Dragonfly wings inspire better windfarms
According to the British dragonfly society the maximum speed of a dragonfly is around between 10 and 15 metres a second and - in old money - that's roughly 25 to 30 miles an hour. But it's not the speed that we're interested in today so much as how these insects could help to improve the blades of wind turbines. Jean-Luc Mauricette is from Anglia Ruskin University. He explained how to Chris Smith...
Jean-Luc - So wind turbines don't work particularly well in turbulent conditions, the ones that you will typically see are the ones that look like propellers. The reason why the propeller types don’t work particularly well is because they always have to be facing the direction of the wind so if they’re off slightly by say 10, 15 degrees then they stop working. So you have to have costly mechanisms so they can always face the direction of the wind.
Chris - I gather that would mean then that you have to spread them out a lot. Obviously they’re not very space economic.
Jean-Luc - Precisely yes, they have to be spread out quite far from each other so they don’t interfere with each other's wakes. We can address this with vertical axis wind turbines which look like egg beaters.
Chris - I'm just picturing that like an egg whisk, vertically mounted egg whisk spinning round. How does that work then?
Jean-Luc - With the vertical axis wind turbines they have omni-directional operation, wherever the wind is coming from they will generate lift. But this also has some inherent problems because on one side the blades will be facing the wind and on the other they will also be working against it. You have to have higher winds to stop the wind turbine rotating. So this is addressed with the dragonfly design. We see an increase in lift and a decrease in drag which is what drives the power generation of the turbines.
Chris - I was going to ask you where did the dragonflies come in and now you’ve handed me a piece of plastic which has got lots of ridges and furrows in it. Tell me what I'm holding here.
Jean-Luc - This is based on a cross-section of a dragonfly wing. And you can see the corrugations there. It looks counterintuitive if you look at a typical profiled airfoil that you would see from a plane wing or a propeller. So that looks like a elongated teardrop turns sideways.
Chris - With a very smooth surface. So what you’re saying is that if we cut a wing across it would be like a teardrop shape turned on its side, but it would be smooth because we're all very familiar with the idea of making smooth surfaces for purposes of being aerodynamic and so on. So why the dragonflies have all these ridges in their wings then?
Jean-Luc - It’s actually similar to what you see on a golf ball. So the dimples in the golf ball will create turbulence which reduces drag but also increases lift as well.
Chris - This seems counterintuitive doesn't it? You make turbulence and that makes you have less drag because what the air doesn't stick to the surface as well when it's all turbulent round the ball, and it will be the same over this wing surface?
Jean-Luc - Yes.
Chris - And how does that actually translate into better performance then of that blade if it's got these corrugations?
Jean-Luc - It increases the lift by causing turbulence and reduces the drag because the passing airflow is not directly interacting with the surface of the blade. With typical wind turbines, you can lose up to 20 percent efficiency because of soiling of the surface. But if the passing wind isn't interacting with the surface directly then you won't have that problem. So dirty blades will not negatively affect performance.
Chris - Have you built a model then? So you know what, if you were to take a wind turbine and design the blades out of a material like this one that you've you've given me as an example based on dragonfly wing. Have you done modelling studies to work out how it would perform and under what sorts of wind conditions and therefore how much better it would perform if you were to do this?
Jean-Luc - Yes. So we've run computational fluid dynamics simulations and they show that we expect around a 26, 27 percent increase in performance.
Chris - Straight away? Just by using this shape informed by a dragonfly wing?
Jean-Luc - Yes yes because as well as reducing drag and increasing the lift of the blade, it also delays stall. So once you reach a certain angle the flow around the wing breaks off and it stops producing lift.
Chris - Is it actually feasible to build with this though because for the people at home it looks like you know when we were little in the classroom and we used to make a fan by folding bits of paper back and forwards on themselves and you end up with a concertinaed surface it looks a bit like that. This surface - could you build something at an industrial scale with that sort of pattern on the blades?
Jean-Luc - Yes absolutely and it actually should be easier because if you break it down into individual components they're essentially a collection of straight lines and they can be created with recycled materials as well. And because the surface doesn't have to be smooth because it's not interacting with the passing airflow then recycled materials have a good place in the manufacture of these blades.
Chris - And you're saying that because these vertically mounted turbines would be more immune to the effects of soiled air, kind of lots of turbulence from adjacent turbines, you could pack a lot more of them into less space so we would therefore generate our generating capacity for a patch of land would increase.
Jean-Luc - Precisely and they would work a lot better in urban environments as well. Typically you don’t see wind turbines in urban environments because of the quick direction changes of the wind and the turbulence. But we see no such problem with these designed.
Chris - What about noise? Because people who live near wind farms say that they find under certain circumstances the noise quite objectionable - sort of low pitched throbbing noise that is chronic it's there all the time and it really does upset some people. Are these likely to be more or less noisy?
Jean-Luc - Well it's interesting that you mention that actually because research shows that they are less noisy because the surface is interacting less with the air itself so there's less of a pressure change. So we see a reduction in noise as well.