Carbon Footprint of Electricity Generation

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This POSTnote provides an overview of the "carbon footprints" of a variety of electricity generation technologies. Background In the UK, the Climate Change Act (2008) requires a reduction in emissions of 80% by 2050 compared with 1990 levels. Carbon dioxide is the most significant Greenhouse Gas〈GHG〉and is produced for example, when fossil fuels are burnt. GHGs other than carbon dioxide, such as methane, are quantified as equivalent amounts of carbon dioxide.Thus, the electricity sector has a key role to play in meeting these budgets. Average emissions from electricity generation fell from 718 gCO2eq/kWh in 1990 to 500 gCO2eq/kWh in 2008. The Committee on Climate Change (CCC) recommends a further reduction to just 50 gCO2eq/kWh by 2030 to support achievement of the national budgets. These figures consider only the emissions caused directly at the point of electricity generation, such as when coal is burnt in a coal-fired power station. To provide a more complete picture of the emissions caused by generation technologies, all stages of their life cycles must be considered. These include their construction and maintenance; the extraction, processing and transport of their fuels (if applicable); and their ultimate decommissioning and disposal. Carbon Footprints A carbon footprint aims to account for the total quantity of greenhouse gas emitted over the whole life cycle of a product or process. (Calculated by the method of life cycle assessment, POSTnote 268) The analysis nevertheless provides a more comprehensive view than considering only direct emissions in isolation. This POSTnote describes the carbon footprints of a variety of electricity generation technologies. Data generally refer to existing rather than future technology, and are international in their scope rather than specific to the UK. Only footprint data from published, peer-reviewed studies were included in the main analysis. The footprints aim to consider all emissions up to and including the process of electricity generation, and ignore:  downstream emissions, such as those caused by the construction of transmission cables and consumer appliances, and;  alternatives to direct electricity generation, such as heating technologies and combined heat-and-power plants. These offer further and sometimes alternative ways of providing energy services to consumers. Fossil-Fueled Technologies Table 1 1st Quartile (gCO2eq/kWh) Median (gCO2eq/kWh) 3rd Quartile (gCO2eq/kWh) Number of Estimates Coal 800 940 990 12 Coal CCS 180 190 220 9 Gas 400 460 610 6 Gas CCS 140 150 200 2 All numbers in this table are approximate value. Please see the POSTnote for detail. Table 1 gives carbon footprint data for coal and gas-fired electricity generation, with and without potential carbon capture and storage (CCS) technology. The footprints are dominated by the emissions produced directly as fuel is burnt during plant operation. Direct emissions are influenced mainly by generating efficiency but also the specific type of fuel (e.g. lignite vs higher-grade coal). Coal The studies show that existing UK plant of 786, 846 and 990 gCO2eq/kWh. In general, the improved generation efficiencies of newer designs of plant (POSTnote 253) give footprints at the lower end of the range shown in Table 1. Gas The lowest carbon footprints are achieved by the most efficient generation technology–combined cycle gas turbines (CCGT)–which predominate in the UK. One UK study gives a footprint of 488gCO2eq/kWh for a CCGT. More recent research from Imperial College London and separately at the University of Manchester is indicating that UK CCGT footprints can be as low as 365gCO2eq/kWh for modern technology, but these estimates are excluded from the figure because they have not yet been peer-reviewed and published. The type and source of gas used for electricity generation can have a significant effect on the carbon footprint. Domestic supplies of North Sea gas are in decline and so imports are increasing, reaching 32% of UK supply in 2009. These come either by pipeline or, as liquefied natural gas (LNG), by ship. Research in the USA estimates that the footprint of electricity from imported LNG is 20-25% higher than from US-produced gas due to the additional energy requirement and hence emissions associated with its processing and shipping. Recent but unpublished estimates suggest that the use of 100% LNG would increase the footprint of modern CCGTs, though figures vary widely from 4% to 31%. Natural gas is composed mainly of methane, which is itself a greenhouse gas. Methane is a more potent greenhouse gas than the CO2, but it also has a tenfold shorter residence time in the atmosphere so its effect reduces more rapidly. The warming potential of methane is generally taken to be 25 times that of CO2, even to 33 in recent modeling research. A shorter 20 year timescale gives a global warming potential of methane of 72 to 105 times that of CO2. The footprint of gas generation is influenced by the “fugitive” emissions of methane that arise during its production and transport. Researchers have found fugitive emissions to be greater than previously thought in the USA, increasing the footprint of US natural gas. They also found that the fugitive emissions and hence footprint of US “shale gas” (POSTnote 374) to be greater than those of “conventional” gas. Carbon Capture and Storage (CCS) CCS technologies (POSTnote 335) have the potential to reduce emissions from fuel combustion considerably, but are yet to be proven feasible at full scale. Modelling of future coal-fired generators with CCS has produced carbon footprints ranging from 160 to 280gCO2eq/kWh. For gas, the carbon footprint of a modelled CCGT with CCS was 140 to 200gCO2eq/kWh (Table 1). Recent but as yet unpublished research at Imperial College London has modelled gas CCS with a much lower footprint of 56gCO2eq/kWh. Low-Carbon Technologies Table 2 Median (gCO2eq/kWh) Number of Estimates Solar PV 70 17 Geothermal 40 3 S/M Wind 30 6 Marine 20 6 Nuclear 18 15 L Wind 15 9 River Hydro 13 4 All numbers in this table are approximate value. Please see the POSTnote for detail. Table 2 summarises footprint data for “low carbon” generation technologies. In many cases, emissions do not arise directly from the operation of the generators and so footprints are dominated by indirect emissions, such as those produced during construction. Solar A range of international footprints for solar photovoltaic (PV) systems is approximately from 20 to 170. For a residential PV system in a typical UK