CH4 ỵ H2O-CO ỵ 3H2 Ca(OH2)-CaO ỵH2O CaCO3-CaO ỵ CO2 FeCO3-FeO ỵ CO2 Metal xH2-metal yH2 ỵ(x y)H2 2-step water splitting: Fe3O4/FeO redox system n/a CH3OH-CO ỵ 2H2 MgO ỵH2O-Mg(OH)2 Every storage method mentioned can play an important role in several concentrated solar power designs The chosen storage scheme must, however, be carefully matched to the size (total power output) and operational procedures associated with a specific plant, as well as to its governing environmental and economic factors Luckily, the developments made to date in all three thermal storage methods offer a great diversity of materials from which one can choose in order to meet varying necessary parameters 10 Energy cycles The conversion of solar thermal energy into electricity generally requires the use of a thermodynamic cycle Several types of cycles are the mainstream options for heat conversion into work They can vary in design and process efficiency, but all cycles use heat harvested from solar collectors to power a generator for electricity production The most common thermodynamic cycle used is the Rankine cycle In this regime, heat is supplied externally (from collectors) to a closed loop system, which usually uses water as its working fluid Cycle operation is outlined in several repeating steps Working fluid is pumped from low to high pressure This requires little input energy for the pump if the fluid is a liquid This is one advantage of the Rankine cycle High pressure liquid is heated in a boiler at a constant pressure to become saturated vapor The vapor is then allowed to expand through a turbine generator to 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 thermodynamic loop If vapor temperature is not very high (wet vapor), condensation can occur during release through the turbine, and fast moving water droplets damage the turbine and reduce its lifetime and efficiency Rankine operations at high temperatures produce ‘dryer vapor’, and can thus considerably increase system performance Solar powered Rankine cycles using low cost collectors for clean water and power generation are reviewed by Garcı´a-Rodrı´guez and Blanco-Ga´lvez [82] The ‘organic’ Rankine cycle utilizes organic fluid such as 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 (70–90 1C) These lower temperatures result in a lower thermodynamic efficiency, but this may be counter-balanced by the lower heat inputs required to drive the system Organic fluids that have boiling points above water can be used, and this may have thermodynamic benefits A comparison of several working fluids for organic Rankine cycle operation of PTCs was carried out by Delgado-Torres and Garcia-Rodriguez [83] The Brayton cycle has also been adapted for CSP electricity generation This cycle uses a gas compressor, a combustion chamber and an expansion turbine General operations of the Brayton cycle begin with ambient air being drawn into a compressor to be pressurized It is then directed into a combustion chamber, where it is heated at a constant pressure Conventionally, this heating is done by burning fossil fuels, but thermal energy harvested from solar collectors performs this task in a CSP power plant The heated air is allowed to expand through a gas turbine (or a series of turbines) to produce electricity The compressor can be powered by the turbine generators Excess heat is exhausted into the atmosphere In 2002 a hybrid open solar Brayton cycle was operated for the first time consistently 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 available everywhere A regeneration mechanism can be incorporated to improve Brayton cycle efficiency Still-warm air that has already passed through the turbine can be circulated back towards the compressor intake and pre-heat air before it enters the combustion chamber Less heat is exhausted out of the system, and less power is consumed by the chamber’s heating mechanism The Brayton cycle generally operates at significantly higher temperatures than the Rankine cycle Despite this fact, the overall efficiencies of large-scale steam generators and gas turbines seem to be similar The combined cycle utilizes a hybrid of the Rankine and Brayton cycles, and can achieve higher efficiencies than either The combined cycle uses the Rankine cycle as a bottoming cycle; heated air is first used to power turbines in the Brayton regime Excess heat, which would otherwise be exhausted into the atmosphere, is instead employed as a heating mechanism for a Rankine (steam) cycle Though it is more efficient, this design is more bulky and expensive to implement A cost-efficiency analysis must be carried out for a given plant size and output in order to evaluate economic viability A discussion of a combined cycle CSP design using solar tower reflector technology is presented by Kribus et al [84] Both hybrid and solar-only power plants are investigated An efficiency study of the combined cycle was done by Donatini et al [85] The project examined the combined cycle integrated in a parabolic trough collector regime using molten salts as the heat transfer fluid Decreasing the cost and improving the efficiency of power production cycles can greatly influence the market