Dual-Path CO₂ Heat Pump Boosts EV Efficiency in Cold Climates
As electric vehicles (EVs) continue to dominate the future of mobility, one persistent challenge remains: maintaining cabin comfort and battery performance in subzero temperatures without sacrificing driving range. Traditional heating methods, such as Positive Temperature Coefficient (PTC) heaters, consume significant battery power, directly reducing an EV’s operational range. In response, automakers and researchers are turning to advanced thermal management systems—among them, CO₂-based heat pump technology—as a more energy-efficient solution. A recent study led by Dr. Wu Yue from SAIC Volkswagen Automotive Company presents a groundbreaking dual-path CO₂ heat pump system designed to simultaneously heat both the passenger cabin and the vehicle’s battery, offering a promising path toward improved energy efficiency and thermal resilience in cold-weather EV operation.
Published in the Chinese Journal of Refrigeration Technology, the research introduces a novel system architecture that diverges from conventional single-purpose heat pumps. Most existing thermal systems in EVs prioritize cabin heating, often neglecting the equally critical need to maintain optimal battery temperature. However, lithium-ion batteries are highly sensitive to low ambient temperatures. As temperatures drop, battery discharge capacity diminishes significantly. For instance, at -10°C, a battery operating at a 2C discharge rate can lose over half its rated capacity, drastically impacting vehicle performance and range. At the same time, excessively high temperatures accelerate battery degradation. Therefore, maintaining the battery within an ideal thermal window—typically between 20°C and 40°C—is essential for longevity and efficiency.
Dr. Wu Yue’s dual-path system addresses this dual challenge by integrating two parallel heating circuits within a single CO₂ heat pump loop. One branch supplies heat to the passenger cabin via an air-side internal heat exchanger, while the other delivers thermal energy to the battery cooling circuit through a plate heat exchanger. This design allows the system to reclaim waste heat from the refrigeration cycle and distribute it strategically to where it is needed most. Unlike PTC heaters, which have a coefficient of performance (COP) consistently below 1, meaning they consume more electrical energy than the heat they produce, heat pumps can achieve COP values greater than 1 by moving heat rather than generating it directly. This fundamental thermodynamic advantage makes heat pumps a superior choice for EV thermal management, especially when paired with a high-performance refrigerant like CO₂.
Carbon dioxide, or R744, has emerged as a leading candidate for next-generation automotive refrigerants due to its environmental and thermodynamic benefits. Unlike synthetic refrigerants such as R134a and R1234yf, CO₂ is a naturally occurring, non-toxic, and non-flammable substance with a global warming potential (GWP) of just 1. As regulatory pressures mount to phase out high-GWP refrigerants, CO₂ offers a sustainable alternative. Moreover, CO₂ excels in cold-climate performance. Its supercritical operation in the gas cooler enables a significant temperature glide during heat rejection, allowing for more effective heat transfer compared to the near-isothermal condensation of traditional refrigerants. This characteristic is particularly advantageous in heating mode, where maximizing heat output at low ambient temperatures is critical.
The system’s architecture features four electronic expansion valves (EXVs) that provide precise control over refrigerant flow distribution. The air heat exchange branch EXV regulates the flow to the cabin heater, the main circuit EXV controls overall system pressure and mass flow, and two EXVs in the water-side loop—the front and rear—manage the flow to the battery heat exchanger. This multi-valve configuration enables dynamic allocation of heating capacity based on real-time demand. For example, during initial cold starts, the system can prioritize battery warming to restore performance, then shift focus to cabin comfort once the battery reaches its optimal temperature.
To validate the system’s performance, Dr. Wu and her team conducted extensive bench testing under various low-temperature conditions. The experimental setup replicated real-world EV thermal loads, with sensors monitoring refrigerant pressure, temperature, coolant flow, and air-side heat exchange. By systematically varying EXV openings and inlet coolant temperatures, the team was able to map the system’s response and identify optimal control strategies.
One of the key findings was the impact of the air-side EXV opening on system performance. When the valve opening exceeded 70%, the cabin heating output plateaued and then declined, while the overall system COP decreased. This behavior is attributed to an imbalance in refrigerant distribution—excessive flow to the air-side circuit starves the water-side loop, reducing the total heat output. Therefore, the study recommends limiting the air-side EXV opening to 70% or less to maintain peak efficiency. Additionally, adjusting this valve allows control of the pressure drop across the evaporator, offering a means of protecting downstream components from excessive pressure fluctuations.
