Life-cycle greenhouse-gas emissions of energy sources

Comparisons of life-cycle greenhouse gas emissions or global warming potential attempt to calculate the global warming potential of electrical energy sources by doing a life cycle assessment of each energy source and presenting the findings of the assessment in units of global warming potential per unit of electrical energy generated by that source; which uses the global warming potential unit, the Carbon dioxide equivalent(e) and the unit of electrical energy, the kilowatt hour(kWh). The assessments attempt to cover the full life of the source, from material & fuel mining, construction, to operation and waste management.

In 2014, the IPCC harmonized the Carbon dioxide equivalent(e) findings of the major electricity generating sources used worldwide by assessing the findings of hundreds of individual scientific assessment papers published from each energy source.

It is important to note that for all technologies, advances in efficiency and therefore reductions in e since the time of publication have not been included. For example, the total life cycle emissions from wind power may have reduced since publication, similarly, due to the timescales over which the studies were conducted, nuclear Generation II reactor's e results are presented and not the global warming potential of Generation III reactors, which are presently under construction in the United States and China. Other limitations found in the literature of these comparisons of energy sources include missing life cycle phases and where to define the cut-off point in the global warming potential the energy source may cause when in a combined electrical grid in the real world, as opposed to the established practice of simply assessing the energy source in isolation.

2012 Yale University systematic review and harmonization of nuclear power data
A Yale University review published in the Journal of Industrial Ecology analyzing life cycle assessment emissions from nuclear power determined that: ""The collective LCA literature indicates that life cycle GHG emissions from nuclear power are only a fraction of traditional fossil sources and comparable to renewable technologies."" It went on to note that for the most common category of reactors, the Light water reactor: ""Harmonization decreased the median estimate for all LWR technology categories so that the medians of BWRs, PWRs, and all LWRs are similar, at approximately 12 g -eq/kWh"" The study noted that: ""the difference between nuclear power life cycle GHG emissions constructed in an electric system dominated by nuclear (or renewables) and a system dominated by coal can be fairly large (in the range of 4 to 22 g -eq/kWh compared to 30 to 110 g -eq/kWh, respectively)""

Although the paper primarily dealt with data from Generation II reactors, it did also summarize the Life Cycle Assessment literature of pre-commercial nuclear technology's.''"FBRs [ Fast Breeder Reactors ] have been evaluated in the LCA literature. The limited literature that evaluates this potential future technology reports median life cycle GHG emissions ... similar to or lower than LWRs and purports to consume little or no uranium ore."

2011 IPCC aggregated results of the available literature
A literature review conducted by the Intergovernmental Panel on Climate Change in 2011, of numerous energy sources emissions per unit of electricity generated, found that the  emission value, that fell within the 50th percentile of all total life cycle emissions studies were as follows.

2008 Benjamin K. Sovacool survey of nuclear power.
A meta analysis of 103 nuclear power life-cycle studies by Benjamin K. Sovacool found that nuclear power plants produce electricity with a mean of 66 g equivalent life-cycle carbon dioxide emissions per kWh, compared to renewable power generators, which produce electricity with 9.5 to 38 g carbon dioxide per kWh and fossil-fuel power stations, which produce electricity with about 443 to 1,050 g equivalent lifecycle carbon dioxide emissions per kWh.

Sovacool thus concludes that nuclear energy technologies are 7 to 16 times more effective at fighting climate change than fossil fuel power plants on a per-kWh basis. Renewable electricity technologies are "two to seven times more effective than nuclear power plants on a per kWh basis at fighting climate change." Sovacool has said that his estimates already include all conceivable emissions associated with the manufacturing, construction, installation and decommissioning of renewable power plants.

On his nuclear power paper, Benjamin K. Sovacool has been criticized by his peers, as it was noted that his paper was overly based on data from Jan Willem Storm van Leeuwen. Beerten et al. state:

"Most recently, Sovacool(2008) calculated a mean value for the overall emissions by averaging the global results of 19 LCA [Life-Cycle Analysis] studies forming a subset of, as stated by the author, 'the most current, original and transparent studies' out of 103 studies. However, a critical assessment reveals that a majority of the studies representing the upper part of the spectrum are studies that can be traced back to the same input data and performed by the same author, namely Storm van Leeuwen. After careful analysis, it must be concluded that the mix of selected LCAs results in a skewed and distorted collection of different results available in the literature. Furthermore, since many studies use different energy mixes and other assumptions, averaging GHG emissions of those studies is no sound method to calculate an overall emission coefficient, as it gives no site specific information needed for policy makers to base their decisions."

