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Three blades is the smallest number that reduces the vibrations due to the blades crossing the support structure. When a blade crosses the support its applied force is reduced because the wind is slower around the support. This reduction in forces creates a yawing torque that can lead to unwanted vibrations. Much of the structural stiffness and bearing requirements are related to these effects. Three blades minimizes the effect because when one blade crosses the support the other two blades are out in a Y shape, shortening the force differential when compared to two blades.

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This is spot on and the real reason. Thank for posting this, I was working on a much larger comment but yours is better because it says the same thing in fewer words.

If tower thump were not an issue then a one bladed turbine would be the best possible turbine (cost efficiency wise).

Further down he's even arguing for turbines with 12 blades, it's a ridiculous number and quite inefficient unless you want very high torque at low rpm such as for directly driving a mechanical waterpump.

Jacques, you are too nice, you could have said you have also the practical experience of what is building a windmill[0]. A very nice read for people enjoying both using a computer and their hands building stuff.

[0]: http://jacquesmattheij.com/how-to-build-a-windmill

Is there any particular reason why that piece never uses the phrase "wind turbine"? That's actually what it is, right?

At the risk of stereotyping; because he's Dutch? :)

Because lay-people call them windmills.

You mean a real, practical, one bladed turbine like these guys? http://www.powerhousewind.co.nz/story.htm

Even though 12 blades are less efficient, the article points out that they may have advantages for the bigger system of a whole wind farm, availability of sites, production, transport, control and non-generating time. It's not beyond belief that all those other advantages could counteract the reduced efficiency. You can't really know from intuition unless someone actually studies it.

Another example - commercial aircraft are known to have a relatively inefficient shape, but that's cheaper to produce than the more complex and more aerodynamic alternatives even though they would reduce fuel costs.

It's significant that high-solidity turbines with many blades are in use. As you say, they're quite common in small-scale settings like water pumping.

An article purporting to 'discover' that design would probably be more shocking if it wasn't already in common use for sensible applications.

cool. I thought I had read in an issue of Popular Science or Discover magazine that one blade was theoretically the best but I don't know enough about it to say for sure.

you're probably thinking about a propeller - as in the type on an airplane or boat that puts energy into a fluid.

I'm not an expert but I'm pretty sure there is a big difference from a device trying to pull energy out of the fluid.

A quick google didn't lead me to an authoritative source, but I know I have read many times that a one-bladed propeller is the most efficient.

Oh, I had thought an infinite number of infinitely thin blades got you the closest to Betz's limit?

Wouldn't that just be a disk?

Interesting. Isn't a fish fin (as in the fins on the side etc) essentially a one-bladed propeller?

The three blade approach also offers better outcomes for repair costs (and animal strikes). High-blade, high-solidity designs are more likely to break in extreme weather (even a stopped windmill takes a lot of strain at 70% solidity) and repair is usually necessary after failure of a single blade.

Between that and your points, my impression is that many-blade designs aren't useful beyond small scale generation, which is exactly where they're still in use. There are an awful lot of three-blade advantages that went completely neglected in this piece.

In the 1980s there was a hype of one and two blade designs.

I always found the one blade designs most fascinating and vividly remember a video of the three one blade rotors of the Monopteros 50. Each blade was 56 m long. There was also GROWIAN, a two blade design with 100 m diameter on a 100 m tower.

All of these projects were abandoned and the towers demolished. The three blade design has won for the reasons you stated.

I searched hard for the Monopteros video but there isn't even a good picture which shows all three towers. By-catch: Wikipedia page about unconventional wind turbines [1]

[1] https://en.wikipedia.org/wiki/Unconventional_wind_turbines

Somewhat related, there are a few airplanes with one-bladed propellors, for example:

http://www.alisport.com/eu/eng/silent2_targa1.htm

In this example, it's used purely because of compactness when stowed.

So, this is a redesign. I was told that old Windmills (used for milling) evolved to have an even number of removable blades. This allowed the mill to continue operating, while repairing a damaged blade -- simply remove the opposing blade, during repairs.

This is generally true. Mill or pumping turbines generally have a lot of blades (for higher torque), which obviously increases the risk of blade failure. That can be offset by having designs which aren't decommissioned by blade loss.

Of course, that's much less appealing for commercial installations, where access to the blades is expensive and difficult.

edit: also, even blade counts on big, high-powered turbines tend to get into problems with resonance by being symmetrical.

For a two blade system, could that vibration problem be addressed by extending the support up beyond the nacelle, so that both blades cross slower wind spots at the same time?

No. That changes whether the turbine is pitching up and down on top of the post, but doesn't change the fact that the turbine shaft is pulled out (or pushed into) the turbine with different force at the crossover point. That will create vibration even more in the long-post model you describe.

There's a question then, why are support towers solid? Maybe they should be made more like the Eiffel Tower?

