Solar thermal collectors are an important element of solar hot water systems for residential purposes. Solar collectors come in a variety of sizes and shapes, but all these devices capture heat so it can be used to warm household water. Many incorporate a fluid, typically water, for heat transfer.
Depending on whether a collector is designed to take advantage of heat conduction, convection, or radiation, it will be designed with slightly different properties and applications.
All solar collectors include something called absorbers. Absorbers can be constructed in different shapes and sizes, but usually are an assemblage of pipes or passageways made of plastic or metal that has a dark coating or color designed to absorb solar radiation and heat. To prevent radiation and heat from escaping, transparent glass or plastic sheeting is often used to cover the absorbers.
Absorbers may have a fluid circulating inside or around them to collect and move the heat. Ordinary water is an effective heat transfer fluid.
Solar collectors are designed to meet performance and durability standards, such as ASHRAE 93-77, 93-1986, and 96-1980, ISO 9806, or EN 12975.
Designs of solar thermal collectors
Low temperature collectors, for applications such as heating the water for swimming pools, may be constructed from plastic without the transparent covers.
Flat plate solar collectors are flat and thin with multiple layers, including an absorber layer. Approximately one-third of all solar collectors are of this type.
Evacuated tube collectors (ETC) incorporate rows of glass tube collectors. Inside each glass tube is an absorber sitting within a vacuum. The absorbers can be shaped like a long metal fin that can be rotated to optimize sunlight striking it, or as a tube within a tube. Half of all collectors used throughout the world are of the evacuated tube type. China is by far the world’s biggest user of ETCs.
Mirrors are sometimes integrated to reduce shading that diminishes heat and light. The benefit of a vacuum around the absorbers is that it reduces the negative impact of ambient temperatures on performance. strong>Unglazed collectors are lower cost and used predominantly in China, Turkey, Germany, and Japan.
Air collectors are flat plate collectors that are adjusted to use air as the heat transfer medium. These systems can supplement space heating and are best suited for daytime needs. Air collectors represent just one percent of all solar collectors used around the globe. Home-made solar thermal collectors
A metal pipe or drum that is painted black, or even a black plastic bag filled with water, can serve as a crude DIY solar thermal collector. Some enterprising individuals make an assemblage of copper strips soldered to copper pipes, with the sun-facing side painted black.
Low temperature usages of 86 degrees Fahrenheit (30 degrees Celsius) can be achieved with economical, bare and unglazed solar collectors used alone or linked to a storage tank.
Measuring the efficiency of solar collector systems
Comparing the cost efficiency of solar thermal systems is a complex matter when each has a different design. For example, you might have an ETC with a flat absorber, an ETC with tubular absorbers, or one that includes reflectors.
The efficiency calculation can be based alternately on either the absorber area (surface receiving the sunlight), the aperture area (space that lets in light), or the gross area (overall dimensions). Each system has a theoretical maximum efficiency, which can be impacted by the system layout. You need to be sure you’re comparing apples to apples when comparing the value of a given system.
The Solar Rating & Certification Corporation provides data sheets that detail the specifications and expected performance for various thermal collectors on days that are clear or cloudy. Usually the solar collectors are positioned to face the south.
SRCC assigns collectors to categories ranging from A to E, with A being the lightest duty (typically for solar pool heating) and E going up to 144 degrees Fahrenheit (typically for industrial processes. Domestic or residential hot water applications are usually considered category C or D.
For more information, see:
Laughton, C. Solar Domestic Water Heating: The Earthscan Expert Handbook for Planning, Design and Installation. Earthscan, 2010.