THE TECHNOLOGY STATUS OF
CONCENTRATED SOLAR POWER
Robert D. Stephens
Global Energy Systems Intelligence Center
14 October 2008
Export Control Classification Number is EAR99, export license is not required for distribution
within GM.
GM INFORMATION -- GM INTERNAL USE ONLY ------ NOT TO BE DISTRIBUTED OUTSIDE GM
Abstract
Concentrated solar power utilizes focused sunlight to convert heat energy into electricity. Several
technologies are in various stages of commercial development. These are: 1) parabolic solar
troughs, 2) power towers, and 3) parabolic dish/Stirling engine systems. All of these power
generating technologies are directed toward providing large-scale grid-power applications.
Parabolic trough technology is in commercial use today in many countries and provides
dispatchable electricity at costs somewhat higher than fossil-fuel generated electricity. Costs are
expected to become competitive with fossil-fuel powered plants via further developments in
technology and by gaining economies of scales as more and larger plants are built.
Power tower systems are in commercial operation on small scales (<20 MW capacities) but larger
plants are in various stages of planning and construction. This technology is expected to provide
electricity at lower costs and higher efficiencies than parabolic trough systems, but this has yet to
be demonstrated.
Parabolic dish/Stirling engine systems are expected to produce electricity at the highest efficiency
of all these technologies. However, costs are the highest due to the absence of mass-produced
Stirling engines. This technology also suffers the disadvantage of having no demonstrated
methodology of thermal storage and consequently is not considered a dispatchable source of
electricity. These systems have the advantage of being modular and in theory, could be deployed
on small scales, e.g. 25kW.
Concentrated solar power systems generate power with nearly zero fossil fuel inputs. As such they
operate with virtually no CO2 emissions and can generate electricity at costs independent of price
fluctuations associated with fossil fuels.
GM INFORMATION -- GM INTERNAL USE ONLY ------ NOT TO BE DISTRIBUTED OUTSIDE GM
Purpose of the Research
The purpose of this work was to evaluate the technical and economic status of concentrated solar
power technology for the generation of electricity.
Conclusions
Concentrated solar power:
1. is commercially available today at costs somewhat higher than electricity generated via
fossil fuels
2. generating costs are independent of fossil fuel price volatility
3. generating costs are expected to become comparable to fossil-fuel-fired plants in the near
future
4. is essentially GHG-emission free
Significance
Long-term solar-power purchase contracts can eliminate power price fluctuations associated with
the costs of fossil fuels and GHG emission burdens. Policy or market incentives might be
necessary for this technology to fully develop in the US.
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
1
Introduction
Concentrated solar power (CSP) systems collect and focus the sun’s rays to produce heat
which is used to generate electricity with conventional boiler/steam turbine systems. A number of
different approaches to CSP are currently being used and others are in development for
commercial application in large, utility-scale solar-powered electric generating facilities.
The major categories of CSP systems covered in this report are:
 Parabolic thermal troughs
 Solar tower
 Parabolic dish/stirling engine
This report will describe each of these technologies and review the current state-of-the-art
and the advantages and disadvantages of each. The report will also review cost and efficiencies
for these technologies.
Parabolic Troughs
THE TECHNOLOGY
The most cost-competitive solar power technology in use today are parabolic trough
systems, an example of which is shown photographically in Figure 1. These systems utilize many
rows of long – typically hundreds of meters long – parabolic, trough-shaped mirrors to collect and
concentrate light on a “receiver”, i.e. a specially designed pipe containing flowing heat transfer fluid
(HTF).
Figure 1. Photograph of a parabolic solar trough system collector and receiver. (Source: Solar
Millenium AG)
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
2
This pipe is anti-reflection coated with multi-layer materials designed to optimize absorbance of the
light focused upon it,while also minimizing emissivity, which is a source of heat loss and decreased
system efficiency. This pipe is positioned at the focal point of the parabolic mirrors and is
contained within a larger glass tube that transmits the light from the mirrors while also enabling
vacuum insulation of the HTF-containing pipe so as to minimize convective heat loss. The mirrors,
generally referred to as the “collector” and the HTF-containing pipe, referred to as the “receiver”
move together in a one-axis solar tracking mode to optimize light collection throughout the day.
The parabolic trough mirrors typically produce about 40X-80X solar flux concentration on the
receiver.
The HTF in-use in commercial systems today is synthetic oil-based and limits the upper
temperature of the HTF to about 390C. Higher temperature HTFs would enable designing
systems with higher efficiency. Alternative HTFs being considered today include molten salt and
pressurized superheated steam.
