Demonstration of free cooling with CO2.

The use of CO2 as a pumped secondary volatile heat transfer medium is demonstrated for a server room application. The condenser of the primary refrigerating system using propane comprises an additional air cooled heat exchanger for condensing CO2 when the ambient temperature is sufficiently low. This free cooling arrangement reduces the compressor operation time to less than 30% of the year in the Danish climate. The performance has been measured and recorded by utilising the building management system and the refrigeration plant controller, and it is evaluated against the design conditions. Characteristics of the control system are analysed, and observations during the first years of operation are reported. Measurements confirm that that the free cooling system alone will maintain the design cooling capacity of 100 kW supplying air at 25°C to the server room, when the ambient temperature is 13°C or lower. The design pressure is 70 bar for the CO2 cycle and evaporation temperatures close to 20°C have been recorded at part load conditions.