Wind Energy Technology Trends
The principal components of a commercial wind turbine are the following: rotor which consists of the blade and hub; nacelle which houses the gearbox and generator; tower; control; foundation; and transformer. Modern wind turbines have 3 blades and their speeds are controlled by either stall or pitch regulation. The rotor of commercial turbines are either connected to the generator through the gearbox and drive train or coupled directly to the generator.
Wind turbine design drivers include the following: low wind and high wind sites; grid compatibility; acoustic performance; aerodynamic performance; visual impact; and offshore expansion. The large unexploited potential of offshore areas drives the recent developments of wind technology. Considerations for the development of large turbines for offshore areas are: low mass nacelle arrangements; large rotor technology and advanced composite engineering; and design for offshore foundations, erection and maintenance.
Turbine Size, Rotor Diameter and Hub Height
The size of commercial wind turbines has steadily increased from 20-60 kW units in 1980s. Commercial prototypes of 1-2 MW turbines appeared in 1990s. The first 4.5 MW prototype was constructed in 2002 and two 5 MW prototypes in 2004. 7-12 MW wind turbines are being talked as the next generation of offshore turbines.
Wind turbine�s power rating is determined by the square of the rotor diameter. Thus, the increase in turbine size is associated with increase in rotor diameter. The 50 kW turbines in the 1980s had diameter of 15 m, the 500 kW units in early 1990s had 40 m diameter, the 2 MW models have 80 m diameter, and the 5 MW prototypes have more than 120 m diameter. The 7-12 MW turbines could have rotor diameter of more than 200 m.
For turbines with the same power rating, there has also been a remarkable increase in rotor diameter. For 1.5 MW wind turbines, for example, turbines manufactured in 1997 have average diameter of 65 m while models in 2003 have an average diameter of 74 m. This is partly due to the optimization of design to maximize energy capture on low wind speed sites.
Hub height selection is site dependent. There is a trade off to be made between the benefits of extra energy derived from elevating the rotor to higher heights and the cost of larger towers. Offshore sites have relatively low wind shears and will not benefit from increasing the height of the towers. Large offshore turbines could be expected to have less than or equal to the rotor diameter with provisions for blade tip clearance in extreme wind conditions.
Power Control and Speed Variation
During high operational wind speeds, power is regulated either by stall or pitch regulation. Stall regulation requires speed control, which is achieved by connecting the generator to the grid. The grid holds the rotor speed constant regardless of changes in wind speed. As the wind speed increases, flow angles over the blade section steepen and the blades become increasingly stalled thus limiting power to the desired levels. Pitch regulation on the other hand involves turning the blades about their long axis to regulate the power. Pitch regulation however requires changes to rotor geometry as well as active control systems to sense blade position, measure output power and instruct changes in blade pitch.
The Danish classic wind turbine designs are stalled regulated but pitch regulation is becoming favored for large wind turbines. Most multi-MW wind turbines available in the market at present are pitch regulated. This is due to the fact that pitch regulation offers better output power quality. Also, for certification purposes, the independent operation of each pitch actuator allows the rotor to be regarded as two independent braking systems.
During the earlier years of wind power generation, wind turbines operate at fixed speed when generating power. At the start up, the rotor is initially parked, then accelerated by the wind until the desired speed is reached during which the connection to the grid is only made. As wind speed continues to increase, the rotor speed is controlled by either stall or pitch regulation. Variable speed designs were later introduced to allow the rotor speed and the wind speed to match in order for the rotor to maintain the best flow geometry to achieve maximum efficiency. The rotor could be connected to the grid at low speeds and would speed up in proportion to the wind speed. Once the rate power is achieved, the rotor is controlled to operate at constant speed through pitch regulation. Two speed systems were also introduced to improve energy capture and noise emissions of stall regulated wind turbines.
At present, pitch regulation and variable speed are the design routes pursued by many manufacturers for large turbines. Of the 52 models with more than 1 MW power rating from 20 wind turbine manufacturers in 2003, 37 models had variable speed, 12 had 2 speed systems, and only 3 with fixed speed. It appears that multi-MW wind turbines require some degree of speed variation and continuous variable speed designs are the preferred choice of the majority of manufacturers. Variable speed is realized in various ways. The details to achieve variable speed from direct drive systems are significantly different from the conventional gearbox and drive train systems.
The conventional design to transmit the energy captured by the rotor to the electric generator is through the drive train and the gearbox. The rotor is attached to a main shaft, which is connected to the gearbox. The gearbox increases the speed of the shaft to a speed required by the generator. A large wind turbine at MW scale could have a three stage gearbox and four or six-pole generator.
Due to various difficulties associated with the gearbox, direct drive systems were also pursued by a number of manufacturers. In direct drive systems, the rotor is directly coupled to the generator. The system has gearless drive train and multi-pole generator. The advantages of direct drive systems are reductions of capital costs, drive train losses, downtime and maintenance cost. On the other hand, direct drive trains are heavier than conventional drive trains since the mass and size of direct drive generators are large.
Some manufacturers developed a new drive system, which is a compromise between the conventional drive train and direct drive. This hybrid system consists of a single-stage of gearing and multi-pole generator. This type of power train avoids the complexity of a multi-stage gearbox and has a lower system mass since it uses a medium speed multi-pole generator.
At present, the conventional drive train dominates the market but direct drive systems and hybrid systems started to appear in the market.
Source: www.ewea.org; Eize de Vries, Thinking Bigger, Are there limits to turbine size. Renewable Energy World May-June 2005.