Theoretical analysis of phase change heat transfer and energy storage in a spherical phase change material with encapsulation

Abstract

Encapsulation of a phase change material (PCM) is a commonly used technique for providing mechanical and chemical stability, and enabling the manufacturing of PCM composites. However, thermal impedance of the encapsulating layer may adversely affect phase change heat transfer in the PCM. Experimental measurement of heat transfer and phase change in an encapsulated PCM is difficult, particularly for the case of micro/nano-encapsulation. As a result, development of robust theoretical phase change heat transfer models in presence of encapsulation is critical. This paper presents theoretical analysis of the problem of phase change heat transfer in a spherical PCM with an encapsulant layer. Temperature distribution in the newly formed phase and the encapsulant layer is determined by solving a spherical two-layer thermal conduction problem in non-dimensional form. The rate of phase change propagation is then determined from the temperature distribution. The effect of thermal contact resistance at the PCM-encapsulant interface is accounted for. Results are shown to be consistent with past work for the special case of no encapsulation. Results are also shown to agree well with numerical simulations. The use of only a few eigenvalues is shown to result in good accuracy. The effect of encapsulant thickness, thermal properties and PCM-encapsulant thermal contact resistance as well as external boundary condition on phase change propagation is analyzed. The non-dimensional model is used to address practical problems in phase change energy storage with typical materials and conditions. This work contributes an important theoretical analysis tool for a problem of much practical importance. Results from this work may help optimize and improve the performance of encapsulated PCMs for energy storage and thermal management.