solar energy materials & solar cells -...

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

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

Science

E-m

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

Review

Innovation in concentrated solar power

David Barlev a,c, Ruxandra Vidu b,c, Pieter Stroeve a,b,c,n

a Department of Electrical and Computer Engineering, University of California Davis, Davis, CA 95616, USAb Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA 95616, USAc California Solar Energy Collaborative (CSEC), University of California Davis, Davis, CA 95616, USA

a r t i c l e i n f o

Article history:

Received 30 October 2010

Accepted 12 May 2011

Keywords:

Concentrated solar power (CSP)

Design

Materials

Heat absorption

Transport

Thermal storage

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.05.020

esponding author at: Department of Chemic

, University of California Davis, Davis, CA 956

ail address: [email protected] (P. Stroeve

a b s t r a c t

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.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2703

2. Concentrating solar collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2704

3. Parabolic trough collectors (PTC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2705

4. Heliostat field collectors (HFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2707

5. Linear Fresnel reflectors (LFR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2711

6. Parabolic dish collectors (PDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2712

7. Concentrated photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2714

8. Concentrated solar thermoelectrics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2716

9. Thermal energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2717

10. Energy cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2719

11. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2720

12. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2722

13. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2723

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2723

1. Introduction

As the world’s supply of fossil fuels shrinks, there is a greatneed for clean and affordable renewable energy sources in orderto meet growing energy demands. Achieving sufficient supplies ofclean energy for the future is a great societal challenge. Sunlight,the largest available carbon-neutral energy source, provides theEarth with more energy in 1 h than is consumed on the planet in

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al Engineering and Materials

16, USA.

).

an entire year. Despite of this, solar electricity currently providesonly a fraction of a percent of the world’s power consumption.A great deal of research is put into the harvest and storage of solarenergy for power generation. There are two mainstream cate-gories of devices utilized for this purpose—photovoltaics andconcentrated solar power (CSP). The former involves the use ofsolar cells to generate electricity directly via the photoelectriceffect. The latter employs different methods of capturing solarthermal energy for use in power-producing heat processes.

Concentrated solar power has been under investigation forseveral decades, and is based on a simple general scheme: usingmirrors, sunlight can be redirected, focused and collected as heat,which can in turn be used to power a turbine or a heat engine to

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Table 1Description and specifications of the four main CSP technologies.

Data compiled from [1,2].

Collectortype

Description Rel. thermodynamicefficiency

Operating temp.range (1C)

Relativecost

Concentrationratio (sun)

Technologymaturity

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

or high-mounted pipe as receiver

Low 50–300 Very low 10–40 Mature One-axis

Solar tower – Large heliostat field with tall tower in

its center

– Receiver: water/HTC boiler at top

– Can be used for continuous thermal storage

High 300–2000 High 150–1500 Most recent Two-axis

Dish-Stirling – Large reflective parabolic dish with Stirling

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)

High 150–1500 Very high 100–1000 Recent Two-axis

D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–27252704

generate electricity. Despite being relatively uncomplicated, thismethod involves several steps that can each be implemented in aplethora of different ways. The chosen execution method of everystage in solar thermal power production must be optimally matchedto various technical, economic and environmental factors that mayfavor one approach over another. Extensive explorations of varioussolar collector types, materials and structures have been carried out,and a multitude of heat transport, storage and electricity conversionsystems has been tested. The progress made in every aspect of CSP,especially in the last decade, was geared towards expanding theefficiency of solar-to-electric power production, while making itaffordable in comparison with near-future fossil fuel derived power.

This work describes the four main types of concentrating solarcollectors (Tables 1 and 2) [1,2] and discusses innovation in eachover the last decade. Progress in the related fields of concentratedphotovoltaics and thermoelectrics will also be presented, alongwith advances made in thermal energy storage methods, energyconversion cycles and CSP applications.

2. Concentrating solar collectors

A solar energy collector is a heat-exchanging device that trans-forms solar radiation into thermal energy that can be utilized forpower generation. The basic function of a solar collector is to absorbincident solar radiation and convert it into heat, which is then carriedaway by a heat transfer fluid (HTF) flowing through the collector. Theheat transfer fluid links the solar collectors to the power generationsystem, carrying thermal energy from each collector to a centralsteam generator or thermal storage system as it circulates.

There are two general categories of solar collectors. The firstincludes stationary, non-concentrating collectors, in which thesame area is used for both interception and absorption of incidentradiation. The second category consists of sun-tracking, concen-trating solar collectors, which utilize optical elements to focuslarge amounts of radiation onto a small receiving area and followthe sun throughout its daily course to maintain the maximumsolar flux at their focus. A comprehensive review of sun-trackingmethods and principles was published by Mousazadeh et al. [3].

Light concentration ratios can be expressed in suns, with a singlesun (1000 W/m2) being a measurement of average incident light flux

per unit area at the earth’s surface. Though more costly, concentratingcollectors have numerous advantages over stationary collectors, andare generally associated with higher operation temperatures andgreater efficiencies. The addition of an optical device to the conven-tional solar collector (receiver) has proved useful in several regards;various concentration schemes can achieve a wide range of concen-tration ratios, from unity to over 10,000 sun [2]. This increases theoperation temperature as well as the amount of heat collected in agiven area, and yields higher thermodynamic efficiencies. Radiationfocusing allows the usage of receivers with very small relative surfaceareas, which leads to significant reductions in heat loss by convection.

Despite the added capital investment necessary for manufacturingthe optical elements of the apparatus, the materials used for thesemirrors/lenses are generally inexpensive compared with thermalcollector materials, which are needed in much smaller amounts ina concentrator scheme. The reduction in receiver size and materialamounts makes expensive receiver conditioning (vacuum insulation,surface treatments, etc.) for higher efficiency and heat loss minimiza-tion economically sensible. Finally, the ability to control the concen-tration ratio of a system allows delicate manipulation of its operationtemperature, which can be thermodynamically matched to specificapplications as needed to avoid wasted heat. It is important to notethat reflective materials used in CSP technologies must meet certainreflectivity and lifetime requirements to be cost-effective. A study ofthe optical durability of solar reflectors was presented by Kennedyand Terwilliger [4] and an investigation specific to aluminum first-surface mirrors was carried out by Almanza et al. [5].

Tyagi et al. [6] investigated the effects of HTF mass flow ratesand collector concentration ratios on various system parameters.Results showed that exergy output (available work from a processthat brings a system to thermal equilibrium), exergetic andthermal efficiencies and inlet temperature increased with solarintensity, as expected. Exergetic and thermal efficiencies andexergy output were found to increase with mass flow rate aswell. Optimal inlet temperature and exergetic efficiency at highsolar intensity were both found to be the decreasing functionsof the concentration level. At low intensity values, however,efficiency first increases and then decreases with increase inconcentration. This behavior results from increased radiativelosses associated with high concentration ratios. Both concentra-tion ratios of solar collectors and the mass flow rates at which

Table 2Schematic diagrams of each CSP technology listed in Table 1.

Figures from [2].

Collector type Schematic diagram

Parabolic trough collector

Linear Fresnel reflector

Heliostat field collector

Parabolic dish reflector

D. Barlev et al. / Solar Energy Materials & Solar Cells 95 (2011) 2703–2725 2705

they operate must be meticulously chosen to achieve optimalperformance.

The four main types of concentrating solar collectors are

(1)
Parabolic trough collectors; (2) heliostat field collectors; (3) linear Fresnel reflectors; and (4) parabolic dish collectors.
Concentrating collectors can achieve different concentrationratios and thus operate at various temperatures. From a theoreticalstandpoint, the efficiency of power producing heat processes is both

proportional and strictly dependent on the operation temperature.In practice, however, the materials chosen for light concentrationand absorption, heat transfer and storage, as well as the powerconversion cycles used are the true deciding factors [7]. Thefollowing sections will describe the aforementioned collectorschemes in detail, and present technological advancements thathave been made in each over the last 10 years.

3. Parabolic trough collectors (PTC)

Parabolic trough technology is the most mature concentratedsolar power design. It is currently utilized by multiple operational

Fig. 2. Schematic flow diagram of Recirculation mode of operation of direct steam

generation.

Figure reproduced with permission from ref. 9, &2005 Elsevier.


large-scale CSP farms around the world. Solar Electric GeneratingSystems (SEGS) is a collection of fully operational PTC systemslocated 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 inArizona and is scheduled to become operational in 2011. PTCseffectively produce heat at temperatures ranging from 50 to400 1C. These temperatures are generally high enough for mostindustrial heating processes and applications, the great majorityof which run below 300 1C.

The parabolic trough collector design features light structuresand relatively high efficiency. A PTC system is composed of asheet of reflective material, usually silvered acrylic, which is bentinto a parabolic shape. Many such sheets are put together inseries to form long troughs. These modules are supported fromthe ground by simple pedestals at both ends. The long, parabolicshaped modules have a linear focus (focal line) along which areceiver is mounted. The receiver is generally a black metal pipe,encased in a glass pipe to limit heat loss by convection. The metaltube’s surface is often covered with a selective coating thatfeatures high solar absorbance and low thermal emittance. Theglass tube itself is typically coated with antireflective coating toenhance transmissivity. A vacuum can be applied in the spacebetween the glass and the metal pipes to further minimize heatloss and thus boost the system’s efficiency.

The heat transfer fluid (HTF) flows through the receiver,collecting and transporting thermal energy to electricity genera-tion systems (usually boiler and turbine generator) or to storagefacilities. The HTF in PTC systems is usually water or oil, where oilis generally preferred due to its higher boiling point and relativelylow volatility. Several water boiler designs have been suggestedby Thomas [8]. The preferred boiling system implements directsteam generation (DSG), where water is the heat transfer fluid.It is partially boiled in the collector and circulated through asteam 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 DSGoperational modes and control systems were developed andtested [9]. Both methods utilize pressure control in addition totemperature control of circulating water. This approach is done toachieve a constant output of steam at a monitored temperaturethroughout most hours of the day (9 am–6 pm). A pressure levelof 100 bar and temperatures of up to 400 1C have been demon-strated. The Once-Through mode (Fig. 1) features a preheated waterfeed into the inlet. As water circulates through the collectors, it isevaporated and converted into superheated steam that is used topower a turbine. In the more water-conservative Recirculation mode(Fig. 2), a water–steam separator is placed at the end of the collectorloop. More water is fed to the evaporator than can be evaporated inone circulation cycle. Excess water is re-circulated through theintermediate separator to the collector loop inlet, where it is mixed

Fig. 1. Schematic flow diagram of Once-Trough mode of operation of direct steam

generation.


with preheated water. This process guarantees good wetting ofabsorber tubes and prevents stratification. Steam is separated fromwater and fed into the inlet of a superheating section. The Recircula-tion regime is more easily controlled than the Once-through regime,but has an increased parasitic load due to the additional processsteps. Usage of water as a HTF inflicts more stress on the absorbertubes than other heat transfer media, due to water’s relatively highvolatility. A simulation of thermohydraulic phenomena under theDSG process was carried out by Eck and Steinmann [10]. Sufficientcooling of the absorber tubes and a moderate pressure drop betweeninlet and outlet can help moderate the stress, reduce corrosion andpromote tube lifetime.

