OT: PC fan > Wind turbine.

I’m currently working on converting a PC fan into a small wind turbine. I have lots of questions but not about PICAXE. I’m hoping that I can borrow some brain power from here. Please see my post at the EPE forum: http://www.chatzones.co.uk/discus/messages/7/8223.html?1219854036

Feel free to post your comments here.

Looks like you'll get a lot more response on that Forum than here.
What amount of power do you hope/expect to generate using a little fan? Small diameter makes me think Small Torque.

Hi Boris

Thats what I was thinkin too dippy.

As a child I spent many a summer holiday in Wales, staying with family in caravans along the sea front.

My grandad set up a wind turbine. I was told he did this using a car alternator plus it's charging circuit, to charge a 12V car battery, providing lights and a modest amount of TV viewing on a small portable set.

I wondered about using a PC fan myself.. but I'm guessing the power output would be tiny, even a small light weight turbine blade wouldn't be able to produce the rpm required and the EMFs generated would also act as a braking force to further add to the disappointment?

boriz said:

I’m currently working on converting a PC fan into a small wind turbine. I have lots of questions but not about PICAXE. I’m hoping that I can borrow some brain power from here. Please see my post at the EPE forum: http://www.chatzones.co.uk/discus/messages/7/8223.html?1219854036

Feel free to post your comments here.

Click to expand...

some good ideas here http://www.google.com/cse?cx=008032414425079535247:kplxrakvu20&q=wind+turbine&sa=Search

I've made a couple of wind turbines.

To make a long story short:

- the power that a wind turbine can maximum generate depends directly on the area swept by blades. An 1.2 m diameter area will give you roughly max 100w. That is, no matter how hard the wind blows, you'll get max 100w

- the power will also depend on the speed (rotation per minute - rpm) of the rotor. For a wind turbine you may expect speeds around 200 rpm in 15kn winds and it will go to about 600 rpm in higher winds before things start to break.

- the power depends on velocity (meters per second - m/s), that is the speed of magnets passing over coils. The bigger the rotor, the lower rpm needed to produce the same amount of power.

- the voltage of generator depends on number of loops in coils and the current on the thickness of copper wire used.


From what I've seen in your pics I wouldn't expect too much power out of your fan.

If you want some power that you can actualy use try a dc motor with low rpm or a dc motor driven at higher voltages ie for 240V dc motor that spins at 3000 rpm converted to a generator will give you 12V at 150 rpm

IMHO avoid classic DC motors, as brush wear is usually alarming when in WT (wind turbine) applications. Hence suggest brushless stepper motors, & near endless ex. HD, floppy,VCR, printer types etc abound.

Don't forget that fan blades,intended to MOVE AIR, are NOT suited to generate WT electricity. There are numerous sites & "YouTube" clips showing blades profiled from plastic drainage pipes etc.

What sort of power output & application are you after? These days,if you're just after a few Watts, of course solar PVs win hands down,with car 12V 2W trickle chargers globally ~US$20. PVs are easier on the neighbours too -small WT can be a suburban visual eyesore, & a magnet for stone throwing boys...

However nifty DIY WTs based around bike hub dynamos can be viable. Forget bottle dynamos of course as they are very inefficient & high rotational speeds are needed. Good old post WW2 Sturmey Archer "Dynohubs" (6V ac @ ½A at 60rpm) are now pretty elusive, but check your local wrecked bike outlet! These "best of British" SA units just never die- I've a ~50yo. one here still going strong. The front hub ones are easiest to work with, but the rear hub (combined with a 3 speed gear box) are perhaps more common. See => http://www.gotwind.org/diy/Dynohub.htm

Sage advice Manuka, I got a solar shed light from the range at the weekend... 6 quid. not tried it on anything picaxe yet but it's designed to tricckle charge 3AAs and sports a 5 inch fluorescent tube light... Nice

As much as that! I recently picked up similar here in NZ for under "a fiver" (~US$10), due to what was revealed as poor soldering, & the PV puts out 8V(OC) at 70mA(SC) - translating into bright sun charging of 3 x AA NiCds at ~60mA. You really can't beat PVs & white LEDs for skinflint energy work, & the so called "shed lights" now abounding make A1 off grid solar engines.

