Summary

Wind power has been used for thousands of years for sailing, and hundreds of years to provide mechanical power. The use of wind-powered water pumps became popular in many countries, using simple designs with many blades rotating slowly, but these proved unsuitable for generating electricity. Modern designs use aerofoil blades, similar to an aircraft wing, and deliver higher efficiencies when driving an electrical generator.

Wind turbine designs initially used gearboxes to convert the slow rotational speed of the turbine into the high speed needed for a generator, but some recent designs allow turbines to run at variable speeds and use electronics to convert the power into a form suitable to feed into the national grid. All horizontal axis wind turbines need to be turned to face the wind to operate efficiently, and most also allow the angle of the blades to be adjusted according to the strength of the wind.

In general, a wind turbine’s generation over a year will average 30% of its rated capacity, although this will vary according to the location. Turbines must obviously be sited where it is windy in order to be economic, although consideration has to be given to their visual impact. One alternative is siting them offshore, but the costs of installation and maintenance are higher than for onshore turbines. The cost of electricity generated for the grid is about 3.2p per kWh from onshore wind and 5.5p per kWh from offshore wind.

By the end of 2007, the global installed capacity wind power was about 94 GW, of which 19.7 GW was added during 2007, and wind supplied about 1.3% of global electricity.

Technology background

Wind turbines extract energy from the wind. People have been harnessing wind power for transport, in the form of sailing ships, for thousands of years. The use of wind turbines for providing mechanical power is also fairly old, with records of wind-powered machines for grinding corn going back to 1000 AD in Persia.

There are two ways in which energy can be extracted from the wind - by drag or by aerofoil lift. The traditional designs of windmill were drag machines, in which the wind pushed the blades around. If the blade is made as an aerofoil shape, it can generate lift, as with an aircraft wing, giving a much higher efficiency for energy transfer. Most wind turbines have blades rotating around a horizontal axis. Wind turbines have also been made with blades rotating around a vertical axis, but these have not proved as successful in practice.

Drag wind turbines

The 20th century development of wind energy was originally based on water pumping. The Dutch used wind pumps to keep their reclaimed land free from water and the idea was exported to the USA and Australia to pump water from boreholes for cattle. These drag wind turbines used a large number of wide blades and ran slowly to provide the high torque required to drive water pumps to lift water. Similar wind-pumps are used in many parts of the developing world such as Southern and Eastern Africa.

Early attempts to use drag wind turbines to drive electric generators proved inefficient, because generators require high speeds and low torque, while a drag turbine supplies low speeds and high torque.

Aerofoil wind turbines

It was not until the development of the aerofoil wing for aircraft, that people realised the potential for using aerofoil blades in wind turbines. The oil price rise in the early 1980s encouraged a new interest in the use of wind power for electricity generation. California, in particular, set up an ambitious wind programme that encouraged the development of a range of different technologies, including vertical axis turbines as well as upwind and downwind horizontal axis machines.

The rapid development of wind energy in the past 20 years has been mainly based around horizontal axis turbines with aerofoil blades driving electric generators. The rated capacity has increased, and large commercial machines are now rated at 2 MW or more. The generator is placed in a nacelle behind the blades, and the whole arrangement is supported on a tower (usually made as a steel tube) well above the ground. A 2 MW machine has a tower about 110m tall, with three 35m blades, usually made of glass reinforced plastic. Blades are sometimes made from wood for smaller models. Most wind turbines need to be kept facing the wind, so a ‘yaw’ mechanism is used drive the nacelle around on a geared wheel, guided by a sensor that checks the wind direction. Smaller wind turbines use a wind vane, behind the blades, that hold them facing the wind. A few systems use turbines downwind of the tower, but these have proved less efficient, as the tower affects the wind flow over the blades.

Generating electricity from wind

A wind generator must be able to supply power at a steady frequency to the grid, even though the wind is constantly changing in speed and direction. The older systems use gearboxes to match the speed of the turbine with that required by the generator, so requiring the turbine to rotate at a relatively constant rate (typically 10 to 30 rpm). The angles of the turbine blades are adjusted as the wind speed varies to maintain the rate of rotation. If the wind speed gets too high (usually above 25 m/s), the blades are ‘furled’ i.e. turned out of the wind and brakes are applied, to prevent the turbine from being damaged by excessive rotational speed.

More recent turbine designs use variable speed generators and electronics to feed power to the grid at the correct frequency, allowing the turbine blades to rotate at a speed more appropriate to the wind conditions. This reduces the noise and inefficiencies introduced by the gearboxes. Sophisticated power control systems are used to match the variable power output from these machines to the grid. Most generators operate at 700 V, but a transformer is used to step the voltage up to 11 kV or 132 kV, so the power can be fed into the grid.

The size of a wind turbine is defined by its rated capacity, which is the power that it can generate at its design wind speed (usually 15 m/s), The wind does not blow steadily at this speed, so the actual power generated is on average about 30% of the rated capacity; this is termed the capacity factor. Since wind is not a guaranteed energy source, there must be back-up systems to supply electricity from another source. For grid-connected systems, this is usually conventional power stations using either fossil fuel, biomass, or nuclear power as the source of fuel. Conventional grid systems require significant reserve capacity to manage the variability of demand, and the failure a large power station, so the variability of wind power does not require additional reserve capacity until wind is a significant contributor to the overall mix – typically around 20%. Also, if wind turbines are installed across a whole country, the overall variability is reduced. For stand-alone systems, such as for island communities not connected to the grid, wind power is often designed with a diesel engine back-up.

Turbine siting

The best site for a wind turbine, obviously, is in a place where there is plenty of wind. The west coast of Europe, especially that of the UK, is exposed to the Atlantic winds, which are fairly reliable. Wind speed increases with height, so the best sites are on the hilly areas in the west of the UK, in places such as Cornwall, Wales, around the Lake District and on the west coast of Scotland. Many of these areas have significant landscape value, so the siting of wind turbines must be done with sensitivity.

The wind flow pattern over the sea is much smother than over mountainous areas of land and the visual intrusion of wind turbines is less important. However, the cost of wind turbines offshore is much higher than onshore. This is because the costs of construction and connection to the national grid are higher, as is the cost of maintenance. Larger turbines with capacities of 5 MW or more are used offshore.

Economics

The cost of a large wind turbine has gradually decreased and is now about £800 per installed kW. Smaller systems are more expensive, typically £2,600 per installed kW for a 15 kW system. The cost of electricity generated for the grid is about 3.2p per kWh from onshore wind and 5.5p per kWh from offshore wind.

Installed capacity

By the end of 2007, the global installed capacity wind power was about 94 GW, of which 19.7 GW was added during 2007, and wind supplied about 1.3% of global electricity. Germany (22.2 GW), the USA (16.8 GW) and Spain (15.1 GW) accounted for almost 60% of the capacity. In the developing world, India (7.8 GW) and China (5.9 GW) are the largest players, accounting for almost 15% of the world total. The UK had the ninth largest number of installations in the world, with an installed capacity of 2.4 GW.

Ashden Award winners using wind power:

Cwmni Gwynt Teg cooperative, UK (2003)
Renewable Devices Swift Turbines, UK (2005)
Good Energy, UK (2006)
Ecotricity, UK (2007)
Seaton Primary School, UK (2007)
Energy Agency, Ayrshire, Scotland (2008)
Sandhills Primary School, Oxford (2008)

Reference:

The main references for wind energy technology are the British Wind Energy briefing sheets.
Wind energy statistics are from the Global Wind Energy Council
A guided tour on wind energy can be found at: http://www.windpower.org/en/tour.htm