Figure 2 shows a simplified schematic diagram of a parabolic solar trough system. The
solar collector is the array of parabolic trough-shaped mirrors along with the HTF-carrying
receivers, which can cover many acres of land. AbengoaSolar, a leading CSP company, reports
that a 100MW CSP system with 7 hours of thermal storage requires 940 acres of land1. In the
case of a recently announced plan for a 280 MW solar farm in Arizona, which – if built - would be
the largest solar facility in the US, the solar collector will cover nearly three square miles. Other,
smaller parabolic trough systems are already in operation in the US. These are listed in Table I.
Solar Field
Solar
Preheater
Steam Turbine
G
Hot Salt
Tank
Solar
Superheater
Steam
Generator
Condensor
Solar
Preheater
Cold Salt
Tank
Cooling
Tower
Expension
Vessel
Deaerator
Figure 2. Schematic diagram of a simplified parabolic solar trough system. (Source: Abengoa
Solar)
In the system shown in Figure 2, the heater is typically a natural gas burner. This burner is
used primarily to heat the HTF when the system is not in operation and ambient temperatures are
low enough to cause freezing of the HTF, which occurs at 13C for the oil-based material
commonly in use today. However, hybrid systems exist in which natural gas can also be burned to
generate electricity when solar thermal energy is not available. The thermal storage system
utilizes molten salt to store heat for up to 13 hours of operation without sunlight. The molten salt is
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
3
Table I Existing parabolic solar trough systems in operation in the U.S.
Name
Solar Electric
Generating
Stations (SEGS)
I-IX1
Nevada Solar One
Saquaro Solar
TOTAL Capacity
Location
Size
(MW)
Technology
On-line date
Status
California
354
Parabolic
Trough
1986-1990
On-line
Nevada
64
2007
On-line
Arizona
1.35
Parabolic
Trough
Parabolic
Trough
2006
On-line
419.35
composed of 60% sodium nitrate and 40% potassium nitrate, although improved alternative
materials are being sought. As the diagram indicates, the HTF can be circulated in a number of
ways that enable varying operating modes. With appropriate use of valves, the HTF can be solar
heated by circulating through the solar collector only. Alternatively, the HTF can be heated via the
natural gas burner, or via the thermal storage system. Likewise, electricity can be generated or not
while using any of these HTF circulation loops.
Parabolic solar trough systems have been used at sites around the world for producing heat
as well as grid power for about two decades. Consequently, the reliability and durability of
parabolic trough mirrors, the HTF, valves and pumps, as well as boilers and turbines have been
demonstrated on a commercial scale. This proven track record in conjunction with recently
invoked government policies and tax credits have incentivized many new US projects.
Announcements of plans for solar power plant construction projects have totaled 1,847 MW of
capacity within just the last two years. Table II summarizes potential new CSP projects in the US
that are in various stages of development ranging from facilities in construction to proposed plans.
A number of technical aspects of parabolic solar trough systems are the focus of active
research and development. These offer potential to both improve performance and reduce costs.
Areas of research include improving mirror performance by improving focus, alignment and
increased aperture, stability of mirror structural support, improved anti-reflection coatings for the
receiver to reduce emissivity, development of higher temperature oils and/or other HTF, and
system components and design criteria for utilizing alternative and/or `higher temperature HTF e.g.
molten salt or pressurized steam.
CSP systems of all types, solar parabolic troughs included, require direct sunlight and do
not operate on diffuse sunlight. Consequently, they must be located in areas of maximal sunlight
and minimal cloud cover. In the US, as in most parts of the world, these locations are desert or
semi-desert areas where water resources are scarce.
A major advantage of using solar thermal energy for electricity generation, as opposed to
photovoltaic (PV) electric generation, is the potential for efficient thermal storage, which can be
used for electricity generation when the sun isn’t shining. These systems are currently being
designed into and used by parabolic trough systems. This is a significant advantage because it
enables the solar power plant to dispatch power to the grid when the load requires it. This load
matching increases the value of the power produced. An example of the type of dispatchability
that this represents relative to solar flux is shown in Figure 3.
1
SEGS I-IX are actually nine separate systems based upon the same design.
