Briefs: CO2 heat pumps for train AC

A paper presented at the 9th Gustav Lorentzen Conference on Natural Refrigerants by A. Haffner, O. Christensen and P. Nekså1 envisaged the opportunity to use R744 (CO2) as a refrigerant in air-conditioning (AC) systems in trains. They claimed that in 2006, 75% of air-conditioning systems in trains used HFC-134a, which has a GWP of 1410, whereas R744 has a GWP of 1(0). The other 25% used HFC-407C. The rolling stock of the railway, tram and metro operators consisted of 175000 units, 65000 of which are equipped with air-conditioning systems. The total refrigerant charge in the railway sector amounts to 1605 tonnes CO2 eq. and the annual leakage rate averages 5%, which is lower than the 40% leakage rate in marine units but still represents substantial emissions. Estimations based on the assumption that these refrigerant ratios remain identical show that HFC emissions from the railway sector could double to 174 tonnes CO2 eq. by 2020 (while emissions from the marine sector will increase threefold to 1141 tonnes CO2 eq.) Using CO2 to replace these refrigerants would have a major positive effect on the environment. Three technical solutions were envisaged to switch back and forth from heating to cooling mode: by changing the direction of the refrigerant flow through the system, by changing the configuration of the air streams through the heat exchanger thanks to hatches, and lastly by rotating by 180° the whole heat pump, placed on a turntable unit. Experiments have proven that prototype R744 air-conditioning systems in railway passenger coaches have lower energy demands than R134a (52% of a traditional existing R134a system and 64% of an optimized R134a system). 1. A. Haffner, O. Christensen and P. Nekså: Reversible R744 Heat Pumps Applied in Public Trains