Innovation in concentrated solar power

Innovation in concentrated solar power This work focuses on innovation in CSP technologies over the last decade. A multitude of advancements has been developed during this period, as the topic of concentrated solar power is becoming more mainstream. Improvements have been made in reflector and collector design and materials, heat absorption and transport, power production and thermal storage. Many applications that can be integrated with CSP regimes to conserve (and sometimes produce) electricity have been suggested and implemented, keeping in mind the environmental benefits granted by limited fossil fuel usage. David Barlev a,c , Ruxandra Vidu b,c , Pieter Stroeve a,b,c,n a Department of Electrical and Computer Engineering, University of California Davis, Davis, CA 95616, USA b Department of Chemical Engineering and Materials Science, University of California Davis, Davis,

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David Barleva,c, Ruxandra Vidub,c, Pieter Stroevea,b,c,n

a

Department of Electrical and Computer Engineering, University of California Davis, Davis, CA 95616, USA

b

Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA 95616, USA

c California Solar Energy Collaborative (CSEC), University of California Davis, Davis, CA 95616, USA

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

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

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 great

need for clean and affordable renewable energy sources in order

to meet growing energy demands Achieving sufﬁcient supplies of

clean energy for the future is a great societal challenge Sunlight,

the largest available carbon-neutral energy source, provides the

Earth with more energy in 1 h than is consumed on the planet in

an entire year Despite of this, solar electricity currently provides only a fraction of a percent of the world’s power consumption

A great deal of research is put into the harvest and storage of solar energy for power generation There are two mainstream cate-gories of devices utilized for this purpose—photovoltaics and concentrated solar power (CSP) The former involves the use of solar cells to generate electricity directly via the photoelectric effect The latter employs different methods of capturing solar thermal energy for use in power-producing heat processes Concentrated solar power has been under investigation for several decades, and is based on a simple general scheme: using mirrors, sunlight can be redirected, focused and collected as heat, which can in turn be used to power a turbine or a heat engine to

Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells

n

Corresponding author at: Department of Chemical Engineering and Materials

Science, University of California Davis, Davis, CA 95616, USA.

E-mail address: pstroeve@ucdavis.edu (P Stroeve).

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generate electricity Despite being relatively uncomplicated, this

method involves several steps that can each be implemented in a

plethora of different ways The chosen execution method of every

stage in solar thermal power production must be optimally matched

to various technical, economic and environmental factors that may

favor one approach over another Extensive explorations of various

solar collector types, materials and structures have been carried out,

and a multitude of heat transport, storage and electricity conversion

systems has been tested The progress made in every aspect of CSP,

especially in the last decade, was geared towards expanding the

efﬁciency of solar-to-electric power production, while making it

affordable in comparison with near-future fossil fuel derived power

This work describes the four main types of concentrating solar

collectors (Tables 1 and 2)[1,2] and discusses innovation in each

over the last decade Progress in the related ﬁelds of concentrated

photovoltaics and thermoelectrics will also be presented, along

with advances made in thermal energy storage methods, energy

conversion cycles and CSP applications

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 for

power generation The basic function of a solar collector is to absorb

incident solar radiation and convert it into heat, which is then carried

away by a heat transfer ﬂuid (HTF) ﬂowing through the collector The

heat transfer ﬂuid links the solar collectors to the power generation

system, carrying thermal energy from each collector to a central

steam generator or thermal storage system as it circulates

There are two general categories of solar collectors The ﬁrst

includes stationary, non-concentrating collectors, in which the

same area is used for both interception and absorption of incident

radiation The second category consists of sun-tracking,

concen-trating solar collectors, which utilize optical elements to focus

large amounts of radiation onto a small receiving area and follow

the sun throughout its daily course to maintain the maximum

solar ﬂux at their focus A comprehensive review of sun-tracking

methods and principles was published by Mousazadeh et al.[3]

Light concentration ratios can be expressed in suns, with a single

sun (1000 W/m2) being a measurement of average incident light ﬂux

per unit area at the earth’s surface Though more costly, 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 in

a 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 functions

of 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 1

Description and speciﬁcations of the four main CSP technologies.

