Carbon intensity

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Also known as emission intensity, life-cycle greenhouse-gas emissions or levelized CO2 emissions, the carbon intensity of an energy source is the amount of CO2 and equivalent greenhouse gases (e.g. Methane) it emits for each unit of energy (usually electricity) it produces. It is important that the emissions incurred in building and decommissioning generating plant, and mining, processing and transporting fuel, etc, are included, not just the emissions from burning fuel itself. Thus non-fossil fuel sources like wind, solar and nuclear do not have zero emissions (although their emissions are very low).

It is debatable what factors should be included in the emissions attributed to an energy source, for example should the emissions of other energy sources or storage needed to back up or compensate for intermittent energy sources like wind and solar be included in their footprint? Wikipedia's article discusses these issues.

Chapter 7 of IPCC Working Group 3's current assessment report (ar5) discusses their findings which are presented in Figure 7.6 on page 539: IPCC wg3 ar5 figure7.6 carbon intensity.png

Measures of Carbon/emission intensity can also be assessed for activities other than energy production, for example per country. Wikipedia discusses methodologies and data.

See also Carbon intensity of nuclear energy

FURTHER READING

Life-cycle greenhouse-gas emissions of energy sources Wikipedia

Measurement of life-cycle greenhouse gas emissions involves calculating the global-warming potential of electrical energy sources through life-cycle assessment of each energy source. The findings are presented in units of global warming potential per unit of electrical energy generated by that source. The scale uses the global warming potential unit, the carbon dioxide equivalent (CO2e), and the unit of electrical energy, the kilowatt hour (kWh). The goal of such assessments is to cover the full life of the source, from material and fuel mining through construction to operation and waste management.
In 2014, the Intergovernmental Panel on Climate Change harmonized the carbon dioxide equivalent (CO2e) findings of the major electricity generating sources in use worldwide. This was done by analyzing the findings of hundreds of individual scientific papers assessing each energy source.
For all technologies, advances in efficiency, and therefore reductions in CO2e since the time of publication, have not been included. For example, the total life cycle emissions from wind power may have lessened since publication. Similarly, due to the time frame over which the studies were conducted, nuclear Generation II reactor's CO2e results are presented and not the global warming potential of Generation III reactors, presently under construction in the United States and China. Other limitations of the data include: a) missing life cycle phases, and, b) uncertainty as to where to define the cut-off point in the global warming potential of an energy source. The latter is important in assessing a combined electrical grid in the real world, rather than the established practice of simply assessing the energy source in isolation.

Table 3.5. Carbon dioxide intensities of fuels and electricity for regions and countries. IPCC (via Internet Archive Wayback Machine)

Nuclear and wind power estimated to have lowest levelized CO2 emissions Josh Rhodes; University of Texas at Austin Energy Institute; 6 Nov 2017

Using data from the Energy Institute’s 2016 Full Cost of Electricity Study, we estimate the levelized carbon intensity for 12 different fuel and technology combinations for newly constructed power plants. The levelized carbon intensity is estimated by dividing a power plants’ emissions over its lifetime by the total expected electricity output. We estimate that nuclear and wind power, at 12 and 14 g CO2-eq (grams of CO2 equivalent) per kWh of electricity, respectively, have the lowest levelized carbon intensity of all the power plant options we considered. While we have calculated different values for nuclear and wind power, the difference is within the margin of error. Solar energy had the third-lowest levelized carbon intensity, at 41-48 g CO2-eq per kWh of electricity.
We tallied the CO2-eq impacts at six stages in a power plant’s lifecycle: 1) upstream, 2) on-going, non-combustion, 3) power generation, 4) carbon sequestration, 5) fugitive methane emissions and 6) downstream. Not all stages applied to every power plant and were omitted from an individual power plant’s charts if they did not apply, e.g., CCS for a non-CCS plant. Data for 1-3 and 6 were taken from NREL’s harmonization of life cycle analysis studies summarized in Table C-1 of Volume 1 of the Renewable Energy Futures Study. That study described the categories as follows. Upstream includes “emissions resulting from raw materials extraction, materials manufacturing, component manufacturing, transportation from the manufacturing facility to the construction site, and on-site construction.” Ongoing non-combustion includes “fuel cycle emissions (where applicable) and emissions resulting from non-combustion-related O&M activities.” Power generation includes emissions “from combustion at the power plant (where applicable) for the purpose of electricity generation.” Downstream includes “emissions resulting from project decommissioning, disassembly, transportation to the waste site, and ultimate disposal and/or recycling of the equipment and other site materials.” Carbon capture and sequestration values are assumed to reduce combustion CO2 by 90%, but CCS plants have more up and downstream embedded emissions because of their larger amount of on-site capital. Fugitive methane emissions were calculated based on the heat rate of the power plant and an assumed 1% leakage rate in US natural gas infrastructure. A global warming potential of 30 was used to convert the leaked CH4 to CO2-eq.
Other notable power plant options that we did not consider are hydroelectric, geothermal, and biomass. Hydroelectric power plants are very clean during operation, but have significant upstream emissions from their construction and from organic matter anaerobically decomposing under the reservoir. Geothermal power plants generally have low emissions. Biomass power plants have high emissions during operation, but on lifecycle have low emissions because of the carbon fixed during the fuel’s growth phase. We did not consider any carbon emissions associated with long-term spent fuel storage, which is particularly relevant for nuclear power. We did not consider coal bed methane emissions, though fugitive emissions from the natural gas system were included.

