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Figure 14-43.—Typical hermetic system schematic wiring diagram. | Up
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Solar Collector Orientation |
CHAPTER 15
SOLAR ENERGY
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 RADIATION SYSTEMS
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-ft2 or 2,453
watt S/m2 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.
COLLECTING SOLAR ENERGY
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.
15-1
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