CO2 Heat Pumps Found to Offer High Efficiency at Low Ambient Temperature

CO₂ (R744) heat pumps in electric vehicles (EVs) are highly efficient at ambient temperature below 0°C (32°F), according to Nina Piesch, a Research Assistant from the Norwegian University of Science and Technology (NTNU).

Piesch based this comment on a research paper – “R744 heat pump solution for electric vehicles,” a survey of previous research – that she presented at the 10^thInternational Institute of Refrigeration (IIR) conference, held in Ohrid, North Macedonia, April 27–29.

CO₂ heat pumps boast a higher efficiency in electric vehicles because of their increased suction vapor density, which is a significant advantage in colder climates, she said.

Moreover, R744 eliminates the need for an auxiliary heater, a high-cost component that’s typically present in conventional systems, she noted, adding that this enhances the overall efficiency of EVs, making them a more desirable and cost-effective option for customers.

Previously, some researchers tested an R744 system for EVs in heating mode at ambient temperatures ranging from 0°C to -20°C (-4 °F). The researchers noted that extremely low outdoor temperatures and high discharge temperatures could pose challenges to component tolerance. To tackle this issue, they introduced two-stage compression with intermediate cooling. This resulted in reducing the discharge temperature along with improved heating capacity and better COP.

Defrosting strategies

Meanwhile, Chinese researcher Anci Wang of Xi’an Jiatong University has recently looked at defrosting strategies in EVs that use transcritical CO₂ heat pumps. This study revealed that traditional defrosting could lead to various false defrosting phenomena and even damage to the compressor and pipes in EVs. Therefore, as a solution, researchers proposed a new defrosting strategy: taking the maximum discharge temperature as the new criterion for defrosting control. This approach improved defrosting performance in EVs.

In a further study by Wang, researchers evaluated electric vehicle battery management with two-phase refrigerant direct cooling in a transcritical CO₂ system. Using a parallel thermal management model for the battery and the cabin, they evaluated battery thermal management based on battery temperature and its uniform temperature differences.

These parameters were regulated by the evaporation temperature and vapor quality in the cold plate. The study found that parallel direct cooling of the battery and cabin using R744 as a working fluid could effectively manage the surface temperature and uniformity of the battery, increasing efficiency.

“In electric vehicles, R744 exhibits superior performance, especially in a heating mode, compared to commonly used refrigerants R1234yf and R134a,” said Piesch.

R744 heat pumps in electric vehicles not only ensure safety since it’s non-flammable but also reduce global environmental impact due to low GWP, she added.

However, while R744 has significant potential, “it requires a shift in thinking and design principles to fully exploit its benefits,” added Piesch.

“Further work should focus on the process design and operating strategies specifically to the properties of R744 to reach even higher performance,” she said. “Higher heat rejection and throttling losses of R744 compared to other refrigerants could provide potential opportunities for efficiency improvements such as expansion work recovery with ejectors.”

Some automobile manufacturers have already implemented CO₂ heat pumps, including Volkswagen in its ID.3 and ID.4 vehicles.

“In electric vehicles, R744 exhibits superior performance, especially in a heating mode, compared to commonly used refrigerants R1234yf and R134a.”

Nina Piesch, Research Assistant from the Norwegian University of Science and Technology (NTNU)

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