Mathematical modeling for transient thermal performance analysis of phase change material-based heat exchanger

Abstract

Thermal energy storage (TES) provides a unique way for the thermal solutions to short-duration thermal load systems. TES system utilizes phase change material (PCM), wherein energy is stored in latent heat during the lean time and released during peak load. Conventionally, in a shell-and-tube PCM heat exchanger, PCM is stored in the shell and heat transfer fluid (HTF) flows in the tube. Numerical modeling based on computational fluid dynamics (CFD) tools like FLUENT and COMSOL are computationally intensive. Most numerical research is based on the enthalpy porosity model for solidification/melting problems. Enthalpy-porosity model requires a mushy zone parameter constant which significantly affects analysis results. The constant has no theoretical basis and is chosen based on experience. Analytical models in literature cannot give reasonably good results, which can be directly utilized for engineering purposes. This paper establishes a mathematical model based on the effectiveness-NTU (e-NTU) technique for transient analysis/rating of such PCM heat exchangers. Transient behavior of PCM heat exchanger due to phase transforming static PCM are modeled using an innovative concept of PCM zones, effective zonal length of heat exchange, and PCM zonal resistances. These functions are dependent on one dimension – total PCM liquid fraction ‘xf’ and a model is established for evaluating thermal performance analysis and optimization of PCM heat exchangers. The model is validated against published experimental results for cumulative energy stored in PCM of a PCM heat exchanger with air as HTF. The root-mean-square error for evaluated & predicted model results is less than 2.5%. The model proposed is uncomplicated and can be further utilized to optimize PCM heat exchangers. Transient analysis of inlet–outlet HTF temperature difference, PCM heat exchanger effectiveness, total PCM liquid fraction, PCM phase fractions, and zonal heat exchange length ratios of PCM heat exchanger for the respective experimental cases is further investigated.