Volumetric and gravimetric energy densities of encapsulated high temperature phase change materials: Materials selection criteria

Abstract

The present work represents an attempt to understand some of the Thermodynamics associated with energy storage in confined phase change materials. The estimation of the phase change properties is performed through thermodynamic paths that connect two points along the liquid–solid saturation curve. The analysis leads to some conceptual differences between the latent heat estimated by other authors and the latent heat obtained in this work. Additionally, the elastic properties of core–shell systems are coupled to the proposed equations at saturation. A set of general equations for the volumetric and gravimetric energy densities in compressible media, is introduced. The proposed equations for energy densities incorporate the isothermal compressibility of liquid and solid phases. The equations introduced, provide a more realistic estimation of energy storage capacity in confined systems than previous models, where the compressibility is not considered. Consequently, it is possible to introduce a high compressible limit, which is shown to represent an ideal behavior, where materials have a maximum energy storage capacity. Thermal performance of confined PCMs can be improved by assessing the energy storage capacity of materials with finite compressibility in comparison with their corresponding high compressible limit. The energy storage efficiency of a particular core–shell configuration, is defined as the ratio between the energy density and its high compressible limit. The key thermodynamic and elastic parameters that contribute to the behavior of several core–shell combinations in relation to their proximity to the high compressible limit, are identified. Several configurations of different high temperature salts and shell materials, are analyzed through a balance between energy storage efficiency and crude energy density values. Experimental results from other authors in copper-alumina systems with a sacrificial layer, are used to validate the results of this work. Finally, the results of the proposed model, are used to outline limitations and advantages of several core–shell configurations, in relation to their energy storage performance. The model represents an attempt to provide some guidance in PCM and shell’s material selection for high temperature applications.