Taking the measure of CO2

By Michael Garry, Aug 30, 2019, 11:34 9 minute reading

Professor Armin Hafner, one of the world’s leading CO2 refrigeration researchers, looks back at the progress made by CO2 over the past two decades – and the road ahead.

Professor Armin Hafner

The modern age of natural refrigerants is said to have begun in the late 1980s when Norwegian Professor Gustav Lorentzen developed the modern transcritical cycle for CO2.

Following in his footsteps is Armin Hafner, since 2016 a professor in refrigeration at the department of energy and process engineering at NTNU (Norwegian University of Science and Technology), in Trondheim, Norway, where Lorentzen also taught. Prior to that he worked for two decades at SINTEF, a Norwegian research organization.

Hafner has done some of the leading research on CO2 refrigeration and heat pump technology, investigating every application of CO2 systems as well as the components that comprise them. Accelerate Magazine conducted the following interview with him on the progress CO2 has made in HVAC&R and its prospects in the global marketplace.


//R744: When did you start working on CO2 refrigeration?

Armin Hafner: My first practical experience with CO2 refrigeration technology was during my diploma thesis at FH-Karlsruhe (now the Karlsruhe University of Applied Sciences) in Germany in 1995. I worked with F.Grässlin to develop a CO2 laboratory system and build a pilot system under the supervision of Prof. J. Reichelt. An open-type BOCK (now GEA) piston compressor was applied in an air-cooled refrigeration system.

During this period, we studied the research work performed at SINTEF/NTH in Trondheim, Norway. The team in Trondheim had started around 1988 developing CO2 refrigeration systems for mobile applications (car air conditioning) and hot-water heat pumps in close cooperation with Hydro Aluminum. I applied for an exchange program at NTNU in Trondheim and was accepted to continue my studies within refrigeration in a Master Program. I graduated in 1996 and began my refrigeration research career at SINTEF.


//R744: How would you describe the progress made by CO2 refrigeration since you began working on it?

Armin Hafner: Since the early days of the revival of CO2 refrigeration, spearheaded by the team led by Professor Gustav Lorentzen, there has been some skepticism from industrial players about applying or converting to this natural high-pressure fluid.

In the beginning, before the turn of the millennium, the number of available components was rather limited. Due to the properties of CO2, the main components of a CO2 refrigeration system could not be taken from the shelf of low-pressure equipment for HFCs; they had to be adapted both with respect to safety and energy efficiency.

Due to the great effort of various research teams around the globe and public funding, it could be demonstrated that, when adapting the system layout to the properties of CO2, the CO2 systems are competitive with current technologies or even more beneficial. As an example, there are more than 5 million CO2 hot-water heat pumps installed in Japan. CO2 is an excellent working fluid for hot water production, if a high temperature lift is required and a low water-inlet temperature can be provided. Therefore, nowadays this kind of technology is also introduced to energy systems in high-performance buildings like hotels etc.

Another example is commercial refrigeration. Due to the excellent transport properties of CO2, energy losses can be significantly reduced in centralized refrigeration systems, which are common in Europe. The shop owners are not very much interested in the type of working fluid that is inside their display cabinets; they care about reliability and cost of ownership. Both are in favor for CO2 and even the safety of their customers and staff can be secured when applying this nonflammable natural working fluid.

Large efforts with great technical achievements have been made within mobile air conditioning (MAC) applying CO2 at the beginning of this century. Due to political decisions, personal preferences and lobbying, the number of systems in the market is limited; however, due to the introduction of new electric powertrain concepts a CO2 revival in this sector is very likely to happen.


//R744: Do you think that CO2 transcritical will continue to grow substantially in food retail or will another technology (hydrocarbons, HFO blends) prevent that growth?

Armin Hafner: Since the introduction of the CO2 refrigeration technology in supermarkets by Costan and Linde nearly 20 years ago, the annual growth rates in Europe have been very high. There is a limit to how many new systems can be produced and implemented every year, due to the production capacity of compressor manufacturers and others. Due to the enforcement of the EU F-Gas Regulation, the demand is even higher than what can be produced, which keeps the cost high and opens the door for alternatives. 

Hydrocarbons, utilized as working fluids in self-contained display cabinets, are a solution which should be applied as these are much more energy efficient than comparable HFC units. Hydrocarbon- based systems for small-format stores where the heat is rejected towards a water loop can be another low-cost solution with a natural working fluid.

Outside of Europe, there is a trend to implement CO2 refrigeration technology in large supermarkets. Most of the global players, which source their refrigeration equipment from global suppliers, do have a strategy to convert to CO2 in existing or new stores. The challenge is to find skilled contractors able to implement and service this technology. However, the know-how is being transferred and a growth in the number of CO2 transcritical systems will occur. Shop owners around the globe are business people; if they see a profit in implementing CO2 technology, they will do it. The environmental benefit will be a side product, which enables leap frogging from HCFC towards natural working fluids in some parts of the world.

