Why do engineers choose expensive rotors blades for wind turbines when sheet metal would do?

Question

I realize that the aerodynamics (or fluid dynamics) of the engineered blades are ideal -- proven through rigorous testing and re-design. But if all of this development and high tech materials make the blades so expensive that we can only put three to a rotor then maybe all of that R&D is wasted time and money. Why couldn't we just make rotors with many uncomplicated and cheep sheet metal blades. The greater number of blades would guarantee that more energy was harnessed in light winds and the pitch of these blades could be rotated in high winds so that they didn't rev at too high an RPM for the dynamo to make efficient energy. It just seems to me that with greater surface area comes greater harnessing of the wind's potential energy. I know that more blades create more drag but wouldn't that be cancelled out by the greater amount of wind energy harnessed and therefore the more electricity produced

Answer 1

No. The problem is that as you add blades, the blades begin to interfere with the efficiency of the other blades.

Imagine it like an airplane's wings: you can add a second set of wings to form a biplane, but that means that the lower set of wings is flying in the downwash from the upper set. In order to counteract that, the lower set of wings needs to be set at a higher angle of attack relative to the fuselage. And if you add a third set of wings, the lowest set is now flying in the downwash from TWO upper sets of wings. And so forth.

With a rotor its worse, because EVERY blade is in the wake of all the others, which means that the most efficient rotor has the fewest number of blades. Two blades creates structural problems with harmonics, so three is the usual practical minimum. More blades just lowers the efficiency of the rotor (and increases the expense) for no net return.

Of course, if you're not concerned with efficiency, a high-solidity multi-blade rotor made out of sheet metal will work -- think of the old-style farm windmills used to pump water, for example. That kind of rotor works well in low speed, high-torque applications. But as the windspeed increases, more and more of the available energy will be wasted by a rotor of that kind, compared to something more aerodynamically efficient. From a cost-benefit perspective, you're better off with the modern low-solidity turbine.

Answer 2

General idea is good. I'll try to demonstrate why it is conceptually not so good.

The design for the rotor blade shape need only to be done once. therefore trying to save cost at the design stage would be not very good conceptually.

Imagine a hard disk. If, because it was time-consuming to format the hard disk in a very efficient format, and you chose to do a less efficient format because it was quicker to format, And you format the hard disk only once, then the problem is that you end up paying for it throughout the lifetime you are using it.

Cheap metal blades would be ok if only one windmill is there, otherwise the farm would be affected.

Also, note that the saving would only be on the blades, the other components, like the racers, the rotor etc would have to be designed stronger/better to compensate for the poor design of the wings.

The energy is not only a function of surface area.

Answer 3

NIce idea, but making an inefficient thing bigger, usually doesn't improve it's efficiency. Sheet metal is fine for wind turbine blades, if it is formed precisely and run a slow speeds. The airfoil shape of an efficient turbine blade is a precise thing and relatively small variations can reduce the efficiency tremendously. Adding more blades doesn't guarantee success, because your are fighting the system's friction. If the blade assembly gets out of balance, then the turbine will quickly shake itself to pieces. Sheet metal is hard to form precisely. It is possible however to form airfoil blades that are designed to be flexible.....a sailboat sail is a good example. But the shape must be precisely formed and maintained as the airfoil runs through the air. All in all, it seems like an easy problem to solve, but in reality, it's a precise piece of engineering.