Collection characteristics of composite solar collector with volume heat trap and surface heat trap.

Abstract

A composite solar collector, in which the working fluid layer was divided into two layers by an opaque trapping surface, was considered. In the composite collector, thermal collection in the upper fluid layer is by a volume heat trap and that in lower fluid layer is by a surface heat trap. The collection characteristics of the collector were examined theoretically and experimentally.In the upper fluid layer of the composite collector natural convection was more suppressed and the temperature gradient was larger than in the volume heat trap collector. Also, in outdoor performance the mean temperature of the composite collector was higher than that of the volume heat trap collector. As solar radiation decreased, however, the difference between the two collectors became small because of thermal accumulation in the insulating layer.Under the present experimental conditions, the collection efficiency was high when the trapping surface was set near the middle of the fluid layer. As the temperature level became high or as the convective heat transfer coefficient became large, the optimum position of the trapping surface shifted to a deeper position. It was found that the optimum position of the trapping surface depended on the thermal conditions.