Knowledge of short-time dynamics of flow and feed systemsin a DSG regime is crucial for successful design and operation.A transient non-linear simulation tool was developed to studydynamic behaviors of the aforementioned PTC system designs, forwhich several feed control systems were suggested [11]. It isimportant to mention that for DSG systems, the temperaturedifference registered between the hottest and the coldest pointsover the external wall of the pipe will increase if feed flow is toohigh [12]. This is a result of non-constant heat transference fromthe receiver to the HTF, and can potentially affect the quality ofproduced steam. A test facility for a solid sensible heat storagesystem was developed for the DSG parabolic trough collectordesign discussed. A performance analysis of the storage systemintegrated with the power plant was implemented by Steinmannet al. [13]. Integration of thermo-chemical storage throughammonia 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 theHTF, recent innovation also promotes the use of ionic liquids(molten salts) for heat transfer media [15], as they are moreheat-resilient than oil, and thus corrode the receiver pipes less.Ionic liquids are, however, very costly, and such an investmentwould have to be weighed against the incurring costs of receivermaintenance and replacement to determine their cost-effectiveness.

PTCs are mounted on a single-axis sun-tracking system thatkeeps incident light rays parallel to their reflective surface andfocused on the receiver throughout the day. Both east–west andnorth–south tracking orientations have been implemented, withthe former collecting more thermal energy annually, and thelatter collecting more energy in the summer months when energyconsumption is generally the highest [2]. The east–west orientationhas been reported to be generally superior [16]. The trackingmechanism must have parabolic collectors for tracing the sun’s pathvery accurately in order to achieve efficient heating of the receivertube. However, trough collectors are generally exposed to winddrag, and must thus be robust enough to account for wind loads andprevent deviations from normal insolation incidence.


A study of turbulent flow around a PTC of the 250 kW solarplants in Shiraz, Iran, was conducted by Naeeni and Yaghoubi[17]. The study investigates stress applied to the collector, takinginto account varying collector angles, wind velocities and air flowdistribution with respect to height from the ground. A secondstudy by the authors models the effects of the same parameterson heat transfer from the PTC receiver tube [18].

In order to make the PTC structure more resilient to externalforces, it is possible to reinforce collector surfaces with a thinfiberglass layer. A smooth, 901 rim angle reinforced trough wasbuilt by a hand lay-up method [19]. The fiberglass layer is addedunderneath the reflective coating (on the inner surface) of theparabolic trough. The reflector’s total thickness is 7 mm, and canwithstand a force applied by a 34 m/s wind with minimaldeviation; deflection at the center of the parabola vertex wasonly 0.95 mm, well within acceptable limits.

Receiver design considerations are crucial for efficient heattransfer to the HTF and heat loss management. Radiative heatlosses from receiver tubes play an important role in collectorperformance. Thermal loss due to the temperature gradientbetween the receiver and the ambient has a significant impacton a system’s thermal efficiency. PTCs operating at high tempera-tures (around 390 1C) can experience up to 10% radiative lossesannually. At this temperature range, thermal loss from receiver tubereaches 300 W/m of the receiver pipe [20]. A loss of 220 W/m wasreported for an operational temperature of 180 1C, with collectorefficiency ranging from 40% to 60% [21]. In both of the aforemen-tioned studies, synthetic oil was used as the heat transfer fluid.The high temperature difference between the receiver tube’sinterior and the ambient also induces a thermal stress, which cancause bending of the pipes. Thermal analysis of an energy-efficientPTC receiver was presented by Reddy et al. [22], and a numericalmodel to evaluate its heat transfer characteristics was proposed. Thenew receiver design features porous inclusions inside the tube,which increase the total heat transfer area of the receiver, alongwith its thermal conductivity and the turbulence of the circulatingHTF (synthetic oil). Heat transfer for this scheme was enhanced by17.5% compared with regular (no inclusions) design, but the systemsuffered a pressure decrease of about 2 kPa.

The use of a heat pipe as a linear receiver for PTCs was proposedby Dongdong et al. [23]. The heat pipe can keep an essentiallyuniform circumferential temperature, despite the uneven illumina-tion provided by trough collectors. Since heat does not flow from theHTF to the heat pipe, smaller heat losses occur during hours of lowinsolation. PTC systems featuring a heat pipe as the receiver have65% thermal efficiency at 380 1C. They are also cheaper to manu-facture because the bellows system generally incorporated intoconventional receiver tubes is not necessary. Lifetime testing ofthe heat pipe receiver with respect to various operation tempera-tures is still being investigated, but meets the general requirements(12–15 years) under operation below 380 1C.

Parabolic trough collector systems generally operate in unsteadystate. For this reason, a dynamic model is essential for effectivedesign and performance prediction of a PTC system. A dynamicsimulation of PTC was conducted by Ji et al. [24], modeling a southfacing, one-axis tracking parabolic trough collector. The simulationcalculated variations in incidence angle of solar beam to collectoraperture, as well as the distribution of concentrated solar radiationalong the focal line. Effects of HTF mass flow rate and receiver tubelength on outlet temperature and system efficiency were investi-gated. An increase in tube length augments outlet temperatures andefficiency, as expected due to greater total insulation. A decrease inmass flow rate increases outlet temperature and slightly decreasessystem efficiency.

The integration of a parabolic trough collector field withgeothermal sources has been suggested by Lentz and Almanza

[25,26]. Hot water and steam from geothermal wells can bedirectly fed into an absorber pipe going through a PTC field. Thecombination of both thermal energy sources increases the volumeand the quality of (directly) generated steam for power produc-tion. Several hybrid designs have been suggested by the authors.

PTCs can also be integrated with solar cells in concentratedphotovoltaics (CPV) modules. Heat-resistant, high-efficiencyphotovoltaic cells can be mounted along the bottom of thereceiver tube to absorb the concentrated solar flux. The perfor-mance of a CPV parabolic trough system with a 37 sun concen-tration ratio was characterized by Coventry [27] at AustralianNational University in 2003. Monocrystalline silicon solar cellswere used, along with the thermal PTC apparatus. Measuredelectrical and thermal efficiencies were 11% and 58%, respectively,producing a total efficiency of 69%. It is important to note thatuneven illumination of the solar cell modules causes a directdecrease in the cells’ performance, and thus optical considera-tions must be weighed carefully.

The mature field of parabolic trough collectors provides anefficient, relatively inexpensive power production scheme. Multipleadvances in reflector and receiver design have been made in the lastdecade to enhance efficiency and reduce losses. Heat collection andtransfer methods have been modeled and tested repeatedly in orderto achieve optimal power output throughout the day. The PTCscheme also lends itself to easy storage schemes, as well as tosimple integration with both fossil fuels and other renewableenergy sources.

4. Heliostat field collectors (HFC)

The most recent CSP technology to emerge into commercialutility was the heliostat field collector design. This expensive,powerful design has so far been incorporated in relatively fewlocations around the world. The 10 MW Solar One (1981) andSolar Two (1995) were the first HFC plants to be demonstrated,built in the Mojave Desert of California. They have since beendecommissioned. Other plants, such as the 11 MW PS10 and 20 MWPS20 in Spain, and the 5 MW Sierra SunTower in California, wererecently completed.

The heliostat field collector design features a large array of flatmirrors distributed around a central receiver mounted on a (solar)tower. Each heliostat sits on a two-axis tracking mount, and has asurface area ranging from 50 to 150 m2. Using slightly concavemirror segments on heliostats can increase the solar flux theyreflect, though this elevates manufacturing costs. Every heliostatis individually oriented to reflect incident light directly onto thecentral receiving unit. Mounting the receiver on a tall towerdecreases the distance mirrors must be placed from one anotherto avoid shading. Solar towers typically stand about 75–150 mheight. A fluid circulating in a closed-loop system passes throughthe central receiver, absorbing thermal energy for power produc-tion and storage. An advantage of HFCs is the large amount ofradiation focused on a single receiver (200–1000 kW/m2), whichminimizes heat losses and simplifies heat transport and storagerequirements. Power production is often implemented by steam andturbine generators. The single-receiver scheme provides for uncom-plicated integration with fossil-fuel power generators (hybridplants) [2].

HFC plants are typically large (10 MW and above), as the benefitfrom an economy of scale is required to offset the high costsassociated with this technology. They can incorporate a very largenumber of heliostats surrounding a single tower. The immense solarflux reflected towards the receiver yields very high concentrationratios (300–1500 suns). HFC plants can thus operate at very hightemperatures (over 1500 1C), which positively impacts collection

Fig. 4. Solar ground reformer integrated with a reflector tower HFC system.


Fig. 5. Brayton cycle and combined cycle efficiencies as a function of the

temperature and gas turbine pressure ratio.



and power conversion efficiencies by enabling the use of higher-energy cycles.

A reflective solar tower design has been suggested, in which asecondary reflector is mounted on the tower, and the centralreceiver is grounded (Fig. 3). A review of the optics of the reflectortower was presented by Segal and Epstein [28]. Since HFCsoperate at such high temperatures, the greatest losses areincurred convectively at the receiver’s surface.

Aside from the convenience associated with having the recei-ver situated at ground level, the optics of the design increase theconcentration ratio, allowing the collector to be smaller anddiminish losses. Transport losses can also be lowered by situatingthe turbine generator in close proximity with the receiver. Never-theless, Segal and Epstein [29] reported that the reflector towerscheme is not more efficient than the solar tower regime, and thatsuperiority of either technology is subject mainly to economic factors.

The integration of a solar reformer with a heliostat field arraywas proposed in 2002. Solar reforming of methane with steam orCO2 is an efficient chemical heat storage method. The syngasproduced can be converted into electricity using a gas turbine orcombined cycle. The suggested reformer rests on the ground, andhas a collector mounted above it (Fig. 4). A solar reflector tower isused to concentrate solar flux from heliostats onto the groundreformer. In this fashion, the power producing unit can be separatedfrom the concentrator field entirely.

Landfill gas and biogas can be used to supplement gas pro-duced by the reformer. The design and operation of a large-scalereformer are discusses by Segal and Epstein [30]. The synthesisgas produced by this technology can also be utilized for theproduction of methanol.

An optimization study of an HFC system’s main parameterswas conducted by Segal and Epstein [7]. The effects of operationtemperature, heliostat field density and the use of a secondaryreflector (reflector tower regime) on power conversion weretested across different energy cycles (Fig. 5). The investigationconcluded that maximum overall efficiency of an HFC system isreached at 1600 K, with an average field density of 35%. Theauthors emphasize that differences between large and small HFCplants with regards to these values are negligible.

The solar tower reflector can also be integrated with concen-trated photovoltaics (CPV). The principle behind this design is tosplit the solar spectrum into PV-used and thermal-used portions. Forexample, monocrystalline silicon solar cells operate at efficienciesranging between 55% and 60% at wavelengths of 600–900 nm. The

Fig. 3. Schematic diagram of solar reflector tower in an HFC system.


rest of the light can be used for electricity generation using Rankine–Brayton cycles, or otherwise be stored for later use. Discussion ofspectrum splitting optics and HFC–CPV hybrid design is given bySegal et al. [31]. The study’s results show that a heat input of55.6 MW yields 6.5 MW from the solar cells array and 11.1 MWfrom a combined energy cycle. This was done under concentrationratios of 200–800 sun.

