Horizontal/Vertical Axis Machines
There are two types of large turbines that are utilized:
- Horizontal Axis Wind Turbines (HAWTs)
- Vertical Axis Wind Turbines (VAWTs)
In most farms, the horizontal axis-type turbines are used because the rotors are more efficient in translating the wind into rotational energy. Vertical axis turbines do produce some energy, but their efficiency is not close to that of horizontal machines. Since their rotors are closer to the ground, they do not get the fast winds to produce comparable amounts of energy. Also, some vertical axis turbines need guy wires, which are impractical for farming.
Because of the disadvantages with vertical axis turbines, horizontal machines are usually used. Most of the technology described on these pages is related to horizontal axis wind turbines.
Horizontal Axis Wind Turbines (HAWT)
All grid-connected commercial wind turbines today are built with a
propeller-type rotor on a horizontal axis (i.e. a horizontal main shaft). The purpose of the rotor, of course, is to convert the linear motion of the wind into rotational energy that can be used to drive a generator. The same basic principle is used in a modern water turbine, where the flow of water is parallel to the rotational axis of the turbine blades.
Vertical Axis Wind Turbines (VAWT)
Classical water wheels let the water arrive at a right angle (perpendicular) to the rotational axis (shaft) of the water wheel. VAWTs are a bit like water wheels in that sense (some vertical axis turbine types could actually work with a horizontal axis as well, but they would hardly be able to beat the efficiency of a propeller-type turbine).
In the figure at the side, Eole C, a 4200 kW Vertical axis Darrieus wind turbine with 100 m rotor diameter at Cap Chat, Québec, Canada. The machine, which was the world's largest wind turbine that time, is no longer operational. The only vertical axis turbine which has ever been manufactured commercially at any volume is the Darrieus machine, named after the French engineer Georges Darrieus who patented the design in 1931 (it was manufactured by the U.S. company FloWind which went bankrupt in 1997). The Darrieus machine is characterised by its C-shaped rotor blades which make it look a bit like an eggbeater. It is normally built with two or three blades.
The basic theoretical advantages of a vertical axis machine are
1. The generator, gearbox etc. may be placed on the ground and a tower may not be needed for the machine.
2. There is no need for a yaw mechanism to turn the rotor against the wind.
The basic disadvantages are
1. Wind speeds are very low close to ground level, where the lower part of the rotor is placed.
2. The overall efficiency of the vertical axis machines is not impressive.
3. The machine is not self-starting. However, this is only a minor inconvenience for a grid connected turbine since the generator may be used as a motor drawing current from the grid to to start the machine.
4. The machine may need guy wires to hold it up, but guy wires are impractical in heavily farmed areas.
5. Replacing the main bearing for the rotor necessitates removing the rotor on both a horizontal and a vertical axis machine. In the case of the latter, it means tearing the whole machine down (that is why EOLE 4 in the picture is standing idle).
- Horizontal vs vertical axis: general differences
- Vertical axis aerodynamic principle: general notes