Solar thermal collectors can be categorized by the temperature at which they efficiently deliver heat.
Low-temperature collectors are unglazed and uninsulated. They operate at up to 18°F (10°C) above ambient temperature, and are most often used to heat swimming pools. At this low temperature, a cover glass would reflect or absorb solar heat more than it would reduce heat loss. Often, the pool water is colder than the air, and insulating the collector would be counterproductive.
Low-temperature collectors are extruded from polypropylene or other polymers with UV stabilizers. Flow passages for the pool water are molded directly into the absorber plate, and pool water is circulated through the collectors with the pool filter circulation pump. The simple collectors available for swimming pool systems cost around $9 per square foot and a complete system with installation is on the order of $18/SF.
Mid-temperature systems place the absorber plate in an enclosure insulated with fiberglass or polyicocyanurate, and with a low-iron cover glass to reduce heat loss at higher temperatures. They produce water 18°–129°F (about 10°–50°C) above the outside temperature, and are most often used for heating domestic hot water (DHW). Reflection and absorption reduce the solar transparency of the glass and reduce the efficiency at low temperature differences, but the glass is required to retain heat at higher temperatures. A copper absorber plate with copper tubes welded to the fins is used. To reduce radiant losses from the collector, the absorber plate is often treated with a black nickel selective surface, which has a high absorptivity in the shortwave solar spectrum, but a low-emissivity in the long-wave thermal spectrum. Such flat plate systems cost as high as $155/SF installed for a single residential system to around $90/SF for a large commercial system.
High-temperature collectors surround the absorber tube with an evacuated borosilicate glass tube to minimize heat loss, and often utilize mirrors curved in a parabolic shape to concentrate sunlight on the tube. Evacuating the air out of the tube eliminates conduction and convection as heat loss mechanisms, and using a selective surface minimizes radiation heat loss.
High-temperature systems are required for absorption cooling or electricity generation, but are used for midtemperature applications such as commercial or institutional water heating as well.
Due to the tracking mechanism required to keep the focusing mirrors facing the sun, high-temperature systems are usually very large and mounted on the ground adjacent to a facility. These collectors are usually used in very large systems and typical installed system cost is on the order of $50/SF.
Selecting the best type of collector will depend on the application. At low temperatures, the inexpensive, unglazed collectors offer the highest efficiency, but efficiency drops off very quickly as temperature increases. Glazed collectors are required to efficiently achieve higher temperatures, and very high-temperature applications require an evacuated tube in order to deliver any useful heat.