Innovation in concentrated solar power

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

Contents lists available at ScienceDirect

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

Review

Innovation in concentrated solar power
David Barlev a,c, Ruxandra Vidu b,c, Pieter Stroeve a,b,c,n
a
b
c

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

a r t i c l e i n f o

abstract

Article history:
Received 30 October 2010
Accepted 12 May 2011

This work focuses on innovation in CSP technologies over the last decade. A multitude of advancements
has been developed during this period, as the topic of concentrated solar power is becoming more
mainstream. Improvements have been made in reﬂector and collector design and materials, heat
absorption and transport, power production and thermal storage. Many applications that can be

integrated with CSP regimes to conserve (and sometimes produce) electricity have been suggested and
implemented, keeping in mind the environmental beneﬁts granted by limited fossil fuel usage.
& 2011 Elsevier B.V. All rights reserved.

Keywords:
Concentrated solar power (CSP)
Design
Materials
Heat absorption
Transport
Thermal storage

Contents
1.
2.
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4.
5.
6.
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10.
11.
12.
13.

Introduction . . . . . . . . . . . . . . . . . . .
Concentrating solar collectors . . . . .
Parabolic trough collectors (PTC) . .

Heliostat ﬁeld collectors (HFC) . . . .
Linear Fresnel reﬂectors (LFR) . . . . .
Parabolic dish collectors (PDC) . . . .
Concentrated photovoltaics . . . . . . .
Concentrated solar thermoelectrics.
Thermal energy storage . . . . . . . . . .
Energy cycles . . . . . . . . . . . . . . . . . .
Applications . . . . . . . . . . . . . . . . . . .
Discussion . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . .

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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

n
Corresponding author at: Department of Chemical Engineering and Materials
Science, University of California Davis, Davis, CA 95616, USA.
E-mail address: (P. Stroeve).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2011.05.020

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

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

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

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

Description

Rel. thermodynamic
efﬁciency

Operating temp.
range (1C)

Relative
cost

Concentration Technology
ratio (sun)
maturity

Tracking

PTC

– Parabolic sheet of reﬂective material
(aluminum, acrylic)
– Linear receiver (metal pipe with heat
transfer ﬂuid)

Low

50–400

Low

15–45

Very mature

One-axis

Linear
Fresnel

– Linear Fresnel mirror array focused on tower Low
or high-mounted pipe as receiver

50–300

Very low

10–40

Mature

One-axis

Solar tower

High
– Large heliostat ﬁeld with tall tower in
its center
– Receiver: water/HTC boiler at top
– Can be used for continuous thermal storage

300–2000

High

150–1500

Most recent

Two-axis

Dish-Stirling

– Large reﬂective parabolic dish with Stirling High
engine receiver at focal point
– Can be used with/out HTC, if heat engine
produces electricity directly from reﬂected
thermal energy (in this case, thermal storage
cannot be achieved by the system)

150–1500

Very high

100–1000

Recent

Two-axis

generate electricity. Despite being relatively uncomplicated, this
method involves several steps that can each be implemented in a
plethora of different ways. The chosen execution method of every
stage in solar thermal power production must be optimally matched
to various technical, economic and environmental factors that may
favor one approach over another. Extensive explorations of various
solar collector types, materials and structures have been carried out,
and a multitude of heat transport, storage and electricity conversion
systems has been tested. The progress made in every aspect of CSP,

especially in the last decade, was geared towards expanding the
efﬁciency of solar-to-electric power production, while making it
affordable in comparison with near-future fossil fuel derived power.
This work describes the four main types of concentrating solar
collectors (Tables 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 transforms solar radiation into thermal energy that can be utilized for
power generation. The basic function of a solar collector is to absorb
incident solar radiation and convert it into heat, which is then carried
away by a heat transfer ﬂuid (HTF) ﬂowing through the collector. The
heat transfer ﬂuid links the solar collectors to the power generation
system, carrying thermal energy from each collector to a central
steam generator or thermal storage system as it circulates.
There are two general categories of solar collectors. The ﬁrst
includes stationary, non-concentrating collectors, in which the
same area is used for both interception and absorption of incident
radiation. The second category consists of sun-tracking, concentrating solar collectors, which utilize optical elements to focus
large amounts of radiation onto a small receiving area and follow
the sun throughout its daily course to maintain the maximum
solar ﬂux at their focus. A comprehensive review of sun-tracking
methods and principles was published by Mousazadeh et al. [3].
Light concentration ratios can be expressed in suns, with a single
sun (1000 W/m2) being a measurement of average incident light ﬂux

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

collectors have numerous advantages over stationary collectors, and
are generally associated with higher operation temperatures and
greater efﬁciencies. The addition of an optical device to the conventional solar collector (receiver) has proved useful in several regards;
various concentration schemes can achieve a wide range of concentration ratios, from unity to over 10,000 sun [2]. This increases the
operation temperature as well as the amount of heat collected in a
given area, and yields higher thermodynamic efﬁciencies. Radiation
focusing allows the usage of receivers with very small relative surface
areas, which leads to signiﬁcant reductions in heat loss by convection.
Despite the added capital investment necessary for manufacturing
the optical elements of the apparatus, the materials used for these
mirrors/lenses are generally inexpensive compared with thermal
collector materials, which are needed in much smaller amounts in
a concentrator scheme. The reduction in receiver size and material
amounts makes expensive receiver conditioning (vacuum insulation,
surface treatments, etc.) for higher efﬁciency and heat loss minimization economically sensible. Finally, the ability to control the concentration ratio of a system allows delicate manipulation of its operation
temperature, which can be thermodynamically matched to speciﬁc
applications as needed to avoid wasted heat. It is important to note
that reﬂective materials used in CSP technologies must meet certain
reﬂectivity and lifetime requirements to be cost-effective. A study of
the optical durability of solar reﬂectors was presented by Kennedy
and Terwilliger [4] and an investigation speciﬁc to aluminum ﬁrstsurface mirrors was carried out by Almanza et al. [5].
Tyagi et al. [6] investigated the effects of HTF mass ﬂow rates
and collector concentration ratios on various system parameters.
Results showed that exergy output (available work from a process
that brings a system to thermal equilibrium), exergetic and
thermal efﬁciencies and inlet temperature increased with solar
intensity, as expected. Exergetic and thermal efﬁciencies and
exergy output were found to increase with mass ﬂow rate as
well. Optimal inlet temperature and exergetic efﬁciency at high
solar intensity were both found to be the decreasing functions

of the concentration level. At low intensity values, however,
efﬁciency ﬁrst increases and then decreases with increase in
concentration. This behavior results from increased radiative
losses associated with high concentration ratios. Both concentration ratios of solar collectors and the mass ﬂow rates at which

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

2705

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

Schematic diagram

Parabolic trough collector

Linear Fresnel reﬂector

Heliostat ﬁeld collector

Parabolic dish reﬂector

they operate must be meticulously chosen to achieve optimal
performance.
The four main types of concentrating solar collectors are
(1)
(2)

(3)
(4)

Parabolic trough collectors;
heliostat ﬁeld collectors;
linear Fresnel reﬂectors; and
parabolic dish collectors.

proportional and strictly dependent on the operation temperature.
In practice, however, the materials chosen for light concentration
and absorption, heat transfer and storage, as well as the power
conversion cycles used are the true deciding factors [7]. The
following sections will describe the aforementioned collector
schemes in detail, and present technological advancements that
have been made in each over the last 10 years.
3. Parabolic trough collectors (PTC)

Concentrating collectors can achieve different concentration
ratios and thus operate at various temperatures. From a theoretical
standpoint, the efﬁciency of power producing heat processes is both

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

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

large-scale CSP farms around the world. Solar Electric Generating

Systems (SEGS) is a collection of fully operational PTC systems
located in the California desert with a total capacity of 354 MW.
SEGS is at present the largest PTC power plant in the world.
Another PTC plant with a 280 MW capacity is being built in
Arizona and is scheduled to become operational in 2011. PTCs
effectively produce heat at temperatures ranging from 50 to
400 1C. These temperatures are generally high enough for most
industrial heating processes and applications, the great majority
of which run below 300 1C.
The parabolic trough collector design features light structures
and relatively high efﬁciency. A PTC system is composed of a
sheet of reﬂective material, usually silvered acrylic, which is bent
into a parabolic shape. Many such sheets are put together in
series to form long troughs. These modules are supported from
the ground by simple pedestals at both ends. The long, parabolic
shaped modules have a linear focus (focal line) along which a
receiver is mounted. The receiver is generally a black metal pipe,
encased in a glass pipe to limit heat loss by convection. The metal
tube’s surface is often covered with a selective coating that
features high solar absorbance and low thermal emittance. The
glass tube itself is typically coated with antireﬂective coating to
enhance transmissivity. A vacuum can be applied in the space
between the glass and the metal pipes to further minimize heat
loss and thus boost the system’s efﬁciency.
The heat transfer ﬂuid (HTF) ﬂows through the receiver,
collecting and transporting thermal energy to electricity generation systems (usually boiler and turbine generator) or to storage
facilities. The HTF in PTC systems is usually water or oil, where oil
is generally preferred due to its higher boiling point and relatively
low volatility. Several water boiler designs have been suggested
by Thomas [8]. The preferred boiling system implements direct

steam generation (DSG), where water is the heat transfer ﬂuid.
It is partially boiled in the collector and circulated through a
steam drum where steam is separated from the water.
The DISS (Direct Solar Steam) project PTC plant in Tabernas,
Spain, is a leading DSG test facility, where two successful DSG
operational modes and control systems were developed and
tested [9]. Both methods utilize pressure control in addition to
temperature control of circulating water. This approach is done to
achieve a constant output of steam at a monitored temperature
throughout most hours of the day (9 am–6 pm). A pressure level
of 100 bar and temperatures of up to 400 1C have been demonstrated. The Once-Through mode (Fig. 1) features a preheated water
feed into the inlet. As water circulates through the collectors, it is
evaporated and converted into superheated steam that is used to
power a turbine. In the more water-conservative Recirculation mode
(Fig. 2), a water–steam separator is placed at the end of the collector
loop. More water is fed to the evaporator than can be evaporated in
one circulation cycle. Excess water is re-circulated through the
intermediate separator to the collector loop inlet, where it is mixed

Fig. 1. Schematic ﬂow diagram of Once-Trough mode of operation of direct steam
generation.
Figure reproduced with permission from ref. 9, &2005 Elsevier.

