ODP _ GWP

ODP _ GWP _TEWI

Ozone Depletion Potential (ODP)

Relative value that indicates the potential of a substance to destroy ozone gas as compared with the potential of chlorofluorocarbon-11 (CFC-11) which is assigned a reference value of 1. Thus, a substance with ODP of 2 is twice as harmful as CFC-11.

Global Warming Potential (GWP)

GWP is perhaps the most commonly used environmental metric. GWP is the index, which compares the global warming impact of an emission of a greenhouse gas in relation to the impact from the emission of similar amount of CO2. The impact is estimated during a time horizon. A time horizon of 100 years is most adopted and normally assumed when no information on time horizon is given (including the values cited in this paper). GWP is an easy metric to use. The smaller the GWP, the lower contribution of a substance to the global warming. Based on GWP criteria the R290 (Propane) and R1270 (Propene) can be considered as the most environmentally friendly refrigerants while the R1234yf is less favorable. R152a, in turn, is the least environmentally friendly refrigerant out of the four candidates, but still its contribution to the global warming is much lower than that of the reference R410A.

Total Equivalent Warming Impact (TEWI)

In addition to the direct impact of the refrigerant (which is conveniently estimated by GWP), any system or process, which requires energy input, indirectly affects the environment. This impact is originated from CO2 emissions from the energy production processes.

TEWI metric is more indicative than the GWP.

In order to indicate the overall environmental impact from a refrigeration system during it operation, another environmental indicator, named TEWI, is used. TEWI accounts for the global warming impact from both direct and indirect emissions and calculated as a sum of both: direct effect of refrigerant released during the lifetime of the equipment and the indirect impact of CO2 emissions from fossil fuels used to generate energy to operate the equipment throughout its lifetime. TEWI can be calculated using the equation below (UNIDO 2009):

TEWI = direct emissions + indirect emissions = (GWP×L×N)+(Ea×β×n),

where

L – annual leakage rate in the system, kg (3% of refrigerant charge annually),

N – life of the system, years (15 years),

n – system running time, years (based on weather data, 4910 hours),

Ea – energy consumption, kWh per year (modelled for each refrigerant),

β – carbon dioxide emission factor, CO2-eq. emissions per kWh (165 g CO2/kWh).