Re "I’m currently working on converting a PC fan into a small wind turbine."

A few years back I did a search on the internet trying to find some real world data from a year or two of datalogged power outputs from small turbines. These usually are rated as "peak" watts, ie power in a cyclone, rather than average watts. The best data I could get was from a site selling good old fashioned multiblade windmills, and they listed their power in terms of gallons of water pumped to a certain head per 24h. This converts quite nicely to watts. I came up with a realistic average watts of 30W per square metre of swept area, (at a height of 12 metres).

This is of course, somewhat lower than the figures you see on websites selling wind generators.

A computer fan is 10cm in diameter. Swept area is 75cm squared, or 0.0075 of a square metre. That gives a power output of 0.225W, if you can get it 12 metres in the air. I've tried these fans down near the ground and sometimes they won't even turn, even if it is quite windy. Plus computer fans are not rated to be out in the elements.

But they could be useful to get some datalogging information. You could calibrate it by driving along in the car with someone holding the fan out the window and note whether you get any power out, and at what speed.

IMHO avoid classic DC motors, as brush wear is usually alarming when in WT (wind turbine) applications.

Click to expand...

Indeed they wear fast, it would be good just to start with something. I've managed to get 0.02A from a small stepper motor spinned with the cordless @2400 rpm. Far better results came from an old washing machine stepper motor that would give out heaps of power but the blades would be too big.

I have now a modified Hugh Piggott's design axial flux generator with 1.1 m diameter blades. It starts to produce 12V @ 180 rpm (about 10 kn - add or take, I couldn't finish the wind meter as I didn't get the lcd to work) and pumps 1A @ 220 rpm (about 15 kn?). On Saturday night winds (Auckland) of about 40 kn it managed to blow a 12V/10W light bulb

Try a floppy drive stepper motor

goto this link for some great ideas recycling all types bits including a stepper moter from a 5.25 Floppy Drive.
http://www.thebackshed.com/Windmill/AssemblyA.asp
Cheers

Doc. This fan is 85mm diameter. But “converting a PC fan into a small wind turbine” means adding larger blades of course

May even use Darrius style VAWT. Turbulent air around here.

Of course this will not produce much power, but it does not need too. It’s an experiment, a study. I will primarily be using this unit to measure wind conditions at the intended site of a real turbine, and to try out various blade designs and efficiency measures etc. EG: I will probably be building my first synchronous rectifier! (analogous to a bridge rectifier, but with ZERO voltage drop).

If I ever manage to charge a couple of AAs with it, I’ll be more than happy. I chose a PC fan mainly because they are so cheap and I have several here that can be tested to destruction without grief. May end up using a PICAXE data logger.

I suppose I could even build a FIFO charging stack for AAs. A bunch of them in a line with a PICAXE charge controller that charges the first one, then switches to the second one etc. Come to think of it. That would actually be useful…

Thanks Rbright.

Boris if as you suggest "It’s an experiment, a study. I will primarily be using this unit to measure wind conditions at the intended site of a real turbine"

Then look at Gills project here where he has just done that:

http://www.thebackshed.com/Windmill/articles/GillsDataLogger.asp

To quote from this page"In Gill's words....

Gill’s Anemometer
V.5.0

I have a need to assess sites for their potential as suitable for wind power generation.
The idea is to gather a week or so of wind data then compare that to the nearest recording site say like a weather station or airport. With a known general variation, the years of weather station records can be converted to site data for a quick more long term assessment."
unquote

This site also has some other nice PicAxe code.....enjoy

Thanks.

I'm hoping I can charge a single AA. From the results so far it could be possible. Might need a voltage doubler tho.

I have set up a Blog for this project at http://gluegun.wordpress.com/. All comments welcome. No registration required.

Great blog. Keep experimenting. I think with the scope traces you will work out the polarity of the coils etc pretty quickly.

