Environmental Engineering Reference
In-Depth Information
cover and selective coating. In concentrating collectors temperatures are generally
even higher.
In line with these requirements, mainly copper and aluminium are used to build
absorbers. As there may be a shortcoming of these materials, provided that the
market increase of solar thermal collectors is continuing, polymeric materials and
steel could become more important in the future. In the simplest case, this basic
material is painted black on the side receiving radiation (maximum absorber tem-
perature approximately 130 °C). For a large number of absorbers, this side is also
coated selectively (maximum absorber temperature approximately 200 °C).
The heat carrier flows through the channels inside the absorber. The energy
proportion of the solar radiation on the absorber converted into heat inside the
absorber is partly transported to the heat carrier (by heat transfer). The system of
pipes in the absorber can vary in terms of pipe material, pipe cross-section, length
and pipe allocation within the collector.
Cover. The transparent cover of collectors ought to be as transparent for solar
radiation as possible and retain the long-wave thermal reflection of the absorber.
At the same time it has to reduce convective thermal losses to the environment.
Suitable materials are glass sheets, synthetic plates or synthetic foils (e.g. made
of polyethylene or Teflon). The high level of material stress often leads to brittle
and tarnished synthetic materials. Furthermore, the outer area can also be
scratched very easily by atmospheric exposure. Thus transmission values of syn-
thetic covers are often not stable long-term. Therefore for most applications glass
is used. For solar collectors mainly safety glass is used that is characterised by a
high level of transparency and resistance to hail. Additionally, low iron contents
can reduce the absorption capacity in the short-wave spectrum. Thus it is avoided
that the glass sheet heats up. Convective thermal losses to the colder environment
are reduced. Often infrared-reflecting layers are vacuum coated on the bottom side
of the cover to reflect the long-wave heat radiation from the absorber to the cover
into the direction of the absorber. Thus losses can be reduced even further.
Collector box. The collector box holds the components required for radiation
transmission, absorption, heat conversion and insulation. It can be made of alu-
minium, galvanised steel plate, synthetic material or wood. It gives the collector
mechanical firmness and makes it environment-proof. However, a low level of
ventilation has to be ensured in order to reduce high or low pressure caused by
temperature fluctuations and remove possible humidity.
Independent of the material, designs can be differentiated as boxes for on-roof
installation on top of the roof tiles and boxes for in-roof integration for collectors
installed on pitched roofs. Boxes installed externally on the roof have a cover (e.g.
made of aluminium) at their back, whereas boxes integrated into the roof do not
need such a cover.
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