The concept of a dual receiver for solar towers was suggested byBuck et al. [32]. The proposed receiver is made of an open volumetricair heater with a tubular evaporator section (Figs. 6 and 7). In thisdesign, the receiver has both a water heating section and an airheating section. Water (HTF) is circulated through, evaporated in thetubular evaporator, and is then superheated by hot air. Feed water isalso preheated using the hot air. This concept essentially combinesdirect steam generation with regular water HTF operations. Theresults (Table 3) of the new design demonstrate numerous benefits,which include a higher receiver thermal efficiency, lower receivertemperature and lower parasitic losses. A 27% gain in annual outputis facilitated by these improvements, compared with the solar airheating system. Separation of evaporation and superheating sectionsalso alleviates thermo-mechanical stress on the receiver to somedegree.

Planning the layout of a heliostat field presents a great optimiza-tion challenge. A novel methodology for layout generation based onyearly normalized energy surfaces (YNES) was presented by Sanchez


and Romero [33]. This ‘Heliostat Growth Method’ (HGM) uses theYNES program to evaluate the usable solar energy flux at each pointin a solar field year-round, given a specific solar tower height. Usingthis 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 thoseaffected by the geometry of the heliostat. This provides greaterinsight and flexibility to the field layout process and its optimization.

A clever design for small-scale ‘tri-generation’ solar powerassisted plant was brought forth by Buck and Friedmann [34]. Thedesign puts together a solar–gas turbine hybrid system, whichincorporates a small heliostat field, a receiver mounted on a solartower, a micro-turbine and an absorption chiller. In this regime,electric power, heating and cooling can all be produced by the

Fig. 7. Schematic plant incorporating dual receiver, outlining three heat transfer stage

Figures reproduced with permission from ref. 32, &2006 Elsevier.

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

Design conditions Receiver outlet temp. (1C)

Receiver efficiency (including recirculation lo

Air temp at blower (1C)

Air mass flow (kg/s)

Annual performance Annual receiver efficiency (including recircul

Capacity factor (%)

Net annual electric energy (GWh)

Fig. 6. Scheme of dual receiver unit from top view (left) and side view (right).


same system. System configurations were assessed for technicalperformance and cost.

Forsberg et al. [35] suggested the use of liquid fluoride salt asan HTF in order to raise the heat-to-electricity conversion effi-ciency of HFCs to about 50%. The molten salt operates attemperatures between 700 and 850 1C, delivering heat to a closedmulti-reheat Brayton cycle using N2 or He as the working fluid.Due to such high operation temperatures, thermal energy storageas sensible heat in graphite is suggested. A schematic diagram ofsuch an HFC plant is shown (Fig. 8). Graphite, a low-cost solidfeaturing a high heat capacity, is compatible with the fluoride saltat high temperatures. The efficiency boost reported by theauthors can greatly reduce electricity costs.

The combination of a single central receiver with molten saltsas the HTF generally allows the highest operation temperatures ofany CSP regime and produces electricity with the highest effi-ciencies. High-efficiency heat storage with molten salts enablessolar collection to be decoupled from electricity generation in asimpler manner than water/steam systems permit [36].

The design and performance of a novel high-temperature airreceiver was presented by Koll et al. [37]. The receiver suggestedis a porous absorber module consisting of extruded parallelchannel structures of silicon carbide ceramics. The inner surfacearea 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 lowheat transfer coefficient. The receiver design is modular andpromotes easy scaling. The hot air is delivered at 700 1C to awater boiler system for steam generation. Steam can be producedconsistently at 485 1C and 27 bar, but these parameters varyaccording to the system’s capacity. Using air as a heat transfermedium greatly reduces capital investment as it is free andreadily available anywhere.

s (preheating, evaporation and superheating).

Reference plant Dual receiver plant

700 500

sses) (%) 79 87

110 80

56 40

ation losses) (%) 66.7 79.4

14.3 18.2

12.5 15.9

Fig. 9. Schematic diagram of torque tube heliostats (TTH).

Figure reproduced with permission from ref. 41, &2008 ASME.

Fig. 8. Solar power tower with liquid-salt heat transport system, graphite heat storage and Brayton power cycle.

Figure reproduced with permission from ref. 35, &2007 ASME.

Fig. 10. Schematic of mini-mirror array design featuring ‘ball-in-socket joint’

tracking mechanism.

Figures reproduced with permission from ref. 42, &2010 ASME.


The heliostat material selection is a crucial aspect of HFCpower plant design. These large mirrors make up about 50% of thetotal system’s cost and must feature high reflectivity and stiffness,be light-weight, easily cleaned and corrosion resistant. Xiaobinet al. [38] suggested the use of PVC composite plastic steel forheliostat fabrication. This polymer material has similar propertiesto metal–aluminum alloys conventionally used, but is not asheavy, and has a significantly longer lifetime. Its stiffness is highrelative to its weight and it is reported by the authors to becheaper. One significant issue with this material is its low heatresilience, a problem which must be contended with in order toensure 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 onvarious environmental conditions.

Conventional heliostat design dictates that cost reduction isimplemented by increasing the area of the mirrors. Doing thisreduces specific drive cost while increasing the torques heliostatsexperienced 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 isvaried from 01 to 1801 and vertical angle is varied from 01 to 901.Moreover, a finite element model was constructed to performcalculations of wind-induced dynamic responses. Increased windtorques result in higher specific weight and drive power. Theusage of torque tube heliostats (TTH) (Fig. 9) is suggested byAmsbeck et al. [41]. TTH systems incorporate arrays of long,narrow mirrors mounted on turning tubes that control theirelevation. An optical performance and a weight estimation of aTTH system were carried out by the authors, and compared with a

regular HFC system of a slightly smaller area. Although the TTHsystem indeed experienced smaller wind torques, it suffered anannual energy output reduction of 3%. Furthermore, the highnumber of moving elements and the more involved control makethis system hardly advantageous compared with the conventionaldesign.

Another novel design to help avoid heavy mirror tracking inthe face of wind loads was suggested by Gottsche et al. [42]. Thisregime utilizes mini-mirror arrays (10�10 cm) made of highquality materials. Each mirror is mounted on a ball-in-socket jointdriven by a step motor (Fig. 10). The mirrors are encased in a clearbox that shields them from the wind. The purpose of this design isto avoid wind loads and save on stiff materials (mainly steel) thatare necessary to make large heliostats resilient to wind torques.Unfortunately, the low-cost achieved by the group was counteredby a 40% drop in optical performance compared with conven-tional HFC systems.

For initial planning of an HFC power plant, a general efficiencyevaluation tool can be quite useful. Collado [43] presented aquick, non-specific evaluation method for annual heliostat fieldefficiency evaluation. The model is a combination of an analytical


assessment of the flux density produced by a heliostat fromZaragoza University, an optimized mirror density distributiondeveloped by University of Houston for the Solar One project,and molten salt receiver efficiencies measured during the SolarTwo project. This model does not take into account manyimpacting factors specific to a particular HFC system and islimited in its accuracy.

Similarly, a new method for approximating geometrical para-meters and sizing of the tower reflector regime was developed bySegal and Epstein [44]. The method utilizes edge rays originatingfrom the heliostat field boundaries and is particularly useful forgeometrical assessment of very large arrays of heliostats. Themethod’s results were compared with real field calculations andfound to be a good first approximation regime.

A simulation using the same ‘edge ray’ principle method wasdeveloped by Xiudong et al. [45]. Its purpose was to promotemore efficient placement of heliostats and obtain a faster gen-erating response of the design and optimization. A novel modulefor the analysis of non-spherical heliostat arrangements has beenincorporated into the simulation. A toroidal heliostat field wasdesigned and analyzed by the authors and proved significantlyless efficient that conventional HFC arrangements. A method forcalculating the annual solar flux distribution of a given area is anadded feature, with the purpose of evaluating feasibility of cropgrowth around heliostat fields.

Heliostat field collector technology has greatly improved overthe last few decades, and continues to draw much attention as asuitable scheme for large solar thermal plants. The exceedinglyhigh temperatures at which they operates it grant HFC plantsexcellent efficiencies, while allowing them to be coupled to avariety of applications. The high capital investment necessary forthe construction of HFC systems is an obstacle, however, andfurther technological advancements in efficiency must be accom-panied by low cost materials and storage schemes for this CSPmethod to become more economical.

Fig. 11. Schematic diagram of the CLFR design.


Fig. 12. Schematic diagram of inverted air cavity receiver.

Source: Wikipedia.

5. Linear Fresnel reflectors (LFR)

Concentrated solar power production using linear Fresnelreflectors is quite similar to the parabolic trough collectorscheme. The two share common principles in both arrangementand operation. In March 2009, the German company NovatecBiosol constructed a LFR solar power plant known as PE 1 that hasan electrical capacity of 1.4 MW. The success of this projectinspired the design of PE 2, a 30 MW plant based on the LFRtechnology, to be constructed in Spain. The 5 MW KimberlinaSolar Thermal Energy plant has been recently completed inBakersfield, California.

Linear Fresnel reflectors incorporate long arrays of flat mirrorsthat concentrate light onto a linear receiver. The receiver ismounted on a tower (usually 10–15 m tall), suspended aboveand along reflector arrays. The mirrors can be mounted on one ortwo-axis tracking devices. The flat, elastic nature of the mirrorsused makes the LFR design significantly cheaper than PTC.Additionally, central receiver units save on receiver materialcosts, which are generally higher than reflector costs. SeveralFresnel reflectors can be used to approximate a parabolic troughcollector shape, with the advantage that the receiver is a separateunit, and does not need to be supported by the tracking device.This makes tracking simpler, accurate and more efficient. A heattransfer fluid circulates through the receiver, collecting and trans-porting thermal energy to power production and storage units.

A significant challenge with LFR systems is light blockingbetween adjacent reflectors. Solving this issue requires eitherincreased spacing between mirrors, which takes up more land, or

increased receiver tower height, which augments the cost.A novel solution to the shading problem is discussed by Millsand Morrison [46] at Sydney University, Australia. Their design ofthe compact linear Fresnel reflector (CLFR) scheme featuresadjacent mirrors oriented towards two separate receivers inopposite directions (Fig. 11). The use of multiple receivers allowsa more compact reflector distribution, avoiding shading andutilizing a portion of solar flux that otherwise goes to waste.Reflectors near the base of a receiver are always oriented towardsit. Yet, when reaching a nearly equidistant point between twoseparate receivers, the mirrors from each will reverse theirorientation, allowing them to come very close together withoutblocking one another. For commercial power production (greaterthan 1 MW scale), it is very reasonable to have multiple receivers,and thus the CLFR design is very useful without incurring extracosts, especially in areas where land is limited.

A useful addition to the CLFR design is the incorporation of aninverted cavity receiver attached to a planar array of boiling tubes(Fig. 12). This structure allows plant operation in a direct steamgeneration (DSG) regime. Mills and Morrison [47] indicate thatthis receiver design bypasses receiver thermal uniformity chal-lenges with parabolic trough DSG system. Design considerationsof the inverted cavity receiver are presented by Singh et al. [48].This work compares thermal performance of circular and rectan-gular absorber tubes, as well as black nickel and black paintcoated tubes. Circular absorbers in the receiver are reported to havea higher thermal efficiency by 8% compared with a rectangularabsorber. Nickel selective surface coating performed 10% betterthan ordinary black paint. A heat loss study of the same variables isalso performed by the authors. Nickel selective-coated absorbersexperience 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 ina CLFR regime was presented by Chavez and Collares-Pereira [50].


