Numerical Validation of Effective Specific Heat Functions for Simulating Melting Dynamics in Latent Heat Thermal Energy Storage Modules

Thermal management systems (TMSs) are routinely expected to mitigate transient heat loads that often exceed nominal operating conditions. Thermal energy storage (TES) devices provide the ability to improve the performance of TMSs by allowing them to temporarily store heat until there is an opportunity to reject it. To design and control TMSs with integrated TES, system level modeling and simulation tools are needed. However, latent heat TES modules, such as those that utilize phase change materials (PCMs), are difficult to simulate using variable time-step ordinary differential equation solvers due to their melting characteristics. In this paper, we validate a novel approximation of the effective specific heat for a latent heat TES module that enables the charging and discharging processes to be simulated as continuous one-phase phenomena. We incorporate the sigmoid-based effective specific heat function into a reduced-order TES model previously derived by the authors and quantify the computational improvement over established approaches for simulating melting and solidification. We validate the proposed approach against our previously published model which uses retroactive updates to simulate melting for a variety of charging and discharging load conditions in a plate-fin heat exchanger geometry using lithium nitrate trihydride as the PCM. We demonstrate improved accuracy for two-dimensional heat transfer problems with time-varying boundary conditions. Furthermore, we quantify the sensitivity of model accuracy and execution speed to these parameters.

II. Introduction

Across a range of applications, an increase in electrification is leading to the need for thermal management systems (TMSs) that can mitigate increasingly transient heat loads that often exceed nominal operating conditions. Designing such systems using traditional steady-state analysis typically results in overdesigned components, increasing size and cost. One way to potentially alleviate the effect of transient heat loads is through the use of thermal energy storage (TES) which can temporarily store thermal energy in the form of latent heat, thereby acting as a buffer until the heat can be safely dissipated or rejected. This can have the additional benefit of circumventing the need for system over-designs to accommodate off-nominal operating conditions.