The collector is the most important and most expensive part of a solar-heating system. Collectors for space and water heating are of two basic types: liquid and air. Liquids may be water, an antifreeze mixture, or various hydrocarbon and silicone heat transfer oils. An air type of collector uses air as the collector medium. For the advantages and disadvantages of air-and liquid- heating systems, see table 15-1. The absorber plate is that part of the collector that absorbs the solar energy and converts it to thermal energy. Some thermal energy is carried to the building or thermal storage unit by the medium that circulates through passages in the absorber plate. The absorber plates are made of metal, plastic, or rubber compounds. The metals commonly used in order of decreasing thermal conductivity are copper, aluminum, and steel. Plastic (polyolefin) and rubber (ethylene propylene compound) is inexpensive. However, because of their low thermal conductivity and their temperature limitations, they are suitable only for low-temperature applications, such as heating water in swimming pools or for use with water- source heat pumps. Figure 15-2 depicts typical cross sections of solar collectors.
Flat-plate collectors are most suitable for low-temperature applications, such as domestic water and space heating. They collect both direct and diffuse radiation. It is not required that they track the sun. Tubes should be 1/2 inch in diameter or greater for low-pressure drop and longer life. The better the attachment of tube-to-plate (such as by soldering), the better the heat transfers.
Liquid and air collectors each have some advantages. Liquid types are most suited to domestic hot water because the collector area is usually smaller. The design procedures for air collectors differ however. Heat transfer oils used in liquid systems offer freeze protection and some corrosion protection, but they also require heat exchangers for heating domestic hot water, as do antifreeze-water mixtures.
Collectors are black and gray in color and have a rough textured surface. The rough-surface absorbs solar rays better than a smooth surface. A smooth, shiny surface will reflect radiant energy away from the collector. Generally, surfaces are made of metal particles, rather than paint, because paint cracks and peels at high temperature.
The transparent covers serve to admit solar radiation to the absorber while reducing convection and radiation heat losses from the collector. The covers also protect the absorber from dirt, rain, and other environmental contaminants.
The materials used as covers include glass or plastic sheets. Glass is most commonly used because of its superior optical properties and durability. Standard plate glass reflects about 8 percent and absorbs about 6 percent of normal incident solar radiation, resulting in a transmissivity of about 86 percent. Glass is subject to impact damage and is more expensive than plastic; however, it does not degrade in sunlight or at high collector temperatures and is more durable than plastic.
Although resistant to impact damage, plastic generally degrades in sunlight and is limited as to the temperatures they can sustain without undergoing serious deformation. In general, acrylic is the most ultraviolet-resistant, and polycarbonate offers good impact and high-temperature properties.Continue Reading