Thank you for your interesting question. I guess that you are speaking about ecoinvent IPCC GWP LCIA results, and comparing those to (usually) direct CO2 emissions in other references. As I will point out further below, I don’t agree that ecoinvent numbers are in general specifically higher than those from other sources if the same type of data is compared, and not pears to apples.
Pre-remark: I don’t have the quantitative evaluation of the electricity datasets in ecoinvent split into direct emissions of various greenhouse gases (GHG) and emissions of GHG related to the remaining life cycle. And I don’t have a written comparison of ecoinvent scores with existing references/literature, even if I of course cross-check our values with existing publications before release of a new version. But we are anyway planning to do such an evaluation and publish the results.
So our (as you say) quite obvious reasoning for the IPCC GWP scores in ecoinvent is of course that ecoinvent includes all GHG and not only CO2, plus includes indirect GHG emissions as well. Further, it is very important not to compare pears with apples. Numbers out there can be valid for very different things:
- Direct CO2 emissions only
- Direct emissions of some/all GHG
- Emissions related to a production mix (other than the supply mixes as modelled in the electricity market datasets)
- Data for different years (changes in key parameters such as efficiency, or change in electricity mix over the years)
- Including indirect emissions of GHG, but only very selective.
- As Bernhard Wohner points out, different allocation or also system boundary choices.
I don’t agree that ecoinvent scores for CO2(eq) emissions/kWh are always higher than numbers indicated in different sources – given that we compare the same fruits and not ananas to strawberries. Of course ecoinvent scores are higher than those e.g. given in the IEA report “CO2 emissions from fuel combustion”. That report gives the direct CO2 emissions related to electricity production in all countries. Here, we of course observe the effect of including life cycle data, all greenhouse gases, and electricity imports to a certain country leading to higher CO2eq/kWh numbers in ecoinvent than in the IEA report. The differences are actually in the same order as e.g. can be found in the following report, table “National and European emission factors for consumed electricity” or “standard CO2 emission factors and CO2-equivalent LCA emission factors…” [http://www.eumayors.eu/IMG/pdf/technical_annex_en.pdf].
In terms of the scores of individual electricity producing technologies, ecoinvent results for IPCC GWP are in general not particularly high or low, if the results are e.g. roughly compared to the NREL LCA harmonization study as shown on http://www.nrel.gov/analysis/sustain_lcah.html and in the related special issue of the Journal of Industrial Ecology (Meta-Analysis of Life Cycle Assessments) http://onlinelibrary.wiley.com/doi/10.1111/jiec.2012.16.issue-s1/issuetoc
Hope this helps.
Karin (editor of the electricity sector)