Which Natural Refrigerant Will Prevail in Mobile Heat Pumps – CO₂ or Propane?

One of the most important applications of natural refrigerants is in mobile air-conditioning and heat pumps in cars, trains and buses – a topic that will take center stage at ATMOsphere MAC Summit 2024 × TU Berlin, to be held September 12–13 at the Technical University (Technische Universität or TU) in Berlin.

The event is jointly organized by ATMOsphere, publisher of NaturalRefrigerants.com, and Technical University Berlin’s Chair of Heat Transfer and Heat Conversion. To register for the event, click here.

Delivering the opening keynote address at the conference – titled “The natural choice – NatRefs for mobile air conditioning and heat pump systems” – will be Stefan Elbel, Professor and Head of the Chair of Heat Transfer and Heat Conversion at TU Berlin. Elbel, an expert on natural refrigerants and the use of ejectors in CO₂ (R744) systems, was for many years Professor at the University of Illinois, Urbana-Champaign, where he was Co-Director of the Air Conditioning and Refrigeration Center. He remains affiliated with Creative Thermal Solutions, Urbana, Illinois, where he served for many years as Chief Engineer and more recently as Chief Executive Officer.

In the following interview with Elbel, held on August 13 at the 16^th IIR-Gustav Lorentzen Conference on Natural Refrigerants at the University of Maryland, he previews the program at the ATMO MAC Summit and looks at the competition between CO₂ and propane (R290) in mobile heat pumps. The interview was edited for brevity and clarity.

What topics will the ATMOsphere MAC Summit 2024 × TU Berlin cover?

It’s everything mobile, including cooling and heating of cars, trains and buses. But I think a lot of folks will come for the automotive side. Right now there’s probably the most change in the car sector. That is driven by EVs (electric vehicles), which need heat pumps for comfort heating and cooling. From the HVAC side, that is actually a good development because depending on where you are in Europe, people may see air-conditioning in [combustion-engine] cars as a luxury. But now, all of a sudden, EV cars also need heat pumps for comfort heating, battery conditioning and faster charging in the winter. So that gives our product a much different attribute and [changes] the way people look at it. And in automotive, probably because it’s a product that is similar around the world, change is implemented relatively quickly.

Which natural refrigerants are most often used in EV heat pumps?

It’s interesting to see a European side, mostly the Germans and more specifically Volkswagen, with a CO₂ solution. And in many other parts of the world – the U.S., for example – we see a focus on propane. It will be interesting to see who wins the race, though it doesn’t really matter because both are good fluid options.

That will be based on which has the best efficiency and the best mileage properties?

Yes, plus the lowest cost.

Which do you suspect will prevail?

It’s a tricky question because a CO₂ system you could make with direct expansion, and there’s no hazard potential. For propane, you need a secondary loop. But in an EV, this is ideal, because you will have a secondary loop anyway because there are inverters you need to cool, and the battery needs to be cool. Those are most likely be done by a secondary loop from the same circuit. Even with CO₂, I think you would also see EV architectures with a secondary loop because there are so many distributed cooling tasks. You could say, if the secondary loop is a must, then propane makes a lot more sense because there you get the additional benefit of better safety.

Time will tell which refrigerant is more favored. We can think how that worked with beverage companies in light commercial between propane and CO₂. It’s clear that propane won because it’s easier to implement and a more conventional system with no special components. All of this is driven by cost. But also what’s needed is that the OEMs convince us that the (R290) concepts are safe because that is the only red flag – what happens with flammability. And if you deal with propane, you need to make sure that systems are tight. So we could see in EV system designs a move towards hermetic systems.

What charge of R290 is generally used in EV heat pumps?

It depends on the system capacities. If you want to have the fast-charging system, then you’re talking about peak capacity of maybe 15 to 20 kilowatts (4.3 to 5.7TR); it used to be just half of that. Of course, you cannot expect to have a 20-kilowatt system with 150 grams. But I think safety concepts could be developed to feature maybe 300 grams – below 500 for sure. People claim that there’s maybe less hazard potential because you don’t have a combustion engine anymore. Of course, you have a 1,000-volt battery, which could spark.

HFO-1234yf has become the dominant refrigerant in car MAC. But, especially in Europe, the gas may be regulated because of its status as a PFAS that turns into another PFAS, trifluoroacetic acid (TFA), in the atmosphere. How will this affect its use in MAC and EVs?

With the appearance of EVs, all of a sudden there were new things that yf cannot easily do. If you rank the fluids, CO₂ at low temperature has by far the best heat pump performance. Propane is somewhere in the middle, much better than yf. And yf is really at a point where it just cannot provide sufficient capacity at even modestly low temperatures. This is a technical reason to get rid of it, and the PFAS scare does the rest, plus the price of the fluid, which nobody likes.

CO₂ is great as a heat pump refrigerant for low temperatures, but it’s not as good at high ambient temperatures, correct?

That is its Achilles’ heel. That reminds us just that there is no such thing as a miracle fluid that does everything. If CO₂ were that great, everybody would be using it. There would have never been a synthetic fluid. So whatever we do is a compromise. Of course, it is possible to boost the performance of a CO₂ system at high ambients – with ejectors, for example – but that translates into additional cost and system complexity. In supermarkets, CO₂ won the race, so the standard supermarket system nowadays in Europe is a relatively complex-looking CO₂ system, and that functions without any problem. So we shouldn’t say that just because of additional complexity, the CO₂ system has no chance. Time will tell. But if there is a simpler option, like propane could be, I think people will put much more attention on it. Even in supermarkets, you have self-contained propane freezer cases.

In terms of cost, how do the natural refrigerant heat pump systems compare HFO-1234yf heat pumps?

It’s hard to say because you’re comparing now very different sales volumes. So even though there are commercially available cars that have CO₂ heat pumps, this is a fraction of the total and CO₂ is much more complex. But I wouldn’t say there’s a large difference between propane technology and yf.

The conference will also look at heat pumps for trains and buses?

Yes. These are similar applications yet very different systems because you have a lot larger cooling capacities, so safety quickly becomes an issue. With mass transport, of course, a lot more people occupy a confined space. So that seems to drive us towards CO₂. But there are train or rail suppliers that make propane systems. So it’s the same thing – which one will win? Again, both are natural fluids and good options.

It will be interesting to see who (CO₂ or propane) wins the race, though it doesn’t really matter because both are good fluid options.

Stefan Elbel, Professor at TU Berlin