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Education and Careers

Calculations for wind energy statistics

Examines emissions reductions, electricity produced, homes equivalent, energy balance and carbon footprint

See also:
Calculating the energy in the wind
Extracting energy from the wind

Emissions Reductions

Every unit (kWh) of electricity produced by the wind displaces a unit of electricity which would otherwise have been produced by a power station burning fossil fuel. This is a generally accepted fact used by many organisations including Government in their environmental calculations. Wind-generated electricity does not replace electricity from nuclear power stations because these operate at 'base load', that is they will be working for the whole time that they are available.

Electricity from wind turbines replaces the output of coal and gas fired power stations as these are the most flexible plant on the system.

Nuclear plant operates at base-load. It is the output from coal-fired and gas plants which is adjusted to meet the electricity demand on the system. In other words, most 'load following' is carried out by coal and gas fired plant.

It is easy to calculate how much carbon dioxide (CO₂) is emitted during the production of electricity from coal-fired, oil-fired or gas-fired power stations as this information is available from the main generators in their annual Environmental Performance Reviews.

BWEA calculations use a static figure representing the energy mix in the UK:

• 430g CO₂/kWh

Emissions reductions can be calculated using the following formulae:

• CO₂ (in tonnes)= (A x 0.3 x 8760 x 430)/1000

where A = the rated capacity of the wind energy development in MW
(note this is not the same as its declared net capacity or dnc)

0.3 is a constant, the capacity factor, which takes into account the intermittent nature of the wind, the availability of the wind turbines and array losses

8760 is the number of hours in a year

A typical turbine being installed onshore in the UK currently has a rated capacity of 2 MW and will therefore contribute emission reductions of

• 2260 tonnes of CO₂ each year

Electricity Produced

The amount of electricity produced by a wind energy development can be estimated by

Electricity produced = B x 0.3 x 8760

where B = the rated capacity of the wind energy development in kW
and constants 0.3 and 8760 have the same meaning as above

This is only an average estimation given that in many places, particularly Scotland and offshore, the wind speeds are higher leading to a greater electricity production per turbine, as power output is a cube of the wind speed.

On average then, a typical onshore turbine in the UK, rated at 2 MW, produces 5.3 million units of electricity each year. This is equivalent to 5,256 MWh or 5.3 GWh.

Homes Equivalent

A more realistic measure of the amount of electricity a wind project generates is to calculate how many households this will supply

Number of households = B x 0.3 x 8760/4700

where 4,700 is the average UK household electricity consumption in kW hours.²

A typical turbine therefore produces enough electricity each year to meet the needs of 1,000 homes.

Energy Balance

The comparison of energy used in manufacture with the energy produced by a power station is known as the 'energy balance'. It can be expressed in terms of energy 'pay back' time, that is the time needed to generate the equivalent amount of energy used in manufacturing the wind turbine or power station.

The average wind farm in the UK will pay back the energy used in its manufacture within three to ten months, and over its lifetime a wind turbine will produce over 30 times more energy than was used in its manufacture.

This compares favourably with coal or nuclear power stations, which deliver only a third of the total energy used in construction and fuel supply. So, if fuel is included in the calculation, fossil fuel or nuclear power stations never achieve an energy pay back. Wind energy not only achieves pay back within a few months of installation but does so from a fuel that is free and inexhaustible.

Carbon footprint

All electricity generation technologies emit CO₂ at some point during their lifecycle, whether from extraction and refining of raw materials, or during manufacture, transport and construction, and fossil-fired power plants will also emit CO2 during combustion of their fuel.

The Parliamentary Office of Science and Technology has published a report on the carbon footprint of electricity, which compares the lifecycle CO₂ emissions of different electricity generation systems currently used in the UK, including fossil-fuelled and ‘low carbon’ technologies. The note concludes that while all electricity generation technologies emit CO2 at some point during their lifecycle, CO2 from renewables is non-operational.

Wind power therefore ranks with one of the lowest carbon footprints at 4.64-5.25g CO2eq/kWh for onshore and offshore development respectively.

References

1. Parliamentary Office of Science and Technology, 1994. Select Committee Briefing: Environmental Aspects of Wind Generation.
2. Digest of UK Energy Statistics, 2005. BWEA regularly updates 'annual homes equivalent' figures based on the latest data for domestic electricity consumption divided by number of households.
3. Parliamentary Office of Science and Technology, October 2006, postnote 268, Carbon footprint of electricity generation, www.parliament.uk/documents/upload/postpn268.pdf