Hybrid modeling of two-phase refrigerant cooling integrated with EV heat pumps for Li-ion batteries under high discharge rates

The rapid development of electric vehicles (EVs) and high-power electronics has increased the demand for efficient battery thermal management systems (BTMS). This study investigates the performance of a two-phase refrigerant cooling (2pRc) using a secondary heat exchanger (SHE) in an EV-integrated heat pump system through numerical simulations and experimental validation. A hybrid modeling approach is utilized to assess the battery and two-phase heat transfer characteristics of a 94 Ah lithium-ion battery under high discharge conditions (2.5C). Experiments were conducted at refrigerant temperatures of 10 °C–20 °C and mass fluxes of 40–70 kg/m2s. The battery model resulted in a heat flux of 52 kW/m2 at 60 % state of charge (SOC) and an internal resistance of 70 mΩ. Two-phase heat transfer analysis revealed that convective boiling was the most important heat transfer mechanism in the 2pRc system. The SHE in the 2pRC system performed optimally at 20 °C with a mass flux of 60 kg/m2s and an inlet vapor quality of 0.1, resulting in high heat transfer efficiency, low pressure drop, and an effectiveness exceeding 80 %. Increasing coolant flow (3.7–4.3 L/min) and inlet temperature (20–30 °C) decreased pressure drop by 16.6 % while increasing battery inlet coolant temperature from 12.6 °C to 22.2 °C. The Simscape-based thermal model demonstrated high accuracy, with deviations within ±10–20 %, indicating reliability for system-level simulations. Additionally, the model predicted that increasing the discharge rate to 2C increased heat flux to 33 kW/m2 and required a rise in coolant flow rate of over 2.5 L/min.