This is a 3 or 6 bladed machine, but I’ve drawn only 2 blades (and 2 towers) in the diagrams to make them easier to understand.
The 20 Megawatt VAWT Exceeds the Betz Limit
The rule of thumb for separating horizontal axis turbines is that two turbines should not be closer than two or three rotor diameters. Apparently, after the streamtube has traveled two or three rotor diameters beyond the turbine, it will have re-absorbed kinetic energy from the surrounding wind to the extent that its energy density will have roughly returned to its original value. It’s easy to see how this will apply to a very large diameter VAWT. Simply imagine a ridiculously large turbine. Say for example the diameter of the VAWT described here is equal to the diameter of New York City. In this case it is obvious that the wind that passes through the downstream 180 degree arc will have just as much energy as the wind that passes through the upstream 180 degree arc. From this example we can see that the maximum fraction of energy that a very large diameter VAWT may extract is 2 x 60% = 120%. Darrieus rotors have a reputation for delivering lower efficiency than their horizontal axis cousins. However, given the doubling of the ceiling on energy extraction, it is reasonable to believe that a very large diameter VAWT may prove to be quite efficient. This is wonderful news, since one frequently cited reason for the gradual disappearance of the Darrieus from the windfarm landscape is that it has the larger weight per power ratio. The extraordinary scalability of the VAWT described here together with its ability to exceed the Betz limit may very well reverse this state of affairs.
Though I believe that a very large machine is possible with the configuration presented here, the design may also be useful for smaller machines. In this case, it seems like the outside towers might be eliminated:
In the interest of clarity, the diagram has a number of simplifications. For example, at least three towers are required, but more than three towers may be used if needed. Why would you need more than three towers? Because the diameter of this machine can be made very very large.
One design alternative would have the generators on the ground. In this case, the central tower is replaced by the torque tube, and the torque tube extends to the ground where it engages the tire that drives the generator. Another alternative would replace the cables in the drive system with symmetrical airfoils. Why use airfoils instead of tubes? To minimize drag. Why airfoils instead of cables? Look again at the diagram that shows the whole machine:
If the (vertical) airfoils are tall, the cable might make them bow too much or might put too much vertical load on the rings. Using a streamlined airfoil to transfer torque to the torque tube would eliminate these problems.
Single Airfoil Support Variation
This post shows improvements on ideas from earlier posts. I won’t duplicate earlier discussions here, but I’ve pasted links below in case you want to read more. One very important point that is explained in earlier posts is that the wind machine proposed here can exceed the Betz limit. In fact, this machine can extract 120% of the kinetic energy flowing through the swept area instead of just 60%.
- 20 Megawatt Direct Drive Darrieus Wind Turbine,
- Highly Scalable Direct Drive VAWT, and
- Direct Drive Linear Turbine With Yawing Oblong Track.