Carbon emissions from electricity generation: the numbers

I’ve gathered together nine life cycle analyses comparing the “cradle-to-grave” CO₂ emissions for electricity from fossil fuels, nuclear and renewables. The analyses are taken from a diverse range of industrial, academic and environmental organisations. Surprisingly, they all agree with each other. Here are the emissions numbers, side-by-side.

Which electricity technologies have the lowest greenhouse gas emissions? Vague claims get thrown about, but the actual numbers often seem hard to find. What’s more, some of the more accessible studies are by bodies with obvious ulterior interests in the results, be they energy companies or environmental groups. To get to the bottom of the question, I’ve gone for a fairly comprehensive trawl though the available data: all the main generating technologies, end-to-end life cycle analyses of each, and analyses from many different organisations spanning the spectrum of views on energy policy.

Life Cycle Analysis

An effective comparison of CO₂ emissions has to be a “cradle-to-grave” assessment spanning the entire life of the generating plant and the fuel cycle. Such an assessment is called a “life cycle analysis”. It takes total lifetime emissions into account. To see why that’s important, let’s take nuclear as an example. CO₂ emissions from a nuclear power plant are low when it’s operating, but does nuclear really provide “low carbon” electricity when the entire fuel cycle is taken into account? What about uranium mining and enrichment? What about the construction of the plant? What about fuel reprocessing, waste disposal and plant decommissioning? A complete “cradle-to-grave” assessment of the technology is the only way to get a true picture of total lifetime emissions.

The question about end-to-end emissions applies to all electricity generating technologies. Wind energy emits no CO₂ in operation, but how much CO₂ is given off to produce the raw materials, the concrete and copper? These are the questions that need to be answered when assessing total greenhouse gas emissions for any generating technology. That’s what a life cycle analysis does. A life cycle analysis is a complete “cradle-to-grave” examination of a process. In the case of CO₂ emissions, a life cycle analysis can provide the hard numbers for the total emissions of an electricity generating technology over its entire life.

Nine Life Cycle Analysis Reports Brought Together

There are several authoritative life cycle analyses of greenhouse gas emissions from electricity generation. Although these are very solid pieces of work, some could have their neutrality questioned for one reason or another. Therefore, I’ve brought together analyses from nine different bodies spanning a range of views on energy policy, to see how their numbers compare. I’ve tried to use reports that have been subject to detailed scrutiny – peer-reviewed academic publications, compilations of peer-reviewed work, analyses submitted to statutory planning processes, and so on. The reports reviewed here compare six electricity generating technologies, coal; gas; solar photovoltaic (PV); nuclear; wind and hydroelectric power. Because any one study could be accused of bias one way or another, I’ve brought together analyses from a range of sources:

• three of the reports are from governmental organisations: the European Commission’s Externalities of Energy (ExternE) project, the IAEA and the UK DTI;
• three are academic studies by research establishments in the US (U. of Wisconsin), Japan (CRIEPI) and Switzerland (Paul Scherrer Institute);
• one is from a environmental organisation with a pro-renewables anti-nuclear energy policy (Sustainable Development Commission);
• and two are from energy companies with commercial interests in nuclear power and coal (British Energy and Vattenfall).

Here are the nine organisations in more detail:

