Generator solution for low RPM VAWT

[Marshall Swearingen]

To those more experienced than me, whose know-how I'm seeking here, I apologize for bringing up another new VAWT question. . . ("Oh boy, here we go, another VAWT idea.")

Our design is exhibiting high torque and low RPM. We're getting good power results, especially in low winds, but our problem is harnessing that as useable power. We are turning a PMG that was marketed as "low RPM" (a Ginlong 500) which seemed like a good fit. We set up a simple off-grid testing system with a 24V battery bank, a Xantrex C40, and a dump load, and are measuring the rectified DC current as a function of wind speed with a transducer and anemometer hooked to a HOBO logger.

At first we were stumped when we weren't registering any current even while the wing was cranking away. Then we realized the generator wasn't producing the voltage needed to charge the batteries. Even after gearing the generator by 6:1, we weren't getting sufficient voltage (like I said, high torque, low RPM. . .). To get any theoretical measure of power at all, we hooked the generator directly to the dump load and are calculating power from its known resistance.

So, my question now is how we can start to adapt the system to produce useable power, to get in the voltage range required to charge a 24V off-grid system, etc. It seems like VAWT folks must have encountered this problem in the past. I can think of a few options:

1) Gear up the generator even more. This would get the genetator up to speed in higher winds, but it would compomise the good power potential in low winds.

2) Find a generator that puts out the needed voltage in low RPM (<100). Does anyone know of such a generator? Does anyone know much about switched reluctance generators?

3) Or, does anyone know of a way to generate electricity from an oscillating shaft, other than converting it to unirotational and driving a generator? Just curious.

Thanks for any help. If anyone is interested in our power results so far, I could share those too.

Marshall

[Rob Beckers]

Hi Marshall,

Nothing against VAWTs here as far as I know. From my own perspective (as someone who sells wind turbines for a living), I like the looks of a VAWT. My issues are with commercially marketed VAWTs that either do not produce anywhere near the energy their makers claim, or if they do, are several times the price of a comparable HAWT. Those objections are not even VAWT specific, the same can be said for many, many HAWTs that are sold commercially.

Anyway, back to the issue at hand. What size is your turbine (swept area), and if you have it, what does the power curve look like in terms of RPM vs. power? Ginlong makes pretty good alternators, but for direct battery charging the alternator needs to be matched to the turbine or it won't work.

-RoB-

[Marshall Swearingen]

Hi Rob,

Thanks for your reply. To answer your questions:

1) Swept area: I haven't thought much about the swept area, and am not entirely sure how to calculate it for this design. The machine now consists of a wing (6' high by 2' wide) that oscillates on an axis parallel to the tower, offset from it by standoff arms that are the same length as the height of the wing. The angle swept by the wing actually seems to vary with wind speed, so it would seem that the swept area actually varies as well. Are you getting at using the swept area as a power estimate to size the generator?

2) Power versus RPM: I just ordered an RPM sensor that can (hopefully) be integrated into the HOBO logger, so we'll have a record of power as a function of both wind speed and RPM. I'm starting to understand that knowing all of those factors is indispensable for making a system that can generate good power. I'll keep this post up to date with what I come up with.

I do have some rough measurements from another similar prototype that show the RPM never getting much above 200RPM, and hovering around 100RPM in the wind speed range we hope to be generating the bulk of the power. Do you (or anyone else) know of an alternator that is made for such low RPM? If I were to make my own alternator, would I be able to wind it to make decent voltage in that RPM range?

Also to keep in mind, the dimensions I gave for the wing are for what I'm thinking of at this time as an experimental model. I'm hoping to build something in the 3kW range with a wing closer to 20' high by 4' wide.

Thanks again,

Marshall

[Rob Beckers]

Hi Marshall,

Yes, swept area is meant as a quick way to get a rough estimate of power vs. wind speed. For regular VAWTs only about half of the swept area should be counted, as the blade(s) move against the wind the other half. In your case I have no idea; I don't know what your turbine looks like, or how an oscillating wing works.

For RPM you don't need a separate sensor. It can simply be derived from the frequency of the voltage coming from the alternator. Depending on the number of poles of the alternator you just need to divide/multiply the number to get to RPM.

You hopefully already know this, but if not: For battery charging directly from a wind turbine the match between turbine and alternator is much more important than for an MPPT setup (where the inverter takes care of optimally loading the turbine). An alternator that is too 'large', in that it is capable of supplying more power at a certain RPM than the turbine can deliver, will overload the turbine. This causes the RPM to drop and the blade(s) to stall. The other way around is not great either, since it means the turbine runs at too high an RPM, and non-optimal TSR (making it inefficient). More in general, direct battery charging is inefficient for wind turbines because power that can be delivered by the alternator tends to go up linearly with RPM (the voltage is fixed by the battery), while power available from the wind turbine follows a cube function. That means you can only match the wind turbine power at a single point (or small range), and the turbine will run at a non-optimal TSR the rest of the curve. Still, many battery charging wind turbines work this way, and they do fine, it's just something to be aware of when matching alternator and turbine. As for too large an alternator vs. turbine; this can be mitigated by adding a series resistance to the alternator (so in effect it doesn't put the brakes on the turbine as much as it would without the resistance).

