Enhancing light transmission and thermal conduction for accelerating direct photo-thermal conversion and storage

Direct absorption/storage solar collectors (DASSC) using composite phase change materials (CPCM) have attracted great attention in the solar energy field for the advantages of less heat exchange processes and high heat storage density. However, the low thermal conductivity of the CPCM has greatly restricted the thermal performance of the DASSC. This study aims to improve the photo-thermal conversion efficiency (PTCE) of DASSC using light transmission and thermal conduction enhancing methods within a new developed CPCM. Firstly, paraffin/expanded graphite CPCM is developed for direct solar absorption, conversion and storage. Secondly, both light transmission and thermal conduction enhancing methods have been investigated to improve the charging rate and PTCE of the solar collector. The paraffin/expanded graphite CPCM exhibits high absorbance in the visible light region with higher radiation energy, which is 3.03 times that of pure paraffin. At low irradiation intensity (400 W/m²), thermal conduction enhancing method performs better than light transmission enhancing method, in which the charging rate are increased by 15.64 % and 13.24 % compared to the non-enhanced collector, respectively. However, with irradiation intensity increase, such as 1000 W/m², the light transmission enhancing method has a slightly better effect than the thermal conduction enhancing method, which increase the PTCE by 8.29 % and 8.25 %, respectively. This research introduces a new view to think about the method for improving the efficiency of DASSC.