-
Department ofb School of Engic Facultad de In
24 January 2013
Keywords:
Concentrated solar power plants
Hourly beam irradiation
Plant simulation
ar radiation and energy demand peak, CSPs experience short term
variations
t 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 dening the
required TES and/or BS, an accurate
estimation of the daily solar irradiation is needed. Local solar
irradiation data are mostly only available
. . . . . .
ide en
plants
. . . . . .
. . . . . .
. 472
. 472
. 473
5. Computing global and diffuse solar hourly irradiation. . . .
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
474
Contents lists available at SciVerse ScienceDirect
Renewable and Sustainable Energy Reviews
Renewable and Sustainable Energy Reviews 22 (2013) 466481E-mail
address: [email protected] (H.L. Zhang).5.2.1. Estimating the
daily irradiation. . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 474
5.2.2. Sequence of days . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
475
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.: 32 16 322695; fax: 32 16
322991.5.1. Background information . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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. . 474
5.2. The adopted model approach and equations . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 4744.
Enhancing the CSP potential . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
4.1. Thermal energy storage systems . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Backup systems . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .2.1.1. Solar power towers . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468
2.1.2. Parabolic trough collector . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469
2.1.3. Linear Fresnel reector . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 472Contents
1. Introduction . . . . . . . . . . . . . . . .
1.1. Solar irradiance as worldw
1.2. Concentrated solar power
2. CSP technologies . . . . . . . . . . . .
2.1. Generalities . . . . . . . . . .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)
briey 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 ndings
and available meteorological data;
(iii) illustrates predictions for different selected STC
locations; and nally (iv) describes the use of the
predictions in simulating the required plant conguration of an
optimum STC.
The methodology and results demonstrate the potential of CSPs in
general, whilst also dening the
design background of STC plants.
& 2013 Elsevier Ltd. All rights reserved.
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. . . . . . . . . . . . . . 467
ergy source . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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. . . . . . . . . . . . . . 467Design methodology
Solar towersAccepted 26 January 2013hours of the day where
sol
on cloudy days and cannoa r t i c l e i n f o
Article history:
Received 17 November 2012
Received in revised form
a b s t r a c t
Concentrated solar power plants (CSPs) are gaining increasing
interest, mostly as parabolic trough
collectors (PTC) or solar tower collectors (STC).
Notwithstanding CSP benets, the daily and monthly
variation of the solar irradiation ux is a main drawback.
Despite the approximate match betweenChemical Engineering, Chemical
and Biochemical Process Technology and Control Section, Katholieke
Universiteit Leuven, Heverlee 3001, Belgium
neering, University of Warwick, Coventry, UK
geniera y Ciencias, Universidad Adolfo Ibanez, Santiago,
ChileConcentrated solar power plants: Review and design
methodology
H.L. Zhang a,n, J. Baeyens b, J. Degreve a, G. Caceres c
ajournal homepage: www.elsevier.com/locate/rser
-
n .
es .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
dening thetion of the dfor worldwi
vious theoretical and experimental ndings into a general
method
calculations from the temperatures recorded at the
locations;
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 467capacity of TES and required BS, an accurate
estima-aily solar irradiation is needed. Solar irradiation data
tionally, a recent technology called concentrated solar
thermo-electrics is described. These CSP technologies are currently
inexperience short term variations on cloudy days and cannotprovide
energy during night hours. In order to improve the overallyield in
comparison with conventional systems, the CSP processcan be
enhanced by the incorporation of two technologies, i.e.,thermal
energy storage (TES) and backup systems (BS). Bothsystems
facilitate a successful continuous and year round opera-tion, thus
providing a stable energy supply in response toelectricity grid
demands. To determine the optimum design andoperation of the CSP
throughout the year, whilst additionally
water/steam) or via a secondary circuit to power a turbine
andgenerate electricity. CSP is particularly promising in regions
withhigh DNI. According to the available technology roadmap [8],
CSPcan be a competitive source of bulk power in peak and
inter-mediate loads in the sunniest regions by 2020, and of base
loadpower by 2025 to 2030.
At present, there are four available CSP technologies (Fig.
2):parabolic trough collector (PTC), solar power tower (SPT),
linearFresnel reector (LFR) and parabolic dish systems (PDS).
Addi-problem for all CSP technologies: despite the close
matchbetween hours of the day in which energy demand peaks andsolar
irradiation is available, conventional CSP technologies
trated on a small area. Using mirrors, sunlight is reected to
areceiver where heat is collected by a thermal energy
carrier(primary circuit), and subsequently used directly (in the
case of5.2.3. Estimation of the hourly diffuse and beam
radiatio
5.2.4. Shortcut estimates, based on recorded temperatur
6. Model parameters . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
6.1. Common measurement methods of solar radiation . . . . . .
.
6.2. Available information. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . .
6.3. Selected locations. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . .
7. Results and discussion . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
7.1. Calculations of H0, H and Hb . . . . . . . . . . . . . . .
. . . . . . . . . .
7.2. Methodology to apply the predictions in CSP design . . . .
. .
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
1. Introduction
1.1. Solar irradiance as worldwide energy source
More energy from the sunlight strikes the earth in 1 h than
allof the energy consumed by humans in an entire year. In fact,
solarenergy dwarfs all other renewable and fossil-based
energyresources combined.
We need energy electrical or thermal but in most caseswhere and
when it is not available. Low cost, fossil-basedelectricity has
always served as a signicant cost competitor forelectrical power
generation. To provide a durable and widespreadprimary energy
source, solar energy must be captured, stored andused in a
cost-effective fashion.
Solar energy is of unsteady nature, both within the day
(daynight, clouds) and within the year (wintersummer). The
captureand storage of solar energy is critical if a signicant
portion of thetotal energy demand needs to be provided by solar
energy.
Fig. 1 illustrates the world solar energy map. Most of
thecountries, except those above latitude 451N or below
latitude451S, are subject to an annual average irradiation ux in
excess of1.6 MW h/m2, with peaks of solar energy recorded in some
hotspots of the Globe, e.g., the Mojave Desert (USA), the Sahara
andKalahari Deserts (Africa), the Middle East, the Chilean
AtacamaDesert 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
signi-cant market position due to their advantages in terms of
higherefciency, lower operating costs and good scale-up potential.
Thelarge-scale STC technology was successfully demonstrated
byTorresol in the Spanish Gemasolar project on a 19.9 MWel-scale
[2].
Notwithstanding CSP benets, the varying solar radiation
uxthroughout the day and throughout the year remains a mainde
locations are mostly only available as monthly select an
appropriate plant conguration, and present designpreliminary
recommendations using predicted hourly beamirradiation data.
In general, the study will demonstrate the global potential
ofimplementing the SPT technology, and will help to determine
themost suitable locations for the installation of SPT plants.
2. CSP technologies
2.1. Generalities
Concentrated solar power (CSP) is an electricity
generationtechnology that uses heat provided by solar irradiation
concen-meestimate the hourly beam irradiation ux from
availablemonthly mean global irradiation data for selected
locations,and compare the results obtained of monthly data
withreview the CSP technologies and discuss solar power
toweradvantages compared to the other technologies;of calculating
the hourly beam irradiation ux. The basis waspreviously outlined by
Dufe and Beckmann [3], and uses the Liuand Jordan [4] generalized
distributions of cloudy and clear days,later modied by Bendt et al.
[5], then by Stuart and Hollands [6]and nally by Knight et al.
[7].
The present paper has therefore the following specicobjectives:.
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. . . . . . . . . . . . . . . . . . . . . . . . 480
averages, and a predictive conversion into hourly data and
directirradiation is needed to provide a more accurate input into
theCSP design. Considering that a CSP plant will only accept
directnormal irradiance (DNI) in order to operate, a clear day
model isrequired for calculating the suitable irradiation data.
