A vertical axis windmill is a windmill in which the turbine blades rotate around a vertical axis (perpendicular to the ground), rather than around a horizontal axis, as in the more traditional propeller-type windmills which you see in large wind farms. The vertical axis powered mill is not a modern invention; in fact, it appeared before the horizontal axis windmill did.
Windmills were first developed to help farmers grind corn and draw water from wells; the earliest known ones are vertical axis systems developed in Persia around 500-900 A.D. A typical early design used vertical sails made of bundles of reeds or wood, attached by horizontal struts to the vertical central shaft, to which the grinding stone was also affixed.
There are numerous modern implementations of the vertical axis wind turbine (VAWT), all of which claim to offer some advantage over horizontal wind turbines. Whether these claims have any validity is open to debate. For example, they claim to be less dangerous to birds.
While they do theoretically present a smaller impact area for a bird to fly into, there are no studies or figures published that back this up. The smaller impact area is not inherent to the design of the vertical mounting, but rather a result of the typically smaller size of the units, since they are marketed as single urban building use units.
Are Vertical Axis Windmills More Efficient?
A second frequent claim is that they are more efficient in generating power, since they can capture wind from all directions. Only a half-truth; horizontal axis turbines can capture wind from any direction as well. In both cases, the turbine blades will only produce power in one direction of turn, so where is the efficiency gain?
Furthermore, vertical axis turbines are touted as being perfect for residential and urban applications, since they can be roof-mounted and do not require a tower. There are two major problems with this idea.
First of all, wind speed near the ground is greatly reduced by the effects of friction between the ground and moving air masses, and wind speed is the energy which we are trying to harness; the higher the speed the greater the amount of electricity can be generated. This is why horizontal turbines are mounted on tall towers.
Secondly, the wind turbulence that exists near the ground- created by buildings, signage, highways and other man-made structures as well as trees- degrades the quality of wind to the point where it is difficult to consistently extract energy from it in an efficient manner.
All the knowledge in the fields of fluid dynamics and aerodynamics back this up.
At this point, you should be asking yourself why all the wind-generating turbines you see are horizontally mounted on towers, and you don’t see any vertical axes turbines mounted on buildings.
Hint- it is not because they are such breakthrough cutting-edge technology that no one has tried it yet.
The next question you need to ask is whether the location where you plan to install the wind turbine can consistently provide the necessary wind speed, taking into consideration trees and neighbouring buildings.
In the selection of windmill design, the most critical data to look at is how many kilowatt-hours of electricity the turbine will generate for a given wind speed. Compare outputs for equal wind speeds, and choose the highest efficiency turbine, regardless of configuration.
All other claimed benefits of the vertical axis windmill- bird friendliness, ease of generator maintenance, low noise, superior aesthetics, are secondary.