Direct Refrigerant Cooling Outperforms Liquid Cooling in EV Battery Thermal Management
As the global automotive industry accelerates its shift toward electrification, one of the most critical challenges remains effective thermal management of lithium-ion battery packs. Overheating during high-rate charging and discharging not only degrades battery performance and lifespan but also poses serious safety risks, including thermal runaway. While liquid cooling has long been the dominant solution in electric vehicles (EVs), new research from Jiangsu University suggests that direct refrigerant cooling—a technology leveraging the vehicle’s existing air conditioning system—could offer superior thermal control, particularly under demanding operating conditions.
A team of researchers led by Professor Chunxian Shan and graduate student Peng Yang at the School of Energy and Power Engineering, Jiangsu University, has conducted a comprehensive experimental study on a direct refrigerant cooling system for EV battery packs. Their findings, published in the Journal of Jiangsu University (Natural Science Edition), demonstrate that this integrated approach outperforms conventional liquid cooling in managing average battery temperature, especially during high-current discharge scenarios.
The study, titled Experiment on Operating Characteristics of Refrigerant Direct Cooling System for Electric Vehicle Battery Packs, introduces a novel battery module design utilizing a harmonica-type cold plate. This configuration allows refrigerant—specifically R134a—to flow directly through microchannels beneath the battery cells, absorbing heat through phase change rather than relying on a secondary coolant loop. By eliminating the need for a separate liquid cooling circuit, the system reduces component count, weight, and potential failure points, contributing to improved vehicle efficiency and safety.
One of the central goals of the research was to compare the thermal performance of direct refrigerant cooling against traditional liquid cooling under identical test conditions. The experimental setup replicated real-world EV operating environments, with the battery module housed in a temperature-controlled chamber simulating ambient conditions ranging from 10°C to 30°C. The battery pack consisted of 16 lithium iron phosphate (LFP) cells arranged in an 8S2P configuration, each monitored with multiple temperature sensors to capture both average temperature and thermal gradients across individual cells and the entire pack.
In the comparative tests, both cooling methods used the same cold plate geometry to ensure a fair evaluation. The liquid cooling system employed water as the coolant, while the direct cooling system used R134a refrigerant. To isolate the effect of cooling mechanism, inlet temperatures for both fluids were set to 16.00°C. Notably, the mass flow rate of the refrigerant (0.25 g/s) was approximately half that of the water coolant (0.53 g/s), giving the liquid system a theoretical advantage in convective heat transfer.
Despite the lower flow rate, the direct cooling system achieved significantly better temperature control. At a 2.0C discharge rate—representing a high-power demand scenario such as rapid acceleration or fast charging—the average battery temperature under direct cooling peaked at 39.60°C, remaining below the critical 40.00°C threshold. In contrast, the liquid-cooled system reached 42.22°C, exceeding the recommended upper limit for optimal lithium-ion battery operation. The natural convection (passive) cooling baseline, as expected, performed the worst, with temperatures rising well above 45°C.
This 2.62°C improvement is attributed to the latent heat absorption during refrigerant evaporation. Unlike liquid cooling, which relies solely on sensible heat transfer, direct refrigerant cooling exploits the phase change from liquid to vapor, enabling far greater heat removal per unit mass of coolant. The researchers noted that this advantage becomes increasingly pronounced under high thermal loads, making direct cooling particularly well-suited for fast-charging applications where heat generation is intense and transient.
The study also explored the influence of key system parameters on cooling performance, including ambient temperature, compressor speed, and valve opening. These variables are crucial for real-world implementation, as they determine how the thermal management system adapts to changing driving conditions and environmental factors.
Ambient temperature had a measurable impact on both refrigerant flow rate and evaporation temperature. As the surrounding temperature increased from 10°C to 30°C, the mass flow rate of refrigerant decreased linearly, while the evaporation temperature rose. This behavior is linked to the thermodynamic response of the expansion valve, which adjusts refrigerant delivery based on the superheat at the evaporator outlet. Higher ambient temperatures elevate the initial battery temperature, increasing the thermal load on the cold plate. However, the system’s feedback mechanism—governed by the expansion valve—responds by modulating flow to maintain stable operation. The results showed that at higher ambient temperatures, the larger temperature differential between the battery and the refrigerant enhanced heat transfer efficiency, leading to a slower rate of temperature rise during discharge.
Compressor speed emerged as a particularly influential factor. The electric scroll compressor used in the system operated between 2,000 and 6,000 rpm, and its speed directly affected both refrigerant flow rate and evaporation pressure. As compressor speed increased, so did the refrigerant mass flow, while the evaporation temperature decreased due to lower pressure in the evaporator. At 3,500 rpm, the system demonstrated exceptional thermal control, maintaining the battery pack average temperature below 40.00°C even during continuous 2.0C discharge. This level of performance is critical for enabling high-power operation without compromising battery health.
