Modern wind energy technology is available for a range of sites, wind speeds and climates. European wind farms are very reliable – they stand ready to operate 97% of the time (this is known as their “availability”) and are generally well integrated into the environment and accepted by the public.
Wind turbines grew constantly in size up to beginning of the century, but in the past three or four years there has been a levelling out of wind turbine size for onshore wind turbines and a focus on increased supply in the 1.5 – 3 MW range. That said, larger wind turbines are still being developed for the offshore wind power market.
The past exponential growth of turbine size was mainly driven by cost factors. Larger wind turbines are more cost-effective as they allow an optimised use of the land available, and the maintenance cost per kW installed is lower. All these factors, together with the psychology of “bigger is better” contributed to the growth of unit size through the 1990s.
The key factor in maintaining design development into the multi-megawatt range has been the development of an offshore market. For offshore wind farm applications, economics requires larger wind turbines in order to limit the proportionally higher costs of infrastructure (foundations, electricity collection and sub-sea transmission) and lower the number of turbines to access and maintain per kW of installed capacity.
Technology trends evolve around various different factors:
Rotor diameters: the industry always works towards larger diameters. The world’s largest wind turbine is currently the Enercon E-126 with a capacity of 6 MW and a diameter of 126 m.
Tip speed: for turbines on land, restrictions on acoustic noise emission often limit how fast the tip can go. These restrictions don’t exist offshore, which gives a clear potential benefit in higher tip speeds.
Pitch versus stall: there are now about four times as many pitch-regulated turbine designs (in which a monitor immediately turns the rotor blades slightly out of the wind if power is too high) on the market as stall-regulated (in which the rotor blades are bolted onto the hub but the blade design counters the lifting force from high winds).
Speed variation: this offers the possibility of increased ‘grid friendliness’, load reduction and some minor energy benefits.
Drive train trends: here the aim is to reduce the mass or keep the right balance between weight and size.
Hub height: when wind turbines were designed exclusively for land use, hub heights increased more or less directly in proportion to diameter. However, hub heights are now growing less than the diameter. This trend has come about because the largest machines are for offshore, where there is reduced wind shear.
Rotor and nacelle mass: manufacturers are continually introducing new concepts and materials in drive train layout, structure and components to reduce mass and cost.
Transport and installation: crane manufacturers now produce designs specially suited to wind farm installation. Often complete hubs are lifted onto nacelles and sometimes hub and blades are lifted individually.
Top Wind Turbines in 2009 (BTM):
1) Vestas Denmark 12.5
2) GE U.S. 12.4
3) Sinovel China 9.2
4) Enercon Germany 8.5
5) Goldwind China 7.2
6) Gamesa Spain 6.7
7) Dongfang China 6.5
8) Suzlon India 6.4
9) Siemens Germany 5.9
10) RePower Germany 3.4
By Elke Zander, www.ewea.org