Shop owners around the globe are business people; if they see a profit in implementing CO2 technology, they will do it," Armin Hafner

//R744: Do you see transcritical CO2 continuing to grow in industrial refrigeration and taking away market share from ammonia systems?

Armin Hafner: Transcritical CO2 refrigeration and heat pump applications are economically beneficial for industrial players. Due to the new developments of large-capacity compressors by several compressor manufacturers, it is nowadays possible to build industrial systems with  a reasonable number of compressors.

Traditionally, ammonia has a strong position within the industrial sector. CO2 must be seen to be complementary to these units, not as a competitor.

The market growth for refrigeration systems with natural working fluids is high, especially when the end user is informed about the personal and environmental risks related to HFCs of all kinds. Therefore, there is a need for – and growing share of – both ammonia and CO2 systems.

In cases when refrigeration systems have to be replaced, like inside fishing vessels, CO2 can offer the advantage of being more compact and being able to increase the cooling capacity without requiring more space for the equipment. At the same time, there are no critical issues related to HSE (Heath Safety Environment)

CO2 will also play an important role in some chiller applications, when both hot water and chilled water are required (hotels, etc.). If flammable working fluids cannot be applied, CO2 units can represent an efficient and reliable alternative, as demonstrated by enex in Italy.


//R744: Will transcritical systems that integrate refrigeration, AC and heating take over from refrigeration-only systems in commercial and industrial installations?

Armin Hafner: This will be a natural development, since the end user is very concerned about the total cost of ownership for installations. The shop owners are very much interested in integrated systems, if they own the entire building. A challenge in getting more of these integrated systems into the market is the structure of the companies. If one of them is responsible for the building and its operation, while another one takes care of the requirements for the shop, a common business model must be developed; this would allow them to invest commonly in an integrated unit, instead of independent units with different owners and service providers.

For industrial applications, the ownership situation is often clearer; therefore, most of the CO2 refrigeration systems can be classified as integrated units within energy processes.


//R744: CO2 has made very little headway in mobile air conditioning because of the dominance of R1234y. Do you see that changing, given the environmental concerns and flammability dangers associated with R1234yf? 

Armin Hafner: The new focus of the automotive sector towards powertrains with alternative energy sources to enable these vehicles to drive over long distances must include energy-efficient heating concepts. As most of the new vehicles will not rely on burning fuel in the future, the amount of surplus heat is very limited. The energy management is even more advanced, since different parts need to be cooled or heated to enable a safe and efficient drive.

Taking all these aspects and flammability/toxicity issues into consideration, the number of options for the active heating and cooling of both the passenger compartment and important parts are limited. CO2 has proven to be the champion, as we will see soon when these new vehicle platforms are introduced by the remaining OEMs.


//R744: What are CO2’s prospects in residential air conditioning and heating?

Armin Hafner: The market for residential AC and heating can be mostly covered on a global base with systems applying the natural working fluid R290. Propane is an excellent working fluid available and affordable on a worldwide basis for all people.

In the case of moderate climates, and modern houses where most of the heating demand is represented by hot water heating, CO2 units can become an option, as the Eco Cute systems have in Japan.


//R744: Much progress has been made in supporting transcritical CO2 in warm ambient climates with adiabatic gas coolers, ejectors, parallel compression, subcooling, and thermal storage. Should the “CO2 equator” issue be considered solved?

Armin Hafner: Yes this is definitely the case. Thanks to a great effort and common research and development there is no CO2 equator anymore. Nowadays the end user can choose among different energy-efficient solutions adapted to their special requests and needs. The important issue is that the systems are designed for the worst case – i.e. able to perform during the hottest hours of a year for the region where the unit is implemented. However, the efficiency advantage of CO2 is achieved during the main operating hours and part-load operation.

In addition, when integrating additional functions like AC, hot water, etc. the total cost of ownership is lower for CO2 systems made in serial production, i.e. large volume orders of standardized units.


//R744: Overall, where do you see the state of CO2 refrigeration in five years? What are the biggest hurdles still to be overcome?

Armin Hafner: The state-of-the-art CO2 refrigeration units by 2025 will be: hot-water heat pumps, commercial refrigeration packs, mobile air conditioning and heat pump units for low/zero emission vehicles, transport refrigeration (truck, trailer and containers), chilling and freezing systems for fishing vessels, integrated chillers providing hot water and cooling, and low-temperature industrial freezing applications for high- quality food.

The biggest hurdles will be: knowledge transfer and practical training on how to adapt and implement CO2 refrigeration systems in regions where technical skills are less based on organized educational programs; and raising awareness of end users to apply systems with natural working fluids.

If contractors provide objective safety information – explaining the content of the material date sheet and the consequences/risks – to end users when offering alternative systems, responsible managers will ask for units with natural working fluids.

By Michael Garry

Aug 30, 2019, 11:34




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