12 Watt wind turbine tail assembly and auto-furling design

Auto-Furling

A wind-turbine has a theoretical maximum efficiency of approximately 60%, meaning 60% of the energy present in the wind can be extracted. Up until that point, the power available is proportional to the cube of the wind speed. Since the aim is to extract useful power during 'average' wind-speeds in the range 7 - 20km/h, the turbine is designed to work in this operating range. If the wind speed were to get up to (say) 40-50km/h, the amount of power generated would rise very rapidly but I'd get more than I could eat and the coils would burn out. Designing the turbine to work in storm-force winds would involve beefing everything up and making the device too heavy to work in low wind speeds.

Large commercial turbines adjust the pitch of the blades so they become less efficient as the wind increases. Smaller machine often use a tilt-back tower that saves the turbine but requires someone to go out and reset it later. A better solution is to use an auto-furling system that turns the turbine blades out of the wind at high wind speeds. This is usually done by using a riser-hinge and adjusting the weight of the tail as to when it kicks in.

My solution is a simplified variation of the riser-hinge without the riser-hinge! The idea is to mount the blade-axis off-centre so that there is a force that tries to turn the blades out of the wind automatically. However, when this happens, the tail will start to swing around into the wind and it will be forced back behind the machine, presenting the blades back to the wind. The tail is hung from the tail boom on standard hinges and is free to swing back and forth (somewhat like a shop sign). When the wind reaches some threshold value, rather than turning the blades back to the wind, the tail will start to 'flap', dropping air becoming less efficient and so the machine will come to equilibrium at an angle to the wind, slowing it down. Well, that was my thinking and I'll delve into some maths later that hopefully shows this may in fact work. The telling test will be when the machine is mounted on a pole.

Going back to look at the frame, you can see that there is a square vertical section welded onto one side that will eventually be where the turbine sits on the mounting pole and it should be free to rotate around the pole. There are two box-section stubs that protrude horizontally past this vertical mounting. It is onto these stubs that the tail is mounted. The back stub sticks out slightly more than the front one so the tail will stick out at an angle of about 5 degrees to counter the fact that the blades will be offset from the mount point. Hopefully this arrangement also balances up the weight of the alternator with the tail.

As you can see from the frame, the stubs have been welded onto the main frame although on the Mk2 larger turbine, they were actually part of the main frame. I discuss a few more measurements in that section as in this first versions, things were more experimental

The tail boom itself was made from 25mm angle iron (mild steel actually). In reality is was made by bolting two sections of angle together as I did not have one 800mm piece. It was then bolted onto the two stubs after drilling appropriate holes. A more permanent arrangement would require welding it in place but as with the rest of the machine, I wanted everything to be modular and easy to dismantle to allow for adjustments. I don't really have a good photo of the tail being attached to the frame, but the picture below shows a front-view of the machine with the tail behind it.

The tail vane was made from a piece of 9mm thick plywood. Its dimensions were carefully chosen by selecting the only piece of scrap 9mm plywood I had to hand. It measured 20cm by 43cm and it looks a bit on the small side to me but that is easy to change later. The picture below shows how it is 'hung' from the angle-steel using small gate hinges.

Now the plan is that at a certain wind speed, the tail will start to hinge up and spill air, thereby be less efficient at turning the blades back into the wind. The wind speed at which this occurs will depend on the weight of the tail so it should be adjustable by putting bolts/weights at the bottom of the tail to make it harder to move. To get an approximate idea of the winds peed at which this will happen, I used the following:

10-Jul-2007