Data compiled from [1,2]

Concentration ratio (sun)

Technology maturity

– Linear Fresnel mirror array focused on tower

or high-mounted pipe as receiver

Solar tower – Large heliostat ﬁeld with tall tower in

its center

– Receiver: water/HTC boiler at top

– Can be used for continuous thermal storage

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)

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they operate must be meticulously chosen to achieve optimal

performance

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 concentration

ratios and thus operate at various temperatures From a theoretical

standpoint, 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

Table 2

Schematic diagrams of each CSP technology listed in Table 1

Figures from [2]

Parabolic trough collector

Linear Fresnel reﬂector

Heliostat ﬁeld collector

Parabolic dish reﬂector

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large-scale CSP farms around the world Solar Electric Generating

Systems (SEGS) is a collection of fully operational PTC systems

located in the California desert with a total capacity of 354 MW

SEGS is at present the largest PTC power plant in the world

Another PTC plant with a 280 MW capacity is being built in

Arizona and is scheduled to become operational in 2011 PTCs

effectively produce heat at temperatures ranging from 50 to

400 1C These temperatures are generally high enough for most

industrial heating processes and applications, the great majority

of which run below 300 1C

The parabolic trough collector design features light structures

and relatively high efﬁciency A PTC system is composed of a

sheet of reﬂective material, usually silvered acrylic, which is bent

into a parabolic shape Many such sheets are put together in

series to form long troughs These modules are supported from

the ground by simple pedestals at both ends The long, parabolic

shaped modules have a linear focus (focal line) along which a

receiver is mounted The receiver is generally a black metal pipe,

encased in a glass pipe to limit heat loss by convection The metal

tube’s surface is often covered with a selective coating that

features high solar absorbance and low thermal emittance The

glass tube itself is typically coated with antireﬂective coating to

enhance transmissivity A vacuum can be applied in the space

between the glass and the metal pipes to further minimize heat

loss and thus boost the system’s efﬁciency

The heat transfer ﬂuid (HTF) ﬂows through the receiver,

collecting and transporting thermal energy to electricity

genera-tion systems (usually boiler and turbine generator) or to storage

facilities The HTF in PTC systems is usually water or oil, where oil

is generally preferred due to its higher boiling point and relatively

low volatility Several water boiler designs have been suggested

by Thomas [8] The preferred boiling system implements direct

steam generation (DSG), where water is the heat transfer ﬂuid

It is partially boiled in the collector and circulated through a

steam drum where steam is separated from the water

The DISS (Direct Solar Steam) project PTC plant in Tabernas,

Spain, is a leading DSG test facility, where two successful DSG

operational modes and control systems were developed and

tested [9] Both methods utilize pressure control in addition to

temperature control of circulating water This approach is done to

achieve a constant output of steam at a monitored temperature

throughout most hours of the day (9 am–6 pm) A pressure level

of 100 bar and temperatures of up to 400 1C have been

demon-strated The Once-Through mode (Fig 1) features a preheated water

feed into the inlet As water circulates through the collectors, it is

evaporated and converted into superheated steam that is used to

power a turbine In the more water-conservative Recirculation mode

(Fig 2), a water–steam separator is placed at the end of the collector

loop More water is fed to the evaporator than can be evaporated in

one circulation cycle Excess water is re-circulated through the

intermediate separator to the collector loop inlet, where it is mixed

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 systems

in 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 Steinmann

et 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

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

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A study of turbulent ﬂow around a PTC of the 250 kW solar

plants in Shiraz, Iran, was conducted by Naeeni and Yaghoubi

[17] The study investigates stress applied to the collector, taking

into account varying collector angles, wind velocities and air ﬂow

distribution with respect to height from the ground A second

study by the authors models the effects of the same parameters

on heat transfer from the PTC receiver tube[18]