Fig. 2. Schematic ﬂow diagram of Recirculation mode of operation of direct steam
generation.
Figure reproduced with permission from ref. 9, &2005 Elsevier.

with preheated water. This process guarantees good wetting of
absorber tubes and prevents stratiﬁcation. Steam is separated from
water and fed into the inlet of a superheating section. The Recirculation regime is more easily controlled than the Once-through regime,

but has an increased parasitic load due to the additional process
steps. Usage of water as a HTF inﬂicts more stress on the absorber
tubes than other heat transfer media, due to water’s relatively high
volatility. A simulation of thermohydraulic phenomena under the
DSG process was carried out by Eck and Steinmann [10]. Sufﬁcient
cooling of the absorber tubes and a moderate pressure drop between
inlet and outlet can help moderate the stress, reduce corrosion and
promote tube lifetime.
Knowledge of short-time dynamics of ﬂow and feed systems
in a DSG regime is crucial for successful design and operation.
A transient non-linear simulation tool was developed to study
dynamic behaviors of the aforementioned PTC system designs, for
which several feed control systems were suggested [11]. It is
important to mention that for DSG systems, the temperature
difference registered between the hottest and the coldest points
over the external wall of the pipe will increase if feed ﬂow is too
high [12]. This is a result of non-constant heat transference from
the receiver to the HTF, and can potentially affect the quality of
produced steam. A test facility for a solid sensible heat storage
system was developed for the DSG parabolic trough collector
design discussed. A performance analysis of the storage system
integrated with the power plant was implemented by Steinmann
et al. [13]. Integration of thermo-chemical storage through
ammonia de-synthesis was theoretically investigated as well,
and efﬁciencies of up to 53% were reported [14].
In contrast with the DSG scheme, which employs water as the
HTF, recent innovation also promotes the use of ionic liquids
(molten salts) for heat transfer media [15], as they are more
heat-resilient than oil, and thus corrode the receiver pipes less.
Ionic liquids are, however, very costly, and such an investment

would have to be weighed against the incurring costs of receiver
maintenance and replacement to determine their cost-effectiveness.
PTCs are mounted on a single-axis sun-tracking system that
keeps incident light rays parallel to their reﬂective surface and
focused on the receiver throughout the day. Both east–west and
north–south tracking orientations have been implemented, with
the former collecting more thermal energy annually, and the
latter collecting more energy in the summer months when energy
consumption is generally the highest [2]. The east–west orientation
has been reported to be generally superior [16]. The tracking
mechanism must have parabolic collectors for tracing the sun’s path
very accurately in order to achieve efﬁcient heating of the receiver
tube. However, trough collectors are generally exposed to wind
drag, and must thus be robust enough to account for wind loads and
prevent deviations from normal insolation incidence.

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

A study of turbulent ﬂow around a PTC of the 250 kW solar
plants in Shiraz, Iran, was conducted by Naeeni and Yaghoubi
[17]. The study investigates stress applied to the collector, taking
into account varying collector angles, wind velocities and air ﬂow
distribution with respect to height from the ground. A second
study by the authors models the effects of the same parameters
on heat transfer from the PTC receiver tube [18].
In order to make the PTC structure more resilient to external
forces, it is possible to reinforce collector surfaces with a thin
ﬁberglass layer. A smooth, 901 rim angle reinforced trough was
built by a hand lay-up method [19]. The ﬁberglass layer is added

underneath the reﬂective coating (on the inner surface) of the
parabolic trough. The reﬂector’s total thickness is 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 temperatures (around 390 1C) can experience up to 10% radiative losses
annually. At this temperature range, thermal loss from receiver tube
reaches 300 W/m of the receiver pipe [20]. A loss of 220 W/m was
reported for an operational temperature of 180 1C, with collector
efﬁciency ranging from 40% to 60% [21]. In both of the aforementioned studies, synthetic oil was used as the heat transfer ﬂuid.
The high temperature difference between the receiver tube’s
interior and the ambient also induces a thermal stress, which can
cause bending of the pipes. Thermal analysis of an energy-efﬁcient
PTC receiver was presented by Reddy et al. [22], and a numerical
model to evaluate its heat transfer characteristics was proposed. The
new receiver design features porous inclusions inside the tube,
which increase the total heat transfer area of the receiver, along
with its thermal conductivity and the turbulence of the circulating
HTF (synthetic oil). Heat transfer for this scheme was enhanced by
17.5% compared with regular (no inclusions) design, but the system
suffered a pressure decrease of about 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 illumination provided by trough collectors. Since heat does not ﬂow from the
HTF to the heat pipe, smaller heat losses occur during hours of low

insolation. PTC systems featuring a heat pipe as the receiver have
65% thermal efﬁciency at 380 1C. They are also cheaper to manufacture because the bellows system generally incorporated into
conventional receiver tubes is not necessary. Lifetime testing of
the heat pipe receiver with respect to various operation temperatures is still being investigated, but meets the general requirements
(12–15 years) under operation below 380 1C.
Parabolic trough collector systems generally operate in unsteady
state. For this reason, a dynamic model is essential for effective
design and performance prediction of a PTC system. A dynamic
simulation of PTC was conducted by Ji et al. [24], modeling a south
facing, one-axis tracking parabolic trough collector. The simulation
calculated variations in incidence angle of solar beam to collector
aperture, as well as the distribution of concentrated solar radiation
along the focal line. Effects of HTF mass ﬂow rate and receiver tube
length on outlet temperature and system efﬁciency were investigated. An increase in tube length augments outlet temperatures and
efﬁciency, as expected due to greater total insulation. A decrease in
mass ﬂow rate increases outlet temperature and slightly decreases
system efﬁciency.
The integration of a parabolic trough collector ﬁeld with
geothermal sources has been suggested by Lentz and Almanza

2707

[25,26]. Hot water and steam from geothermal wells can be
directly fed into an absorber pipe going through a PTC ﬁeld. The
combination of both thermal energy sources increases the volume
and the quality of (directly) generated steam for power production. Several hybrid designs have been suggested by the authors.
PTCs can also be integrated with solar cells in concentrated
photovoltaics (CPV) modules. Heat-resistant, high-efﬁciency
photovoltaic cells can be mounted along the bottom of the
receiver tube to absorb the concentrated solar ﬂux. The performance of a CPV parabolic trough system with a 37 sun concentration ratio was characterized by Coventry [27] at Australian

National University in 2003. Monocrystalline silicon solar cells
were used, along with the thermal PTC apparatus. Measured
electrical and thermal efﬁciencies were 11% and 58%, respectively,
producing a total efﬁciency of 69%. It is important to note that
uneven illumination of the solar cell modules causes a direct
decrease in the cells’ performance, and thus optical considerations must be weighed carefully.
The mature ﬁeld of parabolic trough collectors provides an
efﬁcient, relatively inexpensive power production scheme. Multiple
advances in reﬂector and receiver design have been made in the last
decade to enhance efﬁciency and reduce losses. Heat collection and
transfer methods have been modeled and tested repeatedly in order
to achieve optimal power output throughout the day. The PTC
scheme also lends itself to easy storage schemes, as well as to
simple integration with both fossil fuels and other renewable
energy sources.

4. Heliostat ﬁeld collectors (HFC)
The most recent CSP technology to emerge into commercial
utility was the heliostat ﬁeld collector design. This expensive,
powerful design has so far been incorporated in relatively few
locations around the world. The 10 MW Solar One (1981) and
Solar Two (1995) were the ﬁrst HFC plants to be demonstrated,
built in the Mojave Desert of California. They have since been
decommissioned. Other plants, such as the 11 MW PS10 and 20 MW
PS20 in Spain, and the 5 MW Sierra SunTower in California, were
recently completed.
The heliostat ﬁeld collector design features a large array of ﬂat
mirrors distributed around a central receiver mounted on a (solar)
tower. Each heliostat sits on a two-axis tracking mount, and has a
surface area ranging from 50 to 150 m2. Using slightly concave

mirror segments on heliostats can increase the solar ﬂux they
reﬂect, though this elevates manufacturing costs. Every heliostat
is individually oriented to reﬂect incident light directly onto the
central receiving unit. Mounting the receiver on a tall tower
decreases the distance mirrors must be placed from one another
to avoid shading. Solar towers typically stand about 75–150 m
height. A ﬂuid circulating in a closed-loop system passes through
the central receiver, absorbing thermal energy for power production and storage. An advantage of HFCs is the large amount of
radiation focused on a single receiver (200–1000 kW/m2), which
minimizes heat losses and simpliﬁes heat transport and storage
requirements. Power production is often implemented by steam and
turbine generators. The single-receiver scheme provides for uncomplicated integration with fossil-fuel power generators (hybrid
plants) [2].
HFC plants are typically large (10 MW and above), as the beneﬁt
from an economy of scale is required to offset the high costs
associated with this technology. They can incorporate a very large
number of heliostats surrounding a single tower. The immense solar
ﬂux reﬂected towards the receiver yields very high concentration
ratios (300–1500 suns). HFC plants can thus operate at very high
temperatures (over 1500 1C), which positively impacts collection

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

and power conversion efﬁciencies by enabling the use of higherenergy cycles.
A reﬂective solar tower design has been suggested, in which a
secondary reﬂector is mounted on the tower, and the central
receiver is grounded (Fig. 3). A review of the optics of the reﬂector

tower was presented by Segal and Epstein [28]. Since HFCs
operate at such high temperatures, the greatest losses are
incurred convectively at the receiver’s surface.
Aside from the convenience associated with having the receiver situated at ground level, the optics of the design increase the
concentration ratio, allowing the collector to be smaller and
diminish losses. Transport losses can also be lowered by situating
the turbine generator in close proximity with the receiver. Nevertheless, Segal and Epstein [29] reported that the reﬂector tower
scheme is not more efﬁcient than the solar tower regime, and that
superiority of either technology is subject mainly to economic factors.
The integration of a solar reformer with a heliostat ﬁeld array
was proposed in 2002. Solar reforming of methane with steam or
CO2 is an efﬁcient chemical heat storage method. The syngas
produced can be converted into electricity using a gas turbine or
combined cycle. The suggested reformer rests on the ground, and
has a collector mounted above it (Fig. 4). A solar reﬂector tower is
used to concentrate solar ﬂux from heliostats onto the ground
reformer. In this fashion, the power producing unit can be separated
from the concentrator ﬁeld entirely.
Landﬁll gas and biogas can be used to supplement gas produced by the reformer. The design and operation of a large-scale
reformer are discusses by Segal and Epstein [30]. The synthesis
gas produced by this technology can also be utilized for the
production of methanol.
An optimization study of an HFC system’s main parameters
was conducted by Segal and Epstein [7]. The effects of operation
temperature, heliostat ﬁeld density and the use of a secondary
reﬂector (reﬂector tower regime) on power conversion were
tested across different energy cycles (Fig. 5). The investigation
concluded that maximum overall efﬁciency of an HFC system is
reached at 1600 K, with an average ﬁeld density of 35%. The
authors emphasize that differences between large and small HFC

plants with regards to these values are negligible.
The solar tower reﬂector can also be integrated with concentrated photovoltaics (CPV). The principle behind this design is to
split the solar spectrum into PV-used and thermal-used portions. For
example, monocrystalline silicon solar cells operate at efﬁciencies
ranging between 55% and 60% at wavelengths of 600–900 nm. The

Fig. 3. Schematic diagram of solar reﬂector tower in an HFC system.
Figure reproduced with permission from ref. 7, &2003 Elsevier.

