![Operation of the novel Variable Velocity Receiver proposed in [75] vs. a Traditional External Tubular Receiver. Reprinted from Applied Thermal Engineering, Vol. 128, M.R. Rodríguez-Sánchez, A. Sánchez-González, D. Santana, Feasibility study of a new concept of solar external receiver: Variable velocity receiver, Pages No. 335-344, Copyright (2018), with permission from Elsevier.](https://www.researchgate.net/publication/325329982/figure/fig2/AS:11431281120751918@1676626049941/Operation-of-the-novel-Variable-Velocity-Receiver-proposed-in-75-vs-a-Traditional_Q320.jpg)
![Volumetric receiver used in [78]. Reprinted from Energy. Vol. 94, Roldán, M.I., Fernández-Reche, J. and Ballestrín, J., Computational fluid dynamics evaluation of the operating conditions for a volumetric receiver installed in a solar tower, Pages No. 844-856, Copyright (2016), with permission from Elsevier.](https://www.researchgate.net/publication/325329982/figure/fig3/AS:11431281120708034@1676626050262/Volumetric-receiver-used-in-78-Reprinted-from-Energy-Vol-94-Roldan-MI_Q320.jpg)
![Schematic diagram of the tower and heliostat field for the dual-receiver proposed in [81]. Reprinted from Applied Thermal Engineering, Vol. 91, Luo, Y., Du, X. and Wen, D., Novel design of central dual-receiver for solar power tower, Pages No. 1071-1081, Copyright (2015), with permission from Elsevier.](https://www.researchgate.net/publication/325329982/figure/fig4/AS:11431281120744037@1676626050353/Schematic-diagram-of-the-tower-and-heliostat-field-for-the-dual-receiver-proposed-in_Q320.jpg)
![Reflectance of new coating from Ref. [91]. Reprinted from Solar Energy Materials and Solar Cells, Vol. 174, Rodríguez-Palomo, A., Céspedes, E., Hernández-Pinilla, D. and Prieto, C., High-temperature air-stable solar selective coating based on MoSi 2–Si 3 N 4 composite, Pages No. 50-55, Copyright (2018), with permission from Elsevier.](https://www.researchgate.net/publication/325329982/figure/fig5/AS:11431281120708035@1676626050424/Reflectance-of-new-coating-from-Ref-91-Reprinted-from-Solar-Energy-Materials-and_Q320.jpg)
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Nonlinear Engineering 2019; 8: 10–31
Albert Boretti*, Stefania Castelletto, and Sarim Al-Zubaidy
Concentrating solar power tower technology:
present status and outlook
https://doi.org/10.1515/nleng-2017-0171
Received December 19, 2017; accepted February 21, 2018.
Abstract: The paper examines design and operating data
of current concentrated solar power (CSP) solar tower (ST)
plants. The study includes CSP with or without boost by
combustion of natural gas (NG), and with or without ther-
mal energy storage (TES). Latest, actual specic costs per
installed capacity are high, 6,085 $/kW for Ivanpah So-
lar Electric Generating System (ISEGS) with no TES, and
9,227 $/kW for Crescent Dunes with TES. Actual produc-
tion of electricity is low and less than the expected. Actual
capacity factors are 22% for ISEGS, despite combustion of
a signicant amount of NG exceeding the planned values,
and 13% for Crescent Dunes. The design values were 33%
and 52%. The study then reviews the proposed technol-
ogy updates to improve ratio of solar eld power to elec-
tric power, capacity factor, matching of production and
demand, plant’s cost, reliability and life span of plant’s
components. Key areas of progress are found in materi-
als and manufacturing processes, design of solar eld and
receiver, receiver and power block uids, power cycle pa-
rameters, optimal management of daily and seasonal op-
eration of the plant, new TES concepts, integration of solar
plant with thermal desalination or combined cycle gas tur-
bine (CCGT) installations and specialization of project.
Keywords: renewable energy; concentrated solar power;
solar tower; parabolic trough; natural gas boost; thermal
energy storage; molten salt; steam; Rankine cycles
*Corresponding Author: Albert Boretti, Department of Mechan-
ical and Aerospace Engineering (MAE), Benjamin M. Statler
College of Engineering and Mineral Resources, West Virginia
University, Morgantown, WV 26506, United States, E-mail: al-
boretti@mail.wvu.edu, a.a.boretti@gmail.com
Stefania Castelletto, School of Engineering, RMIT University, Bun-
doora, VIC 3083, Australia, E-mail: stefania.castelletto@rmit.edu.au
Sarim Al-Zubaidy, The University of Trinidad and Tobago, Trinidad
and Tobago, E-mail: sarim.alzubaidy@gmail.com
1Introduction
The basic principles of concentrated solar power (CSP) sys-
tems are covered in previous reference works such as [1–
5]. Lenses or mirrors concentrate the sun light energy on a
small area. The concentrated radiant energy is then con-
verted to heat at high temperature. The heat is nally
transferred to a power cycle working uid (typically wa-
ter/steam). Superheated steam typically drives a Rankine
steam turbine cycle. Concentrators dier in the way they
track the sun and focus the light. The most popular con-
centrating technologies are Parabolic Trough (PT) and So-
lar Tower (ST). Dierent concentrators provide dierent re-
ceiver temperature and peak temperature of the steam for
the power cycle, with correspondingly varying thermal ef-
ciency of the power cycle. In addition to the type of re-
ceiver and the solar eld feeding this receiver, also the re-
ceiver uid (RF) plays a role in the peak temperatures of
the steam. Current RFs include oil, molten salt (MS) or wa-
ter/steam. Intermediate heat exchangers are needed be-
tween oil or MS and water/steam. MS permits thermal en-
ergy storage (TES) in hot and cold reservoir to decouple
electricity production from availability of sun light. While
an additional MS circuit has been proposed as an ap-
pendage to existing CSP plants with oil as RF, MS provides
better outcome when used directly as the RF. Replacement
of oil with MS permits operation at higher temperatures for
higher steam temperature and higher eciency of power
generation. Additionally, it lowers the cost of TES. Direct
use of water/steam as a RF has the advantage of simplic-
ity, cost and sometimes eciency. However, this links the
production of electricity to sun availability. Condensation
of steam usually occurs in air-cooled towers. Water cooled
condensers may permit better power cycle eciencies but
are impractical in mostly desert locations.
By using the combustion of natural gas (NG), it is pos-
sible to drastically improve the match between production
and demand of CSP plants. However, boost by NG is rea-
sonable only if performed in minimal extent, for both ef-
ciency of energy use and regulations concerning emis-
sions of carbon dioxide. The use of NG in a combined cycle
gas turbine (CCGT) plant occurs with a fuel conversion e-
ciency that is about double the eciency of a CSP plant op-
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |11
erated with NG only (ηabove 60% vs. ηaround 30%). The
spreading in between the ηof a CSP plant and a NG fueled
plant is similarly large in cases of cogeneration, where the
gas turbine (GT) plant also features production of process
heat, for heating, cooling, desalination or other activities.
Therefore, it is not ecient to design a CSP ST plant requir-
ing a signicant NG combustion.
The ST technology oers theoretically higher e-
ciency because of higher temperature. However, the tech-
nology is also more demanding from economic and tech-
nical view-points. While ST plants are certainly less
widespread than PT plants [6], there is an open debate in
the literature about which CSP technology may have the
better perspectives.
The world largest CSP plant, Ivanpah Solar Electric
Generating System (ISEGS) uses ST technology. ISEGS is
made up of three installations one close to the other. The
second largest CSP project in the world, the Solar Energy
Generating Systems (SEGS) facility, is based on PT. This
project is made up of 8 dierent installations presently op-
erational. The net capacity of ISEGS is 377 MW, while the
net capacity of Solar Energy Generating Systems (SEGS)
SEGS II-IX is 340 MW. Both facilities use NG to boost the
electricity production. ISEGS uses NG in a greater extent
than the SEGS facilities. Both ISEGS and SEGS lack of TES.
The actual capacity factors (ϵ) of both installations (elec-
tricity produced in a year divided by the product of net ca-
pacity by number of hours in a year) is about 20% disre-
garding the boost by combustion of NG, which is however
not negligible as shown in [8] and here further discussed.
The global market of CSP is dominated by PT plants,
about 90% of all the CSP plants [6]. As per [6], back in
2010 the ST component of CSP was overshadowed by the
PT component, accounting for more than 90% of the to-
tal CSP installed capacity. The situation has not drastically
changed since then [7]. The majority of the larger CSP plant
projects under development/under construction are based
on the solar tower conguration.
The CSP technologies presently do not compete on
price with photovoltaics (PV) solar panels that have pro-
gressed massively in recent years because of the decreas-
ing prices of the PV panels and the much smaller operating
costs. While the uptake of solar energy is still minimal, the
CSP component is largely overshadowed by the PV com-
ponent [9]. While CSP may have the potential to play a key
role in balancing renewable energy production, the tech-
nology is presently living in the shadow of PV [9].
Both installed capacity (power) and electricity pro-
duction (energy) data show the CSP ST technology is over-
shadowed by the CSP PT technology, while overall the CSP
technology suers from the more competitive costs of PV,
while the total solar contribution to the global energy mix
is still minimal [6, 7, 9–11]. According to [7], in 2016 glob-
ally only wind and solar PV power systems have seen a
considerable growth in terms of capacity installed com-
pared to 2015 data. In 2016 globally the hydropower capac-
ity has been 1,096 GW (in 2015 1,071 GW), the bio-power
capacity has been 112 GW (in 2015 106 GW), the geother-
mal power capacity has been 13.5 GW, (in 2015 13 GW), the
CSP capacity has been 4.8 GW (in 2015 4.7 GW). Conversely
in 2016 the global wind power capacity has been 487 GW,
more than 10% higher than the year before (433 GW) and
the global solar PV capacity has been 303 GW, almost 40%
more than the 2015 value of 228 GW. The installed capacity
of CSP is only about 1.5% of the total solar power capac-
ity, where the installed capacity of PV is 98.5% of the total
solar power capacity.