In order to make the PTC structure more resilient to external

forces, it is possible to reinforce collector surfaces with a thin

ﬁberglass layer A smooth, 901 rim angle reinforced trough was

built by a hand lay-up method[19] The ﬁberglass layer is added

underneath the reﬂective coating (on the inner surface) of the

parabolic trough The reﬂector’s total thickness is 7 mm, and can

withstand a force applied by a 34 m/s wind with minimal

deviation; deﬂection at the center of the parabola vertex was

only 0.95 mm, well within acceptable limits

Receiver design considerations are crucial for efﬁcient heat

transfer to the HTF and heat loss management Radiative heat

losses from receiver tubes play an important role in collector

performance Thermal loss due to the temperature gradient

between the receiver and the ambient has a signiﬁcant impact

on a system’s thermal efﬁciency PTCs operating at high

tempera-tures (around 390 1C) can experience up to 10% radiative losses

annually At this temperature range, thermal loss from receiver tube

reaches 300 W/m of the receiver pipe[20] A loss of 220 W/m was

reported for an operational temperature of 180 1C, with collector

efﬁciency ranging from 40% to 60%[21] In both of the

aforemen-tioned studies, synthetic oil was used as the heat transfer ﬂuid

The high temperature difference between the receiver tube’s

interior and the ambient also induces a thermal stress, which can

cause bending of the pipes Thermal analysis of an energy-efﬁcient

PTC receiver was presented by Reddy et al.[22], and a numerical

model to evaluate its heat transfer characteristics was proposed The

new receiver design features porous inclusions inside the tube,

which increase the total heat transfer area of the receiver, along

with its thermal conductivity and the turbulence of the circulating

HTF (synthetic oil) Heat transfer for this scheme was enhanced by

17.5% compared with regular (no inclusions) design, but the system

suffered a pressure decrease of about 2 kPa

The use of a heat pipe as a linear receiver for PTCs was proposed

by Dongdong et al [23] The heat pipe can keep an essentially

uniform circumferential temperature, despite the uneven

illumina-tion provided by trough collectors Since heat does not ﬂow from the

HTF to the heat pipe, smaller heat losses occur during hours of low

insolation PTC systems featuring a heat pipe as the receiver have

65% thermal efﬁciency at 380 1C They are also cheaper to

manu-facture because the bellows system generally incorporated into

conventional receiver tubes is not necessary Lifetime testing of

the heat pipe receiver with respect to various operation

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 unsteady

state For this reason, a dynamic model is essential for effective

design and performance prediction of a PTC system A dynamic

simulation of PTC was conducted by Ji et al.[24], modeling a south

facing, one-axis tracking parabolic trough collector The simulation

calculated variations in incidence angle of solar beam to collector

aperture, as well as the distribution of concentrated solar radiation

along the focal line Effects of HTF mass ﬂow rate and receiver tube

length on outlet temperature and system efﬁciency were

investi-gated An increase in tube length augments outlet temperatures and

efﬁciency, as expected due to greater total insulation A decrease in

mass ﬂow rate increases outlet temperature and slightly decreases

system efﬁciency

The integration of a parabolic trough collector ﬁeld with

geothermal sources has been suggested by Lentz and Almanza

[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 order

to 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 heliostat

is 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 another

to 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

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and power conversion efﬁciencies by enabling the use of

higher-energy cycles

A reﬂective solar tower design has been suggested, in which a

secondary reﬂector is mounted on the tower, and the central

receiver is grounded (Fig 3) A review of the optics of the reﬂector

tower was presented by Segal and Epstein [28] Since HFCs

operate at such high temperatures, the greatest losses are

incurred convectively at the receiver’s surface

Aside from the convenience associated with having the

recei-ver situated at ground level, the optics of the design increase the

concentration ratio, allowing the collector to be smaller and

diminish losses Transport losses can also be lowered by situating

the turbine generator in close proximity with the receiver

Never-theless, Segal and Epstein[29]reported that the reﬂector tower

scheme is not more efﬁcient than the solar tower regime, and that

superiority of either technology is subject mainly to economic factors

The integration of a solar reformer with a heliostat ﬁeld array

was proposed in 2002 Solar reforming of methane with steam or

CO2 is an efﬁcient chemical heat storage method The syngas

produced can be converted into electricity using a gas turbine or

combined cycle The suggested reformer rests on the ground, and

has a collector mounted above it (Fig 4) A solar reﬂector tower is

used to concentrate solar ﬂux from heliostats onto the ground

reformer In this fashion, the power producing unit can be separated

from the concentrator ﬁeld entirely

Landﬁll gas and biogas can be used to supplement gas

pro-duced by the reformer The design and operation of a large-scale

reformer are discusses by Segal and Epstein [30] The synthesis

gas produced by this technology can also be utilized for the

production of methanol

An optimization study of an HFC system’s main parameters

was conducted by Segal and Epstein[7] The effects of operation

temperature, heliostat ﬁeld density and the use of a secondary

reﬂector (reﬂector tower regime) on power conversion were

tested across different energy cycles (Fig 5) The investigation

concluded that maximum overall efﬁciency of an HFC system is

reached at 1600 K, with an average ﬁeld density of 35% The

authors emphasize that differences between large and small HFC

plants 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 to

split the solar spectrum into PV-used and thermal-used portions For

example, monocrystalline silicon solar cells operate at efﬁciencies

ranging between 55% and 60% at wavelengths of 600–900 nm The

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 output

is 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 tion challenge A novel methodology for layout generation based onyearly normalized energy surfaces (YNES) was presented by Sanchez

optimiza-Fig 4 Solar ground reformer integrated with a reﬂector tower HFC system Figure reproduced with permission from ref 30, &2003 Elsevier.