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
4
Table II Potential new CSP projects in the US
Name
Location
Victorville
Hybrid
Palmdale
Hybrid
Bethel Solar
One
Bethel Solar
Two
California
Size (MW)
Solar
Technology
Parabolic
trough
Parabolic
trough
Parabolic
trough
Parabolic
trough
On-line
date
2010
Status
Parabolic
trough
Parabolic
trough
Parabolic
trough
No date
Approved by
PUC
Approved by
PUC (Public
Utility
Commission)
Announced
No date
Announced
No date
Announced
Parabolic
trough
2011
Parabolic
trough
Parabolic
trough
2011
Filed with
California
Energy
Commission
Filed with
PUC
In the
permitting
process
Announced
California
50 solar/513
natural gas
50 solar/520
natural gas
49.4
California
49.4
Harper Lake
California
250
SME-1
California
150
Cuddleback
California
Beacon Solar
California
214
solar/1040
natural gas
250
Mojave Solar
California
553
Solana
Arizona
280
Xcel
Colorado
200
Unknown
Southwest
CSP
200
Unknown
Ivanpah
Nevada,
Arizona, or
New Mexico
California
25MW in
2011
then
20152016
2010
400
Solar tower
2010
Carrizo
California
177
CLFR
2010
FP&LSolar
Stirling Solar
Thermal One
Stirling Solar
Thermal Two
GV1
Florida
California
300
850
California
900
2009
20091012
2012
California
2
CLFR
Dish/Stirling
Engine
Dish/Stirling
Engine
Concentrating
PV
Under
review
Under
review
Announced
Approved by
PUC
Proposed
No date
Announced
California
No date
2008
2008
2011
Under
review
Announced
Announced
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
5
Sunlight
Output
Energy in Storage
0
5
10
15
20
Figure 3. This figure shows the daily solar flux distribution with time relative to the potential power
output from a CSP facility using thermal storage. (Source: Sandia National Laboratories)
EFFICIENCY AND COSTS
The annual solar to net electricity efficiencies of solar trough systems have been reported to
be in the range of 11% to 14%. The lowest of these values were the first parabolic trough systems
built in the late 1980’s. Studies have shown that these efficiencies can be increased by operating
with higher temperature HTF, e.g. molten salt or pressurized steam. DOE has established a 2011
goal of 15.5% annual solar-to-electric efficiency in parabolic trough systems2.
The largest single capital cost factor in a solar trough system is the solar field – including
collectors and mirrors. Consequently, this is an area of focus for cost reductions. DOE’s Solar
Energy Technologies Multi-Year Program Plan, 2007-2011 has set several long-term goals to
reduce the cost of electricity produced by solar trough systems2. These include:
 reducing solar collector costs by 40%, from about $260/m2 to $160/m2
 increasing annual solar field efficiency from 42% to 52%
 increasing the operating temperature from 390C to 450C
 reducing costs of mirrors, structures, and receivers
Longer term goals to achieve further cost reductions include:
 approaches to maintain receiver vacuum and removal of H2 build up
 improved receiver coatings for reduced emittance
 collector/receiver designs for reduced costs
 anti-soiling coatings for mirrors
 the development of US mirror supplies
 improved mirror interconnections
 improved low-cost drives for larger mirrors
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
6
DOE also believes the cost of thermal energy storage systems must be reduced and
targets a 50% cost reduction. The route for achieving this would include eliminating heat
exchangers in the thermal storage unit by using the same fluid for both HTF and thermal storage
media and also increasing the temperature differential between the hot and cold side of the
circulation loop.
The DOE projects that a 100 MW solar parabolic trough plant will achieve peak solar-toelectric efficiencies of 25.6% and annual solar-to-electric efficiency of 15.5% by 2011, using a
thermal storage system capable of 6 hours of energy storage. Installed cost is expected to be
$4100/kW including the storage system with operating and maintenance cost at $0.016 kWh.
These costs yield a levelized cost of electricity (LCOE) of $0.089/kWh. The LCOE costs assume
financing of a utility-scale independent power project. The market for parabolic trough based
power is dispatchable, intermediate-load power valued by DOE at between $0.05/kWh and
$0.08/kWh at natural gas prices of $5/MMBtu.
Of the capital costs associated with parabolic trough systems, a significant portion is
attributable to the cost of the solar field, i.e. the parabolic mirrors and solar receivers. These costs
are estimated as representing 50% of total capital investment. These costs reflect the costs of the
vacuum insulated receivers, as well as the costs associated with producing and installing the
shaped mirrors, and required mountings. Design of parabolic troughs and mountings are complex,
given the need to withstand the force of wind, and receivers require vacuum insulation and special
coatings for maximizing absorptivity and minimizing emissivity.
