Always willing to bring yet more wind energy information to the world, I once again made the sacrifice of going for a drive in the country. Last time I went North, this time, South.
In fact this time I drove down to Glenelg River in Victoria, just East of the South Australian border, just north of the wet stuff at the bottom of Australia that keeps us from walking to the south pole. This trip was only a little over a thousand kilometres. I spent a couple of days there to look at stuff.
I saw a stack of these.
Some, mixed in with these.
And I even saw one of whatever this is.
This thing was actually kind of interesting. I might have to make one out of drinking straws or something. It's most interesting aspect was that it worked like two different styles of wind turbine depending on which way the wind was blowing.
Or not. I'm not sure. But if it didn't work like I think it does, then I've just had an idea.
Sometimes you just have to wait until glue dries before you can progress with your wind turbine construction. This has been one of those times, and this post follows on from the post called "Wind energy - Carving wooden blades".
After undoing all the clamps, I still felt a bit like I had no idea what I was doing. This was partly due to knot getting enough sleep, and partly due simply to the fact that I really don't have any idea what I'm doing.
I'm still not certain I've got the shape correct.
But that's ok. I'll learn some stuff anyway.
Just don't copy this until you see if it works out.
The aim here is to try to make the blade have a steeply angled, thick wing shape at the slow centre, tapering out to a thin, slightly angled wing shape at the faster moving tip.
The cuts I'm making go to approximately 6mm from the edge to around 6mm from the other edge
It seemed like a good idea at the time.
I used a coping saw to make these cuts. A coping saw is the only saw I ever enjoy using. They have rotatable blades so you can cut sideways, and because the blades are very thin, you can cut circles etc. They are the only saw that seems to cut anything. It might be that, because the blades are so thin, you break them all the time. This means your blade is always brand new. Buy a coping saw if you need a small saw. They are very cheap, and are great to work with.
It took around 10 minutes to make the cuts.
Any of the regular readers (hi regular readers) might have come to realise that perhaps I don't use rulers and things as much as I should. This may be one of those times.
If you look carefully, you can see a collection of random looking lines drawn on the wood. At least one of these lines is important. Perhaps two.
This is the under side of the blade. Actually I think this might be the front. This is my problem. I cant tell. I'll cut and see.
Hack away at the little bits of wood between the cuts. They break of in a very satisfying way.
I used a chisel for a bit, then just hit them with the side of the chisel as if it were a hammer.
Then I just used a hammer.
Once again, I'm a little surprised.
It kind of looks a little bit like it might one day resemble a turbine blade.
The wacky angled bit of wood in the foreground is just the top, smallest length of wood on the progressively more angled stack.
The bit sticking out toward the back is the blade I just hacked into a rough shape.
Next up, let the filing begin.
This might take a while. It's not as smooth as it looks in the photo to the left.
I'm tempted to just use the angle grinder, but I thought I should do at least one side by hand to keep it to a basic set of tools.
If it takes too long I'll use the angle grinder for the other side, but this one will be by hand.
And using an angle grinder might set fire to my project anyway.
I used a bench grinder to carve my home made fishing lures in a previous "thing", and that got a little hot but worked quite well and was fast.
Making the wooden turbine blades is proving to be more of a intellectual problem rather than a skill problem. At least so far.
I cut my 2400mm length of 42mm by 11mm pine into lengths I could stack as per the plan in the post called wooden blades. The longest section was cut to 1200mm, then 700mm, 350mm, and the last length was 150mm. I chose these numbers because of some vague image in my head of what it should look like.
I lined them up and drilled a centre hole to fit a bolt I had.
this way I can rotate the stack into position knowing that at least one point is lined up.
Next up was to glue them and line them up so that they were in a rotated stack. Each progressively shorter section was stacked on top of the others with enough rotation to line up the stacks edge with the new items half way point.
After doing this I started to question exactly why I was doing this. My plan had started to evaporate in my head.
I have design doubt.
I cant figure out which side I think is the front, and which way I think this thing will spin.
The only thing left to do was to attach as many clamps I could find and stop thinking about it until it's dry.
Watching glue drying is even less fun than watching paint dry, because you cant even see it.
I'm not sure showing the results, and then the "making the furling model" is really best practice, but I got a bit excited when I made the thing and it actually worked, so I posted the result first.
I tend to strive for 2nd best practice anyway, so it's all good. Here is what I did.
First up I cut some blades out of a 30cm cardboard clingwrap tube.
I used about a quarter of a tube for each blade.
