Twice Area Misconception

This is one of the biggest misconception about an action type VAWT. It is a well known fact that VAWTs use half the area exposed to the wind to generate power. However, it is also a misconception that a HAWT uses the full area it sweeps to generate power. When someone states that a VAWT needs twice the area of a HAWT to generate equal power (s)he immediately assumes that the area swept by a HAWT is a solid surface and that all the air particles that have passed through that area have touched the blades and generated power. This is not the case, some air particles do in fact pass through the HAWT without ever touching a blade. This is not desirable, which causes efficiency of the HAWT to drop.

Before the Jet age, propeller type military air crafts were firing their machine guns through the propeller blades. Picture this: a propeller is rotating at 2000 rpm and a machine gun is firing at 400 rounds per minute, but none of the bullets ever touch a blade. Of course this is the desired effect, if it were touching the propellers you would be shooting yourself instead of the enemy. But why were the bullets not touching the propellers? Because the machine gun was synchronized with the rotating blades...a simple concept!

You may ask what does this simple concept has to do with HAWT? In fact it has a lot do with it. Imagine if you will that a zillion air particles are coming toward a HAWTs blades and that these blades are rotating at their optimum rotation speed. One way or another many of the air particles will make contact with the blades and this will cause the air particles to come in contact with the blades to transfer a portion of their kinetic energy to the blades. At the same time, some air particles, just like the machine gun bullets, will be in sync with the blades and will naturally pass through them without ever touching the blades. The natural synchronization of air particles and blades is not desirable and can not be eliminated. One may assume that by putting more blades on to the HAWT (like the old American Windmills) that this problem can be eliminated. Unfortunately, the solution is not this simple and putting more blades could be problematic (this will be explained later somewhere in this site).

To demonstrate the air particles passing through the blades of a HAWT I made the following flash animation.

In this animation the blue bubbles moving from left to right represent air particles with their full kinetic energy. The green bubbles represent the air particles that have made contact with a blade and therefore have passed some of their kinetic energy to the the blades. By moving the slider next to the air stream up or down you can change the location of the air stream, with respect to the blades. You can also move the stream closer or further away from the rotation center by moving the slider down or up respectively. The red dots on the blades represent where the air particles will hit the blades. Notice that these red dots also change their positions when you move the slider. The black dot represents the air stream when you are looking at the air stream from behind the rotating blades. It is assumed that when the red and black dots are in contact the air particles are hitting the blades and the blue color will change to green. However, when the red and black dots are not in contact there is no color change and this means that the air particles are passing through the blades without ever touching a single blade. The second slider changes the rotation speed of the propeller however it does not change the speed of the air particles. When you move this slider you will observe that the interaction behavior has changed, however there are still some blue air particles passing through the blades even though the blades are rotating faster.

The actual interaction between the blades and the air particles is a much more complex phenomenon than what is described above and shown in this simple flash animation. In any case, this animation demonstrates that air particles will pass through the blades without ever touching a blade no matter how fast the blades are rotating. The slower the rotation of the blade, the more air particles pass, while faster rotation reduces the number of freely passing air particles, there are still some that pass through.

This means that a VAWT does not require twice the are of a HAWT. In actuality during low wind speeds the HAWT may require more area than a VAWT to generate the same amount of power and that is why HAWTs are much more inefficient than VAWT during low speed wind.

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