Concentrated solar power plants: Review and design methodology
H.L. Zhang
a,
n
, J. Baeyens
b
, J. Degr
eve
a
, G. Cac
eres
c
a
Department of Chemical Engineering, Chemical and Biochemical Process Technology and Control Section, Katholieke Universiteit Leuven, Heverlee 3001, Belgium
b
School of Engineering, University of Warwick, Coventry, UK
c
Facultad de Ingenierı
´
a y Ciencias, Universidad Adolfo Iba
´
n
˜ez,
Santiago, Chile
article info
Article history:
Received 17 November 2012
Received in revised form
24 January 2013
Accepted 26 January 2013
Available online 15 March 2013
Keywords:
Concentrated solar power plants
Design methodology
Solar towers
Hourly beam irradiation
Plant simulation
abstract
Concentrated solar power plants (CSPs) are gaining increasing interest, mostly as parabolic trough
collectors (PTC) or solar tower collectors (STC). Notwithstanding CSP benefits, the daily and monthly
variation of the solar irradiation fl ux is a main drawback. Despite the approximate match between
hours of the day where solar radiation and energy demand peak, CSPs experience short term variations
on cloudy days and cannot provide energy during night hours unless incorporating thermal energy
storage (TES) and/or backup systems (BS) to operate continuously. To determine the optimum design
and operation of the CSP throughout the year, whilst defining the requi red TES and/or BS, an accurate
estimation of the daily solar irradiation is needed. Local solar irradiation data are mostly only available
as monthly averages, and a predictive conversion into hourly data and direct irradiation is needed to
provide a more accurate input into the CSP design. The paper (i) briefly reviews CSP technologies and
STC advantages; (ii) presents a methodology to predict hourly beam (direct) irradiation from available
monthly averages, based upon combined previous literature findings and available meteorological data;
(iii) illustrates predictions for different selected STC locations; and finally (iv) describes the use of the
predictions in simulating the required plant configuration of an optimum STC.
The methodology and results demonstrate the potential of CSPs in general, whilst also defining the
design background of STC plants.
& 2013 Elsevier Ltd. All rights reserved.
Contents
1. Introduction ......................................................................................................467
1.1. Solar irradiance as worldwide energy source . . . ...................................................................467
1.2. Concentrated solar power plants................................................................................467
2. CSP technologies . . ................................................................................................467
2.1. Generalities . . ..............................................................................................467
2.1.1. Solar power towers . . . ................................................................................468
2.1.2. Parabolic trough collector ..............................................................................469
2.1.3. Linear Fresnel reflector ................................................................................470
2.1.4. Parabolic dish systems . ................................................................................470
2.1.5. Concentrated solar thermo-electrics ......................................................................471
2.2. Comparison of CSP technologies ................................................................................471
3. Past and current SPT developments ...................................................................................472
4. Enhancing the CSP potential . . .......................................................................................472
4.1. Thermal energy storage systems ................................................................................472
4.2. Backup systems .............................................................................................473
5. Computing global and diffuse solar hourly irradiation. ....................................................................474
5.1. Background information . .....................................................................................474
5.2. The adopted model approach and equations . . . ...................................................................474
5.2.1. Estimating the daily irradiation . . . .......................................................................474
5.2.2. Sequence of days .....................................................................................475
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.rser.2013.01.032
n
Corresponding author. Tel.: þ32 16 322695; fax: þ32 16 322991.
E-mail address: Zhanghl.lily@gmail.com (H.L. Zhang).
Renewable and Sustainable Energy Reviews 22 (2013) 466–481
5.2.3. Estimation of the hourly diffuse and beam radiation .........................................................475
5.2.4. Shortcut estimates, based on recorded temperatures.........................................................475
6. Model parameters . . ...............................................................................................476
6.1. Common measurement methods of solar radiation .................................................................476
6.2. Available information.........................................................................................476
6.3. Selected locations............................................................................................477
7. Results and discussion ..............................................................................................477
7.1. Calculations of H
0
, H and H
b
...................................................................................477
7.2. Methodology to apply the predictions in CSP design ................................................................478
8. Conclusions ......................................................................................................480
References . . . ........................................................................................................480
1. Introduction
1.1. Solar irradiance as worldwide energy source
More energy from the sunlight strikes the earth in 1 h than all
of the energy consumed by humans in an entire year. In fact, solar
energy dwarfs all other renewable and fossil-based energy
resources combined.