As explained in Ref. [8], the installed capacity (power)
is particularly misleading in case of solar if used to indi-
cate the actual annual production of electricity (energy)
by these systems, as the capacity factors (electricity pro-
duced divided by the product of the installed capacity by
the number of hours in a year) of recent CSP plants, for ex-
ample ISEGS [12–15], are only about 20%, even conceding
the benet of a production boost by combustion of NG.
In terms of energy, according to the International En-
ergy Agency [10], the 2014 World electricity generation has
been 23,816 TWh, with Coal/Peat providing the 40.8%, NG
the 21.6%, Hydro the 16.4%, Nuclear the 10.6%, Oil the
4.3%, and Others, including all the Renewables the 6.3%.
This 6.3% is mostly wind. Presently, the total solar elec-
tricity generation in the world is only 1.05% of the total.
According to the United States Energy Information Admin-
istration [11], the net generation in the United States dur-
ing 2015 has been 31.2% by coal, 34.7% by NG, 20.2% nu-
clear, 6.7% by conventional hydroelectric, 5.7% by wind,
and 1.4% all solar. As CSP plants only represent 1.5% of the
worldwide installed capacity of solar electricity plants, the
total CSP contribution to the global energy mix is therefore
less than 0.02% [7, 10]. The situation in the United States
is not far from the world average. The contribution by CSP
ST is then an even smaller 0.002%.
This scenario is expected to drastically change in the
next few years, and there is a clear need to develop new
CSP ST technologies to match the signicant demand.
However, this requires signicant technology updates that
is unclear could be delivered.
Ref. [16] discusses the most relevant drivers and bar-
riers for the deployment of CSP in the EU by 2030. Apart
from supporting policies, the most relevant drivers are the
added value of CSP in terms of energy storage that makes
them more reliable compared to other renewable energy
12 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
technologies, and the substantial cost reductions that are
expected for the technology. The most relevant barriers are
the still very high cost of the technology when compared
to conventional power plants and other renewable energy
technologies, and the uncertainty of policies. Hence, re-
duction of costs of the CSP technology is the key factor for a
growth. Similarly, Ref. [17] discusses the main reasons why
Chinese and Brazilian energy policies so far have not been
focused on CSP. As the high Levelized Cost of Electricity
(LCOE) of large scale deployment of CSP technologies may
aect the competitiveness of national industry in global
markets, a comprehensive answer may only follow global
policies. CSP has not beneted so far from the global de-
mand that has boosted wind and solar photovoltaic with
their subsequent price reductions.
Aims of this paper are to provide an objective assess-
ment of the current costs and performances of CSP ST
plants, and then to survey the proposed technologies the
many issues that strongly limit the current outlook of CSP
ST. As the present costs and performances dier consider-
ably from the planned values, this negatively impacts on
the reliability of the gures circulated in the surveyed peer
review works.
2Methods
The specic renewable energy technology assessment is
performed based on actual costs and electricity produc-
tion data of existing power plants delivering electricity
to the grid and having a nameplate capacity exceeding a
threshold value.
The survey of development trends is then performed
based on a review of the latest literature that propose ad-
vances vs. the current design. These literature claims may
not translate in actual technology improvements as as-
serted in the papers.
3Assessment of present status of
CSP ST technology
Concentrating solar power (CSP) has been so far mostly
proposed and implemented in the parabolic through (PT)
technology. 90% of the CSP plants are indeed PT. Hence,
we start this state-of-the-art paragraph with a reference to
the PT technology to move then to the ST technology. Inci-
dents and accidents are not considered in the present anal-
ysis, similarly to the environmental impacts. The substan-
tial land requirements for CSP are also not included in the
analysis.
3.1 List of CSP ST plants
The number of existing CSP ST plants of signicant size
is very limited, and the time they have been operational
is also minimal. Additionally, not all the data needed are
publicly available. Hence, the full potential of the ST tech-
nology is not shown by the surveys of plants.
In the list of the ST plants of [18], here reproduced as
Table 1, there are only 34 CSP ST plants worldwide. Only 3
above 20 MW of capacity are operational, ISEGS of 377 MW
capacity since 2014, Crescent Dunes Solar Energy Project
(Tonopah) of 110 MW capacity since 2015, and Khi Solar
One of 50 MW capacity since 2016. The 377 MW ISEGS plant
only producing 703,039 MWh/year (2016), the output of a
medium to small scale CCGT plant, has the best data set
covering 3 years.
Without a proper, extensive real-world experience,
costs, reliability and production data over the plant life
span may only be poorly addressed.
3.2 Parabolic trough typical design
parameters
The most common CSP systems are PT. A PT is made up
of a linear parabolic reector concentrating the sun light
onto a tubular receiver. The receiver is located along the
focal line of the reector. The tubular receiver is lled with
a working uid. The RF may be oil, MS or water/steam. The
reectors follow the sun with tracking along a single axis.
The working uid is heated as it ows through the receiver
up to temperatures from 390 to 500 ◦C, depending on the
uid used. If oil or MS, this uid is then used as the heat
driving the production of steam for the power cycle in a
heat exchanger. The shaped mirrors of a PT focus the sun
light on a tube running along the focus line with an 80
times concentration. The sun light is absorbed by tube that
is often in a glass vacuum, and delivered to the RF.
PT are in principle less ecient than ST [19], as the
temperature of the hot source is typically larger in ST in-
stallations, and the eciency of a Carnot machine is well
known to change with the temperature of hot and cold
sources as
η= 1 −TC
TH
(1)
where THis the temperature of the hot source and TCis the
temperature of the cold source. They are however much
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |13
Table 1: List of solar power tower projects (from [18]).
Project capacity
MW status year start
Tamarugal Solar Energy Project 450 under development
Likana Solar Energy Project 390 under development
Ivanpah Solar Electric Generating System (ISEGS) 377 operational 2014
Copiapó 260 under development
Golmud 200 under construction
Aurora Solar Energy Project 135 under development
Huanghe Qinghai Delingha 135 MW DSG Tower CSP
Project 135 under development
NOOR III 134 under construction
Ashalim Plot B (Megalim) 121 under construction
Atacama-1 110 under construction
Crescent Dunes Solar Energy Project (Tonopah) 110 operational 2015
Golden Tower 100MW Molten Salt project 100 under development
Redstone Solar Thermal Power Plant 100 under development
SunCan Dunhuang 100 MW Phase II 100 under construction
Yumen 100MW Molten Salt Tower CSP project 100 under development
MINOS 52 under development
Hami 50 MW CSP Project 50 under development
Khi Solar One 50 operational 2016
Qinghai Gonghe 50 MW CSP Plant 50 under development
Shangyi 50MW DSG Tower CSP project 50 under development
Supcon Solar Project 50 under construction
Yumen 50MW Molten Salt Tower CSP project 50 under construction
Planta Solar 20 (PS20) 20 operational 2009
Gemasolar Thermosolar Plant (Gemasolar) 19.9 operational 2011
Planta Solar 10 (PS10) 11 operational 2007
SunCan Dunhuang 10 MW Phase I 10 operational 2016
Sierra SunTower (Sierra) 5operational 2009
Lake Cargelligo 3operational 2011
ACME Solar Tower 2.5 operational 2011
Jülich Solar Tower 1.5 operational 2008
Sundrop CSP Project 1.5 operational 2016
Jemalong Solar Thermal Station 1.1 operational 2016
Dahan Power Plant 1operational 2012
Greenway CSP Mersin Tower Plant 1operational 2012
simpler and they are less expensive to build and operate.
Hence, the much wider use [6].
Reference PT specications change with the RF and
the availability of TES (data from [8, 19–23]):
•For oil as the RF, the receiver temperature is 390 ◦C,
the peak ux on receiver is about 25 kW/m2, the hot
storage temperature is 390 ◦C, the cold storage tem-
perature is 290 ◦C, and the condenser temperature
for heat rejection is 40 ◦C.
•For MS (nitrate salt) as the RF, the receiver temper-
ature is 500 ◦C, the peak ux on receiver is about
25 kW/m2, the hot storage temperature is 500 ◦C, and
the cold storage temperature is 300 ◦C.
•In case of water/steam as the RF, the receiver tem-
perature is 500 ◦C, the peak ux on receiver is about
25 kW/m2. There are in this case no hot and cold stor-
age tanks.
3.3 Solar tower typical design parameters
A ST concentrates the sun light from a eld of heliostats
on a central tower. The heliostats are dual axis tracking re-
ectors grouped in arrays. They concentrate sunlight on
a relatively small central receiver located at the top of the
tower. The sun light with ST is much more concentrated
than in PT. The RF may be heated to temperatures from 500
to 1000 ◦C depending on the RFs and the solar concentra-
tion design. When MS is used, it serves as the heat driving
the production of steam for the power cycle in a heat ex-
changer. When water/steam is used, then there is no need
14 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
of this heat exchanger. The eld of heliostats focus the
light on top of the tower with a 500 to 1000 times concen-
tration. Light is absorbed by metal tubes and delivered to
the RF, either water/steam or MS (nitrate salt). Due to sun-
light shaded, blocked, absorbed, or spilled, there is a 40%
loss of incident light collected by the RF. Receiver, piping,
and tank thermal losses further reduced the amount of en-
ergy transferred to the power cycle.
The reference ST specications change with the RF
and the availability of TES (data from [8, 20, 24, 25]):
•For MS (nitrate salt) as the RF, the receiver tem-
perature is 565 ◦C, the peak ux on receiver is
1,000 kW/m2, the hot storage temperature is 565 ◦C,
the cold storage temperature is 290 ◦C, and the con-
denser temperature for heat rejection is 40 ◦C.
•In case of water/steam as the RF, the receiver tem-
perature is 550 ◦C, the peak ux on receiver is
>300 kW/m2. The hot and cold storage tanks are not
available in this case.
3.4 Additional information of existing plants
from thermal models
Computational tools are used for heat balance design of
thermal power systems, and for simulation of o-design
plant performance. Hence, thermal models are the best av-
enue to appreciate design variants in CSP ST plants from
the point of view of electricity output.
