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Selecting and Using GWP values for Refrigerants – 2015 Updated version

Selecting and Using GWP values for Refrigerants – 2015 Updated version

07.09.2015

Summary and Recommendations

Refrigerant GWP values have been revised with each new IPCC Assessment Report. Regulations and guidance for the use of GWP values typically use the most up to date GWP values.
The F-Gas Regulation 517/2014 uses Fourth Assessment Report AR4 values and has measures that use GWP limits for applications and servicing. In addition it has a cap and reduction, based on CO2 equivalents, for the HFCs that can be placed on the market, using quotas issued to producers and importers of HFCs.

The GWP values used in the F-Gas Regulation are in the downloadable logbook that can be used to meet the servicing and maintenance requirements.

 

The previous F-Gas Regulation 842/2006 used IPCC Third Assessment Report TAR GWP values. In the future it is expected that IPCC Fifth Assessment Report (AR5) GWP values may be used for future regulations or revisions of current regulations.
All the IPPC Assessment Report values are available here.
For refrigerant users and equipment designers, the values contained in the F-Gas Regulation 517/2014 annex (IPCC Fourth Assessment Report AR4 GWP values) are the most appropriate at present.
Companies designing refrigerants should also be aware of any potential impacts from the use of AR5 values. The Commission is empowered by the F-Gas review article to update the GWPs of the substances listed in the F-Gas Regulation annexes.
Companies voluntarily reporting their emissions as part of their corporate social responsibility reporting could select IPCC Second Assessment Report SAR, TAR or Fourth Assessment Report AR4 values. However it is essential that only one database is used to ensure consistency and to enable trends in emissions to be clearly seen. The source of the GWPs should be referenced. Any change in GWP source values should be noted and the emissions restated when the change is made. The use of 20 and 500 year time horizon GWPs is not recommended.

Global Warming Potential (GWP)

Simple Definition

The GWP of a refrigerant is its global warming impact relative to the impact of the same quantity of carbon dioxide over a 100 year period.

Complete Definition

The Global Warming Potential of a refrigerant is defined as the integrated radiative forcing over a “time horizon” of 100 years following an assumed release of 1kg divided by the integrated radiative forcing over the same period from release of 1 kg of carbon dioxide. Radiative forcing is the specific increase in infrared absorption in Wm 2ppb 1 (Watts per square metre at the Earth’s surface per part per billion concentration of the material). All effects beyond 100 years are disregarded; thus Global Warming Potential captures all of the effect of an HFC but less than 40% of the total effect from CO2.

Why are GWPs used

Basically, the intention is to put all greenhouse gas emissions onto a common scale and GWP, however imperfect, remains the recommended metric to compare future climate impacts of emissions of long-lived gases.

The adequacy of the GWP concept has been widely debated since its introduction; uncertainties and changes arise from the models used to calculate radiative forcing.
 
However, the main problem arises from using CO2 as the reference gas because of the very long “tail” on its atmospheric lifetime.  This is shown in the chart, which compares rates of removal of greenhouse gases from the atmosphere.
The Intergovernmental Panel on Climate Change (IPCC) have concluded that “However as long as it has not been determined, neither scientifically, economically nor politically, what the proper time horizon for evaluating ‘dangerous anthropogenic interference in the climate system’ should be, the lack of temporal equivalence does not invalidate the GWP concept or provide guidance as to how to replace it.”

The 100 year time horizon for GWPs

Wide variations in GWPs may be quoted and mis-used for HFCs. GWP values for time horizons of 20, 100 and 500 years are published by IPCC in their Assessment Reports and the values change between the reports, which are produced roughly every five years.

 

GWPs are calculated relative to CO2 up to the “time horizon”, with all effects beyond that time period being disregarded. Because CO2has an atmospheric lifetime much longer than HFCs, then a shorter time horizon results in higher GWPs for HFCs. However the most commonly used HFCs are removed from the atmosphere quickly compared to CO2 so that short time horizons overstate their relative contribution to global warming.
This is why 100 year time horizon was selected to provide an appropriate compromise between short and long term effects.

