In addition to being toxic or explosive and thereby dangerous to people's health, there are other problems associated with refrigerants. Environmental aspects are increasingly being taken into consideration. Refrigerants can thus also be ranked according to their impact on the stratospheric ozone layer (the Ozone Depletion Potential, ODP) or as greenhouse gases (the Global Warming Potential, GWP).
Ozone Depletion Potential, ODP
Refrigerants containing chlorine or bromine contribute to the breakdown of the ozone layer (see Figure 5.1). The reaction is as follows:
However, the CIO molecule is unstable. It breaks down and reacts with ozone molecules (in accordance with the equation above) repeatedly until a more stable compound is created.
The ODP is the ratio of the impact on ozone of a chemical compared with the impact of a similar mass of CFC-11 (R11). Thus, the ODP of CFC-11 is 1.0 by definition. Other CFCs and HCFCs have ODPs ranging from 0.01 to 1.0. The halons have ODPs ranging up to 10. Carbon tetrachloride has an ODP of 1.2, and methyl chloroform's ODP is 0.11. HFCs have zero ODP because they do not contain chlorine.
Global Warming Potential, GWP
Due to their stability in the atmosphere, CFCs as well as HCFCs and HFCs are often very effective greenhouse gases. The GWP factor is used to reflect their impact on global warming.
The GWP is the ratio of the warming caused by a substance to the warming caused by a similar mass of carbon dioxide. Thus, the GWP of CO2 is 1.0 by definition. CFC-12 has a GWP of 8500, while CFC-11 has a GWP of 5000. Various HCFCs and HFCs have GWPs ranging from 93 to 12100. Water, a substitute in numerous end-uses, has a GWP of 0.
When using GWP values from different sources, it is important to consider that the values may differ due to different integration times or calculation models.
Another measurement of the impact on global warming is the TEWI value (Total Environmental Warming Impact). This includes not only the direct impact of any release of the refrigerant (GWP), but also the impact during the generation and use of primary energy in the system.