penetration of concentrated solar power technologies A few innovative energy cycles have been discussed in the literature, which use multicomponent working fluids or employ additional cycle steps to improve efficiency and limit power consumption A multi-component working fluid features variable boiling temperatures according to its composition This process can yield a better thermodynamic match with different sensible heat sources than can be achieved with a single-component fluid The advantages of using an ammonia/water mixture as a working fluid are reviewed by Goswami et al [86] The mixture is utilized in the bottoming Rankine cycle of a combined cycle operated plant design An innovative addition to the combined cycle was suggested by Kribus [87] A solar triple cycle is proposed, the first of which utilizes is a magneto-hydrodynamic (MHD) cycle This cycle operates at very high temperatures, upwards of 2000 1C It passes hot ionized gas through a magnetic field, resulting in electric current generation The great amount of heat is exhausted into a Brayton and Rankine bottoming cycles connected in series The triple cycle needs to be integrated with an HFC design in order to meeting the high temperature requirement The overall peak conversion efficiency of the solar triple cycle is shown to be significantly higher than the solar combined cycle scheme The sensitivity of this result to several system parameters and the technological feasibility of the triple cycle are examined by the authors The improvement of well-understood energy cycles and the development of new ones greatly extend the potential of all nearly all concentrated solar power production regimes The contributions of advanced/high energy cycles to the overall thermal-to-electric power conversion efficiency can be very significant, and help bring CSP closer to the realm of grid-parity It is important to note that relative costs associated with this step become quite considerable with increased levels of sophistication, a fact that must be weighed against the benefits such clever designs provide 11 Applications In addition to the main objective of electricity production, concentrated solar power technologies offer a large variety of applications for which solar thermal energy can be harnessed Industrial heat processes, chemical production, salt-water desalination, heating and cooling are just a few examples of the plethora of available applications that can be implemented using CSP technologies It is important to note that some applications are CSP technology selective – they require integration with a specific CSP design – while others can be coupled to several of the regimes discussed in this article The use of solar thermal power for water desalination and purification has been discussed repeatedly in the literature The fact that regions of the world where clean drinking water is scarce also have an abundance of solar radiation, which makes this CSP application very worthwhile Desalination is generally done by evaporating salt-water to leave salt behind, then condensing salt free vapor back into its liquid state The process of heating large amounts of water for drinking and agricultural purposes requires immense amount of energy Concentrating solar radiation and converting it to heat is an efficient method by which this process can be achieved using emission-free, renewable energy In addition to boiling the water, thermal power could be used to power absorption chillers, thus using the same power source both for evaporation and condensation of water Several plant designs for solar powered desalination, detoxification and disinfection of water are presented by Blanco et al [88] Designs for both large-scale and small-scale operations are discussed Solar waterdetoxification schematics are presented, which are based on the concept of using near-ultraviolet visible spectrum bands to promote oxidizers generation Solar water disinfection utilizes the same method, but incorporates a supported photocatalyst to generate powerful oxidizers to control and destroy pathogenic water organisms A different desalination design by Alrobaei [89] serves the same purpose using parabolic trough collectors coupled to a gas turbine operating in the combined Rankine/ Brayton cycle A novel application of CSP was presented by Perez-de-losReyes et al et al [90], where an array of six parabolic trough collectors were used to harvest thermal energy for disinfestation of greenhouse soils The system was able to bring soil temperature up to 60 1C, and was reported effective by the authors D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 CSP technologies can supply electricity and heat for chemical production processes The production of hydrogen using concentrated solar power is discussed by Glatzmaier and Blake [91] The authors compare two separate processes involving concentrated solar power and the electrolysis of water In one regime, CSP is used to produce alternating current electricity, which is then supplied to an electrolyzer operating in ambient temperature The other method utilizes high thermal electrolysis of steam This regime was operated at about 1273 K