Author Topic: 6v charging from 2 wind gens  (Read 1636 times)

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mkirk

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6v charging from 2 wind gens
« on: November 26, 2007, 12:18:12 AM »
I have two homebrew wind generators and would like to charge a pair of 6v batteries. Would it be possible to charge the batteries individually at 6v, and take the output from the pair at 12 volts? The gens are built from two Ametek DC motors (38v and 20v). I am having trouble achieving +12 volts, and thought that charging at 6v would might be an alternative. Right now I only have the 38v unit up and flying. It generates around 8-11v in 20 mph wind. I am thinking that the 20v unit will do about the same. I've seen other posts regarding 6v charging systems, but none have mentioned using 2 individual gens with 2 six volt batteries. Any advice will be appreciated.
« Last Edit: November 26, 2007, 12:18:12 AM by (unknown) »

richhagen

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Re: 6v charging from 2 wind gens
« Reply #1 on: November 25, 2007, 09:01:06 PM »
On my electric car, I charge and manage each 12V string independently via isolated chargers.  Such a scheme should be possible, so long as the turbines output is isolated, although I am not sure it is overly practical.  You would still have to prevent overcharge for each 6V battery, in other words, two 6 volt dump load controllers and dump loads.  You would also need a scheme to stop drawing power out at 12V when either of the 6V batteries was discharged as there charging would not be identical.  Rich
« Last Edit: November 25, 2007, 09:01:06 PM by richhagen »
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richhagen

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Re: 6v charging from 2 wind gens
« Reply #2 on: November 25, 2007, 09:07:12 PM »
I rereading your post, I am struck that depending upon the exact specs of your 38V motor, you ought to be able to generate 12V.  The 20V may be problematic because if it may require too high of an RPM to reach 20V.  What type of blades are on these gennys, and I don't suppose you would have an RPM vs. open voltage reading for each - as it might be useful for a prognosis.  Rich
« Last Edit: November 25, 2007, 09:07:12 PM by richhagen »
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sk windpirate

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Re: 6v charging from 2 wind gens
« Reply #3 on: November 25, 2007, 09:23:43 PM »
Re: 6V charging.


What size of prop are you using on this 38v Ametec? It should do a lot better than 11 Volts in a desent wind.


Ron

« Last Edit: November 25, 2007, 09:23:43 PM by sk windpirate »

Jrmobb

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Re: 6v charging from 2 wind gens
« Reply #4 on: November 25, 2007, 11:41:32 PM »
This is a newb question for all you solar masters. Cant he just put the 6v in parallel for charging and hook the inverter or what ever 12v he uses in series. Is there drawbacks to this setup.
« Last Edit: November 25, 2007, 11:41:32 PM by jrmobb »

richhagen

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Re: 6v charging from 2 wind gens
« Reply #5 on: November 26, 2007, 01:10:07 AM »
A recent post regarding what appears to be a similar Ametek motor indicates a cut in RPM of about 400 RPM.  That should be doable with a 4 foot prop in reasonable winds.  You should reach cut in way below 20 miles per hour, even with a larger blade set as you can see from the chart I threw together for you below.





I am therefore wondering about the specifics of your blades, or if there may be some other issue with your installation.  Rich
« Last Edit: November 26, 2007, 01:10:07 AM by richhagen »
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richhagen

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Re: 6v charging from 2 wind gens
« Reply #6 on: November 26, 2007, 01:22:08 AM »
You would be unable to charge and use the inverter at the same time - Unless you had one turbine connected to charge one 6V battery and the other turbine the other, with the outputs from the turbines isolated from each other, still that setup would be more problematic because you would have to monitor and manage each 6 volt battery separately as I noted above.  Rich
« Last Edit: November 26, 2007, 01:22:08 AM by richhagen »
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electrondady1

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Re: 6v charging from 2 wind gens
« Reply #7 on: November 26, 2007, 07:01:45 AM »
would he not simply require a 6 volt charge controller?

such devises exist.

i know there are line loss issues with lower volt systems.

but it should be doable

« Last Edit: November 26, 2007, 07:01:45 AM by electrondady1 »

mkirk

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Re: 6v charging from 2 wind gens
« Reply #8 on: November 26, 2007, 07:27:09 AM »
Thanks for your input Rich. After reading your comments I am now aware of the lack of charging 'balance' between individual 6v genny batteries using this scenario. It does seem like an impractical way to manage charge to the batteries.