A scheme of the Rice CSP ST facility is provided in Fig-
ure 1 (from [26]). The Rice Solar Energy Project was a latest
generation CSP ST project [27] put on hold. The proposed
location was Rice, California (Mojave Desert, near Blythe).
The gross turbine capacity is 150 MW. The land area is
5.706 km2. The solar resource is 2,598 kWh/m2/year. The
planned electricity generation was 450,000 MWh/year.
The heliostat solar-eld aperture area is 1,071,361 m2. The
number of heliostats is 17,170, and every heliostat has an
aperture area of 62.4 m2. The tower height is 164.6 m.
The receiver type is external, cylindrical. The heat-transfer
uid is MS. The receiver inlet temperature is 282 ◦C, and
the receiver outlet temperature is 566 ◦C. The steam Rank-
ine power cycle has a maximum pressure of 115 bar. The
cooling method is dry cooling. The TES is achieved by rais-
ing MS temperature from 282 ◦C to 566 ◦C. The TES e-
ciency is 99%.
A scheme of the ISEGS ST facility discussed here after
(as well as schemes of PT installations such as the Kramer
Junction PT facility) is also provided in [26] and [8]. The
reader is referred to these models for the further detailed
information eventually needed to complement the infor-
mation here provided. The power cycle is a relatively sim-
ple Rankine cycle of limited eciency mostly due to the
Carnot law.
Fig. 1: Thermoflow scheme of the design point balance for the Rice
concentrating ST facility. Courtesy of Thermoflow, www.thermoflow.
com. All data extracted from public available sources, California
Energy Commission.
3.5 Operational parameters
Plant level data of electricity production and NG consump-
tion of CSP plants in the United States are provided in [28].
Data of [28] includes the energy input from both the sun
and the NG. Reference design data are provided in [27].
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |15
Capacity factors of CPS plants with and without NG
boost such as ISEGS (Jan-2014 start, ST, no TES), SEGS
(various starting dates, Oct-1990 SEGS-IX, PT, no TES) and
Solana (Dec-2013 starting date, PT, TES) have been dis-
cussed and compared in [8].
The capacity factor ϵ1is dened as the ratio of the ac-
tual electricity produced in a year E [MWh] with the prod-
uct of net capacity P [MW] by number of hours in a year:
ϵ1=E
P·8760 (2)
This capacity factor does not account for the consump-
tion of NG.
Ref. [8] suggests as a rst option to account for the NG
consumption by multiplying the above capacity factor by
the ratio of the solar energy input QSun to the total solar
energy and NG energy input QSun+ QNG , all in [MWh]:
ϵ2=E
P·8760 ·QSun
QSun +QNG
(3)
Ref. [8] suggests two other way to evaluate the net ca-
pacity factors.
A third capacity factor ϵ3is dened as the ratio of the
actual electricity produced reduced of the electricity pro-
duced by burning the NG in a gas turbine (GT) plant with
eciency ηGT =30%, with the product of net capacity by
number of hours in a year:
ϵ3=E−QNG ·ηGT
P·8760 (4)
A fourth capacity factor ϵ4is nally dened as the ra-
tio of the actual electricity produced reduced of the elec-
tricity produced by burning the NG in a CCGT plant with
eciency ηCCGT =60%, with the product of net capacity by
number of hours in a year:
ϵ4=E−QNG ·ηCC GT
P·8760 (5)
An important parameter not accounted for is the re-
quested electricity generation prole. Without TES, the
costs per installed capacity are lower, because there is cost
associate to the TES and because the turbine is oversized
compared to the solar eld. However, the electricity pro-
duction is strongly linked to the sun availability. More than
a simple comparison between capacity factors, it is impor-
tant to compare planned and actual capacity factors to un-
derstand the maturity of the technology.
3.6 Prior analyses of operational data
The capacity factors of the three largest CSP projects,
•ISEGS (ST, no TES, NG boost),
•SEGS (PT, no TES, NG boost)
•and Solana (PT, TES, no NG boost),
were analyzed in Ref. [8]. Results are summarized below:
•Over the period July 2016 to June 2017 (latest 12
months), without accounting for the NG consump-
tion, ϵ1were 22.98%, 21.59% and 23.67% for ISEGS
1-2-3, 22.54% for SEGS IX and 32.65% for Solana.
•By accounting for the consumption of the NG at the
actual energy conversion eciency ηof the plant,
ϵ2are smaller for ISEGS 1-2-3 at 19.20%, 18.04% and
20.12% and for SEGS IX at 19.91%, while obviously
ϵ2=ϵ1for Solana.
•By considering the energy conversion eciency of a
reference GT plant η=30%, the ϵ3are marginally bet-
ter in ISEGS 1-2-3 and SEGS IX, while ϵ3= ϵ1for Solana.
•Finally, by accounting for the consumption of the
NG at the energy conversion eciency of a reference
CCGT plant η=60%, ϵ4are much smaller for ISEGS 1-
2-3 at 15.83%, 14.80% and 17.07%, and much smaller
for SEGS IX at 17.91%, while ϵ4=ϵ1for Solana.
This analysis conrms the well-established fact [29–
34] that TES plays a signicant role in producing much
higher capacity factors in present installations, as by TES
the power available to the turbine is extended but it is also
reduced.
3.7 A critique to the boost by natural gas
The above analysis shows as the use of NG to boost the
electricity production in a CSP plant is not motivated, be-
ing the fuel energy used at a much lower eciency than in
a CCGT plant, and everything but cost eective. While the
hybridization with fossil fuels may be motivated by poli-
cies pushing renewable energy production rather than the
reduction of fossil fuel usage, the actual benets in terms
of pollution reduction and aordability of energy are quite
questionable [8]. Also considering all the issues of inte-
grating intermittent, variable renewable energy electricity
into the grid [35–37], there is no point for a better environ-
ment and a better economy of fostering hybridization of
CSP with fossil fuel plants as the simple upgrade of tradi-
tional plants burning fossil fuels possibly including much
cheaper solar ponds costs less while also reducing fossil
fuel consumption.
CCGT plants can work with an eciency as high as
62.21% (the 605 MW Bouchain power plant [38, 39]) with-
16 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
out any of the constraints of the renewable energy plants.
The Bouchain power plant replaces a prior coal plant on
25% of the footprint of the old plant. It is generating more
power with 50% less CO2emission.
CCGT plants do not have the constraints of the opera-
tion of renewable energy plants such as solar plants. These
plants have theoretically the opportunity to operate 24/7
all the year round except than the periods of the sched-
uled maintenance. Their capacity factors can exceed 90%
if contributing to the base-load, or at a lesser extent to
match the spikes in the load demand or to supplement the
intermittent renewable energy supply. In 2015, the CCGT
plants of the US have been operated with average capacity
factors of 56% [40].
While many policies dedicated to CSP still consider hy-
bridization to make the overall CSP investment more prof-
itable by increasing the usage of the power plant and sav-
ing the costs of TES, CSP should not be coupled to the
burning of fossil fuels. In our opinion, CSP ST only makes
sense as a standalone technology that does not require any
fossil fuel.
3.8 Analysis of latest costs and production
data
Table 2 provide the latest costs and production data of the
most recent CSP ST and PT installations. Opposite to many
projections in the literature, costs are here actual, as actual
is the production. As there are only 2 CSP ST projects op-
erational worldwide with a signicant capacity >50 MW,
ISEGS and Crescent Dunes, the rst without TES and the
second with TES, we also use data of 2 CSP PT projects
with and without TES, Solana and SEGS, to make a min-
imal statistic based on facts.
The ISEGS CSP ST plant had a cost (2014 values)
of 2,200 m$ for 377 MW of capacity, or 5,9836 $/kW. In
terms of 2017 values (US ination calculator [101]) this
corresponds to 2,294 m$ and 6,084 $/kW. The plant has
no TES. The heliostat solar-eld has an aperture area of
2,600,00 m2, the heat-transfer uid is water/steam, the re-
ceiver inlet temperature is 249 ◦C, the receiver outlet tem-
perature is 565 ◦C. The Rankine power cycle max pres-
sure is 160 bar. The latest production data of ISEGS [8, 12–
14], [15] indicates capacity factors of not more than 22-
23% despite burning substantial amounts of NG translat-
ing in signicant additional generating costs and pollu-
tion, with the actual capacity factor drastically reducing
of more than one third to less than 15% once the consump-
tion of NG is properly accounted for at the fuel energy con-
version eciency of a reference CCGT. The ISEGS power
plant was approved in 2014 to burn up to 525 million cu-
bic feet of NG per year [41] and in 2014, the plant emitted
46,084 metric tons of CO2by burning NG, that is twice the
pollution threshold at which power plants and factories
in California are required to participate in the state’s cap
and trade program [42]. While the planned electricity gen-
eration was an optimistic 1,079,232 MWh/year [12], or an
annual solar-to-electricity eciency of 28.72%, the actual
data [13–15] for the year 2014, 2015 and 2016 indicate an ac-
tual production of 419,085, 653,122 and 703,039 MWh/year
despite the huge consumption of NG 774,525, 1,245,986 and
1,290,308 MMBtu to boost production. NG consumption of
two months of 2014 is missing.
The SEGS installation comprises 8 dierent plants
presently operational, built over a time span of several
years. It consists of SEGS II located at Daggett, SEGS III–
VII located at Kramer Junction and SEGS VIII–IX located
at Harper Lake. SEGS I located at Daggett is not opera-
tional any more. SEGS X had been in construction but it
was never nished. SEGS XI and SEGS XII were planned
but never built. The 80 MW SEGS IX has Terminol as re-
ceiver uid, and a temperature at the exit of the 483,960 m2
solar eld of 390 ◦C. The Rankine power cycle max pres-
sure is 100 bar. [43] claims that a plant like the 30 MW SEGS
VI plant in Kramer Junction could have had in 2002 cost of
3,204 $/kW. This translates in an optimistic specic cost
of 4,398 $/kW 2017 values. [105] claims a capital cost of
99.3 m$ 1989 values for the specic plant, or 3,310 $/kW,
translating in a much larger 2017 cost of 6,584 $/kW. As the
costs for SEGS based on projections and not actual costs
are non-accurate, these values are not included in Table 2.
