Environmental indicators
To evaluate the toxicity of a gas in terms of greenhouse effect, an index is used known as Global Warming Potential (GWP). It represents the relative ability of a greenhouse gas to participate in radiative forcing, and is 1 for CO2.
The contribution of a gas to the greenhouse effect depends on the one hand on the amount of emissions, and on the other hand on two factors taken into account in the calculation of GWP: its absorption properties in the infrared and its residence time in the atmosphere.
This last factor being very variable from one gas to another (several hundred years for CO2, between 10 and 15 years for CH4), it is necessary to agree on an appropriate integration time horizon (ITH). The GWP can then be determined by calculating the cumulative radiative forcing on the ITH chosen. The IPCC has conducted assessments of GWP on ITH of 20, 100 and 500 years.
As is done for greenhouse gas emissions, assessing the harmfulness of a gas in terms of destruction of the ozone layer is performed using an index known as Ozone Depletion Potential (ODP). This index is 1 for CFC 11.
Properties of some pure refrigerants
name | formula | ODP | GWP | M | Tb | Tc | Pc | gamma |
kg/kmol | °C | °C | bar | |||||
R11 | CFCl3 | 1 | 4,000 | 137.4 | 23.8 | 198 | 44.1 | 1.13 |
R 12 | CF2Cl2 | 1 | 8,500 | 120.9 | -29.8 | 111.8 | 41.1 | 1.14 |
R 13 | CClF3 | 1 | 11,700 | 104.5 | -81.4 | 28.8 | 38.7 | 1.17 |
R 22 | CF2HCl | 0.06 | 1,700 | 86.5 | -40.8 | 96.2 | 49.9 | 1.18 |
R 123 | CHCl2CF3 | 0.02 | 93 | 152.9 | 27.9 | 183.7 | 36.7 | 1.08 |
R 32 | CH2F2 | 0 | 580 | 52 | -51.7 | 78.2 | 58 | 1.24 |
R 125 | CF3CHF2 | 0 | 3,200 | 120 | -48.1 | 66.3 | 36.3 | 1.1 |
R 134a | CF3CH2F | 0 | 1,300 | 102 | -26.1 | 101.1 | 40.6 | 1.1 |
R 717 | NH3 | 0 | 0 | 17 | -33.3 | 133 | 114.2 | 1.31 |
R 290 | CH3CH2CH3 | 0 | 3 | 44.1 | -42.1 | 96.8 | 42.5 | 1.14 |
R 600a | CH(CH3)3 | 0 | 3 | 58.1 | -11.7 | 135 | 36.5 | 1.11 |
R 744 | CO2 | 0 | 1 | 44 | 78.4 | 31.1 | 73.7 | 1.3 |
R 718 | H2O | 0 | 0 | 18 | 100 | 374.2 | 221 | 1.33 |
For complex technologies like those used for refrigeration, the latter index is not sufficient, because firstly the refrigerants are normally not directly released into the atmosphere, and secondly there is an indirect effect that is due to CO2 emissions corresponding to the energy consumed by the plant during its lifetime, which itself depends on the structure of the national electricity generation fleet. We therefore define another index, known as Total Equivalent Warming Impact (TEWI).
The TEWI takes into account according to equation (1):
the direct impact based on refrigerant charge m,
gas leakage during the lifetime n (annual percentage f) and at the machine end of life (recycling factor a),
and indirect impact due to energy consumption E.
TEWI = GWP . f . m . n + GWP . m (1 - a) + n . E . b (1)
To fix ideas, f = 5 to 10%, a =0.5, n = 15, b >= 0.1 kg/kWh for France, and b = 0.55 kg/kWh for Europe, the difference reflecting the capacity of the French nuclear power plants.
The calculation of equation (1) may seem a bit complex, but it allows one to compare very different technologies and to account for both direct and indirect effects. As shown in this formula, the value of TEWI can be decreased by reducing the refrigerant charge of facilities (m) and increasing their containment (f), which justifies the efforts being made in this direction by manufacturers.
