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is my water wheel too small?  RSS feed

 
Keith Keahi
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Aloha I'm the rookie idiot to this site and need some help with my hydro water wheel. Just an off grid guy but not a wiz at it so be kind

So I built a 4ft diameter by 18in wheel. I have it geared hoping to get close to 1000 rpm. When the water wheel is free spinning it gets up to 55rpm but once I hook it up to the PMA the resistance slows my wheel down to 12rpm. So when I play with my gearing to get more speed it makes it worst. I have a good flow but my max head is 4ft. I get between 600 to 800gpm. I'm using an undershot setup. I'm guessing I need more torque but am I maxed out for my wheel? Do I have to go bigger? Oh! btw my PMA is a freedom 2 so not sure if it's a good choice of motor I bought.

So any advise would be valuable and kindly taken.
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Wheel
 
Roy Hinkley
Posts: 268
Location: S. Ontario Canada
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Not an expert either but your motor is rated 2800 watts?
Just eyeballing the size of the setup and others I've seen that output about 500 - 800 watts. You may be asking too much from your small(ish) diameter wheel.
 
David Livingston
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Location: Anjou ,France
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Have you thought of converting to an over shot wheel ? You are loosing a lot of energy and not taking advantage of the full four foot of "head" .
Over shot wheel I have been told are more efficient . Would it be possible for you to move the wheel slightly down stream put the wheel in a trough make it over shot . This way you could have a maybe 5 foot wheel over shot wheel thus increacing power out put considerably
See here http://www.whitemill.org/z0028.htm


David
 
Keith Keahi
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That's what I was afraid of. I'm thinking of going with an 8ft wheel but an undershot is my only option cause I can't get my head any higher then it is. I might be able to squeeze out 6 more inches. Gonna research a few wheel options and proper trough design. In the pic I'm only using a third of the flow. But now I'll focus the whole stream into the trough and hope I get enough torque with an 8ft wheel. Do you think that could work?

The PMA they say should give me 2000 but they used to say 2800. I doubt that I would get anything that high even with limitless options and wheel size on this motor but if I can get a 1000 watts then I'm happy. Enough to keep my battery bank happy through the night. Thank you guys for your direction and guidance. And if you got more to teach I'm all ears. If I succeed to get this working then I must credit all you for your help.
 
Keith Keahi
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Oh. Correction. From what your link showed I guess I have a breast shot design. Gonna have to stick with this cause my max head height is 4 to 5ft
 
David Livingston
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I dont think that will make a difference and could even make things worse . Your power comes from the flow of the water , . You can only get a % of the available power . . Your wheel could be 50ft tall you are not going to get any more power than is in the flow . Legthening the run off and hightening the drop will help and having an overshot wheel will increace the power slightly and increace the % of that power you can capture .
For a wheel like yours I would expect there to be an optimum size but certainly not more than 12 ft ie twice the head . But since the max power capture would be 50% available as opposed to 70 % for overshot I would make an overshot . ( if you do the Math its going to give you 30% more power available in theory)
David
 
Keith Keahi
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So your saying with more water it will give me more torque? So if I build a larger wheel I won't get more torque then my 4ft wheel? For some reason I was thinking of a wheel like a breaker bar wrench. Longer the wrench the more torque at the pivot point. So I need to get the trough setup as an overshot and channel as much water as I can to get the toque I need? I'll try that this week and see what happens. Gonna take me a few hrs to build and install the new trough. Thanks for the help. I'll keep you posted in a few days when I complete the over shot.
 
David Livingston
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The energy comes from the water . Its a function of volume speed and height .
The wheel converts this into rotory motion . How efficently it does this depends on the type of wheel and the type of equipment .
For low speed low volume look up a https://en.wikipedia.org/wiki/Pelton_wheel Pelton Wheel .
Or an overshot wheel give the highest conversion rates . The size of the wheel should be as large as possible to increace the height function

David
 
Keith Keahi
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I looked up Pelton wheels but I need pressure and the way my stream is setup and it's real purpose which is to flow into rice like wetland patches I can't alter it. From where my wheel is located that is the only spot that has a vertical drop of around 4ft. Before and after this location I get a drop of a foot at about 80yards. I can't run a long pipe or the water will bypass the diversions where it feeds the patches.
Looks like most Pelton wheels need 10 to 20 ft of head to operate(powerpout.com). They call that low head but for me that hight means 1/2 a mile or more of pipe. Not an option for me. With 4ft of drop it seems my wheel can only be 4ft for an overshot. I'm maxed out on width so now the option your saying is to get more water over it to increase torque. Gonna give that a try.
Sucks that I have these limitations or I would go crazy with a hydro system that could produce some crazy watts.
 
Brian White
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Why not a small gravitational vortex in a rain barrel?  It is way simpler than a waterwheel.  you only need 2 to 4 vanes on the shaft to harness the power.   The original  viking and Roman water wheels were horizontal and nobody really knows how efficient they were.  Some may have had a bit of a vortex going.    I thought permaculture was partly about reducing complexity in power systems.   If so,  the vortex is a far simpler system than a waterwheel.   
 
C. Letellier
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Location: Greybull WY north central WY zone 4 bordering on 3
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assuming the wheel is 18 inches wide I am fairly sure you don't have 600 gallons per minute coming out your chute.  But lets do the math for 600 gallons per minute and 4 ft.  There is .134 cubic feet per gallon  600 x .134 = 80.3 cubic feet per minute.  There are 62.4 lbs per cubic foot.  80.3X 62.4 = 5016 lbs per minute.  And it is falling 4 ft.    20,064 ft-lbs per minute.  60 seconds per minute gives 334 ft-lbs per second.  1 ft-lb per second = 1.356 watts.  334 ft-lbs per second x 1.356 watts seconds per ft-lb = 453 watts of power.  Assuming you can get 50% efficiency that is just over 200 watts peak power output.


 
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