Science Questions

Eddie Cunningham asked the Naked Scientists: Why do wind turbines only have three blades? What do you think?

I imagine the number of blades is the result of an optimisation process. Parameters to optimise will include:
- maximising total energy captured
- minimising total mass (ie material cost/weight, bearing-wear etc)
- minimising downstream turbulence (efficiency and, importantly, noise pollution)
- minimising bird-strike
- (possible) minimising radar/TV cross-section (to reduce interference)
- limiting 'cross section' in gale conditions when the blades are deliberately stalled.

I'm guessing that a 4- or 5-bladed design would capture slightly more wind energy, but degrade most of the other considerations.

Aerodynamics is one of the least intuitive of all bits of Engineering.  Just 'looking at' a design can easily give you the wrong impression about its suitability. There are so many parameters involved and 'they' have certainly looked at all possibilities.

Not an expert on wind turbines but generally, fewer blades are cheaper to make and maintain.  On a wind turbine however, to sink the same power, reducing the count to two blades would require each of the blades to be quite a lot longer, and to generate the same power, each blade attachment would have to be proportionally stronger, pushing the costs back up.  In addition, because the blades would be longer, the tower would have to be higher, further increasing the costs.  On the other hand, reducing the blade count from four to three wouldn't incur the same degree of penalty, and as the turbine has to be at a minimum height anyway, three blades seem to be optimimum, all things considered.

Do the all have only three blades? Windmills one sees on farms have more.

Maybe the farmers did not understand aerodynamics.

Small turbines (like you get on boats) have multiple blades. (Damned noisy, tho)

If you're thinking of the wind-pump windmills one sometimes sees on farms or in western movies, where they are more like a fan than a propeller, I think it's largely due to the blades not being aerodynamically shaped - they're just planks of wood with no aerofoil cross-section, so you need to use lots of them to make up for the inefficiency.  Small turbines that you'd be likely to find on small boats can operate at much higher rotation speeds before they run the risk of shedding blades and so don't need a pitch-change mechanism, much reducing the cost per blade - when the wind gets too strong you just collapse or lower it - so you can afford to use lots of blades and reduce the diameter at the same time.

Why do wind turbines have three narrow blades, but ceiling fans have five wide blades?
Dale E. Berg, a member of the technical staff in the wind energy technology department at Sandia National Laboratories, replies:

The differences between wind turbine and ceiling fan blades arise from the contrasting design criteria: the wind turbine is intended to capture high-velocity wind to generate electricity efficiently; the ceiling fan needs to move air at low velocity with inexpensive components.

To keep drivetrain costs low, a wind turbine must capture the energy in fast-moving air and rotate at relatively high speed—within limits, so as to avoid excessive noise generation. (Slow rotation would increase the torque and require heavier and more expensive drivetrain components.) Such high-efficiency energy conversion dictates the use of lift-type turbine blades, similar to airplane wings, of twisted and tapered airfoil shapes. The blade design creates a pressure difference in wind—high pressure on one side and low pressure on the other—that causes the blades to turn. A combination of structural and economic considerations drives the use of three slender blades on most wind turbines—using one or two blades means more complex structural dynamics, and more blades means greater expense for the blades and the blade attachments to the turbine.

The ceiling fan, on the other hand, is built to keep the occupants of a room comfortable by moving air gently. Its engineers work to minimize noise while the fan rotates at low speed (for safety reasons) and to keep the construction costs, and therefore the purchase price, low. Energy efficiency is not a primary concern, because operation is inexpensive—a typical ceiling fan running 24 hours a day consumes about 60 kilowatt-hours a month, for an average electricity cost of about six dollars. For this reason, most ceiling fans incorporate blades that are comparatively inefficient drag devices; rotating the pitched blades pushes air vertically out of the way. Wide, flat blades are inexpensive to build and work well as drag devices. More blades are better, up to a point, and the usual layout of four or five blades is the result of balancing trade-offs between efficiency and expense.

A 2001 article in Mechanical Engineering chronicled the quest of a man named Danny Parker to create a more efficient ceiling fan. Parker’s initial blade prototype looked a lot like a wind turbine blade, but the end result (because of man­ufacturing, safety and operating concerns) was a hybrid between a standard ceiling fan blade and a wind turbine blade.

http://www.sciam.com/article.cfm?id=wind-turbine-fan-blades

How about using screw formed devices?

Would that work?
(Maybe in concord with some sort of windfunnel creating a stronger airflow.)

this isn't a question or an answer, but an opinion, adult to adult, ok? this is a question many people will search for and they will find this page, so i think it's important they should read an opinion that shares their skepticism, and offers an alternative view to the witewash presented here. wind farm blades are exactly like a PROPELLOR, not a turbine. it's the elephant in the room and that's why this page is here. these are covert weather weapons / defences. they are so bad at catching the wind, the operators have been caught feeding them power FROM the national grid, just so they go round and people think they're doing something. it's also tragic that clean energy projects have been compromised in the process.

Maybe in the olden days the farmers had three blades and now it's a tradition so thy don't want to change it?

lazyhorse is onto something. It's been discovered that the plan is to implement enough of these and run them off the electrical grid. The government would then be able to speed them up and slow them down at will. This would actually speed up and slow down the rotation of the earth and they could get rid of the daylight savings time which causes such a problem twice a year when everyone needs to convert. Not too many people know about this plan.