Airflow Impacts Frosting and Defrosting in EV Heat Pumps
In the rapidly evolving landscape of electric mobility, one persistent challenge continues to hinder driver comfort and vehicle efficiency in cold climates: the performance degradation of thermal management systems due to frost accumulation. As winter temperatures descend, the heat pump systems designed to warm vehicle cabins face a critical obstacle—frost formation on outdoor heat exchangers. This phenomenon not only diminishes heating capacity but also increases energy consumption and compromises passenger comfort. A new experimental study conducted by researchers at Zhongyuan University of Technology has shed light on a crucial variable in this equation: airflow volume.
The research, led by Zhou Guanghui, Wang Rui, Li Haijun, Xu Qi, Cai Xia, Zhang Qingge, Yuan Tiesuo, and Chu Xuejing, investigates how varying indoor fan airflow affects both frosting behavior and defrosting efficiency in a quasi-two-stage compression heat pump system specifically designed for pure electric vehicles operating under low-temperature conditions. Their findings, published in the journal Energy Conservation, offer valuable insights for automotive engineers aiming to optimize thermal systems in next-generation EVs.
As urbanization accelerates across China and global EV adoption climbs, the demand for efficient, reliable cabin climate control grows proportionally. Traditional resistive heating methods, while effective, consume significant battery power, directly reducing driving range—a key concern for consumers. In response, automakers have increasingly turned to heat pump technology, which can deliver up to three times more heat energy than the electrical energy it consumes under favorable conditions. However, when ambient temperatures drop below 5°C and humidity levels rise, the outdoor evaporator coil begins to frost over, obstructing airflow and insulating the heat exchange surface. This leads to a sharp decline in system performance.
To address these limitations, the research team designed and tested an advanced heat pump configuration featuring quasi-two-stage compression with intermediate pressure vapor injection. This architecture allows for higher compression ratios without excessive discharge temperatures, making it particularly suitable for cold-climate operation. The system utilizes R1234yf, a next-generation refrigerant with lower global warming potential compared to traditional R134a, aligning with global environmental regulations and sustainability goals.
The experimental setup was conducted within a standard enthalpy-difference laboratory, ensuring precise control over environmental variables. Testing conditions were established in accordance with GB/T 37123–2018, the Chinese national standard for electrically driven automotive air conditioners, focusing on heating mode operations. Key parameters included an outdoor ambient temperature of 1°C, relative humidity of 70%, compressor speed fixed at 3,600 rpm, and outdoor fan velocity maintained at 4.5 m/s. The primary variable under investigation was indoor fan airflow, tested at three levels: 60%, 80%, and 100% of maximum capacity. This approach allowed the researchers to isolate the impact of cabin-side airflow on the overall thermodynamic behavior of the system, particularly during prolonged heating cycles where frosting inevitably occurs.
One of the most significant observations from the study was the direct correlation between increased indoor airflow and declining evaporating temperature. At 60% fan speed, the evaporator temperature dropped from an initial 1.05°C to -21.03°C as frost accumulated. When fan output was raised to 80%, the starting temperature was slightly lower at 0.46°C, but the final temperature plunged further to -22.55°C. The most aggressive setting—100% fan speed—began at -0.26°C and reached -23.55°C by the end of the frosting cycle. This progressive cooling effect underscores a fundamental principle in heat transfer: higher airflow enhances convective heat exchange on the indoor coil (now acting as a condenser in heating mode), which in turn demands greater heat absorption from the outdoor environment. As the outdoor coil works harder to extract heat from cold, moist air, its surface temperature drops below the dew point and eventually below freezing, accelerating frost nucleation and growth.
However, the benefits of enhanced heat transfer come with trade-offs. While higher airflow improves short-term heating output, it also intensifies the rate at which performance degrades over time due to frosting. The data revealed that at 60% fan speed, system heating capacity decreased by 44.3%, falling from a peak of 2.30 kW to 1.28 kW. At 80% airflow, the reduction was slightly less severe at 42.2%, with output dropping from 2.70 kW to 1.56 kW. The highest airflow setting saw a 42.8% decline, from 3.40 kW to 1.78 kW. These figures highlight a critical balance: although higher airflow delivers stronger initial heating, the system reaches its performance nadir faster as frost builds up.
Similarly, the coefficient of performance (COP)—a measure of energy efficiency—followed a comparable trend. At 60% fan speed, COP fell from 1.42 to 1.01, representing a 28.8% drop. At 80%, it declined from 1.65 to 1.20 (27.2%), and at 100%, from 2.01 to 1.45 (27.8%). The slight improvement in relative efficiency at higher airflow suggests that the system operates closer to its optimal point before frosting dominates. Nevertheless, all configurations experienced substantial efficiency losses as ice layers formed on the microchannel outdoor heat exchanger, which, due to its compact fin structure, is especially prone to blockage.