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.

rest of the light can be used for electricity generation using Rankine–
Brayton cycles, or otherwise be stored for later use. Discussion of
spectrum splitting optics and HFC–CPV hybrid design is given by
Segal et al. [31]. The study’s results show that a heat input of
55.6 MW yields 6.5 MW from the solar cells array and 11.1 MW
from a combined energy cycle. This was done under concentration
ratios of 200–800 sun.
The concept of a dual receiver for solar towers was suggested by
Buck et al. [32]. The proposed receiver is made of an open volumetric
air heater with a tubular evaporator section (Figs. 6 and 7). In this
design, the receiver has both a water heating section and an air
heating section. Water (HTF) is circulated through, evaporated in the
tubular evaporator, and is then superheated by hot air. Feed water is
also preheated using the hot air. This concept essentially combines

direct steam generation with regular water HTF operations. The
results (Table 3) of the new design demonstrate numerous beneﬁts,
which include a higher receiver thermal efﬁciency, lower receiver
temperature and lower parasitic losses. A 27% gain in annual output
is facilitated by these improvements, compared with the solar air
heating system. Separation of evaporation and superheating sections
also alleviates thermo-mechanical stress on the receiver to some
degree.
Planning the layout of a heliostat ﬁeld presents a great optimization challenge. A novel methodology for layout generation based on
yearly normalized energy surfaces (YNES) was presented by Sanchez

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

and Romero [33]. This ‘Heliostat Growth Method’ (HGM) uses the
YNES program to evaluate the usable solar energy ﬂux at each point
in a solar ﬁeld year-round, given a speciﬁc solar tower height. Using
this data, the method splits impacting factors such as shadowing,
blocking, atmospheric attenuation and others into two categories:
those associated with spatial position of the solar tower and those
affected by the geometry of the heliostat. This provides greater
insight and ﬂexibility to the ﬁeld layout process and its optimization.
A clever design for small-scale ‘tri-generation’ solar power
assisted plant was brought forth by Buck and Friedmann [34]. The
design puts together a solar–gas turbine hybrid system, which
incorporates a small heliostat ﬁeld, a receiver mounted on a solar
tower, a micro-turbine and an absorption chiller. In this regime,
electric power, heating and cooling can all be produced by the

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

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

2709

same system. System conﬁgurations were assessed for technical
performance and cost.
Forsberg et al. [35] suggested the use of liquid ﬂuoride salt as
an HTF in order to raise the heat-to-electricity conversion efﬁciency of HFCs to about 50%. The molten salt operates at
temperatures between 700 and 850 1C, delivering heat to a closed
multi-reheat Brayton cycle using N2 or He as the working ﬂuid.
Due to such high operation temperatures, thermal energy storage
as sensible heat in graphite is suggested. A schematic diagram of
such an HFC plant is shown (Fig. 8). Graphite, a low-cost solid
featuring a high heat capacity, is compatible with the ﬂuoride salt
at high temperatures. The efﬁciency boost reported by the
authors can greatly reduce electricity costs.
The combination of a single central receiver with molten salts
as the HTF generally allows the highest operation temperatures of
any CSP regime and produces electricity with the highest efﬁciencies. High-efﬁciency heat storage with molten salts enables
solar collection to be decoupled from electricity generation in a
simpler manner than water/steam systems permit [36].
The design and performance of a novel high-temperature air
receiver was presented by Koll et al. [37]. The receiver suggested
is a porous absorber module consisting of extruded parallel
channel structures of silicon carbide ceramics. The inner surface
area of the channel exceeds that of the aperture by a factor of 50.
This allows the usage of air as the exclusive HTF, despite its low
heat transfer coefﬁcient. The receiver design is modular and
promotes easy scaling. The hot air is delivered at 700 1C to a
water boiler system for steam generation. Steam can be produced

consistently at 485 1C and 27 bar, but these parameters vary
according to the system’s capacity. Using air as a heat transfer
medium greatly reduces capital investment as it is free and
readily available anywhere.

Fig. 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].

Design conditions

Annual performance

Receiver outlet temp. (1C)
Receiver efﬁciency (including recirculation losses) (%)
Air temp at blower (1C)
Air mass ﬂow (kg/s)
Annual receiver efﬁciency (including recirculation losses) (%)
Capacity factor (%)
Net annual electric energy (GWh)

Reference plant

Dual receiver plant

700
79
110

56
66.7
14.3
12.5

500
87
80
40
79.4
18.2
15.9

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

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

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

The heliostat material selection is a crucial aspect of HFC
power plant design. These large mirrors make up about 50% of the
total system’s cost and must feature high reﬂectivity and stiffness,
be light-weight, easily cleaned and corrosion resistant. Xiaobin
et al. [38] suggested the use of PVC composite plastic steel for
heliostat fabrication. This polymer material has similar properties

to metal–aluminum alloys conventionally used, but is not as
heavy, and has a signiﬁcantly longer lifetime. Its stiffness is high
relative to its weight and it is reported by the authors to be
cheaper. One signiﬁcant issue with this material is its low heat
resilience, a problem which must be contended with in order to
ensure heliostat operation temperatures can be accommodated.
Several heliostat cleaning methods are proposed by Xiliang et al.
[39], such as using highly pressurized air/water depending on
various environmental conditions.
Conventional heliostat design dictates that cost reduction is
implemented by increasing the area of the mirrors. Doing this
reduces speciﬁc drive cost while increasing the torques heliostats
experienced by wind loads. A study by Ying-ge et al. [40]
demonstrates the distribution and characteristics of heliostats’
mean and ﬂuctuating wind pressure while wind direction angle is
varied from 01 to 1801 and vertical angle is varied from 01 to 901.
Moreover, a ﬁnite element model was constructed to perform
calculations of wind-induced dynamic responses. Increased wind
torques result in higher speciﬁc weight and drive power. The
usage of torque tube heliostats (TTH) (Fig. 9) is suggested by
Amsbeck et al. [41]. TTH systems incorporate arrays of long,
narrow mirrors mounted on turning tubes that control their
elevation. An optical performance and a weight estimation of a
TTH system were carried out by the authors, and compared with a

Fig. 10. Schematic of mini-mirror array design featuring ‘ball-in-socket joint’
tracking mechanism.
Figures reproduced with permission from ref. 42, &2010 ASME.

regular HFC system of a slightly smaller area. Although the TTH

system indeed experienced smaller wind torques, it suffered an
annual energy output reduction of 3%. Furthermore, the high
number of moving elements and the more involved control make
this system hardly advantageous compared with the conventional
design.
Another novel design to help avoid heavy mirror tracking in
¨
the face of wind loads was suggested by Gottsche
et al. [42]. This
regime utilizes mini-mirror arrays (10 Â 10 cm) made of high
quality materials. Each mirror is mounted on a ball-in-socket joint
driven by a step motor (Fig. 10). The mirrors are encased in a clear
box that shields them from the wind. The purpose of this design is
to avoid wind loads and save on stiff materials (mainly steel) that
are necessary to make large heliostats resilient to wind torques.
Unfortunately, the low-cost achieved by the group was countered
by a 40% drop in optical performance compared with conventional HFC systems.
For initial planning of an HFC power plant, a general efﬁciency
evaluation tool can be quite useful. Collado [43] presented a
quick, non-speciﬁc evaluation method for annual heliostat ﬁeld
efﬁciency evaluation. The model is a combination of an analytical

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

assessment of the ﬂux density produced by a heliostat from
Zaragoza University, an optimized mirror density distribution
developed by University of Houston for the Solar One project,
and molten salt receiver efﬁciencies measured during the Solar
Two project. This model does not take into account many

impacting factors speciﬁc to a particular HFC system and is
limited in its accuracy.
Similarly, a new method for approximating geometrical parameters and sizing of the tower reﬂector regime was developed by
Segal and Epstein [44]. The method utilizes edge rays originating
from the heliostat ﬁeld boundaries and is particularly useful for
geometrical assessment of very large arrays of heliostats. The
method’s results were compared with real ﬁeld calculations and
found to be a good ﬁrst approximation regime.
A simulation using the same ‘edge ray’ principle method was
developed by Xiudong et al. [45]. Its purpose was to promote
more efﬁcient placement of heliostats and obtain a faster generating response of the design and optimization. A novel module
for the analysis of non-spherical heliostat arrangements has been
incorporated into the simulation. A toroidal heliostat ﬁeld was
designed and analyzed by the authors and proved signiﬁcantly
less efﬁcient that conventional HFC arrangements. A method for
calculating the annual solar ﬂux distribution of a given area is an
added feature, with the purpose of evaluating feasibility of crop
growth around heliostat ﬁelds.
Heliostat ﬁeld collector technology has greatly improved over
the last few decades, and continues to draw much attention as a
suitable scheme for large solar thermal plants. The exceedingly
high temperatures at which they operates it grant HFC plants
excellent efﬁciencies, while allowing them to be coupled to a
variety of applications. The high capital investment necessary for
the construction of HFC systems is an obstacle, however, and
further technological advancements in efﬁciency must be accompanied by low cost materials and storage schemes for this CSP
method to become more economical.

increased receiver tower height, which augments the cost.
A novel solution to the shading problem is discussed by Mills

and Morrison [46] at Sydney University, Australia. Their design of
the compact linear Fresnel reﬂector (CLFR) scheme features
adjacent mirrors oriented towards two separate receivers in
opposite directions (Fig. 11). The use of multiple receivers allows
a more compact reﬂector distribution, avoiding shading and
utilizing a portion of solar ﬂux that otherwise goes to waste.
Reﬂectors near the base of a receiver are always oriented towards
it. Yet, when reaching a nearly equidistant point between two
separate receivers, the mirrors from each will reverse their
orientation, allowing them to come very close together without
blocking one another. For commercial power production (greater
than 1 MW scale), it is very reasonable to have multiple receivers,
and thus the CLFR design is very useful without incurring extra
costs, especially in areas where land is limited.
A useful addition to the CLFR design is the incorporation of an
inverted cavity receiver attached to a planar array of boiling tubes
(Fig. 12). This structure allows plant operation in a direct steam
generation (DSG) regime. Mills and Morrison [47] indicate that
this receiver design bypasses receiver thermal uniformity challenges with parabolic trough DSG system. Design considerations
of the inverted cavity receiver are presented by Singh et al. [48].
This work compares thermal performance of circular and rectangular absorber tubes, as well as black nickel and black paint
coated tubes. Circular absorbers in the receiver are reported to have
a higher thermal efﬁciency by 8% compared with a rectangular
absorber. Nickel selective surface coating performed 10% better
than ordinary black paint. A heat loss study of the same variables is
also performed by the authors. Nickel selective-coated absorbers
experience a 20–30% heat loss coefﬁcient reduction. Additionally,
a double glass absorber cover is compared with a single glass cover,
and is found to reduce the heat loss coefﬁcient by 10–15% [49].
An innovative design to further limit wasted solar radiation in

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

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 transporting thermal energy to power production and storage units.
A signiﬁcant challenge with LFR systems is light blocking
between adjacent reﬂectors. Solving this issue requires either
increased spacing between mirrors, which takes up more land, or

2711

Fig. 11. Schematic diagram of the CLFR design.
Figure reproduced with permission from ref. 2, &2004 Elsevier.

Fig. 12. Schematic diagram of inverted air cavity receiver.
Source: Wikipedia.

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

New geometries for reﬂector ﬁelds are explored in this study,
with the purpose of limiting blocking/shading while maximizing
the ﬁeld layout density. The authors propose reformation of the
platform on which reﬂectors are resting (ground) into a waveshaped one (Fig. 13). Individual reﬂectors’ size/shape adjustments
based on their position in the heliostat ﬁeld are also suggested.
A concentration increase of up to 85% of theoretical maximum is
reported under this design.
Dey [51] describes several receiver design considerations for the
CLFR concept. The absorber is a basic inverted air cavity with a glass
encasing that encloses a selective surface. The central design goals
analyzed are (1) maximization of heat transfer between the absorbing surface and the steam pipes, and (2) ensuring uniform absorber
surface temperature to avoid degradation of the selective surface.
Heat calculations are presented for absorber temperature
distribution, and satisfactory absorber pipe separations and sizes
are shown to alleviate temperature differences between the ﬂuid
surface and the absorbing surface. Similar work using ﬁnite
element calculations was done by Eck et al. [52] for three separate

parts of a LFR system–the evaporator, pre-heater and superheater
(Table 4). Thermal loads for each section were modeled and
maximum temperatures were investigated. In the case of the
superheater, the maximum temperature derived was 570 1C,
exceeding the temperature limit of the absorber coating. A novel
step-by-step heat ﬂux reduction method is thus required for safe
and successful operation. Such a control system would adjust
reﬂectors to an off-focus position one by one to prevent overheating while operating at the highest allowed temperature. This
kind of sensitive, intelligent system would surely increase power
plant costs.
A study by Hoshi et al. [53] investigated the suitability of high
melting point phase change materials (PCMs) for storage use in
large-scale CLFR plants (Fig. 14a–c). Several candidates for latent
heat storage materials are discussed, and mathematical models of
charging and discharging heat storage from each are presented.
NaNO2 is emphasized as a particularly suitable contender for largescale latent heat storage due to its high melting point and low cost.
LFR technology offers many of the advantages of PTC systems
while incurring smaller reﬂector costs. It too can be easily coupled
to direct steam generation as well as molten salts for thermal
energy transport. The central receiver regime it incorporates
shrinks costs further, but tags on the challenge of maximizing
the amount solar radiation that can be collected. Innovation in
receiver design and reﬂector organization has made LFR relatively

Fig. 13. Wave platform structure for a CLFR system allows maximization of solar
radiation collected from a given area.
Figure reproduced with permission from ref. 50, &2010 Elsevier.

