Carbon Dioxide was widely used in all manner of refrigeration systems for cooling, chilling and freezing for about 60 years from the mid 1870's onward, but its decline started with the arrival of CFC's in the 1930's. In this paper, it is shown that carbon dioxide used as a refrigerant in both office and hospital air conditioning cooling and heating, and industrial refrigeration applications offers the potential to save considerable amounts of energy when used as a standard refrigerant in a system capable of running in both subcritical and transcritical mode, and attendant CO_2 emissions and cooling water. The high pressure and low critical point characteristics allow CO_2 to be used in the sub-critical mode at a low condensing temperatures within 3 to 5°C of the ambient air used for condenser cooling. Above the low critical point of 31.1 (73.83 bara) transcritical operation is a distinct advantage, as it becomes possible to extract heat at a gliding temperature from the transcritical compressor discharge. In many applications where simultaneous heating and cooling is part of the process, cooling and a lot of heating may be extracted from the same source. This reduces energy and cooling water consumption and attendant CO_2 emissions. There are also some capital cost offsets. Using USA department of Energy and Australian Greenhouse Office data for energy consumption for buildings in the USA and Australia, this study shows that CO_2 cooling has the potential to reduce primary energy consumption, combined cooling water consumption at the power stations and cooled buildings, and CO_2 greenhouse gas emissions by about 40 to 50%, 40 to 50% and 45% to 55% respectively in the existing building stock when coupled with a 25% reduction in duct air velocity and greater savings when implemented in new buildings. Incorporation of energy recovery from exhaust air and economizing cycles will produce additional reductions in all areas. The absence of cooling towers entirely eliminates the danger of legionella disease and because CO_2 is used as a refrigerant, there is no chance of HFC fugitive gases escaping into the environment.
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