The procedure, outlined in the present paper, combines pre-dium
to large-scale operation and mostly located in Spain and
-
Nomenclature
Abbreviations
BS Backup systemCRS Central receiver systemCSP Concentrated
solar power plantCLFR Compact linear Fresnel collectorDNI Direct
normal irradianceDSG Direct steam generationHCE Heat collector
elementHFC Heliostat eld collectorHTF Heat transfer uidISCC
Integrated solar combined cycleLFR Linear Fresnel reectorNREL
National Renewable Energy LaboratoryPDC Parabolic dish collectorPTC
Parabolic trough collectorTES Thermal energy storageS&L Sargent
and LundySNL Sandia National LaboratoriesSTC Solar tower
collector
radiation, at mean earth-sun distance, outside of
theatmosphere
H0 the extra-terrestrial radiation (MJ/m2 day)
Ho,av The monthly average of H0H The daily total radiation
obtained from the registered
measurementsHav The monthly average of HHd The daily diffuse
radiationI The hourly radiationId The hourly solar diffuse
radiationIb The hourly solar beam radiationI0 The hourly
extraterrestrial radiationKT,av Monthly average clearness indexKT
Daily clearness index;kT Hourly clearness index;KT,min Minimum
daily clearness indexKT,max Maximum daily clearness indexKRS
Hargreaves adjustment coefcient (1C
0.5) (0.16/0.19)n The nth-day of the yearn Number of the day of
the month (1, 2,y nd )
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481468Symbols
a Parameter dened by Eq. (17)in the USA as shown in Fig. 3.
Although PTC technology is themost mature CSP design, solar tower
technology occupies thesecond place and is of increasing importance
as a result of itsadvantages, as discussed further.
Fig. 1. World solar e
b Parameter dened by Eq. (18)dr The inverse relative distance
EarthSunF Cumulative distribution function or fraction of days
in
which the daily clearness index in less than a certainspecic
value;
GSC the solar constant1367 W/m2, as energy of the sunper unit
time received on a unit area of the surfaceperpendicular to the
propagation direction of thedk k
ndm Number of the days in a certain month (31, 30 or 28)rt The
ratio of hourly to total radiationrd The ratio of hourly diffuse to
daily diffuse radiationTmax Maximum air temperature (1C)Tmin
Minimum air temperature (1C)ws The sunset hour angle (rad)2.1.1.
Solar power towers
Solar power towers (SPT), also known as central receiversystems
(CRS), use a heliostat eld collector (HFC), i.e., a eld ofsun
tracking reectors, called heliostats, that reect and concen-trate
the sunrays onto a central receiver placed in the top of axed tower
[2,9]. Heliostats are at or slightly concave mirrors
nergy map [1].
w The hour angle of the sun (rad)d The solar declination angle
(rad)g Parameter that denes the exponential distribution
proved by Bouguer law of absorption of radiationthrough the
atmosphere
Latitude of the location (rad)x Dimensionless parameter, dened
by Eq. (9)
-
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 469that follow the sun in a two axis tracking. In the
central receiver,heat is absorbed by a heat transfer uid (HTF),
which thentransfers heat to heat exchangers that power a steam
Rankinepower cycle. Some commercial tower plants now in operation
usedirect steam generation (DSG), others use different uids,
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 efciency at which heat is
converted intoelectricity and reducing the cost of thermal storage.
In addition,the concept is highly exible, where designers can
choose from awide variety of heliostats, receivers and transfer
uids. Someplants 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
ofreectors (usually silvered acrylic) that are curved in one
dimen-sion in a parabolic shape to focus sunrays onto an absorber
tubethat is mounted in the focal line of the parabola. The
reectorsand the absorber tubes move in tandem with the sun as it
dailycrosses the sky, from sunrise to sunset [9,10]. The group
ofparallel connected reectors is called the solar eld.
Typically, thermal uids are used as primary HTF,
thereafterpowering a secondary steam circuit and Rankine power
cycle.
Fig. 2. Currently available CSP Technologie
USA40.1%
Spain57.9%
Iran1.4% Italy
0.4%
Australia 0.2%
Germany0.1%
Fig. 3. Installed operational CSP power (MarchOther congurations
use molten salts as HTF and others use adirect 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 eitherair or vacuum between these two to reduce convective
heat lossesand allow for thermal expansion. The metal tube is
coated with a
s:(a) STP; (b)PTC; (c) LFR; (d) PDC [8].
Parabolic Trough96.3%
Solar Tower3.0%
Parabolic Dish0.1%
Linear Fresnel0.7%
2011), by country and by technology [10].
Fig. 4. Absorber element of a parabolic trough collector
[9].
-
selective material that has high solar irradiation absorbance
andlow thermal remittance. The glass-metal seal is crucial in
redu-cing heat losses.
2.1.3. Linear Fresnel reector
Linear Fresnel reectors (LFR) approximate the parabolic shapeof
the trough systems by using long rows of at or slightly
curvedmirrors to reect the sunrays onto a downward facing
linearreceiver. The receiver is a xed structure mounted over a
towerabove and along the linear reectors. The reectors are
mirrorsthat can follow the sun on a single or dual axis regime. The
mainadvantage of LFR systems is that their simple design of
exiblybent mirrors and xed receivers requires lower investment
costsand facilitates direct steam generation, thereby eliminating
theneed of heat transfer uids and heat exchangers. LFR plants
arehowever less efcient than PTC and SPT in converting solar
energyto electricity. It is moreover more difcult to incorporate
storagecapacity into their design.
A more recent design, known as compact linear Fresnelreectors
(CLFR), uses two parallel receivers for each row of
mirrors and thus needs less land than parabolic troughs
toproduce a given output [11].The rst of the currently operatingLFR
plants, Puerto Errado 1 plant (PE 1), was constructed inGermany in
March 2009, with a capacity of 1.4 MW. The successof this plant
motivated the design of PE 2, a 30 MW plant to beconstructed in
Spain. A 5 MW plant has recently been constructedin California,
USA.
2.1.4. Parabolic dish systems
Parabolic dish collectors (PDC), concentrate the sunrays at
afocal point supported above the center of the dish. The
entiresystem tracks the sun, with the dish and receiver moving
intandem. This design eliminates the need for a HTF and for
coolingwater. PDCs offer the highest transformation efciency of any
CSPsystem. PDCs are expensive and have a low compatibility
withrespect of thermal storage and hybridization [11].
Promotersclaim that mass production will allow dishes to compete
withlarger solar thermal systems [11]. Each parabolic dish has a
lowpower capacity (typically tens of kW or smaller), and each
dishproduces electricity independently, which means that
hundreds
Fig. 5. Concentrated solar thermo-electric technology[11].
yna
Mean gross efciency (as % of direct radiation) 15.4
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481470Mean net efciency (%) 14
Specic power generation (kW h/m2-year) 308
Capacity factor (%) 2350
Unitary investment (h/kW hel) 1.54
Levelized electricity cost (h/kW hel) 0.160.19Table 1Comparison
between leading CSP technologies [8,11,13].
Relative cost Land occupancy Cooling water
(L/MW h)
Thermo-d
efciency
PTC Low Large 3,000 or dry Low
LFR Very low Medium 3,000 or dry Low
SPT High Medium 1,500 or dry High
PDC Very high Small None High
Table 2Comparison for 50 MWel CSP plants with TES.
ParametersPTC with oil, without
storage and back-upSPT with steam, without
storage and back-up
SPT with molten salt, TES storage
and back-up system
14.2 18.1
13.6 14
258 375
24 Up to 75
1.43 1.29
0.170.23 0.140.17mic Operating
T range (1C)Solar concentration
ratio
Outlook for improvements
20400 1545 Limited
50300 1040 Signicant
300565 1501500 Very signicant
1201500 1001000 High potential through
mass production
-
or thousands of them are required to install a large scale plant
likebuilt with other CSP technologies [11].
Maricopa Solar Project is the only operational PDC plant, witha
net capacity of 1.5 MW. The plant began operation on January2010
and is located in Arizona, USA.
