Optimum performance of thermal trap collectors

The “thermal trap effect” in semitransparent material and the trapping system in the conventional flat-plate collectors with two, three or four glass or plastic covers with air-gaps in between are analysed under a common heading of “thermal trap collectors”. In general, a thermal trap collector consists of one or many slabs of semitransparent material of finite thickness with air-gaps in between, and an ideal withdrawal mechanism at the base of the trapping system to withdraw all available energy. This approach makes a comparative study of the two types of collectors possible, and provides data to design the appropriate withdrawal mechanism and operating conditions. A steady state analysis which neglects internal reflections and body radiation shows the existance of an optimum performance in single-layer thermal trap collectors and its dependence on thickness. A model which includes internal reflections is then analysed and the existance of the optimum performance and its dependence on thickness is demonstrated by taking the example of a single slab of methylmethacrylate plastic. The model is extended to multilayer thermal trap collectors and two examples are considered; a multilayer methylmethacrylate thermal trap collector and a multilayer “glass” thermal trap collector. The results show that the two-layer methyl methacrylate thermal trap collector has, in general, a better performance than the corresponding single or three and four-layer systems. But at high withdrawal efficiencies of about 60 per cent, the single layer methyl methacrylate shows its uniqueness and becomes competitive with the two-layer system. But the three and four-layer “glass” thermal trap collectors perform better than the corresponding single and two-layer ones, with the three-layer system having an overall better performance. These results show that the number of slabs in addition to thickness are important parameters in the study of the performance of thermal trap collectors.