Further cost reductions are anticipated through scale-up of plant size and economies of
scale of technology utilization, as well as system efficiency improvements through improved
components, HTF, and thermal storage. DOE also reports upon an independent analysis that
shows LCOE costs of parabolic trough systems going from $0.095 to $0.04 from 2004 to 20202.
They report that 20% of these cost reductions will be realized by scale-up of plant sizes, 54% from
R&D to identify efficiency/performance improvements, and 26% from volume production of
components.
Many of the cost/efficiency improvements of parabolic trough systems are being addressed
by technologies developed by Schott and Ausra. Schott has developed receivers for parabolic
trough systems and have built a factory to scale-up production. These receivers are high-tech
systems that consist of pipes inside of glass tubes. The pipes carry the HTF and the antireflection-coated glass tubes transmit the concentrated solar rays to the HTF-carrying pipes and, in
addition, provide vacuum thermal insulation to these pipes. The pipes have coatings specifically
designed to absorb solar radiation and prevent thermal emissions that represent loss of efficiency.
Mass production of these solar collectors have the potential of reducing their cost.
Ausra is developing technology that addresses some of the R&D goals established by the
DOE, notably solar collector cost reductions and HTF alternatives. Ausra has developed Compact
Linear Fresnel Reflectors (CLFR) as an alternative to parabolic mirrors. In theory, CLFRs could be
used as alternatives to parabolic troughs. CLFRs utilize flat mirrors that can be mass produced,
shipped and handled more efficiently. In addition, they propose mounting CLFRs close to the
ground to avoid wind loads and thereby reducing mounting costs associated with the supporting
structure. In addition, Ausra’s technology utilizes pressurized steam as the HTF. In June of 2008,
Ausra announced the opening of a manufacturing facility in Nevada to produce CLFRs and
receivers, with capacity of 700 MW of solar collectors per year. Mass production at this scale has
the potential of reducing the cost of CSP solar collectors. Ausra is a private company, founded in
2006 with support funding from Khosla Ventures and Kleiner Perkins Caufield & Byers.
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
7
Power Tower
THE TECHNOLOGY
Currently, solar tower systems are used commercially only as small demonstration systems
being operated in conjunction with larger solar trough systems. The only example of this is the PS10 plant built by Abengoa Solar that is in operation in Spain. As such, they do not have a track
record of demonstrated reliability and durability when built at a commercial scale. Abengoa is
building a second power tower system in Spain and Luz II is building a pilot plant in Israel.
A simplified diagram of a power tower system is shown in Figure 4. The system consists of
a field of heliostats – flat mirrors mounted on 2-axis solar tracking systems – that focus solar
radiation on a receiver located high on a tower. Consequently, these systems do not require
circulation of the HTF over such long distances as required by a solar trough system and this
minimizes heat losses in the HTF. Commercial scale systems would use anywhere from hundreds
to thousands of heliostats that are capable of producing solar flux concentrations of 1000X or
higher at the receiver. Consequently, these systems can generate very high power at the receiver.
The higher power at the receiver and the minimization of heat loss in the HTF circulation loop
enables power tower systems to operate at higher temperatures and efficiencies than solar trough
systems. Attaining these higher efficiencies require tower systems to utilize a HTF capable of
operating at higher temperatures. HTFs that are being considered include molten salt, pressurized
steam, and heated air. Systems currently using molten salt operate with temperatures of about
290C on the cold side and 570C leaving the receiver. Superheated pressurized steam systems
are being studied that would use temperatures as high as 550C from the receiver. However,
pressurized steam systems will require the development of alternative thermal storage systems
because the majority of energy in pressurized steam exists in the phase change from vapor to
liquid.
Receiver
Collector
HTF
Figure 4. Artist drawing of a power tower system showing collector, receiver and HTF connection
to power block.
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
8
From the central tower receiver, the HTF circulation is similar to that shown in Figure 2,
with hot HTF used to generate steam to spin a turbine to produce electricity. Power tower
systems, like solar trough systems, can use thermal storage to produce power either when clouds
obscure sunshine or after sundown to dispatch power to meet grid demand loads. The use of
molten salt for both HTF and heat storage medium in power tower systems reduces system
complexity and potentially, costs.
EFFICIENCY AND COSTS
A power tower system rated at 11 MW has been in operation in Spain using pressurized
steam as the HTF. Annual solar-to electric efficiencies of this plant have been reported in the
range of 16% to 19%. Luz II, a company specializing in power tower system design and
construction claims their systems reach an overall 20% thermal to net electricity efficiency. They
are building a demonstration plant in Israel that is scheduled to come on line in 2008.