We need energy – electrical or thermal – but in most cases
where and when it is not available. Low cost, fossil-based
electricity has always served as a significant cost competitor for
electrical power generation. To provide a durable and widespread
primary energy source, solar energy must be captured, stored and
used in a cost-effective fashion.
Solar energy is of unsteady nature, both within the day (day–
night, clouds) and within the year (winter–summer). The capture
and storage of solar energy is critical if a significant portion of the
total energy demand needs to be provided by solar energy.
Fig. 1 illustrates the world solar energy map. Most of the
countries, except those above latitude 451N or below latitude
451S, are subject to an annual average irradiation flux in excess of
1.6 MW h/m
2
, with peaks of solar energy recorded in some ‘‘hot’’
spots of the Globe, e.g., the Mojave Desert (USA), the Sahara and
Kalahari Deserts (Africa), the Middle East, the Chilean Atacama
Desert and North-western Australia.
1.2. Concentrated solar power plants
Concentrated solar power plants are gaining increasing interest,
mostly by using the parabolic trough collector system (PTC),
although solar power towers (SPT) progressively occupy a signifi-
cant market position due to their advantages in terms of higher
efficiency, lower operating costs and good scale-up potential.
The large-scale STC technology was successfully demon-
strated by Torresol in the Spanish Gemasolar project on a
19.9 MW
el
-scale [2].
Notwithstanding CSP benefits, the varying solar radiation flux
throughout the day and throughout the year remains a main
problem for all CSP technologies: despite the close match
between hours of the day in which energy demand peaks and
solar irradiation is available, conventional CSP technologies
experience short term variations on cloudy days and cannot
provide energy during night hours. In order to improve the overall
yield in comparison with conventional systems, the CSP process
can be enhanced by the incorporation of two technologies, i.e.,
thermal energy storage (TES) and backup systems (BS). Both
systems facilitate a successful continuous and year round opera-
tion, thus providing a stable energy supply in response to
electricity grid demands. To determine the optimum design and
operation of the CSP throughout the year, whilst additionally
defining the capacity of TES and required BS, an accurate estima-
tion of the daily solar irradiation is needed. Solar irradiation data
for worldwide locations are mostly only available as monthly
averages, and a predictive conversion into hourly data and
direct irradiation is needed to provide a more accurate input
into the CSP design. Considering that a CSP plant will only
accept direct normal irradiance (DNI) in order to operate, a
clear day model is required for calculating the suitable
irradiation data.
The procedure, outlined in the present paper, combines pre-
vious theoretical and experimental findings into a general method
of calculating the hourly beam irradiation flux. The basis was
previously outlined by Duffie and Beckmann [3], and uses the Liu
and Jordan [4] generalized distributions of cloudy and clear days,
later modified by Bendt et al. [5], then by Stuart and Hollands [6]
and finally by Knight et al. [7].
The present paper has therefore the following specific
objectives:
review the CSP technologies and discuss solar power tower
advantages compared to the other technologies;
estimate the hourly beam irradiation flux from available
monthly mean global irradiation data for selected locations,
and compare the results obtained of monthly data with
calculations from the temperatures recorded at the locations;
select an appropriate plant configuration, and present design
preliminary recommendations using predicted hourly beam
irradiation data.
In general, the study will demonstrate the global potential of
implementing the SPT technology, and will help to determine the
most suitable locations for the installation of SPT plants.
2. CSP technologies
2.1. Generalities
Concentrated solar power (CSP) is an electricity generation
technology that uses heat provided by solar irradiation concen-
trated on a small area. Using mirrors, sunlight is reflected to a
receiver where heat is collected by a thermal energy carrier
(primary circuit), and subsequently used directly (in the case of
water/steam) or via a secondary circuit to power a turbine and
generate electricity. CSP is particularly promising in regions with
high DNI. According to the available technology roadmap [8], CSP
can be a competitive source of bulk power in peak and inter-
mediate loads in the sunniest regions by 2020, and of base load
power by 2025 to 2030.
