The ‘carbon footprint’ of power generation

Most of us have heard of the 'carbon footprint'; that notorious culprit responsible for the accumulation of greenhouse gases in the stratosphere. But many of us don't know exactly what the term means.

A carbon footprint is a measure of the total amount of carbon dioxide (CO2) and other emissions of a defined population. It is usually calculated for the time period of a year.

When you drive a car or ride a vehicle, the engine burns fuel which creates a certain amount of CO2, depending on its fuel consumption and the driving distance. When you cook your food by burning wood or gas, again you generate considerable CO2. Airplanes are said to leave perhaps the biggest carbon footprint; which is why environment-conscious people prefer to travel by surface transport whenever practical.

When you buy food and other household articles, you should be aware that their production emitted large quantities of CO2 and other harmful gases. The generation of the electrical power is another major emitter of polluting gases – in fact all activity requiring the use of fossil fuels like coal, oil, gas, wood, or even cow dung leaves a carbon footprint.

Technically, a carbon footprint is the total amount of CO2 and other greenhouse gases emitted over the full life cycle of a processor product. It is expressed as grams of CO2 equivalent per kilowatt hour of generation (gCO2eq/kWh). However there are other gases even more harmful to the ozone layer than carbon dioxide, such as methane.

The more correct measuring method would be the carbon footprint by capita for the year for any country.

Electricity generation

Generation of electricity is the single major contributor of CO2 and other greenhouse gas emissions. The carbon footprint varies with the technology adopted in the generation and operation of the plant.

Also to compare the impact of these different technologies accurately, the total CO2 amounts emitted through a system's life must be calculated, which means both direct and indirect contribution. Direct is the contribution during operation of the power plant, and indirect arises during other non-operational phases of the life cycle - developing these fuels and other items used in the plant.

Generation technologies based on non-fossil fuel such as wind, solar (photovoltaic), hydro, biomass, wave/tidal and nuclear power are 'low carbon' or 'carbon neutral' because they do not emit CO2 during their operation. However, they are not 'carbon free' since CO2 emissions arise in other phases of their life cycle such as during extraction, construction, maintenance and decommissioning.

The carbon footprint of fossil-fuelled power plants is dominant due to emissions during their operation. Indirect emissions during other life-cycle phases such as raw material extraction and plant construction are relatively minor. Coal burning power systems have the largest carbon footprint of all the electricity generation systems analysed here.

Conventional coal combustion systems result in emissions of the orderof ~1,000 gCO2eq/kWh or more.

Future techniques such as carbon capture and storage (CCS) and co-firing with biomass could reduce the carbon footprint of coal-fired electricity generation.

Carbon footprints are calculated using a method called life cycle assessment. This method is used to analyses the cumulative environmental impact 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. Lower emissions can be achieved using better coals with high calorific values and newer gasification plants (~800gCO2eq/kWh), but this isstill a challenge.

Gas powered electricity generation has a carbon footprint around half that of coal (~500gCO2eq/kWh), because gas has a lower carbon content than coal.

Most of the power generation in India is based on thermal fuel which includes coal, gas and diesel. This is~ 65 per cent. Next is hydro with ~25 per cent and renewables is ~7 per cent, according to figures published by ministry of power for the year 2010.

Renewable and nuclear technologies

In contrast to fossil-fuelled power generation, renewable and nuclear energy systems emit the least greenhouse gases and other atmospheric pollutants minimal, only derived from the process of creating them.

Biomass is obtained from organic matter, either directly from material like willows, grass, rice husks, coconut shells, etc; or indirectly from industrial and agricultural by-products such as wood chips. Biomass is classed as 'carbon neutral' because the CO2 released by burning is equivalent to the CO2 absorbed by the plants during their growth.

However, biomass fuels are much lower in energy and density than fossil fuels. Large quantities of biomass are needed to fuel a power station. Thus biomass electricity generation is most suited to small-scale local generation in village communities or operating as combined heat and power (CHP) plants.

Replacing a component of the fossil fuel with 'carbon neutral' biomass reduces the overall CO2 emissions from these power stations.

