Thermal Design of Compound Parabolic Concentrating Solar-Energy Collectors

A theoretical analysis of the heat exchanges in a Compound Parabolic Concentrator solar energy collector is presented. The absorber configuration considered is that of a tube (with or without a spectrally-selective surface) either directly exposed or enclosed within one or two glass envelopes. The annular cavity formed between the tube and the surrounding envelope can be either air-filled or evacuated. The optimal annular gap, which leads to the best overall collector efficiency, has been predicted for the nonevacuated arrangement. It was found to be approximately 5 mm for the considered geometry. This is about half that recommended by Rabl and Ratzel and gives a 3 percent better overall collector efficiency than obtained with their design. The evacuation of the annular cavity or the application of a selective surface, separately employed, are demonstrated to yield improvements of the same order. It was necessary, for the particular solar radiation data used, both to evacuate the cavity and apply a selective surface if receiver temperatures exceeding 140°C are required. The comparative performances of different CPC designs have also been considered. The theoretical predictions were compared with experimental results and adequate corroboration was obtained.