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Conventional methods of generating electricity burn fuel to provide the energy to drive a wind generator, usually by using the heat to provide steam to drive a turbine. These technologies may use fossil fuels, - coal, oil or gas - or nuclear fuel. Using fossil fuels creates pollution, such as oxides of sulphur and nitrogen which contribute to acid rain, and carbon dioxide which contributes to global climate change.
Although conventional sources of power dominate the energy needs of European countries, wind energy is growing rapidly. Renewable energy sources currently provide nearly 5.4% of the European Union's primary energy needs1 and have the potential to provide much more.
Almost all wind turbines producing electricity for the national grid consist of rotor blades which rotate around a horizontal hub. The hub is connected to a gearbox and generator, which are located inside the nacelle. The nacelle houses the electrical components and is mounted at the top of the tower. This type of turbine is referred to as a 'horizontal axis' machine.
Rotor diameters range up to 80 metres, smaller machines (around 30 meters) are typical in developing countries
Wind turbines can have three, two or just one rotor blades. Most have three.
Blades are made of fibreglass-reinforced polyester or wood-epoxy.
The blades rotate at 10-30 revolutions per minute at constant speed, although an increasing number of machines operate at a variable speed.
Power is controlled automatically as wind speed varies and machines are stopped at very high wind speeds to protect them from damage.
Most have gearboxes although there are increasing numbers with direct drives.
The yaw mechanism turns the turbine so that it faces the wind. Sensors are used to monitor wind direction and the tower head is turned to line up with the wind.
Towers are mostly cylindrical and made of steel, generally painted light grey. Lattice towers are used in some locations. Towers range from 25 to 75 meters in height.
Commercial turbines range in capacity from a few hundred kilowatts to over 2 megawatts. The crucial parameter is the diameter of the rotor blades - the longer the blades, the larger the area 'swept' by the rotor and the greater the energy output. At present the average size of new machines being installed is now super megawatt, 1.3-1.85MW, and there are larger machines on the market. The trend is towards moving to these larger machines as they can produce electricity at a lower price.
There are many different turbine designs, with plenty of scope for innovation and technological development. The dominant wind turbine design is the up-wind, three bladed, stall controlled, constant speed machine. The next most common design is similar, but is pitch controlled. Gearless and variable speed machines follow, again with three blades. A smaller number of turbines have 2 blades, or use other concepts, such as a vertical axis.
Most turbines are upwind of the tower - they face into the wind with the nacelle and tower behind. However, there are also downwind designs, where the wind passes the tower before reaching the blades.
There are two main methods of controlling the power output from the rotor blades. The angle of the rotor blades can be actively adjusted by the machine control system. This is known as pitch control. This system has built-in braking, as the blades become stationary when they are fully 'feathered'.
The other method is known as stall control. This is sometimes known as passive control, since it is the inherent aerodynamic properties of the blade which determine power output; there are no moving parts to adjust. The twist and thickness of the rotor blade vary along its length in such a way that turbulence occurs behind the blade whenever the wind speed becomes too high. This turbulence means that less of the energy in the air is transfered, minimising power output at higher speeds. Stall control machines also have brakes on the blade tips to bring the rotor to a standstill, if the turbine needs to be stopped for any reason.
Most wind turbines start operating at a speed of 4-5 metres per second and reach maximum power at about 15 m/s.
Most important is the windiness of the site. The power available from the wind is a function of the cube of the wind speed. Therefore a doubling of the wind speed gives eight times the power output from the turbine. All other things being equal, a turbine at a site with an average wind speed of 5 meters per second (m/s) will produce nearly twice as much power as a turbine at a location where the wind averages 4 m/s.
Second is the availability of the equipment. This is the capability to operate when the wind is available - an indication of the turbine's reliability. This is typically over 98% for modern machines. Last is turbine arrangement. Turbines in wind farms must be carefully arranged to gain the maximum energy from the wind - this means that they should shelter each other as little as possible from the prevailing wind.
The wind is an intermittent energy resource - it does not blow all the time - but this does not reduce its value as a source of power. The variable output from wind energy poses no special difficulty for power system operation. Electricity demand is constantly fluctuating, and supply and demand have to be matched on a minute to minute basis, 24 hours of the day, every day of the year. The fluctuation caused by the introduction of wind to the system is not discernible above these normal fluctuations, and will not be until electricity generated from wind turbines reaches approximately 20% of the total system supply.
Wind energy effectively 'shaves off' some of the demand which has to be met by conventional generating plant. This is often described as having a 'negative load' effect on the electricity network.
Wind energy coincides well with period of peak electricity demand. Demand often peaks on cold windy winter days - just when wind turbines are at their most productive. Figure 2 illustrates the concept of negative load. It shows the electricity demand over a seven day period in February 1996 in England and Wales, at a time when demand was at one of the highest levels that year. The wind farm output data is the actual output of four wind farms, scaled up to show the effect of 10,000 megawatts of installed capacity. This would meet 10% of England and Wales' annual electricity demand.
From:wind generator manufacturer