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

Data compiled from [52].

Heat transfer coefﬁcient (W/m2 K)
Average ﬂuid temp. (1C)
Max tube temp. (1C)
Min tube temp. (1C)
Temperature drop (K)

Pre-heater

Evaporator

Superheater

1700
140
189
142
47

860
275
325
281
44

500
440
569
455

114

Fig. 14. (a–c) Heat storage materials and their properties. (a) Heat capacity of high
melting point phase change materials. (b) Heat capacity of molten salts. (c) Media
costs of high melting point phase change materials.
Figures reproduced with permission from ref. 53, &2005 Elsevier.

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

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

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

to 1000 sun. At temperatures exceeding 1500 1C, they can produce power efﬁciently by utilizing high energy conversion cycles.
The collector type features a large parabolic-shaped dish, which
must track the sun on a two-axis tracking system to maintain
light convergence at its focal point. A receiver is mounted at the
focus, collecting solar radiation as heat. Two general schemes are
possible for power conversion; the less popular has a heat transfer
ﬂuid system connecting the receivers of several dishes, conducting
thermal energy towards a central electricity generation system. This
design is less convenient as it requires a piping and pumping
system resilient to very high temperatures, and suffers from
transport thermal losses. The more prevalent system mandates
a heat engine be mounted near/at the focal points of individual
dishes. The heat engine absorbs thermal energy from the receiver,

and uses it to produce mechanical work, which an attached
alternator then converts into electricity. A heat-waste exhaust
system must be incorporated to release excess heat from the
system. Finally, a control system is necessary to ensure matching
of the heat engine’s operation to the incoming solar ﬂux.
An advantage of this design is that the reﬂector, collector and
engine can operate as separate units, making fossil-fuel hybridization a relatively simple task. It is important to note however,
that this PDC system does not lend itself to thermal storage
methods.
The Stirling engine is often used for this application, although
gas turbines can also be employed in the Brayton or Rankine/
Brayton combined cycles. Stirling engine performance is better in
temperatures below 950 1C, whereas at higher temperatures,
combined cycle gas turbines can achieve higher efﬁciencies [54].
The operations and speciﬁcations of a 10 kW single dish-Stirling
system were described in detail by Jin-Soo et al. [55].
Due to its high concentration ratios, the parabolic dish collector
is an excellent candidate for concentrated photovoltaics. The usage
of state-of-the-art, high-cost high-performance photovoltaic cells is
justiﬁed when they are utilized at concentrations exceeding
100 sun; a large solar ﬂux focused in a small region of cells can
produce enough power to offset the high capital investment
required. GaAs and multi-junction PV cells are very expensive to
fabricate. Yet, operational module efﬁciencies exceeding 30% have
been demonstrated by multiple manufacturers and veriﬁed by the
National Renewable Energy Lab (NREL). Moreover, these PV technologies are very heat-resistant, and perform better under high
concentration ratios. Incorporating such modules into the parabolic
dish collector apparatus is fairly simple, and can yield results that
are comparable to or better than heat engine systems, potentially
with a longer lifetime. Further discussion of concentrated photovoltaics is developed in a later section.

A numerical simulation of a heat-pipe receiver for the parabolic dish collector was performed by Hui et al. [56]. Using this
type of receiver between the dish and the Stirling engine is
reported to provide power uniformly and nearly isothermally to
the engine heater. This results in improved engine performance.
Heat-pipe utilization also limits convective heat loss from the
receiver.
Parabolic dish collectors are high-cost devices: they are very
large mirrors that must feature nearly perfect concavity to
effectively concentrate solar radiation. They are also very heavy,
and their tracking system must thus be very sensitive and ﬁnely
tuned. A novel suggestion by Kussul et al. [57] to moderate the
high collector cost is to manufacture an approximated parabolic
dish using many small, ﬂat mirrors. A prototype was constructed
by the group, which contains 24 mirrors in the shape of equilateral triangles, each with a side length of 5 cm special nuts are
used to maintain required positions of nodes in the connection
points of mirror apexes. These small mirror arrangements approximate a parabolic collector in a relatively inexpensive way.

2713

At such high operation temperatures, heat losses become
extremely signiﬁcant, and must be contended with to achieve
high efﬁciencies. A detailed two-dimensional simulation of heat
transfer in a modiﬁed cavity receiver of PDC system is presented
by Reddy and Kumar [58]. Combined heat losses due to both
laminar convection and surface radiation from the receiver are
calculated by this model. The modiﬁed cavity receiver (Fig. 15a
and b) has a semi-circle shape that features a small aperture at
the dish’s focal point. The receiver is essentially hollow (air
cavity) and its inner surface is laid with absorber tubes. The
encasing of the tubes is made of insulating material.

Reddy and Kumar published another numerical analysis in
2009 [59], in which a three-dimensional model is used to
estimate receiver heat losses at different dish inclination angles
and various operating temperatures. The model evaluates heat
loss reductions realized through secondary concentrator integration. A cone collector, compound parabolic collector (CPC) and
trumpet reﬂector were compared as second stage concentrators
(Fig. 16a–c), and yielded natural convection heat loss reductions
of 29.23%, 19.81% and 19.16%, respectively.
Another thermal analysis of a PDC system was done by Nepveuat
al. [60]. The authors constructed a thermal energy conversion model
of the 10 kW Eurodish/Stirling unit erected at the CNRSPROMES
laboratory in Odeillo. The model analyzes spillage and radiation
(reﬂection and IR-emission) losses of the reﬂector, and calculates
conduction, convection, reﬂection and thermal radiation losses
through the receiver cavity (Fig. 17). A thermodynamic analysis of
a SOLO Stirling 161 engine is also presented. The model was
compared to experimental results of the solar power system and
was determined a good ﬁt.
An innovative solar thermal power approach was formulated by
Shuang-Ying et al. [61]. This design features a dish concentrator

Fig. 15. (a) Light collection and (b) general schematics of air cavity receiver in a
dish/Stirling system.
Figures reproduced with permission from ref. 58, &2008 Elsevier.

2714

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

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

7. Concentrated photovoltaics
Fig. 16. (a–c). Secondary reﬂectors for a parabolic dish reﬂector system.
Figure reproduced with permission from ref. 59, &2009 Springer.

Fig. 17. Eurodish receiver heat ﬂow and heat loss diagram.
Figure reproduced with permission from ref. 60, &2009 Elsevier.

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

A paradigm shift in PDC design suggested by Feuermann and
Gordon [62] utilizes arrays of mini-dishes coupled with ﬁber
optics that carry solar radiation to a central receiver. Each mirror
is about 20 cm in diameter and has a small ﬂat mirror at its focal
point, to which a single optical ﬁber is attached. The ﬁber
transports collected light to a central receiving unit, where it
can be converted into heat. Low attenuation ﬁbers of high
numerical aperture, coupled with mass produced highly accurate
parabolic dishes, can operate at 80% efﬁciency and yield incredibly high concentration ratios of up to 30,000 sun. Experimental
realization and ﬁeld experience of this proposed system were
carried out by Feuermann et al. [63]. One mm diameter optical
ﬁbers repeatedly transported solar ﬂux levels of 11–12 ksun to a
target as far as 20 m away. The prototype proved impervious to
dust penetration and condensation, and was reportedly constructed solely from off-the-shelf parts and customized items

The concept of concentrated photovoltaics is rapidly becoming a
dominant player in the solar power production market. In March
2010, the 330 kW ‘OPEL Solar’ (Spain) became the ﬁrst operational
utility-grade CPV power plant. CPV systems employ various light
concentration schemes to focus large amounts of solar radiation
onto small solar cell modules. Very small units of high-cost highefﬁciency solar cells are used to absorb the high incoming ﬂux,
which makes the CPV model economically competitive.
Mainstream concentrator technologies utilized are parabolic
dish collectors and Fresnel lenses. Designs using PTC [27] and HFC
[31] systems (discussed in previous sections) have been reported
as well. The type of solar cell technology used in a CPV system is
chosen according to the desired concentration level. While the
performance of most PV technologies increases with solar concentration ratios, excessive heating can be detrimental to the
efﬁciency and lifetime of solar cells. Organic and amorphous
silicon cells are generally too heat-sensitive to be used with

concentrators. Conventional monocrystalline silicon cells can
operate efﬁciently at lower concentrations (1–100 sun) without
needing active cooling mechanisms. Low concentration systems
generally feature wider acceptance angles, and in some cases do
not need to track the sun, reducing their cost.
Two-axis tracking systems are required in high concentration
systems. Gallium arsenide and multi-junction cells are better used
in medium–high concentration systems (100–300 sun, 300 sun and
above). These cells are very expensive to manufacture, but have
exhibited record conversion efﬁciencies and operate well under high
temperature. Still, heat sinks are often integrated with high-concentration CPV modules in order to alleviate high temperature
effects and prolong cell lifetime. Examples of cooling mechanisms
include direct water cooling and thermal conduction by heat pipes,
discussed by Farahat [65].
Due to very high material and manufacturing costs, multijunction cells are signiﬁcantly more expensive than silicon cells
per unit area. Yet, multi-junction cell efﬁciency can be up to 15%
greater than that of silicon cells, which can make a big difference
in performance at high solar concentrations. Furthermore, the
small PV receivers account for only a fraction of the total CPV
system cost, hence system economics may very well favor the use
of multi-junction cells.
Another recently explored concept is the Concentrating Photovoltaics and Thermal (CPVT) design. This scheme produces both
electricity and heat simultaneously in a single system. The heat
can be used for industrial heat processes, heating and cooling of
buildings, or simply to increase electricity output. A parabolic
trough CPVT design was introduced by Coventry [27], and a solar
tower design was suggested by Segal et al. [31]. Small CPVT
systems can be installed in private homes, and can feature a total
energy output of over 50% compared with 10–20% of the basic PV
panels.

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

A novel design for a miniature parabolic dish collector CPVT
system for residential use was presented by Kribus et al. [66].
Analysis of the electric and thermal performance, heat transport
system, manufacturing cost and resulting cost of energy for
domestic water heating is carried out. The reﬂector is made of a
single thermally bent glass sheet coated with silver to produce the
reﬂective surface. An external protective coating prevents exposure
of the silver to the environment. A 32% conversion efﬁciency multijunction module is mounted at the focal point, over a cooling plate
that removes the surplus heat from the cells to a coolant ﬂuid
(typically water). The heated coolant is directed to a heat exchanger
where the transported thermal energy may be used as an additional
energy product. Performance testing of a 0.95 m2 dish area under
direct insolation of 900 W/m2 yielded an electrical output of 172 W
and a thermal output of 530 W, exceeding 60% of the input energy.
Miniature dish collectors can be used to achieve very high
concentration CPV systems. Investigation of this type of system
operating at a concentration ratio of 1000 sun was presented by
Feuermann and Gordon [64]. The system features high-efﬁciency
heterojunction cells as the PV receiver and utilizes optical ﬁbers
for heat conduction towards a passive heat sink. Arrays of these
small systems can be mounted together on large, two-axis
tracking systems. The merits and identiﬁed problems of a similar
design were discussed by Anton et al. [67].
Fresnel lenses used in CPV systems are small and very thin
(3–5 mm), and are generally made of glass, plastic or acrylic resin

Fig. 18. Schematic side-view of a Fresnel lens (left) compared with a circular lens
(right).
Source: Wikipedia.

