Liquid-gas phase change material based thermal storage

Abstract

In this thesis, a thermal storage system based on liquid-gas phase change material with encapsulation is studied. Latent heat thermal storage system with solid-liquid phase change material has been studied, researched, and applied in variety of applications in last decades. However, liquid-gas phase change materials are not as being interested as solid-liquid phase change materials due to its large volume change during the phase change. In this study, water is used as liquid-gas phase change material and polydimethylsiloxane (PDMS) is used as encapsulation material to form the thermal energy storage system. To study the behaviors of liquid-gas change in this system, a method of small step increase in pressure is used to analytically simulate the system. Important parameters at each step are calculated. Simulations by using computer software are also used to help calculating the deformation and stress of the shell, and verifying them with calculated results to prove the formulas used in calculation are valid. In addition, a method of predicting energy storage is developed based on small step increase in pressure. Several results have been found in this study. As the filling of liquid PCM in the spherical cavity increases, the maximum energy stored in the system increases as expected; the mass based energy density decreases as the filling increases, while at the same time the volumetric energy density of the spherical cavity increases. The volumetric energy density of the spherical cavity will reach to its maximum in early stage when internal pressure is increased. The radius ratio of the shell and spherical cavity can affect mass based energy density, but the effect is very small. An optimal radius ratio of 3.7 is found to generate around two percent error in ultimate stress and the validity is verified with simulation. In addition, the rigidity of encapsulation material can affect both mass based energy density and volumetric energy density. As the Young’s Modulus of PDMS increases, the volumetric energy density of the spherical cavity increases.