2.1.5. Concentrated solar thermo-electrics
As well as with photovoltaic systems, direct conversion ofsolar
energy into electricity can also be achieved with concen-trated
solar thermo-electric (CST) technology. Solar thermo-electric
devices can convert solar thermal energy, with its
inducedtemperature gradient, into electricity. They can also be
modiedto be used as a cooling or heating technology [11]. Recently,
CSPtechnologies have been combined with thermo-electrics in orderto
achieve higher efciencies [11]. A concentrated solar
thermo-electric power generator typically consists of a solar
thermalcollector and a thermo-electric generator (Fig. 5). Heat
isabsorbed by the thermal collector, then concentrated and
con-ducted to the thermo-electric generator, where the
thermalresistance of the generator creates a temperature
differencebetween the absorber plate and the uid, which is
proportionalto the heat ux. The current cost of thermo-electric
materialshampers the widespread use of CSTs.
2.2. Comparison of CSP technologies
Within the commercial CSP technologies, parabolic
troughcollector (PTC) plants are the most developed of all
commerciallyoperating plants [12]. Table 1 compares the
technologies on thebasis of different parameters.
In terms of cost related to plant development, SPT and
PDCsystems are currently more expensive, although future
developments and improvements [13] will alter levelized
energycost projections, as presented by Sandia National
Laboratories(SNL) and by Sargent & Lundy Consulting Group
(S&L): SPT will bethe cheaper CSP technology in 2020.
In terms of land occupancy, considering the latest improve-ments
in CSP technologies, SPT and LFR require less land than PTCto
produce a given output. Additionally, PDC has the smallest
landrequirement among CSP technologies [8,12].
Water requirements are of high importance for those
locationswith water scarcity, e.g., in most of the deserts. As in
otherthermal power generation plants, CSP requires water for
coolingand condensing processes, where requirements are
relativelyhigh: about 3000 L/MW h for PTC and LFR plants (similar
to anuclear reactor) compared to about 2000 L/MW h for a
coal-redpower plant and only 800 L/MW h for a combined-cycle
naturalgas power plant. SPT plants need less water than PTC (1500
L/MW h) [8]. Dishes are cooled by the surrounding air, so they
donot require cooling water. Dry cooling (with air) is an
effective
Hea
Liqu
Stea
Stea
Stea
Stea
Mol
Hi-T
Stea
Air
Mol
Pres
Pres
Sup
Air
Wat
Air
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 471Table 3PS20, Sierra sun tower and Gemasolar
technical parameters [19].
Characteristics PS 20 Sierra sun tower Gemasolar
Turbine net capacity 20 MWel 5 MWel 19.9 MWelSolar eld area
150,000 m2 27,670 m2 304,750 m2
Number of heliostats 1,255 24,360 2,650
Heat transfer uid Water Water Molten salt
Receiver outlet temperature 2,550300 1C 440 1C 565 1CBackup fuel
Natural gas Natural gas Natural gas
Storage capacity 1 h (No storage)15 h
(molten salt)
Capacity factor Approx. 27% Approx. 30% 7075%
Table 4Experimental solar power towers [12].
Project Country Power
PSA SSPS-CRS Spain 0.5 MWelEURELIOS Italy 1 MWelSUNSHINE Japan 1
MWelSolar One USA 10 MWelPSA CESA-1 Spain 1 MWelMSEE/Cat B USA 1
MWelTHEMIS France 2.5 MWelSPP-5 Russia 5 MWelTSA Spain 1 MWelSolar
Two USA 10 MWelConsolar Israel 0.5 MWthSolgate Spain 0.3 MWelEureka
Spain 2 MWelJulich Germany 1.5 MWelCSIRO SolarGas Australia 0.5
MWthCSIRO Brayton Australia 1 MWthalternative as proven by the
plants under construction in NorthAfrica [8]. However, it is more
costly and reduces efciencies. Drycooling systems installed on PTC
plants located in hot deserts,reduce annual electricity production
by 7% and increase the costof the produced electricity by about 10%
[8]. However, theefciency reduction caused by dry cooling is lower
for SPT thanfor PTC. The installation of hybrid wet and dry cooling
systemsreduces water consumption while minimizing the
performancepenalty. As water cooling is more effective, operators
of hybridsystems tend to use only dry cooling in the winter when
coolingneeds are lower, then switch to combined wet and dry
coolingduring the summer.
A higher concentrating ratio of the sun enables the
possibilityto reach higher working temperatures and better
thermodynamicefciencies. On SPT plants, the large amount of
irradiation focusedon a single receiver (2001000 kW/m2) minimizes
heat losses,simplies heat transport and reduces costs [13].
In terms of technology outlooks, SPT shows promisingadvances,
with novel HTF being developed and achieving highertemperatures to
improve the power cycle efciencies. Moreover,higher efciencies
reduce the cooling water consumption, andhigher temperatures can
considerably reduce storage costs.
A tentative comparison of 50 MWel CSP plants with TES [13,14]is
presented in Table 2. The capacity factor is dened as the ratioof
the actual output over a year and its potential output if theplant
had been operated at full nameplate capacity. Capacityfactors of
CSP-plants without storage and back-up systems arealways low, due
to the lacking power production after sunset andbefore sunrise.
t transfer uid Storage medium Operating since
id sodium Sodium 1981
m Nitrate salt/water 1981
m Nitrate salt/water 1981
m Oil/rock 1982
m Nitrate salt 1983
ten nitrate Nitrate salt 1984
ec salt Hi-Tec salt 1984
m Water/steam 1986
Ceramic 1993
ten nitrate Nitrate salt 1996
surized air Fossil hybrid 2001
surized air Fossil hybrid 2002
erheated steam Pressurized H2O 2009
Air/ceramic 2009
er/gas 2005
2011
-
A lower cost in SPT technology is mainly due to a lowerthermal
energy storage costs, which benets from a largertemperature rise in
the SPT compared to the PTC systems[14,15]. A higher annual
capacity factor and efciency in SPT ismainly possible due to the
thermal storage, which enables acontinuous and steady day-night
output [14,16].
Additionally, in SPT plants, the whole piping system is
con-centrated in the central area of the plant, which reduces the
sizeof the piping system, and consequently reduces energy
losses,material costs and maintenance [2,8]. In this scenario,
solartowers with molten salt technology could be the best
alternativeto parabolic trough solar power plants. Considering all
mentionedaspects, SPT has several potential advantages. For both
SPT andPTC technology, abundant quality data of main specic
compo-nents are known [3,12,17,18], thus facilitating a more
accurateanalysis of the technology.
3. Past and current SPT developments
The early developments included the PS 10 and a slightlyimproved
PS 20 (Planta Solar 10 and 20) [18] of respectivecapacities 11 and
20 MWel, built near Sevilla. The plant technol-ogies involve
glass-metal heliostats, a water thermal energystorage system (1 h),
and cooling towers. A natural gas back-upis present [18,19]. The
Sierra Sun Tower is the third commercialSPT plant in the world, and
the rst of the United States. It consistof two modules with towers
of 55 m height, total net turbinecapacity of 5 MWel and constructed
on approximately 8 ha. Itbegan production in July 2009. Gemasolar
is the fourth andnewest commercial SPT plant in the world, as it
began productionin April 2011. It is the rst commercially operating
plant to apply
located on 185 ha near Sevilla, Spain. The molten salt
energystorage system is capable of providing 15 h of electricity
produc-tion without sunlight, which enables the plant to provide
elec-tricity for 24 consecutive hours. Table 3 shows the
maincharacteristics of the PS 20, Sierra Sun Tower and Gemasolar
SPT.
Additional pilot-SPT plants have been built and developedaround
the world since 1981, as illustrated in Table 4 [12].
Commercial SPT plants are also being implemented, either inthe
design or in the construction phase, as illustrated in Table
5.Recently additional large-scale projects have been announced
fore.g., Morocco, Chile, the USA, and the Republic of South
Africa.(RSA). The RSA announced an initiative of 5000 MW [20].
Theseprojects are not considered in Table 5, for current lack of
detailedinformation.
