Wind turbines produce electricity by using the natural power of the wind to drive a generator. The wind is a clean and sustainable fuel source, it does not create pollution and it will never run out. Wind energy technology is developing fast, turbines are becoming cheaper and more powerful, bringing the cost of renewably-generated electricity down. Europe is at the hub of this high-tech industry.
The need for clean energy
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.
How wind turbines work
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.
Stall and pitch control
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.
Factors affecting performance
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.
Wind energy production and electricity demand
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.
Figure 2 The negative load effect: how wind energy reduces the need for conventional power.
(Source Econnect Ltd, UK, using data from the National Grid Company and wind farms.)
Figure 3 shows how well wind energy is matched to demand around the year. Electricity demand is higher in the winter, and winter months are also the windiest.
Figure 3 Yearly wind energy output and seasonal electricity demand.
(Source Econnect Ltd, UK, using data from the National Grid Company and wind farms.)
Capacity credit
Another way of looking at the value of wind energy is to look at its capacity credit. The capacity credit of a certain amount of wind energy can be thought of as the amount of conventional plant which could be 'replaced' by wind power, without making the system less reliable. In reality wind turbines are not installed in order that conventional power stations can close prematurely; building wind farms does help avoid the need to build new thermal or nuclear power plants.
Studies on how wind energy can best be integrated into electricity networks have been carried out in several European countries. Each study concludes that wind energy does have a significant capacity credit. In Denmark a capacity credit of 20 - 25% is used in comparative studies of the economy of different new energy technologies, and the country now gets around 20% of it's electricity from wind energy!
Other benefits of wind power
Apart from generating electricity without causing pollution, wind energy has numerous other advantages.
It is widely distributed - more countries have sizeable wind power potential than have large resources of hydro-power or fossil fuel reserves.
It is ideal for generating electricity at a local level - European wind schemes are typically clusters of around 10 - 40 turbines, providing enough electricity for 4,000 to 16,000 households. Some countries such as Denmark and Germany also have a high proportion of single turbines. The electricity can be fed directly into the distribution network, reducing electricity distribution and transmission losses. By contrast, electricity from larger power stations has to be transmitted on high voltage power lines and travel long distances before it gets to the point of use.
Wind energy is good for island communities - the supply can be connected to diesel or solar systems to provide back-up when the wind is not blowing.
Wind energy is low risk - the relatively small unit size of each individual wind turbine (or wind scheme) also reduces the risk of technical failure or industrial action compared with larger generating units.
Wind energy encourages energy diversity - it is sensible for any nation to have a balanced portfolio of energy technologies, rather than to rely heavily on a small number of energy sources. The energy mix among different European countries varies widely, with some countries more dependent on energy imports than others. The UK and Germany have a relatively diverse mix of fuels, whereas others are more dependent on oil (Spain and Greece), coal (Denmark) and nuclear (France and Belgium). Expanding the use of wind energy will increase energy diversity and improve the security of electricity supply. Energy diversity lessens international political sensitivity concerning fossil fuel reserves, volatility of oil and gas prices and the risks associated with nuclear power.
Wind energy capacity in Europe
Europe is the world leader in wind energy, with more installed capacity than any other region of the world. By the end of 1999 there was over 8,500 megawatts of installed capacity - more than double figures for 1996. This growth trend is expected to continue as wind is the fastest growing energy sector worldwide.
The future of wind energy
Improvements in wind energy technology mean that the trends which have led to the dramatic fall in the cost of wind energy are set to continue.
Countries all over the world are setting targets for wind power. It is estimated that 22,000 megawatts of wind energy capacity, in the form of 40,000 wind turbines will be installed in the next 10 years. This represents an annual market of around 2.4 billion Euros . Europe is the hub of this global business, with six companies supplying over half of the world's turbines global wind energy market. Europe stands to benefit greatly from this move towards sustainability.
Glossary and References
1 unit of electricity = 1 kilowatt hour
1,000 kilowatts = 1 megawatt
1 The Declaration of Madrid: An action plan for renewable energy sources in Europe. CEC, 1994
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