Wind Turbines

Tube Turbine Blades

How to turn a drainpipe or gutter into a wind turbine. Be careful ! They may be dangerously fragile, but they are cheap: approximately GBP 2 for a blade and one hour of work. Ordinarily, small wind turbines stuck on a roof are a bad idea: too much noise, and the slow gusty wind found at rooftop height generates little power with horizontal axis turbines. However, if you are just playing around, and not attaching anything dangerous to your house, gutter blades can be worth investigating. The original web posting was really interesting and original, but probably had a significant flaw: putting the trailing edge right on the centreline where it can only create drag.

The GIF diagram below is my optimisation of the original. There is also an ODG (OpenOffice) diagram available. Diagram of improved PVC tube wind turbine blade

Everything really depends on the lift to drag ratio of the turbine blades which dictates which tip speed ratios are efficient but there is another way to reason the change from the original design. First, the basics: to extract energy from the wind, the aerofoil must use its lift to pull the blades and generator around (rotation) and the wind must flow around the blades and out downwind/behind it, or the air will stagnate. This is a nice diagram of wind and turbine rotation. When a turbine is not rotating (apparent wind same as real wind) the greatest lifting force is produced when the aerofoil is facing directly into the wind and the lifting force produces only rotational force. However, in the original design, the trailing edge is at the centre of the pipe perpendicular to the oncoming wind (90 degrees away from static optimum) where the generated lift cannot pull the blade around at all ? Also, if one used a normal aerofoil section, then air approaching the trailing edge (after flowing round the downwind curved aerofoil face) could theoretically be travelling upwind, and the centre of lift of any aerofoil, is nearer the leading edge, than the trailing edge so there is a tendency for the blade to twist with the leading edge being blown downwind and the trailing edge even more upwind.

So, both leading and trailing edge blade cuts needs to be moved towards the leading edge and away from the centreline by the angle of the trailing edge aerofoil (5 to 10 degrees), plus a bit more if the blades are very twisty although maybe stall furling is useful ? I'm measuring distance in degrees away from the open drainpipe centre because I do not know how big your drainpipe is and it is the blade angle that is important. This small optimisation of the original design made a big difference to my tiny turbine (not a scientific test, not properly controlled, but 50mA went to 120 on a tiny stepper motor turbine). There are so many interacting variables it is hard to say anything general for different generators, aerofoils, blade numbers etc. The highest efficiency theoretical wind turbine rotor has many thin blades but almost no-one can be bothered to build one because of the construction effort. Do beware of the wierd detrimental aerodynamics of common aerofoils at small Reynolds numbers. The lift Vs. drag curves have unusual dips at even low angles of attack when the viscosity of the air becomes important with small blades. Actually, simple trigonometry shows that if the blade has a lift to drag ratio of 10, then an angle of only 5 degrees upwind from the plane of rotation would be enough for the lift rotational component to overcome the drag and spin the turbine. If your L/D ratio is worse(lower), then you need to move further from the centre cut of the drainpipe, and you may need more blades. Finally, if the overall curvature of the aerofoil blade is reduced by angling the blade cuts to chop off the inside of the trailing edge, (opposite of original design), then the cuts can be nearer the centre of the drainpipe.

There are no exact dimensions in the above section because everything depends on the number of blades, gearing and, most of all, the diameter of your tube or gutter. Wide thick tubes would save on trouble later but with small Reynolds numbers (below 100,000) really thin aerofoils have an advantage for lift and drag. I guess I should turn all the calculations into an online program that creates a PostScript plot for your computer printer ? I would need some aerodynamics help - anyone interested ? Actually, this PDF seems to have all the answers for blade design.

There are some useful online introductions aerodynamics and aerofoils and lift. There are more pages on more robust tube turbine blades but one might want more reinforcement at the axle or is that flexibility a cheap way to offer a teetering blade which is especially significant for two-bladed turbines.

Another idea I would like to try is the self-furling blade that protects itself in high winds by pointing more upwind to reduce its tip-speed ratio. This is actually really easy if the blade root is hinged at, for example, a 45 degree angle, then increased lift causes the hinge to bend and point the blade more upwind, so slowing its rotation. Try it with an old pop (soda) bottle: less than a minutes work with some scissors.

Here is a variation (new twist) on a Savonius vertical axis rotor that some Indian theoretical engineers with a computer say is more efficient than the standard one with all-vertical edges. I also heard that they are more efficient if the buckets overlap so air can flow from one to the other past the centre axis but that is not shown below. Savonius rotors work mostly through drag so they can never be as efficient as lifting blades but they can be cheap, simple and sturdy so maybe still have their place for experimenters. The axis is shown in red, and the outside edge of the bucket is in blue.

image of a savonius rotor seen from below>

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