Uncertainty for GWP Values- why they have changed over time

GWP values have been refined over the past two decades with the development of atmospheric science. As GWPs of refrigerant are relative to CO2, any change in the calculated global warming impact (radiative forcing) of CO2 directly affects the refrigerant GWP.

In addition, GWPs also depend on the atmospheric lifetime and infra-red absorption spectra (radiative efficiencies) of the refrigerants.  Atmospheric lifetime is linked to the reaction rates for the various processes that convert the refrigerant into very low GWP breakdown products and improved knowledge about atmospheric science and radiative efficiencies for HFCs has led to revisions in their GWPs.

Uncertainty in refrigerant GWP is stated to be ±35%, a value that has remained substantially unchanged singe the Second Assessment Report (SAR). Uncertainties in refrigerant GWPs are dominated by the uncertainty in the reference gas, CO2.

The GWPs listed in the Assessment Reports for 100 year time horizons are shown in the table for the most widely used HFCs. Also shown are the GWPs for methane and nitrous oxide.

 

How important are the differences in GWP

Time Horizon

The use of 100 year time horizons is recommended and indeed these are the only GWPs referenced in the Kyoto Protocol and F-Gas Regulation for compliance and reporting purposes.
GWPs at a 20 year time horizon are sometimes quoted to accentuate the contribution of HFCs to global warming. Using 20 year time horizons for HFCs distorts the relative contribution of CO2 (over 90% of it is ignored) and does not contribute to an informed and objective assessment of the use of HFCs.
Similarly 500 year time horizon GWPs should not be used for HFCs as they do not reflect the agreed balance between short and long term effects.

It is not permissible, from scientific or legal points of view, to mix GWP time horizons or to cherry pick values from the databases in different IPCC Reports.

The GWPs for 20,100 and 500 year time horizons are shown in the table for the most widely used HFCs.  Because AR5 does not contain values for 500 year GWPs, the figures given here are from AR4

Fourth Assessment Report (AR4) GWPs

Time Horizon

20 years 100 years 500 years
HFC-32 CH2F2

2330

675

205

HFC-125 CF3CHF2

6350

3500

1100

HFC-134a CF3CH2F

3830

1430

435

HFC-143a CF3CH3

5890

4470

1590

HFC-152a CH3CHF2

437

124

38

Using IPCC Assessment Report GWP values

Comparing technology options

 

The GWPs of the widely used HFC refrigerant components have changed by a maximum of 26% between the SAR values (1995) and the AR5 values (2013). These changes have essentially no impact on decision making when used for TEWI (Total Equivalent Warming Impact) or LCCP (Life Cycle Climate Performance) calculations, which are used to compare alternative technology options.

Reporting to UNFCCC and compliance for Parties to the Kyoto Protocol

The Rio Convention (United Nations Framework Convention on Climate Change – UNFCCC) references Second Assessment Report GWP values for the purposes of national reporting of greenhouse gas emissions (national “Greenhouse Gas Inventories”). This arrangement is expected to continue. The same values were used during the first commitment period of the Kyoto Protocol to this Convention. It is probable that different arrangements will apply when the Protocol is renegotiated but, as any reductions must be referenced back to a baseline period, any change in GWPs will require a revision of the baseline emissions.  It is worthwhile noting that the GWPs of two of the major GHGs – methane and nitrous oxide (N2O) – have changed by +43% and –15% respectively between SAR and AR5. These changes would also impact on any baseline revisions, so it is not just an issue for HFCs.


[1] The first 2/3 of a CO2 emission is removed from the atmosphere relatively quickly (within 100 years or so). The other 1/3 remains for several thousand years. This affects the choice of time horizon.

[2] From IPCC AR4 Chapter 2 Changes in Atmospheric Constituents and in Radiative Forcing 2.10.1 Definition of an Emission Metric and the Global Warming Potential pages 210 and 211
   

Related Document fs-18-selecting-and-using-gwp-values-for-refrigerants-revised-17-june-2015 (pdf - 120.88 Kb)