4. Enhancing the CSP potential
As stated before, the CSP potential can be enhanced by
theincorporation of two technologies in order to improve
thecompetitiveness towards conventional systems: Thermal
energystorage (TES) and backup systems (BS). Both systems offer
thepossibility of a successful year round operation, providing a
stableenergy supply in response to electricity grid demands
[2,3].
4.1. Thermal energy storage systems
Thermal energy storage systems (TES) apply a simple princi-ple:
excess heat collected in the solar eld is sent to a heatexchanger
and warms the heat transfer uid (HTF) going from thecold tank to
the hot tank. When needed, the heat from the hottank can be
returned to the HTF and sent to the steam generator
l po
el
el
el
el
el
el
el
el
el
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481472molten salts as heat transfer uid and storage
medium [2,19]. It is
Table 5Developing solar power tower projects [19].
Project Country Nomina
BrightSource Coyote springs 1 Nevada, USA 200 MW
BrightSource Coyote springs 2 Nevada, USA 200 MW
BrightSource PG&E 5 California, USA 200 MW
BrightSource PG&E 6 California, USA 200 MW
BrightSource PG&E 7 California, USA 200 MW
Crescent Dunes Solar Energy Project (Tonopah) Nevada, USA 110
MW
Gaskell sun tower California, USA 245 MW
Ivanpah Solar Electric Generating Station (ISEGS) California,
USA 370 MW
Rice Solar Energy Project (RSEP) California, USA 150 MWFig. 6.
Thermal energy storage system in(Fig. 6). In the absence of storage
capacity, on the sunniest hours,
wer output HTF Storage medium Projected to start operation
Water July 2014
Water July 2015
Water July 2016
Water December 2016
Water July 2017
Molten salt Molten salt October 2013
Water May 2012
Water October 2013
Molten salt Molten salt October 2013a parabolic trough collector
plant [8].
-
plant operators defocus some unneeded solar collectors to
avoidoverheating the HTF. Storage avoids losing the daytime
surplusenergy while extending the production after sunset.
Two types of thermal storage are necessary to maintain aconstant
supply through the year, Short and Long term energystorage. Short
term thermal energy storage collects and storessurplus daytime
energy for nighttime consumption. Long termthermal energy storage
is less obvious, since involving storage inspring and summer for
autumn and winter months. Currently,only sensible heat is stored.
The signicant improvement by usinglatent heat storage (phase change
materials) or even chemicalheat storage (reversible
endothermic/exothermic synthesis) is infull development [21], with
chemical heat being considered moresuitable for long term thermal
energy storage.
and to guarantee a nearly constant generation capacity,
especiallyin peak periods. CSP plants equipped with backup systems
arecalled hybrid plants. Burners can provide energy to the HTF, to
thestorage medium, or directly to the power block. The integration
ofthe BS can moreover reduce investments in reserve solar eld
andstorage capacity. CSP can also be used in a hybrid mode by
addinga small solar eld to a fossil fuel red power plant. These
systemsare called integrated solar combined cycle plants (ISCC). As
thesolar share is limited, such hybridization only limits fuel use.
Apositive aspect of solar fuel savers is their relatively low
cost:
Fig. 7. Heat capacity of different storage materials, (kW h/m3)
versus meltingpoints (1K) [11].
Fig. 8. Cost of different storage materials (US$/kW h) versus
melting points (1K) [10].
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 473Thermal storage can be achieved directly or
indirectly. Liquidse.g., mineral oil, synthetic oil, silicone oil,
molten salts, can beused for sensible heat in direct thermal
storage systems. Formolten salts, the desired characteristics for
sensible heat usageare high density, low vapor pressure, moderate
specic heat, lowchemical reactivity and low cost [21]. Indirect
storage is whereHTF circulates heat, collected in the absorbers,
and then pumpedto the thermal energy storage system. The storage
material (solidmaterial) absorbs heat from the HTF in heat
exchangers, while thesolid material and the HTF are in thermal
contact.
The thermal storage capacity can be varied in order to
meetdifferent load requirements, and different options are
possible,depending on the storage capacity included, i.e., (i) with
a smallstorage only, if electricity is only produced when the
sunshine isavailable; (ii) in a delayed intermediate load
conguration, wheresolar energy is collected during daytime, but
with an extendedelectricity production, or a production only when
demand peaks;(iii) in a fully continuous mode, with a sufciently
large storagecapacity to cover electricity production between
sunset andsunrise (e.g., Gemasolar).
In order to select optimum sensible heat storage materials,
theheat capacity plays a major role [11,21], and values are
illustratedin Fig. 7.
Molten single salts tend to be expensive [11],as illustrated
inFig. 8.
The molten nitrate salt, used as HTF and storage medium, is
acombination of 60 wt% sodium nitrate (NaNO3) and 40 wt%potassium
nitrate (KNO3). It is a stable mixture and has a lowvapor pressure.
It can be used within a temperature range of260 1C to 621 1C.
However, as the temperature decreases, itstarts to crystallize at
238 1C and solidies at 221 1C [21].
4.2. Backup systems
CSP plants, with or without storage, are commonly equippedwith a
fuel backup system (BS), that helps to regulate productionFig. 9.
Possible combination of hybridization (a) and sole TES (b) in a
solar plant.
-
irradiation is needed to provide a more accurate input into
the
registered measurements, as discussed in Section 6, and Ho,av
isthe monthly average extraterrestrial irradiation. Ho is
computed
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481474for each day and location by the following
formula:
Ho 24 60=pGSCdr coscosdcoswswssinsind 2With
2CSP design. It is therefore necessary to apply a methodology
thatconverts these values into hourly databases. Considering that
aCSP plant will only accept direct normal irradiance (DNI) in
orderto operate, a clear day model is required for calculating
theappropriate irradiation data.
Although numerous researchers (o2000) have generatedcalculation
procedures for obtaining synthetic data on a daily orhourly basis
[7,2232], the present paper updates and combinesthe essentials of
these different publications into expressions ofdaily distributions
and hourly variations for any selected location,starting from the
monthly average solar irradiation value, bygenerating a sequence of
daily and hourly solar irradiation values.Such a sequence must
represent the trend of solar irradiation in aspecic area, with
respect to the values observed, the monthlyaverage value and its
distribution (the good and bad days).
The essential parameter is a dimensionless clearness
indexvariable, dened as the ratio of the horizontal global
solarirradiation and the horizontal global extra-terrestrial solar
irra-diation, dened as a monthly, a daily, and an
hourlycharacteristic.
In general, the meteorological variable solar radiation
isneither completely random, nor completely deterministic.
Highlyrandom for short periods of time (days, hours), it is
deterministicfor longer periods of time (months, years). The
extra-terrestrialsolar irradiation can be predicted accurately for
any place andtime, since the specic atmospheric conditions of a
given area willdetermine the random characteristics of the solar
irradiation atground level.
5.2. The adopted model approach and equations
5.2.1. Estimating the daily irradiation
Before obtaining hourly data, estimations of daily
irradiationmust be calculated rst, as shown below.
First, it is necessary to compute the monthly average
clearnessindex for each month and location, which is dened as:
KT ,av Hav=Ho,av 1Where Hav is the monthly average irradiation,
obtained from thewith the steam cycle and turbine already in place,
only compo-nents specic to CSP require additional investment.
Fig. 9 shows a typical performance for a CSP plant enhancedwith
thermal energy storage system and backup system, in aconstant
generation at nominal capacity.
5. Computing global and diffuse solar hourly irradiation
5.1. Background information
To determine the optimum design and operation of the
CSPthroughout the year, whilst additionally dening the potential
ofTES and required BS, an accurate estimation of the daily
solarirradiation is needed. Solar irradiation data for worldwide
loca-tions are mostly only available as monthly averages (see
Section6), and a predictive conversion into hourly data and
directHo the extra-terrestrial radiation (MJ/m day)Gsc the solar
constant1367 W/m2, as energy of the sun perunit time received on a
unit area of the surface perpen-dicular to the propagation
direction of the radiation, atmean earth-sun distance, outside of
the atmosphere.
dr the inverse relative distance EarthSun, as denedbelow in Eq.
