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MIL-HDBK-1003/13A
The amount of solar energy collected by a solar collector depends on its
efficiency, which is determined by how it is constructed, its configuration,
and the choice of materials used.  Standards are available as given in
Section 1.2 which can test the instantaneous efficiency of a solar collector
for a specified set of operating conditions.
Other parameters, not related to the physical characteristics of a solar
collector, can affect performance.  The atmosphere reduces the radiation
received on the earth's surface and is also responsible for the scattering of
light which results in diffuse, as distinct from direct, solar radiation.
The diffuse component may represent as much as 25%-30% of the total solar
radiation depending on the weather conditions.  Cloudy days are virtually
100% diffuse solar energy.  Solar flat plate collectors absorb heat from the
diffuse component as well as the direct.  Thus, some heat is available on
partly cloudy days.  The reflectance of the ground (snow, sand, water, etc.)
or nearby objects may also influence the amount of solar energy reaching a
collector.  Therefore, the amount of solar energy received at any location
depends on the hour of the day, the day of the solar year, and the meteoro-
logical conditions.  This amount can vary from about 50 Btu/ft2-hr on a
foggy winter day to as much as 300-375 Btu/ft2-hr on a typical sunny summer
day.
1.3.3 Solar collector orientation.  Even though solar collectors can
collect heat from the diffuse component of solar radiation, solar systems are
designed to use the direct component.  Direct radiation is in the form of
parallel rays coming straight from the sun.  To best capture this energy the
solar collector should be tilted as shown in Figure 1-2 so that it is more
nearly perpendicular to the solar rays.  The "optimum" tilt angle varies even
as the sun changes its position throughout the day and year.  However, since
the solar system cannot be continuously moved, some general rules can be
stated:
a.
For all year domestic hot water (DHW) heating use a tilt angle
equal to the latitude.
b.
For all year DHW heating and winter space heating, use a tilt angle
equal to the latitude plus 10-15 degrees.
c.
For all year DHW heating, winter space heating, and summer cooling
use same as (a).
d.
For winter only space heating use a tilt angle equal to the
latitude plus 10-15 degrees.
e.
For summer space cooling use a tilt angle equal to the latitude
minus 10-15 degrees.
f.
For summer only space cooling and all year DHW heating use the same
as (e).
In addition to choosing the best collector tilt angle, consideration must be
given to the orientation of a collector (i.e., the direction the collector
faces).  Normally true south is the best and most frequent choice.  However,
slightly west of south (10 degrees) may be preferable in some locations if an
early morning haze or fog is a regular occurrence.
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