Energy from the sun reaches earth as direct,
reflected and diffuse radiation.
Direct radiation is highest on a surface
perpendicular to the sun’s rays (angle of
incidence equal to 0 degrees) and provides
the most usable heat.
Diffuse radiation is energy from the sun
that is scattered within the atmosphere by
clouds, dust or pollution and becomes
non-directional. On a cloudy day, 100 per
cent of the energy may be diffuse radiation;
on a sunny day, less than 20 per cent may
be diffuse.
The amount of the sun’s energy reaching
the surface of the earth also depends on
cloud cover, air pollution, location and the
time of year. Figure 1 shows the solar
energy available in five Canadian cities at
different times of the year.
The amount of solar energy reaching a
tilted collector significantly changes the
result. Figure 2 shows the amount of solar
energy received by a horizontal collector,
such as window, for a passive solar design.
Note that even Yellowknife receives a
significant amount during part of the
heating season.