Survey of Energy Resources 2007
Wind Energy - Types of Modern Wind Turbine
Early machines - twenty-five years ago - were fairly small (50-100 kW, 15-20 m diameter) but there has been a steady growth in size and output power. Several commercial types of wind turbine now have ratings of over 2 MW and diameters of around 60-70 m; machines for the offshore market have outputs of up to 5 MW and diameters up to 110 m. The way in which sizes have increased is shown in Fig. 12-3; the average rating of turbines installed in Germany in 1992 was 180 kW and in 2005 it was 1 728 kW - nearly ten times as much.
Machine sizes have increased for two reasons. Larger machines are cheaper and they deliver more energy. The energy yield is improved partly because the rotor is located higher from the ground and so intercepts higher-velocity winds, and partly because they are slightly more efficient. Yields, in kWh per square metre of rotor area, are now double those of 1990 (Welke and Nick-Leptin, 2006). Reliability has also improved steadily and most wind turbine manufacturers now guarantee availabilities of 95%.
The majority of the world's wind turbines have three glass-reinforced plastic blades. The power train includes a low-speed shaft, a step-up gearbox and an induction generator, either four or six-pole. There are numerous other possibilities, however. Wood-epoxy is an alternative blade material and some machines have two blades. Variable-speed machines are becoming more common and many generate power using an AC/DC/AC system, but double-fed induction generators are becoming increasingly common. Variable speed brings several advantages - it means that the rotor turns more slowly in low winds (keeping noise levels down), it reduces the loadings on the rotor and the power conversion system, and the generators are usually able to deliver current at any specified power factor. Some manufacturers build direct-drive machines, without a gearbox. These are usually of the variable-speed type, with power-conditioning equipment.
Towers are usually made of steel and the great majority are of the tubular type. Lattice towers, common in the early days, are now rare, except for very small machines in the range 100 kW and below. Recent increases in the price of steel have re-awakened interest in concrete towers, but there are relatively few examples yet.
As the power in the wind increases with the cube of the wind speed, all wind turbines need to limit their power output in very high winds. There are two principal means of accomplishing this, with pitch control on the blades or with fixed, stall-controlled blades. Pitch-controlled blades are rotated as wind speeds increase so as to limit the power output and once the 'rated power' is reached, a reasonably steady output can be achieved, subject to the control system response. Stall-controlled rotors have fixed blades which gradually stall as the wind speed increases, thus limiting the power by passive means. These dispense with the necessity for a pitch-control mechanism, but it is rarely possible to achieve constant power as wind speeds rise. Once peak output is reached the power tends to fall off with increasing wind speed, and so the energy capture may be less than that of a pitch-controlled machine. The merits of the two designs are finely balanced and until recently roughly equal numbers of each type were being built. Since the turn of the century, however, pitch-controlled machines have become more popular.