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Wind EnergyCan We Rely on the Wind?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. In technical terms this argument boils down to the question; 'can wind energy be regarded as having a capacity credit?' Wind energy can be relied upon, even though the wind is not available 100% of the time. Wind turbines generate electricity for 70-85% of the time, but not always at full output. Most wind turbines start generating power at wind speeds of around 3 or 4 m/s (when the output is a few kilowatts), generate maximum ("rated") power at around 15 m/s and shut down to prevent storm damage at 25 m/s or above. The proportion of time that wind turbine is generating between these wind speeds depends on the average wind speed at the site. Most sites where wind turbines are installed in the UK have wind speeds in the range 7.5 - 9 m/s and so generate for 70-85% of the time. No energy technology can be relied upon 100% of the time.The concept of load factor deals with the day to day productivity of electricity generating plant. Different energy technologies have different load factors, no individual power plants is always available to supply electricity. All power stations 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, as shown in the table, but this is compared to the maximum power the turbine can give out, not it's average.
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 windfarm 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. The relationship between capacity credit and load factorCapacity credit is calculated by determining the reductions of installed power capacity at thermal power stations so that the probability of loss of load at winter peaks is not increased. Or, in other words, how much thermal power plant could be 'replaced' by wind power, without making the system less reliable. For low levels of penetration of wind power into the grid, the capacity credit of wind energy is about the same as the installed capacity multiplied by the load factor. In other words if there is 100 MW of wind energy installed in the country, then this can be relied upon to replace (or avoid the need to build) 30 MW of thermal or nuclear generation capacity. However as the level of wind penetration rises, the capacity credit begins to tail off as shown in the figure.
Spinning reserveSome electricity generating plant is held in a 'warm' state in which it can increase electricity output at short notice. Coal fired power stations take several hours to warm up from cold. To ensure that there is sufficient capacity available to respond rapidly to a sudden increase in electricity demand, some thermal power stations are kept in a condition known as 'spinning reserve'. These plants are burning fuel and are generating electricity, but are not running at full load. Keeping a certain amount of plant as spinning reserve is necessary to ensure that electricity supply can maintained should there be an unexpected loss of generating capacity. For example one of the two 1000MW cross channel links importing electricity from France could fail, a steam turbine could trip out, or a supergrid line could be hit by lightening. To provide an acceptable margin of safety enough plant is held in spinning reserve to cater for the failure of the single largest power source. There does not need to be any extra spinning reserve to cater for the unpredictable nature of the wind: "At the present low levels of wind power, the reserve requirement is not significantly affected, but with higher levels of wind capacity the unpredictability of wind generation might increase the reserve requirements. A study by the former CEGB (Central Electricity Generating Board) estimated that additional reserve might be required once intermittent sources supplied more than 20% of peak demand in England and Wales" Quote from paragraph 23 of the Welsh Affairs Committee report on Wind Energy. Volume 1, HMSO 1994. ConclusionWind energy does have a capacity credit and can therefore be relied upon, even although the wind isn't always available. The 360+ MW of wind energy curently operating in the UK has a capacity credit of over 100 MW, replacing or avoiding the need to build an equivalent amount of thermal or nuclear capacity. Although this capacity credit falls as penetration of wind and other non-firm technolgies into the system increases, this will not be an issue until levels reach approximately 20% penetration. The criticism that extra spinning reserve is neccesary to take into account the intermittent nature of wind is not valid. Spinning reserve will always be needed to cater for unexpected unavailability of the largest single power source, and not just to cater for the currently low levels of electricity generated from the wind. Further InformationFor more information on what happens when the wind stops blowing David Millborrow is our favourite source of information. See these articles and reports for more information.
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