Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 122

4 Solar Thermal Heat Utilisation

4.1 Principles

Part of the solar radiation energy can be converted into heat by using absorbers
(e.g. solar collectors). The absorbers together with the other necessary compo-
nents are the solar system. Solar systems are installations converting solar radia-
tion into heat in order to heat swimming pools, produce domestic hot water, cover
the demand for space heating or supply other heat consumers. In the following,
the physical principles of energy conversion for these forms of solar thermal heat
utilisation are described (see /4-1/, /4-2/, /4-3/).

4.1.1 Absorption, emission and transmission

The basic principle of solar thermal utilisation is the conversion of short-wave
solar radiation into heat. This energy conversion process can also be described as
photo-thermal conversion. If radiation incidences on material a certain part of the
radiation is absorbed. A body's capacity to absorb radiation is called absorbing
capacity or absorption α , where α reflects the share of absorbed radiation as part
of the entire radiation on matter. An ideal black body absorbs radiation at every
wavelength and therefore has an absorption coefficient equal to one.
The emission ε represents the power radiated by a body. The relationship be-
tween absorption α and emission ε is defined by Kirchhoff's law . For all bodies
the ratio of specific radiation and the absorption coefficient is constant at a given
temperature, and in terms of its amount, equal to the specific radiation of the black
body at this temperature. This ratio is exclusively a functionality of temperature
and wavelength. Matter with a high absorption capacity within a defined wave
range also has a high emission capacity within that same wave range.
In addition to absorption and emission, also reflection and transmission play a
role. The reflection coefficient ρ describes the ratio of the reflected to the incident
radiation. The transmission coefficient τ defines the ratio of the radiation transmit-
ted through a given material to the entire radiation incident, according to Equation
(4.1). Thus the sum of absorption, reflection and transmission is one.

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