Production data and NG consumption of the SEGS fa-
cility is estimated from the latest SEGS IX plant data, the
more recent and best performing. The plant has no TES.
The capacity factors of SEGS IX are not far from the val-
ues of ISEGS, despite SEGS IX started operation almost 25
years before ISEGS.
The cost of Solana (PT, TES) is approximately
2,000 m$, 10% less than the ISEGS ST facility that was
completed only two months later, however for 34% less
net capacity [8]. The plant has TES. Solana has a specic
cost of 8,258 $/kW 2017 values. The production data of
Solana is much better than SEGS and ISEGS, with latest
capacity factors of about 33%, even if smaller than the
planned 43.1%. Solana is by far the best performing large
CSP plant in the United States. The solar-eld aperture
area is 2,200,000 m2, the heat-transfer uid is Therminol
VP-1 - Xceltherm MK1. The solar-eld Inlet Temperature
is 293 ◦C, the solar-eld outlet Temperature is 393 ◦C.
6 hours TES is obtained by using MS with two tanks. The
Rankine power cycle max pressure is 100 bar.
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |17
The 110 MW Crescent Dunes Solar Energy Project [44,
45], the only other project of reasonable size in the world
featuring the CSP ST technology and currently opera-
tional, had a cost of 975 m$ 2015 values, corresponding
to 1,015 m$ of 2017. This corresponds to 8,864 $/kW 2015
values or 9,227 $/kW 2017 values. The heliostat solar-eld
has an aperture area of 1,197,148 m2, the heat-transfer uid
is MS, the receiver inlet temperature is 288 ◦C, the re-
ceiver outlet temperature is 565 ◦C. The plant has TES and
no boost by NG. The TES has 2-tanks, storage capacity 10
hours, TES achieved by raising salt, temperature from 288
to 565 ◦C. The Rankine power cycle max pressure is 110 bar.
The project started operation in November 2015. While
the planned electricity generation was 500,000 MWh/year
(capacity factor 51.89%), the actual electricity produced
in 2016 was only 127,308 MWh/year. During the year 2016,
the plant was operated 10 months January to October. The
plant has been so far out of service for about one half of the
lifetime, from November 2015 to July 2017. Crescent Dune
was indeed shut-down in October 2016 due to a leak in a
MS tank and it returned to operation only in July 2017. By
using the actual production of 2016, the capacity factor is
only 13%.
From Table 2 we may conclude that PT may possibly
permit lower costs than ST, while TES signicantly add to
the cost of the plant, but apart from reliability issues, it
also improves the electricity production.
According to the latest construction cost data for elec-
tric generators installed in 2015 by the US Energy Informa-
tion Administration [46], capacity weighted costs in $/kW
were 1,661 $/kW for wind, 696 $/kW for NG, 2,921 $/kW
for solar photovoltaic. According to the latest (2017) out-
look of cost and performance characteristics of new gen-
erating technologies by the US Energy Information Ad-
ministration [47], solar thermal is about 3,908 $/kW, so-
lar photovoltaic 2,169 $/kW, wind 1,576 $/kW, wind o-
shore 4,648 $/kW, conventional combustion turbine is
1,040 $/kW, advanced combustion turbine is 640 $/kW.
We may also conclude from Table 2 that the values of [47]
are optimistic, i.e. the actual costs dier considerably
from the estimates of expert panels. While Table 8 - Cost
and performance characteristics of new central station
electricity generating technologies of [47] reports a base
overnight cost in 2016 (2016 $/kW) of 3,908 for solar ther-
mal, ISEGS, Solana and Crescent Dunes have 2017 costs of
6,085, 8,258 and 9,227 $/kW.
It must be mentioned that the capacity weighted cost
is a measure largely in favor of renewable energy plants,
typically delivering much smaller capacity factors than
fossil fuel plants. The actual costs of CSP per installed ca-
pacity of ISEGS, SEGS, Solana and Crescent Dunes are gen-
erally high, and much larger than the planned values. Ad-
ditionally, the actual production of electricity of ISEGS,
SEGS, Solana and Crescent Dunes has been much less than
the planned values, and by far much smaller than the val-
ues potentially achievable in CCGT plants [8]. In the cases
of ISEGS and SEGS, the NG consumption has further im-
pacted on the economy and the emissions [8], even if sur-
prisingly this consumption is not properly accounted for.
Opposite to SEGS, the costs of ISEGS and Solana, that
started production almost simultaneously, are accurate,
similarly to Crescent Dunes, that started production only
one year later.
Maintenance or rectication costs are unknown and
hence not included in the table.
Data for ISEGS are from [12–15], data for Solana are
from [48, 49], data for Crescent Dunes are from [44, 45],
and data for SEGS IX are from [50, 51] as per the retrieved
date.
3.9 Discussion of the costs and production
data analysis
It may be argued that the above analysis is based on a small
population. This is unfortunately the best possible data
base if we want to rely on plants built and operational.
As it is not uncommon that ambitions renewable energy
projects especially CSP are never completed (Rice was put
on indenite hold in 2014 for nancial issues, while SEGS
X was just started, and SEGS XI and XII were only planned,
when their developer led for bankruptcy in 1992), we pre-
fer to discuss the state of the art of a technology based on
actual data of costs and electricity production and not on
expectations that may prove to be wrong.
The latest list of CSP projects worldwide published
by the National Renewable Energy Laboratory [27] in-
cludes 184 projects. However, 10 projects are currently
non-operational, and 78 are under construction, contract
or development. Of the 96 operational, only 7 have net ca-
pacity more than 100 MW and only 4 have net capacity
more than 150 MW. Discussing the CSP contribution to the
global energy mix, it does not make any sense to consider
small size plants. CSP plants of capacity not exceeding
150 MW, with capacity factors of the order of 0.2, translate
in annual power outputs not exceeding 262.8 GWh, that is
not certainly indication of an important contribution.
All the 4 plants of net capacity exceeding 150 MW are
in the United States. These are the 377 MW Ivanpah So-
lar Electric Generating System (ISEGS) and the 250 MW
each Solana Generating Station mentioned in the above
analysis, plus Genesis Solar Energy Project and Mojave So-
18 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
Table 2: Actual costs and performances of the ISEGS, SEGS, Solana and Crescent Dunes plants. Notes: (1) SEGS IX only. (2) January to Octo-
ber 2016.
Project ISEGS 1-3 SEGS II-IX SOLANA Crescent
Dunes
Type ST PT PT ST
TES no no yes yes
NG combustion yes yes no no
year start Jan-14 Oct-90 (1) Dec-13 Oct-15
capacity MW 377 340 250 110
planned solar electricity generation MWh/year 1079232 NA 944000 500000
planned solar capacity factor 32.68% NA 43.11% 51.89%
latest 12 months capacity factor not
accounting for the NG consumption 22.75% 22.54% (1) 32.65% 13.21% (2)
latest 12 months capacity factor accounting for
the NG consumption η=60% 15.90% 17.91% (1) 32.65% 13.21% (2)
2017 reference cost m$ 2294 NA 2065 1015
2017 specic cost $/kW 6085 NA 8258 9227
lar Project. Genesis and Mojave Solar Project are parabolic
trough but no thermal energy storage. The 7th largest CSP
plant in the world, the 110 MW Crescent Dunes Solar En-
ergy Project, is also in the United States. We also included
Crescent Dunes in our analysis. The 5th and 6th plants are
not in the United States. The 146 MW Noor I is in Mo-
rocco, and the 125 MW Dhursar is in India. While Noor I is
parabolic trough, Dhursar is Fresnel. For Noor I, we do not
have same data of actual costs and electricity production
we have mined for the United States plants. ISEGS started
production January 2014, Solana October 2013. Genesis
March 2014, Mojave Solar Project December 2014 and Cres-
cent Dunes November 2015. Hence, all of them are very re-
cent. This is the state of the art of the technology.
The SEGS plants are also included in the analysis.
The total 354 MW of installed capacity unfortunately came
from 8 dierent individual plants. By considering the two
largest 80 MW plants in Harper Lake a single installation
(as it is done for the 3 installations of Ivanpah), we match
the requirement of more than 150 MW of power. As SEGS
has been built in the 1980s, this latter plant gives an idea
how much we have progressed over the last three decades.
The latest (2016) actual electricity production of
ISEGS; Solana; Genesis; Mojave Solar Project; Crescent
Dunes and SEGS IX are analyzed in [106]. From their net
capacities, the actual capacity factors are 21.29%; 29.39%;
28.50%; 28.53%; 13.21% and 22.54%. The planned capacity
factors are conversely 32.68%; 43.11%; 26.48%; 27.40% and
51.89% while this parameter is not available for SEGS IX.
In case of Ivanpah and in lesser extent SEGS IX, the capac-
ity factors drastically reduce by accounting for the natural
gas combustion. The result of the further expanded data
base of [106] conrm the results of Table 2. The more con-
solidated parabolic trough technology without any molten
salt thermal energy storage of Genesis and Mojave Solar
Project appears to be superior to the most sophisticated so-
lutions still suering of the lack of maturity and reliability
such as Ivanpah and Crescent Dunes and in a minor extent
Solana.
4Survey of development trends in
CSP ST technology
In Section 2 we provided the actual data of power plants
currently operational, including energy outputs and costs
of ISEGS (ST), SEGS (PT), Solana (PT, MS TES) and Cres-
cent Dunes (ST, MS TES). ISEGS and Crescent Dunes are
the state-of-the-art of the operational CSP ST technology.
Based on the real-world experience, the outlook of the CSP
ST technology is less brilliant than expected by projec-
tions. Hence, there is an urgent need to further progress
the technology rather than promoting production of sub-
standard plants. Here we survey the developments being
sought to improve the current CSP ST technology, with the
advertence that the discrepancies in between actual and
projected costs and production data make the claims of the
literature speculative. Goal of the survey is only to list the
most promising development trends.
