Thermal Performance Measurements on CPC Collectors
S. Fischer* and H. Mdller-Steinhagen
University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW), Pfaffenwaldring 6,
70550 Stuttgart, Germany
* Corresponding Author, fischer@itw. uni-stuttgart. de
This paper presents the results of a CPC collector test according to EN 12975-2:2006  using both accepted test procedures: the test method under steady state conditions and the quasi-dynamic test procedure. It is shown that only the test under quasi-dynamic conditions is suited to determine collector parameters that characterise the thermal performance of concentrating collectors such as CPC collectors to a desirable extent.
Keywords: Collector test, CPC collector, concentrating collectors, thermal performance
CPC collectors (see figure 1) use a Compound Parabolic Concentrator as reflector in order to concentrate the solar irradiance on the absorber. Due to this concentration CPC collectors have, in comparison to the aperture area, a smaller absorber area which results in smaller heat losses. With reference to the operating temperature, CPC collectors therefore present a logical link between flat plate collectors and evacuated tube collectors.
CPC collectors available on the market usually have a concentration ratio in the range of C = 1 - 2. These concentration ratios, however, result in a smaller conversion of the diffuse irradiance compared to flat plate and evacuated tube collectors. During the determination of the thermal performance of CPC collectors this fact must be taken into account.
The European standard EN 12975 /1/ allows test methods to determine the thermal performance of solar thermal collectors with different test methods. At present the test method under steady state conditions as well as the method under quasi-dynamic conditions are used to determine the thermal performance of CPC collectors.
The present paper compares the results gained with the two different test methods. The comparison will show that the method under steady state conditions is not suitable to determine the thermal performance of CPC collectors correctly.