In regards to blade specifics, I have been using 4' diameter pvc cutout blades. My first hub had 3 blades that were about 3" wide at the hub end, and 1.5" at the tip. The blades were cut from 4" PVC. These did not perform very well even in high speed winds. Yesterday I cut a set of 2 props for a 2 prop system (also 4' diameter), hoping that this would help the genny achieve higher prop speed.. Last night after writing this post I went out and checked open circuit voltage in high winds.. I was getting 25+ volts! The genny is located in a 12 acre open field on a hilltop (15' pole) so I don't think wind is the problem. So maybe it will be just a matter of refining my blades to achieve the cutin speed.


Are properly carved wooden blades an option? I have heard that they are much more efficient given the proper shape. Also according to your plot, 3' blades look like a better way to go. Sorry for the newbie observations, I am just getting started.. Thanks again.

« Last Edit: November 26, 2007, 07:27:09 AM by mkirk »

mkirk

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Re: 6v charging from 2 wind gens
« Reply #9 on: November 26, 2007, 07:35:57 AM »
Thanks for the input. As far as 6 volt charge controllers go.. I would probably need a 6 volt controller attached to each battery, with seperate diversion loads? I had plans to build my own 12 volt controller based on the following page:


http://www.fieldlines.com/story/2004/9/20/0406/27488


Maybe this could be adapted for 6 volt operation.

« Last Edit: November 26, 2007, 07:35:57 AM by mkirk »

richhagen

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Re: 6v charging from 2 wind gens
« Reply #10 on: November 26, 2007, 07:55:36 AM »
Since he wants to pull out 12V it becomes a bit more cumbersome.  It seems like he should be able to get 12V charging with a decent prop on the Ametek 38V though.  Rich
« Last Edit: November 26, 2007, 07:55:36 AM by richhagen »
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richhagen

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Re: 6v charging from 2 wind gens
« Reply #11 on: November 26, 2007, 08:39:28 AM »
I would certainly have a look at DanB's blade page which is accessable under the wind section off of the front page of the Otherpower site.  He has some pretty good instructions on carving wooden blades.  It doesn't seem to me that your PVC blades are performing properly.  


The previous chart only showed the blade rpm at the tip speed ratio, it didn't show the power you could capture at that speed.  






Note that on the legend, while the colors are correct, the bottom most curve is listed on the top of the legend, I meant to correct that to make it a bit less confusing, but I have not done so yet. The curves show the power at 35% which is possible within a narrow band of wind speeds for an efficient axial flux machine.  






This is the curve showing the kinetic energy in the wind passing through a 4 foot diameter swept area.  The bad news is that you will likely only achieve an efficiency of 25% or less with a tape drive mill.


While you can spin up a smaller blade set faster, the power into the shaft will be lower once you exceed the cut in speed because the available power in the wind passing through the blades is less in an amount proportionate to the swept area.  For the 4 foot set, you are probably looking at about 65 Watts of total kinetic energy in a 10Mph breeze, of which you will probably only capture about an Amp or 12 to 13 watts.  Now the swept area in a 4 foot diameter blade is (16/9) or 1.77 times greater than for a 3 foot diameter blade set.  


If the blades are so small that they are greatly mismatched in speed and power to the alternator, they will not be able to push the speed up as the wind speed increases, they will tend towards stall, and the power out will be reduced further.  Conversly if the blades are to large, the drag placed on them from the alternator will not be sufficient to keep them from running away to destruction in higher winds.  Looking at the chart from the earlier post, with a 4 foot blade, you would be aiming for a cut in wind speed of about 8MPH and with a 5 foot at about 10MPH.  You would probably get overall more power out with a larger prop, but it will be idle more unless you carve it for a faster TSR.  My logic on this is that for battery charging you want to get power into the battery as much of the time as possible, for grid tie, you want to maximize the overall power produced.  I would probably not go larger than 5 foot, or you may not reach cut in most of the time.  Most places have an average wind speed of 11 MPH and a wind speed distribution that puts the wind below that speed much of the time.  Higher TSR blades tend to be noisier, and since they are thinner with less solidity, they can be more problematic at startup for a given blade diameter.  This is just my opinion, but in your case, I would likely stick to a 4 foot diameter, TSR7, since in periods of higher winds you will likely not have the battery capacity to store the power you make anyway.  