On the bright side, as explained in [52, 53], what is
called the “learning rate”, i.e. the cost reduction follow-
ing the expansion of a technology, may give hope. The
short-term survival of CSP depends on accelerated expan-
sion and sucient cost decrease. Ref. [52] identied the
learning rate of CSP and found that it exceeded 20% in the
last 5 years. Each time the global CSP capacity doubled,
investment costs decreased by over 20%. While this g-
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |19
ure may be optimistic, as it is larger than prior estimates
such as [102–104], and it is still based on a very scattered
and incomplete data base, for sure there is a learning rate.
Ref. [52] found the learning rate to be highly volatile, re-
ecting the start-stop pattern of the expansion of CSP ex-
perienced so far. Hence, continuity in the R&D eort as
well as in the development of an industrial product may
ultimately deliver the sought benets from CSP. Latest cost
reductions mentioned in [53] are certainly an indication
of the benets of expanding the CSP ST technology know-
how.
Design, construction and operating technical and eco-
nomic issues are considered in the literature to various
extents. CSP ST have many variants for receivers, work-
ing uids, power cycles, type, number and layout of he-
liostats, height of tower, condenser, turbine, heat exchang-
ers and TES. As an example of a preliminary introductory
survey, Ref. [54] examines some of the main parameters
of existing plants, solar energy to electricity conversion ef-
ciency, and mirror and land area per MWeof capacity,
packing density, conguration of the eld layout, receiver
size, tower height and cost of the plant.
4.1 Energy storage and heat transfer fluid
Here we report on the improvement being sought by using
a TES with dierent options for the receiver and the TES
uids. This is the main area of development being consid-
ered, as the added value of CSP compared to other renew-
ables is the ability to produce electricity potentially 24/7
through TES.
TES is the key to achieve high capacity factors and
avoid NG boost. TES allows improved dispatch-ability
(generation on demand) of power from a CSP plant [19].
TES drastically increases the annual capacity factor [8].
The MS TES technology is the best avenue to generate non-
intermittent electricity with CSP and achieve capacity fac-
tors above 0.3, and potentially up to 0.4. A 10 hour TES
eliminates the need for a NG back up or boost of electricity
production at sunrise and in the evening peak hours [8].
Next generation CSP plants will very likely consist of
four major units, solar eld to concentrate the sun light en-
ergy, ST MS receiver to convert the solar energy into ther-
mal energy, TES section to store the thermal energy us-
ing the MS, and nally power block generating electric-
ity through a steam turbine. While the cost will further in-
crease because of the TES, it will be paid back by the in-
creased production and dispatch-ability.
The current best RF and TES uid is MS that, how-
ever, has the drawback of having low degradation temper-
ature and high melting temperature, in addition to other
downfalls such as corrosion and heat tracing. Solar salt,
60% NaNO3and 40% KNO3, is used as a low-cost RF and
TES uid. MS temperatures typically go up to 565 ◦C. This
permits superheated steam generation. MS has good heat
transfer characteristics [55]. As major downfalls, the salt is
freezing below 220 ◦C, heat tracing is required, and drain-
ing of receiver and other system components during the
night must be provided. Furthermore, the salt may de-
grade at temperatures higher than 600 ◦C and depending
on salt quality it can generate corrosion of metallic com-
ponents [55].
Alternative uids are therefore under investigation for
a broader range of operation and for cost and performance
advantages, as RF and/or the TES uid. The power block
uid is usually water/steam, but other uids are also con-
sidered for the power block, as it is discussed in another
paragraph. There is obviously the opportunity to use a
single uid as receiver, TES and power block uid. Wa-
ter/steam is the most obvious example.
Ref. [56] reviewed various types of RF including air,
water/steam, thermal oils, organic uids, molten-salts and
liquid metals. The dierent alternatives were compared
with reference melting temperature, thermal stability and
corrosion with stainless steels and nickel based alloys the
piping and container materials. MS shows advantages op-
erating up to 800 ◦C.
Dierent alternatives for the RF are mentioned in [55].
The presentation includes alternative RF as well as re-
ceiver technologies. MS, water/steam, air in open/closed
systems, liquid metals, solid particles and other gases are
considered as heat transfer medium. Classication is by
maturity of technologies. It includes MS and water/steam
as state of the art technology, open volumetric air receiver
as “rst-of-its-kind” technology, then pressurized air re-
ceivers as technology in pilot phase, liquid metals and
solid particles as technology under development. The dif-
ferent receiver technologies proposed in [55] are reviewed
in a subsequent specic paragraph.
The impact of the uid in a at plate, high tempera-
ture, TES unit with at slabs of phase change materials is
studied in [57]. Six gaseous and liquid uids are compared.
For the capacity rate considered, liquid sodium was the
best performing (99.4% of the ideal electricity to grid). So-
lar salt achieved a 93.6% performance. Atmospheric air, air
at 10 bars, s-CO2at 100 bar and steam at 10 bar achieved
performances between 87.9% and 91.3%. The work con-
cludes that gases are comparable to liquids as TES uids
for the specic application and it mentions that gases may
also be used as the working uid in the power block.
20 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
CSP TES systems are also reviewed in [58]. Various as-
pects are discussed including trend of development, dier-
ent technologies of TES systems for high temperature ap-
plications (200–1000◦C) with a focus on thermochemical
heat storage, and storage concepts for their integration in
CSP plants. TES systems are considered a necessary option
for more than 70% of the new CSP plants being developed.
Sensible heat storage technology is the most used TES in
CSP plants in operation, for their reliability, low cost, easy
to implementation and large experimental feedback. La-
tent and thermochemical energy storage (TCES) technolo-
gies have much higher energy density. This gives them bet-
ter perspectives for future developments. TCES are specif-
ically covered in a following paragraph. New concepts for
TES integration include coupled technology for higher op-
erating temperature and cascade TES of modularized stor-
age units for intelligent temperature control.
The current commercial TES systems used in CSP
plants either steam accumulators or MS are reviewed
in [59]. The economic value of the TES system is assessed
by the calculation of the LCOE, an economic performance
metric, of the TES itself rather than the full plant. Calcula-
tions were done for dierent plant congurations and stor-
age sizes varying from 1 to 9 h of operation at full capacity.
LCOE is shown to be a valid argument for the selection of
the TES, even if other aspects not included also play a rel-
evant role.
The opportunity to adopt particle suspensions as RF,
TES uid, and power block uid is considered in [60]. Val-
ues of the heat transfer coecient up to 1,100 W/m2/K
(bare tubes) and 2,200 W/m2/K (nned tubes) were ob-
tained for operation of a pilot plant at low supercial gas
velocities of 0.04–0.19 m/s limiting heat losses by the ex-
haust air. Despite additional costs for particle handling
and an appropriate boiler, the required overall investment
and operating costs are signicantly lower than the refer-
ence MS system, leading to a reduction in LCOE from ap-
proximately 125 €/MWh to below 100 €/MWh.
The developments of the last ve years and expected
for the near future of the most important components of
a CSP ST are water/steam, air or CO2power cycles; wa-
ter/steam, MS, liquid metals, particles or chemically react-
ing uids and the RF; design of heliostats; design of re-
ceivers, volumetric, tubular, solar particle receivers; TES
and hybridization are all reviewed in [61]. They conclude
that there will certainly be an increased number of CSP ST
in the near future, but with a signicant hold-back until
standardization and experience is gained. Within the next
5 years, plants will use either MS or water/steam as the re-
ceiver uid, but most of them will have MS TES. In a 10
years’ time, more plants will be MS with TES. However, ac-
cording to [61], “The commercial plant designs in 10 years
will look not much dierent than the commercial plant de-
signs today”, which is a not desirable option.
Thermochemical energy storage has been proposed to
replace the TES. This opportunity is in a very early stage
of development. In addition to the classic TES design with
two tanks of a properly selected TES uid, TCES systems
have been also proposed. TCES is the reversible conversion
of solar-thermal energy to chemical energy.
The TCES systems are reviewed in [62]. TCES has high
energy density and low heat loss over long periods than
the MS TES. CSP plants with TCES systems are modelled,
and sample computational results are provided for ammo-
nia and methane systems with two gas storage options.
The gas storage is identied as the main cost driver. The
compressor electricity consumption is identied as the
main energy driver.
Ref. [63] reports of a pilot-scale redox-based TCES sys-
tem. The storage unit is made of inert honeycomb sup-
ports (cordierite) coated with 88 kg of redox active ma-
terial (cobalt oxide). When crossing respectively the re-
duction/oxidation temperature of the Co3O4/CoO pair, the
heat absorbed or released by the chemical reaction allows
to store or release energy at constant temperature. Within
the limit of a campaign of 22 thermochemical charge/dis-
charge cycles, there was no measurable cycle-to-cycle
degradation. The system average capacity was very close
to the ideal case. The TCES system oers a storage capac-
ity of 47.0 kWh vs. the 25.3 kWh of the same volume of
a sensible-only storage unit made of uncoated cordierite
honeycombs.
4.2 Power cycles and power cycles fluids
Here we report on the improvement being sought by us-
ing dierent power cycles and power cycle uids. This is
an area of development receiving signicant attention that
may possibly produce results within the next decade.
Supercritical steam [64] and supercritical CO2
(sCO2) [65] power cycles are being considered to improve
the conversion eciency thermal-electric cycles. Ref. [66]
computed the thermodynamic irreversibility such as con-
vective and radiative loss on tower receiver and thermal
resistance in heat exchangers. Increasing the receiver
working temperature increases both thermal and exergy
conversion eciencies only until an optimum tempera-
ture is reached. The optimum temperature increases with
the concentration ratio. Increasing the concentration
ratio, the conversion eciency increases only until an
optimum concentration ratio is reached. Increasing the
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |21
end reversible engine eciency increases the thermal
conversion eciency until a maximum value is reached.
Then, the conversion eciency drops dramatically.
The performance of an integrally geared compressor-
expander recuperated recompression cycle with sCO2as
the working uid is modeled in Ref. [67]. Mostlythrough re-
duced power block cost and a better cycle model, the LCOE
is computed to be 5.98 c/kWh.