(3)
ws the sunset hour angle, as dened in Eq. (4) [10]d the solar
declination angle, as dened by Eq. (5) the latitude of the location
(rad)n the nth day of the year (1365)
dr 10:033cos2pn=365 3The sunset hour angle, when the incidence
angle is 901, as isneeded for CSP plants [33], is dened as:
cosws tantand 4The declination angle is dened by the equation of
Cooper [34] as:
d 23:45sin2p284n=365 5As a result, the daily extra-terrestrial
irradiation can be expressedby Eq. (6)
Ho 24 60=pGSCdrcoscosdcoswswssinsind 6Liu and Jordan [4] studied
the statistical characteristics of solar
irradiation, using the clearness index (a measure of the
atmo-spheric transmittance) as a random variable. They
demonstratedthat the hourly clearness index was related to the
monthlyaverage value. Bendt et al. [5] thereafter proposed a
frequencydistribution of daily clearness index values, staring from
monthlyaverage values. Initially based upon irradiation studies in
the USA,this approach has been validated for different worldwide
loca-tions [3336].
The distribution to the frequency of days with a value of
theclearness index KT has an exponential correlation throughout
themonth ranging between the minimum and maximum
valuesrecorded.
The correlation is expressed as:
fKT egKT,min2egKT
h i= egKT ,min2e
gKT ,max
h i7
Where g is a dimensionless parameter that denes the
particularexponential distribution, given by:
g1:4981:184x227:182e1:5x=KT ,maxKT ,min 8Where x is also a
dimensionless parameter given by:
x KT ,maxKT ,min=KT ,maxKT ,av 9The minimum and maximum values
of KT KT,max and KT,minrespectively, are given by:
KT ,min 0:05 10
KT ,max 0:63130:267KT,av11:9KT ,av0:758 11To obtain a daily
clearness index, Knight et al. [7] dene daily KTas a function of
ndk the day of the month and ndm as the numberof days of the month,
with (ndk 0.5 )/ndma:
KT 1=g ln 12aegKT,minaegKT,,max
n oh i12
Finally, the daily total irradiation, H, is obtained following
Eqs.(1) and (12), where the daily clearness index is multiplied
withdaily extra-terrestrial irradiation H0.
H KT :H0 13In summary, with all mentioned equations solved,
articial
months with articial daily total radiations (H) are created,
where
months are ordered from the lowest to highest radiation
level.
-
5.2.2. Sequence of days
Daily total radiation data results from Eqs. (1) and (13)
areobtained in a predened sequence through the month by
varyingradiation levels in an ascending and descending pattern;.
How-ever, the sequence of days in which they succeed each other
isunknown, and obviously does not strictly follow an ascending
or
I0=H0, where is I0 the hourly extra-terrestrial irradiation,
the
As a result, hourly global and beam irradiation data for
everyday of the year (typical year of 365 days) are obtained for
eachlocation, which will be used as an input for the heliostat
eld.
The results of the calculations will be given and discussed
inSection 7.
5.2.4. Shortcut estimates, based on recorded temperatures
The previous methodology related the radiation ux to thesunshine
duration. A considerable amount of information is todayavailable on
the relationship between the solar irradiation andother
meteorological parameters such as cloud-cover, amount ofrain,
humidity and/or temperature. The parameter that has thelargest
measurement network is the ambient temperature, and ashortcut
method to relate the extra-terrestrial solar irradiation tothe
average daily solar irradiation.
These different methods were reviewed by Gajo et al.
[40],relating H to Tmax, Tmin or Tmean.
The authors found that the original Hargreaves method per-formed
overall best for different locations.
The Hargreaves method predicts KT as:
KT kRSTmax2Tmin0:5 20The adjustment coefcient kRS is empirical
and differs for
interior or coastal regions:
for interior locations, where land mass dominates and airmasses
are not strongly inuenced by a large water body,
reg
gh
-20
-20
-25
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 475hourly diffuse irradiation Id is obtained as the
ratio of hourlydiffuse to daily diffuse irradiation rd, which is
dened as:
rd Id=Hd p=24fcoswcosws=sinwspwscosws=180g19
Finally, hourly beam irradiation Ib is calculated by subtracting
Idfrom I.
Table 6Sequence model of the daily clearness indexes.
Mean clearness index (KT,av) Sequence of days throu
KT,avo0.45 24-28-11-19-18-3-2-4-90.45oKT,avo0.55
24-27-11-19-18-3-2-4-9K 40.55 24-27-11-4-18-3-2-19-9descending
order, but rather present a random occurrencesequence.
Knight et al.[7] and Graham et al. [37,38] apply a
separatemethodology to obtain the 31 clearness indexes which
succeedeach other in a month (with 31 days) and propose a
particularsequence to organize the clearness indexes as shown in
Table 6.This technique is currently used to generate typical years
insimulation programs such as TRNSYS [23].
5.2.3. Estimation of the hourly diffuse and beam radiation
As CSP plants accept only DNI, diffuse irradiation is
subtractedfrom the global irradiation to obtain the beam
irradiation, whichis the one we are interested. Direct irradiation
follows a constantdirect direction, whilst diffuse irradiation is
the part of the globalirradiation that follows different directions
due to interactionswith the atmosphere (See Fig. 10).
The daily diffuse irradiation (Hd ) is dened by the
Erbscorrelations [39]: the daily total diffuse fraction depends on
thesunset hour angle (ws ) and is dened as:For wsr81.41
Hd=H 120:2727KT2:4495K2T11:951K3T9:3879K4T if KTo0:715 0:143 if
KTZ0:715
14For wsZ81.41
Hd=H 10:2832KT2:5557K2T0:8448K3T ifKTo0:715 0:175 ifKTZ0:715
15
With H and Hd calculated for each day.The hourly irradiation (I)
is obtained by the ratio of hourly to
daily total irradiation (rt) which is dened by the
followingequation from CollaresPereira and Rabl [39] as function of
thehour angle (w in radians) and the sunset hour angle (wS):
rt I=H p=24abcoswfcoswcosws=sinwspwscosws=180g16
With a and b constants given by:
a 0:4090:5016sinws60p=180 17
b 0:66090:4767sinws60p=180 18Based on Liu and Jordan [4],
assuming that Id=Hd is the same asT,avFig. 10. Direct and diffuse
irradiation.
the month
-14-23-8-16-21-26-15-10-22-17-5-1-6-29-12-7-31-30-27-13-25
-14-23-8-16-21-7-22-10-28-6-5-1-26-29-12-17-31-30-15-13-25
-14-23-8-16-21-26-22-10-15-17-5-1-6-29-12-7-31-20-28-13-30ional
station, either because homogeneous climate conditions
tionkRS0.16; for coastal locations, situated on or adjacent to
the coast of a
large land mass and where air masses are inuenced by anearby
water body, kRS0.19.
The temperature difference method is recommended for loca-s
where it is not appropriate to import radiation data from a
-
Ha(kW
4.9
3.8
3.4
7.8
6.1
7.2
6.1
7.3
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481476Table 7Selected locations with basic data.
Location Latitude
(rad)
Longitude
(rad)
Hav (Jan)
(kW h/m2 day)
Chuquicamata, Chile 22.5 68.9 8.32Upington, RSA 28.5 21.08
7.93Geraldton, Australia 28.78 114.61 8.28Sevilla, Spain 37.41 5.98
2.56Font Romeu, France 42.5 2.03 1.81
Marrakech, Morocco 31.6 8 3.49Sainshand, Mongolia 44.89 110.14
2.21
Ely, USA 39.3 114.85 2.56
a Data taken from nearby Albi.b Data taken from nearby Ulan
Bator.do not occur, or because data for the region are lacking. For
islandconditions, the methodology of Eq. (20) is not appropriate
due tomoderating effects of the surrounding water body.
Since Tmax and Tmin data are indeed widely available,
theHargreaves KT-values can be used in the methodology of
Section5.1, and results of the both methods will be illustrated in
Section 7.
