An investigation on packed-bed collectors

Abstract

This work is aimed at investigating the performance characteristics of packed-bed solar collectors in order to improve solar-energy collection with considerable reduction in cost. Solid and hollow spheres, metallic screens, and other irregular shapes are used as absorbing materials. Water is in direct contact with the absorbing materials, thus effecting heat-transfer by convection and radiation rather than by conduction. The materials absorb the transmitted solar radiation and emit long-wave radiation. Water, being a strong absorber of long-wave radiation especially in the the infra-red region of the solar spectrum, absorbs directly from the solar radiation and intercepts the emitted long-wave radiation from the collector materials. The result is a slightly higher water exit temperature than the bed temperature. The average collector efficiency is about 80 percent for most of the materials tested and the variation remains within ± 10 percent. Over-all efficiency ranging from 20 to 60 percent is obtained and this compares favorably with the performance of most ordinary flat-plate collectors, although temperatures obtained are not as high.