A disadvantage of the solar tower system is the cost of heliostat mounting, two-axis tracking
on each individual heliostat, and construction costs of the central tower. Currently, solar tower
systems are less proven in commercial applications than solar parabolic trough systems and
consequently are not as commercially attractive despite the potential for higher operating
efficiencies and lower costs.
DOE estimates that current pricing for a solar tower system is about $7200/kW 2. They
estimate that heliostats would contribute approximately 50% of the capital costs. Sandia National
Laboratories modeled the LCOE of solar tower systems, with the results for a molten salt system
ranging from $0.054/kWh to $0.087/kWh for heliostat pricing ranging from $80/m2 to $200/m2,
respectively3. Based on these estimates, Sandia established a heliostat price goal of $100/m2 for
solar tower systems to be economically attractive.
Sandia also organized a workshop of 30 heliostat and manufacturing experts to gather
information upon which predictions were made for potential costs for large-scale production of
heliostats. Two production figures – 5,000/yr and 50,000/yr - were used to predict pricing for the
148 m2 heliostats designed by Advanced Thermal Systems. Sandia estimated heliostat pricing of
$164/m2 and $126/m2 for the 5,000/yr and 50,000/yr cases, respectively3.
Solar Dish
THE TECHNOLOGY
Solar dish power systems utilize a parabolic dish to focus the sun’s rays onto a receiver. In
practice, the parabolic dish is typically a number of separate parabolic facets that together form a
parabolic-shaped reflector. In these systems, the solar flux at the receiver can typically exceed
2000X over the density of direct solar insolation. Consequently, with a solar insolation of
1000w/m2, a nominal value in the American southwest, a dish of 10m in diameter could provide a
25Kw output. Solar dish systems are typically designed for operation in the range of from 10kW to
25kW outputs. These systems utilize 2-axis solar tracking to continuously use sunlight throughout
the day. A photograph of a solar dish system is shown in Figure 5.
The receivers being considered for solar dish applications are Stirling and Brayton cycle
heat engines. Stirling engines have demonstrated thermal-to-electricity conversions efficiencies of
40% demonstrating that these systems have potentially high operating efficiencies. Indeed,
recently a system at Sandia National Laboratories in Albuquerque, NM attained a record in solarto-net electricity conversion efficiency of 31.25%. The first advanced prototype of such a system
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
9
holds the record for 29% efficiency over sustained operation4. DOE has established a 2011 target
of 24% as the annual solar-to-electric operating efficiency2.
To generate electrical output, the Stirling or Brayton engines are interfaced to either an
induction generator, which provides synchronized AC output directly to the grid or to alternators,
whose output are rectified and then converted to AC via inverters.
The major advantage of solar dish/stirling engine systems over other CSP systems include:
1) start-up is nearly instantaneous as opposed to long daily heat-up times for the HTFs
used in solar trough and power tower systems, and
2) elimination of the inefficiency of HTF heat-up time in parabolic trough and power tower
systems
3) the systems are modular and can, therefore, be easily scaled, and
4) they can potentially achieve high solar-to-grid efficiencies
5) there is no need for cooling towers and associated water consumption and/or parasitic
losses from dry cooling.
Figure 5. A photo of an array of parabolic solar dish systems. (Source: Sandia National
Laboratories)
There are, however, significant disadvantages to the solar dish systems currently being
tested and demonstrated, most importantly being no available means of energy storage.
Consequently, these systems do not provide dispatchable power. This disadvantage could be
overcome if the heat engine used can be hybridized to utilize the combustion of fossil fuel as the
heat source.
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
10
In 2005, Southern California Edison announced an agreement with Stirling Engine Systems
to build a 500 MW solar dish system on 4500 acres northeast of Los Angeles, CA. This facility
would consist of 20,000 solar dishes with the option to add an additional 14,000 dishes to provide a
total of 850 MW. Each dish would be 37-feet in diameter and generate 25 kW each. To date, this
facility has not been built.
Sandia National Laboratories in Albuquerque, NM has six solar dish/Stirling engine systems
that were installed by Stirling Engine Systems, Inc. Sandia and Stirling Engine Systems, Inc. plan
to benchmark baseline performance metrics of these systems by monitoring the mean time
between unplanned maintenance as well as the mean time between unplanned component
failures. These data should aid in the identification of potential future design improvements as well
as development of system cost models.
Issues associated with commercialization of solar dish technology include the cost of
Stirling engines and their durability and reliability. Currently, Stirling engines are produced in low
quantities at high costs. For commercialization to take place, engine costs will need to be reduced
by an order of magnitude or more2. However, these cost reductions are not likely to be achieved
by economies of scale alone.