Fig 5 Brayton cycle and combined cycle efﬁciencies as a function of the temperature and gas turbine pressure ratio.

Figure reproduced with permission from ref 7, &2003 Elsevier.

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

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and Romero[33] This ‘Heliostat Growth Method’ (HGM) uses the

YNES program to evaluate the usable solar energy ﬂux at each point

in a solar ﬁeld year-round, given a speciﬁc solar tower height Using

this data, the method splits impacting factors such as shadowing,

blocking, atmospheric attenuation and others into two categories:

those associated with spatial position of the solar tower and those

affected by the geometry of the heliostat This provides greater

insight and ﬂexibility to the ﬁeld layout process and its optimization

A clever design for small-scale ‘tri-generation’ solar power

assisted plant was brought forth by Buck and Friedmann[34] The

design puts together a solar–gas turbine hybrid system, which

incorporates a small heliostat ﬁeld, a receiver mounted on a solar

tower, a micro-turbine and an absorption chiller In this regime,

electric power, heating and cooling can all be produced by the

same system System conﬁgurations were assessed for technicalperformance and cost

Forsberg et al.[35]suggested the use of liquid ﬂuoride salt as

an HTF in order to raise the heat-to-electricity conversion 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 N2or He as the working ﬂuid.Due to such high operation temperatures, thermal energy storage

efﬁ-as 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 salt

at high temperatures The efﬁciency boost reported by theauthors can greatly reduce electricity costs

The combination of a single central receiver with molten salts

as 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 suggested

is 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

Fig 7 Schematic plant incorporating dual receiver, outlining three heat transfer stages (preheating, evaporation and superheating).

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

Table 3

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

Reference plant Dual receiver plant

Annual performance Annual receiver efﬁciency (including recirculation losses) (%) 66.7 79.4

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

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The heliostat material selection is a crucial aspect of HFC

power plant design These large mirrors make up about 50% of the

total system’s cost and must feature high reﬂectivity and stiffness,

be light-weight, easily cleaned and corrosion resistant Xiaobin

et al.[38]suggested the use of PVC composite plastic steel for

heliostat fabrication This polymer material has similar properties

to metal–aluminum alloys conventionally used, but is not as

heavy, and has a signiﬁcantly longer lifetime Its stiffness is high

relative to its weight and it is reported by the authors to be

cheaper One signiﬁcant issue with this material is its low heat

resilience, a problem which must be contended with in order to

ensure heliostat operation temperatures can be accommodated

Several heliostat cleaning methods are proposed by Xiliang et al

[39], such as using highly pressurized air/water depending on

various environmental conditions

Conventional heliostat design dictates that cost reduction is

implemented by increasing the area of the mirrors Doing this

reduces speciﬁc drive cost while increasing the torques heliostats

experienced by wind loads A study by Ying-ge et al [40]

demonstrates the distribution and characteristics of heliostats’

mean and ﬂuctuating wind pressure while wind direction angle is

varied from 01 to 1801 and vertical angle is varied from 01 to 901

Moreover, a ﬁnite element model was constructed to perform

calculations of wind-induced dynamic responses Increased wind

torques result in higher speciﬁc weight and drive power The

usage of torque tube heliostats (TTH) (Fig 9) is suggested by

Amsbeck et al [41] TTH systems incorporate arrays of long,

narrow mirrors mounted on turning tubes that control their

elevation An optical performance and a weight estimation of a

TTH system were carried out by the authors, and compared with a

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 G ¨ottsche 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 is

to 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 countered

by a 40% drop in optical performance compared with tional HFC systems

conven-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

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.