and, thermodynamically, required less energy than ambient temperature electrolysis A solar collector can provide both AC electricity and thermal energy to the system in this design Heat conversion into electricity followed by the electrolysis of water is a process that involves several lossy steps and thus has a low overall efficiency Kolb et al [92] suggested utilizing solar towers for large scale production of hydrogen The authors proposed an alternative design, by which hydrogen is produced using a thermo-chemical process This regime features an HFC, a solid-particle receiver, a particle thermal energy storage system and a sulfuric acid cycle Such a thermo-chemical plant is said to produce hydrogen at a much lower cost than solar-electrolyzer plants of similar size Hydrogen production is an effective chemical storage medium for thermal energy, and can be used for many industrial processes as well The production of zinc can also be achieved using CSP technology A 300 kW solar chemical pilot plant was demonstrated in the framework of the EU-project SOLZINC [80] Production was implemented using a carbothermic reduction process of zinc oxide This process makes zinc production possible at temperatures of 1300–1500 K, compared with the ZnO dissociation process, which requires temperatures exceeding 2000 K A ‘beam-down’ HFC regime was used to concentrate solar radiation onto a dual-cavity solar chemical reactor The top cavity is a solar absorber, and the bottom one is a reaction chamber containing a ZnO/C packed bed Demonstration of the plant yielded 50 kg/h of 95% purity zinc The measured conversion efficiency was 30% Zinc can be used in batteries and fuel cells, and can be reacted with water to produce high purity hydrogen gas This is an exothermic reaction, and can itself be used for power generation, making zinc a possible thermo-chemical storage candidate The product of this reaction is in turn ZnO, which can then be used again for zinc production A process for carbon dioxide recycling was reviewed by Hartvigsen et al [93] Co-electrolysis of CO2 and steam can be applied to produce synthesis gas in a large-scale fashion This process not only reduces CO2 emissions into the atmosphere, but can utilize syngas for further clean energy production Carbon 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 processes 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 plant supplies heat to a coal-fired power station The usage of solar thermal power for superplastic forming processes is suggested by Lytvynenko and Schur [94] The process 2721 discussed is used for forming of sheet metals Utilization of CSP for this process is reported to be efficient and cost-effective Thermal treatment of crude oil using a parabolic trough collector system was suggested by Mammadov et al [95] Concentrated solar energy can also be used for driving the endothermic reaction that produces lime (calcination reaction) Running this reaction at above 1300 K is reported to reduce emissions of the process by 20–40%, depending on the manufacturing plant [96] An economic assessment for a large-scale (25 MW) plant based on this process found estimates lime cost to be roughly twice the current price of conventionally produced lime This process produces very high purity lime, and its prices might be competitive with fossil-fuel based calcination processes for chemical and pharmaceutical sectors requiring unadulterated lime Solar power can be utilized for temperature control of buildings, providing both heating and cooling mechanisms A high efficiency solar cooling process is outlined by Gordon and Choon Ng [97] A cascade of mini-dish collector and gas micro-turbine produces electricity that drives a mechanical chiller, with turbine heat rejection running absorption chiller A special feature of this system is that energy can be stored compactly as ice The compactness of the solar mini-dish system is conducive for small-scale ultra-high-performance solar cooling systems The utilization of Fresnel lenses was also suggested for lighting and temperature control of buildings [98] A collection system using a Fresnel lens concentrator and a solar receiver generally absorbs between 60% and 80% of incoming radiation The remaining solar flux can be distributed in the interior space for illumination and heating needs On days when solar radiation is high, this provides cooling of interior spaces as well as brightness control During low solar intensity periods, the absorber can be shifted off-focus to permit 100% of light to be distributed around the interior (Fig 24a–c) The receiver can be of PV type, thermal type or a hybrid of the two, and will collect solar energy for heat and/or electricity generation A parabolic trough collector system was constructed at the Carnegie Mellon University to study the potential of this CSP regime in solar heating and cooling [99] The collective area of the mirrors was 52 m2 The collector system was coupled to a 16 kW double effect, water–lithium bromide (LiBr) absorption chiller and a heat recovery heat exchanger Generation of hot and chilled water was available depending on the season Under optimal design, the system was able to achieve 39% of