Carbon footprints of power generation using various technologies – Useful for selecting clean technology:
All electricity generation systems have a ‘carbon footprint’, that is, at some points during their construction and operation carbon dioxide (CO2) and other greenhouse gases are emitted to the atmosphere.
To compare the impacts of various different technologies accurately, the total CO2 amounts emitted throughout a system’s life must be calculated. Emissions can be both, direct – arising during operation of the power plant, and indirect – arising during other non-operational phases of the life cycle. Fossil fuelled technologies (coal, oil, gas) have the largest carbon footprints, because they burn these fuels during operation. Non-fossil fuel based technologies such as wind, photovoltaic (solar), hydro, biomass, wave / tidal and nuclear are often referred to as ‘low carbon’ or ‘carbon neutral’ because they do not emit CO2 during their operation. However, they are not ‘carbon free’ forms of generation since CO2 emissions do arise in other phases of their life cycle such as during extraction, construction, maintenance and decommissioning.
A ‘carbon footprint’ is the total amount of CO2 and other greenhouse gases, emitted over the full life cycle of a process or product. It is expressed as grams of CO2 equivalent per kilowatt hour of generation (gCO2eq/kWh), which accounts for the different global warming effects of other greenhouse gases.
Calculating carbon footprints - Carbon footprints are calculated using a method called life cycle assessment (LCA). This method is used to analyze the cumulative environmental impacts of a process or product through all the stages of its life. It takes into account energy inputs and emission outputs throughout the whole production chain from exploration and extraction of raw materials to processing, transport and final use. The LCA method is internationally accredited by ISO 14000 standards.
a. Fossil fuelled technologies - The carbon footprint of fossil fuelled power plants is dominated by emissions during their operation. Indirect emissions during other life cycle phases such as raw material extraction and plant construction are relatively minor.
i) Coal burning power systems have the largest carbon footprint of all the electricity generation systems analyzed here. Conventional coal combustion systems result in emissions of the order of >1,000 gCO2eq/kWh. Lower emissions can be achieved using newer gasification plants (<800gco2eq/kwh),>
ii) Oil accounts for only a very small proportion (about 1%) of the electricity generated in most of the countries. It is primarily used as a back-up fuel to cover peak electricity demand periods. The average carbon footprint of oil-fired electricity generation plants is ~650gCO2eq/kWh.
iii) Current gas powered electricity generation has a carbon footprint around half that of coal (~500gCO2eq/kWh), because gas has a lower carbon content than coal. Like coal fired plants, gas plants could co-fire biomass to reduce carbon emissions in the future.
b. Low carbon technologies - In contrast to fossil fuelled power generation, the common feature of renewable and nuclear energy systems is that emissions of greenhouse gases and other atmospheric pollutants are ‘indirect’, that is, they arise from stages of the life cycle other than power generation.
i) Biomass - Biomass is obtained from organic matter, either directly from dedicated energy crops like short-rotation coppice willow and grasses such as straw, or indirectly from industrial and agricultural by-products such as wood-chips. The use of biomass is generally classed as ‘carbon neutral’ because the CO2 released by burning is equivalent to the CO2 absorbed by the plants during their growth. However, other life cycle energy inputs affect this ‘carbon neutral’ balance, for example emissions arise from fertilizer production, harvesting, drying and transportation.
Biomass fuels are much lower in energy and density than fossil fuels. This means that large quantities of biomass must be grown and harvested to produce enough feedstock for combustion in a power station. Transporting large amounts of feedstock increases life cycle CO2 emissions, so biomass electricity generation is most suited to small-scale local generation facilities,
ii) Photovoltaic (PV) - Photovoltaic (PV), also known as solar cells, are made of crystalline silicon, a semi-conducting material which converts sunlight into electricity. The silicon required for PV modules is extracted from quartz sand at high temperatures. This is the most energy intensive phase of PV module production, accounting for 60% of the total energy requirement. Life cycle CO2 emissions for photovoltaic power systems are currently 58gCO2eq/kWh. However, future reductions in the carbon footprint of PV cells are expected to be achieved in thin film technologies which use thinner layers of silicon, and with the development new semi-conducting materials which are less energy intensive.
iii) Marine technologies (wave and tidal) - There are two types of marine energy devices; wave energy converters and tidal (stream and barrage) devices. Marine based electricity generation is still an emerging technology and is not yet operating on a commercial scale.
iv) Hydro - Hydropower converts the energy from flowing water, via turbines and generators, into electricity. There are two main types of hydroelectric schemes; storage and run-of -river. Storage schemes require dams. In run-of-river schemes, turbines are placed in the natural flow of a river. Once in operation, hydro schemes emit very little CO2, although some methane emissions do arise due to decomposition of flooded vegetation. Storage schemes have a higher footprint, (~10-30gCO2eq/kWh), than run-of-river schemes as they require large amounts of raw materials (steel and concrete) to construct the dam.
v) Wind - Electricity generated from wind energy has one of the lowest carbon footprints. As with other low carbon technologies, nearly all the emissions occur during the manufacturing and construction phases, arising from the production of steel for the tower, concrete for the foundations and epoxy/fibreglass for the rotor blades. Emissions generated during operation of wind turbines arise from routine maintenance inspection trips. This includes use of lubricants and transport. Onshore wind turbines are accessed by vehicle, while offshore turbines are maintained using boats and helicopters. The manufacturing process for both onshore and offshore wind plant is very similar, so life cycle assessment shows that there is little difference between the carbon footprints of onshore (4.64gCO2eq/kWh) versus offshore (5.25gCO2eq/kWh) wind generation.
vi) Nuclear - Nuclear power generation has a relatively small carbon footprints (~5gCO2eq/kWh). Since there is no combustion, (heat is generated by fission of uranium or plutonium), operational CO2 emissions account for <1%>