Experimental study of the heat transfer performance of PCMs within metal finned containers

Abstract

Latent heat thermal energy storages (LHTES) are particularly attractive methods owing to these factors: meet the time shift between energy supply and demand; provide a high energy storage capacity; store and release heat at a relatively constant temperature; provide constant comfort thermal environment without temperature swings when it is applied for space heating or cooling. Nevertheless, the efficiency of using the LHTES techniques is heavily affected by the low thermal conductivities of phase change materials (PCMs). This characteristic of PCMs prolongs the charging and discharging cycle and barriers the widely practical application of LHTES. Hence, researchers generated a lot of related technologies, such as metal fines, carbon fibres, metal honeycomb structure, etc, to overcome this issue and aimed to achieve reasonable thermal conductivities. The objective of this paper is to study the heat performance of two kinds of PCMs within three different types of metal finned structures (straight fins, honeycomb and square finned structure) at the volume ratios of 1.8, 2.7, and 3.6 %, respectively. Two organic PCMs, paraffin wax RT 25 (phase transform at 25 °C) and RT 42 (phase transform at 42 °C) are employed as the heat storage media. The characteristics of them with the thermal conductivity enhancers (TCEs) during the melting and solidification process were investigated experimentally. The results indicate that the heat transfer improvements during the melting process are more efficiency than the solidification process for all of the three structures and both PCMs. To be specific, for paraffin RT 25, the heat transfer efficiencies were increased by 25, 33, and 37 %, in the finned, honeycomb and square cell structured container during the melting process, and increased by 8, 12, and 17.1 %, respectively for the solidification processes. The similar effect happened for paraffin RT 42, the heat transfer efficiencies were increased by 28, 33, and 40 % during melting process, and increased by 17, 28, and 35 %, respectively during freezing process. The performance of the TCEs on the RT 42 is slightly better than that of RT 25, especially during the solidification process due to its higher heat transfer rate between the PCM and TECs induced by a relative higher melting temperature. Meanwhile, the efficiencies of the volume ratios of the TECs were examined. The results show that straight fins have the best efficiency compared to others.