The energy in the wind is captured by the blades of a wind turbine, which are normally mounted on a horizontal shaft (although vertical axis turbines also exist). The wind causes the shaft to turn, which drives the electrical generator. The size of wind turbines varies greatly, from small a few hundred watts to 3MW. Wind turbines are sized according to their rated power, that is the power output at a given wind speed, usually 12 m/s. However, the relation between wind speed and power output is not linear. In fact, the energy content of the wind varies with the cube of the wind speed i.e. doubling the wind speed would give 2 3 = 8 times as much energy. Also, the speed of the wind increases with height above the ground. It is therefore important to have a good understanding of local wind conditions before installing a turbine. For larger turbines, an anemometer will have to be mounted on a mast at the site of the proposed turbine in order to have reliable wind speed and direction data over the course of a year.
On average, a wind turbine generates electricity for 70-85% of the time, although not always at the rated power. Its load factor, the proportion of a turbine's theoretical energy output over a year, is around 30%. This value is compared to 50% for a conventional power station.
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Wind power is one of the more cost effective renewable energy technologies. For a company or organisation, it is a very noticeable way of showing positive environmental action. For grid-connected systems, any excess electricity that is produced can be sold back to the grid. Wind turbines can also be used for off-grid applications, either for direct energy consumption (e.g. resistive heat load) or for battery charging for later use. If the site is well chosen, wind power can be a reliable source of power.
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Most wind turbines start operating ('cut in') at around 3-4 m/s. Larger wind turbines are viable where the average wind speed is 6-7m/s. Smaller turbines can still produce useful amounts of electricity at an average wind speed of 4 m/s. Turbines should be sited away from obstacles that could disrupt the wind pattern e.g. buildings, trees etc..
Planning consent is required for the installation of a wind turbine. It is important to have contact with the planners at the earliest possible stage and to involve the public in the consultation process. A wind turbine could cause shadow flicker. The noise levels vary from machine to machine but turbines are a lot quieter now compared to models that came out 10-15 years ago! Roof-mounted turbines, which have been specially developed to operate at low noise levels and with low vibration should be on the market within the coming year.
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Although the initial capital outlay can be quite high, a well-positioned wind turbine can recoup this cost relatively quickly. Installed costs are size dependent: £500/100W for small, battery charging turbines; £3000/kW for medium sized, grid-connected turbines; £700/kW for large 2MW turbines.
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