Analytical solution and parametric design of bio-PCM-based passive BTMS for cylindrical lithium-ion cells under lumped model assumptions

Abstract

The present study proposes a parametric investigation of a passive battery thermal management system (BTMS) utilizing bio-based phase change materials (bPCMs). The thermal network approach allocates to the cell and the bPCM two thermal nodes to accurately capture the surface and core temperatures of the cell as well as the two concentric layers of the bPCM. A novel analytical solution to the thermal network model is introduced for the first time in the context of cell-bPCM configuration. The resolution is implemented through the zero-order hold discretisation technique, which involves piecewise time integration. Validation against experimental and computational fluid dynamics data under variable load demonstrates the model's predictive capability. Notably, this work features an extensive parametric analysis, examining both bPCM and cell thermo-physical parameters, thereby providing new insight into their effects on thermal performance over consecutive charge-discharge cycles. Results indicate that a 6 mm bPCM layer thickness was identified as optimal, providing a balance between thermal performance and system compactness. A lower melting point within the operating range leads to earlier activation of latent thermal absorption. The heat of fusion showed diminishing benefits beyond 200 kJ/kg, while bPCM thermal conductivity mainly improved internal homogeneity rather than peak suppression. Variations in bPCM density showed negligible impact on peak cell temperature but influenced thermal storage capacity. Furthermore, the study encompasses the effects of battery format, where the cell radius was found to be inversely proportional to temperature spikes observed, while cells with higher heat capacity showed improved resilience to thermal spikes. Notably, increasing the ambient heat transfer coefficient from 5 to 100 W/m 2 .K significantly enhances heat dissipation to the environment and promotes thermal recovery of the bPCM between cycles, reducing peak cell temperatures by up to 3 °C. Additionally, analysis of the BTMS under realistic driving conditions (WLTC, JC08, CLTC, NEDC, UDDS) underscores the system's ability to maintain the cell operating temperature within its optimal range (<36.2 °C), with temperature differences below 6 °C across all driving scenarios examined. This work provides a scalable tool for BTMS design and sizing, facilitating the integration of sustainable solutions into electric vehicles.