cooling and 20% of heating energy for the interior space off the building it was connected to (Pittsburgh, PA) The design of a solar absorption refrigeration system directly powered by a LFR concentrator has also been suggested [100] Evaluation of the technical feasibility of LFR integrated solar-GAX Fig 24 a–c Receiver shifting from focus of linear Fresnel lens can be used to manage the amount of solar radiation introduced into a building, providing temperature control Dashed lines represent diffuse sunlight (a) Receiver is in 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 configurations is performed in order to obtain optimal operation conditions The study validates this technology as more than satisfactory; the numerical simulation demonstrated that this scheme answers both quantity and quality of the advanced cooling system’s energy demands Furthermore, the operation conditions obtain higher global system efficiencies than previously used technologies For example, the LFR system experienced a 17.9% efficiency increase compared with single effect water–lithium bromide cycle coupled in an indirect form with a PTC system A great deal of work has also been done to develop small-scale, solar powered food (fruit, vegetables and nuts) dryers that can be built with local materials [101–107] However, the existing dryer designs are suited to cloudless, dry environments and they dry too slowly in hazy situations, typical of many tropical developing countries Excessively slow drying allows product degradation caused by microbial decay, insects and naturally occurring enzymes Some existing designs are also expensive and relatively inefficient, and have low capacity (o50 kg/day) Adding a solar concentrating surface increases the heat output of solar devices operating in cloudy or hazy conditions [108] With indirect solar dryers this can be accomplished by adding glazed concentrated solar panels to the system Concentrating solar panels can be used to inexpensively increase the heat output for indirect dryers Additionally, they can be used to focus a greater light flux onto the drying zone in direct dryers, allowing them to operate in low-insolation environments The reflective surfaces can be as sophisticated as precision-machined, polished surfaces or as simple as cardboard covered in aluminum foil The development of a multitude of CSP applications is beneficial in many regards; such applications help turn many carbon emitting industrial processes into ‘clean’ ones, conserve large amounts of electricity that would otherwise be used up and promote a general environmentally friendly approach to energy consumption for both industries and individuals Furthermore, the growing number of these applications aids CSP technologies in taking root, increasing the demand for solar thermal power and advancing it into world markets 12 Discussion The variety of available CSP technologies and the advancements made in each can bring a sense of uncertainty as to which technology works best This is a complicated issue because of the many factors that need to be considered in selecting a particular CSP design Every regime features advantages and disadvantages that must be accounted for in accordance with the size, location, purpose and budget of the specific CSP plant one wishes to build Advantages of the parabolic trough collector CSP regime include relatively low costs, mature and well-tested technologies and easy coupling to fossil fuel/geothermal energy sources PTC systems are becoming more efficient with the incorporation of novel receiver designs such as the heat pipe receiver, which significantly limit convective heat losses while reducing receiver cost The reinforcement of PTCs with a light fiberglass structures grants them great stability against wind loads, which further boosts the efficiency as it provides for more accurate sun-tracking The incorporation of direct steam generation into PTC systems is generally a very positive scheme to produce high quality steam at a constant rate throughout daylight hours, and the usage of water as a heat transfer fluid is generally cheaper than synthetic oils or ionic liquids That being said, water is a more volatile substance than other HTFs and will exert more stress on PTC absorber pipes, which may increase maintenance costs It also needs to be readily available at the site, since, unlike oils and molten salts, it is being directly converted into steam for power production, and must thus be constantly replenished Water transport costs are thus another issue that requires attention Using the Recirculation DSG mode in PTC operation will aid water conservation to some degree The choice to use synthetic oils may be the best option in a PTC site where water is not abundant While ionic liquids can be used as heat transfer media, they are very expensive to manufacture and may thus be better suited for higher temperature operations, such as those of heliostat field collectors The operational temperatures of PTCs can exceed 400 1C, high enough for a plethora of industrial heat processes, yet too low for the more efficient, high energy conversion cycles available for power production Despite the