Page 137 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM DESIGN CONSIDERATIONS 113

SUN

Figure 3.58 Solar
9:00 AM 12:00 NOON 3:00 PM hour angle.

The average amount of solar energy striking the surface of the earth is established
by measuring the sun’s energy rays that impact perpendicular to a square-meter area,
referred to as the solar constant S. The amount of energy at the top of the earth’s
2
atmosphere, measured by satellite instrumentation, is 1366 W/m . Owing to the scat-
tering and reflection of solar rays on entering the atmosphere, solar energy looses
30 percent of its power, and as a result, on a clear, sunny day, the energy received on
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the earth’s surface is reduced to about 1000 W/m . The net solar energy received on the
surface of the earth is also reduced by cloudy conditions and is also subject to the
incoming angle of radiation.
Calculation of solar insolation is as follows:

I = S × cos Z

where S = 1000 W/m 2
Z = (1/cos) × (sin L × sin i + cos L × cos I × cos H)
L = latitude
H = hour angle = 15 degrees × (time − 12)

Time in the preceding formula is the hour of the day from midnight.

SUN
ZENITH

Z

SURFACE OF THE EARTH
Figure 3.59 Solar zenith angle.