The main circuit EXV, positioned after the parallel branches, plays a crucial role in system-wide optimization. Tests conducted at -12°C ambient temperature revealed that a 30% opening of the main EXV yielded the highest system COP and heating capacity. At this setting, the refrigerant pressure and superheat levels were optimized, ensuring efficient compressor operation and effective heat rejection in the gas cooler. Deviations from this setting—either too open or too closed—led to suboptimal performance, underscoring the importance of precise valve control in CO₂ systems, which are highly sensitive to operating conditions due to their transcritical nature.
The dual EXVs in the water-side loop offer granular control over battery heating. By adjusting the front and rear valves, the system can modulate the flow rate through the plate heat exchanger, thereby regulating the amount of heat transferred to the battery coolant. Experimental results showed that varying the rear EXV opening from 5% to 100% could shift the proportion of total heating capacity delivered to the battery from 52% to 71%, with a corresponding improvement in system COP from 2.1 to 2.8. Similarly, adjusting the front EXV allowed even greater flexibility, increasing the water-side heating share to 77% and boosting COP to 3.1. These findings demonstrate that the dual-valve configuration is highly effective for load balancing and performance tuning.
An especially valuable insight from the study is the system’s stability across a wide range of coolant inlet temperatures. As the battery warms up during operation, the inlet temperature to the plate heat exchanger rises from subzero levels toward ambient. The tests showed that when the inlet temperature increased from -16°C to 0°C, the total system heating capacity varied by less than 5%, and the COP changed by less than 8%. This thermal robustness is critical for real-world applications, where battery temperature is constantly changing. It indicates that the system can maintain consistent performance without requiring frequent recalibration or control adjustments, simplifying integration into vehicle thermal management software.
The implications of this research extend beyond technical performance. By enabling simultaneous cabin and battery heating with high efficiency, the dual-path CO₂ heat pump can significantly reduce the energy burden on the EV’s high-voltage battery. This translates directly into extended driving range, particularly in winter conditions where heating demands are highest. For consumers, this means fewer range anxiety concerns and greater confidence in EV ownership in colder regions. For automakers, it represents a competitive advantage in markets where thermal comfort and efficiency are key purchase drivers.
Moreover, the use of CO₂ aligns with global sustainability goals. As governments implement stricter regulations on refrigerant emissions and lifecycle environmental impact, CO₂-based systems are poised to become the standard in next-generation EVs. The technology is already gaining traction in premium models from manufacturers such as Tesla, BMW, and Volkswagen, where thermal efficiency and environmental responsibility are paramount.
Dr. Wu’s work also highlights the importance of system-level thinking in EV design. Rather than treating cabin climate control and battery thermal management as separate subsystems, the dual-path approach integrates them into a unified, intelligent network. This holistic strategy not only improves energy efficiency but also reduces component count, weight, and complexity—key factors in vehicle design and manufacturing.
Looking ahead, the principles demonstrated in this study could be extended to include additional thermal functions, such as motor and power electronics cooling, or integration with waste heat recovery from regenerative braking. Future iterations might incorporate predictive algorithms that anticipate thermal loads based on driving patterns, weather forecasts, and route topography, enabling proactive thermal management.
The research also underscores the need for advanced control strategies. With multiple EXVs and dynamic load conditions, the system requires sophisticated software to optimize performance in real time. Machine learning and adaptive control techniques could be employed to continuously refine valve settings based on historical data and current operating conditions, further enhancing efficiency.
In conclusion, the dual-path CO₂ heat pump system developed by Dr. Wu Yue and her team at SAIC Volkswagen represents a significant advancement in EV thermal management. By efficiently delivering heat to both the cabin and the battery, the system addresses two of the most pressing challenges in cold-weather EV operation. Its robust performance, high COP, and adaptability make it a compelling solution for the next generation of electric vehicles. As the automotive industry accelerates toward electrification, innovations like this will be essential in ensuring that EVs are not only environmentally friendly but also practical, comfortable, and reliable in all conditions.
The study, titled “Study on Performance of Two-way Heating CO₂ Heat Pump System for Electric Vehicles,” was published in the Chinese Journal of Refrigeration Technology (doi: 10.3969/j.issn.2095-4468.2024.02.204) by Wu Yue of SAIC Volkswagen Automotive Company.