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SOLAR COLLECTOR ORIENTATION Although solar collectors can collect heat from the diffuse component of solar radiation, solar systems are designed to use radiation. Direct radiation is in the form of parallel rays coming straight from the sun. To capture this energy, tilt the solar collector, as shown in figure 15-1, so it is nearly perpendicular to the solar rays. DISADVANTAGES Leaking, freezing, and corrosion can be problems. Corrosion inhibitors needed with water when using steel or aluminum. There are liquids which are noncorrosive and nonelectrolytic; however, they are toxic and some of them are flammable. A separate collector loop using a nonfreezing fluid and a heat exchanger or, alternatively, a draining water or inhibited water system is required to prevent freezing. In warm regions, where freezing is infrequent, electric warmers or recirculation can be used. In addition to choosing the best collector tilt angle, take into consideration the direction that the collector faces. Normally, true south is the best and most frequent choice, However, 10 degrees west of south maybe preferable in some locations if early morning haze or fog is a regular occurrence. Equally important as collector location is keeping the collectors out of the shade, especially between 0900 and 1500 hours, when most of the useful energy collection occurs. In summary although many buildings do not have a "perfect" solar orientation, there are still many places with good solar energy potential.
COLLECTORS 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 liquidheating 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 watersource 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. |
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