In addition to electricity generation, certain renewable resources—non-concentrating solar thermal technologies, low- and moderate-temperature geothermal, and biomass—can displace fossil fuels at the point of use, particularly in residential and commercial buildings and in light industry and agriculture. These resources are referred to as distributed renewables.

Solar Thermal

Applications for distributed solar thermal include water heating, space heating and cooling, and heat for industry and agriculture. Because the solar collector does not rely on concentrating the sun’s energy and can use both direct and diffuse radiation, distributed solar thermal systems are applicable to the entire United States. However, solar insolation and costs vary across the United States.

The most prevalent and well-developed applications are for heating swimming pools and potable water (in homes and laundries), with performance standards overseen by the U.S. Solar Rating and Certification Corporation.9 Systems include one or more collectors (which capture the sun’s energy and convert it into usable heat), a distribution structure, and a thermal storage unit. Associated piping, heat exchangers, and storage tanks use technology found in conventional HVAC and water-heating systems.

The unique component is the solar collector. The flat-plate collector is most common in the United States, but the use of evacuated-tube collectors is growing rapidly. For heating swimming pools, which is the country’s single largest application of solar thermal, the collector is an unglazed polymer absorber through which the pool water is circulated. Energy is delivered at moderate temperatures, usually less than 10°C above ambient. For domestic hot water, flat-plate collectors employ a copper plate absorber, usually coated with a wavelength-selective layer to reduce radiative losses. The absorber and tubes that contain the working fluid are mounted in an insulated box with a tempered glass cover. In evacuated-tube collectors, the absorber is mounted in an evacuated glass tube, and energy is delivered at temperatures up to 100°C.

Advances in manufacturing, materials, and industry standards have resulted in significant improvements since the 1980s in both the performance and the reli-

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