Solar Thermal Power Production.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
In addition to using the
warmth of the sun directly, it is possible (in areas with high level of solar
radiation) to use the heat to make steam to drive a turbine and produce
electricity. If undertaken on a large scale, solar thermal electricity is very
cost-competitive. The first commercial applications of this technology appeared
in the early 1980’s, and the industry grew very rapidly. Today, utilities in the
U.S. have installed more than 400 megawatts of solar thermal generating
capacity, providing electricity to 350,000 people and displace the equivalent of
2,3 million barrels of oil annually. Nine plants in California’s Mojave Desert
are generating 354 MWe of solar electric capacity, and have accumulated 100
plant-years of commercial operating experience. The technology is maturing
to the point where officials say it can compete directly with conventional power
technologies in many regions of USA. A number of opportunities for solar thermal
projects may open soon in other regions of the world. India, Egypt, Morocco, and
Mexico have active programs that will receive grants from the Global Environment
Facility, and independent power producers are designing power projects in
Greece, Spain, and the US.
According to the way how the heat is
produced solar thermal power plants can be divided between solar concentrators
(mirrors) and solar ponds.
SOLAR CONCENTRATORS
Solar thermal electric power plants generate heat by using lenses and
reflectors to concentrate the sun’s energy. Because the heat can be stored,
these plants can generate power when it is needed, day or night, rain or
shine.
Large mirrors - of the point focusing type or the line
focusing variety - can concentrate solar beams to such an extent that water can
be converted to steam with enough power to drive a generating turbine. Enormous
fields of such mirrors have been constructed by Luz Corp. in the Californian
desert, for the production of 354 MW of electric power. Such systems can convert
solar to electric power with an efficiency of about 15%.
All solar
thermal technologies except solar ponds achieve high temperatures by utilizing
solar concentrators to reflect sunlight from a large area to a smaller receiver
area. A typical system consists of the concentrator, receiver, heat transfer,
storage system and a delivery system.
The sun’s heat can be collected
in a variety of different ways. Today‘s technology includes solar parabolic
troughs, solar parabolic dish and power towers. Because these technologies
involve a thermal intermediary, they can be readily hybridized with fossil fuel
and in some cases adapted to utilize thermal storage. The primary advantage of
hybridization and thermal storage is that the technologies can provide
dispatchable power (dispatchability means that power production can be shifted
to the period when it is needed) and operate during periods when solar energy is
not available. Hybridization and thermal storage can enhance the economic value
of the electricity produced and reduce its average cost.
Solar Parabolic Troughs
 |
These systems use parabolic trough-shaped mirrors to focus sunlight
on thermally efficient receiver tubes that contain a heat transfer fluid.
Fluid is heated to almost 400 deg.C and pumped through a series of heat
exchangers to produce superheated steam which powers a conventional
turbine generator to produce electricity. A transparent glass tube placed
in focal line of the trough may envelop the receiver tube to reduce heat
loss. Parabolic troughs usually employ single-axis or dual-axis tracking.
In rare instances, they may be stationary. |
 Nine
trough systems, built in the mid to late 1980’s by Luz International set up
electricity-generating plants in the southern California desert with a total
installed capacity of 354 MW, making parabolic troughs the largest solar thermal
electric generating producers to date. These plants supply electricity to the
Southern California Edison utility grid. In 1984 Luz International installed
Solar Electric Generating System I (SEGS I) in Daggett, California. It has an
electricity capacity of 13,8 MW. Oil is heated in the receiver tubes to 343°C to
produce steam for electricity generation. SEGS I contains six hours of thermal
storage, and uses natural gas-fueled super heaters to supplement the solar
energy when solar energy is not available. Luz also constructed additional
plants, SEGS II through VII, with 30 MW capacity each. In 1990, Luz completed
construction of SEGS VIII and IX in Harper Lake, each with 80 MW capacity. As a
result of numerous regulatory and policy obstacles, Luz International and four
subsidiaries filed for bankruptcy on November 25, 1991. Three companies now
operate and maintain SEGS I - IX under the same contract that Luz International
had negotiated with Southern California Edison. Plans to construct SEGS X, XI,
and XII were canceled, eliminating 240 MW of additional planned capacity.
 |
Cost projections for trough technology are higher than those for
power towers and dish/engine systems (see bellow) due in large part to the
lower solar concentration and hence lower temperatures and efficiency.
