Numerical investigation of hexagonal cell packing with zig-zag cooling channels for enhanced thermal and spatial efficiency in electric vehicle battery modules

The rapid expansion of electric mobility has intensified the demand for efficient thermal management systems (TMS) in lithium-ion (Li-ion) battery modules, whose performance and safety are highly sensitive to temperature. Conventional cylindrical cells paired with serpentine liquid-cooling channels are widely employed, yet their geometric limitations often constrain cooling uniformity, packing density, and volumetric energy efficiency. This study investigates an alternative architecture integrating hexagonal Li-ion cells arranged in a honeycomb configuration with zig-zag liquid-cooling channels, inspired by natural space-optimization principles. A coupled thermal–fluid numerical model was developed using the finite volume method to simulate transient conjugate heat transfer across a seven-cell single-row module, in which a discrete thermal interface material (TIM) layer explicitly represents the contact resistance between each cell and its adjacent cooling-channel wall, and the battery active material is modeled as thermally orthotropic, with a high axial conductivity and lower, equal radial and tangential conductivities. Volumetric heat generation was derived from the irreversible (Joule) heating term of the Bernardi equation using manufacturer-datasheet internal resistance for a commercial 21700 cell, and applied as a constant source term over charging durations of 1 h, 30 min, and 20 min, corresponding to 1C, 2C, and 3C charge rates, respectively. Inlet boundary conditions for the two channel geometries were matched on the basis of equal coolant pumping power, rather than equal inlet velocity, to provide a physically consistent basis for comparison. Results show that the hexagonal–zig-zag configuration consistently achieves lower maximum and average cell temperatures, with the advantage widening from 0.54 K at 1C to 4.88 K (1.6%) at 3C, while thermal resistance is reduced by approximately 41.6% and the Temperature Uniformity Index improves by up to 1.4% relative to the cylindrical–serpentine baseline. Under the equal-pumping-power basis, the pressure-drop penalty of the hexagonal channel falls to only 4.65%, a substantially fairer comparison than a fixed-velocity basis would suggest. Separately, a spatial packing analysis of a 448-cell module confirms a 15.6% improvement in packing efficiency for the hexagonal arrangement. The model was benchmarked against independently reported liquid-cooled 21700 module data, with both the predicted temperature range and the magnitude of thermal improvement falling within the bounds of comparable published studies. Overall, the findings demonstrate that geometry-driven optimization of cell shape and cooling-channel topology, evaluated under realistic contact-resistance and equal-pumping-power conditions, offers a robust pathway toward more thermally efficient and spatially compact electric vehicle battery modules.