At present, there are four available CSP technologies (Fig. 2):
parabolic trough collector (PTC), solar power tower (SPT), linear
Fresnel reflector (LFR) and parabolic dish systems (PDS). Addi-
tionally, a recent technology called concentrated solar thermo-
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22 (2013) 466–481 467
electrics is described. These CSP technologies are currently in
medium to large-scale operation and mostly located in Spain and
in the USA as shown in Fig. 3. Although PTC technology is the
most mature CSP design, solar tower technology occupies the
second place and is of increasing importance as a result of its
advantages, as discussed further.
2.1.1. Solar power towers
Solar power towers (SPT), also known as central receiver
systems (CRS), use a heliostat field collector (HFC), i.e., a field of
sun tracking reflectors, called heliostats, that reflect and concen-
trate the sunrays onto a central receiver placed in the top of a
fixed tower [2,9]. Heliostats are flat or slightly concave mirrors
Fig. 1. World solar energy map [1].
Nomenclature
Abbreviations
BS Backup system
CRS Central receiver system
CSP Concentrated solar power plant
CLFR Compact linear Fresnel collector
DNI Direct normal irradiance
DSG Direct steam generation
HCE Heat collector element
HFC Heliostat field collector
HTF Heat transfer fluid
ISCC Integrated solar combined cycle
LFR Linear Fresnel reflector
NREL National Renewable Energy Laboratory
PDC Parabolic dish collector
PTC Parabolic trough collector
TES Thermal energy storage
S&L Sargent and Lundy
SNL Sandia National Laboratories
STC Solar tower collector
Symbols
a Parameter defined by Eq. (17)
b Parameter defined by Eq. (18)
d
r
The inverse relative distance Earth–Sun
F Cumulative distribution function or fraction of days in
which the daily clearness index in less than a certain
specific value;
G
SC
the solar constant¼1367 W/m
2
, as energy of the sun
per unit time received on a unit area of the surface
perpendicular to the propagation direction of the
radiation, at mean earth-sun distance, outside of the
atmosphere
H
0
the extra-terrestrial radiation (MJ/m
2
day)
H
o,av
The monthly average of H
0
H The daily total radiation obtained from the registered
measurements
H
av
The monthly average of H
H
d
The daily diffuse radiation
I The hourly radiation
I
d
The hourly solar diffuse radiation
I
b
The hourly solar beam radiation
I
0
The hourly extraterrestrial radiation
K
T,av
Monthly average clearness index
K
T
Daily clearness index;
k
T
Hourly clearness index;
K
T,min
Minimum daily clearness index
K
T,max
Maximum daily clearness index
K
RS
Hargreaves adjustment coefficient (1C
0.5
) (0.16/0.19)
n The nth-day of the year
n
dk
Number of the day of the month (1, 2, y nd
k
)
ndm Number of the days in a certain month (31, 30 or 28)
r
t
The ratio of hourly to total radiation
r
d
The ratio of hourly diffuse to daily diffuse radiation
T
max
Maximum air temperature (1C)
T
min
Minimum air temperature (1C)
w
s
The sunset hour angle (rad)
w The hour angle of the sun (rad)
d
The solar declination angle (rad)
g
Parameter that defines the exponential distribution
proved by Bouguer law of absorption of radiation
through the atmosphere
ø Latitude of the location (rad)
x
Dimensionless parameter, defined by Eq. (9)
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22 (2013) 466–481468
that follow the sun in a two axis tracking. In the central receiver,
heat is absorbed by a heat transfer fluid (HTF), which then
transfers heat to heat exchangers that power a steam Rankine
power cycle. Some commercial tower plants now in operation use
direct steam generation (DSG), others use different fluids, includ-
ing molten salts as HTF and storage medium [9]. The concentrat-
ing power of the tower concept achieves very high temperatures,
thereby increasing the efficiency at which heat is converted into
electricity and reducing the cost of thermal storage. In addition,
the concept is highly flexible, where designers can choose from a
wide variety of heliostats, receivers and transfer fluids. Some
plants can have several towers to feed one power block.
2.1.2. Parabolic trough collector
A parabolic trough collector (PTC) plant consists of a group of
reflectors (usually silvered acrylic) that are curved in one dimen-
sion in a parabolic shape to focus sunrays onto an absorber tube
that is mounted in the focal line of the parabola. The reflectors
and the absorber tubes move in tandem with the sun as it daily
crosses the sky, from sunrise to sunset [9,10]. The group of
parallel connected reflectors is called the solar field.