However, with long operating experience, continued technology
improvements, and operating and maintenance cost reductions, troughs are
the least expensive, most reliable solar thermal power production
technology for near-term applications. |
 
Solar Parabolic Dish/engine
 |
These systems use an array of parabolic dish-shaped mirrors
(similar in shape to a satellite dish) to focus solar energy onto a
receiver located at the focal point of the dish. Fluid in the receiver is
heated up to 1000°C and is utilized directly to generate electricity in a
small engine attached to the receiver.Engines currently under
consideration include Stirling and Brayton cycle engines. Several
prototype dish/engine systems, ranging in size from 7 to 25 kW have been
deployed in various locations in the USA. High optical efficiency and low
start up losses make dish/engine systems the most efficient of all solar
technologies. A Stirling engine/parabolic dish system holds the world’s
record for converting sunlight into electricity. In 1984, a 29% net
efficiency was measured at Rancho Mirage,
California. |
 |
In addition, the modular design of dish/engine systems make them a
good match for both remote power needs in the kilowatt range as well as
hybrid end-of-the-line grid-connected utility applications in the megawatt
range.
This technology has been successfully demonstrated in a
number of applications. | One such
application was the STEP project in the state of Georgia (USA). The Solar Total
Energy Project (STEP) was a large solar parabolic dish system that operated
between 1982 and 1989 in Shenandoah, Georgia. It consisted of 114 dishes, each 7
meters in diameter. The system furnished high-pressure steam for electricity
generation, medium-pressure steam for knitwear pressing, and low-pressure steam
to run the air conditioning system for a nearby knitwear factory. In October
1989, Georgia Power shut down the facility due to the failure of its main
turbine, and lack of funds for necessary plant repairs.
 A
cooperative venture between Sandia National Lab and Cummins Power Generation
is recently attempting to commercialize 7.5 kilowatt (kW) dish/engine
systems. The systems are out of the component stage and into the validation
stage. When they accumulate sufficient running time, they will be ready for the
marketplace. Cummins hopes to sell 10,000 units a year by 2004. Other companies
are also entering into parabolic dish/Stirling technology. Stirling Technology,
Stirling Thermal Motors, and Detroit Diesel have teamed up with Science
Applications International Corporation in a $36 million joint venture with the
Department of Energy, to develop a 25 kW membrane dish/Stirling system.
The National Renewable Energy Laboratory (NREL) and the Cummins Engine
Company are testing two new receivers for dish/engine solar thermal power
systems: the pool-boiler receiver and the heat-pipe receiver. The pool-boiler
receiver operates like a double boiler on a stove. It boils a liquid metal and
transfers the heat energy to an engine on top. The heat-pipe receiver also uses
a liquid metal, but instead of pooling the liquid, it uses a wick to transfer
the molten liquid to a dome receiver.
Solar Central Receivers or Power Towers
 |
These systems use a circular field array of heliostats (large
individually-tracking mirrors) to focus sunlight onto a central receiver
mounted on top of a tower which absorbs the heat energy that is then
utilized in driving a turbine electric generator. A computer-controlled,
dual-axis tracking system keeps the heliostats properly aligned, so that
the reflected rays of the sun are always aimed at the receiver. Fluid
circulating through the receiver transports heat to a thermal storage
system, which can turn a turbine to generate electricity or provide
heat directly for industrial applications. Temperatures achieved at the
receiver range from 538°C to 1482°C. | The first
power tower “Solar One” built near Barstow in Southern California, successfully
demonstrated this technology for electricity generation. This facility operated
in the mid-1980’s, used a water/steam system to generate 10 MW of power. In
1992, a consortium of U.S. utilities decided to retrofit Solar One to
demonstrate a molten-salt receiver and thermal storage system. The addition of
this thermal storage capability makes power towers unique among solar
technologies by promising dispatchable power at load factors of up to 65%. In
this system, molten-salt is pumped from a “cold” tank at 288 deg.C and cycled
through the receiver where it is heated to 565 deg.C and returned to a “hot”
tank. The hot salt can then be used to generate electricity when needed. Current
designs allow storage ranging from 3 to 13 hours.