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assessment of the ﬂux density produced by a heliostat from

Zaragoza University, an optimized mirror density distribution

developed by University of Houston for the Solar One project,

and molten salt receiver efﬁciencies measured during the Solar

Two project This model does not take into account many

impacting factors speciﬁc to a particular HFC system and is

limited in its accuracy

Similarly, a new method for approximating geometrical

para-meters and sizing of the tower reﬂector regime was developed by

Segal and Epstein[44] The method utilizes edge rays originating

from the heliostat ﬁeld boundaries and is particularly useful for

geometrical assessment of very large arrays of heliostats The

method’s results were compared with real ﬁeld calculations and

found to be a good ﬁrst approximation regime

A simulation using the same ‘edge ray’ principle method was

developed by Xiudong et al [45] Its purpose was to promote

more efﬁcient placement of heliostats and obtain a faster

gen-erating response of the design and optimization A novel module

for the analysis of non-spherical heliostat arrangements has been

incorporated into the simulation A toroidal heliostat ﬁeld was

designed and analyzed by the authors and proved signiﬁcantly

less efﬁcient that conventional HFC arrangements A method for

calculating the annual solar ﬂux distribution of a given area is an

added feature, with the purpose of evaluating feasibility of crop

growth around heliostat ﬁelds

Heliostat ﬁeld collector technology has greatly improved over

the last few decades, and continues to draw much attention as a

suitable scheme for large solar thermal plants The exceedingly

high temperatures at which they operates it grant HFC plants

excellent efﬁciencies, while allowing them to be coupled to a

variety of applications The high capital investment necessary for

the construction of HFC systems is an obstacle, however, and

further technological advancements in efﬁciency must be

accom-panied by low cost materials and storage schemes for this CSP

method to become more economical

5 Linear Fresnel reﬂectors (LFR)

Concentrated solar power production using linear Fresnel

reﬂectors is quite similar to the parabolic trough collector

scheme The two share common principles in both arrangement

and operation In March 2009, the German company Novatec

Biosol constructed a LFR solar power plant known as PE 1 that has

an electrical capacity of 1.4 MW The success of this project

inspired the design of PE 2, a 30 MW plant based on the LFR

technology, to be constructed in Spain The 5 MW Kimberlina

Solar Thermal Energy plant has been recently completed in

Bakersﬁeld, California

Linear Fresnel reﬂectors incorporate long arrays of ﬂat mirrors

that concentrate light onto a linear receiver The receiver is

mounted on a tower (usually 10–15 m tall), suspended above

and along reﬂector arrays The mirrors can be mounted on one or

two-axis tracking devices The ﬂat, elastic nature of the mirrors

used makes the LFR design signiﬁcantly cheaper than PTC

Additionally, central receiver units save on receiver material

costs, which are generally higher than reﬂector costs Several

Fresnel reﬂectors can be used to approximate a parabolic trough

collector shape, with the advantage that the receiver is a separate

unit, and does not need to be supported by the tracking device

This makes tracking simpler, accurate and more efﬁcient A heat

transfer ﬂuid circulates through the receiver, collecting and

trans-porting thermal energy to power production and storage units

A signiﬁcant challenge with LFR systems is light blocking

between adjacent reﬂectors Solving this issue requires either

increased 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 allows

a 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 towards

it 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 considerations

of 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 have

a 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 in

a CLFR regime was presented by Chavez and Collares-Pereira[50]

Fig 11 Schematic diagram of the CLFR design.

Fig 12 Schematic diagram of inverted air cavity receiver.

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New geometries for reﬂector ﬁelds are explored in this study,

