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MIL-HDBK-1003/13A
2.1.9 Collector efficiency and heat losses.  In the preceding sections,
many details as to the construction and choice of components of a solar
collector have been given.  All of these features contribute to how well a
collector will perform or how efficient it will be.  Solar collectors,
depending on their construction and materials, suffer from several kinds of
heat losses.  They can lose heat by convection of wind blowing over their top
and bottom surfaces.  As the collector temperature increases above the
temperature of the surrounding air, the radiation heat losses increase.  This
results in lower heat collected (lower efficiency) at higher collector
temperatures.  Heat can be lost by conduction from the back and sides of a
collector.  To evaluate the effects of all these parameters individually
would involve detailed and difficult calculations.
Fortunately, collector performance can be compared much more easily by a
single graph depicting collector efficiency versus the parameter [DELTA]T/I.
collector efficiency is defined as the ratio of the heat collected to the
Insolation (I) falling on the surface of the collector.  Also:
[DELTA]T = Ti - Ta
where Ti = temperature of fluid entering collector (inlet).
Ta = ambient air temperature.
Figure 2-7 gives the efficiency of some typical flat plate solar collectors.
The most efficient solar collector would convert 100% of the sun's energy
falling on it to usable heat.  As shown in Figure 2-7, this is impossible so
the designer looks for a collector that converts the greatest percentage of
solar energy to heat, at the required temperature, and at the lowest cost.
It is important that each collector be tested according to an exacting
standard.  The early standard for testing solar collectors, was NBSIR 74-635
published by the National Bureau of Standards (Hill and Kusada, 1974).  This
is the standard the previous edition of this report used to report collector
efficiencies.  Subsequently, the American Society of Heating, Refrigerating,
and Air Conditioning Engineers, Inc. (ASHRAE) sponsored the development of a
uniform method of testing solar collectors to form the preliminary standard
93-P and finally the version in use today, ASHRAE Standard 93-77, "Methods of
Testing to Determine the Thermal Performance of Solar Collectors."  This
method uses the Hottel-Whillier equation and is generally accepted in the
solar industry.
The differences between the NBS and the ASHRAE standard are as follows:
a.
ASHRAE requires the use of gross collector area rather than
aperture or net area used in NBS.
b.
ASHRAE uses the collector inlet temperature as one of its
parameters whereas NBS uses the average absorber plate temperature
defined as the sum of the inlet and outlet temperatures divided by
two.
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