• ExternE: ^[1]
  ExternE – Externalities of Energy is a research project of the European Commission. In its words, “the ExternE project is the first comprehensive attempt to use a consistent ‘bottom-up’ methodology to evaluate the external costs associated with a range of different fuel cycles. The European Commission launched the project in collaboration with the US Department of Energy in 1991.”
• Sustainable Development Commission: ^[2]
  The Sustainable Development Commission is the UK Government’s independent advisory body on sustainable development. It is chaired by the leading environmentalist Jonathon Porritt. The SDC opposes nuclear power and supports tidal and wind energy.
• University of Wisconsin, Fusion Technology Institute: ^[3]
  The University of Wisconsin’s study is a life cycle assessment produced for a Ph.D. thesis at the university’s Fusion Technology Institute, a fusion engineering research centre.
• Central Research Institute of Electric Power Industry, Japan: ^[4]
  The Japanese report is a life cycle analysis from Japan’s Central Research Institute of Electric Power Industry, which researches fossil, nuclear and renewable energy.
• Paul Scherrer Institute, Switzerland: ^[5]
  The Swiss life cycle assessment is by the Paul Scherrer Institute. The PSI is one of the most prestigious science and engineering research centres in Switzerland.
• Department of Trade and Industry, UK: ^[6]
  The UK figures are from the UK Government’s 2006 Energy Review, produced by the DTI (now BERR). The DTI figures draw on analyses by the OECD. I’ve included these as the “official” view, that is, the data the government itself would look to.
• International Atomic Energy Authority: ^[7]
  The IAEA describes itself as “the world’s center of cooperation in the nuclear field.” The report used here is from its Planning and Economic Studies Section.
• British Energy Group plc & Vattenfall AB: ^[8] [9]
  And finally, the last two sets of numbers are from the energy industry itself. British Energy operates Britain’s nuclear power stations. The life cycle analysis I’ve included here was part of an Environmental Product Declaration for Torness nuclear power station. Vattenfall AB is a Swedish electricity generating company, its energy mix is 45% fossil, 33% nuclear, 21% hydro and 1% renewable. The Vattenfall report is a summary of the various life cycle analyses Vattenfall produces for its own needs.

The Actual Numbers, At Last

I’ve brought together nine studies from bodies spanning a wide range of views on energy policy. Here are the CO₂ emissions numbers from these nine reports in one handy table. The data are “cradle-to-grave” Life Cycle Analyses of the various energy technologies, covering everything from mining to waste disposal.

Table 1.  Total lifetime releases of CO₂ from electricity generating technologies

	Coal	Gas	Solar PV	Nuclear	Wind	Hydro
	kg CO2/MWeh
ExternE [1]	815	362	53	20	7	-
UK SDC [2]	891	356	-	16	-	-
U. of Wisconsin [3]	974	469	39	15	14	-
CRIEPI, Japan [4]	990	653(a)	59	21	37	18
Paul Scherrer Inst. [5]	949(b)	485	79	8	14	3
UK Energy Review [6]	755	385	-	11–22	11–37	-
IAEA [7]	968(c)	440(c)	100(c)	9–21	9–36	4–23
Vattenfall AB [8]	980	450	50	6	6	3
British Energy [9]	900	400	-	5	-	-
The units are kilograms of CO2–equivalent emitted per megawatt-hour of electricity generated.  The figures refer to total emissions for the whole life cycle of the generating plant. Notes: (a) The figures for gas refer to combined cycle where available, the CRIEPI figure is for thermal. (b) The PSI data are the “minimum” value data-set from the PSI report. (c) Figures for best 1990s technology.

What Does It All Mean?

So, what do these numbers show us?

The first point is that all these reports get roughly similar results. Call me a cynic, but I found that slightly surprising. There is some spread in the numbers, reflecting different national circumstances and assumptions, but they all tell more-or-less the same story.

The data show there is an enormous gulf separating coal and gas at the high end from wind, nuclear and hydro at the low end. Solar photovoltaic has somewhat higher life cycle emissions that the lowest three, but it is still a clear factor of ten lower than coal.

The results are surprisingly clear-cut. Nine life cycle analyses from organisations as diverse as Vattenfall, the Paul Scherrer Institute and the Sustainable Development Commission all roughly agree on the numbers. Coal and gas have by far the highest life cycle CO₂ emissions. Solar PV is about ten times lower, and wind, nuclear and hydro are lower still.

Extra Bonus Section: Geothermal and Tidal Barrage

Most of these studies don’t consider geothermal energy in their comparisons. After receiving an inquiry about geothermal in the comments, I’ve added two studies of geothermal emissions. One is part of the Japanese CRIEPI ^[4] report used above, and I’ve added a second study by the Idaho National Engineering and Environmental Laboratory ^[10].

I’ve added a further report on geothermal emissions, from the International Geothermal Association ^[11].