EFFICIENCY AND COSTS
The DOE projects that dish/Stirling system performance will reach 30% solar-to-electric
efficiency and an annual solar-to-electric efficiency of 24%. Installed costs for this system is
projected at $4500/kW with operating and maintenance costs of $0.05/kWh and an LCOE of
$0.25/kWh.
Discussion
APPLICATIONS
CSP is a technology best suited for meeting electric grid demand. Unlike PV technology,
CSP requires large scale power blocks for electricity generation and, therefore, is not well suited to
small-scale applications. For grid power, however, CSP has two oft-cited strengths: 1) output
power tracks solar flux, which is well correlated with afternoon peak grid loads, and 2) with thermal
storage, CSP can also easily be dispatched to meet evening peak loads.
LAND USE
The solar collectors used in concentrated solar power plants can occupy large tracts of
land. Consequently, land use issues are often sited as a disadvantage of CSP. However, land
required for solar power is less than needed for wind power and also less than needed for hydro
power when the space of the reservoir created by a hydroelectric dam is taken into consideration5.
CSP also requires less land than is used for conventional coal fired power plants when the land
area of coal mines are considered.
The amount of land necessary for CSP depends upon a number of parameters, most
notably:
 solar insolation at the site
 the use of thermal storage
 solar field overbuild (for either optimizing power output during off-peak solar flux or to utilize
solar thermal storage during peak solar flux)
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
11
The level of solar insolation is highly variable in time and space. Figure 6 shows a US map
of solar insolation for systems using two-axis tracking. Solar insolation for parabolic trough
systems, which utilize single axis tracking will have values slightly below, but well correlated to
those shown. CSP is considered feasible at levels of 6 to 7 kWh/day or higher, consequently land
areas in the American southwest represent best opportunities for CSP development.
A study done by the DOE determined that seven states – AZ, CA, CO, NV, NM, TX, and UT
had both the solar insolation and available land to generate up to 6800GW6. The Western
Governor’s Association Solar Task Force investigated the availability of “prime” CSP sites in the
western US by considering solar insolation, political sensitivities to siting, and access to existing
transmission lines and identified 200 GW of potential capacity. This represents about 20% of all
US generating capacity7.
System designers have wide flexibility in how to optimize system design relative to output
capacity and the size of the solar field. The aperture – or area of solar collection – can be
designed to collect solar flux that exceeds the electric generating capacity of the boiler/turbine
systems to enable either or both of two useful operating capabilities: 1) thermal storage to enable
dispatchable power to meet peak loads, and/or 2) operation of the boiler/turbines at optimal
efficiency at times of the day or year when solar insolation is less than peak. The use of thermal
storage dictates that some of the solar energy collected will be directed to storage rather than
immediate electric power generation. Consequently, a system with storage is typically designed to
collect more solar energy than can be used for immediate electric generation. Likewise, a system
designed to maintain optimal turbine output over varying solar insolation will be designed to have a
larger solar field to enable providing enough power to the turbines when solar insolation decreases
from peak. These systems benefit from using thermal storage because the excess solar power
collected at times of peak solar insolation can be stored for generating dispatchable power at times
when solar insolation is low or absent.
Given system design options, it is difficult to calculate the land areas required for any given
electric generation rate and CSP technology. This makes it difficult to answer the often asked
questions regarding land area required to satisfy any given demand. In general, Abengoa Solar
provides estimates for land requirements for parabolic trough systems with and without storage.
Without storage, a 100 MW facility is reported to require 475 acres, i.e. 4.75 acres/MW of capacity.
With 7 hours of thermal storage, that same 100 MW facility requires 940 acres of land, i.e. 9.4
acres/MW 7 of capacity. One important difference in these two facilities being, of course, that the
one with storage will have a higher utilization factor and therefore be capable of generating power
over a greater portion of each day, for example, during evening peak grid loads. For reference,
wind power typically requires about 40 acres/MW at recommended wind turbine spacings.
Land requirements are likely to be different for power tower systems. Abengoa Solar
reports that a 20 MW power tower system built at a site with 2120 kWh/m2/yr would require 235
acres of land1, i.e. about 12 acres/MW.
We also have actual operational data for parabolic trough technology to provide
approximate answers to questions regarding land requirements to meet demands. Nevada Solar
One was constructed by Acciona and has been in operation since mid-2007. It is a 64 MW facility
with no thermal storage2 covering 321 acres8 in Nevada, ie. 5 acres/MW of capacity, a value in
reasonable agreement with Abengoa’s figures. Projected annual production for this facility is
130,000 MWh/yr, or 405 MWh/yr/acre.