Advanced power cycles under consideration for CSP
are reviewed in [68]. Supercritical steam turbines are at-
tractive at large scale but presently commercial products
are too large for today’s CSP ST plants. SCO2closed loop
Brayton cycles are early in their development but promise
high eciency at reasonable temperatures across a range
of capacities. In perspective, these cycles may signicantly
lower the costs. GT combined cycles driven by CSP are one
of the highest eciency options available. Other bottom-
ing and topping cycle congurations are also considered.
High temperature component demonstration is indicated
as a critical factor.
Three dierent sCO2power cycles applied to a high
temperature ST CSP system are considered in Ref. [69].
Maximum temperatures are up to 800 ◦C. The uid trans-
ferring energy from the receiver to the power block is KCl-
MgCl2MS. The highest eciency at design conditions is
achieved by the Recompression with Main Compression
Intercooling (RMCI) conguration with a solar energy to
electricity eciency of 24.5% and a maximum temperature
of 750 ◦C. The capacity factor is 18.4%. The performance
decay from design to average yearly conditions is mostly
due to the optical and thermal eciencies reduction re-
spectively of −10.8% and −16.4%.
Several current sCO2Brayton cycles for integration
into a MS CSP ST system are reviewed in [70]. The in-
tercooling cycle can generally oer the highest eciency,
followed by the partial cooling cycle, and the recompres-
sion cycle. The pre-compression cycle can yield higher ef-
ciency than the recompression cycle when the compres-
sor inlet temperature is high. The increase in the hot salt
temperature cannot always result in an eciency improve-
ment. The partial cooling cycle can oer the largest spe-
cic work, while the recompression cycle and the split ex-
pansion cycle yield the lowest specic work. The MS tem-
perature dierences with the simple recuperation cycle,
the partial-cooling cycle, and the pre-compression cycle
are slightly larger than those with the recompression cy-
cle, the split expansion cycle, and the intercooling cycle.
Reheating can decrease the system eciency in the cases
with high hot MS temperature. Larger MS temperature dif-
ference may be achieved without reheating than with re-
heating. While current sCO2Brayton cycle oer high e-
ciency, challenges for integrating them includes the spe-
cic work that is relatively small, and the temperature dif-
ference across the solar receiver that is narrow.
More ecient Rankine power cycles are studied in [71].
The temperature and pressure of the main steam and the
reheating pressure aect the temperature of the MS in
the receiver. If the temperature increases, the receiver e-
ciency decreases but the power block eciency increases.
If the pressure at the inlet of the turbine increases, the
eciency of the power block increases even more than
by increasing the temperature. The reheating pressure
is the most inuential factor on the plant eciency. A
high reheating pressure decreases the plant eciency. The
best eciencies were obtained for the supercritical cycle
with a low reheating pressure and high temperature. The
subcritical cycle at high pressure and temperature per-
formed closely. The investment cost of the dierent cy-
cles increases with the pressure and the temperature of
the power block. Subcritical cycles are less expensive than
supercritical cycles even if the cost increase is balanced
by the eciency increase. Subcritical cycles working at
16 MPa and supercritical cycles working with low reheat-
ing pressure deliver the same cost per MWe.
Energy and exergy analyses of sCO2recompression
Brayton cycles are proposed in Ref. [72]. The heliostat eld
layout is optimized for the optical performance on an an-
nual basis. A recompression Brayton cycle uses the heat
collected at the receiver. An auxiliary boiler is added prior
to the turbine to keep the turbine inlet temperature con-
stant. The net power output is constant 40 MW. The highest
exergy destruction occurs in the heliostat eld. The second
highest exergy destruction happens in the boiler’s com-
bustion chamber. The combustion exergy destruction rate
increases during the winter months when the solar radia-
tion decreases.
The thermal performance of an array of pressurized air
solar receiver modules integrated to a GT power cycle for
a simple Brayton cycle, a recuperated Brayton cycle, and a
combined Brayton-Rankine cycle are studied in [73]. The
solar receiver’s solar energy to heat eciency decreases
at higher temperatures and pressures. The opposite is true
for the power cycle’s heat to work eciency. The optimal
operating conditions are achieved with a preheat stage for
a solar receiver outlet air temperature of 1300 ◦C and an
air cycle pressure ratio of 9, yielding a peak solar energy to
electricity eciency of 39.3% for the combined cycle.
Alternative cycles’ technology certainly needs more
work before introduction in full scale CSP ST plants where
water/steam Rankine cycles are the best short-term (10
years’ time window) solution. According to [61], citing a
private communication, only in perhaps 10 years from
22 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
now, there could be a shift to supercritical CO2-based
plants.
4.3 Design of solar eld and receiver
Here we report on the improvement being sought by re-
designing solar eld and receiver. This is an area of devel-
opment also receiving signicant attention, that may pos-
sibly produce signicant results within the next decade.
According to Ref. [54], the annual solar energy to elec-
tricity conversion eciency corresponds to an average of
about 16%. The packing density has an average of about
20%.
A classication by maturity of receiver technologies is
proposed in [55] and has been included in a prior para-
graph. In addition to MS and water/steam state of the art
technologies, open volumetric air receiver, pressurized air
receivers,liquid metals and solid par ticlesare all technolo-
gies being developed at dierent stages of evolution. As
the receiver design is not decoupled from the design of the
solar eld, here we couple together these two aspects.
Gas receivers, liquid receivers, and solid particle re-
ceivers are reviewed in [74]. Higher thermal-to-electric ef-
ciencies of 50% and higher may be achieved by using
sCO2closed-loop Brayton cycles and direct heating of the
CO2in tubular receiver designs, external or cavity, for high
uid pressures of about 20 MPa and temperatures of about
700 ◦C. Indirect heating of other uids/materials that can
be stored at high temperatures such as advanced MS, liq-
uid metals, or solid particles are also possible, but with
additional challenges such as stability, heat loss, and the
need for high-temperature heat exchangers.
As per [55], strategies aimed at improving MS systems
include higher temperature MS, higher steam parameters,
smaller heat exchanger, smaller storage, less critical re-
ceiver temperature operation. Means to improve the re-
ceiver eciency include reduction of thermal losses, cav-
ity arrangement, face down can design, standard vacuum
absorber for rst temperature step, and selective coatings
for higher absorption of solar radiation [55]. Optimization
of operation includes real time aim point strategy for ho-
mogenous receiver temperature, solar pre-heating of re-
ceiver, faster start-up and elimination of draining of re-
ceiver during clouds [55].
The improvement of the solar ux intercepted by the
receiver to increase the peak ux is considered in [75]. They
propose a new receiver, named Variable Velocity Receiver
(VVR), Figure 2, consisting of a Traditional External Tubu-
lar Receiver (TETR) equipped with valves permitting the di-
vision of each panel in two independent panels. This in-
creases the velocity of the heat transfer uid in specic
zones of the receiver avoiding tube overheating. The novel
design also permits better aiming strategies, for an im-
proved optical eciency of the solar eld and a possible
reduction of the number of heliostats.
The size of the solar eld required by a VVR is 12.5%
smaller than the size required by a traditional TETR. Addi-
tionally, the VVR provides advantages for the winter op-
eration when the panels can be split in two, increasing
the number of passes and the velocity of the heat transfer
uid.
High temperatures, thermal shocks and temperature
gradient from a high, non-homogeneous and variable
ux on the receiver walls are responsible for signicant
stresses. These stresses reduce the life-span of the receiver.
Ref. [76] proposes an open loop approach to control the
ux density distribution delivered on a CSP ST at plate re-
ceiver. Various distributions of aiming points on the aper-
ture of the receiver are considered. The approach provides
interesting indications for the control of the heliostats that
may drastically improve the life-span of the component.
The optimization of a solar eld layout with heliostats
of dierent size is considered in [77]. Although the use of a
single heliostat size is openly questioned in the literature,
there are no tools to design elds with heliostats of dier-
ent sizes in the market. The paper addresses the problem
of optimizing the heliostat eld layout with two heliostats’
sizes.
Ref. [78] numerically studied the inuence of wind
and return air on a volumetric receiver. Figure 3 presents
a sketch of the receiver. The volumetric receiver is a highly
porous material which absorbs solar radiation at dierent
depth through its thickness. The eective area for solar ab-
sorption is larger than that of thermal radiation losses. A
fan draws air through the absorbent pores, and the convec-
tive ow captures the heat absorbed. Thanks to the volu-
metric eect [79], the absorber thermal radiation loss is re-
duced.
Ref. [80] optically simulated the solar light radiation
transmission from the heliostat eld to a pressurized vol-
umetric receiver. The optical eciency of the heliostats’
eld and the local heat ux distribution within the ab-
sorber is computed as a function of time and date, he-
liostats tracking error and receiver mounting height. The
heat ux distribution within the absorber is non-uniform.
The maximum heat ux density at the top area is up to
2.58·109W/m3. The pattern of eld eciency and maxi-
mum heat ux density of the absorber resembles those
of the solar altitude angle during a day/year. The annual
mean eld eciency and the maximum heat ux of the ab-
sorber decrease as the tracking error increases. As the re-
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |23
Fig. 2: Operation of the novel Variable Velocity Receiver proposed in [75] vs. a Traditional External Tubular Receiver. Reprinted from Applied
Thermal Engineering, Vol. 128, M.R. Rodríguez-Sánchez, A. Sánchez-González, D. Santana, Feasibility study of a new concept of solar exter-
nal receiver: Variable velocity receiver, Pages No. 335-344, Copyright (2018), with permission from Elsevier.
ceiver mounting height increases, both these parameters
are marginally increasing.
Fig. 3: Volumetric receiver used in [78]. Reprinted from Energy, Vol.
94, Roldán, M.I., Fernández-Reche, J. and Ballestrín, J., Computa-
tional fluid dynamics evaluation of the operating conditions for a
volumetric receiver installed in a solar tower, Pages No. 844-856,
Copyright (2016), with permission from Elsevier.
A dual-receiver with a surrounding solar eld is pro-
posed in [81], Figure 4. The design couples an external
boiling receiver and a cavity superheating receiver. The
design provides a simple yet controllable heat ux distri-
bution on both sections. The dual-receiver may produce
superheated steam of 515 ◦C and 10.7 MPa with a solar
heat absorbing eciency of 86.55%. The eciency im-
provement compared to two-external cylindrical receivers
is 3.2%.