Building solar, wind or nuclear plants creates an insignificant carbon footprint compared with savings from avoiding fossil fuels, a new study suggests. SIMON EVANS; Carbon Brief; 8 Dec 2017

The research, published in Nature Energy, measures the full lifecycle greenhouse gas emissions of a range of sources of electricity out to 2050. It shows that the carbon footprint of solar, wind and nuclear power are many times lower than coal or gas with carbon capture and storage (CCS). This remains true after accounting for emissions during manufacture, construction and fuel supply.
“There was a concern that it is a lot harder than suggested by energy scenario models to achieve climate targets, because of the energy required to produce wind turbines and solar panels and associated emissions,” explains project leader Dr Gunnar Luderer, who is an energy system analyst at the Potsdam Institute for Climate Impacts Research (PIK).
Luderer tells Carbon Brief: “The most important finding [of our research] was that the expansion of wind and solar power…comes with life-cycle emissions that are much smaller than the remaining emissions from existing fossil power plants, before they can finally be decommissioned.”
Carbon debt
Critics sometimes argue that nuclear, wind or solar power have a hidden carbon footprint, due to their manufacture and construction. This large “carbon debt”, and the related debt of energy, must be paid off if they are to cut emissions over their lifetime.
Factories churning out solar panels use large amounts of electricity, often sourced from coal-fired power stations in China. Wind turbines and nuclear plants need a lot of steel and concrete. And the centrifuges that separate nuclear fuel also rack up a big electricity bill.
Yet zero-carbon sources of electricity are not the only ones to have a hidden, indirect carbon and energy footprint.
For coal and gas, these lifecycle energy uses and emissions come from extraction machinery and fuel transport. Significantly, they also come from methane leaks at pipelines, well heads or coal mines. These lifecycle emissions continue, even if coal or gas plants add CCS, which also may not capture 100% of emissions at the power plant.
What’s more, the indirect energy uses and emissions of each technology will shift over time, due to changing fuel sources, advances in manufacturing and the evolution of global electricity supplies.
The new research, from lead author Michaja Pehl and colleagues, comprehensively measures the lifecycle energy use and greenhouse gas emissions of different sources of electricity, between now and 2050. It then compares these hidden footprints in a world that cuts emissions in line with a 2C climate goal and a world that stops further climate action.
Embodied energy
The first stage of the work is to add up the energy needed to build power stations and to provide them with the fuel and other inputs they need to run. This is called “embodied energy use”. It is the inverse of “energy return on investment” (EROI).
The study finds that electricity from fossil fuels, hydro and bioenergy has “significantly higher” embodied energy, compared to nuclear, wind and solar power.
For example, the study finds that 11% of the energy generated by a coal-fired power station is offset by energy needed to build the plant and supply the fuel, as the chart below shows. This is equivalent to saying that one unit of energy invested in coal power yields nine units of electricity.
Nuclear power is twice as good as coal, with the energy embedded in the power plant and fuel offsetting 5% of its output, equivalent to an EROI of 20:1. Wind and solar perform even better, at 2% and 4% respectively, equivalent to EROIs of 44:1 and 26:1.

Understanding future emissions from low-carbon power systems by integration of life-cycle assessment and integrated energy modelling Michaja Pehl, Anders Arvesen, Florian Humpenöder, Alexander Popp, Edgar G. Hertwich, Gunnar Luderer; Nature Energy; 8 Dec 2017

Both fossil-fuel and non-fossil-fuel power technologies induce life-cycle greenhouse gas emissions, mainly due to their embodied energy requirements for construction and operation, and upstream CH4 emissions. Here, we integrate prospective life-cycle assessment with global integrated energy–economy–land-use–climate modelling to explore life-cycle emissions of future low-carbon power supply systems and implications for technology choice. Future per-unit life-cycle emissions differ substantially across technologies. For a climate protection scenario, we project life-cycle emissions from fossil fuel carbon capture and sequestration plants of 78–110 gCO2eq kWh−1, compared with 3.5–12 gCO2eq kWh−1 for nuclear, wind and solar power for 2050. Life-cycle emissions from hydropower and bioenergy are substantial (∼100 gCO2eq kWh−1), but highly uncertain. We find that cumulative emissions attributable to upscaling low-carbon power other than hydropower are small compared with direct sectoral fossil fuel emissions and the total carbon budget. Fully considering life-cycle greenhouse gas emissions has only modest effects on the scale and structure of power production in cost-optimal mitigation scenarios.