CO2 Heat Pump Boosts EV Efficiency in Cold Climates
As electric vehicles (EVs) continue to dominate global automotive trends, one persistent challenge remains: maintaining cabin comfort and preserving driving range in frigid winter conditions. Conventional heating methods, particularly the use of Positive Temperature Coefficient (PTC) heaters, have long been a significant drain on battery resources, leading to reduced range and increased consumer anxiety. However, a recent breakthrough in thermal management technology could reshape how EVs handle cold-weather operation. A new study published in Chinese Journal of Refrigeration Technology demonstrates that a CO2-based heat pump system not only meets stringent heating performance standards in extreme cold but also dramatically improves energy efficiency compared to traditional R134a systems.
The research, conducted by a team from Jilin University’s State Key Laboratory of Automotive Simulation and Control in collaboration with industry partners, presents compelling evidence that natural refrigerant CO2—long considered a promising but technically challenging alternative—is now a viable and superior solution for next-generation EV climate control. With winter temperatures dropping as low as -23°C, the tested CO2 heat pump system delivered stable, comfortable cabin heating while cutting energy consumption by over a third compared to conventional setups. This advancement could mark a turning point in the quest for year-round EV usability, especially in northern climates where cold-weather performance has been a major barrier to widespread adoption.
What sets this study apart is its real-world validation. Unlike many laboratory-based experiments, the team conducted full-scale road tests under actual winter conditions, measuring cabin air delivery, footwell temperatures, power draw, and cumulative energy use. The results were not only technically impressive but also highly relevant to everyday drivers who demand both warmth and range when temperatures plummet. The findings suggest that CO2 heat pump technology is no longer just a theoretical advantage—it is a practical, high-performance solution ready for integration into mass-market electric vehicles.
At the heart of the innovation is the use of carbon dioxide (R744) as a refrigerant. Unlike synthetic refrigerants such as R134a, which have high global warming potential (GWP), CO2 is a natural substance with a GWP of just 1 and zero ozone depletion potential. In recent years, as regulatory pressure mounts to phase out high-GWP refrigerants under international agreements like the Kigali Amendment, automakers have been actively seeking sustainable alternatives. CO2 has emerged as a frontrunner, particularly in Europe and Japan, where manufacturers like Toyota and Volkswagen have already introduced CO2 heat pumps in select models.
However, CO2 systems operate differently from traditional vapor-compression cycles. They function in a transcritical cycle, meaning the refrigerant does not undergo a conventional condensation phase. Instead, it releases heat through a gas cooler, where high-pressure supercritical CO2 transfers thermal energy to the cabin air. This process requires higher operating pressures—often exceeding 100 bar—but offers superior heat transfer characteristics, especially in cold environments where traditional systems struggle to extract heat from the outside air.
The research team designed a fully functional CO2 heat pump system capable of seamless switching between heating and cooling modes. The system includes a variable-speed compressor, internal heat exchanger (IHX), electronic expansion valves, and dual evaporators and condensers configured for both cabin heating and cooling. By precisely controlling solenoid valves and expansion devices, the system reverses refrigerant flow to deliver either warm air in winter or cool air in summer. This flexibility is critical for EVs, which must maintain thermal comfort across diverse climates without relying on fossil-fueled engines for waste heat.
One of the most significant findings of the study was the system’s ability to deliver consistent and comfortable foot-level heating even at -23°C. In automotive thermal design, footwell temperature is a key indicator of passenger comfort, as cold feet are often the first sign of inadequate heating. The researchers instrumented test vehicles with multiple K-type thermocouples at air outlets and near occupants’ feet to capture real-time temperature data. During road tests conducted at -15°C and -23°C, the CO2 system rapidly warmed the cabin, with outlet air temperatures rising from subzero levels to over 50°C within minutes.
At -15°C ambient temperature, the system achieved stable foot-level air delivery temperatures of 50.5°C for the driver and 57.9°C for the front passenger after 30 minutes of driving followed by 10 minutes of idling. Foot temperatures reached a comfortable 33.1°C and 41.5°C respectively, well within the thresholds defined by vehicle technical specifications (VTS). Even more impressively, at -23°C—nearly 10 degrees colder—the system not only maintained functionality but actually improved relative performance. Average foot-level outlet temperatures increased by approximately 2.86% compared to the -15°C test, demonstrating that the CO2 heat pump becomes more effective as ambient temperatures drop, a phenomenon rarely seen in conventional systems.
This counterintuitive result underscores a fundamental advantage of CO2 refrigerant: its excellent low-temperature heat absorption capability. While R134a systems often require supplemental PTC heating when outdoor temperatures fall below freezing, the CO2 system was able to meet heating demands almost entirely through the heat pump cycle. The researchers noted that temperature fluctuations were minimal, with rear and front zones maintaining balanced output, a crucial factor for passenger comfort and system reliability.
To benchmark performance against existing technology, the team conducted a direct comparison with a conventional R134a heat pump system augmented with a PTC heater. The test conditions were standardized: -22°C ambient temperature, 80 km/h cruising speed, and consistent fan settings. The results revealed a stark contrast in both performance and efficiency. In the initial 300 seconds, the CO2 system warmed the cabin faster, with the driver’s footwell air temperature rising at an average rate of 0.21°C per second—9.52% faster than the R134a/PTC combination.
More importantly, the energy consumption difference was dramatic. The R134a system relied heavily on the PTC heater, which spiked to 8.5 kW during startup before stabilizing at around 3.2 kW, while the compressor consumed an additional 1.2 kW. In contrast, the CO2 system’s compressor operated at a peak of 4.2 kW before settling to a steady 3.2 kW with minimal fluctuation. When comparing total power draw, the CO2 system used 28.6% less power than the combined compressor and PTC load of the R134a system.
