Experimental and numerical study of latent heat thermal energy storage with high porosity metal matrix under intermittent heat loads

The potential mismatch between the ever-increasing demand for usable energy and characteristically intermittent supply from the renewable energy sources can be effectively reduced by latent heat thermal energy storage system (LHTES) using phase change material (PCM). The low thermal conductivity of PCM leads to thermal stratification in LHTES. In the present study, a lab-scale shell and tube LHTES with the metal matrix as thermal conductivity enhancer is designed for medium temperature solar applications (similar to 200 degrees C) and the thermal performance of LHTES is evaluated for different operating conditions. A commercial-grade organic PCM and metal matrix are placed in the annulus, while a commercially available thermic oil used as a heat transfer fluid (HTF), flows through the finned internal tube. A simplified dynamic numerical model comprising of two energy equations is developed considering the absence of local thermal equilibrium between PCM and HTF. A newly developed correlation for interfacial heat transfer coefficient and the effective thermal conductivity of PCM and metal matrix are introduced in the model, which is validated against the experiment. The model is extended to study the effect of porosity of metal matrix on the dynamic performance of LHTES under several cycles of charging and discharging operations. It is found that at a porosity of 0.85, the fluctuation in HTF outlet temperature is less with an improvement in cumulative energy fraction. A significant saving of computational time can be achieved for large scale simulations of LHTES with metal matrix using the simplified dynamic model with reasonable accuracy.