Development of a stable inorganic phase change material for thermal energy storage in buildings

Abstract

Building energy consumption is influenced evidently by solar radiation. To achieve a stable indoor temperature by minimizing the heat fluctuations resulted from solar radiation, latent heat thermal energy storage systems with phase change materials (PCMs) in building envelope have been studied. Though inorganic PCMs have promising potential applications among all kinds of PCMs, the supercooling and segregation are the major issues which have restricted their practical applications. The purpose of this study is to lessen the supercooling degree of an industrial grade PCM (CaCl2·6H2O) by using a nucleating agent - flake graphite. A super absorbent polymer (SAP) was also used as a thickener to prevent segregation of CaCl2·6H2O during phase transition. The combined method of thermogravimetric analysis/differential thermal analysis (TGA-DTA) was firstly proposed to evaluate the segregation of inorganic PCM innovatively. The results showed that using 0.5 wt% flake graphite not only eliminated the supercooling of CaCl2·6H2O, but also improved its thermal conductivity for better thermal performance. The results of Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) demonstrated that the flake graphite did play an important role on the crystallization of CaCl2·6H2O. Numerical simulation demonstrated that using modified CaCl2·6H2O within walls of buildings in Hong Kong and Changsha are economically feasible with the payback periods of 18.3 years and 8.4 years respectively.