Beerten et al. proceed to discuss reasons why LCA analysis for nuclear power plants can give such widely varying estimates. For example, life-cycle greenhouse-gas emissions of nuclear power depend on the enrichment method, the carbon intensity of the electricity used for enrichment, the efficiency of the plant, as well as on chosen mining technologies. Averages and means from multiple sources can be skewed by inharmonious data, clustering bias, by outliers and so on.

In response to these criticisms, particularly in reference to Sovacool applying his methodology to nuclear power but using other researchers' results, from different methodologies, as the source of his above tabled Wind and Solar energy figures, he and colleague Daniel Nugent embarked on studying these other energy sources. Their paper reports that wind energy has a mean value of 34.11 grams of -eq/kWh and solar PV a mean value of 49.91 grams of -eq/kWh, with the minimum for wind being 0.4 g and the maximum 364.8 g and a minimum for Solar PV of 1 g and a maximum of 218 g.

Missing life cycle phases
Although the life cycle assessments of each energy source should attempt to cover the full life cycle of the source from cradle-to-grave, they generally limit the assessment to looking at only the construction and operation phase of the energy sources life cycle, the most rigorously studied phases are those of material and fuel mining, construction, operation, and waste management. However missing life cycle phases, are present for a number of energy sources. At times the assessments variably and sometimes inconsistently include the contribution from the energy source's facility decommissioning, that is, the global warming potential of the process to return the power supply site to greenfield status.

For example the process of hydroelectric dam removal is usually excluded as it is a rare practice with little practical data available. Dam removal however may become increasingly common as dams age, with an example being the decommissioning of the Bull Run Hydroelectric Project, which was the largest concrete dam ever removed in the United States as of 2012. Larger dams, such as the Hoover Dam, and the Three Gorges Dam are intended to last "forever" with the aid of maintenance, a period that is not quantified. Therefore decommissioning estimates are generally omitted for some energy sources, while other energy sources include a decommissioning phase in their assessments.

The median value presented of 12 g -eq/kWhe for nuclear power found in the 2012 Yale University nuclear power review, a paper which also serves as the origin of the 2014 IPCC's nuclear value, does however include the contribution of facility decommissioning with an "Added facility decommissioning" global warming potential in the full nuclear life cycle assessment.

GHG from Utility-Scale Wind power
High electric grid penetration by Intermittent power sources e.g. wind power, sources which have low capacity factors due to the weather, either requires the construction of energy storage projects, which have their own emission intensity or it requires more frequent back up than the reserve requirements necessary to back up more dependable/baseload power sources, such as hydropower and nuclear energy. This higher dependence on back up/Load following power plants to ensure a steady power grid output has the knock on effect of more frequent inefficient(in e g/kWh) throttling up and down of these other power sources in the grid to facilitate the intermittent power source's variable output. When one includes the intermittent sources total effect it has on other power sources in the grid system, that is, including these inefficient start up emissions of backup power sources to cater for wind energy, into wind energy's total system wide life cycle, this results in a higher real world wind energy emission intensity than the direct g/kWh value-which looks at the power source in isolation and excludes all down stream detrimental/inefficiency effects it has on the grid. In a 2012 paper that appeared in the Journal of Industrial Ecology it states.

""The thermal efficiency of fossil-based power plants is reduced when operated at fluctuating and suboptimal loads to supplement wind power, which may degrade, to a certain extent, the GHG benefits resulting from the addition of wind to the grid. A study conducted by Pehnt and colleagues (2008) reports that a moderate level of [grid] wind penetration (12%) would result in efficiency penalties of 3% to 8%, depending on the type of conventional power plant considered. Gross and colleagues (2006) report similar results, with efficiency penalties ranging from nearly 0% to 7% for up to 20% [of grid] wind penetration. Pehnt and colleagues (2008) conclude that the results of adding offshore wind power in Germany on the background power systems maintaining a level supply to the grid and providing enough reserve capacity amount to adding between 20 and 80 g -eq/kWh to the life cycle GHG emissions profile of wind power."'"

