Page 19 - Applied Photovoltaics

1.4 SOLAR RADIATION
Although radiation from the sun’s surface is reasonably constant (Gueymard, 2004;
Willson & Hudson, 1988), by the time it reaches the earth’s surface it is highly
variable owing to absorption and scattering in the earth’s atmosphere.

When skies are clear, the maximum radiation strikes the earth’s surface when the sun
is directly overhead, and sunlight has the shortest pathlength through the atmosphere.
This pathlength can be approximated by 1/cosij where ij is the angle between the sun
and the point directly overhead, as shown in Fig. 1.4. This pathlength is usually
referred to as the Air Mass (AM) through which solar radiation must pass to reach the
earth’s surface. Therefore
AM = 1 cos ij (1.3)

This is based on the assumption of a homogeneous, non-refractive atmosphere, which
introduces an error of approximately 10% close to the horizon. Iqbal (1983) gives
more accurate formulae that take account of the curved path of light through
atmosphere where density varies with depth.

Figure 1.4. The amount of atmosphere (air mass) through which radiation from
the sun must pass to reach the earth’s surface depends on the sun’s position.

When ij = 0, the Air Mass equals 1 or ‘AM1’ radiation is being received; when ij =
60°, the Air Mass equals 2 or ‘AM2’ conditions prevail. AM1.5 (equivalent to a sun
angle of 48.2° from overhead) has become the standard for photovoltaic work.

The Air Mass (AM) can be estimated at any location using the following formula:

AM = 1 + ( ) hs 2 (1.4)

where s is the length of the shadow cast by a vertical post of height h, as shown in
Fig. 1.5.

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