Because a lattice tower has much worse properties when it comes to interacting with the wind. The behavior of the air behind the rotor is as important as getting quality air to flow through the rotor.

Some early windparks used lattice towers, and some of those survive to the present but they are now known to be worse performance wise than tubular towers.

A Shukhov tower [0] has minimal wind loading, due to its hyperboloid sections.

[0] https://en.wikipedia.org/wiki/Shukhov_Tower

That profile is not very well suited to having a windmill on top of it. The problem is that the nacelle is tilted backwards only a few degrees so you need a very slender tower (or you'll have tower strikes in high winds).

The cooling towers of a nuclear plant follow the same principle. The sides are formed from straight structural members angled in opposite directions (doubly ruled), and the gaps are filled with concrete.

The profile of a hyperbolic wind tower may be altered such that the cross section at maximum height minus blade radius is at a minimum. The largely conical profile of the existing Shukhov towers were used for power transmission lines and a radio broadcast antenna.

Check this out: http://www.pbs.org/wgbh/nova/next/tech/proposed-hyperboloid-...

Interesting! You'd think that the major wind power manufacturers would have looked at all possibilities. One major immediate concern is that these look quite ugly and fat compared to the relatively thin and slender supports that the latest generation of turbines uses. I'd love to see a side-by-side comparison of these towers and the various alternatives.

http://cache1.asset-cache.net/gc/78480146-crowley-ridge-wind...

Is one example of lattice towers in production that I know of (but not like the ones you describe and I have never seen those being used for windmills).

As mentioned in Jobs quote near the top of the article, you often find that people tend to do things in a certain way just because that is what has been done before. No one bothers to experiment because the old way has always been good enough.

This is why is is sometimes difficult to avoid premature optimization, because post-maturity optimization tends not to happen, even when it may be warranted. I know of no wind farm anywhere that is used for side-by-side comparisons of wind turbine technologies. It would very likely have to be owned by a university and supported by public funds.

I have never seen a hyperboloid lattice or diagrid structure in person, but they are not entirely unheard of for shapes that do not require a lot of space efficiency in the tower portion, such as for lighthouses, observation decks, water towers, radio towers, etc.:

https://en.wikipedia.org/wiki/Adziogol_Lighthouse

https://en.wikipedia.org/wiki/Kobe_Port_Tower

https://en.wikipedia.org/wiki/File:Ciechanow_water_tower.jpg

https://en.wikipedia.org/wiki/Corporation_Street_Bridge

https://en.wikipedia.org/wiki/Aspire_Tower

https://en.wikipedia.org/wiki/Canton_Tower

>No one bothers to experiment because the old way has always been good enough.

If you are a manufacturer though, you have all of the incentives to come up with something more efficient though. Wind turbines aren't controlled by a monopoly are they?

Lattice towers were common in the past, and are still the best weight/material to strength choice. Other considerations have led to the tubular steel tower dominating, though a new generation of enclosed lattice towers is being developed.

https://renewables.gepower.com/wind-energy/technology/space-...

Probably to be more durable, because of the closed surface - the Eiffel tower has very high maintenance costs as it has to be steadily re-painted. It would probably not be necessary that much for modern steels, but unlike wind turbines the Eiffel tower also does not stand close to the coast or directly in the sea, where highly corrosive winds blow.

Additionally the enclosed space is useful. Inside the wind tower is a lot of equipment. There is the electronics for converting the power and monitoring the wind tower. There is a lift for ease of access to the nacelle. Current wind towers are 80m tall, next generation wind towers are more than 100m to access more reliable higher strength winds.

I don't think this is a strong driver. GE 1.5 MW turbines only have a relatively small space near the base of the tower that is consumed by power electronics and control systems.

Siemens has largely gone to a system where everything is in the nacelle. This way it makes it easier to completely test and connect the power electronics and control, since more of the work is done at the factory instead of in the field. Labor and equipment is much more expensive in the field than in the factory.

They kill a lot of birds too -- birds perch on the bars and then fly off only to be stuck by a blade they didn't see.

I know it's not really what you mean, but jacket foundations are fairly common in offshore wind. The emerged part is still always a solid tube.

https://www.iscpc.org/images/gallery/Jacket_Foundations_EWE....

Random question, why not 1 blade and a counterweight?

One-blade systems do have a counterweight. There are two parts to the vibration problem: One is that the flow field on the lee side of the tower is quite a bit different from the rest of the blade's sweep, so there is a once-per-turn torque pulse in that part of the path. The other is that the air flow field above the height of the nacelle is different from that below the nacelle. Since you are still in the planetary boundary layer, the airspeed varies with height.

by that single explanation, 12 blades would be even better...

it was also explained to that in very cold weather, the three blade design helps prevent any single blade from gathering too much ice from the wind? so if the top one gets a lot of ice its weight will shift it around? i am no engineer, could be nonsense.

Looks like a support structure innovation is needed

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