The Facts on Wind Generation
Wind energy is often criticised for being unreliable. Critics claim that wind energy can never replace existing power stations, or remove the need for new power stations to be built, because the wind cannot be relied upon. Wind energy can be relied upon; even though wind is not available 100% of the time. In fact, no energy technology can be relied upon 100% of the time.
Different energy technologies have different load factors. No individual power plant is always available to supply electricity. All plants are unavailable at certain times, whether for routine maintenance or for unexpected reasons. The load factor of an energy technology is the ratio (expressed as a percentage) of the net amount of electricity generated by a power plant to the net amount which it could have generated if it were operating at its net output capacity. Wind farms can be treated statistically in exactly the same way as conventional power plant. For any type of power plant it is possible to calculate the probability of it not being able to supply the expected load.
As wind is variable, the probability that it will not be available at any particular time is higher. Wind energy has a lower load factor than many other technologies. The load factor of wind varies according to the site and the type of turbine, but it is generally around 30%. It is higher during the winter than the summer. An average wind farm with an installed capacity of say 5 MW will produce an output of 13,140 MWhours/year, i.e. 30% of what it would produce if it were operating continually at maximum output.
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-fired power stations as these are the most flexible plant on the system.
Nuclear plant operates at base-load, as do almost all gas plants. It is the output from coal fired plant which is adjusted to meet the electricity demand on the system. In other words, most ‘load following’ is carried out by coal-fired plant. It is important to realise that gaseous emissions from conventional power sources are decreasing, due to increases in efficiency and the use of pollution abatement equipment.
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, i.e. as 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 five 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 calculations, fossil fuel or nuclear power stations never achieve an energy pay back.
The average UK household electricity consumption is 4,345 kWh. (Digest of UK Energy Statistics 1998).