installation, which ranges from 75 to 116gCO2eq/kWh under different operating conditions. Many studies ignore the disposal stage for PV, often citing the uncertainty that arises because few installed systems have yet reached the end of their lives. Geothermal Geothermal electricity plants use heat from deep underground to drive conventional steam-powered generators. Recent international studies estimated footprints of 15 to 53 gCO2eq/kWh, it is also suggested that footprints are very much influenced by the geological conditions at the plant’s location. The plants must be constructed and operated effectively. Nuclear The majority of estimates fall below 26 gCO2eq/kWh. The variation of carbon footprints arises from nuclear-specific issues, particularly the grade of the uranium ore and the method of uranium enrichment. There are also uncertainties regarding waste disposal and decommissioning. Recent but unpublished research estimates a footprint of 6.4 gCO2eq/kWh for a new build plant. Marine Marine technologies are based on wave power (caused by the wind) or tidal power (POSTnote 324). Two UK studies are carried out by researchers in Edinburgh: one is of a first-generation wave device (“Pelamis”), with a range of 12 to 39 gCO2eq/kWh, and the other is of a first-generation tidal stream turbine (“Seagen”), with a range of 10 to 20 gCO2eq/kWh. The researchers included recycling as a disposal option. Recycling may create carbon savings. International standards (POSTnote 268) allow this approach to carbon footprinting. Tidal barrages are another option for marine electricity generation (POSTnote 324). A study for various possible Severn barrages provided estimates of carbon footprints between -20 and 50 gCO2eq/kWh. The negative footprints (net emissions reductions) arise from a high level of assumed carbon sequestration, resulting from a deposition of silt upstream of the barrage. Wind Table 2 shows international footprint estimates for onshore wind turbines, distinguishing broadly between large (greater than 500 kW in rated power) and smaller scales. Location can have an important effect on the carbon footprint. One study indicates that a micro-wind turbine would have a carbon footprint of around 38 gCO2eq/kWh for locations with an average annual wind speed of 4.5 m/s. This is the minimum wind speed recommended for small wind systems by the industry trade body, RenewableUK. The same study indicated that locations with average wind speeds of 6 m/s would give footprints of 20 gCO2eq/kWh, while locations of 3 m/s would give 96 gCO2eq/kWh. For offshore wind, two peer-reviewed studies give footprints of 9 and 13 gCO2eq/kWh. The Thanet offshore wind farm, which opened in September 2010 off the coast of Kent, is made up of 300 Vestas V90 turbines. A recent study estimated the carbon footprint of an offshore V90 turbine to be 5.2 gCO2eq/kWh for an installation off the coast of Denmark. Hydro Hydro-electric plants produce electricity from flowing water, either within a river (“run-of-river” technologies) or from water released from a dammed reservoir. The studies summarise four international estimates for run-of-river hydro technologies, which range from 2 to 13 gCO2eq/kWh. Reservoir schemes appear to have higher carbon footprints than run-of-river devices due to the extra materials required for dam construction (POSTnote 268). In addition, in areas flooded when the reservoir fills, decaying plant material can produce methane. Uncertainty remains over such emissions and reservoir hydro has thus been excluded from Table 2. Bioenergy Bioenergy is a fuel obtained from organic matter, from dedicated energy crops or as a by-product or waste of other processes. Depending on the source, bioenergy is processed to produce solid biomass (e.g. wood chips), biogas (e.g. from landfill) or bioliquids (e.g. biodiesel). Because of diversity of bioenergy options and methods of production and the scarcity of UK-specific, peer-reviewed studies, bioenergy is excluded from Table 2. Reports have found that the carbon footprint of electricity from bioenergy is generally, but not always, lower than the least carbon intensive fossil-fuel option, gas-fired CCGTs. For example, electricity generated through combustion of short-rotation coppice wood chips has an estimated carbon footprint of 60 to 270 gCO2eq/kWh. An earlier report for the government suggested that electricity generated via two alternatives to direct combustion – gasification and pyrolysis–has a lower carbon footprint. It gave footprints as low as 25 gCO2eq/kWh for electricity from the gasification of wood chips from forestry residue or short rotation coppice. In addition to being used on its own for electricity generation, biomass can be blended and “co-fired” with coal. Recent research has considered various options for replacing up to 10% of coal with biomass. It found that the biomass component reduces GHG emissions by 88 to 97% compared to the coal it displaces. If used in conjunction with CCS, bioenergy-based electricity has the scope to provide net reductions in emissions. Atmospheric carbon dioxide is absorbed by vegetation as it grows, and, using CCS, could be captured and stored when it is subsequently burnt for electricity generation. The government is introducing a limit of 285 gCO2eq/kWh in 2013 for electricity generated from solid biomass and biogas. Concluding Remarks The construction stage is a significant contributor to the footprint of most low-carbon technologies. In such cases, footprints are sensitive to the inclusion of recycling credits. There are many other impacts of electricity generation beyond the emission of greenhouse gases. LCA assesses a wider range of environmental impacts, such as the production of particulates or requirements for water, and can be combined with other techniques from the physical sciences and from economics to provide more comprehensive assessments.  All electricity generation technologies emit greenhouse gases at some point in their life cycle and hence have a carbon footprint.  Fossil-fuelled generation has a high carbon footprint, with most emissions produced during plant operation. “Carbon capture and storage” could reduce these significantly, though this is unproven at full scale.  Nuclear and renewable generation generally have a low carbon footprint. Most emissions are caused indirectly, such as during the construction of the technology itself.  Carbon footprints are sensitive to factors including the technology’s operating conditions and country of its manufacture.  Further studies for the UK would improve the evidence base.
Last Modified: 2015/06/24