To narrow these general ideas down to a particular PV system we need
more detailed local information. Fortunately, the great surge of interest in
solar energy in recent years has spawned data on average sunlight conditions
for many cities and locations around the world (one valuable source
of information is provided by NASA. 2 ) The data is often presented in the
form of 12 monthly mean values of global (direct and diffuse) daily radiation
on a horizontal surface, expressed in kWh/m 2 . radiation is not
included since it does not affect horizontal surfaces and anyway is highly
site - dependent. Sometimes the proportions of direct and diffuse light are
found by practical measurements with specialised instruments; sometimes
they are inferred from the global figure and a clearness index summarising
the amount of light scattering caused by clouds and particles in the local
climate. Figure 3.12 (a) shows a typical distribution for a West European
city such as London or Amsterdam with a temperate climate giving plenty
of ‘ sunshine and showers ’ in summer and cloudy skies in winter. The height
of each bar represents global radiation, composed of direct (yellow) and
diffuse (orange) components. The daily average over the whole year is
about 2.8 kWh/m 2 , giving an annual total of about 1050 kWh/m 2 . Part (b)
of the figure is for the Sahara Desert. Here the extremely sunny, hot and
reliable climate produces a daily average of about 6 kWh/m 2 and an annual
total of about 2200 kWh/m 2 . Most of the radiation is direct.