Long term thermal stability of organic PCMs

Thermal energy storage using phase change materials (PCMs) have been a focal point in the efficient energy utilisation in buildings for over 30 years. The possible use of PCM in buildings is becoming more and more attractive due to the large energy storage density and nearly isothermal nature of the PCM storage when compared to sensible heat storage. Although there is large amount of information available on this topic, literatures show very little information about the long term thermal performance of phase change materials. In this research, changes in thermal characteristics of two commercial organic PCMs when exposed to a constant temperature above their melting point were examined. The thermal characteristic (i.e. melting range and latent heat of fusion) of Rubitherm 21 (RT21, a paraffin mixture) and propyl stearic and palmitate mixture with a melting point in the range of 18-25 °C were tested after an exposure to storage temperatures of 30 and 55 °C. The results obtained indicated that the paraffin based mixture such as RT21 experienced a significant irreversible physical change with time. The data collected and analysis indicated a shift in the peak melting point from 21 to 28 °C and increase in latent heat of fusion from 134 to 170 J/g over a period of 120 days when kept at a temperature of 55 °C. On the other hand, the mixed esters experienced almost no change in mass due to their lower vapour pressure.