New geometries for reflector fields are explored in this study,with the purpose of limiting blocking/shading while maximizingthe field layout density. The authors propose reformation of theplatform on which reflectors are resting (ground) into a wave-shaped one (Fig. 13). Individual reflectors’ size/shape adjustmentsbased on their position in the heliostat field are also suggested.A concentration increase of up to 85% of theoretical maximum isreported under this design.

Dey [51] describes several receiver design considerations for theCLFR concept. The absorber is a basic inverted air cavity with a glassencasing that encloses a selective surface. The central design goalsanalyzed are (1) maximization of heat transfer between the absorb-ing surface and the steam pipes, and (2) ensuring uniform absorbersurface temperature to avoid degradation of the selective surface.

Heat calculations are presented for absorber temperaturedistribution, and satisfactory absorber pipe separations and sizesare shown to alleviate temperature differences between the fluidsurface and the absorbing surface. Similar work using finiteelement calculations was done by Eck et al. [52] for three separateparts of a LFR system–the evaporator, pre-heater and superheater(Table 4). Thermal loads for each section were modeled andmaximum temperatures were investigated. In the case of thesuperheater, the maximum temperature derived was 570 1C,exceeding the temperature limit of the absorber coating. A novelstep-by-step heat flux reduction method is thus required for safeand successful operation. Such a control system would adjustreflectors to an off-focus position one by one to prevent over-heating while operating at the highest allowed temperature. Thiskind of sensitive, intelligent system would surely increase powerplant costs.

A study by Hoshi et al. [53] investigated the suitability of highmelting point phase change materials (PCMs) for storage use inlarge-scale CLFR plants (Fig. 14a–c). Several candidates for latentheat storage materials are discussed, and mathematical models ofcharging and discharging heat storage from each are presented.NaNO2 is emphasized as a particularly suitable contender for large-scale latent heat storage due to its high melting point and low cost.

LFR technology offers many of the advantages of PTC systemswhile incurring smaller reflector costs. It too can be easily coupledto direct steam generation as well as molten salts for thermalenergy transport. The central receiver regime it incorporatesshrinks costs further, but tags on the challenge of maximizingthe amount solar radiation that can be collected. Innovation inreceiver design and reflector organization has made LFR relatively

Table 4FEM analysis results of thermal loads for three LFR system sections.

Data compiled from [52].

Pre-heater Evaporator Superheater

Heat transfer coefficient (W/m2 K) 1700 860 500

Average fluid temp. (1C) 140 275 440

Max tube temp. (1C) 189 325 569

Min tube temp. (1C) 142 281 455

Temperature drop (K) 47 44 114

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.


Fig. 13. Wave platform structure for a CLFR system allows maximization of solar

radiation collected from a given area.


inexpensive in comparison with other CSP technologies. It readilycouples to thermal storage methods and numerous applications.

6. Parabolic dish collectors (PDC)

Parabolic dish reflectors are point-focus collectors. As such,they can achieve very high light concentration ratios, reaching up

Fig. 15. (a) Light collection and (b) general schematics of air cavity receiver in a

dish/Stirling system.



to 1000 sun. At temperatures exceeding 1500 1C, they can pro-duce power efficiently by utilizing high energy conversion cycles.The collector type features a large parabolic-shaped dish, whichmust track the sun on a two-axis tracking system to maintainlight convergence at its focal point. A receiver is mounted at thefocus, collecting solar radiation as heat. Two general schemes arepossible for power conversion; the less popular has a heat transferfluid system connecting the receivers of several dishes, conductingthermal energy towards a central electricity generation system. Thisdesign is less convenient as it requires a piping and pumpingsystem resilient to very high temperatures, and suffers fromtransport thermal losses. The more prevalent system mandatesa heat engine be mounted near/at the focal points of individualdishes. The heat engine absorbs thermal energy from the receiver,and uses it to produce mechanical work, which an attachedalternator then converts into electricity. A heat-waste exhaustsystem must be incorporated to release excess heat from thesystem. Finally, a control system is necessary to ensure matchingof the heat engine’s operation to the incoming solar flux.An advantage of this design is that the reflector, collector andengine can operate as separate units, making fossil-fuel hybridi-zation a relatively simple task. It is important to note however,that this PDC system does not lend itself to thermal storagemethods.

The Stirling engine is often used for this application, althoughgas turbines can also be employed in the Brayton or Rankine/Brayton combined cycles. Stirling engine performance is better intemperatures 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-Stirlingsystem were described in detail by Jin-Soo et al. [55].

Due to its high concentration ratios, the parabolic dish collectoris an excellent candidate for concentrated photovoltaics. The usageof state-of-the-art, high-cost high-performance photovoltaic cells isjustified when they are utilized at concentrations exceeding100 sun; a large solar flux focused in a small region of cells canproduce enough power to offset the high capital investmentrequired. GaAs and multi-junction PV cells are very expensive tofabricate. Yet, operational module efficiencies exceeding 30% havebeen demonstrated by multiple manufacturers and verified by theNational Renewable Energy Lab (NREL). Moreover, these PV tech-nologies are very heat-resistant, and perform better under highconcentration ratios. Incorporating such modules into the parabolicdish collector apparatus is fairly simple, and can yield results thatare comparable to or better than heat engine systems, potentiallywith a longer lifetime. Further discussion of concentrated photo-voltaics is developed in a later section.

A numerical simulation of a heat-pipe receiver for the para-bolic dish collector was performed by Hui et al. [56]. Using thistype of receiver between the dish and the Stirling engine isreported to provide power uniformly and nearly isothermally tothe engine heater. This results in improved engine performance.Heat-pipe utilization also limits convective heat loss from thereceiver.

Parabolic dish collectors are high-cost devices: they are verylarge mirrors that must feature nearly perfect concavity toeffectively concentrate solar radiation. They are also very heavy,and their tracking system must thus be very sensitive and finelytuned. A novel suggestion by Kussul et al. [57] to moderate thehigh collector cost is to manufacture an approximated parabolicdish using many small, flat mirrors. A prototype was constructedby the group, which contains 24 mirrors in the shape of equi-lateral triangles, each with a side length of 5 cm special nuts areused to maintain required positions of nodes in the connectionpoints of mirror apexes. These small mirror arrangements approx-imate a parabolic collector in a relatively inexpensive way.

At such high operation temperatures, heat losses becomeextremely significant, and must be contended with to achievehigh efficiencies. A detailed two-dimensional simulation of heattransfer in a modified cavity receiver of PDC system is presentedby Reddy and Kumar [58]. Combined heat losses due to bothlaminar convection and surface radiation from the receiver arecalculated by this model. The modified cavity receiver (Fig. 15aand b) has a semi-circle shape that features a small aperture atthe dish’s focal point. The receiver is essentially hollow (aircavity) and its inner surface is laid with absorber tubes. Theencasing of the tubes is made of insulating material.

Reddy and Kumar published another numerical analysis in2009 [59], in which a three-dimensional model is used toestimate receiver heat losses at different dish inclination anglesand various operating temperatures. The model evaluates heatloss reductions realized through secondary concentrator integra-tion. A cone collector, compound parabolic collector (CPC) andtrumpet reflector were compared as second stage concentrators(Fig. 16a–c), and yielded natural convection heat loss reductionsof 29.23%, 19.81% and 19.16%, respectively.

Another thermal analysis of a PDC system was done by Nepveuatal. [60]. The authors constructed a thermal energy conversion modelof the 10 kW Eurodish/Stirling unit erected at the CNRSPROMESlaboratory in Odeillo. The model analyzes spillage and radiation(reflection and IR-emission) losses of the reflector, and calculatesconduction, convection, reflection and thermal radiation lossesthrough the receiver cavity (Fig. 17). A thermodynamic analysis ofa SOLO Stirling 161 engine is also presented. The model wascompared to experimental results of the solar power system andwas determined a good fit.

An innovative solar thermal power approach was formulated byShuang-Ying et al. [61]. This design features a dish concentrator

Fig. 17. Eurodish receiver heat flow and heat loss diagram.


Fig. 16. (a–c). Secondary reflectors for a parabolic dish reflector system.

Figure reproduced with permission from ref. 59, &2009 Springer.


cascaded with an alkali metal thermal-to-electric converter (AMTEC)through a coupling heat exchanger. The proposed system employsa heat-pipe receiver for isothermal energy transfer from thedish to the AMTEC unit. Theoretical investigation of this system’sperformance predicts a 20.6% peak thermal-to-electric conversionefficiency. Effects of various parameters on the overall conversionefficiency of the parabolic dish/AMTEC system are discussed in detail.Increasing the geometric concentration and tilting angles of the dishboth result in efficiency enhancement. The authors report that thisdesign has a potential to become a leading low-cost renewableenergy source because of its passive nature.

A paradigm shift in PDC design suggested by Feuermann andGordon [62] utilizes arrays of mini-dishes coupled with fiberoptics that carry solar radiation to a central receiver. Each mirroris about 20 cm in diameter and has a small flat mirror at its focalpoint, to which a single optical fiber is attached. The fibertransports collected light to a central receiving unit, where itcan be converted into heat. Low attenuation fibers of highnumerical aperture, coupled with mass produced highly accurateparabolic dishes, can operate at 80% efficiency and yield incred-ibly high concentration ratios of up to 30,000 sun. Experimentalrealization and field experience of this proposed system werecarried out by Feuermann et al. [63]. One mm diameter opticalfibers repeatedly transported solar flux levels of 11–12 ksun to atarget as far as 20 m away. The prototype proved impervious todust penetration and condensation, and was reportedly con-structed solely from off-the-shelf parts and customized items

relying on pre-existing technologies. The mini-dish scheme wasalso suggested for integration with high concentration photovol-taics [64].

Innovation in parabolic dish reflector technology has promotedthis highly efficient yet expensive technology towards the goal ofbeing reasonably affordable. Novel improvements in reflectorstructure and collector design continue to boost the thermalefficiency of this concentrated solar power scheme. The use of aStirling engine at a PDC’s focus helps alleviate the losses and costsassociated with heat transport. However, this regime does notcomply with thermal storage in a simple manner, a significantissue in the scope of year-round power production.

7. Concentrated photovoltaics

The concept of concentrated photovoltaics is rapidly becoming adominant player in the solar power production market. In March2010, the 330 kW ‘OPEL Solar’ (Spain) became the first operationalutility-grade CPV power plant. CPV systems employ various lightconcentration schemes to focus large amounts of solar radiationonto small solar cell modules. Very small units of high-cost high-efficiency solar cells are used to absorb the high incoming flux,which makes the CPV model economically competitive.

Mainstream concentrator technologies utilized are parabolicdish collectors and Fresnel lenses. Designs using PTC [27] and HFC[31] systems (discussed in previous sections) have been reportedas well. The type of solar cell technology used in a CPV system ischosen according to the desired concentration level. While theperformance of most PV technologies increases with solar con-centration ratios, excessive heating can be detrimental to theefficiency and lifetime of solar cells. Organic and amorphoussilicon cells are generally too heat-sensitive to be used withconcentrators. Conventional monocrystalline silicon cells canoperate efficiently at lower concentrations (1–100 sun) withoutneeding active cooling mechanisms. Low concentration systemsgenerally feature wider acceptance angles, and in some cases donot need to track the sun, reducing their cost.