Typically, thermal fluids are used as primary HTF, thereafter
powering a secondary steam circuit and Rankine power cycle.
Other configurations use molten salts as HTF and others use a
direct steam generation (DSG) system.
The absorber tube (Fig. 4), also called heat collector element
(HCE), is a metal tube and a glass envelope covering it, with either
air or vacuum between these two to reduce convective heat losses
and allow for thermal expansion. The metal tube is coated with a
Fig. 2. Currently available CSP Technologies:(a) STP; (b)PTC; (c) LFR; (d) PDC [8].
USA
40.1%
Spain
57.9%
Iran
1.4%
Italy
0.4%
Australia
0.2%
Germany
0.1%
Parabolic
Trough
96.3%
Solar
Tower
3.0%
Parabolic
Dish
0.1%
Linear
Fresnel
0.7%
Fig. 3. Installed operational CSP power (March 2011), by country and by technology [10].
Fig. 4. Absorber element of a parabolic trough collector [9].
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22 (2013) 466–481 469
selective material that has high solar irradiation absorbance and
low thermal remittance. The glass-metal seal is crucial in redu-
cing heat losses.
2.1.3. Linear Fresnel reflector
Linear Fresnel reflectors (LFR) approximate the parabolic shape
of the trough systems by using long rows of flat or slightly curved
mirrors to reflect the sunrays onto a downward facing linear
receiver. The receiver is a fixed structure mounted over a tower
above and along the linear reflectors. The reflectors are mirrors
that can follow the sun on a single or dual axis regime. The main
advantage of LFR systems is that their simple design of flexibly
bent mirrors and fixed receivers requires lower investment costs
and facilitates direct steam generation, thereby eliminating the
need of heat transfer fluids and heat exchangers. LFR plants are
however less efficient than PTC and SPT in converting solar energy
to electricity. It is moreover more difficult to incorporate storage
capacity into their design.
A more recent design, known as compact linear Fresnel
reflectors (CLFR), uses two parallel receivers for each row of
mirrors and thus needs less land than parabolic troughs to
produce a given output [11].The first of the currently operating
LFR plants, Puerto Errado 1 plant (PE 1), was constructed in
Germany in March 2009, with a capacity of 1.4 MW. The success
of this plant motivated the design of PE 2, a 30 MW plant to be
constructed in Spain. A 5 MW plant has recently been constructed
in California, USA.
2.1.4. Parabolic dish systems
Parabolic dish collectors (PDC), concentrate the sunrays at a
focal point supported above the center of the dish. The entire
system tracks the sun, with the dish and receiver moving in
tandem. This design eliminates the need for a HTF and for cooling
water. PDCs offer the highest transformation efficiency of any CSP
system. PDCs are expensive and have a low compatibility with
respect of thermal storage and hybridization [11]. Promoters
claim that mass production will allow dishes to compete with
larger solar thermal systems [11]. Each parabolic dish has a low
power capacity (typically tens of kW or smaller), and each dish
produces electricity independently, which means that hundreds
Fig. 5. Concentrated solar thermo-electric technology[11].
Table 1
Comparison between leading CSP technologies [8,11,13].
Relative cost Land occupancy Cooling water
(L/MW h)
Thermo-dynamic
efficiency
Operating
T range (1C)
Solar concentration
ratio
Outlook for improvements
PTC Low Large 3,000 or dry Low 20–400 15–45 Limited
LFR Very low Medium 3,000 or dry Low 50–300 10–40 Significant
SPT High Medium 1,500 or dry High 300–565 150–1500 Very significant
PDC Very high Small None High 120–1500 100–1000 High potential through
mass production
Table 2
Comparison for 50 MW
el
CSP plants with TES.
Parameters
PTC with oil, without
storage and back-up
SPT with steam, without
storage and back-up
SPT with molten salt, TES storage
and back-up system
Mean gross efficiency (as % of direct radiation) 15.4 14.2 18.1
Mean net efficiency (%) 14 13.6 14
Specific power generation (kW h/m
2
-year) 308 258 375
Capacity factor (%) 23–50 24 Up to 75
Unitary investment (h/kW h
el
) 1.54 1.43 1.29
Levelized electricity cost (h/kW h
el
) 0.16–0.19 0.17–0.23 0.14–0.17
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22 (2013) 466–481470
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