6. Model parameters
6.1. Common measurement methods of solar radiation
Solar radiation can be measured with pyranometers, radio-meters
or solarimeters. The instruments contain a sensor installedon a
horizontal surface that measures the intensity of the totalsolar
radiation, i.e., both direct and diffuse radiation from
cloudyconditions. The sensor is often protected and kept in a
dryatmosphere by a glass dome that should be regularly wipedclean.
Where pyranometers are not available, solar radiation isusually
estimated from the duration of bright sunshine. Theactual duration
of sunshine, n, is measured with a CampbellStokes sunshine
recorder. This instrument records periods ofbright sunshine by
using a glass globe that acts as a lens. Thesun rays are
concentrated at a focal point that burns a hole in aspecially
treated card mounted concentrically with the sphere.The movement of
the sun changes the focal point throughout theday and a trace is
drawn on the card. If the sun is obscured, the
Fig. 11. Calculated values of H0.v (Jul)
h/m2 day)
Jan Jul
Tmax (1C) Tmin (1C) Tmax (1C) Tmin (1C)
9 25.2 5.8 21.1 1.5
9 36.7 20.5 21.5 1.5
1 33.6 19.3 19.7 9.1
0 15.9 7.6 35.7 19.9
7 9.5a 0.1a 26.9a 14.2a
6 21.4 6.6 37.3 19.8
1 18.5b 34.7b 23.5b 9.4b8 3.3 13 31.3 9.7trace is interrupted.
The hours of bright sunshine are indicated bythe lengths of the
line segments.
6.2. Available information
There are two reliable sources that provide information on
thetwo of the most basic meteorological parameters: monthly
meantemperature and solar radiation. These sources are the
NASAwebsite [41] and TUTIEMPO [42]. NASA has produced a grid mapof
the longitude. The solar radiation data are an estimate that
hasbeen produced from satellite-based scans of terrestrial
cloud-cover. Note that NASA does not provide the mean-daily
maximumand minimum temperature. TUTIEMPO on the other hand
pro-vides daily mean, maximum and minimum temperature data forany
given location. The data are based on measurements carriedout by a
wide network of meteorological stations and hence theselatter data
are very reliable. Note that the NASA data are availableon a
mean-monthly basis, whereas TUTIEMPO are downloadableon a
day-by-day basis. It is important to remember that NASAdata are
based on satellite observations that represent inferredvalues of
irradiation; in contrast, TUTIEMPO provides ground-measured data
for temperature. Hence, if reliable regressions areavailable
between irradiation and mean temperature, then thelatter data may
be used to obtain more realistic estimates ofirradiation.
Fig. 12. Calculated average monthly clearness index by the model
[Eq. (1)]and by the Hargreaves method [Eq. (21)] at Upington
(RSA).
-
n in (a), January (summer) and (b), July (winter).
Fig. 14. The monthly average Id/I ratio for Chuquicamata
(Chile).
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 477Having developed the underlying equations of the
calculationmethod in Section 5, and using the data acquisition of
Section 6,the present section will illustrate the use of the data
obtained.
The monthly extra-terrestrial irradiation, H0, computed by
Eq.(6), is illustrated in Fig. 11 for different latitudes in both
hemi-spheres and shows the seasonal dependence, whilst also
illustrat-ing the maximum and minimum values obtained
throughout6.3. Selected locations
To illustrate the use of the methodology of Sections 5,
8locations were selected because of being already associated
withCSP, or announced as potential sites for STP. The essential
data ofthe locations are given is Table 7.
7. Results and discussion
7.1. Calculations of H0, H and Hb
Fig. 13. The total daily radiation for Upingtothe year.To
proceed with the calculation of the monthly average
clearness index, KT,av Eq. (1) is used together with
NASA-data[41]. Results are illustrated as example in Fig. 12 for
the Upingtonlocation. The Figure also includes the results obtained
from theHargreaves method [Eq. (21)] using TUTIEMPO-data [42].
Clearly,both methods provide similar results of KT,av for most of
themonths, however with higher Hargreaves-values in Spring
andAutumn. The model approach thus provides slightly more
con-servative KT,av values, and is recommended for design.
The daily total irradiation is thereafter obtained by
applyingthe daily clearness index, KT, and the daily
extra-terrestrialirradiation H0. Fig. 13 shows the model-predicted
total dailyirradiation, ordered in ascending daily pattern for
Upington, fora summer month (January) and a winter month (July).
Themonthly average H, calculated by Eq. (13) in January and July,
is7.92 kW h/m2-day and 3.92 kW h/m2-day, respectively.
Similarevolutions can be obtained for the other selected
locations.Applying the Knight et al. [7] sequence model for the
dailyclearness indexes, as function of KT,av transforms the
ascendingnature of the consecutive days into a wave-function,
althoughmonthly average values of H remain unchanged.
A similar evolution can be predicted using daily
KT-valuesresulting from the daily Tmax and Tmin data, according to
Eq. (21).
Fig. 15. The daily solar beam irradiation, Hb, on the 15th of
January and onthe 15th of July , for different locations in both
the Southern and Northern
Hemisphere.
-
These results are also shown in Fig. 13. Clearly, the
Hargreavesapproach provides a more constant H-value throughout
themonth since not respecting an ascending daily pattern.
Themonthly average value of H (HRG) is closely related to the
modelpredictions: 7.56 kW h/m2-day in January, and 4.07 kW
h/m2-dayin July, i.e., a deviation of 4.5% and 3.8% only with the
pre-citedvalues of the model-predicted average values.
The most important result towards CSP design requires thedirect
(beam) irradiation, obtained by withdrawing the diffuseirradiation,
Hd, from the total irradiation, H, according to Eqs. (14)and
(15).
The ratio of the diffuse to total irradiation is illustrated
forChuquicamata in Fig. 14: the more cloudy winter season
(AprilAugust) is reected in the higher value of the ratio.
The resulting beam radiation, Hb, for representative days
inSummer and winter, for all locations, is shown in Fig. 15,
whereasa more detailed monthly average evolution for some
locations, isshown in Fig. 16.
Finally, a complete hourly evolution can be predicted by the
The efciencies of the essential components has been reportedby
S&L, and represented in Table 9.
DNI is calculated on hourly bases The total energy ux reected by
the heliostat eld is
calculated The expected nominal capacity of the plant is
selected 21 consecutive days of lowest radiation levels are
selected to
coincide with the maintenance period, thus limiting lossesduring
plant stand-still
From a given starting day of the year, e.g., January 1st., at
6:00a.m., and repeated for all hours of the year, the
followingdifferent options need to be assessed:
If the solar thermal ux exceeds the required value tooperate the
plant at nominal capacity, only solar thermalenergy will be used,
whilst excess solar energy is stored inthe HTF hot storage tank.
The BS-system is not used, andadditional excess solar thermal
energy cannot be used;
If the solar thermal ux is insufcient to meet the
nominalcapacity, but enough thermal energy is stored in the
hottank, no BS is needed, and the plant will operate oncombined
solar radiation and stored energy;
If the combined solar thermal ux and energy stored
areinsufcient, the plant needs to operate in its hybrid
con-guration, using the BS to meet the thermal requirements.
The detailed simulations are extensive, and are not included
in
Pro
nnual overall CSP efciency (%) 13.0 13.7 16.1 16.6 17.3 18.1
Source of estimation S&L SNL S&L SNL S&L SNL
Table 9Values of CSP-component efciencies.
Component Efciency (%)
Solar eld 4850
TES 499Power block 40
Fig(Ch
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481478Considering that about 10% of the generated
electricity will beused internally for the plant utilities (mostly
pumping), 90% of thecombined efciencies do indeed vary between 14
and 18%.
The nal CSP performance simulation follows the strategy ofFig.
18, with a specic algorithm to be used, in terms of DNI, TESand BS,
as previously presented in Fig. 9.model, as illustrated in Fig. 17,
where the radiation ux can beseen to increase from sunrise to noon,
and thereafter decreasingagain till sunset.