Rich Hagen

« Last Edit: November 26, 2007, 08:39:28 AM by richhagen »
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wooferhound

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Re: 6v charging from 2 wind gens
« Reply #12 on: November 26, 2007, 08:48:11 AM »
« Last Edit: November 26, 2007, 08:48:11 AM by wooferhound »

mkirk

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Re: 6v charging from 2 wind gens
« Reply #13 on: November 26, 2007, 01:06:13 PM »
Thanks to all who replied. Rich, the data you provided was very informative.. It looks like I'll have to take a closer look at my blade construction, before I can move on to the rest of the system. While I'm at it..


Being that my generator is in a field approximately 250 feet from the house, would it be feasible to have the batteries/inverter close to the mill and run 120 VAC to the house (UF-B cable)? I am worried about losses in the long drop considering these are DC motors. Unfortunately there is no barn/shed on site to put the setup in, so I'd have to fabricate some kind of insulated box for the batteries and an all weather utility type box for my charge controller and inverter. I realize that it sub-optimal to store batteries in the great outdoors but I can't see dropping a lot of $$ for heavy gauge wire for the run to the house. I probably should cut my losses and go with a AC type wind generator, right?

« Last Edit: November 26, 2007, 01:06:13 PM by mkirk »

richhagen

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Re: 6v charging from 2 wind gens
« Reply #14 on: November 26, 2007, 03:45:56 PM »
Now this is just my opinion, but 250 feet is a good distance at 12V.  For a larger power system, this would require pretty large cables to keep the losses to a minimum.  Pushing an amp or two may not be that bad though.  Here is a wire resistance calculator that I wrote a while back.  It is in Visual Basic 6, so it will only run on Windows machines though.

http://www.otherpower.com/images/scimages/742/WIRECALC.exe

For your situation you have 500 feet of conductor, 250 there and back if you leave the batteries at the house.  For 10 guage wire you have about .5 Ohms of resistance which is pretty sizable.  The power lost in the conductor is equal to the square of the current times the resistance.  If you were pushing only an amp through 500 feet of cable of that size you would lose a half a Watt, and your voltage drop would be the product of the current and the resistance or a half a volt.  Now at 10 amps, the situation becomes a bit worse, you would lose 50 Watts to resistance and your voltage drop would be 5 volts - so your mill will be spinning a bit faster as well. You actually want a bit of play there as it helps keep the blades out of stall, although 5 volts is probably a bit much and for 4 or 5 foot blades on a tape drive mill I don't think the alternator is strong enough to stall the blades too much anyway.   It is not as bad as it sounds, because to push out 10 amps from your mill with 4 foot blades you are probably near 20Mph anyway, and the blades will want to spin up quite a bit faster anyway.  Since your losses are proportionate to the square of the power output, your losses become large when you can most afford them for battery charging, when you have a lot of power coming in.  


It really depends upon how much power you want from your system.  Are you intending to power your house, or just a hobby setup to power a circuit and a light or two.  If you intend to push a large amount of power, and are stuck at 12V, then it may make sense to have the batteries and inverter nearer to the source.  For example, the 10 amps at 12V or about 120Watts would only be an Amp of current at 120V, so your losses through 10 guage would be back to a half a Watt and a half a volt voltage drop.  It is however a tradeoff, you have to go to the inverter and batteries to check settings and that tends to be more problematic.  If you add additional generation, say solar, or a generator backup, they have to connect to where the batteries and inverter are at as well.  If you are plunking down the time and money for a turbine, tower, inverter, batteries, and other equipment, you will probably plunk down some more green for a larger conductor - although for a larger system, I would probably not choose 12V, in your case it is what you already have.  If you double up the conductors, or double the cross sectional area of the wire, your losses are cut in half.  


Make your system work for you, Rich


You can always make it work for now and make adjustments as your system grows.  That seems to be what most people here do anyway.  

« Last Edit: November 26, 2007, 03:45:56 PM by richhagen »
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