This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Solar thermal energy may be used to provide energy for a variety of uses including building space heating, refrigeration and air conditioning, domestic hot water, hot water and steam for industrial processes, drying and electric power generation.

The technology

In a typical solar thermal system, water or other heat-transfer fluid is heated by solar collectors and then circulated through equipment where the energy is used. Solar collector types include flat-plate collectors, evacuated-tube collectors and concentrating collectors such as parabolic troughs. The Canadian Renewable Energy Network describes the technologies.

Solar collectors can attain high temperatures—more than 500° F in commercially-available parabolic solar troughs and more than 300° F in evacuated-tube collectors. However, systems are generally not designed to deliver such high temperatures. Most systems using evacuated-tube or parabolic-trough collectors will deliver hot water at 195° F or less. For power generation or some industrial process heating, the system may be designed to achieve higher temperatures.

Cost-effectiveness

Because of economies of scale, solar thermal systems can be cost-effective for commercial, industrial and institutional facilities that require large volumes of mid- to high-temperature hot water, even if conventional fuel costs are relatively low. Solar thermal energy is most cost-effective in facilities that have a relatively constant energy requirement over the course of the day, week and year, or that have higher needs during the summer and during the day. Hotels, laundries, kitchens, prisons and military bases, for example, have relatively constant water heating needs.

Facilities with large, relatively constant cooling requirements that may be met by solar absorption cooling include computer data centers and cold storage facilities. CanREN provides background on solar cooling.

Generally, the bigger the project, the more cost effective it is. If the system is large enough, parabolic-trough collectors can be much less expensive than flat-plate collectors or evacuated-tube collectors. Parabolic troughs may be appropriate for projects with hot water requirements greater than about 10,000 gallons per day or with peak energy requirements for heating or cooling of at least 2 million Btu/h.

While economies of scale can bring costs down, often cost-effectiveness depends on Federal, state and utility incentives. Incentives are becoming more available, and the Database of State Incentives for Renewables and Efficiency contains a comprehensive listing of incentives available throughout the United States. The Solar Energy Industries Association also has a guide to Federal tax incentives.

Other considerations

• Energy can be stored for use at night and during brief cloudy periods.

• Systems generally require a large area for the collector field. A parabolic-trough project sized for a heating load of 3,000,000 Btu/h will require approximately 20,000 square feet of collectors. Including the necessary space between the collectors themselves, the total collector field will be three times this, or about an acre and a half.

• Parabolic troughs require direct sunlight and so require a tracking system. This means the solar resource must be better than for flat plate collectors or evacuated tube collectors, which make better use of indirect light.

• Parabolic troughs are typically ground-mounted because of the large areas that are usually required to meet heating requirements. If trough collectors are roof-mounted, the stresses they transmit to the structure due to wind loading must be considered.

• Sizing affects cost-effectiveness. Often the most cost-effective system will be sized to just meet the full summer demand and 50 percent to 80 percent of the annual demand.

• Solar thermal energy is not restricted to southern latitudes. The number of clear, sunny days each year is more important than latitude. Eastern Oregon and Eastern Washington, for example, have good solar resources. Also, many solar thermal projects have been installed in New Jersey recently because of state incentives there.

Manufacturers

There are many suppliers of flat-plate and evacuated-tube collectors. There are at least three companies currently manufacturing modular solar parabolic troughs for commercial and industrial applications:

• Acciona (formerly Solargenix), North Carolina

• Solucar (formerly Industrial Solar Technology Corp.), Colorado

• Solitem, Germany

In 2005, the International Energy Agency published a list of various types of medium-temperature (up to 250°F) solar collectors commercially available or in pre-production at the time.

Case Studies

• Roof Mount Parabolic Troughs; Fort Sam Houston Army Medical Base, San Antonio, Texas. Industrial Solar Technology Corp., June 2003.

• Heating Water with Solar Energy Costs Less at the Phoenix Federal Correctional Institution; parabolic trough case study, Federal Energy Management Program, 1998.

• Evacuated-Tube Heat-Pipe Solar Collectors Applied to the Recirculation Loop in a Federal Building, June 2004. Social Security Administration, Philadelphia, Penn.