Renewable Energy - Energy and the Environment - page 156

2

1

0 0

0.4 0.8 1.2 1.6 2.0 2.4 2.8

Wavelength ( m)

Figure 7.5 The spectral distribution of the solar beam irradiance as a function of wavelength. The dashed
line is extraterrestrial irradiance, and the solid line is clear-sky ground-level irradiance.

The portion of the incoming beam irradiance that is absorbed or scattered by the atmosphere
depends upon the wavelength of the incoming light. Scattering predominates for short wavelengths
(blue), and molecular absorption predominates for long wavelengths (red). The net beam irradiance
at ground level—that is, what remains after scattering and absorption—is shown as a solid line
in Figure 7.5. The sharp dips in spectral irradiance in the red and infrared regions are caused by
molecular absorption by carbon dioxide, water vapor, and ozone (which are greenhouse gases),
while the ultraviolet absorption is caused by diatomic oxygen and ozone. Under the best of con-
ditions, for a clear sky and the sun overhead, about 80% of the extraterrestrial beam irradiance
reaches the earth's surface.
The distinction between beam and diffuse radiation is important to the design and functioning
of solar energy collection systems. If the collector is a flat surface exposed to the sky, it will collect
both beam and diffuse radiation. But if an optical focusing system is used to intensify the solar
radiation impinging on the collector, only beam radiation is collected, which is a lesser amount
than the total of beam and diffuse. The proportions of beam and diffuse radiation depend upon
the amount of cloud cover, with a clear sky providing mostly beam radiation and a heavy cloud
cover resulting in entirely diffuse radiation. 15 Summed over daytime periods, this relationship is
exhibited in Figure 7.6. The abscissa is the ratio of daily total solar radiation on a horizontal flat
plate to the extraterrestrial value and is called the clearness index, while the ordinate is the ratio of
the daily total diffuse radiation to daily total radiation. As the sky becomes less clear, Figure 7.6
shows that both the beam and diffuse radiation decrease, but the former decreases proportionately
faster. Focusing systems collect little energy on cloudy days.
The amount of solar energy falling upon a solar collector depends upon its orientation with
respect to the sun's direction, as viewed from the earth's surface. The solar direction varies with
the local hour of the day and the day of the year. Fixed collectors remain in the same position
year-round. Common types of fixed collectors are horizontal (e.g., solar pond), tilted at an angle

15 If one can't see the sun through the clouds and no shadows form, the light is all diffuse.