Evaluation of thermophysical properties for thermal energy storage materials - determining factors, prospects and limitations

Abstract

Thermal Energy Storage (TES) is representing a promising technology for energy conservation and utilizing fluctuating renewable energy sources and waste heat. While Sensible Thermal Energy Storage (STES) systems make use of the enthalpy change due to heat capacity and temperature gradient, Latent Thermal Energy Storage (LTES) systems utilize phase change enthalpies of different phase transitions and Thermochemical Energy Storage (TCES) systems use the heat of chemical reactions or sorption of the storage material. LTES and TCES promise higher energy densities based on higher enthalpy changes during phase transitions or chemical reactions at different temperature levels compared to STES. Due to that, a detailed knowledge of the thermophysical properties is needed, in order to identify the actual energy density but also thermal transport and thermal expansion of the observed storage material. In this work, a literature review on already published TES materials and their thermophysical properties is done. Furthermore the state-of-the art in measurement methodologies for thermophysical properties is shown and applied to different Phase Change Material (PCM) and Thermochemical Material (TCM). Focusing on specific heat capacity, phase transition enthalpy, reaction enthalpy, thermal expansion, density, thermal diffusivity and thermal conductivity, different already standardized but also new measurement methodologies were conducted, compared and evaluated.