SOLAR THERMAL
ENERGY
Introduction
Although most research into the use of solar energy in recent years has been on photovoltaic
technology, where sunlight is converted directly into electricity, there are many applications of
solar thermal energy such as heating, drying and water distillation.
Many solar thermal technologies have existed for centuries and are well understood. They have
established manufacturing bases in many sun-rich countries. Unlike photovoltaic technologies
manufacturing can be done on a small scale without using expensive equipment. More
sophisticated solar thermal technologies do exist that generate electricity (often on a large
scale) but these are not covered in this technical brief.
Solar technologies that rely entirely on energy absorb from the sun and have no moving
components, are referred to as passive solar technologies where as active solar technologies
may have some additional input such as a pump to drive the system.
The nature and availability of solar radiation
Solar irradiation, or insolation is the “rate of delivery of direct solar radiation per unit of
horizontal surface”, measured in W/m2. (Merriam-webster.com)
The earth revolves around the sun with its axis tilted
at an angle of 23.5 degrees. It is this tilt that gives
rise to the seasons. The strength of sun is dependent
upon the angle at which it strikes the earth’s surface,
and so, as this angle changes during the year, so the
solar insolation changes. Thus, in northern countries,
in the depths of winter, where the sun is low in the
sky to the south, the radiation strikes the earth’s
surface obliquely and solar energy is low.
23.5° max
Winter
Summer
The two phenomena described above provide an
Summer
explanation for the variations of solar irradiation with
season and lattitude.
Figure 1: The angle of the earth to
the sun changes throughout the year.
Illustration: Practical Action / Neil Noble
The total solar irradiation received in a day can vary
from 0.5 kWh/m2/day in the UK winter to 5 kWh/m2 in
the UK summer and can be as high as 7 kWh/m2/day in desert regions of the world, such as
regions of Nigeria (Solar Water Heating in Nigeria, 2006) and the Sahara in Algeria. (Survey of
Energy Resources, 2010) Many tropical regions do not have large seasonal variations and
receive an average 6 kWh/m2/day throughout the year.
The diagram below shows the approximate percentages of direct and diffuse solar insolation
that reaches the surface of the earth. As the direct insolation forms a larger proportion of the
total received, it follows that varying factors such as the weather, i.e. cloud cover, and the time
of day will greatly affect the amount of solar insolation reaching the surface of the earth
(Powerfromthesun.net). It is interesting to note that whilst both direct and diffuse radiation is
useful, diffuse radiation cannot be concentrated.
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