 |
“Solar Two”, a power tower electricity generating plant in
California, is a 10-megawatt prototype for large-scale commercial power
plants. This facility first generated power in April 1996, and is
scheduled to run for a 3-year test, evaluation, and power production phase
to prove the molten-salt technology. It stores the sun’s energy in molten
salt at 550 deg.C, which allows the plant to generate power day and night,
rain or shine. The successful completion of Solar Two should facilitate
the early commercial deployment of power towers in the 30 to 200 MW range
(source: Southern California Edison). |
   
Technology Comparison
 |
Table below highlights the key features of the three solar
technologies. Towers and troughs are best suited for large, grid-connected
power projects in the 30-200 MW size, whereas, dish/engine systems are
modular and can be used in single dish applications or grouped in dish
farms to create larger multi-megawatt projects. Parabolic trough plants
are the most mature solar power technology available today and the
technology most likely to be used for near-term deployments. Power towers,
with low cost and efficient thermal storage, promise to offer
dispatchable, high capacity factor, solar-only power plants in the near
future. The modular nature of dishes will allow them to be used in
smaller, high-value applications. Towers and dishes offer the opportunity
to achieve higher solar-to-electric efficiencies and lower cost than
parabolic trough plants, but uncertainty remains as to whether these
technologies can achieve the necessary capital cost reductions and
availability improvements. Parabolic troughs are currently a proven
technology primarily waiting for an opportunity to be developed. Power
towers require the operability and maintainability of the molten-salt
technology to be demonstrated and the development of low cost heliostats.
Dish/engine systems require the development of at least one commercial
engine and the development of a low cost
concentrator. |
Characteristics of solar thermal electric power systems (as of
1993).
| |
Parabolic
Trough |
Dish/Engine |
Power
Tower |
|
Size |
30-320 MW |
5-25 kW |
10-200 MW |
|
Operating Temperature (ºC/ºF) |
390/734 |
750/1382 |
565/1049 |
|
Annual Capacity Factor |
23-50 % |
25 % |
20-77 % |
|
Peak Efficiency |
20%(d) |
29.4%(d) |
23%(p) |
|
Net Annual Efficiency |
11(d)-16% |
12-25%(p) |
7(d)-20% |
|
Commercial Status |
Commercially Scale-up Prototype |
Demonstration |
AvailableDemonstration |
|
Technology Development Risk |
Low |
High |
Medium |
|
Storage Available |
Limited |
Battery |
Yes |
|
Hybrid Designs |
Yes |
Yes |
Yes |
|
Cost USD/W |
2,7-4,0 |
1,3-12,6 |
2,5-4,4 | (p) = predicted; (d)
= demonstrated;
Comparison of Major Solar Thermal Technologies.
|
Parabolic
Trough |
Parabolic
Dish |
Power
Tower |
| Applications |
Grid-connected electric plants; process heat for industrial
use. |
Stand-alone small power systems; grid support |
Grid-connected electric plants; process heat for industrial
use. |
| Advantages |
Dispatchable peaking electricity; commercially available with
4,500 GWh operating experience; hybrid (solar/fossil)
operation. |
Dispatchable electricity, high conversion efficiencies;
modularity; hybrid (solar/fossil) operation. |
Dispatchable base load electricity; high conversion
efficiencies; energy storage; hybrid (solar/fossil)
operation. |
Solar Thermal Power Cost and Development Issues
The cost of electricity from solar thermal power
systems depends on a multitude of factors. These factors include capital and
operating and maintenance cost, and system performance. However, it is important
to note that the technology cost and the eventual cost of electricity generated
is significantly influenced by factors “external” to the technology itself. As
an example, for troughs and power towers, small stand-alone projects will be
very expensive. In order to reduce the technology costs to compete with current
fossil technologies, it will be necessary to scale-up projects to larger plant
sizes and to develop solar power parks where multiple projects are built at the
same site in a time phased succession. In addition, since these technologies in
essence replace conventional fuel with capital equipment, the cost of capital
and taxation issues related to capital intensive technologies will have a strong
effect on their competitiveness.
COST VERSUS VALUE
Through
the use of thermal storage and hybridization, solar thermal electric
technologies can provide a firm and dispatchable source of power. Firm implies
that the power source has a high reliability and will be able to produce power
when the utility needs it. As a result, firm dispatchable power is of value to a
utility because it offsets the utility’s need to build and operate new power
plants. Dispatchability implies that power production can be shifted to the
period when it is needed. This means that even though a solar thermal
plant might cost more, it can have a higher value.