drawbacks mentioned, it should be noted that the maturity and successful experience to date with PTC technology put it at the forefront of CSP regimes While other CSP methods may exceed PTC efficiencies or be better geared towards storage and applications, the fact that large (upwards of 100 MW) power stations based on the PTC scheme have been operational for several years and continue to be built proves this technology both successful and economical An interesting comparison can be made between the concepts of linear Fresnel reflectors and parabolic trough collectors LFR systems prove the cheapest of all CSP regimes, utilizing flat mirrors instead of concave ones, and having incorporating centralized receiver systems that save on receiver material Though they reach an operational temperature of only about 300 1C, they can still be used in a variety of applications The use of DSG works well with LFRs, and the reasoning needed to select a particular type of HTF for this type of method is very similar to that of PTCs A multitude of phase change materials have been proposed for use in LFR latent heat storage systems Although these substances are costly, they can preserve thermal energy effectively for overnight usage The shading issue that accompanies LFR systems is a maximization problem to which many solutions have been suggested The compact LFR regime greatly reduces shading between neighboring reflectors, and allows significantly greater collection of available sunlight The formation of a wave-shaped 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 feature that can help boost thermal efficiency The coating of absorber tubes with Nickel also aids the heat loss issue, and the two could be used in tandem for maximum heat loss reduction The linear Fresnel reflector method is suited for lower efficiencies than the rest of its CSP counterparts, but it does so with the benefits of a significantly more affordable technology The relatively young but very powerful CSP concept of heliostat field collectors has come leaps and bounds over the last few decades The immense flux a large collection of heliostats can direct towards the central receiving unit generates very high temperatures (up to 2000 1C), and can thus operate very efficiently using complex energy conversion cycles, such as the combined cycle and the magneto-hydrodynamic (MHD) cycle High operation temperatures may boost electricity production efficiencies, but are accompanied with both a higher thermal stress on many components of the HFC system and a challenging convection heat loss problem The usage of air as a heat transfer medium becomes available at these high temperatures, which helps relieve some of the stress heated liquids would exert on system components and significantly reduces HTF costs This hybridized air–water heating system can produce steam at very high temperatures (485 1C) at a constant rate The special design suggested for a receiving unit that has a very large inner surface area compared with its aperture is an excellent solution to help minimize convective losses, but will increase costs due to its complicated structure The dual receiver concept for solar towers D Barlev et al / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 is another novel design that can help boost the total power output by a significant portion The quest for cheap materials for heliostat fabrication is a crucial one, as the large mirrors can make up close to half the cost of an HFC plant The use of PVC composite plastic steel offers a light yet stiff structure, which helps ease stress on mirror trackers while increasing their accuracy (high stiffness materials are more wind resistant) The torque tube heliostat (TTH) scheme suggested for wind load reduction is not very effective; it increases the cost and decreases the energy output of the system without significantly diminishing wind stress The suggested use of minimirror arrays resulted in similar results Experimentation and modeling for non-spherical arrangements of heliostat fields presents some potential to increase the amount of solar flux collected from a given area, but the great height of the solar tower makes heliostat shading a non-vital issue The design incorporating a reflector tower and a ground receiver is helpful in reducing transport losses, and makes good organizational sense The HFC scheme can easily couple to all three thermal storage methods discussed, giving it a big advantage over other CSP regimes It is, however, very costly, and large amounts of power must be produced at high conversion efficiencies to make HFCs a more economically viable technology The parabolic dish collector system operates somewhat differently compared with the aforementioned CSP regimes, as very large dish is a power generating system within itself The mounting of a Stirling engine (or a Brayton/combined cycle engine) at a dish’s focus allows it to operate at very high temperatures throughout the day (usually up to 1000 1C) PDCs are heavy and expensive structures that must track the sun very accurately to fulfill their maximum potential The structural design to incorporate many small mirrors to form the large dish