Non-Topical Article: Reversible CO2 Heat Pump with Ejectors for Efficient Heating and Air-Conditioning

CO2 first appeared as a refrigerant in the middle of the 19th century. In the first half of the 20th century, CO2 was replaced by the so-called safety refrigerants. As the ozone hypothesis first emerged in 1974, politicians reacted and adopted the Montreal Protocol in 1987. Since then, regulations and bans on synthetic refrigerants have been steadily increased. In the meantime, we arrived at the fourth generation of synthetic refrigerants, the so-called HFOs. These are currently being propagated by the chemical and refrigerant industry as a supposed solution, although their decomposition products can accumulate in surface water, are difficult to degrade and can thus also end up and cumulate in drinking water. It is to be expected that the HFO refrigerants, like their three previous generations, do not represent a long-term solution and the only future-proof refrigerants are the natural ones. These are mainly carbon dioxide (CO2), ammonia (NH3) and the group of hydrocarbons (propane, etc.).

A renowned shopping center in the area of Lucerne, Switzerland, was modernized in 2019/2020. In this context, the existing fossil heating and air conditioning system was replaced by two reversible air-to-water CO2 heat pumps with ejectors. The two heat pumps cover a heating capacity of 1.1 MW and an air conditioning capacity of 1.3 MW. Higher investment and life cycle costs were accepted in order to develop sustainable and forward-looking technologies with natural refrigerants. The project was supported by the Swiss Federal Office of Energy. Since the commissioning of the heat pumps, their operation has been investigated, and valuable experience has been gained.

The commercial refrigeration system of the supermarket within the shopping mall is based on a CO2 booster system with ejectors. The waste heat of the CO2 booster system is used in priority for domestic hot water and facility heating. Any additional heating or air-conditioning demand is covered by the two reversible air-to-water CO2 heat pumps. The reversible heat pumps are also equipped with ejectors to eventually achieve the highest possible efficiency.

In summary, the reversible air-to-water CO2 heat pumps with ejectors have been operating successfully since autumn 2019. Valuable knowledge has been gained about their operating behaviour and the individual operating modes. In the meantime, a follow-up Jobsite based on the same CO2 technology has already been successfully commissioned. The measured increase in efficiency at the mentioned follow-up Jobsite underline that relevant conclusion was drawn from the first Jobsite and that these were successfully incorporated.

The outlook to the follow-up Jobsite confirms that adjustments in the system design and operation result in an increase in the COP of around +8 % compared to the model. It can be assumed that a further increase in efficiency can be achieved in further follow-up Jobsites. In addition to the energy optimizations, the system was simplified, and investment and operating costs were reduced. Considering the ecological concerns with synthetic refrigerants, as well as the fact that CO2 is neither toxic nor flammable and, finally, the progress achieved in terms of efficiency, makes the authors confident that reversible air-to-water CO2 heat pumps with ejectors will play a relevant role in future for heating and air-conditioning of modernized buildings.


Jonas Schönenberger, Head of Research and Development, Frigo-Consulting AG

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