Three blades and stall

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Three blades, constant speed, stall regulation and induction generators are the preferred design options of wind turbine owners in Germany according to new data from the Institute for Solar Energy Technology (ISET). The data sheds some light on which of the much discussed wind turbine configurations -- two or three blades, constant speed or variable speed, pitch or stall -- have actually dominated the market over the past ten years (fig 1).

Prior to 1990, pitch control was used for the majority of machines, but the preference is now for stall regulated designs, up from 48% of machines in 1988 to 54% in 1997. At 54% of all machines, the margin is slim, however. The trend towards three blades is stronger, with over 94% of machines in 1997 having this configuration, whereas in 1988 the figure was only 60%. The decreasing numbers of machines with pitch control and two blades is probably connected to the falling share of the German market held by Dutch company Lagerwey with its popular small turbine design.

The preference for constant speed rotors and induction machines is clear. In 1997, two-thirds of machines were constant speed -- up from 58% in 1988 -- and 69% had induction generators, up slightly from 65% in 1988. The changing trends for rotor speed and generator type have been more gradual, while down-wind machines have all but disappeared: 7% of turbines installed in 1988 were down wind but in 1997 their market share was 2%.

Perhaps the most dramatic changes of all is in machine size (fig 2). Average rated power has increased from 30 kW in 1987 to over 500 kW today. This is the average, not the maximum, and ISET notes on its web site that the latest machines have rotors diameters of up to 66 m and ratings of 1.5 MW or more.

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