Felix Nowak-Walenta, Thermal Management Test Engineer at Volkswagen, and Carsten Wachsmuth, Research and Development, detailed how the German automaker’s CO2 (R744) heat pump system benefits the thermal management operation of its electric vehicles (EVs) at the ATMOsphere (ATMO) MAC Summit 2024 × TU Berlin.
The event, organized by ATMOsphere and Technische Universität Berlin’s Department of Heat Transfer and Heat Conversion, was held September 12–13 in Berlin. ATMOsphere is the publisher of NaturalRefrigerants.com.
Volkwagen’s EV heat pump system can source heat from the vehicle’s drivetrain components, the ambient air or both. It features a 5.5kW (1.6TR) CO2 compressor manufactured by South Korea-based Hanon Systems.
“Our motivation to develop the PFAS-free unit came from CO2’s environmental benefits and thermodynamic properties that allow us to use low ambient air as an energy source,” said Wachsmuth. “We have several hundred thousand of these systems in the market.”
According to Nina Piesch, a Research Assistant from the Norwegian University of Science and Technology, R744 heat pumps exhibit “superior performance” in EV heating mode compared to commonly used synthetic refrigerants R1234yf and R134a.
Volkswagen first offered a CO2 heat pump in its ID.3 and ID.4 EVs in 2020. It has since brought the technology to more ID models as well as the Audi Q4 e-tron and Cupra BORN electric vehicles. In addition, the newly released Electric Ford Explorer, which uses VW’s electric vehicle platform, also offers a CO2 heat pump as an optional extra.
Refrigeration system configuration
The thermal system contains two circuits: refrigerant and coolant.
“We have more complexity on the refrigerant side but less on the coolant,” said Wachsmuth, adding that the system “mostly” operates in a transcritical process in air-conditioning (AC) and heat pump modes.
The refrigerant circuit contains a scroll compressor with a 5.3cm3 (0.3in3) displacement, a front–end gas cooler, an accumulator and internal heat exchanger (IHX), an evaporator, an interior gas cooler, a chiller and both expansion and shut off–valves.
“The system contains three expansion valves, allowing use of the evaporator and the chiller,” Wachsmuth explained. “The five shut–off valves allow us to switch operational modes.”
The coolant circuit uses two pumps, two electronic switching valves and a thermostatic switching valve. Heat sources in the circuit include the electric motor, pulse inverter, voltage converter, high voltage (HV) PTC (positive temperature coefficient) and HV battery. Heat sinks include the outside air heat exchanger, chiller and HV battery.
According to Nowak-Walenta, the system contains a 6kW (1.7TR) capacity PTC to supplement the heat pump if needed.
“Our motivation to develop the PFAS-free unit came from CO2’s environmental benefits and thermodynamic properties that allow us to use low ambient air as an energy source.”
Carsten Wachsmuth, Research and Development at Volkswagen
Thermal management
The vehicle’s thermal management system integrates the drive components, the refrigerant circuit and cabin comfort control. The operational modes include:
- Passive cooling of the electric motor(s) and pulse inverter
- Active cooling of the battery and passenger cabin, with coolant circulating solely between the battery and the chiller
- Coolant heat pump, with the heat coming from the battery or drivetrain components
- Ambient air heat pump, with absorbed heat transferred to the cabin via the interior gas cooler and evaporator
- Reheat, commonly used in summer, with the unit providing efficient dehumidification using heating and cooling
“In passive cooling, the thermostatic valve opens, depending on the temperature of the coolant, to cool the electric motor and drive components with outside air,” Nowak-Walenta said.
The battery is added to the loop by opening a valve connected to the exterior heat exchanger, he explained. “This is an efficient, simple way to cool the battery using a small amount of power to operate the pump.”
Depending on the vehicle’s operating speed and ambient temperature, the refrigerant cycle provides active battery and cabin cooling. “Our expansion valves, installed as a double unit, function bidirectionally, allowing flow through the chiller, evaporator or both to meet the cooling requirements,” said Nowak-Walenta.
In winter, the unit harvests heat from the battery, drivetrain components and ambient air for cabin comfort. “R744 is a good refrigerant for winter heat pump operation,” said Nowak-Walenta. In this mode, he explained that the AC gas cooler acts as an evaporator to collect heat from the outside air.
“The reheat cycle is the cherry on top,” Nowak-Walenta said. “By investing energy into the compressor once, you can use the reheat cycle in the summer to run the evaporator to dehumidify the cabin. It’s very efficient.”
Design challenges
While CO2 offers numerous advantages, Wachsmuth noted that working with the refrigerant brings several challenges.
“The higher operating pressure gives a heat transfer advantage, but it also creates challenges in regard to material strength and leakage,” Wachsmuth said, explaining that conventional or propane (R290) systems operate at 30bar (435psi), while CO2 operates at up to 130bar (1,885psi). Operating temperatures of up to 165°C (329°F) create other issues with oils and materials.
According to Nowak-Walenta, Volkswagen developed components and lines to handle the higher pressures and temperatures, but refrigerant leaks still occur. “We are still learning to optimize these systems,” he said.
Wachsmuth said the company also developed new control logic to provide more control freedom in transcritical operations.
Costs are an issue, with Ford offering a CO2 heat pump in the Electric Explorer as an optional extra with a price tag of more than €1,000 ($1,110).
“If you’re the first to do something, the price and suppliers aren’t there yet,” Nowak-Walenta said. “We’re fighting an uphill battle in terms of price, but we’re hoping to convince others that we’re on the right track so that they can follow us or even surpass us.”
