Experimental study and performance evaluation of direct refrigerant cooling/heating thermal management systems for full-scale commercial battery packs

Direct refrigerant cooling (DRC) is emerging as a pivotal solution for electric vehicle (EV) battery thermal management (BTM), offering superior heat dissipation over conventional liquid cooling. However, existing research is largely confined to numerical simulations or module-level experiments of small-scale systems, which may not fully capture the two-phase refrigerant behavior in long-path cold plates or the thermal characteristics of full-scale pack zones. Consequently, the overall thermal performance of commercial full-scale packs under high-power direct cooling and heating remains poorly understood. This study designed and built an 8 kW dual-mode direct cooling/heating platform for a commercial 87 kWh lithium iron phosphate pack, enabling independent regulation and mode switching of the compressor and expansion valve. By varying the DRC operating state, charge/discharge rate, and ambient temperature, the temperature distribution of the pack was revealed in both modes. In cooling mode, the observed cooling behavior is consistent with effective utilization of refrigerant latent heat: the maximum cell temperature ( T max ) stays as low as 39 °C at low rates (≤1.5C), is held at 45 °C with a temperature difference (Δ T ) below 5 °C at 2.0C, and reaches 54 °C at 3.0C once cooling capacity is exceeded; the system's theoretical cycle-averaged coefficient of performance (COP cyc , the charge-averaged COP th ) ranges from 3.69 to 4.89. In direct heating mode, rapid preheating is achieved, while substantially greater thermal non-uniformity is observed, with Δ T reaching 18 °C after preheating at −10 °C. Increasing compressor speed shortens the preheating time by 33.1% but is accompanied by increased temperature non-uniformity. From a battery-side hardware perspective, pressure-bearing capability is a key requirement for applying DRC to liquid-cooling cold plates. Since refrigerant-compatible cold plates (e.g., R134a-based solutions) have been demonstrated in some EV applications, existing liquid-cooled pack architectures may be retrofitted for DRC with limited battery-side modifications. These results demonstrate strong cooling performance for the full-scale commercial pack and, at the same time, identify the pronounced thermal non-uniformity of the direct-heating mode (Δ T up to ~21 °C) as a key challenge—together establishing experimental benchmarks for the dynamic control of transient thermal demands and temperature uniformity in subsequent DRC thermal management systems.