Two-axis tracking systems are required in high concentrationsystems. Gallium arsenide and multi-junction cells are better usedin medium–high concentration systems (100–300 sun, 300 sun andabove). These cells are very expensive to manufacture, but haveexhibited record conversion efficiencies and operate well under hightemperature. Still, heat sinks are often integrated with high-con-centration CPV modules in order to alleviate high temperatureeffects and prolong cell lifetime. Examples of cooling mechanismsinclude direct water cooling and thermal conduction by heat pipes,discussed by Farahat [65].

Due to very high material and manufacturing costs, multi-junction cells are significantly more expensive than silicon cellsper unit area. Yet, multi-junction cell efficiency can be up to 15%greater than that of silicon cells, which can make a big differencein performance at high solar concentrations. Furthermore, thesmall PV receivers account for only a fraction of the total CPVsystem cost, hence system economics may very well favor the useof multi-junction cells.

Another recently explored concept is the Concentrating Photo-voltaics and Thermal (CPVT) design. This scheme produces bothelectricity and heat simultaneously in a single system. The heatcan be used for industrial heat processes, heating and cooling ofbuildings, or simply to increase electricity output. A parabolictrough CPVT design was introduced by Coventry [27], and a solartower design was suggested by Segal et al. [31]. Small CPVTsystems can be installed in private homes, and can feature a totalenergy output of over 50% compared with 10–20% of the basic PVpanels.


A novel design for a miniature parabolic dish collector CPVTsystem for residential use was presented by Kribus et al. [66].Analysis of the electric and thermal performance, heat transportsystem, manufacturing cost and resulting cost of energy fordomestic water heating is carried out. The reflector is made of asingle thermally bent glass sheet coated with silver to produce thereflective surface. An external protective coating prevents exposureof the silver to the environment. A 32% conversion efficiency multi-junction module is mounted at the focal point, over a cooling platethat removes the surplus heat from the cells to a coolant fluid(typically water). The heated coolant is directed to a heat exchangerwhere the transported thermal energy may be used as an additionalenergy product. Performance testing of a 0.95 m2 dish area underdirect insolation of 900 W/m2 yielded an electrical output of 172 Wand a thermal output of 530 W, exceeding 60% of the input energy.

Miniature dish collectors can be used to achieve very highconcentration CPV systems. Investigation of this type of systemoperating at a concentration ratio of 1000 sun was presented byFeuermann and Gordon [64]. The system features high-efficiencyheterojunction cells as the PV receiver and utilizes optical fibersfor heat conduction towards a passive heat sink. Arrays of thesesmall systems can be mounted together on large, two-axistracking systems. The merits and identified problems of a similardesign 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.

Fig. 19. Photovoltaic cell arrays encased with Fresnel lenses, mounted on a two-axis s

Figure reproduced with permission from ref. 68, &2007 Springer.

(polymethylmethacrylate, PMMA). They are flat on one side andridged on the other. The Fresnel lens structure is composed ofmany concentric rings, which are thinner towards the center.Each ring is slightly angled to concentrate incident light onto thefocal point of the lens (Fig. 18).

Linear Fresnel lenses operate in a similar manner, but feature afocal line instead of a focal point. A linear Fresnel collector caninclude an array of these lenses positioned side-by-side. The arrayis mounted on a sun-tracking device. Every lens is mounted on asmall axis through the center of its length, which can orient it tofollow the sun. The entire collector unit can track the sun alongthe second dimension, providing the system with a two-axistracking regime (Fig. 19). An optical and thermal performancesimulation for this type of system was done by Mallick and Eames[68]. The effects of varied spacing between linear lenses within anarray on the efficiency are presented. Linear Fresnel lenses alsocan be coupled with small secondary concentrators to minimizethe PV receiver area needed [69].

Optimization of concentration level, cell technology, receiversize and shape and heating/cooling management is necessary toachieve high performance systems. A study of the energetic andthermal characteristics of a small CPV system was conducted byMirzabaev et al. [70]. The module was based on a Fresnel lens andan AlGaAs–GaAs PV receiver, and compared several receiver sizesand contact shapes (tetragonal and circular). Analysis of theFresnel lens solar collector thermal efficiency was done by Zhaiet al. [71], and was found to be about 50% when using anevacuated tube receiver on a clear day.

One problem with the use of conventional Fresnel lenses forconcentrated photovoltaic is uneven illumination of the solar cellreceiver. Non-uniform intensity distributions can result in localheating and ohmic drops in CPV systems, preventing maximumpower extraction. Several innovative designs to overcome thisissue have been presented in the last 5 years. Ryu et al. [72]devised a new concept of a modular array of Fresnel lenses forlow-medium concentration CPV systems, which is based on theconcept of superposition. A two-dimensional array of lenses isconstructed. Each lens is slightly larger than the PV receiver itself.Individual lens facets are angled to direct normal incident lightonto specific regions of the solar cell module (Fig. 20a and b).Proper determination of the facet angle for each lens in the array

un-tracker.

Fig. 20. (a) Modular Fresnel lenses concept for concentrated photovoltaics. (b) Cross-sectional view of modular Fresnel lenses array.


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 theperformance and concentration efficiency are presented, alongwith illustrations of the new concept.

When investing in high-quality solar cells, it is desirable tointegrate them with systems that achieve very high concentrations.At such conditions, however, Fresnel lenses have a very narrowacceptance angle range (on the order of 711), and the system mustinclude very fine tracking mechanisms for efficient absorption tooccur. The design of a cylindrically symmetric Fresnel lens wasexplored by Yu-Ting and Guo-Dung [73]. A simulation of a CPVsystem incorporating this technology was carried out at highconcentration (300–400 sun). A couple of system designs waspresented. The most successful design (Fig. 21) incorporated thecylindrical Fresnel lens, two reflective surfaces, a biconic lens and alight pipe. This structure, though fairly complicated, expanded theacceptance angle to 7101. Theoretical discussion of the opticalcapabilities of a cylindrical lens was presented by Gonzalez [74].Both a concentration level of 70% of the theoretical maximum and a100% geometrical optical efficiency were reported. The lens alsofeatured very uniform illumination of the receiver, an importantattribute for concentrated photovoltaic systems.

The integration of solar cells with CSP technologies requires acautious balancing of the advantages and issues of each. On one hand,

electricity is produced directly by solar cells, which removes the needfor complicated heat transport and large boiler/turbine systems.

On the other hand, the efficiencies associated with CSP aloneare generally higher, and collected solar energy can be storedthermally, a benefit solar cells do not enjoy. Combining state-of-the-art solar cells with high-concentration reflectors allows agreat amount solar flux to be converted to electric power at highefficiency, while keeping solar cell expenses to a minimum (asonly a small photovoltaic cell area is needed). The combined CPVTscheme yields very high conversion efficiencies, but is inevitablymore complicated and thus more costly to execute. Still, furtherprogress in solar cell and reflector designs will reduce theseexpenses, making this type of power production scheme moreaffordable.

8. Concentrated solar thermoelectrics

Conversion of solar energy into electricity directly can also beachieved using the concept of thermoelectrics. Recent developmentsin thermoelectric applications have been exploring ways to utilizeCSP to generate electricity. Solar thermoelectric devices can converta solar thermal energy (typically waste heat) induced temperaturegradient into electricity. They can also be modified to performcooling or heating. One advantage of thermoelectric methods(compared with heat engines) is their increased reliability, as suchdevices could work 10–30 years with little technical problems [75].Moreover, thermoelectric generators are a flexible source of cleanenergy capable of meeting a wide range of requirements.

Hybrid systems that combine thermoelectric and photovoltaicare under development. This type of system allows harvesting ofsolar radiation in both the ultraviolet and infrared ranges of thespectrum. Such a hybrid can also reduce wasted thermal energy,since it ‘functionalizes’ a wide temperature range for powerproduction. While most silicon solar cell performance begins todegrade at temperatures approaching 100 1C, thermoelectricdevices actually perform better at temperatures over 200 1C.

A solar thermoelectric power generator typically consists of athermal collector and a thermoelectric generator. Heat isabsorbed by the thermal collector, then concentrated and con-ducted over the thermoelectric generator by a fluid pipe. Thethermal resistance of the generator creates a temperature differ-ence between the absorber plate and the fluid, which is propor-tional to the incoming heat flux. The current produced by the


thermoelectric generator is in turn proportional to the tempera-ture difference.

To increase the efficiency of current solar thermoelectricdevices, two main things must be accomplished: (i) improvedthermal transmission of the solar collector and (ii) higher con-centration of the solar radiation onto the hot side of the thermo-electric device. Since thermoelectrics made of high qualitymaterials are relatively expensive, a key design considerationfor these solar generators is minimal use of thermoelectricmaterials. Naturally, amounts used must be adjusted in accor-dance with desired power requirements.

The use of solar concentrating elements can augment themagnitude of the heat flux absorbed by a thermoelectric device,contributing to a higher temperature gradient across it. Amongthe single line focusing parabolic trough collector, the compoundparabolic concentrator and the two-stage concentrator, the latteruses a secondary receiver to further concentrate the incident solarradiation. A design for a two-stage solar concentrator has beenproposed [76], which is well-suited to commercially availablethermoelectric devices for small scale power generation. The two-stage 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 thehot side temperature of the generator. Solar concentration mustbe greater than 20 sun to effectively irradiate a thermoelectricdevice [76,77]. Schematics of two solar thermoelectric regimesthat incorporate concentrators are shown (Fig. 22a and b). Bothschematics are based on the two-stage concentrator design,where the second concentrator also acts as a receiver and cangenerate a larger temperature difference across the thermoelec-tric device. The receiver can combine a thermionic converter (TIC)with a thermoelectric converter (TEC) to use thermal energy moreefficiently (Fig. 22a). The TIC is a cylindrical cavity-type solarreceiver made of graphite, which is heated in a vacuum by thesolar concentrator. Once the TIC emitter is uniformly heated up to1800 K, a hot side generator temperature of 1800 K can beachieved [78]. The thermoelectric device can also be attacheddirectly to the absorber plate of the receiver (Fig. 22b).

The field of solar thermoelectric power generation, its couplingwith two-stage solar concentrators in particular, is a very recentinnovation in the scope of CSP. Many solar thermoelectric designsare 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).

promote great interest in its exploration and motivate continuedresearch of design and materials.

9. Thermal energy storage

A significant complication with the utilization of solar thermalpower as a primary source of energy is the variable supply of solarflux throughout the day, as well as throughout the year. Althoughthere is a reasonable match between the hours of the day in whichboth available solar energy and electricity consumption peak, night-time energy usage must be taken into consideration. Additionally,seasonal and weather changes greatly influence the amount of solarthermal energy that can be harvested. An affordable, reliable energystorage method is thus a crucial element in a successful year-roundoperation of a thermal solar power plant.

The cyclical availability of solar energy determines two typesof thermal storage are necessary for maintaining a constantsupply of solar thermal power driven electricity. The first isshort-term storage, where excess energy harvested daily is storedfor nighttime usage. The second is long-term storage in whichexcess energy is stored during spring and summer months inorder to complement the smaller energy flux available in winter.

