A cheap air solar collector for drying systems / Ein billiger Warmluft Solarkollektor

Posté le 12 septembre 2009 @ 6:20 par Manfred

I built and tested a new type of solar collector to produce warm air (not water).

It is called transpired solar collector. The principle is to use a sheet of densely perforated black material as an absorber.

The heat transfer takes place as the air is sucked through the absorber using a fan, which ensures a good conduction / convection, theoretically better than in the conventional flat thermal solar collectors. The lower is the air flow, the higher is the temperature rise, but the lower is the efficiency (there are more heat losses).

This type of collector is already commercially available as sheets of trapezoidal corrugated metal, called UTC (unglazed transpired collectors).

However this solution has two drawbacks : the collectors must be positioned vertically in order to avoid accumulation of dust and infiltration of rain, and the metal is an expensive material. Those two drawbacks make those collectors unsuitable for drying systems in developing countries.

Reading through the scientific literature on the subject, I found out it was theoretically possible to use the same principle with a fabric instead of a metal sheet. Only could I not find any example of such a collector been actually built. So I decided to give it a try.

To solve the problem of dust and rain, I came with the (unoriginal) idea of adding a transparent cover on top of the absorber. However I did not use glass, but a simple transparent plastic cover instead. Its role is not so much to create a greenhouse effect as to protect from rain, dust, but also wind (which obviously increases the heat losses through forced convection).

I built one such collector, made of black polypropylene fabric for the absorber (around 2€/m2), a transparent UV-stabilized polyethylene sheet for the cover (same price) and a wooden frame (actually much more expensive due to the huge chipboard plate at the back).

I ran tests during three days (13th to 15th July), with and without the transparent cover, in order to assess its influence on the efficiency of the system, and with two different air flows. The fluctuations of the radiation being much faster than the fluctuations of temperature in the collector (due to its thermal inertia), an averaging of the the measured values was necessary.

The pressure drops through the fabric was measured separately on a bench as well as during the tests.

These series of tests were of course not enough to draw a proper efficiency curve, but they allowed me to conclude that :

  • the system works
  • under the test conditions, an average efficiency of 50% was found
  • the pressure drop through the fabric was very low, so a powerful fan is not required
  • after building one such collector, I consider it is possible to build it for much cheaper simply by reducing the wooden casing to an inner frame surrounded with fabric.
  • the estimated cost of such a collector is in the range of 0,03 € / Wp (collector only, not including the ducting or the fan).

A slideshow presenting the building and the testing of the solar collector (in German) / eine Diavorfürung des Aufbaus und der Probe vom Solarkollektor

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