If future electric vehicles (EVs) require full-secondary loop thermal management systems for heating and cooling passengers and components, propane (R290) could gain an edge over CO2 (R744) in the auto industry, according to Prof. Stefan Elbel, Head of the Department of Heat Transfer and Heat Conversion at Technische Universität Berlin (TU Berlin).
“If the efficiency of the thermal management system evolves to include a secondary loop for element heating and cooling, then, in my opinion, the innovation will favor propane,” Elbel told Marc Chasserot, CEO and Founder of ATMOsphere, during an interview on the sidelines of the ATMOsphere (ATMO) MAC Summit 2024 × TU Berlin.
ATMOsphere, the publisher of NaturalRefrigerants.com, organized the ATMO MAC (mobile air-conditioning) Summit in partnership with TU Berlin’s Department of Heat Transfer and Heat Conversion. The event was held September 12–13 in Berlin.
Current MAC designs for CO2-based systems use a single refrigeration circuit to cool and heat the cabin and electric drivetrain. Propane systems can be designed with a single refrigeration circuit, too, but given the flammability of R290, they require a reduced refrigerant charge to mitigate safety risks.
However, a lower charge reduces the system’s ability to effectively heat/cool an EV’s battery and other components. Given this, the industry is investigating full-secondary loop systems that preserve charge levels by using a secondary fluid – like a water/glycol mixture – for heating and cooling the cabin.
Elbel said if EVs, even those with CO2-based MAC systems, require a full-secondary loop, then propane could gain favor because of its cooling performance in high ambient temperatures and lower cost.
“While R744 carries the performance advantage in low ambient temperatures, not too many places in the world regularly experience −40°C [−40°F] temperatures,” Elbel noted. “In addition, given that the core components of a propane system compare to conventional MAC units, it translates to a minimum cost differential.”
Propane advantages/drawbacks
Research conducted by Ford Motor Company and presented at the summit by Angelo Patti, Product Development Engineer, found that an R290 full-secondary loop heat pump system provides a “superior solution” for battery EVs compared to traditional direct expansion and semi-secondary loop systems.
“R290 is the best global option for battery electric vehicles if safety is addressed,” he said.
In addition, German train operator Deutsche Bahn announced at the ATMO MAC Summit that it plans to have 2,000 R290 HVAC units – manufactured by rail equipment supplier Faiveley Transport (Wabtec) – in service on passenger trains by 2027. In a presentation, Peter Danzer, Project Leader, said Deutsche Bahn has had 16 units operating since mid-2023 and has encountered no issues.
CO2 advantages/drawbacks
In the race to produce an efficient MAC system for EVs, CO2 currently leads the way, with hundreds of thousands of electric cars in Europe sold with R744 heat pumps.
German automaker Volkswagen first offered an EV CO2 thermal management solution in 2020, using a CO2 compressor manufactured by South Korea-based Hanon Systems. The company has since brought the system to more ID vehicles and also offers it in the Audi Q4 e-tron and Cupra BORN EVs. In addition, the newly released Electric Explorer by Ford uses VW’s EV platform and features an optional CO2 heat pump.
“As a heat pump fluid, CO2 offers excellent thermodynamics, especially in low-ambient temperatures,” Elbel said. “However, the complexities of CO2 systems and poor performance at elevated ambient temperatures require extra components to maintain efficiency and come with a higher price tag.”
“Given that the core components of a propane system compare to conventional MAC units, it translates to a minimum cost differential.”
Prof. Stefan Elbel, Head of the Department of Heat Transfer and Heat Conversion at TU Berlin