Thermal energy storage can be divided into three maincategories: sensible heat storage, latent heat storage and chemicalstorage. Sensible heat storage involves heating a solid or liquidand insulating it form the environment until the stored thermalenergy is ready to be used. Latent heat storage involves the phasechange (generally solid–liquid) of the storage material. The heat-induced phase change stores a great deal of thermal energy whilemaintaining a constant temperature, and can be easily utilized fornighttime energy storage if kept under proper isolation. A plotdemonstrating sensible and latent heat storage is given (Fig. 23).Chemical storage is implemented using harvested thermal energyin 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, exother-mic reaction will release the heat with minimal losses for electricitygeneration at a later time. Chemical storage is thus most suitable forlong-term or seasonal storage.

Sensible heat storage can employ a large variety of solid andliquid materials. It can be put into practice in a direct or indirectmanner. For storage in solids such as reinforced concrete, solidNaCl and silica fire bricks, an indirect storage method must beimplemented. This type of system uses a heat transfer fluid tocirculate through absorbers, collect heat and transport it to thestorage tank. The HTF is then put in thermal contact with thestorage solids, allowing them to absorb the heat convectively.Sensible heat storage in liquids can be achieved in a directfashion, i.e. the heat storage liquids themselves are used as heattransfer fluids, and are transported to an insulating storagetank after circulating through the solar absorbers. Mineral oil,synthetic oil, silicone oil, nitrate, nitrite and carbonate salts,

Fig. 23. Sensible vs. latent heat storage.



as well as liquid sodium, can all be used for sensible heat storage.Desired characteristics of ‘sensible-heat-storage-friendly’ moltensalts include high density, low vapor pressure, moderate specificheat, low chemical reactivity and low cost. One big disadvantageof molten salts is that they are usually quite pricey. Detailedcharacteristics of storage materials (Table 5a and b) are given byGil et al. [79].

Latent heat storage in the solid–liquid phase transition ofmaterials 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 betweencharging and discharging thermal energy. Additionally, PCM materi-als generally feature higher densities than sensible heat storagematerials. The interest in PCM latent heat storage systems isincreasing, mainly due to potential improvements in energy effi-ciency and nearly isothermal energy storage and release. In additionto the few commercially available PCMs today, many organic andinorganic compounds are being investigated for latent heat storagepurposes (Table 5c–e). A disadvantage of PCMs is their low thermal

Table 5a–f. Various thermal storage materials and their properties.

Data compiled from [79].

(a) Sensible heat storage liquid materials and their properties

Storage medium HIETCsolar salt

Mineraloil

Syntheticoil

Temp. (cold) (1C) 120 200 250

Temp. (hot) (1C) 133 300 350

Avg. density (kg/m3) n/a 770 900

Avg. thermal conductivity (W/m K) n/a 0.12 0.11

Avg. heat capacity (kJ/kg K) n/a 2.6 2.3

Volume specific heat capacity (kWht/m3) n/a 55 57

Cost per kWh (US$/kWh) n/a 4.2 43.0

(b) Sensible heat storage solid materials and their properties

Storage Medium Sand-rockMineral Oil

ReinforcedConcrete

Temp. (cold) (1C) 200 200

Temp. (hot) (1C) 300 400

Avg. density (kg/m3) 1700 2200

Avg. thermal conductivity (W/m K) 1.0 1.5

Avg. heat capacity (kJ/kg K) 1.30 0.85

Volume specific heat capacity (kWh/m3) 60 100

Cost per kWh (US$/kWh) 4.2 1.0

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

Name Type Phase changetemp. (1C)

Density(kg/m3)

RT110 Paraffin 112 n/a

E117 Inorganic 117 1450

A164 Organic 164 1500

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

Compound Phase changetemp. (1C)

Density(kg/m3)

MgCl2-6H2O 115–117 1450 (liquid, 120

1570 (solid, 20 1

Hitec: KNO3–NaNO2–NaNO3 120 n/a

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

Mg(NO3)–2H2O 130 n/a

KNO3–NaNO2–NaNO3 132 n/a

68% KNO3–32% LiNO3 133 n/a

KNO3–NaNO2–NaNO3 141 n/a

Isomalt 147 n/a

LiNO3–NaNO3 195 n/a

conductivity, which results in slow charge–discharge rates. Onesuggested initiative for alleviating this problem involves the fabrica-tion of PCM composite materials; mixing pure PCMs with graphite,for example, can boost thermal conductivity and promote fasterenergy storing and releasing.

Since sensible and latent thermal energy storage schemes canonly retain their energy efficiently for so long, the need for long-term, cross-seasonal storage is made possible by thermo-chemicalstorage processes. Thermal energy storage in heat intensiveendothermic reactions has the possibility to realize higher energyefficient processes the thermal storage regimes. Potentially highenergy densities can be stored using chemical storage.

Reformation of methane and CO2 [30], metal–oxide/metalconversions [80] and ammonia synthesis/dissociation [14,81]are just a few examples of heat-assisted chemical reactionsthat can store solar thermal energy in their endothermic reac-tion products and release it at a later time/place by the reverseprocess. Numerous heat-storing chemical reactions are listed(Table 5f).

Siliconeoil

Nitritesalts

Nitratesalts

Carbonatesalts

Liquidsodium

300 250 265 450 270

400 450 565 850 530

900 1825 1870 2100 850

0.10 0.57 0.52 2.0 71.0

2.1 1.5 1.6 1.8 1.3

52 152 250 430 80

80.0 12.0 3.7 11.0 21.0

NaCl(Solid)

Cast Iron CastSteel

SilicaFire Bricks

MagnesiaFire Bricks

200 200 200 200 200

500 400 700 700 1200

2160 7200 7800 1820 3000

7.0 37.0 40.0 1.5 5.0

0.85 0.56 0.60 1.00 1.15

150 160 450 150 600

1.5 32.0 60.0 7.0 6.0

Specific heat(kJ/kg K)

Thermalconductivity(W/m K)

Latent heat(kJ/kg)

n/a n/a 213

2.61 0.70 169

n/a n/a 306


Thermal conductivity(W/m K)

Latent heat(kJ/kg)

1C) n/a 0.570 (liquid, 120 1C) 165

C) 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 n/a n/a

n/a n/a 75

n/a n/a 275

n/a n/a 252

Table 5 (continued )

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


Density(kg/m3)


Thermal conductivity(W/m K)

Latent heat(kJ/kg)

40%KNO3–60%NaNO3 220 n/a n/a n/a n/a

54%KNO3–46%NaNO3 220 n/a n/a n/a n/a

NaNO3 307 2260 n/a 0.5 174

KNO3/KCl 320 2100 1.21 0.5 74

KNO3 333–336 2.11 n/a 0.5 266

KOH 380 2.044 n/a 0.5 149.7

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

AlSi12 576 2700 1.038 160 560

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

MgCl2 714 2140 n/a n/a 452

80.5% LiF–19.5% CaF2 eutetic 767 2100 1.97 1.7 790

NaCl 800–802 2160 n/a 5.0 492

NaCO3–BaCO3/MgO 500–850 2600 n/a 5.0 n/a

LiF 850 n/a n/a n/a 1800 (MJ/m3)

Na2CO3 854 2533 n/a 2.0 275.7

KF 857 2370 n/a n/a 452

K2CO3 897 2290 n/a 2.0 235.8

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

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


Latent heat(kJ/kg)

Latent heat(kJ/L)

Isomalt: ((C12H24O11–2H2O)þ(C12H24O11)) 147 275 n/a

Adipic acid 152 247 n/a

Dimethylol propionic acid 153 275 n/a

Pentaerythritol 187 255 n/a

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

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

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

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

(f) Chemical storage materials and reactions

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

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

Methane/water n/a 500–1000 CH4þH2O’-COþ3H2

Hydroxides 3.0 GJ/m3 500 Ca(OH2)’-CaOþH2O

Calcium carbonate 4.4 GJ/m3 800–900 CaCO3’-CaOþCO2

Iron carbonate 2.6 GJ/m3 180 FeCO3’-FeOþCO2

Metal hydrides 4.0 GJ/m3 200–300 Metal xH2’-metal yH2þ(x�y)H2

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

Aluminum ore alumina n/a 2100–2300 n/a

Methanolation–demethanolation n/a 200–250 CH3OH’-COþ2H2

Magnesium oxide 3.3 GJ/m3 250–400 MgOþH2O’-Mg(OH)2


Every storage method mentioned can play an important role inseveral concentrated solar power designs. The chosen storagescheme must, however, be carefully matched to the size (totalpower output) and operational procedures associated with a specificplant, as well as to its governing environmental and economicfactors. Luckily, the developments made to date in all three thermalstorage methods offer a great diversity of materials from which onecan choose in order to meet varying necessary parameters.

10. Energy cycles

The conversion of solar thermal energy into electricity generallyrequires the use of a thermodynamic cycle.

Several types of cycles are the mainstream options for heatconversion into work. They can vary in design and processefficiency, but all cycles use heat harvested from solar collectorsto power a generator for electricity production.

The most common thermodynamic cycle used is the Rankinecycle. In this regime, heat is supplied externally (from collectors)

to a closed loop system, which usually uses water as its workingfluid. Cycle operation is outlined in several repeating steps.Working fluid is pumped from low to high pressure. This requireslittle input energy for the pump if the fluid is a liquid. This is oneadvantage of the Rankine cycle. High pressure liquid is heated in aboiler at a constant pressure to become saturated vapor. Thevapor is then allowed to expand through a turbine generator toproduce electricity. Next, it is condensed at a constant pressure tobecome a saturated liquid, and is transferred back into the pump’sreservoir. The working fluid is constantly re-used in this thermo-dynamic loop. If vapor temperature is not very high (wet vapor),condensation can occur during release through the turbine, andfast moving water droplets damage the turbine and reduce itslifetime and efficiency. Rankine operations at high temperaturesproduce ‘dryer vapor’, and can thus considerably increase systemperformance. Solar powered Rankine cycles using low costcollectors for clean water and power generation are reviewedby Garcıa-Rodrıguez and Blanco-Galvez [82].

The ‘organic’ Rankine cycle utilizes organic fluid such astoluene or n-pentane for working fluids. The cycle operation


process is identical, but can operate at lower temperatures(70–90 1C). These lower temperatures result in a lower thermo-dynamic efficiency, but this may be counter-balanced by thelower heat inputs required to drive the system. Organic fluids thathave boiling points above water can be used, and this may havethermodynamic benefits. A comparison of several working fluidsfor organic Rankine cycle operation of PTCs was carried out byDelgado-Torres and Garcia-Rodriguez [83].

The Brayton cycle has also been adapted for CSP electricitygeneration. This cycle uses a gas compressor, a combustionchamber and an expansion turbine. General operations of theBrayton cycle begin with ambient air being drawn into a com-pressor to be pressurized. It is then directed into a combustionchamber, where it is heated at a constant pressure. Convention-ally, this heating is done by burning fossil fuels, but thermalenergy harvested from solar collectors performs this task in a CSPpower plant. The heated air is allowed to expand through a gasturbine (or a series of turbines) to produce electricity. Thecompressor can be powered by the turbine generators. Excessheat is exhausted into the atmosphere. In 2002 a hybrid opensolar Brayton cycle was operated for the first time consistentlyand effectively in the frame of the EU SOLGATE program. Air washeated from 570 K to over 1000 K in the combustor chamber. Oneclear advantage of the Brayton cycle is that air is cheap andavailable everywhere. A regeneration mechanism can be incorpo-rated to improve Brayton cycle efficiency. Still-warm air that hasalready passed through the turbine can be circulated backtowards the compressor intake and pre-heat air before it entersthe combustion chamber. Less heat is exhausted out of thesystem, and less power is consumed by the chamber’s heatingmechanism. The Brayton cycle generally operates at significantlyhigher temperatures than the Rankine cycle. Despite this fact, theoverall efficiencies of large-scale steam generators and gas tur-bines seem to be similar.

