Traditional wind turbines are mounted on horizontal shafts and rotate in a vertical plane. These are referred to as horizontal axis wind turbines
(HAWTs). Alternative designs consist of rotating blades mounted on vertical shafts, known as vertical axis wind turbines
(VAWTs). Although VAWTs have existed for hundreds of years, they have not gained commercial acceptance due to their lower efficiency in converting wind energy to electrical power. However, new research suggests that these types of wind turbines may be better suited for wind farm installations than previously thought.
There are two major types of vertical axis wind turbines:
- Savonius-style turbines - these are very simple turbines that consist of several scoops mounted on a vertical shaft (Figure 1). The open face of the scoop experiences greater drag forces from the wind than the curved back face of the scoop. This differential drag causes the Savonius turbine to spin. A wind anemometer (Figure 2) is perfect example of a Savonius turbine. The history of this type of turbine can be traced back to an Italian engineer in 1622; however, the Finnish engineer Sigurd Johannes Savonius perfected the design and invented the Savonius turbine in 1922. Savonius turbines are less efficient than vertical axis turbines that utilize lift as opposed to drag, but simpler in design, and thus are used when cost or reliability is much more important than efficiency.
- Darrieus-style turbines - these turbines consists of a number of curved aerofoil blades mounted on a vertical shaft (Figure 3). This design of wind turbine was patented by Georges Jean Marie Darrieus, a French aeronautical engineer in 1931, although examples of similar devices can be found much earlier in history. Darrieus turbines tend to be more efficient than Savonius turbines.
VAWTs have a number of significant advantages
- Omnidirectional natural of the rotor: VAWTs are equipped with rotor blades that can pick up wind coming from any direction as compared to HAWTs which need to face the direction of the wind and require a complex yaw system to orientate the rotor.
- Performance in turbulant wind flow: The geometry of VAWTS allows them to work more efficiently than HAWTs in turbulant winds. See OWOE Amazing Energy: Capture Mobility Wind Turbine for a turbine specifically designed for turbulent wind flow.
- Turbine spacing: Because of VAWTs effectiveness in turbulent wind flow, they can be spaced closer together, requiring half or less of the area of a similar number of HAWTs (Figure 4). In addition, by locating two counter-rotating VAWTs in close proximity, i.e., one rotating clockwise and the other counter-clockwise, one can actually increase the output of each turbine independently.
- Lower center-of-gravity (cg): Since the VAWT gearbox and generator are located at the base of the turbine, regardless of turbine height, a VAWT has a significantly lower cg than a HAWT. This can result in a lighter structure. Recently, a number of researchers have been investigting use of VAWTs for floating offshore wind platforms, where the lower cg should lead to smaller platforms and reduced cost (see Figure 5).
- Starting wind speed: VAWTs have a lower starting wind speed than HAWTs which allows the turbines to generate electricity down to 2 to 3 m/s.
- Social and environmental impact: VAWT rotor blades are configured more closely around the turbine shaft. This minimizes the rotating blade shadow on the ground (referred to as "shadow flicker") and makes the blades easier to spot for birds and other flying animals. In addition, VAWTs emit less noise during operations.
- Installation and maintenance: Due to the smaller size of VAWTs, they are easier to transport, set up, and maintain. Major components, such as the generators, are built closer to the ground.
- Appearance: VAWT configurations allow modern geometrical designs that can be tailored to visually complement buildings, campuses, and parks.
Simple physics indicate that a single HAWT is more efficient than a single VAWT given the same wind velocity. HAWT blades can be turned into the wind such that they are always sweeping the most undisturbed air possible. Wake eddies do not interfere with the motion of the blades as they dissipate downwind. VAWT blades, by comparison produce maximum torque on the rotor at only a few points as they travel around the shaft. The remainder of the time the blades must travel through the turbulance that they have created. However, a number of more recent studies spearheaded by Professor John Dabiri
have shown that VAWTs arrayed in layouts that enable them to extract energy from adjacent wakes and from above the wind farm could potentially achieve power densities (watts of power per square meter of land area) significantly greater than those of wind farms consisting of HAWTs.