The evolution of CO2

Since the early days of the revival of CO2 refrigeration, there has been some scepticism from industry players about applying or converting to this natural high-pressure fluid. 

CO2 001A Norwegian professor predicts that CO2 transcritical systems will gain popularity over the next few years. Image credit: Accelerate24

Professor Armin Hafner of the department of energy and process engineering at the Norwegian University of Science and Technology looks at the use of CO2 through the decades.

With early records going back as far as the 1980s, Norwegian Professor Gustav Lorentzen, a thermodynamic scientist, is known as the father of the modern transcritical cycle for CO2. He rediscovered how CO2 could be used as a refrigerant in heating and cooling applications and developed the modern thermodynamic transcritical cycle in 1988-1991.

In 1988 Lorentzen designed a concept for a new, but simple and efficient way of regulating CO2 systems. This idea became the turning point in the re-invention of CO2 cooling technology. Meanwhile, the Japanese corporation Denso had familiarised itself with Lorentzen’s dissertation in 1993 and was evaluating the concept as a basis for a new air-condition application in cars. A series of communications between Lorentzen and Denso followed, and the result of the collaboration was a fundamental step in the innovation of EcoCute, which was commercialised in 2000s.

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 hydrofluorocarbons (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 five 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.

Another example is commercial refrigeration. Due to the excellent transport properties of CO2, energy losses can be significantly reduced in centralised refrigeration systems, which are common in Europe. Shop owners are not very interested in the type of working fluid that is inside their display cabinets; they care about reliability and cost of ownership. Both are in favour for CO2 and even the safety of their customers and staff can be guaranteed when using this non-flammable 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.

Transcriticals in supermarkets

Since the introduction of the CO2 refrigeration technology in supermarkets by Costan and Linde nearly 20 years ago, the annual growth rate 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.

Outside of Europe, there is a trend to implement CO2 refrigeration technology in large supermarkets. Most of the global players, who 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.

Transcritical CO2 refrigeration and heat pump applications are economically beneficial for industrial players. However, 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. Therefore, there is a need for – and growing share of – both ammonia and CO2 systems.

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

Transcritical systems in HVAC

There will be a natural development to transcritical systems that integrate refrigeration, air conditioning and heating from refrigeration-only systems in commercial and industrial installations. Since the end user is very concerned about the total cost of ownership for installations, shop owners are very interested in integrated systems.

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.

The CO2 equator issue is solved thanks to the support made in transcritical CO2 in warm ambient climates with the use of adiabatic gas coolers, ejectors, parallel compression, subcooling, and thermal storage. 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 scenario. In addition, when integrating additional functions like air conditioning and hot water, the total cost of ownership is lower for CO2 systems made in serial production.

Industry challenges

By 2025, the state-of-the-art CO2 refrigeration units will be: hot-water heat pumps, commercial refrigeration packs, mobile air conditioning and heat-pump units for low to 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 organised educational programmes; 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 and risks – to end users when offering alternative systems, responsible managers will ask for units with natural working fluids.

Source: r744 






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