Over time, this efficiency advantage translated into a 35.4% reduction in cumulative energy consumption. As the test progressed, the gap between the two systems widened, indicating that the CO2 heat pump’s efficiency benefits compound over longer drives. For EV owners, this means significantly less range loss during winter operation. A vehicle that might lose 40% of its range in cold weather with a conventional system could see that penalty reduced to 25% or less with CO2 heat pump technology, effectively extending usable range by tens of kilometers in real-world conditions.
The implications of these findings extend beyond passenger comfort. As automakers strive to meet increasingly strict emissions and efficiency regulations, every watt-hour saved contributes to a vehicle’s overall sustainability. The switch to CO2 refrigerant also aligns with broader environmental goals. With a GWP of 1, CO2 is thousands of times less harmful to the climate than R134a (GWP = 1,300) if released into the atmosphere. While leakage rates in modern systems are low, the inherent safety and sustainability of CO2 make it an ideal long-term solution for eco-conscious manufacturers.
Moreover, the study highlights the importance of system integration and control strategy. The CO2 heat pump’s performance was not solely due to the refrigerant choice but also to optimized component selection and intelligent valve management. The internal heat exchanger, for example, played a critical role in improving cycle efficiency by subcooling the high-pressure refrigerant and superheating the low-pressure return gas. The use of electronic expansion valves allowed for precise flow control, adapting to changing thermal loads and ambient conditions.
From a manufacturing perspective, the transition to CO2 systems presents both opportunities and challenges. The higher operating pressures require stronger materials and more robust sealing, which can increase component costs. However, as production scales and supply chains mature, these costs are expected to decline. The research team emphasized that the long-term benefits—improved range, lower energy use, and compliance with environmental regulations—far outweigh the initial investment.
Another advantage of the CO2 system is its potential for integration with other vehicle thermal management functions. Modern EVs generate heat from batteries, power electronics, and motors, all of which require cooling. A CO2-based system can potentially recover waste heat from these components and redirect it to the cabin, further improving overall efficiency. Future iterations of the technology could include bidirectional heat exchange, enabling simultaneous battery warming and cabin heating during cold starts—a capability that would dramatically improve both performance and longevity in winter conditions.
The study also addressed reliability concerns. Some critics have argued that CO2 systems are more complex and prone to failure due to high-pressure operation. However, the road tests conducted over multiple cycles and temperature extremes showed no signs of degradation or instability. The compressor operated smoothly, and temperature control remained precise throughout the experiments. The researchers attributed this reliability to careful system design and the use of proven components adapted for high-pressure service.
For consumers, the most tangible benefit will be the elimination of “range anxiety” during winter months. Many EV drivers report hesitation about long trips in cold weather, knowing that heating demands can cut their range in half. With a CO2 heat pump system, that concern is significantly mitigated. The ability to maintain a warm cabin without sacrificing range makes EVs more practical for daily use in colder regions, potentially accelerating adoption in markets like Scandinavia, Canada, and the northern United States.
Automotive engineers and thermal management specialists will find the study’s methodology particularly valuable. The use of real-world road testing, rather than controlled chamber environments, provides data that closely reflects actual driving conditions. The inclusion of multiple measurement points—both at air outlets and near occupants—ensures a comprehensive assessment of thermal comfort. The comparison with a production-grade R134a/PTC system adds credibility, as it reflects the current state of the art rather than an idealized benchmark.
The research also opens new avenues for future development. The team noted that further optimization of compressor speed control, expansion valve timing, and gas cooler airflow could yield additional efficiency gains. Integration with predictive climate control algorithms—using weather forecasts and route data to pre-condition the cabin—could enhance performance even further. As artificial intelligence and connected vehicle technologies advance, CO2 heat pumps could become part of a fully adaptive thermal ecosystem.
In conclusion, this study represents a significant step forward in electric vehicle thermal management. It demonstrates that CO2 heat pump technology is not only technically feasible but also superior in performance and efficiency to conventional systems, especially in cold climates. The results validate years of research and development in natural refrigerants and provide a clear roadmap for automakers seeking to improve winter range and passenger comfort.
As the automotive industry moves toward a zero-emission future, every component must be re-evaluated for sustainability and efficiency. The CO2 heat pump system studied here exemplifies this shift—combining environmental responsibility with tangible performance benefits. For EV drivers, it promises warmer cabins, longer range, and greater confidence in all seasons. For the industry, it offers a scalable, future-proof solution to one of the last major challenges in electric mobility.
The transition to CO2 refrigerant is no longer a question of if, but when. With mounting regulatory pressure, consumer demand for efficiency, and now, proven real-world performance, the adoption of CO2 heat pumps is poised to accelerate. This study, grounded in rigorous testing and practical application, provides the evidence needed to drive that change. As automakers race to deliver better EVs, the lessons learned from this research will undoubtedly influence the design of next-generation thermal systems.
By proving that a natural refrigerant can outperform synthetic alternatives in the most demanding conditions, the team has not only advanced the state of the art but also contributed to a more sustainable automotive future. Their work underscores the importance of interdisciplinary collaboration between academia and industry in solving complex engineering challenges. As electric vehicles become the norm, innovations like this will ensure they are not only clean but also comfortable, reliable, and practical for everyone, everywhere.
By Lou Hui (Wuhu Chery Technology Co., Ltd.), Wang Jun (Ethermal Automotive Technology (Changshu) Co., Ltd.), Teng Haixu, Liu Guidan, Li Xiaotong, Li Bin (State Key Laboratory of Automotive Simulation and Control, Jilin University). Published in Chinese Journal of Refrigeration Technology, DOI: 10.3969/j.issn.2095-4468.2024.04.206