A Multi-Tube Cavity Receiver (MTCR) was optically
modelled in [82]. The solar ux exhibits a signicant non-
uniformity, showing a maximum ux of 5.1·105W/m2on
the tubes. When considering the random eect on the solar
ux distribution, it is a good practice to treat the tracking
errors as the random errors of the tracking angles. Multi-
point aiming strategy of tracking helps to homogenize the
ux and reduce the energy mal-distribution among the
tubes. The tubes absorb 65.9% of the energy. The optical
loss can be reduced signicantly by the cavity eect, espe-
cially when the coating absorptivity is relatively low.
Heliostats account for about 50% of the capital cost
of CSP ST plants. In conventional heliostats with verti-
cal pedestals and azimuth-elevation drives, the support
structure contributes 40–50% of this cost due to heavy
cantilever arms required by the large spanning structures.
Additional costs are imposed by expensive, dicult to
maintain, drive mechanisms. Ref. [83] shows that a tri-
pod heliostat substantially addresses these shortcomings
for heliostats with aperture areas of 62 to 100 m2. Ray-
tracing simulations are included to estimate the perfor-
mance penalties due to deformation under gravity and
wind loads. The additional energy collection by a less-
sti, larger heliostat more than osets the waste due to
the greater deformation. The economics of CSP ST plants
are strongly dependent on the cost of the heliostats rather
than their optical performance. The cost of a tripod helio-
stat is reduced to $72/m2which is less than half that of the
conventional systems.
The thermal performance of a cavity receiver in a CSP
ST plant that relies on the spatial relationship of its poly-
hedral geometric inner surfaces is studied in [84]. Based
on model results, the thermal eciency of the cavity re-
ceiver is shown to increase with the increase of incident
24 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
Fig. 4: Schematic diagram of the tower and heliostat eld for the
dual-receiver proposed in [81]. Reprinted from Applied Thermal
Engineering, Vol. 91, Luo, Y., Du, X. and Wen, D., Novel design of
central dual-receiver for solar power tower, Pages No. 1071-1081,
Copyright (2015), with permission from Elsevier.
heat ux. When the width-depth ratio decreased, the cav-
ity eciency increased rst and then decreased. The total
heat loss of the receiver varied dierently with the increase
of the heat absorption area to the aperture area ratio.
The thermal eciency of multi-cavity CSP ST receivers
was modelled in [85]. There is an optimal aperture ux
that maximizes the local eciency. This optimum is con-
strained by the maximum receiver working temperature.
For this aperture ux, the thermal eciency, receiver tem-
perature, and RF temperature are calculated for an opti-
mized ux distribution. In the proposed case study, it was
found that a RF with a minimum convection coecient be-
tween 250 and 500 W/m2/K, permits to achieve a receiver
thermal eciency greater than 90%.
An array of high temperature pressurized air based
solar receivers for Brayton, recuperated, and combined
Brayton-Rankine cycles was investigated in [86]. The clus-
ter of 500 solar receiver modules, attached to a hexagon-
shaped secondary concentrator and arranged side-by-side
in a honeycomb-type structure following a spherical y-
eye optical conguration, yield a peak solar energy to elec-
tricity eciency of 37%.
Ref. [87] studied beam-down concentrating solar
tower (BCST). BCST are known for easy installation and
maintenance as well as lower convection heat loss of the
central receiver. A point-line-coupling-focus BCST system
using linear Fresnel heliostat as the rst stage concen-
trator (heliostat) and hyperboloid/ellipsoid reector as
the tower reector is proposed. Theoretical investigation
on the ray concentrating mechanism with two commonly
used reector structures, namely, hyperboloid and ellip-
soid, indicate that the ellipsoid system is superior in terms
of interception eciency over the hyperboloid system due
to smaller astigmatism at the central receiver aperture, es-
pecially at larger facet tracking error [87]. The ellipsoid re-
ector shows signicantly lower tower reector shading ef-
ciency. This is the result of the larger tower reector sur-
face area compared to that of the hyperboloid reector. The
total optical eciency of the hyperboloid system is always
better than that of the ellipsoid system. This eciency gap
decreases as the ratio increases. The hyperboloid tower re-
ector is claimed to be more promising and practical for
the system investigated.
Volumetric air receivers were studied in [88]. This com-
ponent consists of a high temperature resistant cellular
material which absorbs radiation and transfers the heat
to an air ow which is fed from the ambient and from re-
circulated air. It is called volumetric, because the radia-
tion may penetrate the “volume” of the receiver through
the open, permeable cells of the material. In this way, a
larger amount of heat transfer surface supports the solid
to gaseous heat transfer in comparison to a tubular closed
receiver. The heated air is directed to the steam generator
of a conventional steam turbine system. Ref. [88] uses an
advanced cellular metal honeycomb structure. It consists
of winded pairs of at and corrugated metal foils. Several
variations of the pure linear honeycomb structure have
been introduced to increase local turbulence and radial
ow.
While some of these technologies may be easily imple-
mented in future installations, those more sophisticated
and innovative certainly require further studies.
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |25
4.4 Manufacturing and materials
New materials and manufacturing processes are urgently
needed to reduce costs and improve reliability and life-
span of current designs. New materials are also needed
for operation of components with higher temperatures and
with reduced heat losses to also improve eciency.
While there is an abundant literature about new de-
sign of cycles, solar elds and receivers, TES system, and
receiver and power block uids, manufacturing processes
and new materials are only marginally covered in the liter-
ature despite their huge impacts on costs and production
of the CSP ST plants. Manufacturing of solar plant compo-
nents is almost ignored.
Materials studies are mostly focused on the coating
of the receiver. Solar receivers are presently mostly coated
with a high sunlight absorptivity layer applied over the
bare surface of the absorber receiver’s tubes. Pyromark
2500 is the present standard coating. The coating en-
hances absorptivity and light-to-heat conversion. Ref. [89]
studied the eect of the optical properties absorptivity and
emissivity of these coatings on the thermal performance
of a MS external receiver. Solar selective and non-selective
coatings were analyzed and compared against the stan-
dard coating. The thermal eciency increases up to 4%
with the absorptivity of the coating. The emissivity has a
very minor eect on the thermal performance of the re-
ceiver at its nominal working temperature. The eciency
only increases 0.6% when the emissivity of the coating de-
creases from 0.9 to 0.5. Improving the absorptivity of a non-
selective coating leads to higher thermal eciency than
using a selective coating for current MS temperatures. For
superheated steam cavity receivers, the eect of using a se-
lective coating is noticeable at temperatures greater than
500 ◦C.
Ref. [90] also studied solar absorber coatings. The
LCOE metric is used to attribute value to any high-
temperature absorber coating. The LCOE gain eciency
is demonstrated on three dierent solar absorber coat-
ings: Pyromark 2500, lanthanum strontium manganite ox-
ide (LSM), and cobalt oxide (Co3O4). These coatings were
used in a 100 MWe central tower receiver. Depending on
the coating properties, an optimal reapplication interval
was found that maximizes the LCOE gain eciency. Py-
romark 2500 paint enables a higher LCOE gain eciency
(0.182) than both LSM (0.139) and Co3O4(0.083). The solar
absorptance is by far the most inuential parameter. The
cost-eectiveness of Pyromark can be outperformed by a
coating that would have a high initial solar absorptance
(>0.95), a low initial degradation rate (<2·10−6cycle−1),
and a low cost (<$500 k per application).
A novel MoSi2–Si3N4 hybrid composite, Figure 5 was
studied in [91]. The MoSi2–Si3N4 absorber deposited onto
Inconel substrate and capped with a Si3N4/Al2O3 layer on
top is a promising selective coating for receivers operat-
ing in air at temperatures about 600 ◦C. Stacks with the
Inconel/MoSi2-Si3N4/Si3N4/ Al2O3 structure on Inconel
substrate show indeed good thermal stability in air.
Fig. 5: Reflectance of new coating from Ref. [91]. Reprinted from So-
lar Energy Materials and Solar Cells, Vol. 174, Rodríguez-Palomo,
A., Céspedes, E., Hernández-Pinilla, D. and Prieto, C., High-
temperature air-stable solar selective coating based on MoSi 2–Si 3
N 4 composite, Pages No. 50-55, Copyright (2018), with permission
from Elsevier.
4.5 Integrated solar combined cycle
systems
Here we report on the integration of CSP ST with CCGT or
other plants, such as desalination plants.
Despite current policies may favor the opportunity of
integrating CSP ST with CCGT, as explained in a prior sec-
tion we do not believe CSP ST should be coupled to the
burning of fossil fuels. While the use of NG in a boiler that
supplement the solar eld in the production of steam is
common practice but it is not the best one as NG could
be better used at double the thermal eciency in a CCGT
plant, it is more interesting opportunity to consider even
if still questionable the coupling of a CSP ST plant with a
CCGT plant.
Integrated solar combined cycle systems (ISCCS) are
reviewed in Ref. [92]. ISCCS consist of three major compo-
nents, CCGT, ST steam generator and solar eld. The study
indicates that very limited research has been directed so
far toward the development of ISCCS with ST. Most of the
26 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
ISCCS plants in operation today employ the PT technol-
ogy. No known commercial ST ISCCS plant is operational
in 2015. The study of ISCCS with ST is therefore an area of
potential improvements still unexplored.
CSP PT and CSP ST coupled to a CCGT were modelled
in [93]. The solar Rankine cycle is a single reheat regenera-
tive Rankine cycle. The CCGT plant features a commercial
gas turbine, with a dual pressure heat recovery steam gen-
erator. MS is the uid to transfer heat to the water/steam of
the solar Rankine cycle. Synthetic oil is used in the CCGT
plant. The CSP ST has a higher collection eciency than
the CSP PT. The combined cycle is more ecient than the
solar Rankine cycle. The CCGT plant coupled with a CSP
ST is found to deliver the highest annual solar-to-electric
eciency of 21.8%.
As the integration of renewables with conventional
power sources is presently discouraged, it is not expected
that power plant burning fossil fuels will be integrated
with solar elds, even if this is by far the best opportunity
to convert solar thermal energy in electricity.
