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I have a stream of water, it flows at 1 gallon per minute into a water wheel. The water wheel produces about 75lbs of torque. How much electricity can I make with this? Where can I get more info on how to compete this project?

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  • \$\begingroup\$ How much head (how far does the water fall in passing through the wheel) have you got? Does it enter with an pressure (ie, is it a "Pelton wheel" where the water hits the wheel at high speed (using head pressure from a reservoir high above the wheel) or is is a simple flow of water from a stream or pond under no pressure as it hits the wheel? \$\endgroup\$
    – Ecnerwal
    Dec 7 '15 at 17:53
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    \$\begingroup\$ To calculate power, you need to measure torque and speed of rotation of the wheel when the wheel is under some resistance. 75 lbs is not a measure of torque. It is a measure of force. To convert it to torque, you would need to specify how far it is from the center of the water wheel. \$\endgroup\$
    – mkeith
    Dec 7 '15 at 18:14
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One simple sanity check is that you will get less power out (due to imperfect efficiency) than the power that would be required to have a pump circulate water from the outflow to the input. If the head pressure is low, you'll find that a very low power pump will pump 1 GPM up a few feet - so your power out will be lower than that. If your 1 GPM is coming in under pressure from a source a few hundred feet above the wheel, you might have some serious power.

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75lbs is not a measure of torque . 75lbs-in, or ft-lbs, or Newton-Metres. is a measure of torque. To use it, you would also need to know the speed of the wheel when it is generating this torque, in rpm or radians/second.

(If that's the static torque, as you let the speed increase, the torque will decrease, and you have to find the actual torque at the most practical speed, usually the speed where it produces most power)

Now if you measure torque in Nm and speed in radians/second, both SI units, just multiply them and you have the mechanical power in Watts.

If you measure torque in ft-lbs and speed in RPM, multiplying them by 2 * pi gives you the available power in who knows what units, but if you can find the relevant conversion factor you can translate it into horsepower or watts. I usually find it easier just to use SI units throughout.

The electrical power you can get is some fraction of this, maybe 50% in the toy DC motor range, to 90% for a modern generator in the kilowatt range.

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