Effect of graphite addition on the charging/discharging characteristics of an inorganic phase change material

The performance of latent heat thermal storage units (LHTS) largely depends on the thermophysical properties of the phase change materials (PCMs) employed. Several suggestions have been made in the past by numerous researchers on the enhancement of thermal conductivity of PCMs. This paper aims to study the effect of mixing graphite powder to a commercial inorganic type PCM (HS89) suited for applications close to 90°C. The melting (charging) and freezing (discharging) of the inorganic PCM have been studied with two different graphite fractions viz.10 and 20 wt% respectively and compared with that of the pure PCM case (i.e. with no graphite added to the PCM). The results show that increasing the graphite fraction to above 10 wt% delays the charging process during the initial stages whereas accelerates during the later stages of charging. The time for complete charging and discharging of the PCM reduced by 56.5% and 39% respectively with 20 wt% graphite and 47.8% and 5.6% with 10 wt% graphite. The effect of graphite addition to the inorganic PCM has been found to be more significant during charging compared to discharging.