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Electroplated black nickel, black chrome, copper oxide or anodized aluminum
are common types of selective coatings.  Cost of selective surface coatings
may be greater than an extra sheet of glass, but much research is being done
to produce low cost, easily applied coatings.  The stability of black nickel,
chrome and aluminum in the presence of moisture has not yet been proven.
Longterm stability in the presence of moisture or other expected
environmental factors (salt air, etc.) must be included in specifications for
selective surfaces.  Table 2-2 is a summary of absorber coatings both
selective and nonselective.
2.1.3 Collector covers (glazes).  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 material used for covers include glass and/or plastic sheets.  Glass is
most commonly used because of its superior optical properties and durability.
Standard plate glass reflects about 8% and absorbs about 6% of normal
incident solar radiation, resulting in a transmissivity of about 86%.  Yet it
is essentially opaque to long-wave thermal radiation from the absorber.
Transmission of solar radiation into the collector can be increased by
minimizing the reflectance and the absorptance of the glass covers.
Absorptance of solar radiation by the collector can be increased with the use
of thinner tempered glass and by using glass that has a low iron content.
Although glass is subject to impact damage and is more expensive than
plastic, it does not degrade in sunlight or at high collector temperatures,
and is generally considered to be more durable than plastic.  Impact damage
may be reduced with the use of tempered glass and small collector widths.
Also 1/2-inch wire mesh may be hung over glass covers for protection, but the
effective absorber area will be reduced by approximately 15%.  In general,
screens are not recommended.
Most plastic covers transmit the solar spectrum as well or better than glass
glazing.  Unfortunately, they transmit infrared radiation well also,
increasing radiation losses from the collector.  Table 2-3 compares the
different characteristics of glass and plastic covers (Montgomery, 1978).
Although resistant to impact damage, plastics generally degrade in sunlight
and are limited as to the temperatures they can sustain without undergoing
serious deformation.  Often they do not lie flat, resulting in a wavy
appearance.  In general, acrylic is the most UV resistant and FRP Plastics
offer good impact and high temperature properties.  Teflon FEP film has good
transmittance and high temperature properties, but is limited in strength.
Some collectors using plastic covers are designed to have stagnation
temperatures no higher than 200 deg. - 275 deg. F.  However, plastic covers
have been developed to withstand 400 deg. F.  The manufacturer should be


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