I very much doubt you will get enough power to charge an AA or even compensate for the self discharge rate. But if the aim is to collect data then it should be quite possible to get some useful information.

Not so sure about rectifiers. You will lose 0.6V and this may upset your readings as you may underestimate readings at lower speeds. You might need a rectifier built round an op amp that is designed to cancel out the diode voltage drop. Then you can convert AC to DC accurately. If you have the load resistor before the rectifier, then the DC reading will reflect the AC volts and if you know the resistor value you can work out the amps and the watts. Of course, running an op amp will consume power rather than generate it, but if the aim is to collect data, that should not be an issue.

Another way would be just one diode and lop off the peaks and put that power into a RC network.

But before you get to that stage, I suspect you may come against the issue of cogging and the power needed just to get the fan to spin.

If you have a spare fan, you could build a little wind tunnel out of cardboard or a piece of plastic downpipe. You could have a fan at one end then your turbine at the other. You can then calculate the efficiency of the system - watts in vs watts out. If the watts in are a constant, you can then tweak things and improve the efficiency. There will be a maximum efficiency for a lot of things eg if you have bigger blades, you can collect more energy, but the generator runs slower = less efficient.

For computer fans, optimum efficiency is probably close to the speed they run at - ie very fast.

This is a great experiment - keep us posted with the data.

Some quick calculations




wind energy = 0.5 x air density x wind capture area x wind velocity cubed

where air density is approx 1.2kg/m^3
wind velocity is in m/s
wind capture area is in m^2

blade efficiency is approx 0.2-0.3 at best for a regular home built turbine. your fan will be a lot less, we'll say 0.1. Efficiency in the coils we can guess to be around 65%.

So for an 85mm diameter fan, wind capture area = 0.0425^2 x pi = 0.005673
and lets calculate for a 30km/h wind = 8.333m/s

=.5x1.2x.005673x578

=1.96 watts flowing through the fan


= 0.2 watts at the shaft

= 0.13 watts generated before rectifier, and all loads


And that is probably being generous... In actual fact, on a computer fan, the hub uses up a lot of space, and therefore is not acting as a wind swept area. So you'd be getting 0.05 watts in a healthy 30 km/h wind speed.


Best to use a battery to power the picaxe, and use the fan only as an RPM sensor.

Thanks all.

Doc. I plan on using a synchronous rectifier. It has ZERO voltage drop but is equivalent to a bridge rectifier with a 2 diode drop. It works by monitoring the waveform and switching the output polarity when the wave crosses zero.

http://gluegun.wordpress.com/

Hmm - like active switching using a mosfet to grab some power during the peaks?

Looking at your blog - you are learning a lot as you go. Without wanting to spoil your fun, I think I can see where you are heading:

Ok, start with a working motor. Iron cores on the coils and very low power magnets. Swap out for stronger magnets. Now the magnets attract the iron former too much, so you get cogging. Ok, make the blades bigger to overcome cogging. Next, cut off some iron (post 30th August - I'll bet that will be a fiddly operation).

Now there is less iron, and less cogging. But you will then discover you don't get as many volts with less iron.

Ultimately, you could do away with the iron altogether, but you will find you need much stronger magnets, and a completely different layout of the coils as the iron tends to focus the magnetic field.

The next thing you will find is that motors and alternators work best at high speed. And the bigger you make your blades, the slower they turn.

There are many compromises here. Keep doing experiments as there is a lot to learn.

As a general principle though, motors are the product of over a hundred years of optimising. And an alternator made from a motor works best when you run it at the speed, load, and currents that it was designed to work at as a motor.

Boriz: If you just want to "charge a single AA", then simply use the innards of a US$2 solar garden lamp.

Doc. Yep. Already looking for sources of cheap/recycled strong magnets and pre wound coils

Manuka. Yep. But that's a different project. Maybe the next one.

Aren't most PC fans brushless/electronically commutated and therefore unable to function as generators?

I have seen DC washing machine motors suggested as being very suitable for conversion to generators.

