Solar energy technology Page 19

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Flat plate collectors

The function of flat plate collectors is relatively easy. The sunbeams (direct and diffuse) pass a transparent collector cover and are then absorbed by an energy absorber. Such absorbers generally consist of specially coated tubes. At the surface of the absorber the radiation of the sun is converted into heat and is transmitted by the absorber to a heat carrier liquid which is flowing through the absorber pipes. The heat insulation at the bottom and at the sides of the these collectors as well as the transparent cover on the collector front prevent heat losses. Nevertheless, some heat losses always occur. These losses are primarily caused by:

Today special, highly transparent glass, partly pressurised (i.e. security glass) is used as front cover material. The absorber coatings differ in their effectiveness to convert the radiation into heat - the so called "selectivity". Varnish coatings have average absorption rates of 90-95% and emission rates of 80-90%. Selective coatings have average absorption rates of 90-94% and very small emission rates of only 6-15%. Highly effective absorber coatings increase the effectiveness of the collectors, particularly during periods with little sunshine. So, they are very important for the total performance of the whole system. As heat insulation materials on the rear side of the collector polyurethane hard foam, mineral wool or air vacuums are being used. Diagrams 16 and 17 show the function of a flat plate collector.

Diagram 16: Sectional view of a flat plate collector Diagram 17: Energy flow in a flat plate collector

English translation of technical terms and sentences in diagram 17:

Solare Einstrahlung (direkt, diffus)
= Solar radiation (direct, diffuse)
Konvektive Verluste = Convection losses
Reflexionsverluste (Abdeckung, Absorber) = Reflection losses
Solarer Gewinn = Solar energy gain
Eigenabstrahlung (Abdeckung, Absorber) = Own emission (cover, absorber)
Leitungsverluste = Conduction losses


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