Figure 1。2  750 kW, 48 m diameter Wind Turbine, Denmark (Reproduced by permission of NEG MICON)

Figure 1。3  Wind Farm of Variable-Speed Wind Turbines in Complex Terrain (Reproduced by permission of Wind Prospect Ltd)

Figure 1。4  1 MW Wind Turbine in Northern Ireland (Reproduced by permission of Renew-able Energy Systems Ltd)

  The stimulus for the development of wind energy in 1973 was the price of oil and concern over limited fossil-fuel resources。 Now, of course, the main driver for use of wind turbines to generate electrical power is the very low C  emissions (over the entire life cycle of manufacture, installation, operation and de-commissioning)

Figure 1。5  Wind Farm of Six Pitch-regulated Wind Turbines in Flat Terrain (Reproduced by permission of Wind Prospect Ltd)

and the potential of wind energy to help limit climate change。 In 1997 the Commis-sion of the European Union published its White Paper (CEU, 1997) calling for 12 percent of the gross energy demand of the European Union to be contributed from renewables by 2010。 Wind energy was identified as having a key role to play in the supply of renewable energy with an increase in installed wind turbine capacity from 2。5 GW in 1995 to 40 GW by 2010。 This target is likely to be achievable since at the time of writing, January 2001, there was some 12 GW of installed wind-turbine capacity in Europe, 2。5 GW of which was constructed in 2000 compared with only 300 MW in 1993。 The average annual growth rate of the installation of wind turbines in Europe from 1993-9 was approximately 40 percent (Zervos, 2000)。 The distribution of wind-turbine capacity is interesting with, in 2000, Germany account- ing for some 45 percent of the European total, and Denmark and Spain each having approximately 18 percent。 There is some 2。5 GW of capacity installed in the USA of which 65 percent is in California although with increasing interest in Texas and some states of the midwest。 Many of the California wind farms were originally

constructed in the 1980s and are now being re-equipped with larger modern wind turbines。

  Table 1。1 shows the installed wind-power capacity worldwide in January 2001 although it is obvious that with such a rapid growth in some countries data of this kind become out of date very quickly。

  The reasons development of wind energy in some countries is flourishing while in others it is not fulfilling the potential that might be anticipated from a simple consideration of the wind resource, are complex。 Important factors include the financial-support mechanisms for wind-generated electricity, the process by which the local planning authorities give permission for the construction of wind farms,and the perception of the general population particularly with respect to visual impact。 In order to overcome the concerns of the rural population over the environ-mental impact of wind farms there is now increasing interest in the development of sites offshore。

1。2 Modern Wind Turbines

The power output, P, from a wind turbine is liven by the well-known expression:

                         P= 

where ρ is the density of air (1。225 kg/ ),   is the power coefficient, A is the rotor swept area, and U is the wind speed。

  The density of air is rather low, 800 times less than that of water which powers hydro plant, and this leads directly to the large size of a wind turbine。 Depending on the design wind speed chosen, a 1。5 MW wind turbine may have a rotor that is more than 60 m in diameter。 The power coefficient describes that fraction of the power in the wind that may be converted by the turbine into mechanical work。 It has a theoretical maximum value of 0。593 (the Betz limit) and rather lower peak values are achieved in practice (see Chapter 3)。 The power coefficient of a rotor varies with the tip speed ratio (the ratio of rotor tip speed to free wind speed) and is only a maximum for a unique tip speed ratio。 Incremental improvements in the power coefficient are continually being sought by detailed design changes of the rotor and, by operating at variable speed, it is possible to maintain the maximum power coefficient over a range of wind speeds。 However, these measures will give only a modest increase in the power output。 Major increases in the output power can only be achieved by increasing the swept area of the rotor or by locating the wind turbines on sites with higher wind speeds。