Solar collector orientation

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Design: Solar Heating of Buildings and Domestic Hot Water

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:

For all year domestic hot water (DHW) heating use a tilt angle

equal to the latitude.

For all year DHW heating and winter space heating, use a tilt angle

equal to the latitude plus 10-15 degrees.

For all year DHW heating, winter space heating, and summer cooling

use same as (a).

For winter only space heating use a tilt angle equal to the

latitude plus 10-15 degrees.

For summer space cooling use a tilt angle equal to the latitude

minus 10-15 degrees.

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.