MIL-HDBK-1003/13A
Some deviations from these tilt and orientation angles are allowable without
significantly affecting performance. As shown in Figures 1-3 and 1-4, the
tilt angle may vary +/- 10 degrees and the orientation angle up to 20 degrees
either side of true south (National Solar Heating and Cooling Info Center,
1979). For these deviations the solar collectors would still collect 95%-
100% of their rated capacity in most locations of the U.S. Additional
deviations would require more collector area to capture the same amount of
energy. As a very approximate rule of thumb, for each deviation of 10
degrees beyond that shown in Figures 1-3 and 1-4 add 10% more collector area.
If you must choose between an east roof and a west roof, use the west roof in
the western coastal area. Other areas will require local weather
considerations.
As important as collector location, is keeping the collectors out of the
shade, especially between 9 a.m. and 3 p.m., when most of the useful energy
collection occurs. The designer should try to anticipate any future struc-
tures or vegetation (trees) that could block future solar access. In
summary, although many buildings will not have a "perfect" solar orientation,
there can still be many places with good solar energy potential.
1.3.4 Advantages and disadvantages. Solar energy is inherently
nonpolluting, provides substantial freedom from the effects of fuel price
increases, and saves valuable fossil fuels. Disadvantages are that
collectors perform poorly in cold cloudy weather, when most needed; and room
heat exchangers and industrial unit heaters must be larger than in
conventional systems due to the relatively low temperature of heating fluid.
The disadvantages may be circumvented by good design; where fuel costs are
high enough (as discussed in the examples, Section 4), a solar system will
prove cost effective. Solar systems designed for combined heating and
cooling will utilize the collector all year and thus are potentially more
cost effective.
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