The EIA reports that total US electricity generation in 2006 was 4051 billion kWh9. Based
upon our example utilizing Nevada Solar One, the projected land area to produce total US
electricity generated in 2006 would be 10 million acres or about 6400 mi2. This is, however, a
purely academic exercise because there are no expressed plans to design and build a CSP facility
2
The HTF in the solar field allows approximately 30 minutes of operation without direct sunshine.
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
12
with adequate storage for 24 hour operation. Consequently these plants are not intended to fully
replace conventional electric generating capacity.
Figure 6. Map of solar insolation for two-axis solar tracking concentrators.
CO2 EMISSIONS
CSP also has the potential of providing significant environmental benefits by reducing
emissions associated with electricity generation. For example, total CO2 emissions from electricity
generation nationwide in the US has been reported10 as 2,460 million metric tons for 2006, i.e. 5.79
x 10-4 metric tons of CO2/kWh generated10. Using again our example of Nevada Solar One, which
is projected to generate 130,000 MWh/yr, this facility would reduce CO2 emissions by about 75,000
metric tons per year.
Conclusions
CSP parabolic trough systems are in commercial use today in many locations around the
world. These systems provide dispatchable power that can match peaks in grid load. Cost of
generation is still somewhat higher than plants powered by fossil fuels. However, both the capital
costs and O&M costs are now well characterized and expected to decrease because of component
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
13
and efficiency improvements as well as efficiencies of scale from construction of more and larger
plants.
CSP power tower technology is less mature than parabolic trough technology. One small
plant is in operation and a number of other plants are being constructed. These plants offer the
potential of significantly higher operating efficiencies and lower electric generating costs than
parabolic trough systems. However, this is yet to be demonstrated and until systems have
adequate operational history, O&M costs and utilization factors are unknown. These unknowns
represent investment risks that have discouraged broader use of this technology.
CSP parabolic dish/Stirling engine systems offer the potential of operating with the highest
net solar-to-electric efficiencies of any of these CSP technologies. However, there are currently no
commercial systems in operation. This technology also has the disadvantage of having no
demonstrated thermal storage technology and consequently, it is not viewed as a technology that
provides dispatchable power. These systems currently have very high costs that are primarily
associated with Stirling engines, which are currently produced on an as-needed basis.
Consequently, this technology is likely to realize substantial cost reductions if mass production of
Stirling engines becomes a reality. A potential advantage of parabolic dish technology is its
modularity, enabling it to be potentially used as smaller (e.g. 25 kW) power sources.
These CSP technologies are most applicable as grid power sources and as such, have
been and will be implemented as power plants with capacities of 50 MW or larger, probably
substantially larger. Given that these plants still operate at a cost disadvantage to fossil fuel
generating plants, they require tax credits or other incentives for construction of new facilities to
take place. These large scale plants require years for planning, financing, and approvals and
consequently, incentives such as the federal solar investment tax credit, first passed in 2005 and
expired in 2008, should extend over many years if it is to encourage future plant construction.
Potential long term advantages of CSP as a source of power are the near-zero CO2
emissions and the elimination of fuel-price fluctuations.
Appendices
APPENDIX A: ABBREVIATIONS
CLFR
CSP –
HTF
LCOE
NREL
O&M
PV
Compact Linear Fresnel Reflector
Concentrated Solar Power
Heat Transfer Fluid
Levelized Cost of Electricity
National Renewable Energy Laboratory
Operating and maintenance
Photovoltaic
APPENDIX B: CONCENTRATED SOLAR POWER SUPPLIER COMPANIES
Abengoa – design and build large scale solar power plants based upon parabolic trough, central
solar power, and concentrated photovoltaic technologies.
http://www.abengoasolar.com/sites/solar/en/ac_nosotros.jsp
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
14
Acciona Energia – owns Nevada Solar One, a solar thermal-electric plant and is involved in
developing all aspects of solar energy, as well as wind power, hydropower, and biofuels.
http://www.acciona-energia.com/default.asp?x=000201
Ausra – develops and deploys large scale solar power plants. Developed the the Compact Linear
Fresnel Reflector (CLFR) solar collector and steam generation system. http://www.ausra.com
BrightSource Energy – owner of Luz II, Ltd. Design and build large scale solar power plants.
http://www.brightsourceenergy.com/
eSolar – designs and builds heliostats for solar tower applications.
