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 MIL-HDBK-1003/13A
Each additional cover, whether it be glass or plastic, reduces convection
heat losses but results in added expense and less solar radiation transmitted
to the absorber. Most commercially available collectors come with one or two
covers. The decision to use one or two covers depends on the type of
absorber coating, the required collection temperatures, average ambient air
temperature, the local wind conditions, and of course, the cost of the
covers.
As stated in Section 2.1.2, the use of a selective surface is about equal to
using one additional cover. Thus for most cases, only one glass cover is
needed if the absorber has a selective coating. In fact, one study indicated
that winter performance was actually reduced by the use of two glass covers
with a selective surface compared to one cover with the selective surface.
Two covers are generally recommended for use in Northern climates where
winter ambient air temperatures are low. For flat-plate collectors used
mostly for winter heating, one rule of thumb is to use one glass cover where
average winter air is greater than, 45 deg. F, and two glass covers in colder
climates. Table 2-4 gives some approximations in the selection of collector
covers.
2.1.4 Collector insulation. Insulation behind and to the side of the
absorber serves to reduce conduction losses. Usually, this insulation
consists of 1-6 inches of high-temperature fiberglass batting or semi-rigid
board or even mineral wool. Styrofoam and urethane foams are usually not
used because they may deform at high temperatures or give off gases (which
may be toxic). The insulation should be separated from the absorber plate by
1/2 to 3/4 inch and have a reflective foil facing the absorber plate. If
fiberglass insulation is used, it should not be typical construction grade
which contains phenolic binders that may "outgas" at the stagnation
temperature of the collector. In all cases, specifications should call for
insulations that are not flammable, have a low thermal expansion coefficient,
do not melt or outgas at collector stagnation temperatures (300 deg. - 400
deg. F), and (whenever possible) contain reflective foil to reflect thermal
radiation back to the absorber.
2.1.5 Collector housings.
The housing or collector box serves to:
a. Support the collector components.
b. Protect the absorber and insulation from the environment.
c. Reduce convection and conduction losses from the absorber.
Many housing designs are available on the market. They are constructed of
metals, wood, plastics, concrete, and other materials. The most commonly
used materials are aluminum, galvanized sheet metal, fiberglass laminates,
high temperature thermoplastics, and wood (Montgomery, 1978). It is
recommended that wood be avoided for use as a structural member, spacer, or
anchor for panels due to its susceptibility to deterioration and
flammability.
All structural materials are suitable if properly used. However, most
commercially available housings consist of a galvanized sheet metal box with
an anodized aluminum frame which fits on top of the box. Some housings are
designed to be integrated directly into the roof or wall structure, thus
reducing construction costs.
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