It is also clear that the selection of the CSP nominal
capacitywill be a compromise between the seasons, accounting for
thecapability of thermal storage, and the use of a backup
system.
7.2. Methodology to apply the predictions in CSP design
Having established the annual, monthly and daily levels ofdirect
(beam) solar irradiation, its impact on the power yield ofthe CSP
can be assessed. To do so, it should be remembered thateach of the
operations of the overall CSP-layout has its ownefciency, reected
in its overall efciency. The projected overallefciency of CSP
plants was assessed by S&L and SNL, aspresented in Table 8,
including projected increased efcienciesas a result of present and
future improvements.Fig. 16. Evolution of the average monthly
direct (beam) irradiation in 8 locations.Ye
Athear of projection 2004 2004 2008 2008 2020 2020le 8jected
overall CSP efciency.Tab. 17. Hourly evolution at the 15th of the
respective months, in Chuquicamataile).present paper. They will be
reported upon in a follow-up
-
stri
ce si
orage anks
Steam generation Turbine Grid
10%
e components of the SPT.
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 479paper, considering the application of the solar
tower collector,with molten salt HTF/TES, and with natural gas-red
BS. Theturbine capacity will be 19.9 MWel, chosen because of
theextensive data available for the Gemasolar plant (Spain).
Suchsimulations will be carried out for those locations where
theannual average daily irradiation ux exceeds4 kW h/m2-day.
Due to parasitics (electricity use within the plant for
pumps,cooling towers, compressors, instrumentation and controls,
light-ing, heat tracing,.), estimated at 10% for a Gemasolar-type
hybridapplication, the net output to the grid will be 17.9 MWel
during theperiod of operation. The annual power yield of the hybrid
Gema-solar plant is 110 GW h/year [2]. The specic energy
production,i.e., the ratio of total annual grid energy and the
rated net power
Heliostats Field Characteristics
Solar Radiation Data
Thermal Energy Storage Characteristics
Plant Efficiencies
Plant Re
Fig. 18. Performan
Heliostat field Receiver
Thermal field
circuitStt
Parasitics (
Fig. 19. Sequence of thoutput of the plant is therefore
110,000/17.96145 h/year. Fortotal of 8760 h/year, the overall yield
is 70.1%, assumed realistic inview of the annual maintenance period
(zero production), shortduration disturbances, and very low solar
irradiation in winter. Aswill be shown hereafter for the
Chiquicamata example, 69.5 GW h/year will be generated from the
solar energy alone (the remainingturbine power being generated by
the back-up natural gas system).The average net solar yield thus
represents 69,500/17.93882 h/year, or 44.3% (for the 8760 h annual
operation).
For the simulations, it is considered that the plant works witha
thermodynamic cycle in a steady state.
If a total energy production needed from the turbine is19.9
MWel, the efciencies of the different CSP components willdetermine
the hourly heat ow along the system, from heliostatsto turbine and
grid. The system ow sheet is illustrated in Fig. 19,and the
additional required information for each step of thesequence is
given below in different notes.
The total energy of 19.9 MWel in the steam turbine must
bereached each hour, with part of this energy added to the
moltensalts in the receiver (reected by the heliostat eld), and the
restof the energy added by the storage system, and/or by the
back-upsystem (when receiver and storage energies are
insufcient)according to Fig. 9 and the strategy of Fig. 18. By
simulating theplant performance at the heliostat and receiver
levels, the addi-tional energy required by the storage system and
the backupsystem can be determined. Molten salt properties at each
point ofthe thermodynamic cycle are xed and knownctions
Hourly Stored Energy
Demand Supplied by Solar Energy
Short and Long Term Backup Requirements
mulation method.
)Note 1. Heliostat eld
Since the heliostats follow the sun by a two-axis tracking,
nocorrection for the incident angle ymust be made (cos y1), and
Ibcorresponds to the real hourly irradiation at the heliostat
eld.
First, the energy reected by the heliostat eld must be
calculatedeach hour, where hourly radiation data is extracted from
thecalculations of Sections 5 and 6 before. The total energy
reectedby the heliostat eld and concentrated in the receiver is
thendetermined by the heliostat eld efciency and the heliostat
eldreective surface area. The heliostat eld efciency (ZHF ) is
mostlycharacterized by its reectivity, optical efciency, heliostat
corrosionavoidance and cleanliness. A value of 48 to 50% is
commonly used(Table 9).
Note 2. Receiver
The reected energy is concentrated in the receiver, which actsas
a heat exchanger where circulating molten salts absorbs
solarenergy. Total energy absorbed by molten salts is determined
bythe receiver efciency, where receiver thermal losses are
primar-ily driven by the thermal emissivity of the receiver
panels
Fig. 20. Electricity generation through the year in a
Chuquicamata SPT, only withsolar resource conguration.
-
e Ch
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481480(radiation losses) and by the receiver temperature.
Commonly,radiation losses are 51% [14].
Note 3. THF Circuit and THF storage
Piping and tank losses are very limited, due to the
efcientisolation applied, normally again o1% [14].
Note 4. Steam generation and reheated Rankine cycle
The efciencies to be considered include the design point
turbinecycle efciency; start-up losses; partial load operation, and
steamgeneration system efciency. Losses due to minimum turbine
loadrequirements do not apply since the plant has a thermal storage
andback-up system. Commonly, a value of 40% is assigned to the
overallefciency of the thermal power block of the plant (Table
9).
Note 5. Parasitics (in-plant energy use)
The parasitic consumption considers internal electricity
usagemostly in heliostat tracking, THF pumps, condensate pumps,
Fig. 21. Monthly backup requirements in thfeedwater pumps,
cooling water pumps, cooling tower fans andelectrical heat tracing
system, but additionally in instrumenta-tion, controls, computers,
valve actuators, air compressors, andlighting. A maximum 10% [13]
was measured by Sargent andLundy Consulting Group
Considering component efciencies, a simple estimation ofsolar to
electricity efciency (Zsolar) of the plant can be obtainedby using
component efciencies to calculate the total efciency ofenergy
transformation.
Zsolar ZHF Zrec Zpiping Zstorage Zcycle 1Zparasitic A
Where,
ZHF: Heliostat eld efciency,Zrec: Receiver efciency,Zpiping:
Piping efciency,Zstorage: Thermal storage efciency,Zcycle: Power
block gross efciency,Zparasitic: Parasitics,A Plant availability
(capacity factor).
Initial results of the simulation for the
Chuquicamatainitiative, reveal that the solar generation will
account for69.5 GW h/year, in the case of a conservative 13%
overallefciency.
Figs. 20 and 21 provide some indications of the
simulatedresults. The zero production between days 150 and 180
corre-spond with the supposed annual shut-down period for
overallmaintenance.
Provided overall efciencies will increase over the comingyears,
due to technical improvements, the solar energy contribu-tion will
increase, thus reducing the required backup, as will bediscussed in
a follow-up paper, considering the overall design ofthe SPT
plant.
8. Conclusions
To determine the optimum design and operation of the
CSPthroughout the year, whilst dening the required TES and/or BS,an
accurate estimation of the direct daily solar irradiationis
needed
The paper develops the underlying equations to calculate the
uquicamata SPT, in hybrid operating mode.monthly
extra-terrestrial irradiation, H0,the monthly average clearness
index, KT,av the daily total
irradiation, and the direct (beam) irradiation.Results of the
model approach is given for 8 selected locations,
in both Northern and Southern hemisphere.Having established the
annual, monthly and daily levels of
direct (beam) solar irradiation, its impact on the power yield
ofthe CSP can be assessed. The projected overall efciency of
CSPplants was assessed and included in a CSP performance
simula-tion, according to a proposed strategy. Initial simulation
resultsare illustrated for a 19.9 MWel Solar Power Tower project,
withmolten salts as HTF, and operating in an hybrid way
(includingheat storage and back up fuel). In the assesses example,
solargeneration will account for 69.5 GW h/year, in the case of
aconservative 13% overall efciency. Provided overall efciencieswill
increase over the coming years, due to technical improve-ments, the
solar energy contribution will increase, thus reducingthe required
backup, as will be discussed in a follow-up paper,considering the
overall design of the SPT plant.
