Types of Wind Turbines.

Wind Turbines can be classified in two big groups: HAWT (Horizontal Axis Wind Turbines) which its rotor is connected to a horizontal shaft, and VAWT (Vertical Axis Wind Turbines) which have a vertical shaft.

HAWT

Since 1980’s, most of the commercial wind turbines are included in this group. The HAWT consist of a tower and a nacelle that is mounted on the top of the tower. The nacelle contains the generator, gearbox and the rotor.

Different mechanisms exist to point the nacelle and blades to the wind direction. If the wind turbines are small they can be oriented by a tail vane. Furthermore, if they are downwind rotors, they can oriented automatically. On large turbines, the nacelle is moved by an electric yawed system. However, we must say downwind Rotors are noisier since the once-per-revolution tower passage of each blade is heard as a low frequency noise (Hansen 2008).

The turbines with one, two, three and four blades are designed to generate electricity power. On the other side, turbines with 20 or more blades are used for mechanical water pumping. The number of rotor blades is indirectly linked to the Tip Speed Ratio (λ). It means, wind turbines with few numbers of blades have a high Tip Speed Ratio but they might need to be started if the wind speed reaches the operation range.

At this moment, three-bladed wind turbines dominate the wind market. They are more aesthetical and quieter than two-bladed. However, two-bladed rotors have a tower top lighter and hence, the supporting structure can be less heavy and the most important, the costs lower. Therefore, it is overseen the number of this kind of turbines will increase in the off-shore farms (Ackerman2005).

The HAWT can be designed following three philosophies:

  • Following the design of Gedser (1950): High resistance against winds, low tip speed ratio and optimized for reliability.

  • The Hütter turbine (developed in the 1950’s in Germany) is focused on high performance.

  • The last one, it is the philosophy that it is having more followers. Low speed ratio for less visual disturbance and better power quality because short term wind speed variations (within seconds) are not directly translated into power output fluctuations.

VAWT

The big advantage of this group is that the generator and gearbox can be installed at the ground level, making easier surveillance and services tasks. In addition, they don’t need to be oriented to the wind direction. We have three models:

1- Savonious model is manufactured commercially but just for market niches, like battery charging, motor starting, advertisement posters, or rotorvents.

2- Darrious turbine must need an starter motor and its swept area is 2/3 D2, the Tip Speed Ratio around 6 and the Power Coefficient Cp just above 0,3.

3- Giromill is self-starting, but the blade angles have to be adjusted in relation to the wind direction to start. The swept area is obvious, Height times Diameter. It is more efficient than a Darrieus turbine and has a wider range of Tip Speed Ratios, but it is not as efficient as the HAWT. In addition, blades must bear strong loads and the large prototypes have surrendered to the winds.

Sources: Wind Power in Power Systems by Thomas Ackermann, Aerodynamics of Wind Turbines by Hansen, Developing Wind Power Projects by Tore Wizelius.

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