I have just bought 66 magnets like this: http://www.amazon.com/Buzz-Magnets/dp/B000PHHJ98/ref=pd_sxp_grid_pt_1_1/002-7831273-1951224

Mine are packaged slightly differently (different manufacturer I guess) and they are 600mm long. I got them all from a local pound shop. £1 per pair.

The poles are on the sides, not on the ends, making these suitable for a home built generator. Now I’m looking for a cheap source of ready wound coils. The coils on the end of a small TV tube would be good.

@LizzieB
Yep. But if you look at the blog, you’ll see I removed the circuit first and connected straight to the coils.

Re coils, inductors may be an option though most are wound so as to contain the magnetic flux (eg potcores). There are some that are wound on a ferrite rod though. You will need to decide if you want an iron core or not. The iron greatly amplifies the output, but the downside is cogging. If you don't use iron, you need the coil wound such a way that one side of the coil is close to the magnet and the other side is some distance away - otherwise the flux gets cancelled out. That implies a big(ish) coil - maybe 3-5cm in diameter.

Hugh Piggott is an expert on this sort of thing - winding coils yourself, potting them in epoxy, gluing magnets and potting them etc.

What sort of size generator are you building? This started as a computer fan, but I think it grew into something with 30cm blades on it? Anytime you add blades on, you greatly increase the forces on the bearings in high wind. It may be better to start by defining the diameter of the blades, and then selecting a bearing that can cope with the load, then work out the pitch of the blades, and then work out the RPMs, and then work out the best coil/magnet design for that RPM. You may find that steers you towards a motor of a particular design, but bear in mind that most motors have bearings that are designed for radial loads (ie loads out sideways), and for a turbine you have loads coming from the front. That implies thrust bearings. Steppers, and hobby motors do not have thrust bearings. A computer fan does (sort of), but it can only handle grams of force, and you will overload the bearings if you glue bigger blades on. Actually, not many things have thrust bearings. A cordless drill does. And the bearing from a car or trailer does. Oops, suddenly this has increased in scale! It is no accident though that Hugh Piggott uses the assembly from a car wheel - you get a thrust bearing and you also get the brake as well.

There are some cost/benefit equations here as well. Wind starts to work at diameters of 1 metre or more. Below that size, you still have to build lots of complicated stuff, but you get less and less power. At power levels of under 10 watts, I think solar may be simpler and more cost effective.

“What sort of size generator are you building?”

Beats me. I’m making it up as I go. Prolly something bigger than this, and smaller than this

It depends mostly on what salvage I find. I'm a bit of a womble.

Build the hamster one. That looks great - lots of info in that article.

i was looking at a small vertical one to stick on the roof , the design i was looking at was similar to a 3 vane rotating vent only much much bigger and with 4 vanes each around 500mm by 700mm and made completely out of perspex and aluminium and instead of generating power intermitantly it would compress air into a small 3000litre bank of air tanks (they were going to be undergound) then on demand generate electricity and charge batteries off that my parents up in queensland is taking up the idea instead their only problem is that wind in their spot isn't as plentiful as my roof top where 71% of the time there is a decent enough wind to blow you off you off balance if your not careful

I'm interested in building a wind generator too. I like the vertical style and have an idea or two to improve on the standard issue, which will probably not end being an improvement

Where can you get parallel wing section in UK? i.e. aero section but not tapering along its length. Metre lengths would be nice.

Try your local hobby store. I once had a cheap "trainer" aircraft called a "Yamamoto". Each wing was about 3' by 9". The only tapering was the last two inches at the tip.
Helicopter blades would be another option. These are available from 6" to 48" from hobby outlets or bigger from "real" outlets. They also offer you different aerofoil shapes for different 'flight' styles.

Thanks for that BB.

Guys- for modest energy needs, simple solar PVs now win hands down. Given the solar resource in Queensland maybe that should be -ah - "sunny side up". 1W-10W level wind turbines (WT) are just not worth the effort & expense when superior "sky mining" resources now exist. Wind farming only becomes viable at the kW-MW level.

