Although solar water heating systems all use the same basic method for capturing and transferring solar energy, they do so with three specific technologies that distinguish different collectors and systems. The distinctions are important because different water heating needs in various locations are best served by certain types of collectors and systems.
Materials and components used in solar water heating systems vary depending on the expected operating temperature range.
Low-temperature systems (unglazed) usually operate at low temperature, up to 18°F (10°C) above ambient temperature, and are most often used for heating swimming pools. Often, the pool water is colder than the air, and insulating the collector would be counter-productive. Low-temperature collectors are extruded from polypropylene or other polymers with ultraviolet 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.
Mid-temperature systems produce water 18°F to 129°F (10°C to 50°C) above outside temperature, and are most often used for heating domestic hot water. However, it is also possible to use mid-temperature solar water heating collectors for space heating in conjunction with fan-forced convection coils or radiant floors.
Mid-temperature collectors are usually flat plates insulated by a low-iron cover glass and fiberglass or polyisocyanurate insulation. Reflection and absorption of sunlight in the cover glass reduces 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. In order 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 short-wave solar spectrum, but a low-emissivity in the long-wave thermal spectrum.
High-temperature systems use evacuated tubes around the receiver tube to provide high levels of insulation and often use focusing curved mirrors to concentrate sunlight. High-temperature systems are required for absorption cooling or electricity generation, but are used for mid-temperature 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.
Solar collector efficiency varies depending on how warm the collector inlet temperature is relative to the ambient air temperature. The following figure shows typical efficiency curves for the three collector types. Note that inexpensive, unglazed collectors are very efficient at low ambient temperatures, but efficiency drops off very quickly as temperature increases. They offer the best performance for low temperature applications, but glazed collectors are required to efficiently achieve higher temperatures. Evacuated tubes are necessary to attain higher water temperatures, as needed in cooling and some industrial process heat applications.
TYPES OF SOLAR WATER HEATING SYSTEMS
Solar water heating system types are classified by the following types: