Effect of a low cost parabolic reflector on the charging efficiency of an evacuated tube collector/storage system with a PCM

Abstract

This paper presents a novel concept of using a phase change material (PCM) to store thermal energy directly within a heat pipe evacuated tube collector equipped with a compound parabolic concentrator (CPC). The excellent insulating properties of evacuated tubes and the use of latent heat are significant advantages of a PCM integrated evacuated tube collector/storage (ETC/S) over traditional solar water heaters. However, during the charge cycle of the ETC/S, direct solar radiation only reaches the exposed area of the evacuated tubes, which results in uneven heating of the PCM due to a lower energy input in the shaded area. This can be prevented by using a CPC to concentrate the solar radiation on the shaded area of the evacuated tubes, thereby raising the temperature of the PCM and quantity of stored heat. Therefore, a polished, thin aluminum sheet was used as low cost CPC with a concentration ratio of 1.2x. Technical grade paraffin with an onset melting temperature of 51.24°C was used as the PCM. The results from this study showed that the application of the CPC caused the temperature of paraffin on the shaded side of the evacuated tubes to increase more rapidly, especially during and after melting of the paraffin. Furthermore, the use of a CPC in a PCM integrated ETC/S improved the average gross charging efficiency from 31% to 36% and the maximum charging efficiency from 40% to 49%.