Page 35 - Applied Photovoltaics

where Ȝ is the wavelength of light. Changes in spectral content can have a
significant effect on the output of a solar cell. However, this effect is often
neglected, since silicon solar cells absorb almost no light of wavelength
greater than 1.1 ȝm and module reflections increase at oblique angles of
incident light, corresponding to longer wavelengths and increasing air mass.

1.8.4 Radiation on tilted surfaces
Since photovoltaic modules are commonly mounted at a fixed tilt, it is often
necessary to estimate insolation on such tilted surfaces from that on the horizontal.
This requires separate direct and diffuse components, as discussed above. Various
models are available with a range of assumptions about the sky distribution of diffuse
radiation (Duffie & Beckman, 1991; NASA, 2004a). Simple models are preferred if
the input data is itself derived by modelling, such as, for example, from sunshine
hours data (Perez et al., 2001). Here, we consider only surfaces tilted towards the
equator, although models are presented elsewhere for arbitrary orientations (Lorenzo,
1989).

Telecom method
Where insolation data is available in the form of direct and diffuse components, the
following approach can be used to determine the corresponding insolation incident on
a solar panel tilted at an angle ȕ to the horizontal (after Mack, 1979).
First, we can assume the diffuse component D is independent of the tilt angle (which
is a reasonably close approximation provided tilt angles are not much more than
about 45°). Lorenzo (2003) discusses several more comprehensive models
considering, for example, the higher intensities close to the solar disk and near the
horizon with clear skies.

Secondly, the direct component on the horizontal surface S is to be converted into the
direct component S ȕ incident on a plane tilted at angle ȕ to the horizontal as illustrated
in Fig. 1.15.

Figure 1.15. Light incident on a surface tilted to the horizontal (after Mack, 1979).

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