can help mitigate some of the required costs The use of an intermediate heat pipe receiver as a link between the reflective dish and the heat engine can be quite positive, as it promotes uniform and nearly isothermal power delivery to the heat engine, boosting its efficiency The heat pipe receiver also helps mitigate convective heat losses The suggested modified air cavity receiver can serve a similar purpose The use of heat engines and high energy conversion cycles makes PDC power production highly efficient PDC systems 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 thermal storage methods, a very serious disadvantage in the scope of large power production plants The use of thermoelectric materials with parabolic dish collectors is an interesting and fresh idea, but current efficiencies of this scheme are quite low and further investigation of thermoelectric materials and their integration with CSP technologies must be carried out The mini-dish concept for CSP is reported to yield record efficiencies and fantastically high concentration ratios, while maintaining fairly low system costs The development of this concept in the coming years may be proved the best execution of the PDC concept The up-and-coming field of concentrated photovoltaics presents a medium between CSP and photovoltaics that shows great promise State-of-the-art solar cells can be coupled to any of the four main CSP regimes in order to absorb very high solar concentrations that can be directly converted into current High quality silicon cells can be used at concentration of up to 100 sun without exhibiting degradation in efficiency For higher solar concentrations, multi-junction cells can be utilized The costbenefit analysis of CPV systems takes into account the price of both the CSP method used and the solar cells chosen for particular systems The costs of the latter are generally quite high and must be offset by high conversion efficiencies to make economic sense Silicon cells also require a cooling mechanism at higher concentrations, which may result in their ‘out-the-door’ cost to become similar to that of the very expensive multi-junction cells It seems that on the whole, photovoltaic power production is less efficient 2723 than CSP, but the latter comes with much higher initial capital investments The integration of Fresnel lenses with solar cells is thus a great venture, since the lenses are relatively cheap to manufacture and can concentrate light very well Uniform illumination issues were considered by several researchers, to which the answer of cylindrically symmetrical Fresnel lenses proved a formidable solution Fresnel lens CPV systems that can track the sun have been developed, to further enhance radiation collection and boost power output throughout the day The concentrated photovoltaic thermal regime is also of interest, as it permits power harvesting of both regimes simultaneously and can result in extremely high conversion efficiencies The mounting of solar cells along the absorber tube of PTC systems, or at a portion of the focal region of parabolic dishes (and mini-dishes), has been shown to be quite successful The installment of PV cells on an HFC receiver for high energy photon absorption made significant contributions to the overall system efficiency Unlike CPV systems, CPVTs can store a large portion of collected energy for later use, but the trade-off from this advantage is based in the HTF costs, which CPV systems not have The field of concentrated solar thermoelectrics seems to draw much attention as well, but is currently in its infancy developmental stages and is far from commercial power generation capabilities in any scale The great variety of application that can be incorporated into concentrated solar power provides further incentive to invest in it Industrial processes can utilize thermal energy directly to save on the costs of fossil fuels while maintaining an environmentally conscientious image Desalination of water could be done cheaply (in the long run), and temperature control of homes could begin producing power instead of consuming it In agriculture, CSP can be used for food drying, roasting of beans and nuts and cooking Furthermore, concentrated solar power can be used for sterilization of surgical tools in remote areas The CSP applications mentioned in this work are all novel ideas that are potentially very useful, but each of them (like the CSP technologies that fuel them) must stand the test of economics in order to penetrate world markets and become universal 13 Conclusion Over the past few decades, great progress has been made in every facet of concentrated solar power technology Striving towards a sustainable, ‘clean’ energy based culture has instilled many with the drive to help rid society of its dependence on fossil fuels With the sun being an obvious and overabundant form of renewable 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the demand for solar thermal power and advancing it into... commercial power generation capabilities in any scale The great variety of application that can be incorporated into concentrated solar power provides further incentive to invest in it Industrial