Experimental study of storage capacity and discharging rate of latent heat thermal energy storage units

The key performance indicators of thermal energy storage (TES) units are the effective storage capacity and discharging rate. As it happened in building cooling applications, a latent heat thermal energy storage (LHTES) unit, which is a TES unit using phase-change-materials (PCM), when not properly designed, could have an effective storage capacity significantly lower than that of a stratified-water-storage (SWS) tank. This study experimentally demonstrates how the effective storage capacity and discharging rate of PCM-based TES units can be improved by enhancing heat transfer in PCM. To do this, four basic components of shell-and-tube LHTES units were filled with four different PCM composites, respectively, and tested under both laminar and turbulent flow conditions (Re: 500-14,500). The four selected PCM composites were composed of pentadecane and expanded graphite (EG), and achieved effective thermal conductivities (k(eff)) of 0.21 W/(m-K), 1 W/(mR), 8.6 W/(m.K) and 20 W/(m.K)), respectively. It was found that the test unit using pentadecane-EG composite with k(eff) = 8.6 W/(m.K) can have an effective storage capacity twice that of an ideal SWS tank operating under Re = 4300 for the radiant cooling application condition. It has long been known that enhancing heat transfer in PCM could improve the performance of PCM based TES units. However, this is the first experimental confirmation that a PCM based TES unit can achieve an effective storage capacity higher than that of an ideal SWS tank; and the discharging rate of a PCM based TES unit could be improved under a realistic, practical operating condition.