While integration of CSP ST with CCGT does not make
too much sense, there is certainly the scope of integrat-
ing CSP ST with Multi-eect desalination (MED) to pro-
duce electricity and clean water in remote areas. As power
and water supply are the two major issues humanity will
face during this century, a robust growth of CSP around
the world may be integrated with desalination for the next
renewable energy breakthrough [94].
In desalination, seawater is separated into a low con-
centration of salts freshwater stream and a high con-
centration of salts brine. The most relevant desalination
technologies are thermal desalination and membrane de-
salination. Thermal desalination utilizes heat, often by
steam, to change phase of the seawater from liquid to va-
por. Membrane desalination utilizes pressure, and hence
electricity driven pumps, to force water through a semi-
permeable membrane. In general, membrane desalination
has advantages in terms of energy requirements and it
is preferred where salinity is not very high. Seawater re-
verse osmosis (SWRO) membrane processes require less
energy than multi-eect distillation (MED) thermal pro-
cesses. However, Ref. [95] suggests that, for several loca-
tions, for example the Arabian Gulf, CSP plus MED may
require 4% to 11% less input energy than CSP plus SWRO.
This introduces an interesting opportunity for selected lo-
cations where MED may be competitive with SWRO. While
SWRO does not need any integration of the desalination
plant with the CSP plant, as the electricity needed can
be produced everywhere, MED may be easily and conve-
niently integrated with a CSP saving the condenser costs.
MED produces high quality water from sea or brack-
ish water. Concentration of total dissolved solids (TDS) is
25 mg/l or less. MED units range from about 100 m3/day
up to 36,400 m3/day. While single units may be utilized in
smaller volume applications, multiple units may be com-
bined to further increase capacity [94].
In desert installations, far from the coast, the con-
denser is air cooled, and this limits the expansion of steam
in turbine. While in coastal locations the condenser may
certainly be, water cooled for better performances of the
plant, alternatively, the condenser may also be replaced by
a MED thermal desalination module. The steam generated
is superheated to 380 ◦C to 580 ◦C and the steam tempera-
ture for the MED is not higher than 135 ◦C [94]. Hence, the
steam has sucient energy to produce electricity before
entering the MED. If power is the main product, a water
condenser may work better. However, where water is more
precious than power, and MED is competitive with SWRO,
integration of CSP ST with MED is a local renewable energy
break-through.
Ref. [96] proposed solar thermal sea water desali-
nation, however adopting multi-stage ash evaporation
(MSF) rather than MED. The theoretical study considers
a ST with a volumetric solar receiver, a power cycle wa-
ter/steam Rankine, MS as the receiver uid, MS TES plus
the MSF. The seawater is heated by the saturated steam-
water mixture coming from the steam turbine. This elimi-
nates the condenser. Considering the advantages MED has
vs. MSF [94], presently the primary thermal desalination
option, the advantages mentioned in [96] will be further
strengthen when using MED.
4.6 Regionalization
As a nal area of concern, here we report on the improve-
ment being sought in CSP ST technology by specializing
the design for a specic geographical location. Places with
lot of sun to make plausible CSP ST plant may have very dif-
ferent climate conditions. Proximity to coast, availability
of land, orography of land, coupling to desalination, avail-
ability of NG, prevailing weather conditions, sand storms,
wind load, rainfall, all play a key role to reshape one sin-
gle design to match local conditions. Despite some design
concepts may certainly be shared between many dierent
CSP installations, regionalization plays a signicant role
in providing the sought outcomes in terms of performance,
cost and life span of a plant.
The technical, nancial and policy drivers and barri-
ers for adopting CSP ST technologies in India were studied
in [54]. Especially CSP ST with external cylindrical or cav-
Albert Boretti et al., Concentrating solar power tower technology: present status and outlook |27
ity receivers with storage look promising. This technology
is particularly relevant to the Jawaharlal Nehru National
Solar Mission (JNNSM) aimed at achieving grid-connected
solar power of 1800 MW by 2022.
Ref. [97] reviews the CSP plants installed in India and
discusses the growth of the electricity generated by CSP in
India, with targets grown to 100,000 MW by 2022.
The design and construction of a CSP ST demonstra-
tion plant in Saudi Arabia, an area of extreme solar in-
tensity and temperatures, was reported in [98]. The so-
lar receiver was made of alloy steel. Ten heliostats were
chosen, featuring two motors were used to control the he-
liostat rotational and elevation movements. The thermal
uid was a MS mixture 60% NaNO3and 40% KNO3. Cold
and hot storage tanks were manufactured from steel in-
sulated with calcium silicate from all sides. A one-meter
high and one and a half-meter diameter cylindrical ves-
sel was adopted for each of the cold and hot tanks. The
design thermal power was 13 kW. The thermal power re-
leased by the MS was 12.31 kW. The thermal power trans-
ferred to the water/steam was 11.26 kW. The work proves
the value of small demonstration plants. Small demonstra-
tion plant is needed for regionalization in every location
where conditions may dier considerably from the areas
of well-established designs to perform a proper regional-
ization of the design.
The energy and exergy analyses of sCO2recompres-
sion Brayton cycles of Ref. [72] is performed for dierent
locations in Saudi Arabia. The exercise returns a ranking
by location based on the selected CSP ST conguration.
Ref. [99] simulated the behavior of the Spanish GEMA-
SOLAR plant under dierent climates. The analysis is per-
formed for dierent locations of mainland China. An es-
timation of both annual energy production and return of
the investment was provided. Simulations were made with
and without hybridization with combustion of fossil fuels
and with same or modied nominal power. Annual over-
all eciencies were about 14% for the 20 MW power plant
(GEMASOLAR nominal power). Down-scaled plants were
able of maintaining an eciency of 14.97% for a 10 MW
power plant.
Ref. [100] compares under the Algerian climate a
Rankine cycle with a tubular water/steam receiver and
a Brayton cycle with volumetric air receiver. The tubular
receiver Rankine cycle is economically slightly disadvan-
taged vs. the volumetric air receiver Brayton cycle, but it
works better especially under lower solar radiation inten-
sity. The GT requires higher operating temperatures which
are usually dicult to reach throughout the year.
5Conclusions
Our analysis builds on actual data of costs and operation
of CSP ST facilities.
There are only two CSP ST plants of capacity more
than 100 MW operational in the world. The largest one,
the 377 MW capacity ISEGS, with no TES, has not pro-
duced more than 703,039 MWh/year in the 3rd year of life
(2016), up from the 419,085 MWh/year of the 1st year of life
(2014), despite the combustion of hugeamounts of NG, still
1,290,308 MMBtu in the 3rd year of life (2016). The planned
electricity generation was 1,079,232 MWh/year since 2014.
Actual construction cost has been 6,085 $/kW (2017 val-
ues). The other plant, the 110 MW Crescent Dunes facility,
with TES, has produced in 2016 only 127,308 MWh/year vs.
the planned 500,000 MWh/year. Actual construction cost
has been 9,227 $/kW (2017 values).
The electricity production and the reliability of the
plant have been so far worse than the expected. Addi-
tionally, the costs have been much larger than what was
planned. The real-world experience thus casts consider-
able doubts on the number being proposed for the CSP ST
technology by expert panels and the literature. The CSP
ST plant technology is still very far from the standards of
conventional power plants in the power industry, where
the actual costs and performances are usually close to
the planned values. More experience must be gathered to
proper develop a technology that appears to be still in its
infancy.
Having said that, there are certainly many develop-
ment trends being sought that even if not game chang-
ers can make the CSP ST technology much more compet-
itive at least vs. other renewables, wind or solar photo-
voltaics. As actual numbers are very far from plans, we an-
alyze all these technology updates being sought with the
due skepticism. The current trends in the development of
CSP ST installations have been reviewed. Improvements
are being sought for eciency of plant, installation cost,
life-span and operation cost. Materials and manufacturing
processes, design of solar eld and receiver, including u-
ids, cycle and materials, optimal management of daily and
seasonal operation of the plant, new TES concepts, inte-
gration of solar plant with thermal desalination, integra-
tion of solar plant with CCGT installations and nally, spe-
cialization and regionalization of the project specication,
are the key areas of progress of CSP ST technology.
While it is expected that CSP ST installations will grow
considerably in the next few years, there is not yet a bet-
ter solution all-inclusive than the use of MS as RF and TES
uid, with classic solar eld heliostats and receivers, driv-
28 |Albert Boretti et al., Concentrating solar power tower technology: present status and outlook
ing a water/steam superheated Rankine cycle steam cy-
cle. Manufacturing is a major keyword to cover. The dier-
ent alternatives that are presently under study at dierent
stages of development may only progress slowly, benet-
ing from real world experiences requiring time rather than
simulations or laboratory experiments. Cost of plants are
not expected to reduce drastically, even if convergence on
few selected designs of heliostats and receivers could be
benecial to their improvement and cost reduction, with
manufacturing of components in large scale and signi-
cant feed-backs from real world operation expected to be
a major driver of the developments.
Acknowledgement: The authors received no funding.
Conicts of Interest: The authors declare no conict of
interest.
Authors’ contributions: The authors equally contributed
to the review of the papers and the writing of the
manuscript.
Symbols
ηeciency
ϵcapacity factor
E electric energy
P electric power
Q thermal energy
sCO2supercritical carbon dioxide
Acronyms
BCST beam-down concentrating solar tower
CSP concentrated solar power
CCGT combined cycle gas turbine
GT gas turbine
ISCCS Integrated solar combined cycle system
ISEGS Ivanpah Solar Electric Generating System
LCOE Levelized Cost of Electricity
MED multi eect distillation
MS molten salt
MTCR Multi Tube Cavity Receiver
NG natural gas
PT Parabolic Trough
PV photovoltaic
RF receiver fluid
RMCI Recompression with Main Compression Intercooling
SEGS Solar Energy Generating Systems
ST Solar Tower
SWRO sea water reverse osmosis
TCES thermochemical energy storage
TES thermal energy storage
TETR Traditional External Tubular Receiver
VVR Variable Velocity Receiver
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