CHARACTERISTICS

Unlike PV systems, which use sunlight to directly produce electricity, solar thermal systems generate electricity with heat from concentrated sunlight. Solar thermal collectors use mirrors and lenses to concentrate and focus sunlight onto a receiver mounted at the system's focal point. The receiver absorbs and converts sunlight into heat. The heat is then transported to a steam generator or engine where it is converted into electricity.

There are three main types of solar thermal electric systems: parabolic troughs, parabolic dishes and central receiver systems: (1) trough systems—or parabolic trough collectors—use mirrored troughs to focus energy on a fluid-carrying receiver tube located at the trough's focal point. There are 354 MW of trough systems installed in southern California; (2) dish systems use parabolic mirrors to concentrate and focus incoming solar energy onto a receiver mounted above the dish at the focal point. Each dish produces 5-50 kW of electricity that can be used independently or linked together to increase generating capacity; (3) central receivers systems—or "Power Towers"—use thousands of individual tracking mirrors (heliostats) to reflect solar energy onto a receiver located atop a tall tower. The world's largest central receiver plant is a 10 MW power plant near Barstow, California.

Solar energy technologies offer a clean, renewable and domestic energy source. In the U.S., solar thermal power plants produce about 480 million kWh of energy each year displacing 325,000 tons of CO2 (6.8 tons of CO2 /kWh) annually.

Solar energy technologies have made huge technological and cost improvements, but except for certain niche markets—such as remote power applications—are still more expensive than traditional energy sources. Researchers continue to develop technologies that will make solar energy technologies—particularly power generating technologies—cost-competitive with fossil fuels. Current research efforts are focused on developing lighter, more efficient system components and energy storage technologies

SIZE:
Parabolic troughs: 30-80 MW_e units now in deployment; 160 MW_e modules proposed. Central receiver systems: 80-200 MW_e Parabolic dish-Stirling engines: 5-50 kW_e

FEATURES:
Parabolic troughs: require ~2 hectares of land per MW_e of generating capacity, based on a daily mean direct normal insulation value of 6-7 kWh/m². Central receiver systems: require 3-5 hectares land/MW_e. Parabolic dish-Stirling engines: require about 0.7 ha/MW_e.

COST:
Parabolic troughs: $2,890/kW_e (1991) Central receiver systems: $3,300/kW_e—100 MW_e or $2,800/kW_e—200 MW_e (1991) Parabolic dish-Stirling engines: $1,700-$3,000/kW_e

CURRENT USAGE:
Over 354 MW_e parabolic troughs, primarily in California.

POTENTIAL USAGE:
Developing countries, where half the population is currently without electricity and sunlight is usually abundant, represent the biggest and fastest growing market for power producing technologies. The largest potential U.S. application is for power production.

ISSUES ASSOCIATED WITH IMPLEMENTING ACTION

• Fuel source—sunlight—fluctuates seasonally and is nonexistent at night. For dispatchability, solar thermal technologies must have storage capacity or back-up power source, which increases the overall cost of system. However, resource availability does match daytime demand peaks.
• Requires large quantities of land for base-load electricity generation or for peaking electricity generation; may have negative impact on habitats.
• There have been reliability problems with high temperature requirements and corrosive effects on solar mirrors.
• Costs per kWh produced by stand-alone solar thermal sources are currently higher than with conventional sources.

EMISSION ESTIMATE:
6.8 tons/kWh of CO2 avoided

COST-EFFECTIVENESS:
$88-178/ton of CO2 avoided (net)

SECONDARY EFFECTS:
Produces no emissions unless a hybrid natural gas system is used (parabolic troughs only) in which case emissions remain minimal with the exception of NOx @ 31.8 grams/MWh.

RESOURCES

• California Energy Commission. 1990. Solar Thermal Electric - Parabolic Troughs, Energy Staff Report (US).
• The Centre for the Analysis and Dissemination of Demonstrated Energy Technologies (CADDET) provides an on-line directory of solar-thermal technical reports and development status. http://www.caddet-re.org/

CONTACTS

European Solar Energy Association
Bonn, Germany
Tel: +49-228-362373 and 362375
Fax: +49-228-361279
inter_office@eurosolar.org
http://www.eurosolar.org

Solar Energy Industry Association
Scott Sklar
Executive Director
Washington, DC
Tel: (202) 383-2600
Fax: (202) 383-2670
info@seia.org
http://www.seia.org