The impact of thermal properties on performance of phase change based energy storage systems

Abstract

Phase change materials (PCMs) are used commonly for thermal energy storage and thermal management. Typically, a PCM utilizes its large latent heat to absorb and store energy from a source. The rate of energy stored (W) and energy storage density (J/m3) over a certain time period are both important performance parameters of a phase change based energy storage system. While significant experimental research has been carried out to improve thermal conductivity of PCMs, there is a lack of theoretical understanding of how thermal conductivity and other thermophysical properties affect these performance parameters. This paper presents a theoretical heat transfer model to predict the rate of energy storage and energy storage density as functions of PCM thermal properties. Using perturbation method based techniques, expressions for these parameters are derived for two geometries, first for a simplified assumption of constant temperature at the source-PCM interface, and then for a more realistic scenario of time-dependent interface temperature. Results indicate that while increasing thermal conductivity results in improvement in rate of energy stored, the energy storage density does not change for a Cartesian system and actually decreases for cylindrical system. This shows that using a high thermal conductivity PCM may not be ideal when energy must be stored compactly because while this increases the total energy absorbed, it also results in greater rate of melting, which reduces the energy storage density. Results also provide guidelines for material selection for phase change based energy storage systems. For example, a trade-off in the choice between materials of disparate thermal properties is identified in terms of whether the rate of energy stored or energy storage density is paramount. Differences in the performance of Cartesian and cylindrical systems is investigated. Theoretical results presented in this work highlight various performance trade-offs related to the thermal properties of the PCM and help understand the impact of thermal conductivity enhancement on phase change energy storage performance.