Centuries
ago, solar thermal energy was employed by ancient civilizations
in the utilization of solar energy for heating and drying purposes. In
recent times, a diverse array of thermal processes has seen the development of
solar energy applications for power generation, water heating, mechanical crop
drying, and water purification, among other uses. The most significant
applications, considering the range of working temperatures in solar thermal
processes, are as follows:
For
temperatures below 100°C: water heating for domestic and swimming pool
use, heating of buildings, and the implementation of evaporative systems such
as distillation and dryers.
For
temperatures below 150°C: air conditioning, cooling, and the heating of
water, oil, or air for industrial purposes.
For
temperatures ranging from 200 to 2000°C: the generation of electrical and
mechanical power.
For
temperatures below 5000°C: the utilization of solar furnaces for the
treatment of materials.
In cases
where a temperature exceeding 100°C is necessary for the processes, the solar
energy flux alone cannot sufficiently raise the working fluid temperature to
such levels. Instead, a concentration of the energy flux using mirrors or
lenses becomes imperative. The ratio of the energy flux received by the
energy absorber to that captured by the collector must then exceed one. Designs
often easily achieve concentration levels of hundreds of suns.
Classified
as low-, medium-, or high-temperature heat exchangers, solar collectors come in
three primary types: flat plate, evacuated tube, and concentrating. Despite
substantial geometric distinctions, their shared objective is to convert solar
radiation into heat to fulfill various energy requirements. The heat
generated by solar collectors can either directly meet energy demands or be
stored. To align energy demand and production, an assessment of the
collector's thermal performance is essential. The instantaneous useful energy
collected results from an energy balance applied to the solar collector.
Properly
evaluating the energy output of a solar collector necessitates consideration of
the physical properties of the materials involved. Solar radiation, predominantly
consisting of short wavelengths, penetrates a translucent cover and impacts
the energy receiver. Low-iron glass, chosen for its high transmissivity,
commonly serves as a glazing cover, significantly minimizing heat losses.
Achieving optical characteristics for the energy receiver that closely resemble
those of a blackbody, particularly high absorptivity, is crucial.
Properties
with high thermal conductivity can be enhanced by incorporating selective
coatings. This, combined with radiation absorption, results in an elevation
of the receiver's temperature; the shortwave radiation undergoes
transformation into long-wave radiation. The glazing material essentially turns
opaque under the new wavelength conditions, promoting the greenhouse effect. A
synergistic combination of the cover's high transmissivity to solar radiation
and the receiver's high absorptivity yields exceptional performance for a
well-constructed solar collector.
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