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Mr Sonnekalb
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Carbon dioxide (CO2) is 1,430 times less harmful to the climate than the conventional hydrofluorocarbon (HFC) refrigerant R134a. Rapid substitution of synthetic HFC with CO2 is desirable from a climate protection point of view. As CO2 is a high-pressure refrigerant, the air-conditioning systems must be designed for the transcritical pressure range (above 73 bar). New gaskets, oils and materials have been developed so the system can be used without problems in the small high speed vehicle compressors in use today. The so-called transcritical operating method offers adequate performance and efficiency when operating these air-conditioning systems at ambient temperatures of 50°C. Where the operating method is subcritical, its performance and efficiency decreases strongly at ambient temperatures of less than 25°C.
The result of the transcritical operating method is that the refrigerant does not have a liquid phase at the high-pressure stage, but remains in the form of supercritical gas. This requires a different system architecture. The redesign of refrigerant-bearing components has resulted in smaller dimensions, making their use in vehicles feasible. On the other hand, the high operating pressures of the CO2 cooling system place a high load on the gaskets, compressors and heat exchangers used.


  • Carbon dioxide has the greatest thermal output potential of all refrigerants and can be used within a wide evaporating temperature range from +20°C to –55°C.

  • Substitution of synthetic HFCs with CO2 is an important measure for climate protection.

  • By replacing HCF with CO2, the emission of greenhouse gases can be reduced without jeopardising jobs in the affected industries.

  • The level of hazardous substances emitted and product life cycle costs are reduced.

  • Increase in passenger comfort on buses and trains.

Innovative aspects (ecologic, social and economic)

Unlike conventional air conditioning systems, the transcritical CO2 system can also provide heating by changing to heat pump mode. In relation to primary energy, the heat pump operates 30-80 % more efficient than a conventional engine-independent air heating system.

Application examples


National Programme Funds

Current stage of development

Already on the market

Intellectual Property Rights

Patent granted

Country of origin


Country of application



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