wind turbine Blade Design (Aerodynamics part 2)
I want to go a lot deeper into the topic of how wind turbines work, focusing here on the blades in the rotor. The reason for doing this is to learn how to design the blades for specific targets, instead of general sizing. By thoroughly testing the electric generator that the rotor blades will drive, we should be able to design a rotor to match that generator, at least in most conditions.
A simple declaration, but there is a huge amount of information to be put together to make it happen. At least the information can be sorted into groups. At least by breaking it down it should be easier to tackle.
Properties of the Air
Where you live has a strong influence on the behaviour of a wind turbine. And it’s not only the average wind speed to be taken into account. Wind turbines are often found in very remote places, and sometimes they are “remote” because of their extreme environment!
The wind speed is a pretty obvious factor in wind turbine performance. So obvious that I don’t need to get into it now. I will come back to wind speed again many times so it doesn’t need a special introduction.
Air Density is a factor that does need consideration right away. It’s usually taken for granted, or folks don’t even know much about it. The density of air varies – a lot actually. Temperature, pressure regularly change the density of the air that surrounds you. Not only does this make things happen such as “hot air rises”, but it also affects the performance of a wind turbine. The WT has to take energy from that air, but as we’ll see soon, the energy is in the form of moving mass. Reduce the mass and the energy is reduced, too.
Some facts about air density: Sea level air density, at standard air pressure and temperature is 1.225 kg per cubic meter, or 0.0765 pound per cubic foot. If the temperature rises to 30C (86F) the density goes down 5%. If at the same time the air pressure is low (say 99 kiloPascals) then the air density is another 2% lower. The wind turbine gets only 93% of the energy it should have!
The last property I want to introduce is one that is probably unknown to the general audience: Reynold’s Number. This number is a strange factor but it has a fascinating way of governing the SIZE of the things people build. The reason for this is that the air itself doesn’t scale up and down, so why should we expect to scale up and down wings and rotor blades?! Air flowing over surfaces builds up boundary layers of slow-moving air, no matter how fast or how big or small the wings are. When we look at the properties of airfoils, we must be cautious of Reynold’s Number because it prevents the performance information from simply being scaled down.
The typical shape of an airfoil, easily recognizable on any aircraft wing, is well known to all of us, and yet there is a staggering variety of them out there. Some with big round leading edges, some are flat on one side, some have an extra curl on the trailing edge. What’s it all for?
Different airfoils can do different things. I suppose I should start by saying that ALL of them gain a certain coefficient of lift when the leading edge is raised to a certain angle of attack. Some will also know already well that the action of having lift immediately creates drag, too. Airfoils are often tested for other things like pitching moment and pressure distribution, but these factors won’t help our discussion and I will consider only lift and drag.
Very broadly speaking, airfoils that have a blunt or big round leading edge resist stalling, and those with a sharp leading edge will stall pretty abruptly. We can also say, generally, that an airfoil with a rough surface suffers from more drag than a smooth one. The different types and shapes of airfoils exist to take advantage of such rules, or to circumvent them! An airfoil known as the Wortmann FX 63-127, for example, has a blunt leading edge, thickness mostly near the front, and a reversed curve at the trailing edge. It was designed to have docile stall and to work at very low Reynold’s Numbers (there’s that mystery factor again). The Wortmann FX is quite a success, since it is found on dozens of aircraft types.
Perhaps better known and more common are NACA airfoils. They were developed in the 1920’s through the 1940’s during the first detailed scientific evaluations of just what airfoils should look like. 100 years later, we have dozens of airfoil families to choose from. Wind turbine developers and scientific agencies like the National Renewable Energy Laboratories (NREL) have shown a preference for the Selig and Wortmann families.
When looking at an airfoil, the questions a builder of wind turbines ask are many:
- How big will it be?
- Does it stall abruptly or smoothly?
- Does it resist contamination?
- What are the Lift and Drag properties?
- Is there enough space for the structure inside?
I will state my preference right away: I think wood is an excellent material for small wind turbine blades, and I can give many reasons why. But let’s look at the alternatives first. Many deserve a fair consideration.
Metals, like aluminum and steel are so common, it’s hard to believe more wind turbine blades aren’t metallic. You certainly can make a small wind turbine blade with metal, but even aluminum which is lighter than steel, will leave you with a very heavy set. Going back to the airfoils we saw earlier, the material required to “fill up” the space inside the airfoil makes it very heavy. Then consider if you can make the blade hollow – this too is possible, but manufacturing quickly begins to require advanced techniques, such extrusion dies, or welding. It can become too difficult for the average home builder – and this primer is meant mostly for people who want an accessible project that can be done in the basement or garage.
Plastic is also an appealing material – it’s cheap and plentiful, and light too. Well, actually, it’s too light, from a my point of view. The blades of a wind turbine must be strong enough to support their weight, even when spinning and hundreds or even thousands of revolutions per minute. Plastics generally can’t do that. PVC and nylon are about as strong as it gets, and they are marginal for the abuse typically given out to wind turbine blades by mother nature. They are also very flexible – though I have noticed that excessive flexibility comes more from the way people have tried building their plastic blades, not the inherent flexibility of the material. If it was done as a solid casting, with steel fittings to attach it to the central hub, I expect a plastic blade would be strong enough for harsh duty. Again, this specific technique is beyond the average hobbyist.
Plastics can be reinforced, either by carbon fibers or glass fibers. Here we definitely have a workable material – very light and strong, and it can be moulded into almost any shape the builder can imagine. Including wind turbine blades. I have tried it myself and the result was a very very light set of blades, however my technique was poor and their shapes were not well matched to each other. With practice, and some extra effort invested in making molds for consistent shapes, the fiberglass blades would turn out to be excellent. Carbon Fiber is more expensive, but it is so much stronger that the blade can be made even lighter than fiberglass – if such a thing were necessary. Fiberglass is cheap, and about the only drawbacks I can think of are the the fumes, and the numerous tools that get used up rather quickly.
Back to wood again, and the reason for my preference is that with wood I can RELAX. No cure times forcing me to rush, no skin peeling off my hands, and I will take the scent of sawdust over epoxy solvents any day!
My thoughts on this… coming soon.