Power in low vs stall in high winds

[Jason Wilkinson]

Please explain why, if i get power in low winds , my turbine will stall in high winds
 is not the more the merrier ?
  Jason

[Flux]

For a turbine prop to work effectively its speed needs to rise directly with wind speed, if you hold the prop speed constant it comes off the peak of its power curve very quickly, small deviations from the ideal speed don't badly affect the power out, but stall starts to affect things quite badly and when you hit hard stall the power falls drastically, in fact so drastically that you only get a tiny fraction of the power available, even though the theoretical power is rising as wind speed cubed. You can be forgiven for believing that in high winds there is so much power available that it ought to work well if it works ok in low winds.

The main trouble is direct battery charging where the alternator output is clamped to constant voltage. For an alternator to be efficient it needs to have low resistance, but a low resistance alternator clamped to a fixed voltage is forced to run at near constant speed.

In high winds the prop curve is fairly steep and is a good match to a reasonably efficient alternator. In low winds the power curve is very flat and is a poor match to a decent alternator. To get enough speed rise to avoid hard stall if you cut in at low wind speeds you have to use an inefficient alternator and this produces poor output and heating problems in high winds, but even then it gives better results than a highly efficient alternator causing hard stall.

It becomes a trade off between prop efficiency and electrical efficiency and the better the low wind performance you go for the worse the trade off. Unless it is a very poor wind site going for a low cut in speed to get results in very light winds will murder the high wind results and remember there is very little energy in low winds.

If you only get very low winds, then even this very low energy may be worth having and if you don't get many really windy days the trade off may work even if the top end performance is dreadful.

If you have a reasonable site you my do much better loosing a bit of low wind performance to have better output on the average days where most of your real energy comes from.

There are some tricks that help you get some of the best of both worlds but without some form of mppt you will need to make compromises.

I did a series of articles covering this problem years ago and I believe you can still find them in the FAQ section.

Flux

[gww]

Have you read the sticky above, right now, on this 1st page of the forum sub section wind "stalling and adding line resistance".  Alot of good info and I believe flux adresses your spicific question of power in low and stall in high winds.  It has to do with the turbines efficiancy.  Honestly I don't type or spell good enough to recap it here.  It is also where I got the spicific info on fixing my specific turbines from hughs portion of the post. 
I hope this helps.
gww

Flux beat me to it as I was typing but I posted anyway.

[gww]

My question is; for small turbines that are in low wind sites, is there an mppt that has tare losses small enough that you actually gain more power then you lose during low wind periods?
gww

[Flux]

Yes you will gain with mppt. As for cost effectiveness that depends on your site, needs and how much you value the wind power. Adding more solar may be better use of money, depends on how many sun less days you get and the wind regime.

Low power electronic converters can be over 90% efficient, you can't get close to that with alternator or blade efficiency.

Even a little boost converter to aid the low winds rather than wind the alternator for an un realistic low cut in can make a worthwhile improvement. The buck converter method works better as it greatly reduces line losses.

Flux

[gizmo]

I've been getting good results with cap doublers. Sort of a poor mans MPPT, and only suited to alternators with a high pole count, like a F&P or hub motor. Basically the doubler will let the windmill generate a few amps below its normal cut in speed. I'm using one on a OEM hub motor based windmill, and see up to 5 amps ( about 250watts ) before the main rectifier starts to pass power. The windmill is good for over 800w, but has a high cut in speed at 48v, since its ideally suited to 24v. The cap doubler means its making power in light winds where it would otherwise be generating nothing.

But like I said, its only suitable for alternators with lots of poles.

Glenn

[Flux]

That is a good method for the high frequency alternators, they are less inclined to stall but if you choose a low cut in speed they reactance limit far too early.

I suspect the performance with booster on 48v is way better than the same stator direct at 24v even though on first sight it better matches 24v.

Star delta changing doesn't work well as the curve is too steep in star, but the capacitive reactance far better approximates the flat prop curve in low winds.

Flux

[gww]

jason w
If you are unhappy with me sort of highjacing your thread for my stuff please post so.  If you believe it fits the subject and helps (I hope I am not out of line) then here goes.

Gizmo
I have a smartdrive that I haven't got to yet.  I still don't understand exactly how to use caps or how I wan't the stator wired if I use caps.  I have been checking "the back shed) forum daily and guess next I will look for picktures.

Flux.
I read your "matching the load"  post however it was a bit above my head.  I can't even keep in my head around the terms buck and boost and which one double voltage and which one reduces it.  Sooo I always ask For my spicific turbine.  I have the ten coil, tweleve magnet rotors turbine, 48 volt.  I understand the taking of a 24 volt turbine and putting it on a 48 volt system with a voltage doubler.  Would a turbine like mine only need a voltage reducer or would it need a bit of both or do I need to read your "matching the load" post five or six more times cause I don't have a clue yet.

Thanks
gww

[Flux]

Hugh's design will be about optimum for direct charging in most wind areas and the alternator as it stands would not be the best for mppt but would benefit from a buck converter mppt.

The buck converter reduces volts and a boost converter increases volts.

Let's look at both options briefly. Imagine a 32v stator used for 48v, the cut in will be something in the region of 12 mph and at that point the prop would be running faster than ideal tsr. In high winds the alternator will work well, with less turns and thicker wire it will better match the high wind condition and also will be able to deliver more current with the thicker wire.

