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 MIL-HDBK-1003/13A
2.0 SOLAR SYSTEM COMPONENTS.
This section will cover in detail the typical components of a flat plate
solar collector as shown previously in Figure 1-2.
2.1 Collectors. The collector is the most important and one of the most
expensive parts of a solar heating system. It must be long-lived and well
insulated, yet its cost must be minimized. Collectors of primary interest
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. Air-type collectors use air as the collector fluid. The
absorber plate is that part of the collector which absorbs the solar energy
and converts it to thermal energy. A portion of the thermal energy is
carried to the building or thermal storage unit by the fluid which circulates
through passages in the absorber plate. The absorber plates can be made of
metal, plastic, or rubber compounds. The metals commonly used in order of
decreasing thermal conductivity are copper, aluminum, and steel. Plastics
(polyolefins) and rubbers (ethylene propylene compounds) are relatively
inexpensive, but due to their low thermal conductivity and their temperature
limitations, they are suitable only for low temperature applications, such as
heating swimming pool water or for use with water source heat pumps. Typical
cross sections of solar collector types are shown in Figure 2-1.
Other major components of a solar collector include:
a.
Absorber plate coating - To enhance the heat transfer and protect
the absorber plate.
b.
One or more transparent covers - To reduce thermal losses by
radiation (using the "greenhouse effect") and by convection (wind,
etc.). Spacings are nominally 1/2 inch or more.
c.
Insulation - One to three inches are used to reduce heat loss
through the side and back of the absorber plate.
d.
Collector box or housing - To provide a rigid mounting to hold the
components.
Must be weatherproofed.
e.
Gaskets and seals - To insure a weathertight seal between
components while allowing thermal expansion of the components.
Normally these seals remain ductile to accomplish their purpose.
Flat-plate collectors are most suitable for low temperature applications such
as domestic hot water and space heating.
They collect both direct and
diffuse radiation.
It is not required that they track the sun, thus initial
cost and maintenance are minimized.
A properly designed flat-plate
collector has a life expectancy of 10 to 25 years, or sometimes longer.
All
copper and glass systems currently exhibit the longest lives.
Using
softened water will help.
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 transfer, but the
greater the manufacturing cost.
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