Transient Simulation of Heat Exchangers with Phase Change Material 2020-01-0156

Heat exchangers with Phase Change Material (PCM) are finding more energy storage applications for both Internal Combustion Engine Vehicles (ICEVs) and Electric Vehicles (EVs). These applications include cold storage evaporators for stop-start cars, thermal storage system for EV cabin heating and cooling, and other Heating, Ventilation, and Air Conditioning (HVAC) and Power Train Cooling (PTC) peak load shaving applications.

The energy stored in a PCM heat exchanger is typically charged/discharged using refrigerant, coolant, or air, depending on the system design of different applications. Due to the low thermal conductivity of state-of-art PCM, the PCM heat exchangers generally rely on aluminum fins to enhance the speed of charging and discharging of the stored energy. Different fin shape, height and density will result in different PCM freezing/melting rate.

In this paper, two different fin designs (folded-sine-wave fin and off-set-strip fin) are simulated with Computational Fluid Dynamics (CFD) to compare the melting time of a hot PCM with respect to different operating parameters (coolant temperature, coolant convection heat transfer coefficient, and PCM initial temperature). Next, a 1-Dimensional (1-D) approximation of the 2-Dimensional (2-D) transient heat conduction problem is proposed. The equivalent thermal conductivity of the 1-D approximation is determined using similar CFD approach to match the melting time of the original 2-D problem. The equivalent thermal conductivity of the 1-D approximation can be used as a single metric to evaluate different fin designs. Finally, a Simulink model is developed to simulate the transient performance of a full PCM heat exchanger using the 1-D equivalent thermal conductivity, which can be incorporated into system simulation to optimize heat exchanger design and estimate in-vehicle performance. The model is validated against test data of a heat storage PCM heat exchanger, and its capability of simulating peak load shaving application is demonstrated.