Photovoltaic cells are made of crystalline silicon, a semi-conducting material which can convert 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 per cent of the total energy requirement.

The life cycle CO2 emissions for photovoltaic power systems is currently in the ~60 gCO2eq/kWh range. Future reductions in the carbon footprint of PV cells are expected with the advancement of thin film technologies which use thinner layers of silicon, and with the development new semi-conducting materials like copper, iridium and selenium, production of which is less energy intensive.

The life cycle CO2 emissions for PV systems operating in India are lower than elsewhere at ~30 gCO2eq/kWh, because of its tropical location and weather -there are 3,000 hours of sunlight in a year on average, so overall operating hours are greater and energy output is higher.

Hydro power converts the energy from flowing water into electricity through turbines and generators. 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 put into 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-off- river schemes or micro hydro schemes, as they require large amounts of raw material like steel and concrete for construction. Run-of-river schemes have very small reservoirs, or none at all in some cases during the monsoon flow; so do not give rise to any emissions during their operation.

Carbon footprints for this type of hydro scheme are some of the lowest of all electricity generation technologies (~ 5gCO2eq/kWh).

Electricity generated from wind energy also has one of the lowest carbon footprints. As with most of other low carbon technologies, all the emissions occur during the manufacturing and construction phases, arising from the production of steel for the tower, concrete for the foundation and epoxy/fiberglass for the rotor blades. These account for 98 per cent of the total life cycle CO2 emissions.

Emissions generated duringoperation of wind turbines arise from routine maintenance inspection trips to the site location. This includes use of lubricants and transport. On shore wind turbines are accessed by vehicle, while offshore turbines are maintained using boats and helicopters.

The manufacturing process for both onshore and offshore wind plants is very similar, so life cycle assessment shows that there is hardly any difference between the carbon footprints of onshore (~4.8gCO2eq/kWh)versus offshore (~5.5gCO2eq/kWh) wind generation. The footprint of an offshore turbine is marginally greater because it requires larger foundations.

Nuclear power generation has a relatively small carbon footprint (~5gCO2eq/kWh). Since there is no combustion (heat is generated by fission of uranium or plutonium) operational CO2 emissions account for just ~1% of the total. Most emissions occur during uranium mining, enrichment and fuel fabrication. The most energy intensive phase of the nuclear cycle is uranium extraction, which accounts for ~40% of the total CO2 emissions.

Government policy

India relies on coal for most of its electricity; and this accounts for more than a third of the country's CO2 emissions. However, despite rapid economic growth, the report notes that India's emissions are about a quarter of those from China or the United States.

The Indian government claims to be a front-runner among developing nations for emissions disclosure with its first national survey of greenhouse gases in more than a decade. The government study based on 2007 data showed a sharp increase in industrial activity since the last assessment in1994.

This has helped made India the world's fifth biggest emitter of greenhouse gases after China, the US, Europe and Russia.

Since1994, emissions from electricity, cement and waste have more than doubled, in addition to substantial rises in the transport and residential sectors. Awareness is spreading among the public. But despite push from the concerned government groups and non-government organisations, there is still lot to be done.

India will have to turn its attention towards the impacts of climate change, as continuous warming and the changing rainfall pattern over the Indian region may jeopardise India's development by adversely impacting natural resources such as water, forests, coastal zones and mountains, on which more than 70 per cent of the rural population is dependent.

Conclusions

The greatest potential for carbon footprint reduction is in conventional fossil fuelled electricity generation, using improved combustion technologies, better and more efficient fuels, carbon capture and storage, and co-firing with biomass wherever it is possible. Another is bringing up the percentage of low carbon footprint technologies like renewable energies - solar, wind, hydro, microhydro, and so on.

Our lifestyle itself contributes largely to carbon the footprint. For instance, the more you buy processed food, clothes, furniture etc, the more you are contributing to the carbon footprint. Thus it comprises of two layers, primary footprint - which monitors your carbon emission directly (through energy consumption), and secondary footprint, relating to indirect carbon emissions such as through food preference, fashion, recycling practices and even in recreation activities.