BENEFITS
Solar
thermal power plants create two and one-half times as many skilled, high paying
jobs as do conventional power plants that use fossil fuels.
California Energy Commission study shows that even with existing tax
credits, a solar thermal electric plant pays about 1,7 times more in federal,
state, and local taxes than an equivalent natural gas combined cycle plant. If
the plants paid the same level of taxes, their cost of electricity would be
roughly the same.
POTENTIAL
Utilizing only 1% of the earth’s
deserts to produce clean solar thermal electric energy would provide more
electricity than is currently being produced on the entire planet by fossil
fuels.
FUTURE
Over 700
megawatts of solar thermal electric systems should be deployed by the year 2003
in the U.S. and internationally. The market for these systems should exceed
5,000 megawatts by 2010, enough to serve the residential needs of 7 million
people which will save the energy equivalent of 46 million barrels of oil per
year.
SUMMARY
Solar
thermal power technologies based on concentrating technologies are in different
stages of development. Trough technology is commercially available today, with
354 MW currently operating in the Mojave Desert in California. Power towers are
in the demonstration phase, with the 10 MW Solar Two pilot plant located in
Barstow (USA), currently undergoing testing and power production. Dish/engine
technology has been demonstrated. Several system designs are under engineering
development, a 25 kW prototype unit is on display in Golden (USA), and
five to eight second-generation systems have been scheduled for field validation
in 1998. Solar thermal power technologies have distinct features that make them
attractive energy options in the expanding renewable energy market
world-wide.
Solar thermal electricity generating systems have come a
long way over the past few decades. Increased research and development of solar
thermal technology will make these systems more cost competitive with fossil
fuels, increase their reliability, and become a serious alternative for meeting
or supplying increased electricity demand.
Solar Ponds
Neither focusing mirrors nor solar cells can generate electricity at
night. For this purpose the daytime solar energy must be stored in storage
tanks, a process which occurs naturally in a solar pond.
Salt-gradient solar ponds have a high concentration of salt near the
bottom, a non-convecting salt gradient middle layer (with salt concentration
increasing with depth), and a surface convecting layer with low salt
concentration. Sunlight strikes the pond surface and is trapped in the bottom
layer because of its high salt concentration. The highly saline water, heated by
the solar energy absorbed in the pond floor, can not rise owing to its great
density. It simply sits at the pond bottom heating up until it almost boils
(while the surface layers of water stay relatively cool)! This hot brine can
then be used as a day or night heat source from which a special organic-fluid
turbine can generate electricity. The middle gradient layer in solar pond acts
as an insulator, preventing convection and heat loss to the surface. Temperature
differences between the bottom an surface layers are sufficient to drive a
generator. A transfer fluid piped through the bottom layer carries heat away for
direct end-use application. The heat may also be part of a closed-loop Rankine
cycle system that turns a turbine to generate electricity.
 |
1. High salt concentration
2. Middle layer.
3. Low salt concentration.
4. Cold water in and hot
water out. | This type of power station has been
tested at Beit Ha’Arava (Israel) near the Dead Sea. Israel leads the world in
salt-gradient solar pond technology. Ormat Systems Inc. has installed several
systems in the Dead Sea. The largest is a 5 MW electric system. This 20 hectare
pond converts sunlight to electricity at an efficiency of about 1%. It consists
of a pond of water with very high salinity in its lower depths. Although the
solar pond operated successfully for several years, in 1989 it was shut down for
economical reasons. The largest solar pond in the USA is a 0,3 hectare pond in
El Paso, Texas, which has operated reliably since its start in 1986. The pond
runs a 70 kW (electric) organic Rankine-cycle turbine generator, and a 20 000
litres per day desalting unit, while also providing process heat to an adjacent
food processing company. The pond has reached and sustained temperatures higher
than 90 deg. C in its heat-storage zone, generated more than 100 kW of electric
power during peak output , and produced more than 350 000 litres of potable
water in a 24 hour period. During five year operation, it has produced more than
50 000 kWh of electricity. A man-made, salt-gradient solar pond was built
in Miamisburg, (Ohio, USA) and it heats a municipal swimming pool and a
recreational building.
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