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Q & A: electricity source?

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Most recent answer: 03/13/2010
Q:
I am attempting to produce electricity to run my single story home. Last year the maximum of 1100 watts was used. I am considering a pump that produces 3000 gallons per hour that will spray through a 2 1” nozzles onto a TURGO RUNNER TURBINE. The turbine will in turn drive a generator to produce electricity. The generator needs 3750 RPM to produce 10,000 watts of power. My question is: Will the 3000 gallons per hour be sufficient to produce the amount of rotation needed for the generator? The Turgo Runner is 133 mm in diameter. I have gone to http//www.1728.com/flowrat.htm and it calculates 2” Diameter with a 3000 gallons per hour would produce a velocity of 2.2918e+3 (a number I don’t understand guessing it would be 2,291.8) feet per minute. If my guess is correct then the circumference of the Turgo Runner is approximately 22” and the pump would turn it at 1250 rpm (2,291.8 fpm * 12” / 22”). Can you confirm the calculation or show me the errors of my way. Hope I have given you enough information. Thanks, Jack
- Jack Horton
Corona, CA USA
A:
The key strategy in designing an energy supply is to trace the flow of energy (specifically, Helmholtz free energy, to be precise) from the source to the point of use.

What will power your pump? If it will be electrically powered, then the scheme is a sure loser. The electrical power output will be less than the power consumed by the pump. This conclusion is not an probable engineering judgment but a definite certainty based on the laws of thermodynamics. The Second Law says that free energy is lost at every stage.

If your pump will be powered by something else (for example, gasoline) then the question is whether your device is the best way to convert the free energy of the fuel into electrical power. Since one can get gasoline-powered generators, which skip the steps of pumping water around, it almost certainly is a relatively inefficient way to do that. Frictional losses from the flowing viscous water would throw away some of your energy. The same argument would apply if your pump were, for example, driven by a wind turbine. Direct electrical generation is more efficient.

If you do intend to burn some sort of fossil fuel to make electricity, then you need to compare the efficiency of your set up with that of the large commercial plants from which most of us buy our electricity. Although there are some losses in transmission, the efficiencies of these large plants typically beat anything you can do at home.

This is not to say that there's no role for pumps. Say your energy comes from some combination of wind and solar. These are great sources, but they aren't always on when you need them. You need some way of storing the energy they supply for periods when they're off. One way is via batteries, but these have some drawbacks. Another way can be to pump water up into raised tanks or ponds. It can then run a generator on the way back down.  There's some inefficiency, but since that energy would otherwise be thrown out, it can be a good system anyway.

You'll notice that I didn't say anything about the specifics of your pipes, etc. That's not just because I'm lazy. Keeping track of the basic flow of energy is far more reliable than trying to piece together detailed calculation of particular sub-systems.

Mike W.

(published on 03/13/2010)

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