The energy that the earth receives from the sun is electromagnetic radiation. Most of the energy is received in the visible and infrared portions and a small amount is received as ultraviolet radiation. Energy from the sun travels approximately 90 million miles in just over 8 minutes to reach earth. If 100 percent of the solar constant were to be collected on an area the size of the United States, we could absorb enough energy in 32 minutes to supply the energy needs of the entire world for a year.
Solar insolation is the amount of solar energy per unit area per unit of time that strikes the surface of the earth. If measurements were made of the solar energy available in outer space, a fixed amount could be determined. This fixed amount of energy is called the solar constant. The solar constant is as follows: 428 Btu/hr-ft or 2,453 watt S/m to 1,940 Langleys/min. Langleys (L) is the most common measurement used. At most, 70 to 80 percent of this amount strikes the surface of the earth; the remainder is absorbed or reflected by the atmosphere. Those solar rays that hit the earth's surface on a clear day are, for the most part, parallel to each other. When there is haze, cloud cover, smog, or dust in the air, the parallel pattern is broken and the rays are deflected in many different directions by these particles of water or dust in the atmosphere. This is the reason why light and heat appear to come at us from all directions; the term used for this is diffuse radiation.
With the right solar collector, diffuse radiation can be useful. Because of the filtering effect, the average solar intensity on the ground is about 1,400 Btu per square foot per day. This is equal in a square mile to the productivity of a large hydroelectric power plant.
Collection of solar energy is based on the high absorption of radiant energy by dull black surfaces and on the "greenhouse effect." The latter refers to the ability of glass to transmit visible radiation and to prevent the loss of heat from the collector plate that radiates at longer wavelengths (infrared frequencies). Glass (or plastic) cover plates are generally used over flat-absorber plates to reduce heat loss (fig. 15-1). The heated absorber plate has tubes that allow fluid to circulate through the plate and receive heat. The heated fluid heats potable water, closed spaces, or drives an absorption air-conditioner.
Figure 15-1. - Cross section of a typical solar heat collector with heavy back insulation and two cover sheets.Continue Reading