Forty-five members of the scientific community from European universities and research institutions have released a position paper supporting the uptake of hydrocarbons in heat pumps and foreseeing a gradual shift to hydrocarbon-based units.

The effort was led by researchers at Sweden’s KTH Royal Institute of Technology and the Fraunhofer Institute in Germany.

The academics maintain that “a transition to propane [R290] for outdoor installations [over] three to five years and for indoor heat pump [over] three to eight years seems to be realistic, depending on the different applications and capacity ranges.”

The use of natural refrigerants leads to an overall sustainable operation of heat pumps,” report the authors, suggesting that “it is a clear decision for environmental protection but also provides the industry with a long-term reliable perspective, allowing the focus to lie on further optimization rather than on adopting systems to new refrigerants every few years.”

The scientists argue for an early announcement of “clear and ambitious fade-out dates” for synthetic refrigerants, considering different development time spans for different product classes (indoor, outdoor, monobloc split, multi-split and VRF) and application areas (residential, commercial and industrial).

They add, “any development of new products should clearly focus on natural refrigerants.”

The authors have been assisting the European heat pump industry with its product improvements for almost three decades. With the revision of the EU F-gas Regulation and the impending PFAS Restriction Intention, they have decided to “comment on the ongoing debate on the technical feasibilities and changes that would occur when using propane (R290) refrigerant in heat pumps.”:

On energy efficiency

Based on the properties of hydrocarbons, say the academics, “they can be expected to provide low pressure drops and a system efficiency as high, or even higher, than synthetic alternatives.

The superiority of hydrocarbons is not only theoretically proven but is also supported by system efficiency comparisons carried out by using data from thousands of systems from almost the entire EU market that use either propane or other refrigerants, the authors add.

“Many years of experience using hydrocarbons as refrigerants already exists,” say the researchers, specifying that “the favourable thermodynamic and transport properties of propane and other hydrocarbons make them very well suited as refrigerants.”

On safety

Regarding the issue of flammability, the academics acknowledge that “it is well known that propane and other hydrocarbons are highly flammable, and of course, this needs to be considered carefully when designing systems.”

However, they note, the use of natural gas (e.g., LNG as fuel in cars), which is common and accepted almost all over the world, “involves similar risks,” and hydrocarbons used in heat pumps “are contained in hermetic systems.”

In addition, they specify that international safety standards are already in place and are constantly being improved. “New standards will most probably allow a wider use of hydrocarbon refrigerants, as more detailed safety precautions are being developed,” they write, referencing IEC 60335-2-40 ED7, adopted last year.

Referring to the progress made by the Fraunhofer Institute on the LC150 project, the academics elaborate further on the optimization of components and design to deliver higher capacities even with restricted charges. “An important factor for increasing safety, or broadening the use of hydrocarbons to larger systems, is to reduce the refrigerant charge per kW heating capacity in heat pumps,” they note.

The researchers point out that charge “is an area which has been neglected by the industry up to now since the amount of charge is not an important factor when using non-flammable fluids.” Recent research and development, says the paper, have shown that efficient systems can be operated with about 10g of propane per kW (1.4oz/TR), as compared to about 100g of propane per kW (12.5oz/TR) required with typical designs.

On components

A recent survey showed that several hundred designs of hydrocarbon heat pumps from about 48 manufacturers are commercially available in Europe, the researchers note, adding that these numbers clearly indicate that the components required for producing such heat pumps are available on the market.

“All of the basic research and development work to optimize the design of heat exchangers as well as compressors using low-GWP refrigerant has been done in the past 15 years,” they write. “This has led to widely available components and design rules for scaling up capacities,” refuting a belief often held by industry.

Sectors’ conversion to hydrocarbons

Going from nonflammable synthetic refrigerants to flammable hydrocarbons requires “careful consideration of the safety,” both at production sites and for products, the authors write. But a similar transition was made about 30 years ago when the home refrigerator industry switched from using R12 to isobutane (R600a) in three to five years, they point out.

Another example of production lines’ conversion is reported and documented by the Montreal Protocol Multilateral Fund for the Implementation, where a successful transition of a production line of split-systems from HCFC22 to R290 is reported.

The researchers conclude that “for air-to-water heat pumps the transition has started” since most European heat pump manufacturers have shifted their focus to R290 for new products. In fact, six out of the top 10 heat pump models in requests-for-funding in Germany in 2022 (all air-to-water) were charged with R290.

“Other product groups will follow this transition and the dynamics of its demonstrated path,” suggest the authors.

The signatories are below, and the position paper can be found here.

Prof. Björn Palm, Assoc. Prof. Joachim Claesson, Prof. Per Lundqvist, Prof. Rahmatollah Khodabandeh, Prof. Hatef Madani, Assoc. Prof. Samer SawalhaKTH Royal Institute of Technology, Sweden
Dr. Thore Oltersdorf, Dr. Lena SchnabelFraunhofer Institute for Solar Energy Systems, Freiburg, Germany
Prof. José Gonzálvez Maciá, Ass. Prof. Emilio Navarro PerisTechnical University of Valencia (UPV), Spain
Prof. Mario Motta, Assoc. Prof. Luca MolinaroliPolytechnic University of Milan, Italy
Prof. Dirk Müller, Christian Vering, M.S.RWTH Aachen, Germany
Prof. Tobias SchragTechnische Hochschule Ingolstadt, Germany
Prof. William SuenUniversity College London, United Kingdom
Prof. Dariusz MikielewiczGdańsk University of Technology, Poland
Prof. Branimir PavkovićUniversity of Rijeka, Croatia
Prof. Risto CiconkovUniversity of Skopje, North Macedonia
Prof. Vincent LemortUniversity of Liege, Belgium
Prof. Liutauras VaitkusKaunas University of Technology, Lithuania
Prof. Jaime Sieres AtienzaUniversity of Vigo, Spain
Prof. Armin Hafner, Prof. Trygve M. EikevikNorwegian University of Science and Technology, Norway
Markus Müller, Dr. Karl SteinjanInstitut für Luft- und Kältetechnik Dresden, Germany
Dr. Paul ByrneUniversity of Rennes, France
Peter TomleinSZ CHKT – Slovak Association for Refrigeration, AC and HPs SV IIR -Slovak committee for cooperation with IIR
Assoc. Prof. Uroš Milovančević, Assoc. Prof. Milena Otović, Vladimir Černicin, M.S., Teaching AssistantUniversity of Belgrade, Chair for Refrigeration and Heat Pumps, Serbia
Prof. Tonko Ćurko, Prof. Vladimir SoldoUniversity of Zagreb, Croatia
Prof. Piero ColonnaTU Delft, Netherlands
Prof. Dr. Andrej KitanovskiUniversity of Ljubljana, Slovenia
Prof. Jacek Smolka, Prof. Andrzej J. Nowak, Dr. Michal Haida, Dr. Michal PalaczDepartment of Thermal Technology, Silesian University of Technology, Gliwice, Poland
Prof. René RiebererGraz University of Technology, Austria
Prof. Konstantinos StergiaropoulosUniversity of Stuttgart, Germany
Mick Eschmann, Prof. Stefan BertschOST-Eastern Switzerland University of Applied Sciences Heat Pump Test Center WPZ
Prof. Michael KauffeldKarlsruhe University of Applied Sciences, Germany
Prof. Beat WellingHochschule Luzern, Switzerland
Prof. Christiane ThomasTU Dresden, Germany