I made the first cut straight along the tube, and the next cut from a quarter of the tube away. Cut to one centimetre away from the first cut at the tip.
That's not so clear. If they look a bit like the picture they will probably work. I wasn't too fussy with angles and things because this wont matter if it falls to bits.
I'd be fussy with the real thing, because vibration will be an issue.
Next I delicately removed some excess bits of an old computer chip fan (I used a hammer and a screwdriver), so that I could mount the blades on it and use it as a bearing. It would have been possible to even generate a small amount of electricity if this fan wasn't broken.
It turns out, many electric motors will produce electricity if you rotate them.
And then a drop of super glue to mount the blades in place.
In this case, my blades will be rotating clockwise.
As mentioned in a previous post on blade twist, the leading edge bits that sticks out from the centre line in the middle of the blades, are there because we want a steep angle of attack, and a fatter blade profile at the centre. This is because a given spot near the centre moves slower than a given spot at the tip.
I cut a tail out of cardboard, and screwed it onto a stick. The total tail length should be around the length of one of your blades.
From what I've read, the tail area should be somewhere around 15% of the swept area of the blades.
So 16% of pi X (blade length X blade length)
But I just cut something that looked about right.
I drilled a hole through the stick so it wouldn't split when I put a nail through it. The nail will be the pivot point for the tail.
The nail goes through on an angle of around 20 deg, so that the bottom points away from the tail.
Now all we need to do is make a base plate with a few holes drilled in it to mount everything on.
It should look like this.
But instead of a pipe set at an angle I just drilled a hole at an angle through some wood.
Mine looked like this with the computer chip fan bearing mounted.
To create the angles required, and to make your tail do the right thing as described in the post called genius furling, there are a few angles and dimensions to get right.
- Picture a square (black)
-mark out a 50 deg angle (green)
-and weld a pipe on a 20 deg angle (yellow), pointing back along the line made by the 50 deg angle.
This creates an offset between the turbine and the tail.
-this offset (blue) should be around 5% of the length of one blade.
What all this does, is makes your tail and the offset blades try to meet in strong winds because the tail is always trying to point down wind. This means the blades turn away from the wind, thus protecting them from spinning too fast and either burning out all your wiring, or total catastrophic destruction of everything. The weight of the tail resists this meeting, because it has to move up if its to move to the side, due to the 20 deg angle. Adjusting the weight of the tail adjusts the wind speed required to furl the system.
Ingenious. (and sadly not my idea)
It's described better in the post I called genius furling.
Here is a video showing the test turbine working. This video shows the angles a bit better than the last as I attached a drinking straw pointing straight back at 90 deg to the blades. Especially in the last few seconds of the video, the straw shows the angle of the turbine in relation to the tail as the wind speed changes (using a fan). The wind direction stays the same throughout the video.
A wind turbine blade needs to present to the wind at different angles depending on which point on the blade we are looking at. Because the inside of the rotation turbine moves slower than the tip, the inside blade edge needs to look more like a bi-plane wing, and the tip needs to look more like a modern jet wing.
Try this. It wont make a wing, but it will demonstrate how we might make some twist.
Take a cardboard roll, and draw two lines along its length but on an angle, so that the lines don't quite run parallel to the sides.
Cut along the lines.
And you get this.
No great surprise, but it does show a way we might make a wing shape with a twist in it.
Our wind doesn't just need to be twisted, but it also needs to be thicker and broader in near the centre, and tapering off to thin and narrow at the tip.
That means if we draw a triangle like thing on our cardboard roll, we should get something a bit like what we need.
This should give us a bi-plane style wing at one end, a modern jet wing at the other, and everything else in between.
The little square bit at the far end is where I would bolt it onto the central hub if I was going to add 2 more blades and make a turbine.
We start to see something approaching what we need. It has a big thick wing shape near the centre, and a thin modern jet looking wing shape at the tip. It also has a twist in it so that the angle of attack reduces as we get nearer the tip. This is a good thing because the tip is travelling so much faster than the near centre. The faster it rotates, the more the apparent wind appears to be coming from the direction of the next blade, rather than at 90 deg to the turbine.
I put some sticky tape around our new turbine blade model to try to get a reflection that might make the shape clear.
I think this captures it a bit.
Now, none of this is my idea by the way. In fact many other people have made blades like this and they seem to work OK on smaller turbines. They just don't make them out of cardboard rolls.
The best part about all this is it looks like I might be able to use PVC yet again.
