My friend just brought this question up, and I really don’t know of a real answer.

Why on wind turbines do they only use 1 prop, instead of using for instance 2 at the top parallel to each other, spaced out like 10m. but creating independent power (2 power lines running up to the top of the turbine.) Or why not put multiple turbines on the 1 shaft. Save space, rotate more? more power?

I would assume the 2nd one wouldn’t get as much wind behind the 1st one and the money would be better to be spent on it’s own pole.

I agree, the bottom one would not get as much wind. I also suspect that the “back draft” (or whatever you might call it - the wind blowing off or deflected off and around the edges of the top one as it turns) would affect the wind hitting the one below it.

I suppose you could try a fun experiment taping two pinwheels together to find out.

:::blowing::: 

Just teasing

I’ve also wondered why they settle for three blades instead of a whole series of them like in the old time farm wind-mills that were used to pump water. 

This is an area where I have no knowledge, and the reasoning is certainly not intuitive, at least to me.  Just north of Palm Springs they have a huge farm of these, and a few are experimental.  I’ll never understand the one that looks like an up-ended egg-beater. but I’ve seen it actually working.

Occam

Occam - 17 July 2008 07:04 PM

I’ll never understand the one that looks like an up-ended egg-beater. but I’ve seen it actually working.

I think you’re referring to the vertical axis wind turbine.  Wikipedia discusses this type and many more HERE.

Occam, look up the tesnic wind turbine on youtube it’s a very refined very efficient low wind “eggbeater” and the best I’ve found so far, make sure you watch both parts to see the advantage.

Occam - 17 July 2008 07:04 PM

I’ve also wondered why they settle for three blades instead of a whole series of them like in the old time farm wind-mills that were used to pump water. ... Occam

I’m sure this was based on many experiments and cost analyses. The old time farm pumpers were designed with materials and methods of the time.

These turbines are expensive and enormous.  When one fails, the blades are flung for over a mile.  Moreover, to be cost-effective they have to operate for years without substantial maintenance.  Altogether this means simple design with high-strength materials.

Finally, you might ask yourself why modern turboprop planes have only 2-4 blades on their props, but submarine props have 5+ with a very curvaceous look.  Different fluids, different engineering requirements, lots of simulation time.

Hypnos - 17 July 2008 11:37 PM

These turbines are expensive and enormous.  When one fails, the blades are flung for over a mile. ...

You don’t want to be near one in freezing weather either. They sling chunks of ice off the blades a considerable distance.

Water props and air props work in different ways, a wind turbine blade works like a sail boat or a airplane wing by generating lift not just by being pushed. Also why would you go near hundreds of spinning blades in the first place? Water propellers work in a different manor because the fluid is more dense. And as for why they only use 3 blades… not positive but I think they can vary the pitch of the blades and I think length and being light weight is just as important as surface area but these things are usually about 2 semi trucks long so adding blades shouldn’t be necessary. Oh and I mentioned the tesnic wind turbine in my last post but couldn’t put up a link cause I was on my cell but here ya go.

Darrieus (typical vertical turbine) vs. Tesnic (improved vertical turbine)
http://www.youtube.com/watch?v=98aCW1aTGu8
http://www.youtube.com/watch?v=IdadfFnCgWM