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MIL-HDBK-1003/13A
Advances in collector cost reduction will probably be made in the direction
of cheaper manufacturing processes.  Some collectors not made from tube and
sheet may not tolerate DHW line pressures.  Specifications for pressurized
collector circuits should require collectors which will take proof test
pressure equal to 150% of expected circuit pressure.
In hot climates, it is important to reduce roof heat load due to collector
heat gain in summer; this can be accomplished by venting the space between
collector plate and glazes with dampers or by covering the collectors.  A
normal amount of dirt and dust on the glass cover will reduce heat collected
by about 5%.  Normal rainfall is usually sufficient to relieve this problem.
Except for warm climates with high insolation (I > / - 1400 Btu/ft2-Day),
two cover glasses may be optimum (see Section 2.1.3).  In warm climates, one
glass is optimum.  Many plastics have an undesirable transparency to infrared
radiation, to which glass is nearly opaque, so the desired "greenhouse
effect" is not so pronounced with plastic materials as with glass.  However,
losses by radiation from the collector are small compared with convective
losses due to wind; thus plastics can be employed to reduce breakage and
cost, but with some loss in collector performance.  Plastics with maximum
opaqueness to infrared and maximum transparency to ultraviolet (UV) and
visible radiation and with high resistance to UV degradation should be
specified.  Collector orientation should follow the guidelines given in
Section 1.3.3.  Collector sizing will be given in Section 3.0.  The following
sections give more detailed information on collector designs and components.
2.1.1 Liquid and air-type collectors.  Liquid and air type collectors each
have some advantages which are summarized in Table 2-1 (Kimbell, 1978).
Liquid types are more suited to DHW, the collector area is usually smaller,
and more information is available about liquid systems.  Collectors for
heating air do not require protection from freezing and have minimal
corrosion problems, leaks do not cause serious damage, they may cost less per
unit area, and are better suited to direct space heating for residences where
duct-work is already present.  However, since leaks in air systems are less
easily detected, they can degrade system performance if not corrected.
Wherever this manual discusses liquid collectors, air collectors are
included, and cost analyses apply equally to both.  The design procedure for
air collectors differs, however.  Heat transfer oils used in liquid systems
offer freeze protection and some corrosion protection, but they also require
heat exchangers for heating domestic hot water, as do antifreeze-water
mixtures.
2.1.2 Selective surfaces.  Some collectors are manufactured with a black
coating which absorbs the high frequency incoming solar radiation very well
and which emits low frequency infrared radiation poorly.  This is a highly
desirable combination of properties for a collector.  The absorptance should
be 0.9 or higher and emittance may be 0.1 or lower.  Such coatings are
approximately equal in effect to one cover glass.  Thus, a selective coating
plus one cover glass may be expected to be about equal in efficiency to a
collector with two cover glasses and a flat black painted surface.
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