http://www.esolar.com/index.html
Flabeg – provides mirrors for parabolic trough, central solar tower, and solar dish applications.
http://www.flabeg.com/en/index.html
Flagsol designs and builds parabolic trough collectors. http://www.flagsol.com/
Infinia Corp. – develops and builds Stirling engines. http://www.infiniacorp.com/main.php
Luz II – subsidiary of BrightSource Energy, they develop utility-scale solar thermal power plants
based on power tower technology using superheated pressurized steam as the HTF.
http://www.luz2.com/
Rioglass Solar – a subsidiary of Abengoa Solar, Rioglass makes parabolic trough mirrors for CSP
systems. They claim to make 5 million mirrors per year.
Sener – designed and manufactures the SENERTrough parabolic collector, also is
developing molten salt-based solar power tower technology.
http://www.sener.es/SENER/index.aspx?&lang=en
Schott – designs and manufactures thermal systems for parabolic trough solar systems,
consisting of concentric anti-reflection coated glass tubes and steel heat transfer fluid tubes
specially coated to reduce emmisivity. http://www.us.schott.com/solar/english/index.html
SkyFuel – delivers and operates solar fields for CSP plants. They have developed two
technologies – SkyTrough, parabolic trough collectors, and Linear Power Tower systems based on
Fresnel reflectors. http://www.skyfuel.com/
Solel – designs and builds thermal receivers for parabolic trough solar systems.
http://www.solel.com/
Solargenix – designs and builds parabolic trough collectors called the LS-2.
http://www.solargenix.com/
Solar Millenium – develop, construct, and operate parabolic solar trough power plants. They are
also developing solar chimney technology.
http://www.solarmillenium.com/
Solar Reserve – licensed by United Technology to build utility scale power plants based upon
central solar tower molten salt technology. http://www.solar-reserve.com/company.html
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
15
Stirling Energy Systems – a developer of solar power generation equipment for utility-scale
power plants. The Company’s unique technology, the SunCatcher™, combines a mirrored
concentrator dish with a high-efficiency Stirling engine specially designed to convert sunlight to
electricity. http://www.stirlingenergy.com/about_overview.htm
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
16
References
1) Abengoa Solar at http://www.abengoasolar.com/sites/solar/en/tec_ccp.jsp
2) DOE Energy Efficiency and Renewable Energy, Solar Energy Technologies Program MultiYear Program Plan 2007-2011.
3) Kolb, G. J., Jones, S. A., Donnelly, M. W., Gorman, D., Thomas, R., Davenport, R., Lumia,
R. “Heliostat Cost Reduction Study”, Sandia Report SAND2007-3293, June 2007.
4) Andraka, C. E., Powell, Mark, Dish Stirling Development for Utility-Scale
Commercialization, SolarPACES 2008, Las Vegas, Nevada, March, 2008.
5) Anderson, D. and Ahmed, K., The Case for the Solar Energy Investments, World Bank
Technical Paper Number 279 – Energy Series, World Bank, Washington, D. C. , February
1995, ISBN 0-8213-3196-5.
6) NREL, “Analysis of Concentrating Solar Power Plant Siting Opportunities: Discussion Paper
for WGA Central Station Working Group” July, 2005.
7) CERA Report “Concentrating Solar Power – US Demand Heats Up”, April, 2008.
8) Acciona Energia website at: http://www.accionaenergia.com/default.asp?x=00020401&z=000105&item=307
9) DOE/EIA Report#: DOE/EIA-0383 (2008).
10) Energy Information Administration/Electric Power Annual 2006, p. 47.
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
17
DISTRIBUTION LIST
GM Research & Development and Strategic Planning
Executive
Larry Burns
Alan Taub
GM Intelligence
Clay Phillips
Chemical and Environmental Sciences Laboratory
Scott Jorgensen
James Spearot
Powertrain Systems Research Laboratory
Gary Smyth
Madhusudan Raghavan
Materials & Processes Laboratory
Mark Verbrugge
Fuel Cell Activities
John Ferris
Britta Gross
Byron McCormick
Ian Sutherland
Environment, Energy, and Safety Policy
Robert F. Babik
David W. Lake
Elizabeth A. Lowery
Mary E. Stanek
WFG Energy & Utility Services
Tom Neelands
Mike Demsky
Kamesh Gupta
GM China
Benny Zhang
Kristin Zimmerma
GM INFORMATION -- GM INTERNAL USE ONLY -- NOT TO BE DISTRIBUTED OUTSIDE GM
18