Tidal barrages are also omitted from the main reports listed above, because large-scale tidal schemes aren’t widely available. The UK has a specific interest in them though; a barrage across the river Severn could provide 2 GW of average power, or about 4% of the UK’s electricity needs. The project is moving up the political agenda, so I’ve added the lifecycle CO₂ emissions here. I’ve taken the numbers from a report prepared by Black & Veatch ^[12] for the UK Sustainable Development Commission. The report includes a lifecycle emissions analysis for the Severn barrage.

These are the additional figures for geothermal and tidal energy:

Table 2. 
Lifetime CO₂ emissions from geothermal and tidal electricity generation

		Geothermal	Tidal
		kg CO2/MWeh
CRIEPI [4]		23	-
INEEL [10]		82(a)	-
IGA [11]		122	-
UK SDC [12]		-	2(b)
The units are kilograms of CO2–equivalent emitted per megawatt-hour of electricity generated.  The figures refer to total emissions for the whole life cycle of the generating plant. Notes: (a) Figure for CO2 only. (b) The tidal figure is for the Severn tidal barrage.

Carbon emissions for geothermal electricity are slightly lower than emissions for solar photovoltaic, and somewhat higher than wind, nuclear and hydro.

The Severn Barrage’s lifecycle emissions are dominated by the initial construction phase, but the figures are striking. This project has the lowest lifecycle CO₂ emissions of all the electricity generating technologies I’ve looked at here.

References

1. ExternE National Implementation Germany, W. Krewitt et al., Externalities of Energy European Commission Research Project, Nov. 1997  (WebCite cache)
2. The Role of Nuclear Power In a Low Carbon Economy, SDC Position Paper, UK Sustainable Development Commission, Mar. 2006 (page 5)  (WebCite cache)
3. Life-Cycle Assessment of Electricity Generation Systems and Applications for Climate Change Policy Analysis, P.J. Meier, Ph.D. thesis, University of Wisconsin, Aug. 2002 (page 70)  (WebCite cache)
4. Energy Technology Life Cycle Analysis that Takes CO2 Emission Reduction Into Consideration, Central Research Institute of Electric Power Industry, Japan, Annual Research Report, 1995  (WebCite cache)
5. Greenhouse Gas Emissions From Energy Systems: Comparison and Overview, R. Dones et al., Paul Scherrer Institut Annual Report 2003, Annex IV (Table 2, page 38)  (WebCite cache)
6. UK Govt. Energy Review: The Energy Challenge, 2006, Chapter 5, Electricity Generation, (Table 5.3, page 116)  (WebCite cache)
7. Greenhouse Gas Emissions Of Electricity Generation Chains: Assessing the Difference, J.V. Spadaro et al., IAEA Bulletin 42/2/2000, (page 21)  (WebCite cache)
8. Vattenfall’s Life Cycle Studies of Electricity, Vattenfall AB, Oct. 1999 (upper graph on page 16)  (WebCite cache)
9. Environmental Product Declaration of Electricity from Torness Nuclear Power Station, British Energy, May 2005  (WebCite cache)
10. Geothermal Electrical Production CO2 Emissions Study, K.K. Bloomfield et al., Idaho National Engineering and Environmental Laboratory, 1999  (WebCite cache)  (alternative link)
11. Geothermal Power Generating Plant CO₂ Emission Survey, R. Bertani et al., Newsletter of the International Geothermal Association, No. 49, July 2002 (Figure 1, page 2)  (WebCite cache)
12. Tidal Power in the UK: Research Report 3 – Severn barrage proposals, Black & Veatch for the Sustainable Development Commission, Oct. 2007  (page 60)  (WebCite cache)

 < Edit March 3, 2008:  Add “Geothermal” section. > 

 < Edit June 12, 2008:  Add “Severn Tidal Barrage” section. > 

 < Edit June 20, 2008:  Add “IGA geothermal data” section. > 

 < Edit Sept 23, 2008:  Fix table entries in “UK Energy Review” row of Table 1.
Thanks to Peter Lang for pointing out the error in this comment. > 

 < Edit Nov 1, 2008:  Cache all references onto WebCite. >