Solar energy technology Page 20

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Vacuum flat plate collectors

These kinds of flat plate collectors have a vacuum inside. In the pressure reduced vacuum there are fewer air molecules that could transport heat to the outside by convection or conduction which would then be lost. Thus the vacuum has a heat damming function and so these collectors need less rear side insulation. However, one frequent problem with almost all vacuum flat plate collector designs was that they were easily leaking and the air that had penetrated into the collector had to be removed with special vacuum pumps again. For this reason, vacuum flat plate collectors have almost completely disappeared from the market today. They have been replaced by flat plate collectors with noble gas fillings.

Vacuum tube collectors

Vacuum tube collectors function in a similar way as vacuum flat plate collectors with the main difference that selectively coated vacuum tubes are used as absorbers. Furthermore, inside the absorber is a so called "heat pipe". When the vacuum tube (the absorber) is heated up by the radiation energy of the sun a so called "cooling liquid" that is inside the heat pipe of the vacuum tube evaporates, moves up and transfers the heat over a heat exchanger (condenser) to a heat carrying liquid. Afterwards, the cooling liquid condenses and moves down again in the heat pipe in order to absorb new solar heat. The heat that was transferred to the heat carrying liquid is then directly supplied to the consumer just as in other collector types too. The cooling liquids used today do not contain any "FCKW" anymore for environmental reasons and instead mostly now consist of a water-glycol mixture. Diagram 18 and 19 show vacuum tube collectors with a so called "dry integration", i.e. the tubes that are part of the system can be individually replaced without having to empty the whole system first. Furthermore, the absorber surfaces can also be individually adjusted.

Diagram 18: Function of a vacuum tube collector with "dry integration"

English translation of technical terms and sentences in diagram 18:

= Heat exchanger pipe
Wärmeträgerflüssigkeit = Heat carrying liquid
Wärmedämmung = Heat insulation
Kondensator = Condenser
Kollektorgehäuse = Collector box/case
Wärmeeinstrahlung = Heat/solar radiation
Senkfühler = Dip-in sensing element
Glasröhre = Glass tube
Füllung Wärmerohr = Filling of the heat pipe
Absorber selektiv beschichtet = Absorber with selective coating
Wärmerohr -Querschnitt = Heat pipe cross-section


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