What makes this study particularly valuable is its focus not only on frosting but also on defrosting dynamics. The researchers employed reverse-cycle defrosting, a method that reverses the refrigerant flow using a four-way valve, turning the indoor unit into an evaporator and the outdoor coil into a condenser. This allows hot refrigerant gas to melt the accumulated frost. Unlike auxiliary electric heaters, this method uses waste heat from the system itself, minimizing additional energy draw from the battery.
During defrosting trials, the team observed that higher indoor airflow significantly reduced defrosting duration. At 60% fan speed, the process took 272 seconds. This was reduced to 248 seconds at 80% airflow—a savings of 8.82%—and further shortened to 236 seconds at 100% airflow, marking a 13.24% reduction compared to the lowest setting. Concurrently, the average condensing temperature during defrosting rose from 14.46°C at 60% airflow to 16.26°C at 80%, and 17.36°C at 100%. This temperature increase is crucial: higher condensing temperatures mean more thermal energy is delivered to the frosted coil, accelerating ice melt.
The mechanism behind this improvement lies in improved heat distribution. With greater airflow across the indoor heat exchanger (now functioning as an evaporator during defrost), the refrigerant absorbs heat more efficiently from the cabin air, leading to higher superheat and more stable compressor operation. This results in hotter discharge gas being sent to the outdoor unit, enhancing the defrosting capability. Additionally, faster defrost cycles mean less time spent in a non-heating mode, preserving cabin comfort and reducing the need for supplemental heating.
Another key finding relates to frosting duration. Contrary to what one might expect, higher indoor airflow actually shortened the time required for frost to fully develop on the outdoor coil. At 60% airflow, it took 103 minutes for the system to reach full frosting. At 80%, this was reduced to 98 minutes (4.85% faster), and at 100%, to just 95 minutes (7.77% faster). This indicates that while high airflow accelerates performance degradation, it also triggers the defrost control logic sooner, potentially leading to more frequent but shorter defrost cycles. For vehicle manufacturers, this presents an opportunity to fine-tune defrost initiation strategies based on real-time airflow and load conditions.
The implications of this research extend beyond academic interest. As automakers strive to extend EV range in cold weather, every kilowatt-hour saved in cabin heating translates to additional miles on the road. By understanding how airflow influences both frosting and defrosting, engineers can design smarter control algorithms that dynamically adjust fan speeds based on ambient conditions, occupancy, and thermal demand. For instance, during mild cold spells, maintaining moderate airflow could delay frosting and extend heating efficiency. During intense heating requests, temporarily increasing airflow might be acceptable if it ensures rapid defrosting and sustained comfort.
Moreover, the use of R1234yf in a quasi-two-stage system demonstrates a viable pathway for cold-climate EVs. This refrigerant, though more expensive and slightly less efficient than some alternatives, offers environmental advantages and compatibility with existing service infrastructure. Combined with vapor injection technology, it enables reliable operation even in sub-zero environments, a necessity for markets in northern China, Scandinavia, Canada, and the northern United States.
The study also underscores the importance of system-level thinking in EV thermal management. While much attention is given to battery cooling, cabin heating remains a major energy sink. Optimizing airflow is a low-cost, software-driven solution that does not require hardware changes. Simple adjustments to fan control logic, informed by empirical data like that presented in this paper, can yield measurable improvements in efficiency and user satisfaction.
Looking ahead, future research could explore the interaction between airflow and other variables such as compressor modulation, refrigerant charge, and advanced defrost detection methods. Integrating machine learning models to predict frosting based on real-time sensor inputs—including humidity, temperature, and pressure—could enable predictive defrosting, avoiding unnecessary cycles and further improving efficiency.
In conclusion, the work by Zhou Guanghui, Wang Rui, and their colleagues provides a comprehensive analysis of airflow’s role in the performance of electric vehicle heat pump systems under cold, humid conditions. Their results demonstrate that while higher indoor fan speeds accelerate frosting, they also enhance defrosting speed and efficiency, ultimately leading to better system responsiveness and shorter downtime. For the automotive industry, this means that intelligent airflow management is not merely a comfort feature but a critical component of energy-efficient thermal design. As EVs continue to penetrate colder markets, such insights will be instrumental in delivering reliable, comfortable, and sustainable transportation solutions.
Zhou Guanghui, Wang Rui, Li Haijun, Xu Qi, Cai Xia, Zhang Qingge, Yuan Tiesuo, Chu Xuejing, Zhongyuan University of Technology, Energy Conservation, doi:10.3969/j.issn.1004-7948.2024.01.008