Miljøprojekt, 1073/2006 – Sammenligning af energiforbruget til køling i supermarkeder med transkritisk CO2 og kaskade system

Sammenligning af energiforbruget til køling i supermarkeder med transkritisk CO2 og kaskade system

Summary and conclusions

Basically, CO₂ is inferior to conventional refrigerants as regards COP in applications. However, due to a number of factors conventional refrigerants are not utilised fully and therefore CO₂ can in many cases compete with regard to COP and in that way energy consumption. The result of the investigation shows that the examined transcritical system under Danish conditions has an energy consumption that is comparable with cascade systems and conventional systems with HFC refrigerants.

The results show that in applications the energy consumption for a transcritical system is app. 4 % lower than for a cascade system. That is because we in Denmark only have app. 50 hours with temperatures above 25°C. At 25°C the efficiency is almost the same for the two systems.

For a freezing system, the results show that the transcritical system in Bellinge uses app. 10 % more energy than a corresponding cascade system. However, the high energy consumption appears as the applied construction has not been optimised. By using a system design that has been optimised for low temperatures the results show that it will not be possible to reduce the energy consumption to a level that is app. 4 % lower than in connection with a cascade system.

There are especially two reasons why it is possible for a transcritical system to obtain an energy consumption that can be compared with a cascade system or a conventional system.

First of all, a CO₂ system can utilise the gas cooler area better during transcritical operation than what is possible during condensing operation. In practice that means that the temperature difference between air into the gas cooler and CO₂ out of the gas cooler is app. half of what it would be for a condensing refrigerant.

Secondly, in connection with transcritical CO₂ systems it is necessary to control the high pressure and therefore a sophisticated control system is required. Such a control system is not currently used in conventional systems and that gives them a higher energy consumption than what is possible in connection with more advanced control.

The implementation of that type of control on a cascade or conventional system would also substantially reduce the energy consumption compared with today.