2715

(polymethylmethacrylate, PMMA). They are ﬂat on one side and
ridged on the other. The Fresnel lens structure is composed of
many concentric rings, which are thinner towards the center.
Each ring is slightly angled to concentrate incident light onto the
focal point of the lens (Fig. 18).
Linear Fresnel lenses operate in a similar manner, but feature a
focal line instead of a focal point. A linear Fresnel collector can
include an array of these lenses positioned side-by-side. The array
is mounted on a sun-tracking device. Every lens is mounted on a
small axis through the center of its length, which can orient it to
follow the sun. The entire collector unit can track the sun along
the second dimension, providing the system with a two-axis
tracking regime (Fig. 19). An optical and thermal performance
simulation for this type of system was done by Mallick and Eames
[68]. The effects of varied spacing between linear lenses within an
array on the efﬁciency are presented. Linear Fresnel lenses also
can be coupled with small secondary concentrators to minimize
the PV receiver area needed [69].
Optimization of concentration level, cell technology, receiver
size and shape and heating/cooling management is necessary to
achieve high performance systems. A study of the energetic and
thermal characteristics of a small CPV system was conducted by
Mirzabaev et al. [70]. The module was based on a Fresnel lens and
an AlGaAs–GaAs PV receiver, and compared several receiver sizes

and contact shapes (tetragonal and circular). Analysis of the
Fresnel lens solar collector thermal efﬁciency was done by Zhai
et al. [71], and was found to be about 50% when using an
evacuated tube receiver on a clear day.
One problem with the use of conventional Fresnel lenses for
concentrated photovoltaic is uneven illumination of the solar cell
receiver. Non-uniform intensity distributions can result in local
heating and ohmic drops in CPV systems, preventing maximum
power extraction. Several innovative designs to overcome this
issue have been presented in the last 5 years. Ryu et al. [72]
devised a new concept of a modular array of Fresnel lenses for
low-medium concentration CPV systems, which is based on the
concept of superposition. A two-dimensional array of lenses is
constructed. Each lens is slightly larger than the PV receiver itself.
Individual lens facets are angled to direct normal incident light
onto speciﬁc regions of the solar cell module (Fig. 20a and b).
Proper determination of the facet angle for each lens in the array

Fig. 19. Photovoltaic cell arrays encased with Fresnel lenses, mounted on a two-axis sun-tracker.
Figure reproduced with permission from ref. 68, &2007 Springer.

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

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

electricity is produced directly by solar cells, which removes the need

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

8. Concentrated solar thermoelectrics
Fig. 21. Wide acceptance angle design for cylindrical Fresnel lens.
Figure reproduced with permission from ref. 73, &2009 SPIE.

must be implemented, and can vary across different systems
(according to size, output, etc.). Mathematical evaluations of the
performance and concentration efﬁciency are presented, along
with illustrations of the new concept.
When investing in high-quality solar cells, it is desirable to
integrate them with systems that achieve very high concentrations.
At such conditions, however, Fresnel lenses have a very narrow
acceptance angle range (on the order of 711), and the system must
include very ﬁne tracking mechanisms for efﬁcient absorption to
occur. The design of a cylindrically symmetric Fresnel lens was
explored by Yu-Ting and Guo-Dung [73]. A simulation of a CPV
system incorporating this technology was carried out at high
concentration (300–400 sun). A couple of system designs was

presented. The most successful design (Fig. 21) incorporated the
cylindrical Fresnel lens, two reﬂective surfaces, a biconic lens and a
light pipe. This structure, though fairly complicated, expanded the
acceptance angle to 7101. Theoretical discussion of the optical
capabilities of a cylindrical lens was presented by Gonzalez [74].
Both a concentration level of 70% of the theoretical maximum and a
100% geometrical optical efﬁciency were reported. The lens also
featured very uniform illumination of the receiver, an important
attribute for concentrated photovoltaic systems.
The integration of solar cells with CSP technologies requires a
cautious balancing of the advantages and issues of each. On one hand,

Conversion of solar energy into electricity directly can also be
achieved using the concept of thermoelectrics. Recent developments
in thermoelectric applications have been exploring ways to utilize
CSP to generate electricity. Solar thermoelectric devices can convert
a solar thermal energy (typically waste heat) induced temperature
gradient into electricity. They can also be modiﬁed to perform
cooling or heating. One advantage of thermoelectric methods
(compared with heat engines) is their increased reliability, as such
devices could work 10–30 years with little technical problems [75].
Moreover, thermoelectric generators are a ﬂexible source of clean
energy capable of meeting a wide range of requirements.
Hybrid systems that combine thermoelectric and photovoltaic
are under development. This type of system allows harvesting of
solar radiation in both the ultraviolet and infrared ranges of the
spectrum. Such a hybrid can also reduce wasted thermal energy,
since it ‘functionalizes’ a wide temperature range for power
production. While most silicon solar cell performance begins to
degrade at temperatures approaching 100 1C, thermoelectric

devices actually perform better at temperatures over 200 1C.
A solar thermoelectric power generator typically consists of a
thermal collector and a thermoelectric generator. Heat is
absorbed by the thermal collector, then concentrated and conducted over the thermoelectric generator by a ﬂuid pipe. The
thermal resistance of the generator creates a temperature difference between the absorber plate and the ﬂuid, which is proportional to the incoming heat ﬂux. The current produced by the

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

thermoelectric generator is in turn proportional to the temperature difference.
To increase the efﬁciency of current solar thermoelectric
devices, two main things must be accomplished: (i) improved
thermal transmission of the solar collector and (ii) higher concentration of the solar radiation onto the hot side of the thermoelectric device. Since thermoelectrics made of high quality
materials are relatively expensive, a key design consideration
for these solar generators is minimal use of thermoelectric
materials. Naturally, amounts used must be adjusted in accordance with desired power requirements.
The use of solar concentrating elements can augment the
magnitude of the heat ﬂux absorbed by a thermoelectric device,
contributing to a higher temperature gradient across it. Among
the single line focusing parabolic trough collector, the compound
parabolic concentrator and the two-stage concentrator, the latter
uses a secondary receiver to further concentrate the incident solar
radiation. A design for a two-stage solar concentrator has been
proposed [76], which is well-suited to commercially available
thermoelectric devices for small scale power generation. The twostage solar concentrators comprise of a primary, one-axis PTC,
with a secondary, symmetrical CPC mounted at its focus.
Several designs have been suggested to further increase the
hot side temperature of the generator. Solar concentration must
be greater than 20 sun to effectively irradiate a thermoelectric
device [76,77]. Schematics of two solar thermoelectric regimes

that incorporate concentrators are shown (Fig. 22a and b). Both
schematics are based on the two-stage concentrator design,
where the second concentrator also acts as a receiver and can
generate a larger temperature difference across the thermoelectric device. The receiver can combine a thermionic converter (TIC)
with a thermoelectric converter (TEC) to use thermal energy more
efﬁciently (Fig. 22a). The TIC is a cylindrical cavity-type solar
receiver made of graphite, which is heated in a vacuum by the
solar concentrator. Once the TIC emitter is uniformly heated up to
1800 K, a hot side generator temperature of 1800 K can be
achieved [78]. The thermoelectric device can also be attached
directly to the absorber plate of the receiver (Fig. 22b).
The ﬁeld of solar thermoelectric power generation, its coupling
with two-stage solar concentrators in particular, is a very recent
innovation in the scope of CSP. Many solar thermoelectric designs
are not fully developed or are still in their initial stage. However,
the usefulness and diversity of applications this concept offers

Fig. 22. Schematic of two-stage concentrator design featuring a (a) thermionic
converter and (b) thermoelectric device (only).

2717

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

9. Thermal energy storage
A signiﬁcant complication with the utilization of solar thermal
power as a primary source of energy is the variable supply of solar
ﬂux throughout the day, as well as throughout the year. Although
there is a reasonable match between the hours of the day in which

both available solar energy and electricity consumption peak, nighttime energy usage must be taken into consideration. Additionally,
seasonal and weather changes greatly inﬂuence the amount of solar
thermal energy that can be harvested. An affordable, reliable energy
storage method is thus a crucial element in a successful year-round
operation of a thermal solar power plant.
The cyclical availability of solar energy determines two types
of thermal storage are necessary for maintaining a constant
supply of solar thermal power driven electricity. The ﬁrst is
short-term storage, where excess energy harvested daily is stored
for nighttime usage. The second is long-term storage in which
excess energy is stored during spring and summer months in
order to complement the smaller energy ﬂux available in winter.
Thermal energy storage can be divided into three main
categories: sensible heat storage, latent heat storage and chemical
storage. Sensible heat storage involves heating a solid or liquid
and insulating it form the environment until the stored thermal
energy is ready to be used. Latent heat storage involves the phase
change (generally solid–liquid) of the storage material. The heatinduced phase change stores a great deal of thermal energy while
maintaining a constant temperature, and can be easily utilized for
nighttime energy storage if kept under proper isolation. A plot
demonstrating sensible and latent heat storage is given (Fig. 23).
Chemical storage is implemented using harvested thermal energy
in reversible synthesis/de-synthesis endothermic reactions. The heat
‘invested’ in producing/dissociating a certain material (ammonia,
methane, etc.) can be easily stored indeﬁnitely. The reverse, exothermic reaction will release the heat with minimal losses for electricity
generation at a later time. Chemical storage is thus most suitable for
long-term or seasonal storage.
Sensible heat storage can employ a large variety of solid and
liquid materials. It can be put into practice in a direct or indirect
manner. For storage in solids such as reinforced concrete, solid

NaCl and silica ﬁre bricks, an indirect storage method must be
implemented. This type of system uses a heat transfer ﬂuid to
circulate through absorbers, collect heat and transport it to the
storage tank. The HTF is then put in thermal contact with the
storage solids, allowing them to absorb the heat convectively.
Sensible heat storage in liquids can be achieved in a direct
fashion, i.e. the heat storage liquids themselves are used as heat
transfer ﬂuids, and are transported to an insulating storage
tank after circulating through the solar absorbers. Mineral oil,
synthetic oil, silicone oil, nitrate, nitrite and carbonate salts,

Fig. 23. Sensible vs. latent heat storage.
Figure reproduced with permission from ref. 79, &2010 Elsevier.