-RoB-

[Marshall Swearingen]

Hi Rob,

So I'm getting that there's no "silver bullet" for low RPM turbines, and that optimizing power from a high-torque low-RPM VAWT will involve gearing up and appropriately sizing the alternator. I was just reading about a Savonius generator geared up by nearly 18:1, so I guess there's precedent for this.

Thanks for your help,

Marshall

[Rob Beckers]

No, that's not what I meant: There are low-RPM alternators (depending a bit on how low the RPM really needs to be). It's too soon to tell if there's an off-the-shelve solution for your problem or not. To identify what works for your case one would have to know the RPM vs. power. On top of that, if it's for battery charging then the battery voltage it's meant for is important.

-RoB-

[Marshall Swearingen]

Okay. I was just looking at the power data for the Ginlong 3500, which I'm thinking of using for a bigger 3kW unit, and it proves your points. On that scale, using MPPT, it already looks much more viable. Looks like a good fit with an Aurora 3.5kW inverter. The tracking voltage window of the Aurora is 50-530V, and the Ginlong hits 50V at only 25RPM (though with nearly no power), so any power produced by the Ginlong would be useable by the Aurora. The Ginlong shouldn't exceed the amperage or voltage limits of the Aurora. And even in the lower RPM range of 100-150RPM, the Ginlong is putting out around 1kW. I suppose torque also comes into play too---can our wing turn the Ginlong? . . .

For now, working on getting the RPM measurements. . .

Thanks,

Marshall

[Rob Beckers]

Power is the product of torque and RPM, in other words, if you know how much power you can load onto the turbine at a given RPM then that already defines the torque on the turbine. You can tell the Aurora how much power to extract at a given RPM, and that way set the torque on the turbine. As long as the alternator can actually deliver that power (at the given RPM) it will work fine.

The Ginlong alternators work well, but their rated power tends to be at too high a voltage for use with the Aurora inverters. Keep in mind that voltage for the alternator is 3-phase AC, while the Aurora is DC. You can multiply AC phase-to-phase voltage by 1.4 to get DC (that is approximate, but close enough for government work). In short, to keep some margin you don't want DC to get over 550V, and that means AC should not exceed 390V phase-to-phase. This is not only under load, but also (or especially) unloaded, for example when the grid is down. Going over 600V will with 100% certainty let the magic smoke out of the Aurora (I've seen it happen), it also sets a flag in memory that the guys at Power-One can read out and they will not repair under warranty in that case.

To make a long story short, the Ginlongs are good alternators but you end up with a larger model than normally needed because of the need to extract power at low RPMs and stay within the voltage limits of the inverter. It adds a bit to the turbine cost.

Hope this helps!

-RoB-

[Glenn Littleford]

2) Find a generator that puts out the needed voltage in low RPM (<100). Does anyone know of such a generator? Does anyone know much about switched reluctance generators?

If you can find one, a F&P Smartdrive motor will make usefull power at low RPM. Dependng on the model, in un-modified form, 20 to 40 RPM will give you 12volts. I've used a F&P on a Lenz2 turbine and it worked OK. http://www.thebackshed.com/Windmill/articles/Lenz2.asp
The problem is you will only make 50 or so watts, from a device that can make over 400 watts at high RPM ( 300 to 500 RPM ). But if you can pick up the F&P Smartdrive's cheap enough, there is nothing stopping you using several together to increase your current, so long as your VAWT is big enough to drive them.

Glenn

[Rob Beckers]

Marshall, to get back to this: The Ginlong alternators are wired up in wye by default. Did you ask them if they can make them wired in delta as well? That would lower their voltage at rated power by a factor of 0.57, and put them right in the sweet spot for the Aurora inverters (or any other inverter with a 600V DC window). Ask Jimmy at Ginlong, I'd like to know as well. If they can make 'm in delta they suddenly become a whole lot more interesting (and it doesn't require any design or production changes on their part, just soldering connections differently).

-RoB-

[Marshall Swearingen]

Hey Rob,

That is an interesting idea. We're going to be hooking up our 3kW system soon, and if we run into a problem with the voltage and decide to go the route of a modified Ginlong, I'll let you know how it goes.

Marshall

[Guy Dewdney]

Hi - first post.

I have the 5kw Ginlong with an Aurora inverter - and can attestify for the superb quality of both - genuinely surprised at the quality of the Ginlong - really really good.

Hope that helps you make a decision.

Guy

[Rob Beckers]

Just heard from someone who managed to order a Ginlong alternator wired in delta (instead of the default wye configuration). So that seems to be possible. This brings rated power for their alternators down to about half the voltage of what it would normally be in their specs, right in the sweet-spot for Aurora grid-tie inverters.

In short, this makes Ginlong alternators much more interesting for DIY turbines. Talk to Jimmy Wang at Ginlong to get a delta-wired version (and no, they don't pay me for this :sad2:).

-RoB-

[Marshall Swearingen]

Hey Rob, thanks for passing the word along. Good to know.