The low wind performance will be lousy, but you can use a fairly small boost converter to bring cut in down to about 6 mph. With suitable control you can keep the prop happy up to about 14mph and at that point the main rectifier will take over and you have the performance of the higher efficiency alternator. At 48v the line losses can normally be made low ( not easy for 12v).

Now let's do it the other way round, if we wind a stator for say 80v we shall get cut in at perhaps 6 mph but by about 8mph it will be stalling, probably by 12 mph it will hit hard stall and again the performance will be lousy but it will be bad in high winds.

Regard the buck converter as a dc transformer that has a 1:1 ratio at cut in, but increases the step down ratio as the wind picks up. The ratio may be close to 2:1 step down at twice cut in wind speed so the alternator is producing about 100v to feed the battery at 50v. In high winds the ratio may be 3:1 or even more. With a 3:1 step down the alternator is only supplying 1/3 of the battery current. The loss is proportional to current squared so the winding and line loss will be 1/9 of the direct case. In addition to this significant electrical gain, you have brought the prop from stall to the peak of its power curve and this will gain you even more than the electrical improvement.

It is fairly to see why you gain so much at maximum power but not so obvious is a gain in the normal operating wind speeds from 10 to 20 mph, this is partly from improved electrical efficiency but probably more from better prop matching.

We tend to think that direct charging machines are running fairly efficiently in the region above cut in and I was surprised how quickly the stall issue comes into play. That is why it is not a good idea to cut in below 8 or 9mph without mppt.

I hope this basically helps you understand where the improvement comes from. For use with a buck converter mppt the higher the alternator efficiency and the harder it stalls directly connected the better it will work. You can use a bigger alternator with more magnet and copper with very high efficiency without having to worry about the prop matching problem.

Flux

[gww]

Flux I made 9' blades an will wait to see how that goes on one.  On the other one which should be a bit stronger turbine,  If I were to make a buck converter or go with 9.5' blade, which would most likely be best?  Also the buck converter would be (dc volts?) and could be placed at ground level even if the rectfiers are on the tower?

Thanks
gww

[Flux]

Yes  a buck converter uses dc, the best place for it is near the battery so that it reduces line loss.

Now the bad bit, producing a reliable buck converter to handle a couple of kW is no small task, I don't regard it as a home project except for electronic experts ( that doesn't include me) That is why I only gave guide lines. It is really Midnite solar who transformed the basic idea into a reliable device. A converter going up in flames is no big deal, but a run away 9ft turbine with no load is not a nice concept.

I mainly concentrated on the boost converter as it is smaller easier and more importantly a failure will stall the turbine or it will continue to run with good high wind performance, quite safely but with poor low wind results.

A reliable buck converter needs careful layout, you will almost certainly find many of the useful drivers are surface mount and without lots of protection circuitry it will be difficult to use. Mine has been reliable but could not survive a loss of battery and it is only on a 6ft machine with manual shut down if it lost its load.

There are many articles on the web that tell you how to make buck converters but they are for little power supplies for computers, it is a big leap to change that for a reliable 3kW 200v unit.

Flux

[gww]

Flux
So for an $800 dollar midnight cc and clipper or a 9.5 foot blade, better to try the blade and see where that goes.  To your point of boost perhaps being better able to be done on home scale, I was looking again at your "post matching the load"  and noticed the mention of F&P smartdrive.  Also your responce to caps being used on the smart drive.  Will the boost make an unmodified smart drive a usable turbine or would splitting the coils and trying to match the blade be a better route to go?
Thanks
gww

[Flux]

The F & P motor is not available in the uk , I am sure Glen and the Back Shed people will be able to help you with the details. 
 In its natural state the F & P has many properties that make it a poor choice for wind power. Because it is cheap , many people have put a lot of effort into making it much more useful.

In its original state it cogs and reactance limits badly. It is inherently a high voltage machine and many of the reconnection schemes are to make it suitable for lower voltage. Various versions are available and by mixing rotors and stators the cog issue has been minimised.

One virtue (or vice depending on how you look at it) is that it has many poles and produces quite a high frequency at low speed. This makes the reactance problem difficult but it also makes the voltage doubler viable with reasonable capacitors. I will let those familiar with the things to advise you on the best winding connections for your application.

An interesting trick which seems to have been tried is to series resonate the reactance with capacitors in the line, at resonance the reactance vanishes and the only factor limiting output is resistance ( similar to the air gap axials). By placing the resonance in the right place you can get close to the ideal cube law loading. I don't know whether this can be done with the boost circuit to cover the whole speed range but it probably can. A lot of ingenuity has gone into creating a very useful device from something that was at first glance not very suitable,

I don't think it has much real advantage over the normal axial design except the low cost but the cheapness is a good incentive to use it to best advantage.

Flux

[gww]

Flux
It is cheep cause I already have it and got it free.  Even though I have yet to get one to work to its potential,  I love hughs design because even a dummy like me can build it without a machine shop.
Thank you for your time
gww

[Jason Wilkinson]

Oh no,no,no,GWW,not at at all  look at what Flux has contributed  we all learn don't we ? at least i do , keep posting
Jason

[gww]

Jason
I surly learn from flux a thank him for his willingness to help.  I get to reading a post and it sets my brain to something I am working on and I selfishly steal other peoples post.  Part of this is because the post clues me in on something I don't even know I want to know, but I do.  I thank you for posting and also for being generous enough to let me get what I needed also. 
Again, thank you
gww