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Advanced battery thermal management for electrical-drive vehicles using reciprocating cooling flow and spatial-resolution, lumped-capacitance thermal model
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The thermal management of traction battery systems for electrical-drive vehicles directly affects vehicle dynamic performance, long-term durability and cost of the battery systems. The time-efficient yet accurate computational model for the battery thermal management system is essential to improve the performance, safety, and life time of the battery systems. In this analysis, the thermal management system is divided into two different perspectives: pack level and cell level thermal management system. For the pack level modeling, a new battery thermal management method using a reciprocating air flow for cylindrical Li-ion (LiMn2O4/C) cells was numerically analyzed using (i) a two-dimensional Computational Fluid Dynamics (CFD) model and (ii) a lumped-capacitance thermal model for battery cells and a flow network model. The results of the CFD model were validated with the experimental results of in-line tube-bank systems which approximates the battery cell arrangement considered for this study. The numerical results showed that the reciprocating flow can reduce the cell temperature difference of the battery system by about 4°C (72 % reduction) and the maximum cell temperature by 1.5°C for a reciprocation period of 120 seconds as compared with the uni-directional flow case. Such temperature improvement attributes to the heat redistribution and disturbance of the boundary layers on the formed on the cells due to the periodic flow reversal. From the cell level concern, the spatial-resolution, lumped-capacitance thermal models for cylindrical battery cells under high Biot number (Bi>1) conditions where the classical lumped-capacitance thermal model is inapplicable because of the significant temperature variation in the battery cells was presented in this analysis. The improved lumped-capacitance thermal models were formulated using first- and second-order Hermite integral approximations. For a validation of the results from the lumped-capacitance models, one-dimensional, transient analytical (exact) solutions using the Green function were obtained for the cylindrical Li-ion battery cells. It was found from the comparison of the results from the computational models that the spatial-resolution, lumped-capacitance thermal models accurately predict the temperatures (core, skin and area-averaged) of the battery cell under various battery duty cycles for a wide range of the Biot numbers covering air cooling to liquid cooling conditions. The battery heat generation was approximated by uniform volumetric joule and reversible (entropic) losses.