Even the most trivial oversight can wreck a WT setup, & naturally this usually happens during storms when maintenance may be impossible. I was witness to a US$30k WT thresh itself to bits in a gale after just one week aloft. All due to failure of a substandard bolt. In total contrast low profile solar PVs (which didn't exist when I first began) usually "just work"- I've had a swag of 2-20W PVs up here for ~10 years,which have not had a single fault. Even bird droppings are largely self cleansed when it rains.

You want cheap ($ level) solar PVs? Grab some solar garden lamps - see my recent Forum posting on this.

I've spent ~50 years dabbling with small DIY wind turbines,both for energy and amusement,& can tell anyone about to begin similar tinkering that IMMENSE OVER ENGINEERING IS ESSENTIAL if you want reliability. WTs additionally need to be mounted at height for smooth winds,& the elevated servicing/installation can be hazardous- as of course can the spinning blades.This may cause significant budget blow outs, with boring old support poles & guys etc the major expense.

EXTRA: Aside from annoyed NIMBY neighbours,again I mention the "Don Quixote" effect that spinning blades have on stone throwing boys, rascals with air rifles & frustrated dusk hunters with shotguns.

You're right Stan. "IMMENSE OVER ENGINEERING IS ESSENTIAL" - absolutely.
As is immense overuse of jargon. "Sky Mining" - I think I'm going to be sick...

But Stan, your method surely depends where you live. And how many watts are these little Garden lamps?
And how many watts on a cloudy day? Or a cloudy, windy day? Or a windy night? Or on Iceland during the winter?
Windy? Say, maybe we can use that....?

Large scale is always better obv. But even quite a few of those IMMENSELY OVER engineered Wind Turbines fail big time.

Economics are arguable for any of them, and the parameters in your equation depend where you live.

I wouldn't poo-poo any method. Equally there is no magic free supply of power; capital costs, mainatenance costs.
But all these arguments have been recycled and disputed ad nauseum.

Oh and when I went looking for some cheap PVs I was told that they'd all been bought by a Kiwi.

Have to agree with Manuka on this one.

Wind turbines work on an exponential level of power to size.

A 4m blade will be good for 2 to 3 KW but a 1m blade will find it hard to generate 100W.

Vertical axis wind turbines (VAWTs) are even worse, as they operate on drag rather than lift, and many commercial VAWT designs have flopped with millions of $$ wasted.

One particular scientist I recall labeled the vertical axis design acronym better suited to Virtually A Waste of Time (VAWT).

But if you just want a couple of watts, or if you are in strong turbulent conditions, or if you just want to experiment, then no problems at all.

I'm playing with the Fisher and Paykel washing machine motors at the moment.

They are easy to get from scrap heaps, and they generate up to around 400W.

Again a quick plug to www.thebackshed.com.au which has stacks of info on this.

Dippy- "sky mining" is jargon? It's more like poetry. How about "wind = the earth breathing" ?

This thread initially hankered after "charging a single AA",& the abundant solar resource in most parts of the populated world means dead simple PVs can now readily manage this. That most of the worlds impoverished live with the greatest solar resource is of course tragically ironic.

Here in the UK (North) we are a bit sun starved. A PV system would need to be designed to work mainly with overcast conditions, that being the most common up here. Effectively making your PV panels maybe 4 or 5 times bigger and more expensive. Not that I don’t like PV mind. I fully intend to consider all renewable possibilities eventually. Just started with wind coz there’s plenty of it here.

BTW. When you own a bit of land. How deep does it go? Haw far down can you drill/dig and it still be your soil? I only ask coz with a deep enough hole, you can get geothermal energy quite easily (once you have the hole).

Well (haha), you'll need a long shovel Boriz. They did something in Southern UK a while back and dug down over a mile. So good luck. (Maybe forget about it...)

I don't know the legal aspect but you can have domestic bore holes 100 metres deep for example.

You may consider a Ground Source heat pump arrangement a bit easier.

100m might be enough. As long as there is temperature differential that can be exploited. I have seen low temperature Stirling engines powered by the heat of a hand.

Of course the important factor is the difference in temperature between the upper and lower plates, but some of these things work on <5 degrees C difference.

Get digging then mate
Hope it's worth it. Good luck.