The combined cycle utilizes a hybrid of the Rankine andBrayton 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 theatmosphere, is instead employed as a heating mechanism for aRankine (steam) cycle. Though it is more efficient, this design ismore bulky and expensive to implement. A cost-efficiency analysismust be carried out for a given plant size and output in order toevaluate economic viability. A discussion of a combined cycle CSPdesign using solar tower reflector technology is presented by Kribuset al. [84]. Both hybrid and solar-only power plants are investigated.An efficiency study of the combined cycle was done by Donatiniet al. [85]. The project examined the combined cycle integrated in aparabolic trough collector regime using molten salts as the heattransfer fluid.

Decreasing the cost and improving the efficiency of powerproduction cycles can greatly influence the market penetration ofconcentrated solar power technologies. A few innovative energycycles have been discussed in the literature, which use multi-component working fluids or employ additional cycle steps toimprove efficiency and limit power consumption.

A multi-component working fluid features variable boilingtemperatures according to its composition. This process can yielda better thermodynamic match with different sensible heatsources than can be achieved with a single-component fluid.The advantages of using an ammonia/water mixture as a workingfluid are reviewed by Goswami et al. [86]. The mixture is utilizedin the bottoming Rankine cycle of a combined cycle operatedplant design.

An innovative addition to the combined cycle was suggestedby Kribus [87]. A solar triple cycle is proposed, the first of which

utilizes is a magneto-hydrodynamic (MHD) cycle. This cycleoperates at very high temperatures, upwards of 2000 1C. It passeshot ionized gas through a magnetic field, resulting in electriccurrent generation. The great amount of heat is exhausted into aBrayton and Rankine bottoming cycles connected in series. Thetriple cycle needs to be integrated with an HFC design in order tomeeting the high temperature requirement. The overall peakconversion efficiency of the solar triple cycle is shown to besignificantly higher than the solar combined cycle scheme. Thesensitivity of this result to several system parameters andthe technological feasibility of the triple cycle are examined bythe authors.

The improvement of well-understood energy cycles and thedevelopment of new ones greatly extend the potential of all nearlyall concentrated solar power production regimes. The contributionsof advanced/high energy cycles to the overall thermal-to-electricpower conversion efficiency can be very significant, and help bringCSP closer to the realm of grid-parity. It is important to note thatrelative costs associated with this step become quite considerablewith increased levels of sophistication, a fact that must be weighedagainst 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 ofapplications for which solar thermal energy can be harnessed.Industrial heat processes, chemical production, salt-water desali-nation, heating and cooling are just a few examples of theplethora of available applications that can be implemented usingCSP technologies. It is important to note that some applicationsare CSP technology selective – they require integration with aspecific CSP design – while others can be coupled to several of theregimes discussed in this article.

The use of solar thermal power for water desalination andpurification has been discussed repeatedly in the literature. Thefact that regions of the world where clean drinking water is scarcealso have an abundance of solar radiation, which makes this CSPapplication very worthwhile. Desalination is generally done byevaporating salt-water to leave salt behind, then condensing saltfree vapor back into its liquid state. The process of heating largeamounts of water for drinking and agricultural purposes requiresimmense amount of energy. Concentrating solar radiation andconverting it to heat is an efficient method by which this processcan be achieved using emission-free, renewable energy. In addi-tion to boiling the water, thermal power could be used to powerabsorption chillers, thus using the same power source both forevaporation and condensation of water. Several plant designs forsolar powered desalination, detoxification and disinfectionof water are presented by Blanco et al. [88]. Designs for bothlarge-scale and small-scale operations are discussed. Solar water-detoxification schematics are presented, which are based on theconcept of using near-ultraviolet visible spectrum bands topromote oxidizers generation. Solar water disinfection utilizesthe same method, but incorporates a supported photocatalyst togenerate powerful oxidizers to control and destroy pathogenicwater organisms. A different desalination design by Alrobaei [89]serves the same purpose using parabolic trough collectorscoupled to a gas turbine operating in the combined Rankine/Brayton cycle.

A novel application of CSP was presented by Perez-de-los-Reyes et al. et al. [90], where an array of six parabolic troughcollectors were used to harvest thermal energy for disinfestationof greenhouse soils. The system was able to bring soil tempera-ture up to 60 1C, and was reported effective by the authors.

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].


CSP technologies can supply electricity and heat for chemicalproduction processes. The production of hydrogen using concen-trated solar power is discussed by Glatzmaier and Blake [91]. Theauthors compare two separate processes involving concentratedsolar power and the electrolysis of water. In one regime, CSP isused to produce alternating current electricity, which is thensupplied to an electrolyzer operating in ambient temperature. Theother method utilizes high thermal electrolysis of steam. Thisregime was operated at about 1273 K and, thermodynamically,required less energy than ambient temperature electrolysis.A solar collector can provide both AC electricity and thermalenergy to the system in this design.

Heat conversion into electricity followed by the electrolysis ofwater is a process that involves several lossy steps and thus has alow overall efficiency. Kolb et al. [92] suggested utilizing solartowers for large scale production of hydrogen. The authorsproposed an alternative design, by which hydrogen is producedusing a thermo-chemical process. This regime features an HFC,a solid-particle receiver, a particle thermal energy storage systemand a sulfuric acid cycle. Such a thermo-chemical plant is said toproduce hydrogen at a much lower cost than solar-electrolyzerplants of similar size. Hydrogen production is an effective chemi-cal storage medium for thermal energy, and can be used for manyindustrial processes as well.

The production of zinc can also be achieved using CSPtechnology. A 300 kW solar chemical pilot plant was demon-strated in the framework of the EU-project SOLZINC [80]. Produc-tion was implemented using a carbothermic reduction process ofzinc oxide. This process makes zinc production possible attemperatures of 1300–1500 K, compared with the ZnO dissocia-tion process, which requires temperatures exceeding 2000 K.A ‘beam-down’ HFC regime was used to concentrate solar radia-tion onto a dual-cavity solar chemical reactor. The top cavity is asolar absorber, and the bottom one is a reaction chamber contain-ing a ZnO/C packed bed. Demonstration of the plant yielded50 kg/h of 95% purity zinc. The measured conversion efficiencywas 30%. Zinc can be used in batteries and fuel cells, and can bereacted with water to produce high purity hydrogen gas. This isan exothermic reaction, and can itself be used for power genera-tion, making zinc a possible thermo-chemical storage candidate.The product of this reaction is in turn ZnO, which can then beused again for zinc production.

A process for carbon dioxide recycling was reviewed byHartvigsen et al. [93]. Co-electrolysis of CO2 and steam can beapplied to produce synthesis gas in a large-scale fashion. Thisprocess not only reduces CO2 emissions into the atmosphere, butcan utilize syngas for further clean energy production. Carbondioxide can be recovered from concentrated sources, such as fossilpower plants. Using high concentration CSP technologies forendothermic electrolysis reactions can employ both thermal andelectrical inputs such that the conversion efficiency within thesolid oxide electrolysis cell is 100%. Large-scale implementationof synthetic fuel production from CO2 enables greater use ofintermittent renewable energy sources.

The large amount of thermal energy that can be harvested usingsolar concentrators makes them a lucrative option for integrationwith industrial heat processes. A substantial fraction of these pro-cesses run below 300 1C, an operational temperature achievable bymost solar concentrator regimes. An article discussing heat processintegration of parabolic trough systems in Cyprus was presented byKalogirou [16]. CSP can be integrated with existing fossil fuel powerplants, and provide thermal energy to aid their operation. An exampleis presented by Mills et al. [47], in which a linear Fresnel reflectorplant supplies heat to a coal-fired power station.

The usage of solar thermal power for superplastic formingprocesses is suggested by Lytvynenko and Schur [94]. The process

discussed is used for forming of sheet metals. Utilization of CSPfor this process is reported to be efficient and cost-effective.Thermal treatment of crude oil using a parabolic trough collectorsystem was suggested by Mammadov et al. [95].

Concentrated solar energy can also be used for driving theendothermic reaction that produces lime (calcination reaction).Running this reaction at above 1300 K is reported to reduceemissions of the process by 20–40%, depending on the manufactur-ing plant [96]. An economic assessment for a large-scale (25 MW)plant based on this process found estimates lime cost to be roughlytwice the current price of conventionally produced lime. Thisprocess produces very high purity lime, and its prices might becompetitive with fossil-fuel based calcination processes for chemicaland pharmaceutical sectors requiring unadulterated lime.

Solar power can be utilized for temperature control of build-ings, providing both heating and cooling mechanisms. A highefficiency solar cooling process is outlined by Gordon and ChoonNg [97]. A cascade of mini-dish collector and gas micro-turbineproduces electricity that drives a mechanical chiller, with turbineheat rejection running absorption chiller. A special feature of thissystem is that energy can be stored compactly as ice. Thecompactness of the solar mini-dish system is conducive forsmall-scale ultra-high-performance solar cooling systems.

The utilization of Fresnel lenses was also suggested for lightingand temperature control of buildings [98]. A collection system using aFresnel lens concentrator and a solar receiver generally absorbsbetween 60% and 80% of incoming radiation. The remaining solarflux can be distributed in the interior space for illumination andheating needs. On days when solar radiation is high, this providescooling of interior spaces as well as brightness control. During lowsolar intensity periods, the absorber can be shifted off-focus to permit100% of light to be distributed around the interior (Fig. 24a–c). Thereceiver can be of PV type, thermal type or a hybrid of the two, andwill collect solar energy for heat and/or electricity generation.

A parabolic trough collector system was constructed at theCarnegie Mellon University to study the potential of this CSPregime in solar heating and cooling [99]. The collective area of themirrors was 52 m2. The collector system was coupled to a 16 kWdouble effect, water–lithium bromide (LiBr) absorption chillerand a heat recovery heat exchanger. Generation of hot and chilledwater was available depending on the season. Under optimaldesign, the system was able to achieve 39% of cooling and 20% ofheating energy for the interior space off the building it wasconnected to (Pittsburgh, PA).

The design of a solar absorption refrigeration system directlypowered by a LFR concentrator has also been suggested [100].Evaluation of the technical feasibility of LFR integrated solar-GAX


cycle is carried out. A parametric study for several design configura-tions is performed in order to obtain optimal operation conditions.The study validates this technology as more than satisfactory; thenumerical simulation demonstrated that this scheme answers bothquantity and quality of the advanced cooling system’s energydemands. Furthermore, the operation conditions obtain higher globalsystem efficiencies than previously used technologies. For example,the LFR system experienced a 17.9% efficiency increase comparedwith single effect water–lithium bromide cycle coupled in an indirectform 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 bebuilt with local materials [101–107]. However, the existing dryerdesigns are suited to cloudless, dry environments and they drytoo slowly in hazy situations, typical of many tropical developingcountries. Excessively slow drying allows product degradationcaused by microbial decay, insects and naturally occurringenzymes. Some existing designs are also expensive and relativelyinefficient, and have low capacity (o50 kg/day).