Other studies


In terms of individual studies, a wide range of estimates are made for many fuel sources which arise from the different methodologies used. According to Sovacool, those on the low end tend to leave parts of the life cycle out of their analyses, while those on the high end often make unrealistic assumptions about the amount of energy used in some parts of the life cycle.

In 2007 the Intergovernmental Panel on Climate Change stated that total life-cycle GHG emissions per unit of electricity produced from nuclear power are below 40 g -eq/kWh (10 g -eq/kWh), similar to those for renewable energy sources.



The Swedish utility Vattenfall did a study in 1999 of full life cycle emissions of nuclear, hydro, coal, gas, solar cell, peat and wind which the utility uses to produce electricity. The net result of the study was that nuclear power produced 3.3 grams of carbon dioxide per kW-hr of produced power. This compares to 400 for natural gas and 700 for coal (according to this study). The study also concluded that nuclear power produced the smallest amount of 2 of any of their electricity sources.

Another report, Life-Cycle Energy Balance and Greenhouse Gas Emissions of Nuclear Energy in Australia, conducted by the University of Sydney in 2008 produced the following results: nuclear = 60-65 g /kWh; wind power = 20 g/kWh; solar PV = 106 g/kWh. The likely range of values from this study produced the following results: nuclear = 10-130 g /kWh; wind power = 13-40 g /kWh; solar PV = 53-217 g /kWh. Furthermore, the study criticised the Vattenfall report : "it omits the energy and greenhouse gas impacts of many upstream[mining] contributions".

In a study conducted in 2006 by the UK's Parliamentary Office of Science and Technology (POST), which used figures from Torness Nuclear Power Station-an Advanced gas-cooled reactor, nuclear power's life cycle was evaluated to emit the least amount of carbon dioxide (very close to wind power's life cycle emissions) when compared to the other alternatives (fossil fuel, coal, and some renewable energy including biomass and PV solar panels).

A 2005 study, issued by Jan Willem Storm van Leeuwen, reported that carbon dioxide emissions from nuclear power plants per kilowatt hour could range from 20% to 120% of those for natural gas-fired power stations depending on the availability of high grade ores. Although the study was heavily criticized, the paper went on to be used by anti-nuclear organizations to claim that nuclear power is not suitable for a warming world.

Heat from thermal power plants
Thermal power plants, those that produce thermal/heat energy, with common low carbon power examples of which being biomass, nuclear and geothermal energy stations, directly add heat energy to the earth's global energy balance. According to David JC MacKay, assuming that all future energy is derived from these thermal power stations operating with their present thermal efficiency of ~30%, and that the world population is 10 billion in 100 years time(~2100) with each individual enjoying a per capita energy usage rate similar to that of the average European standard of living of 125 kWh per day, the extra power contributed by this thermal energy use to the planet would be a global surface area average of 0.1 Watt per square meter, which is one fortieth of the 4 W/m^2 that is believed to be likely if a doubling of atmospheric CO2 concentrations occur, and a little smaller than the "0.25 W/m^2 effect" of Solar variations. "Under these assumptions, human power production would just show up as a contributor to global climate change."

Potential heating from wind turbines
An MIT peer-reviewed study suggested that using wind turbines to meet 10 percent of global energy demand in 2100 could have a warming effect, causing temperatures to rise by 1 C-change in the regions on land where the wind farms are installed, including a smaller increase in areas beyond those regions. This is due to the effect of wind turbines on both horizontal and vertical atmospheric circulation. Whilst turbines installed in water would have a cooling effect, the net impact on global surface temperatures would be an increase of 0.15 C-change. Author Ron Prinn cautioned against interpreting the study "as an argument against wind power, urging that it be used to guide future research". "We’re not pessimistic about wind," he said. "We haven’t absolutely proven this effect, and we’d rather see that people do further research".