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

as well as liquid sodium, can all be used for sensible heat storage.
Desired characteristics of ‘sensible-heat-storage-friendly’ molten
salts include high density, low vapor pressure, moderate speciﬁc
heat, low chemical reactivity and low cost. One big disadvantage
of molten salts is that they are usually quite pricey. Detailed
characteristics of storage materials (Table 5a and b) are given by
Gil et al. [79].
Latent heat storage in the solid–liquid phase transition of
materials is considered a good alternative for sensible heat storage.
From an energy perspective, storage using phase change materials
(PCM) can operate in relatively narrow temperature ranges between

charging and discharging thermal energy. Additionally, PCM materials generally feature higher densities than sensible heat storage
materials. The interest in PCM latent heat storage systems is
increasing, mainly due to potential improvements in energy efﬁciency and nearly isothermal energy storage and release. In addition
to the few commercially available PCMs today, many organic and
inorganic compounds are being investigated for latent heat storage
purposes (Table 5c–e). A disadvantage of PCMs is their low thermal

conductivity, which results in slow charge–discharge rates. One
suggested initiative for alleviating this problem involves the fabrication of PCM composite materials; mixing pure PCMs with graphite,
for example, can boost thermal conductivity and promote faster
energy storing and releasing.
Since sensible and latent thermal energy storage schemes can
only retain their energy efﬁciently for so long, the need for longterm, cross-seasonal storage is made possible by thermo-chemical
storage processes. Thermal energy storage in heat intensive
endothermic reactions has the possibility to realize higher energy
efﬁcient processes the thermal storage regimes. Potentially high
energy densities can be stored using chemical storage.
Reformation of methane and CO2 [30], metal–oxide/metal
conversions [80] and ammonia synthesis/dissociation [14,81]
are just a few examples of heat-assisted chemical reactions
that can store solar thermal energy in their endothermic reaction products and release it at a later time/place by the reverse
process. Numerous heat-storing chemical reactions are listed
(Table 5f).

Table 5
a–f. Various thermal storage materials and their properties.
Data compiled from [79].
(a) Sensible heat storage liquid materials and their properties
Storage medium

HIETC
solar salt

Mineral
oil

Synthetic
oil

Silicone
oil

Nitrite
salts

Nitrate
salts

Carbonate
salts

Liquid
sodium

Temp. (cold) (1C)
Temp. (hot) (1C)
Avg. density (kg/m3)
Avg. thermal conductivity (W/m K)
Avg. heat capacity (kJ/kg K)
Volume speciﬁc heat capacity (kWht/m3)

Cost per kWh (US$/kWh)

120
133
n/a
n/a
n/a
n/a
n/a

200
300
770
0.12
2.6
55
4.2

250
350
900
0.11
2.3
57
43.0

300
400
900
0.10

2.1
52
80.0

250
450
1825
0.57
1.5
152
12.0

265
565
1870
0.52
1.6
250
3.7

450
850
2100
2.0
1.8
430
11.0

270
530

850
71.0
1.3
80
21.0

(b) Sensible heat storage solid materials and their properties
Storage Medium

Sand-rock
Mineral Oil

Reinforced
Concrete

NaCl
(Solid)

Cast Iron

Cast
Steel

Silica
Fire Bricks

Magnesia
Fire Bricks

Temp. (cold) (1C)

Temp. (hot) (1C)
Avg. density (kg/m3)
Avg. thermal conductivity (W/m K)
Avg. heat capacity (kJ/kg K)
Volume speciﬁc heat capacity (kWh/m3)
Cost per kWh (US$/kWh)

200
300
1700
1.0
1.30
60
4.2

200
400
2200
1.5
0.85
100
1.0

200
500
2160
7.0
0.85
150
1.5

200
400
7200
37.0
0.56
160
32.0

200
700
7800
40.0
0.60
450
60.0

200
700
1820
1.5
1.00
150
7.0

200
1200
3000
5.0
1.15

600
6.0

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

Type

Phase change
temp. (1C)

Density
(kg/m3)

Speciﬁc heat
(kJ/kg K)

Thermal
conductivity
(W/m K)

Latent heat
(kJ/kg)

RT110
E117
A164

Parafﬁn
Inorganic

Organic

112
117
164

n/a
1450
1500

n/a
2.61
n/a

n/a
0.70
n/a

213
169
306

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

Phase change
temp. (1C)

Density
(kg/m3)

Speciﬁc heat
(kJ/kg K)

Thermal conductivity
(W/m K)

Latent heat
(kJ/kg)

MgCl2-6H2O

115–117

1450 (liquid, 120 1C)
1570 (solid, 20 1C)

n/a

165

Hitec: KNO3–NaNO2–NaNO3
Hitec XL: 48%Ca(NO3)2–45%KNO3–7%NaNO3
Mg(NO3)–2H2O
KNO3–NaNO2–NaNO3
68% KNO3–32% LiNO3
KNO3–NaNO2–NaNO3
Isomalt
LiNO3–NaNO3

120
130
130
132
133
141
147
195

n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a

n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a

0.570 (liquid, 120 1C)
0.598 (liquid, 140 1C) 0.694
(solid, 90 1C) 0.704 (solid, 110 1C)

n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a

n/a
n/a
n/a
275
n/a
75
275
252

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

2719

Table 5 (continued )
(d) Inorganic substances with potential use as phase change materials
Compound

Phase change
temp. (1C)

Density
(kg/m3)

Speciﬁc heat
(kJ/kg K)

Thermal conductivity
(W/m K)

Latent heat
(kJ/kg)

40%KNO3–60%NaNO3
54%KNO3–46%NaNO3
NaNO3
KNO3/KCl
KNO3
KOH
MgCl2/KCl/NaCl
AlSi12
AlSi20
MgCl2
80.5% LiF–19.5% CaF2 eutetic
NaCl
NaCO3–BaCO3/MgO
LiF
Na2CO3
KF
K2CO3
KNO3/NaNO3 eutetic

220
220
307
320
333–336
380
380
576
585
714
767
800–802
500–850
850
854
857
897
n/a

n/a
n/a
2260
2100
2.11
2.044
1800
2700
n/a
2140

2100
2160
2600
n/a
2533
2370
2290
n/a

n/a
n/a
n/a
1.21
n/a
n/a
0.96
1.038
n/a
n/a
1.97
n/a
n/a
n/a
n/a
n/a
n/a
n/a

n/a
n/a

0.5
0.5
0.5
0.5
n/a
160
n/a
n/a
1.7
5.0
5.0
n/a
2.0
n/a
2.0
0.8

n/a
n/a
174
74
266
149.7
400
560
460
452
790
492
n/a

1800 (MJ/m3)
275.7
452
235.8
94.25

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

Phase change
temp. (1C)

Latent heat
(kJ/kg)

Latent heat
(kJ/L)

Isomalt: ((C12H24O11–2H2O) þ(C12H24O11))
Adipic acid
Dimethylol propionic acid
Pentaerythritol
AMPL ((NH2)(CH3)C(CH2OH)2)
TRIS ((NH2)C(CH2OH)3)
NPG ((CH3)2C(CH2OH)2)
PE (C(CH2OH)4)

147
152
153

187
112
172
126
260

275
247
275
255
28.5
27.6
44.3
36.9

n/a
n/a
n/a
n/a
2991.4
3340 (kJ/kmol)
4602.4 (kJ/kmol)
5020 (kJ/kmol)

(f) Chemical storage materials and reactions
Compound

Material energy density

Reaction temp. (1C)

Chemical reaction

Ammonia
Methane/water
Hydroxides
Calcium carbonate
Iron carbonate
Metal hydrides
Metal oxides (Zn and Fe)
Aluminum ore alumina
Methanolation–demethanolation
Magnesium oxide

67 kJ/mol
n/a
3.0 GJ/m3
4.4 GJ/m3
2.6 GJ/m3
4.0 GJ/m3
n/a
n/a
n/a
3.3 GJ/m3

400–500
500–1000
500
800–900
180

200–300
2000–2500
2100–2300
200–250
250–400

NH3 þ DH’-1/2N2 þ 3/2H2
CH4 þ H2O’-CO þ 3H2
Ca(OH2)’-CaO þH2O
CaCO3’-CaO þ CO2
FeCO3’-FeO þ CO2
Metal xH2’-metal yH2 þ(x À y)H2
2-step water splitting: Fe3O4/FeO redox system
n/a
CH3OH’-CO þ 2H2
MgO þH2O’-Mg(OH)2

Every storage method mentioned can play an important role in
several concentrated solar power designs. The chosen storage
scheme must, however, be carefully matched to the size (total
power output) and operational procedures associated with a speciﬁc
plant, as well as to its governing environmental and economic
factors. Luckily, the developments made to date in all three thermal
storage methods offer a great diversity of materials from which one
can choose in order to meet varying necessary parameters.

10. Energy cycles
The conversion of solar thermal energy into electricity generally
requires the use of a thermodynamic cycle.
Several types of cycles are the mainstream options for heat

conversion into work. They can vary in design and process
efﬁciency, but all cycles use heat harvested from solar collectors
to power a generator for electricity production.
The most common thermodynamic cycle used is the Rankine
cycle. In this regime, heat is supplied externally (from collectors)

to a closed loop system, which usually uses water as its working
ﬂuid. Cycle operation is outlined in several repeating steps.
Working ﬂuid is pumped from low to high pressure. This requires
little input energy for the pump if the ﬂuid is a liquid. This is one
advantage of the Rankine cycle. High pressure liquid is heated in a
boiler at a constant pressure to become saturated vapor. The
vapor is then allowed to expand through a turbine generator to
produce electricity. Next, it is condensed at a constant pressure to
become a saturated liquid, and is transferred back into the pump’s
reservoir. The working ﬂuid is constantly re-used in this thermodynamic loop. If vapor temperature is not very high (wet vapor),
condensation can occur during release through the turbine, and
fast moving water droplets damage the turbine and reduce its
lifetime and efﬁciency. Rankine operations at high temperatures
produce ‘dryer vapor’, and can thus considerably increase system
performance. Solar powered Rankine cycles using low cost
collectors for clean water and power generation are reviewed
by Garcı´a-Rodrı´guez and Blanco-Ga´lvez [82].
The ‘organic’ Rankine cycle utilizes organic ﬂuid such as
toluene or n-pentane for working ﬂuids. The cycle operation

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

process is identical, but can operate at lower temperatures
(70–90 1C). These lower temperatures result in a lower thermodynamic efﬁciency, but this may be counter-balanced by the
lower heat inputs required to drive the system. Organic ﬂuids that
have boiling points above water can be used, and this may have
thermodynamic beneﬁts. A comparison of several working ﬂuids
for organic Rankine cycle operation of PTCs was carried out by
Delgado-Torres and Garcia-Rodriguez [83].
The Brayton cycle has also been adapted for CSP electricity
generation. This cycle uses a gas compressor, a combustion
chamber and an expansion turbine. General operations of the
Brayton cycle begin with ambient air being drawn into a compressor to be pressurized. It is then directed into a combustion
chamber, where it is heated at a constant pressure. Conventionally, this heating is done by burning fossil fuels, but thermal
energy harvested from solar collectors performs this task in a CSP
power plant. The heated air is allowed to expand through a gas
turbine (or a series of turbines) to produce electricity. The
compressor can be powered by the turbine generators. Excess
heat is exhausted into the atmosphere. In 2002 a hybrid open
solar Brayton cycle was operated for the ﬁrst time consistently
and effectively in the frame of the EU SOLGATE program. Air was
heated from 570 K to over 1000 K in the combustor chamber. One
clear advantage of the Brayton cycle is that air is cheap and
available everywhere. A regeneration mechanism can be incorporated to improve Brayton cycle efﬁciency. Still-warm air that has
already passed through the turbine can be circulated back
towards the compressor intake and pre-heat air before it enters
the combustion chamber. Less heat is exhausted out of the
system, and less power is consumed by the chamber’s heating
mechanism. The Brayton cycle generally operates at signiﬁcantly
higher temperatures than the Rankine cycle. Despite this fact, the
overall efﬁciencies of large-scale steam generators and gas turbines seem to be similar.

