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Chapter 2 CO2 and the transcritical cycle 2.1 Refrigerant progression history Natural substances (Ammonia, Carbon Dioxide etc) were used in the early applications right after the start of refrigeration industry however this was the discovery of synthetic refrigerants that declined and eventually obsolete the use of natural refrigerants. Synthetic refrigerants were considered to be more safe and more energy efficient at that time while natural substances had their own limitations. These were the excellent thermodynamic and thermophysical properties that helped halogenated refrigerants to have dominance in the refrigeration and air-conditioning field. As the time passed real consequences of halogenated refrigerants become clear. Major environmental problems were due to emission of chlorine and fluorine atoms present in these refrigerants [3]. Followings are the major environmental problems with conventional synthetic refrigerants, 2.1.1 Ozone Layer Depletion Ozone layer filters highly intense harmful ultraviolet radiation to harmless radiation for plant and human life on earth. Unfiltered ultraviolet radiations react with the CFC molecules and disintegrate them. The chlorine atom removed from CFCs reacts with ozone molecule and convert it to oxygen molecule, so in this way protective ozone layer is getting thin and allowing more ultraviolet radiation to reach the earth surface [7]. 2.1.2 Global warming The second major problem associated with refrigerants is of global warming, it is measured with an index called global warming potential (GWP). GWP gives the ability of heat trapped by the greenhouse gas in the atmosphere, CO2 is considered as a reference with GWP of unity. Greenhouse gases absorb the infrared radiation from the earth and thus increases the earth surface temperature. CFCs were initially replaced with HFCs to protect the ozone layer, later on HFCs found to be involved in global warming, research revealed that HFC’s have relatively long atmospheric life span and high values for GWP in comparison with that of chlorine based refrigerants [3]. GWP and ODP of natural refrigerants (R717, R744) are shown in figure 6 with other synthetic refrigerants. 5PDF Image | heat recovery from R744 based refrigeration system
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