GLOBAL WARMING POTENTIAL

Global warming is possibly the most severe environmental issue faced by civi­lization today. The risk posed by its effects has been described in terms of envi­ronmental disaster due to huge future climate changes. Global warming is the increasing of the world’s temperatures, which results in melting of the polar ice caps and rising sea levels. It is caused by the release into the atmosphere of so-called ‘greenhouse’ gases, which form a blanket and reflect heat back to the earth’s surface, or hold heat in the atmosphere. The most infamous greenhouse gas is carbon dioxide (CO2), which once released remains in the atmosphere for 500 years, so there is a constant build-up as time progresses. The exact extent of the contribution arising from man’s activities may be uncertain, but in any case it is vital to keep it to a minimum and conserve fossil fuel reserves,

I. e. minimize greenhouse gas emissions.

A major cause of CO2 emission is in the generation of electricity at power stations. The CO2 emission factor (kg of CO2 emitted per kWh of electric­ity supplied) is dependent on the UK fuel mix for electricity generation. For coal-fired power stations, the figure is relatively high, for gas-fired stations it is lower and for hydroelectric, wind power or nuclear stations it is zero.

Electricity suppliers may claim various mixes of generation type and hence differing emission factors, but the best presently available average UK figure is 0.422kg CO2/kWh (TEWI Guidelines, IOR/BRA). This value is an average of predicted values for 2005 and 2010. It is estimated that refrigeration compres­sors in the UK consume 12.5 billion kWh per year.

The global warming potential (GWP) of a gas may be defined as an index comparing the climate impact of its emission to that of emitting the same amount of carbon dioxide. The integrated effect over a fixed time allows for time decay of the substance. A time horizon of 100 years is usually adopted, although this is much less than the lifetime of CO2 in the atmosphere. The refrigerant only affects global warming if released into the atmosphere.

The GWP values for HFC refrigerants can be seen in Table 3.1, for exam­ple, R134a has a GWP of 1300, which means that the emission of 1 kg of R134a is equivalent to 1300 kg of CO2, The choice of refrigerant affects the lifetime warming impact of a system and the term total equivalent warming impact (TEWI) is used to describe the overall impact. It includes the effects of refrigerant leakage, refrigerant recovery losses and energy consumption. TEWI should be calculated when comparing system design options for specific appli­cations. Comprehensive method details with calculation examples are given in the Guidelines. Figures 3.3 and 3.4 show the equation used and an example for a medium temperature R134a installation.

The largest element of the TEWI for the vast majority of refrigeration and air-conditioning systems is energy consumption. Figure 3.4 shows the domi­nant effect of the energy consumption element, which if increased by 10% has a similar effect to a doubling of the refrigerant charge and leakage. Column (a)

TEWI	TOTAL EQUIVALENT WARMING IMPACT (GWP X L X n) + (GWP X m [1 — arecovery] + (n X Eannual X P) |<- Leakage -»{«- Recovery losses -»{«- Energy consumption — H
TEWI
Indirect global -*| warming potential
|<- direct global warming potential

GWP L N M ^recovery Eannual Я

Global warming potential [CO2-related]

Leakage rate per year [kg]

System operating time [Years]

Refrigerant charge [kg]

Recycling factor

Energy consumption per year [kWh] CO2-Emission per kWh (Energy-Mix)

Figure 3.3 Method for calculation of TEWI values

200

Ј 150 O

100

50

Evap. temp. Condensing Temp. M L Cooling load E

-10°C 40°C 10 kg 1 kg 13.5 kW 6 kW (X5000 h/a) 0.422 kg CЬ2/kWh 0.75 15 years 1 300

10 kg, E = 6 20 kg, E = 6 10 kg, E = Refrigerant charge, Energy

Figure 3.4 Comparison of TEWI values, data corresponds to the effect of increased refrigerant charge and increased power consumption

0

N GWP

Data for (a) Medium temperature R134a

250

Shows the baseline data, with the effect of double charge and 10% energy con­sumption increase in columns (b) and (c), respectively. The less the amount of energy needed to produce each kW of cooling the less will be the effect on global warming.

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