The combined cycle utilizes a hybrid of the Rankine and
Brayton cycles, and can achieve higher efﬁciencies than either.
The combined cycle uses the Rankine cycle as a bottoming cycle;
heated air is ﬁrst used to power turbines in the Brayton regime.
Excess heat, which would otherwise be exhausted into the
atmosphere, is instead employed as a heating mechanism for a
Rankine (steam) cycle. Though it is more efﬁcient, this design is
more bulky and expensive to implement. A cost-efﬁciency analysis
must be carried out for a given plant size and output in order to
evaluate economic viability. A discussion of a combined cycle CSP
design using solar tower reﬂector technology is presented by Kribus
et al. [84]. Both hybrid and solar-only power plants are investigated.
An efﬁciency study of the combined cycle was done by Donatini
et al. [85]. The project examined the combined cycle integrated in a
parabolic trough collector regime using molten salts as the heat
transfer ﬂuid.
Decreasing the cost and improving the efﬁciency of power
production cycles can greatly inﬂuence the market penetration of
concentrated solar power technologies. A few innovative energy
cycles have been discussed in the literature, which use multicomponent working ﬂuids or employ additional cycle steps to
improve efﬁciency and limit power consumption.
A multi-component working ﬂuid features variable boiling
temperatures according to its composition. This process can yield
a better thermodynamic match with different sensible heat
sources than can be achieved with a single-component ﬂuid.
The advantages of using an ammonia/water mixture as a working
ﬂuid are reviewed by Goswami et al. [86]. The mixture is utilized
in the bottoming Rankine cycle of a combined cycle operated
plant design.
An innovative addition to the combined cycle was suggested

by Kribus [87]. A solar triple cycle is proposed, the ﬁrst of which

utilizes is a magneto-hydrodynamic (MHD) cycle. This cycle
operates at very high temperatures, upwards of 2000 1C. It passes
hot ionized gas through a magnetic ﬁeld, resulting in electric
current generation. The great amount of heat is exhausted into a
Brayton and Rankine bottoming cycles connected in series. The
triple cycle needs to be integrated with an HFC design in order to
meeting the high temperature requirement. The overall peak
conversion efﬁciency of the solar triple cycle is shown to be
signiﬁcantly higher than the solar combined cycle scheme. The
sensitivity of this result to several system parameters and
the technological feasibility of the triple cycle are examined by
the authors.
The improvement of well-understood energy cycles and the
development of new ones greatly extend the potential of all nearly
all concentrated solar power production regimes. The contributions
of advanced/high energy cycles to the overall thermal-to-electric
power conversion efﬁciency can be very signiﬁcant, and help bring
CSP closer to the realm of grid-parity. It is important to note that
relative costs associated with this step become quite considerable
with increased levels of sophistication, a fact that must be weighed
against the beneﬁts such clever designs provide.

11. Applications
In addition to the main objective of electricity production,
concentrated solar power technologies offer a large variety of
applications for which solar thermal energy can be harnessed.
Industrial heat processes, chemical production, salt-water desalination, heating and cooling are just a few examples of the
plethora of available applications that can be implemented using

CSP technologies. It is important to note that some applications
are CSP technology selective – they require integration with a
speciﬁc CSP design – while others can be coupled to several of the
regimes discussed in this article.
The use of solar thermal power for water desalination and
puriﬁcation has been discussed repeatedly in the literature. The
fact that regions of the world where clean drinking water is scarce
also have an abundance of solar radiation, which makes this CSP
application very worthwhile. Desalination is generally done by
evaporating salt-water to leave salt behind, then condensing salt
free vapor back into its liquid state. The process of heating large
amounts of water for drinking and agricultural purposes requires
immense amount of energy. Concentrating solar radiation and
converting it to heat is an efﬁcient method by which this process
can be achieved using emission-free, renewable energy. In addition to boiling the water, thermal power could be used to power
absorption chillers, thus using the same power source both for
evaporation and condensation of water. Several plant designs for
solar powered desalination, detoxiﬁcation and disinfection
of water are presented by Blanco et al. [88]. Designs for both
large-scale and small-scale operations are discussed. Solar waterdetoxiﬁcation schematics are presented, which are based on the
concept of using near-ultraviolet visible spectrum bands to
promote oxidizers generation. Solar water disinfection utilizes
the same method, but incorporates a supported photocatalyst to
generate powerful oxidizers to control and destroy pathogenic
water organisms. A different desalination design by Alrobaei [89]
serves the same purpose using parabolic trough collectors
coupled to a gas turbine operating in the combined Rankine/
Brayton cycle.
A novel application of CSP was presented by Perez-de-losReyes et al. et al. [90], where an array of six parabolic trough
collectors were used to harvest thermal energy for disinfestation

of greenhouse soils. The system was able to bring soil temperature up to 60 1C, and was reported effective by the authors.

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

CSP technologies can supply electricity and heat for chemical
production processes. The production of hydrogen using concentrated solar power is discussed by Glatzmaier and Blake [91]. The
authors compare two separate processes involving concentrated
solar power and the electrolysis of water. In one regime, CSP is
used to produce alternating current electricity, which is then
supplied to an electrolyzer operating in ambient temperature. The
other method utilizes high thermal electrolysis of steam. This
regime was operated at about 1273 K and, thermodynamically,
required less energy than ambient temperature electrolysis.
A solar collector can provide both AC electricity and thermal
energy to the system in this design.
Heat conversion into electricity followed by the electrolysis of
water is a process that involves several lossy steps and thus has a
low overall efﬁciency. Kolb et al. [92] suggested utilizing solar
towers for large scale production of hydrogen. The authors
proposed an alternative design, by which hydrogen is produced
using a thermo-chemical process. This regime features an HFC,
a solid-particle receiver, a particle thermal energy storage system
and a sulfuric acid cycle. Such a thermo-chemical plant is said to
produce hydrogen at a much lower cost than solar-electrolyzer
plants of similar size. Hydrogen production is an effective chemical storage medium for thermal energy, and can be used for many
industrial processes as well.
The production of zinc can also be achieved using CSP
technology. A 300 kW solar chemical pilot plant was demonstrated in the framework of the EU-project SOLZINC [80]. Production was implemented using a carbothermic reduction process of
zinc oxide. This process makes zinc production possible at

temperatures of 1300–1500 K, compared with the ZnO dissociation process, which requires temperatures exceeding 2000 K.
A ‘beam-down’ HFC regime was used to concentrate solar radiation onto a dual-cavity solar chemical reactor. The top cavity is a
solar absorber, and the bottom one is a reaction chamber containing a ZnO/C packed bed. Demonstration of the plant yielded
50 kg/h of 95% purity zinc. The measured conversion efﬁciency
was 30%. Zinc can be used in batteries and fuel cells, and can be
reacted with water to produce high purity hydrogen gas. This is
an exothermic reaction, and can itself be used for power generation, making zinc a possible thermo-chemical storage candidate.
The product of this reaction is in turn ZnO, which can then be
used again for zinc production.
A process for carbon dioxide recycling was reviewed by
Hartvigsen et al. [93]. Co-electrolysis of CO2 and steam can be
applied to produce synthesis gas in a large-scale fashion. This
process not only reduces CO2 emissions into the atmosphere, but
can utilize syngas for further clean energy production. Carbon
dioxide can be recovered from concentrated sources, such as fossil
power plants. Using high concentration CSP technologies for
endothermic electrolysis reactions can employ both thermal and
electrical inputs such that the conversion efﬁciency within the
solid oxide electrolysis cell is 100%. Large-scale implementation
of synthetic fuel production from CO2 enables greater use of
intermittent renewable energy sources.
The large amount of thermal energy that can be harvested using
solar concentrators makes them a lucrative option for integration
with industrial heat processes. A substantial fraction of these processes run below 300 1C, an operational temperature achievable by
most solar concentrator regimes. An article discussing heat process
integration of parabolic trough systems in Cyprus was presented by
Kalogirou [16]. CSP can be integrated with existing fossil fuel power
plants, and provide thermal energy to aid their operation. An example
is presented by Mills et al. [47], in which a linear Fresnel reﬂector
plant supplies heat to a coal-ﬁred power station.

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

2721

discussed is used for forming of sheet metals. Utilization of CSP
for this process is reported to be efﬁcient and cost-effective.
Thermal treatment of crude oil using a parabolic trough collector
system was suggested by Mammadov et al. [95].
Concentrated solar energy can also be used for driving the
endothermic reaction that produces lime (calcination reaction).
Running this reaction at above 1300 K is reported to reduce
emissions of the process by 20–40%, depending on the manufacturing plant [96]. An economic assessment for a large-scale (25 MW)
plant based on this process found estimates lime cost to be roughly
twice the current price of conventionally produced lime. This
process produces very high purity lime, and its prices might be
competitive with fossil-fuel based calcination processes for chemical
and pharmaceutical sectors requiring unadulterated lime.
Solar power can be utilized for temperature control of buildings, providing both heating and cooling mechanisms. A high
efﬁciency solar cooling process is outlined by Gordon and Choon
Ng [97]. A cascade of mini-dish collector and gas micro-turbine
produces electricity that drives a mechanical chiller, with turbine
heat rejection running absorption chiller. A special feature of this
system is that energy can be stored compactly as ice. The
compactness of the solar mini-dish system is conducive for
small-scale ultra-high-performance solar cooling systems.
The utilization of Fresnel lenses was also suggested for lighting
and temperature control of buildings [98]. A collection system using a
Fresnel lens concentrator and a solar receiver generally absorbs
between 60% and 80% of incoming radiation. The remaining solar

ﬂux can be distributed in the interior space for illumination and
heating needs. On days when solar radiation is high, this provides
cooling of interior spaces as well as brightness control. During low
solar intensity periods, the absorber can be shifted off-focus to permit
100% of light to be distributed around the interior (Fig. 24a–c). The
receiver can be of PV type, thermal type or a hybrid of the two, and
will collect solar energy for heat and/or electricity generation.
A parabolic trough collector system was constructed at the
Carnegie Mellon University to study the potential of this CSP
regime in solar heating and cooling [99]. The collective area of the
mirrors was 52 m2. The collector system was coupled to a 16 kW
double effect, water–lithium bromide (LiBr) absorption chiller
and a heat recovery heat exchanger. Generation of hot and chilled
water was available depending on the season. Under optimal
design, the system was able to achieve 39% of cooling and 20% of
heating energy for the interior space off the building it was
connected to (Pittsburgh, PA).
The design of a solar absorption refrigeration system directly
powered by a LFR concentrator has also been suggested [100].
Evaluation of the technical feasibility of LFR integrated solar-GAX

Fig. 24. a–c. Receiver shifting from focus of linear Fresnel lens can be used to
manage the amount of solar radiation introduced into a building, providing
temperature control. Dashed lines represent diffuse sunlight. (a) Receiver is in
focus, blocking light. (b) Receiver is out of focus. (c) Ventilation mode [98].