However, the researchers also observed a trade-off between average temperature control and thermal uniformity. While higher compressor speeds improved overall cooling, they also increased the temperature gradient within individual cells. This phenomenon occurs because enhanced cooling at the cell base—where the cold plate is located—creates a steeper vertical temperature profile. The top and middle sections of the cell, where heat is primarily generated at the electrode tabs, remain relatively warmer, leading to a larger internal temperature difference.
This observation underscores a fundamental challenge in battery thermal management: optimizing for average temperature does not necessarily ensure uniform temperature distribution. Lithium-ion batteries perform best when the temperature difference across the pack is kept below 5.00°C, as excessive gradients can lead to uneven aging, reduced capacity, and increased risk of failure. In the direct cooling system tested, the temperature difference across the entire pack remained within acceptable limits under most conditions, but exceeded 5.00°C during high-rate discharge at lower compressor speeds.
The researchers also investigated the role of valve opening in regulating refrigerant flow to the battery evaporator. By adjusting a control valve in the refrigerant line, they were able to vary the mass flow rate from 0 to 0.70 g/s and achieve evaporation temperatures as low as 5.20°C. Increasing the valve opening improved cooling performance by allowing more refrigerant to flow through the cold plate, thereby lowering the average battery temperature. However, similar to the compressor speed effect, this improvement came at the cost of increased thermal non-uniformity.
At a valve opening of 11.2%, the battery pack temperature difference reached 9.30°C, with the majority of this variation stemming from internal cell gradients rather than differences between cells. In fact, the study revealed that under conditions of high thermal non-uniformity, the temperature difference within individual cells accounted for up to 88% of the total pack temperature difference. This finding highlights the importance of cell-level thermal design and suggests that future improvements in direct cooling systems should focus not only on overall heat removal but also on minimizing internal temperature gradients.
The energy efficiency of the system was also evaluated, particularly in relation to compressor power consumption. As compressor speed increased, so did the input power, following a nonlinear trend. While higher speeds delivered greater cooling capacity, the coefficient of performance (COP) of the refrigeration system decreased. The most energy-efficient operating point was found at 2,000 rpm, where the COP was maximized. This indicates a potential optimization strategy: using lower compressor speeds during moderate thermal loads to conserve energy, and ramping up speed only when necessary during high-power operation.
From a system integration perspective, the direct refrigerant cooling approach offers several advantages. By sharing the refrigerant loop with the cabin air conditioning system, it eliminates the need for a separate liquid cooling circuit, reducing complexity, cost, and weight. This integration also enables advanced thermal management strategies, such as using waste heat from the battery to assist in cabin heating during cold weather, thereby improving overall vehicle efficiency.
However, the technology is not without challenges. The precise control of refrigerant flow and pressure is essential to prevent issues such as liquid carryover or insufficient cooling. The presence of a liquid accumulation zone at the outlet of the microchannel cold plate was observed to cause anomalous temperature drops in the downstream cells, indicating a need for improved flow distribution design. Additionally, the use of refrigerants like R134a, while effective, raises environmental concerns due to their global warming potential. Future iterations of the system may benefit from the adoption of next-generation, low-GWP refrigerants.
The experimental methodology employed in this study stands out for its focus on real battery discharge behavior rather than simulated heat sources. Many previous studies have relied on electrical heaters to mimic battery heat generation, which fails to capture the dynamic thermal response of actual electrochemical processes. By testing with a real battery pack under controlled discharge conditions, the Jiangsu University team provided more accurate and applicable insights into the performance of direct cooling systems.
The implications of this research are significant for EV manufacturers seeking to enhance battery performance, safety, and longevity. As fast-charging infrastructure expands and consumers demand shorter charging times, the ability to manage high thermal loads will become increasingly important. Direct refrigerant cooling, with its superior heat removal capacity and system integration benefits, represents a promising path forward.
Moreover, the findings contribute to a growing body of evidence supporting the transition from liquid to phase-change-based cooling in high-performance applications. While liquid cooling remains adequate for many current EVs, the next generation of vehicles—particularly those targeting ultra-fast charging and high-power operation—may require more advanced thermal solutions. The work by Shan, Yang, and their colleagues provides a solid experimental foundation for such developments.
In conclusion, the research demonstrates that direct refrigerant cooling is not only feasible but also superior to conventional liquid cooling in managing the average temperature of EV battery packs under high thermal loads. Key parameters such as compressor speed and valve opening offer effective means of control, though they must be carefully balanced to avoid excessive thermal gradients. The integration of battery cooling with the vehicle’s HVAC system presents a compelling opportunity to simplify thermal architecture and improve overall efficiency. As the automotive industry continues to push the boundaries of battery technology, innovations in thermal management will play a crucial role in unlocking the full potential of electric mobility.
Chunxian Shan, Peng Yang, Aikun Tang, Dengfu Xia, School of Energy and Power Engineering, Jiangsu University. Journal of Jiangsu University (Natural Science Edition), DOI: 10.3969/j.issn.1671-7775.2024.01.005