References
[1]
/http://www.alternative-energy-resources.net/solarenergydisadvantages.htmlS.
-
[2]
/http://www.torresolenergy.com/TORRESOL/gemasolar-plant/enS.[3]
Dufe J, Beckmann W. Solar engineering of thermal processes. 3rd
ed.. USA:
John Wiley and Sons; 2006.[4] Liu BYH, Jordan RC. The
interrelationship and characteristic distribution of
direct, diffuse and total solar radiation. Solar Energy
1960;4:119.[5] Bendt P, Collares-Pereira M, Rabl A. The frequency
distribution of daily
insolation values. Solar Energy 1981;27:15.[6] Stuart R,
Hollands K. A probability density function for the beam
transmit-
tance. Solar Energy 1988;40:4637.[7] Knight K, Klein K, Dufe J.
A methodology for the synthesis of hourly weather
data. Solar Energy 1991;46:10920.[8] OECD/IEA, technology
roadmap, concentrating solar power, 2010.[9] Muller-Steinhagen H,
Trieb F. Concentrating solar power: a review of the
technology. Ingenia 2004;18:4350.[10] Llorente I, Alvarez JL,
Blanco D. Performance model for parabolic trough solar
thermal power with thermal storage: comparison to operating
plant data.Solar Energy 2011;85:244360.
[11] Barlev D, Vidu R, Stroeve P. Innovation in concentrated
solar power. SolarEnergy Materials & Solar Cells
2011;95(10):270325.
[12] SolarPACES, annual Reports, 2006, 2007, 2008, 2009, 2010.
Downloadablefrom:
/http://www.solarpaces.org/Library/AnnualReports/annualreports.htmS.
[13] Sargent and Lundy Consulting Group, assessment of parabolic
trough andpower tower solar technology cost and performance
forecasts, NationalRenewable Energy Laboratory, 2003.
[14] Ortega J, Burgaleta J, Tellex F. Central receiver system
solar power plant usingmolten salt as heat transfer uid, May
2008.
[15] Pitz-Paal R, Dersch J, Milow B, Tellez F, Ferriere A,
Langnickel U., et al.Development steps for concentrating solar
power technologies with max-imum impact on cost reduction, 2005.
Available at: /http://pre.ethz.ch/
[24] Pissimanis D, Karras G, Notaridou V, Gavra K. The
generation of a typicalmeteorological year for the city of Athens.
Solar Energy 2000;40:40511.
[25] Aguiar RJ, Collares-Pereira M, Conde JP. Simple procedure
for generatingsequences of daily radiation values using a library
of Markov transitionmatrices. Solar Energy 1988;40:26979.
[26] Aguiar RJ, Collares-Pereira MTAG. a time-dependent,
autoregressive, Gaus-sian model for generating synthetic hourly
radiation. Solar Energy1992;46:16774.
[27] Amato U, Andretta A, Bartoli B, Coluzzi B, Cuomo V. Markow
Processes andFourier analysis as a tool to describe and simulate
daily solar irradiance. SolarEnergy 1986;37:17994.
[28] Graham VA, Hollands KGT, Unny TE. A time series model for
K, withapplication to global synthetic weather generation. Solar
Energy 1988;40:8392.
[29] Graham VA, Hollands KGT. A method to generate synthetic
hourly solarradiation globally. Solar Energy 1990;44:33341.
[30] Gordon JM, Reddy TA. Time series analysis of daily
horizontal solar radiation.Solar Energy 1988;41:21526.
[31] Mora-Lopez LL, Sodracj de Cardona M. Characterization and
simulation ofhourly exposure series of global radiation. Solar
Energy 1997;60:25770.
[32] Mora-Lopez LL, Sodracj de Cardona M. Multiplicative ARMA
models togenerate hourly series of global irradiation. Solar Energy
1998;63:28391.
[33] Tham Y, Muneer T, Davison B. Estimation of hourly averaged
solar irradia-tion: evaluation of models. Building Services
Engineering Research andTechnology 2010;31(1):925.
[34] Muneer T, Fairooz F. Quality control of solar radiation and
sunshinemeasurements-lessons learnt from processing worldwide
database. BuildingServices Engineering Research and Technology
2002;23(3):15166.
[35] Hawas MM, Muneer T. Study of diffuse and global radiation
characteristics inIndia. Energy Conversion and Management
1984;24(2):1439.
[36] Badescu V, Gueymard CA, Cheval S, Oprea C, Baciu M,
Dumitrescu A, et al.Computing global and diffuse solar hourly
irradiation on clear sky. Reviewand testing of 54 models. Renewable
and Sustainable Energy Reviews
H.L. Zhang et al. / Renewable and Sustainable Energy Reviews 22
(2013) 466481 481sun.net/book.htmlS.[18] Abengoa solar website.
/http://www.abengoa.esS.[19] N.R.E.L. Website, with the
collaboration of SolarPaces, /http://www.nrel.gov/
csp/solarpaces/S.[20]
/www.guardian.co.uk/environment/2010/oct25/
south-africa-solar-power-plantS.[21] Fernandes D, Pitie F,
Caceres G, Baeyens J. Thermal energy storageHow
previous ndings determine current research priorities. Energy
2012;39(1):24657.
[22] Klein SA, Beckman WA. TRNSYS V14.2 (1996). A transient
system simulationtool.
[23] Petrakis M, Kambezidis HD, Lykoudis S, Adamopoulos AD,
Kassomenos P,Michaelides IM, et al. Generation of a typical
meteorological year for NicosiaCyprus. Renewable Energy
1998;13(3):3818.application to global synthetic weather generation.
Solar Energy 1988;40(2):8392.
[38] Graham VA, KGT. Hollands. A method to generate synthetic
hourly solarradiation globally. Solar Energy 1990;44(6):33341.
[39] Tham Y, Muneer T, Davidson B. Estimation of hourly averaged
solar irradia-tion: evluation of models. Building Service
Engineering Research and Tech-nology 2010;31(1):925.
[40] Gajo EJ, Etxebarria S, Tham Y, Aldali Y, Muneer T.
Inter-relationship betweenmean-daily irradiation and temperature,
and decomposition models forhourly irradiation and temperature.
International Journal of Low-carbonTechnologies 2011;6:2237.
[41]
/http://eosweb.larc.nasa.gov/cgi-bin/sse/[email protected].[42]
/http://www.tutiempo.net/en/S.[17] Stine W, Geyer M. Power from the
Sun, 2001, /http://www.powerfromthe 2012;16:163656.[37] Graham VA,
Hollands KGT, Unny T. A time series model for Kt
withpublications/journals/full/j138.pdfS.[16] Pitz-Paal R, Dersch
J, Milow B, Romero M, Tellez F, Ferriere A., et al. European
Concentrated Solar Thermal Road-Mapping-Executive Summary,
CEECOSTARContract: SES6-CT-2003-502578. (2005) 144.
Concentrated solar power plants: Review and design
methodologyIntroductionSolar irradiance as worldwide energy
sourceConcentrated solar power plants
CSP technologiesGeneralitiesSolar power towersParabolic trough
collectorLinear Fresnel reflectorParabolic dish systemsConcentrated
solar thermo-electrics
Comparison of CSP technologies
Past and current SPT developmentsEnhancing the CSP
potentialThermal energy storage systemsBackup systems
Computing global and diffuse solar hourly irradiationBackground
informationThe adopted model approach and equationsEstimating the
daily irradiationSequence of daysEstimation of the hourly diffuse
and beam radiationShortcut estimates, based on recorded
temperatures
Model parametersCommon measurement methods of solar
radiationAvailable informationSelected locations
Results and discussionCalculations of H0, H and HbMethodology to
apply the predictions in CSP design
ConclusionsReferences