2722

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

cycle is carried out. A parametric study for several design conﬁgurations is performed in order to obtain optimal operation conditions.
The study validates this technology as more than satisfactory; the
numerical simulation demonstrated that this scheme answers both
quantity and quality of the advanced cooling system’s energy
demands. Furthermore, the operation conditions obtain higher global
system efﬁciencies than previously used technologies. For example,
the LFR system experienced a 17.9% efﬁciency increase compared
with single effect water–lithium bromide cycle coupled in an indirect
form with a PTC system.
A great deal of work has also been done to develop small-scale,
solar powered food (fruit, vegetables and nuts) dryers that can be
built with local materials [101–107]. However, the existing dryer
designs are suited to cloudless, dry environments and they dry
too slowly in hazy situations, typical of many tropical developing
countries. Excessively slow drying allows product degradation
caused by microbial decay, insects and naturally occurring
enzymes. Some existing designs are also expensive and relatively
inefﬁcient, and have low capacity (o50 kg/day).
Adding a solar concentrating surface increases the heat output
of solar devices operating in cloudy or hazy conditions [108].
With indirect solar dryers this can be accomplished by adding
glazed concentrated solar panels to the system. Concentrating
solar panels can be used to inexpensively increase the heat output
for indirect dryers. Additionally, they can be used to focus a
greater light ﬂux onto the drying zone in direct dryers, allowing
them to operate in low-insolation environments. The reﬂective
surfaces can be as sophisticated as precision-machined, polished
surfaces or as simple as cardboard covered in aluminum foil.
The development of a multitude of CSP applications is beneﬁcial in many regards; such applications help turn many carbon

emitting industrial processes into ‘clean’ ones, conserve large
amounts of electricity that would otherwise be used up and
promote a general environmentally friendly approach to energy
consumption for both industries and individuals. Furthermore,
the growing number of these applications aids CSP technologies
in taking root, increasing the demand for solar thermal power and
advancing it into world markets.

12. Discussion
The variety of available CSP technologies and the advancements made in each can bring a sense of uncertainty as to which
technology works best. This is a complicated issue because of the
many factors that need to be considered in selecting a particular
CSP design. Every regime features advantages and disadvantages
that must be accounted for in accordance with the size, location,
purpose and budget of the speciﬁc CSP plant one wishes to build.
Advantages of the parabolic trough collector CSP regime
include relatively low costs, mature and well-tested technologies
and easy coupling to fossil fuel/geothermal energy sources. PTC
systems are becoming more efﬁcient with the incorporation of
novel receiver designs such as the heat pipe receiver, which
signiﬁcantly limit convective heat losses while reducing receiver
cost. The reinforcement of PTCs with a light ﬁberglass structures
grants them great stability against wind loads, which further boosts
the efﬁciency as it provides for more accurate sun-tracking. The
incorporation of direct steam generation into PTC systems is
generally a very positive scheme to produce high quality steam at
a constant rate throughout daylight hours, and the usage of water
as a heat transfer ﬂuid is generally cheaper than synthetic oils or
ionic liquids. That being said, water is a more volatile substance
than other HTFs and will exert more stress on PTC absorber pipes,

which may increase maintenance costs. It also needs to be readily
available at the site, since, unlike oils and molten salts, it is being

directly converted into steam for power production, and must thus
be constantly replenished. Water transport costs are thus another
issue that requires attention. Using the Recirculation DSG mode in
PTC operation will aid water conservation to some degree. The
choice to use synthetic oils may be the best option in a PTC site
where water is not abundant. While ionic liquids can be used as
heat transfer media, they are very expensive to manufacture and
may thus be better suited for higher temperature operations, such
as those of heliostat ﬁeld collectors. The operational temperatures
of PTCs can exceed 400 1C, high enough for a plethora of industrial
heat processes, yet too low for the more efﬁcient, high energy
conversion cycles available for power production. Despite the
drawbacks mentioned, it should be noted that the maturity and
successful experience to date with PTC technology put it at the
forefront of CSP regimes. While other CSP methods may exceed PTC
efﬁciencies or be better geared towards storage and applications,
the fact that large (upwards of 100 MW) power stations based on
the PTC scheme have been operational for several years and
continue to be built proves this technology both successful and
economical.
An interesting comparison can be made between the concepts
of linear Fresnel reﬂectors and parabolic trough collectors. LFR
systems prove the cheapest of all CSP regimes, utilizing ﬂat
mirrors instead of concave ones, and having incorporating centralized receiver systems that save on receiver material. Though
they reach an operational temperature of only about 300 1C, they
can still be used in a variety of applications. The use of DSG works
well with LFRs, and the reasoning needed to select a particular

type of HTF for this type of method is very similar to that of PTCs.
A multitude of phase change materials have been proposed for
use in LFR latent heat storage systems. Although these substances
are costly, they can preserve thermal energy effectively for overnight usage. The shading issue that accompanies LFR systems is a
maximization problem to which many solutions have been
suggested. The compact LFR regime greatly reduces shading
between neighboring reﬂectors, and allows signiﬁcantly greater
collection of available sunlight. The formation of a wave-shaped
platform further enhances solar radiation collection. The inverted
air cavity receiver is reported to have substantial mitigating
effects over heat loss in LFR during LFR operation, an important
feature that can help boost thermal efﬁciency. The coating of
absorber tubes with Nickel also aids the heat loss issue, and the
two could be used in tandem for maximum heat loss reduction.
The linear Fresnel reﬂector method is suited for lower efﬁciencies
than the rest of its CSP counterparts, but it does so with the
beneﬁts of a signiﬁcantly more affordable technology.
The relatively young but very powerful CSP concept of heliostat ﬁeld collectors has come leaps and bounds over the last few
decades. The immense ﬂux a large collection of heliostats can
direct towards the central receiving unit generates very high
temperatures (up to 2000 1C), and can thus operate very efﬁciently using complex energy conversion cycles, such as the
combined cycle and the magneto-hydrodynamic (MHD) cycle.
High operation temperatures may boost electricity production
efﬁciencies, but are accompanied with both a higher thermal
stress on many components of the HFC system and a challenging
convection heat loss problem. The usage of air as a heat transfer
medium becomes available at these high temperatures, which
helps relieve some of the stress heated liquids would exert on
system components and signiﬁcantly reduces HTF costs. This
hybridized air–water heating system can produce steam at very

high temperatures (485 1C) at a constant rate. The special design
suggested for a receiving unit that has a very large inner surface
area compared with its aperture is an excellent solution to help
minimize convective losses, but will increase costs due to its
complicated structure. The dual receiver concept for solar towers

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

is another novel design that can help boost the total power output
by a signiﬁcant portion. The quest for cheap materials for heliostat
fabrication is a crucial one, as the large mirrors can make up close to
half the cost of an HFC plant. The use of PVC composite plastic steel
offers a light yet stiff structure, which helps ease stress on mirror
trackers while increasing their accuracy (high stiffness materials are
more wind resistant). The torque tube heliostat (TTH) scheme
suggested for wind load reduction is not very effective; it increases
the cost and decreases the energy output of the system without
signiﬁcantly diminishing wind stress. The suggested use of minimirror arrays resulted in similar results. Experimentation and
modeling for non-spherical arrangements of heliostat ﬁelds presents
some potential to increase the amount of solar ﬂux collected from a
given area, but the great height of the solar tower makes heliostat
shading a non-vital issue. The design incorporating a reﬂector tower
and a ground receiver is helpful in reducing transport losses, and
makes good organizational sense. The HFC scheme can easily couple
to all three thermal storage methods discussed, giving it a big
advantage over other CSP regimes. It is, however, very costly, and
large amounts of power must be produced at high conversion
efﬁciencies to make HFCs a more economically viable technology.
The parabolic dish collector system operates somewhat differently compared with the aforementioned CSP regimes, as very

large dish is a power generating system within itself. The mounting
of a Stirling engine (or a Brayton/combined cycle engine) at a dish’s
focus allows it to operate at very high temperatures throughout the
day (usually up to 1000 1C). PDCs are heavy and expensive
structures that must track the sun very accurately to fulﬁll their
maximum potential. The structural design to incorporate many
small mirrors to form the large dish can help mitigate some of the
required costs. The use of an intermediate heat pipe receiver as a
link between the reﬂective dish and the heat engine can be quite
positive, as it promotes uniform and nearly isothermal power
delivery to the heat engine, boosting its efﬁciency. The heat pipe
receiver also helps mitigate convective heat losses. The suggested
modiﬁed air cavity receiver can serve a similar purpose. The use of
heat engines and high energy conversion cycles makes PDC power
production highly efﬁcient. PDC systems do not require the use of
heat transfer media, which helps decrease their cost. The ﬂip side
of this coin is the fact that PDCs cannot be easily coupled to
thermal storage methods, a very serious disadvantage in the scope
of large power production plants. The use of thermoelectric
materials with parabolic dish collectors is an interesting and fresh
idea, but current efﬁciencies of this scheme are quite low and
further investigation of thermoelectric materials and their integration with CSP technologies must be carried out. The mini-dish
concept for CSP is reported to yield record efﬁciencies and
fantastically high concentration ratios, while maintaining fairly
low system costs. The development of this concept in the coming
years may be proved the best execution of the PDC concept.
The up-and-coming ﬁeld of concentrated photovoltaics presents a medium between CSP and photovoltaics that shows great
promise. State-of-the-art solar cells can be coupled to any of the
four main CSP regimes in order to absorb very high solar
concentrations that can be directly converted into current. High

quality silicon cells can be used at concentration of up to 100 sun
without exhibiting degradation in efﬁciency. For higher solar
concentrations, multi-junction cells can be utilized. The costbeneﬁt analysis of CPV systems takes into account the price of
both the CSP method used and the solar cells chosen for particular
systems. The costs of the latter are generally quite high and must
be offset by high conversion efﬁciencies to make economic sense.
Silicon cells also require a cooling mechanism at higher concentrations, which may result in their ‘out-the-door’ cost to become
similar to that of the very expensive multi-junction cells. It seems
that on the whole, photovoltaic power production is less efﬁcient

2723

than CSP, but the latter comes with much higher initial capital
investments.
The integration of Fresnel lenses with solar cells is thus a great
venture, since the lenses are relatively cheap to manufacture and
can concentrate light very well. Uniform illumination issues were
considered by several researchers, to which the answer of cylindrically symmetrical Fresnel lenses proved a formidable solution.
Fresnel lens CPV systems that can track the sun have been developed,
to further enhance radiation collection and boost power output
throughout the day. The concentrated photovoltaic thermal regime
is also of interest, as it permits power harvesting of both regimes
simultaneously and can result in extremely high conversion efﬁciencies. The mounting of solar cells along the absorber tube of PTC
systems, or at a portion of the focal region of parabolic dishes (and
mini-dishes), has been shown to be quite successful. The installment
of PV cells on an HFC receiver for high energy photon absorption
made signiﬁcant contributions to the overall system efﬁciency.
Unlike CPV systems, CPVTs can store a large portion of collected
energy for later use, but the trade-off from this advantage is based
in the HTF costs, which CPV systems do not have. The ﬁeld of

concentrated solar thermoelectrics seems to draw much attention as
well, but is currently in its infancy developmental stages and is far
from commercial power generation capabilities in any scale.
The great variety of application that can be incorporated into
concentrated solar power provides further incentive to invest in
it. Industrial processes can utilize thermal energy directly to save
on the costs of fossil fuels while maintaining an environmentally
conscientious image. Desalination of water could be done cheaply
(in the long run), and temperature control of homes could begin
producing power instead of consuming it. In agriculture, CSP can
be used for food drying, roasting of beans and nuts and cooking.
Furthermore, concentrated solar power can be used for sterilization of surgical tools in remote areas. The CSP applications
mentioned in this work are all novel ideas that are potentially
very useful, but each of them (like the CSP technologies that fuel
them) must stand the test of economics in order to penetrate
world markets and become universal.

13. Conclusion
Over the past few decades, great progress has been made in
every facet of concentrated solar power technology. Striving
towards a sustainable, ‘clean’ energy based culture has instilled
many with the drive to help rid society of its dependence on fossil
fuels. With the sun being an obvious and overabundant form of
renewable energy, it is no wonder that it has been the subject of
so much attention, especially at the turn of the 20th century. The
variety of technologies with which we can harness the sun’s
energy continues to grow, and improvements in every element of
each concentrated solar power production regime are constantly
added onto form more efﬁcient, robust, economical and environmentally safe facilities.
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