with the purpose of limiting blocking/shading while maximizing

the ﬁeld layout density The authors propose reformation of the

platform on which reﬂectors are resting (ground) into a

wave-shaped one (Fig 13) Individual reﬂectors’ size/shape adjustments

based on their position in the heliostat ﬁeld are also suggested

A concentration increase of up to 85% of theoretical maximum is

reported under this design

Dey[51]describes several receiver design considerations for the

CLFR concept The absorber is a basic inverted air cavity with a glass

encasing that encloses a selective surface The central design goals

analyzed are (1) maximization of heat transfer between the

absorb-ing surface and the steam pipes, and (2) ensurabsorb-ing uniform absorber

surface temperature to avoid degradation of the selective surface

Heat calculations are presented for absorber temperature

distribution, and satisfactory absorber pipe separations and sizes

are shown to alleviate temperature differences between the ﬂuid

surface and the absorbing surface Similar work using ﬁnite

element calculations was done by Eck et al.[52]for three separate

parts of a LFR system–the evaporator, pre-heater and superheater

(Table 4) Thermal loads for each section were modeled and

maximum temperatures were investigated In the case of the

superheater, the maximum temperature derived was 570 1C,

exceeding the temperature limit of the absorber coating A novel

step-by-step heat ﬂux reduction method is thus required for safe

and successful operation Such a control system would adjust

reﬂectors to an off-focus position one by one to prevent

over-heating while operating at the highest allowed temperature This

kind of sensitive, intelligent system would surely increase power

plant costs

A study by Hoshi et al.[53]investigated the suitability of high

melting point phase change materials (PCMs) for storage use in

large-scale CLFR plants (Fig 14a–c) Several candidates for latent

heat storage materials are discussed, and mathematical models of

charging and discharging heat storage from each are presented

NaNO2is emphasized as a particularly suitable contender for

large-scale latent heat storage due to its high melting point and low cost

LFR technology offers many of the advantages of PTC systems

while incurring smaller reﬂector costs It too can be easily coupled

to direct steam generation as well as molten salts for thermal

energy transport The central receiver regime it incorporates

shrinks costs further, but tags on the challenge of maximizing

the amount solar radiation that can be collected Innovation in

receiver design and reﬂector organization has made LFR relatively

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

Table 4

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

Data compiled from [52]

Pre-heater Evaporator Superheater Heat transfer coefﬁcient (W/m 2

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.

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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, which

must track the sun on a two-axis tracking system to maintain

light convergence at its focal point A receiver is mounted at the

focus, collecting solar radiation as heat Two general schemes are

possible for power conversion; the less popular has a heat transfer

ﬂuid system connecting the receivers of several dishes, conducting

thermal energy towards a central electricity generation system This

design is less convenient as it requires a piping and pumping

system resilient to very high temperatures, and suffers from

transport thermal losses The more prevalent system mandates

a heat engine be mounted near/at the focal points of individual

dishes The heat engine absorbs thermal energy from the receiver,

and uses it to produce mechanical work, which an attached

alternator then converts into electricity A heat-waste exhaust

system must be incorporated to release excess heat from the

system Finally, a control system is necessary to ensure matching

of the heat engine’s operation to the incoming solar ﬂux

An advantage of this design is that the reﬂector, collector and

engine can operate as separate units, making fossil-fuel

hybridi-zation a relatively simple task It is important to note however,

that this PDC system does not lend itself to thermal storage

methods

The Stirling engine is often used for this application, although

gas turbines can also be employed in the Brayton or Rankine/

Brayton combined cycles Stirling engine performance is better in

temperatures below 950 1C, whereas at higher temperatures,

combined cycle gas turbines can achieve higher efﬁciencies[54]

The operations and speciﬁcations of a 10 kW single dish-Stirling

system were described in detail by Jin-Soo et al.[55]

Due to its high concentration ratios, the parabolic dish collector

is an excellent candidate for concentrated photovoltaics The usage

of state-of-the-art, high-cost high-performance photovoltaic cells is

justiﬁed when they are utilized at concentrations exceeding

100 sun; a large solar ﬂux focused in a small region of cells can

produce enough power to offset the high capital investment

required GaAs and multi-junction PV cells are very expensive to

fabricate Yet, operational module efﬁciencies exceeding 30% have

been demonstrated by multiple manufacturers and veriﬁed by the

National Renewable Energy Lab (NREL) Moreover, these PV

tech-nologies are very heat-resistant, and perform better under high

concentration ratios Incorporating such modules into the parabolic

dish collector apparatus is fairly simple, and can yield results that

are comparable to or better than heat engine systems, potentially

with a longer lifetime Further discussion of concentrated

photo-voltaics is developed in a later section

A numerical simulation of a heat-pipe receiver for the

para-bolic dish collector was performed by Hui et al.[56] Using this

type of receiver between the dish and the Stirling engine is

reported to provide power uniformly and nearly isothermally to

the engine heater This results in improved engine performance

Heat-pipe utilization also limits convective heat loss from the

receiver

Parabolic dish collectors are high-cost devices: they are very

large mirrors that must feature nearly perfect concavity to

effectively concentrate solar radiation They are also very heavy,

and their tracking system must thus be very sensitive and ﬁnely

tuned A novel suggestion by Kussul et al.[57]to moderate the

high collector cost is to manufacture an approximated parabolic

dish using many small, ﬂat mirrors A prototype was constructed

by the group, which contains 24 mirrors in the shape of

equi-lateral triangles, each with a side length of 5 cm special nuts are

used to maintain required positions of nodes in the connection

points 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 presented

by 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 in

2009 [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 reductions

of 29.23%, 19.81% and 19.16%, respectively

Another thermal analysis of a PDC system was done by Nepveuat

al.[60] The authors constructed a thermal energy conversion model

of the 10 kW Eurodish/Stirling unit erected at the 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 of

a 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 15 (a) Light collection and (b) general schematics of air cavity receiver in a dish/Stirling system.

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