Why is there only one set of blades per wind turbine?

[Infinitybyzero]

I'm a civil/structural engineer and I was just recently thinking about one of the biggest issues for maximizing the efficiency of a wind turbine - the structure itself. The mega turbines that engineers want already have foundations of titanic proportions because as we all know, flag pole structures require a hell of a lot to keep them upright and stable. So my immediate thought in response to this is: can you allow for higher blade speeds and larger blades if you perhaps tied 4 towers together? I'm wondering if there would be a cost savings on the overall structure if you could take advantage of spreading out the overturning force from the wind and the dynamic load from the turbine on the turbine structure over a frame rather than just a cantilevered column.

 

[Bandit127]

I only discovered this the other day, but the load on a wind turbine is far from consistent. Wind contains gusts, eddies and probably other turbulences with a wide range of frequencies. The guy I was talking to was describing a project to dynamically adjust the pitch of the blades to reduce stress on the gearbox that was happening from one side of a rotation to the other.

Bearing that in mind, I think a flag pole structure might provide flex to help even out the force on the turbine and generator components.

 

[CWatters]

It would be impossible to tie the towers together at the top because they have to rotate depending on the wind direction.

 

[256bits]

higher blade speeds

The tips are traveling quite fast already.
http://www.aweo.org/windmodels.html

 

[CWatters]

I think the UK currently has the largest installed turbine at 640 feet with each blade 262ft long. However I read something about a turbine in development that's over 900ft tall.