Wind Power
My latest little project has been designing and building a wind turbine to keep the old batteries topped up when there's not much sun around.
I could have simply gone out and bought new a Rutland 913 or Marlec turbine for around £300, but I didn't have £300 and thought it would be a fun project. It's still ongoing, so I don't recommend you use my designs yet until I have found out if it actually works!
The first step in any wind power project is to get hold of a copy of Hugh Piggott's Windpower Workshop. Hugh is the country's leading expert on small to medium DIY wind turbines and the book is a treasure trove of information. If you're too stingy to buy the book, a lot of its contents are on Hugh's website Scoraig Wind
Designing and building a wind turbine is a complicated business, but I was able to make some major simplifications due to the small size of my machine:
- I used 110mm plastic pipe for the blades, rather than carving them from wood.
- By keeping the rotor down to about 1m diameter, I can get away without a furling system (a mechanism that points the windmill out of high winds to prevent damage).
- The peak output of the windmill is only about 50W (4A) into a 220Ah battery bank. For such a small current there is no need for a regulator, just a diode to prevent discharging.
The generator
One of the most annoying parts of the job was finding a suitable motor to use as a generator. To be any use, it has to generate at least 13V at attainable rpm.
Motors also need to be of the permanent magnet (PMG) type, preferably DC, brushless and rated for continuous use. Ametek make a 99V DC motor which is brilliant but hard to find - if anyone as a good supply of these motors, please let me know!
I eventually found a motor on Ebay rated at 2000 rpm at 24V / 30W. To generate 13V it has to be spun at about 1600 rpm which is feasible but still fast.
The blades
Hugh explains the blade design process well, and I'm not going to repeat it here. One thing he does not mention though is the suitability of plastic pipe as a raw material for the blades on small windmills.
PVC or ABS pipe has just the right curvature for the blades. It's easy to get hold of and easy to cut with a jigsaw. If you make a twin-bladed windmill you can do both in one piece which is very strong.
Choose a pipe with a diameter around 10% of your final turbine. Start by drawing a dead straight line along the length of the pipe. Wrapping a piece of paper around it really helps keep things square.
Next, draw the outline of the blades. I started with the same calculations as Hugh. To get the right setting angle (β), simply shift the centre of the blade slightly away from the centre line on the pipe (towards the leading edge). My pipe was close to 360mm circumference, so I simply shifted it 1mm for each degree of setting angle β.
The first rotor I made was very thin and whippy, designed to run at high rpm and had a tip speed ratio (TSR) of 10, which means the tips move 10 times faster than the wind.
Rotor design 1
Blades = 2; Diameter = 1.2m; TSR = 10;
Rated speed = 10m/s (1600 rpm); Angle of attack α = 4°;
| Station | Radius (mm) | Calculated chord width (mm) | Actual width on pipe (mm) | Setting angle β (°) |
|---|---|---|---|---|
| 1 | 0 | 94 | 94 | 14 |
| 2 | 150 | 54 | 54 | 5 |
| 3 | 300 | 35 | 35 | 2 |
| 4 | 450 | 26 | 26 | 1 |
| 5 | 600 | 20 | 20 | 0 |
This rotor had some problems. It was so slim with such a low angle of attack it was very reluctant to start. Also, I failed to realise that the chord width is not the same as the width of the blade measured along its surface due to the curvature of the pipe. I also thought station 1 referred to the centre, but in fact it is the first one out (150mm).
For the second rotor, I tried a TSR of 8 to make the blades broader, stronger and better starting. This necessitated reducing the diameter to 1m (from 1.2m) to keep the speed up.
Rotor design 2
Blades = 2; Diameter = 1m; TSR = 8;
Rated speed = 10m/s (1525 rpm); Angle of attack α = 4°;
| Station | Radius (mm) | Calculated chord width (mm) | Actual width on pipe (mm) | Setting angle β (°) |
|---|---|---|---|---|
| 1 | 100 | 116 | 85 | 19 |
| 2 | 200 | 65 | 70 | 8 |
| 3 | 300 | 44 | 45 | 4 |
| 4 | 400 | 34 | 35 | 2 |
| 5 | 500 | 27 | 27 | 1 |
This rotor starts and runs just fine. But today a gentle gusting breeze is only producing 2-4 V. I need a windy day (10 m/s plus) to see if it can get up to the full voltage and design rpm. I also need to balance the blades as it vibrates a lot!
Housing and tower
A nice opportunity to practice a bit of metalwork.
My design is very simple, with a 3mm aluminium base plate that is bent up to form the front and back of the housing. The motor and tail boom are bolted directly to it. A very large (16mm) bolt sticks out the bottom and fits neatly into a bit of steel tube bolted to the tower to form the yaw axis.
A piece of 1.5mm ally sheet is bent over to form a lid. Another piece of ally, solid this time, was drilled and glued to the motor shaft to act as a mount for the blades.
The tower is an aluminium pole about 4m long scavanged from an old TV aerial. I made a hinge at the bottom so it can be easily lowered, which is isolated from the roof by 20mm of rubber.
The tower is held up by three guys in the form of tie-down ratchet straps. It can be easily raised and lowered by one person. These too need isolating as they vibrate in the wind badly. Eventually I will replace them with proper wire rope.
Thoughts
Building the wind turbine was an interesting project, but it's too early to say how successful it was. Probably I would have been better off spending the £100 or so it cost on more solar panels!
Check back for an update once we've had a windy spell. Hey, if it works I might even get around to painting it!