Adding a solar concentrating surface increases the heat outputof solar devices operating in cloudy or hazy conditions [108].With indirect solar dryers this can be accomplished by addingglazed concentrated solar panels to the system. Concentratingsolar panels can be used to inexpensively increase the heat outputfor indirect dryers. Additionally, they can be used to focus agreater light flux onto the drying zone in direct dryers, allowingthem to operate in low-insolation environments. The reflectivesurfaces can be as sophisticated as precision-machined, polishedsurfaces or as simple as cardboard covered in aluminum foil.

The development of a multitude of CSP applications is bene-ficial in many regards; such applications help turn many carbonemitting industrial processes into ‘clean’ ones, conserve largeamounts of electricity that would otherwise be used up andpromote a general environmentally friendly approach to energyconsumption for both industries and individuals. Furthermore,the growing number of these applications aids CSP technologiesin taking root, increasing the demand for solar thermal power andadvancing it into world markets.

12. Discussion

The variety of available CSP technologies and the advance-ments made in each can bring a sense of uncertainty as to whichtechnology works best. This is a complicated issue because of themany factors that need to be considered in selecting a particularCSP design. Every regime features advantages and disadvantagesthat 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 regimeinclude relatively low costs, mature and well-tested technologiesand easy coupling to fossil fuel/geothermal energy sources. PTCsystems are becoming more efficient with the incorporation ofnovel receiver designs such as the heat pipe receiver, whichsignificantly limit convective heat losses while reducing receivercost. The reinforcement of PTCs with a light fiberglass structuresgrants them great stability against wind loads, which further booststhe efficiency as it provides for more accurate sun-tracking. Theincorporation of direct steam generation into PTC systems isgenerally a very positive scheme to produce high quality steam ata constant rate throughout daylight hours, and the usage of wateras a heat transfer fluid is generally cheaper than synthetic oils orionic liquids. That being said, water is a more volatile substancethan other HTFs and will exert more stress on PTC absorber pipes,which may increase maintenance costs. It also needs to be readilyavailable at the site, since, unlike oils and molten salts, it is being

directly converted into steam for power production, and must thusbe constantly replenished. Water transport costs are thus anotherissue that requires attention. Using the Recirculation DSG mode inPTC operation will aid water conservation to some degree. Thechoice to use synthetic oils may be the best option in a PTC sitewhere water is not abundant. While ionic liquids can be used asheat transfer media, they are very expensive to manufacture andmay thus be better suited for higher temperature operations, suchas those of heliostat field collectors. The operational temperaturesof PTCs can exceed 400 1C, high enough for a plethora of industrialheat processes, yet too low for the more efficient, high energyconversion cycles available for power production. Despite thedrawbacks mentioned, it should be noted that the maturity andsuccessful experience to date with PTC technology put it at theforefront of CSP regimes. While other CSP methods may exceed PTCefficiencies or be better geared towards storage and applications,the fact that large (upwards of 100 MW) power stations based onthe PTC scheme have been operational for several years andcontinue to be built proves this technology both successful andeconomical.

An interesting comparison can be made between the conceptsof linear Fresnel reflectors and parabolic trough collectors. LFRsystems prove the cheapest of all CSP regimes, utilizing flatmirrors instead of concave ones, and having incorporating cen-tralized receiver systems that save on receiver material. Thoughthey reach an operational temperature of only about 300 1C, theycan still be used in a variety of applications. The use of DSG workswell with LFRs, and the reasoning needed to select a particulartype of HTF for this type of method is very similar to that of PTCs.A multitude of phase change materials have been proposed foruse in LFR latent heat storage systems. Although these substancesare costly, they can preserve thermal energy effectively for over-night usage. The shading issue that accompanies LFR systems is amaximization problem to which many solutions have beensuggested. The compact LFR regime greatly reduces shadingbetween neighboring reflectors, and allows significantly greatercollection of available sunlight. The formation of a wave-shapedplatform further enhances solar radiation collection. The invertedair cavity receiver is reported to have substantial mitigatingeffects over heat loss in LFR during LFR operation, an importantfeature that can help boost thermal efficiency. The coating ofabsorber tubes with Nickel also aids the heat loss issue, and thetwo could be used in tandem for maximum heat loss reduction.The linear Fresnel reflector method is suited for lower efficienciesthan the rest of its CSP counterparts, but it does so with thebenefits of a significantly more affordable technology.

The relatively young but very powerful CSP concept of helio-stat field collectors has come leaps and bounds over the last fewdecades. The immense flux a large collection of heliostats candirect towards the central receiving unit generates very hightemperatures (up to 2000 1C), and can thus operate very effi-ciently using complex energy conversion cycles, such as thecombined cycle and the magneto-hydrodynamic (MHD) cycle.High operation temperatures may boost electricity productionefficiencies, but are accompanied with both a higher thermalstress on many components of the HFC system and a challengingconvection heat loss problem. The usage of air as a heat transfermedium becomes available at these high temperatures, whichhelps relieve some of the stress heated liquids would exert onsystem components and significantly reduces HTF costs. Thishybridized air–water heating system can produce steam at veryhigh temperatures (485 1C) at a constant rate. The special designsuggested for a receiving unit that has a very large inner surfacearea compared with its aperture is an excellent solution to helpminimize convective losses, but will increase costs due to itscomplicated structure. The dual receiver concept for solar towers


is another novel design that can help boost the total power outputby a significant portion. The quest for cheap materials for heliostatfabrication is a crucial one, as the large mirrors can make up close tohalf the cost of an HFC plant. The use of PVC composite plastic steeloffers a light yet stiff structure, which helps ease stress on mirrortrackers while increasing their accuracy (high stiffness materials aremore wind resistant). The torque tube heliostat (TTH) schemesuggested for wind load reduction is not very effective; it increasesthe cost and decreases the energy output of the system withoutsignificantly diminishing wind stress. The suggested use of mini-mirror arrays resulted in similar results. Experimentation andmodeling for non-spherical arrangements of heliostat fields presentssome potential to increase the amount of solar flux collected from agiven area, but the great height of the solar tower makes heliostatshading a non-vital issue. The design incorporating a reflector towerand a ground receiver is helpful in reducing transport losses, andmakes good organizational sense. The HFC scheme can easily coupleto all three thermal storage methods discussed, giving it a bigadvantage over other CSP regimes. It is, however, very costly, andlarge amounts of power must be produced at high conversionefficiencies to make HFCs a more economically viable technology.

The parabolic dish collector system operates somewhat differ-ently compared with the aforementioned CSP regimes, as verylarge dish is a power generating system within itself. The mountingof a Stirling engine (or a Brayton/combined cycle engine) at a dish’sfocus allows it to operate at very high temperatures throughout theday (usually up to 1000 1C). PDCs are heavy and expensivestructures that must track the sun very accurately to fulfill theirmaximum potential. The structural design to incorporate manysmall mirrors to form the large dish can help mitigate some of therequired costs. The use of an intermediate heat pipe receiver as alink between the reflective dish and the heat engine can be quitepositive, as it promotes uniform and nearly isothermal powerdelivery to the heat engine, boosting its efficiency. The heat pipereceiver also helps mitigate convective heat losses. The suggestedmodified air cavity receiver can serve a similar purpose. The use ofheat engines and high energy conversion cycles makes PDC powerproduction highly efficient. PDC systems do not require the use ofheat transfer media, which helps decrease their cost. The flip sideof this coin is the fact that PDCs cannot be easily coupled tothermal storage methods, a very serious disadvantage in the scopeof large power production plants. The use of thermoelectricmaterials with parabolic dish collectors is an interesting and freshidea, but current efficiencies of this scheme are quite low andfurther investigation of thermoelectric materials and their integra-tion with CSP technologies must be carried out. The mini-dishconcept for CSP is reported to yield record efficiencies andfantastically high concentration ratios, while maintaining fairlylow system costs. The development of this concept in the comingyears may be proved the best execution of the PDC concept.

The up-and-coming field of concentrated photovoltaics pre-sents a medium between CSP and photovoltaics that shows greatpromise. State-of-the-art solar cells can be coupled to any of thefour main CSP regimes in order to absorb very high solarconcentrations that can be directly converted into current. Highquality silicon cells can be used at concentration of up to 100 sunwithout exhibiting degradation in efficiency. For higher solarconcentrations, multi-junction cells can be utilized. The cost-benefit analysis of CPV systems takes into account the price ofboth the CSP method used and the solar cells chosen for particularsystems. The costs of the latter are generally quite high and mustbe offset by high conversion efficiencies to make economic sense.Silicon cells also require a cooling mechanism at higher concen-trations, which may result in their ‘out-the-door’ cost to becomesimilar to that of the very expensive multi-junction cells. It seemsthat on the whole, photovoltaic power production is less efficient

than CSP, but the latter comes with much higher initial capitalinvestments.

The integration of Fresnel lenses with solar cells is thus a greatventure, since the lenses are relatively cheap to manufacture andcan concentrate light very well. Uniform illumination issues wereconsidered by several researchers, to which the answer of cylindri-cally 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 outputthroughout the day. The concentrated photovoltaic thermal regimeis also of interest, as it permits power harvesting of both regimessimultaneously and can result in extremely high conversion efficien-cies. The mounting of solar cells along the absorber tube of PTCsystems, or at a portion of the focal region of parabolic dishes (andmini-dishes), has been shown to be quite successful. The installmentof PV cells on an HFC receiver for high energy photon absorptionmade significant contributions to the overall system efficiency.Unlike CPV systems, CPVTs can store a large portion of collectedenergy for later use, but the trade-off from this advantage is basedin the HTF costs, which CPV systems do not have. The field ofconcentrated solar thermoelectrics seems to draw much attention aswell, but is currently in its infancy developmental stages and is farfrom commercial power generation capabilities in any scale.

The great variety of application that can be incorporated intoconcentrated solar power provides further incentive to invest init. Industrial processes can utilize thermal energy directly to saveon the costs of fossil fuels while maintaining an environmentallyconscientious image. Desalination of water could be done cheaply(in the long run), and temperature control of homes could beginproducing power instead of consuming it. In agriculture, CSP canbe used for food drying, roasting of beans and nuts and cooking.Furthermore, concentrated solar power can be used for steriliza-tion of surgical tools in remote areas. The CSP applicationsmentioned in this work are all novel ideas that are potentiallyvery useful, but each of them (like the CSP technologies that fuelthem) must stand the test of economics in order to penetrateworld markets and become universal.

13. Conclusion

Over the past few decades, great progress has been made inevery facet of concentrated solar power technology. Strivingtowards a sustainable, ‘clean’ energy based culture has instilledmany with the drive to help rid society of its dependence on fossilfuels. With the sun being an obvious and overabundant form ofrenewable energy, it is no wonder that it has been the subject ofso much attention, especially at the turn of the 20th century. Thevariety of technologies with which we can harness the sun’senergy continues to grow, and improvements in every element ofeach concentrated solar power production regime are constantlyadded onto form more efficient, robust, economical and environ-mentally safe facilities.

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