The five studies I've chosen to summarize here are referenced at the end of this post. The studies were conducted by a variety of organizations for different purposes over a 9-year period between 1998 and 2006. I've summarized the results of the five studies in the graphic below, which displays the range and the average value of the emissions estimates from the five studies for each of the generation types. All generation types were not evaluated by each study, and in some cases (Wave/Tidal and Oil) only point estimates were give. (Note to reader: g CO2/kWh = kg CO2/MWh = MT CO2/GWh). You'll probably need to click on the image to see the expanded version if you want to read the values from the chart.
Several conclusions can be drawn from the chart above and from a more detailed review of the actually reports from which the data are taken:
(1) there is no significant difference between the life-cycle carbon footprint of hydro, nuclear, and wave/tidal power; and all three electricity generation sources are substantially better (by two orders of magnitude or more) than coal and natural gas.
(2) wind and biomass have ~ twice the CO2 emission footprints of hydro, nuclear, and wave/tidal - but still far superior to natural gas, coal, and oil
(3) solar-PV appears to have a significantly higher CO2 emission footprint than hydro, nuclear, wave/tidal, wind and biomass. There's some interesting details in the analysis. While the majority of studies place it's CO2 footprint near that of hydro, nuclear, and wave/tidal, some studies estimate a significantly higher footprint (hence the range shown the plot). Some of this may be due to different assumptions regarding the specific solar-PV technologies employed, differing assumptions regarding the deployment location of the solar-PV systems,and some may be due to the specific analysis methodologies employed. I find it difficult to believe solar-PV's CO2 footprint could be even close to that of natural gas, but I need to understand this better.
(4) the three fossil sources (natural gas, coal, and oil) are all problematic unless/until we overcome the carbon capture/storage challenge . Breakthroughs in carbon capture and sequestration technologies for these fossil-driven electricity sources would tremendously improve our chances of achieving the global green house gas emission reductions we need.
Last words... if you're interested in reducing CO2 emissions, and you believe in dealing with the facts, you must give serious thought to the "low-carbon portfolio": hydro, nuclear, wave/tidal, geothermal, and biomass. I'm not putting solar in this category just yet. Solar probably belongs in the low-carbon portfolio, but I want to understand the wide variation in CO2 emissions footprints noted above, before I draw that conclusion..
1. "ExtremE - Externalities of Energy. National Implementation In Germany,"Krewitt, Mayerhofer, Friedrich, et al., IER, Stuggart, 1998
2. "Hydropower-Internalized Costs and Externalized Benefits," Frans H. Koch, International Energy Agency (IEA), Implementing Agreement for Hydropower Technologies and Programs, Ottawa, Canada, 2000
3. "A guide to life-cycle green house gas (GHG) emissions from electric supply technologies", Daniel Weisser, PESS/IAEA, IAEA Bulletin 2000
4. "Life-Cycle Assessment of Electricity Generation Systems and Applications for Climate Change Policy Analysis," Paul J. Meier, University of Wisconsin - Madison, August, 2002 (http://fti.neep.wisc.edu/pdf/fdm1181.pdf)
5. "Carbon Footprint of Electricity Generation", Science and Technology Postnote # 268, UK Parliamentary Office of Science and Technology, October 2006 (http://www.parliment.uk/documents/upload/postpn268.pdf)