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urban and rural environments is shown in Fig. 5.18. Further data on the effects of dust
may be found in Hammond et al. (1997).

Figure 5.18. Short-term performance degradation in urban and rural environments
(after Ross & Smokler, 1986).

5.8 THERMAL CONSIDERATIONS
For crystalline silicon in particular, it is desirable for modules to operate at as low a
temperature as possible, since:
x cell output is increased at lower temperatures (see Section 3.3)
x thermal cycles and stress are reduced
x degradation rates approximately double for each 10°C increase in
temperature.
To reduce module degradation rates, infrared radiation, the wavelengths of which are
too long to be absorbed by the cells, should ideally be rejected; however no cost-
effective method has yet been developed for this. The modules and the solar array
must therefore take full advantage of radiative, conductive and convective cooling
and absorb the minimum of unused radiation. Typically, about half the heat loss from
a module is by convection and half by radiation.
Different encapsulation types, giving vastly different thermal properties, have been
used by manufacturers to meet different market needs, as illustrated by some of the
different modules offered by a typical manufacturer:
x marine module
x injection moulded module
x mini module

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