Carbon Footprints of Electrical Power from Different Sources

Understanding Carbon Footprints

(This article first appeared in the NEC Newsletter, published by the New England Coalition on Nuclear Pollution.)

Carbon footprints are the subject of a lot of discussion these days. Unfortunately, most people do not understand them well enough to be able to distinguish fact from fiction.  Many nuclear industry advocates claim nuclear power produces no greenhouse gasses, but renewable energy does, and they back their statements with misleading data that appears to be factual. Fortunately, there are objective ways of measuring carbon footprints, and these provide a path to understanding the truth.

Carbon dioxide (CO2) is a greenhouse gas.  Burning coal produces a predictable number of grams of CO2 per kilowatt-hour of electricity (kWh).  It happens that other gasses also contribute to the greenhouse effect. Methane, for example, has about 20 times the greenhouse effect of CO2.  So if we found a way to produce power that emitted a gram of methane per kWh, that power source would have a carbon footprint of equivalent to emitting 20 grams of CO2.  Carbon footprints are measured in grams of carbon dioxide, or the equivalent, per kilowatt-hour (gCO2eq/kWh).

The carbon footprint of all sources has to be considered on a “cradle-to-grave” basis, meaning that the carbon footprint of a solar panel has to consider the emissions arising from making, delivering, installing, operating, maintaining, and disposing of the panel. In the case of a nuclear power plant, the carbon footprint has to include construction of the plant and its equipment, mining the uranium, refining, enriching, transportation, operations, waste disposal, and decommissioning, among other things.

Even given an objective analysis, estimates of carbon footprints range somewhat. The range for nuclear is particularly wide, partly because some costs are unknown, and partly because some studies are clearly faulty or even biased.  The carbon footprints of various energy sources provided by Benjamin Sovacool in 2008 are in the accompanying table.

gCO2eq/kWh Power source
960-1050 Coal
778 Oil
443 Combined cycle natural gas
66 Nuclear (the range is from 17 to 200)
38 Geothermal, dry rock
32 Photovoltaic (new technology)
14-35 Biomass (various technologies)
13 Solar thermal, concentrated
10-13 Hydroelectric
11 Biogas, anaerobic digestion
9-11 Wind

Data in the chart is largely derived from Benjamin Sovacool’s Valuing the greenhouse gas emissions from nuclear power: A critical survey.  Data for new technology photovoltaic is from recent industry sources.

The number given for nuclear power was derived from an earlier metastudy (study of many studies) on the carbon footprints of nuclear power. The range of numbers in the raw data was extremely broad, from 1.4 to 288 gCO2eq/kWh.  Prof. Sovacool applied objective standards to the processes used for the studies and rejected most because they were improperly documented, failed to address all sources of greenhouse gasses connected to the technology, or for other reasons.  The final figure, 66 gCO2eq/kWh, is an average of those that remain.  It is interesting to note that estimates of carbon footprints from nuclear power have increased over time.  The most recent studies of carbon emissions from nuclear plants average at 91 gCO2eq/kWh, so the value of 66 given here may be low.

There are certain sources of CO2 that are never considered, even in the most careful studies.  One important issue is the amount of damage done to the atmosphere arising from efforts at recovery from such disasters as Fukushima Daiichi, Chernobyl, and Kyshtym.  The amount of greenhouse gasses released from these efforts is never taken into account.

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