E. Walter Le Roy wrote:

We have a 7 foot water wheel in a mountain stream and would like to hook up
an alternator, and light up the fish pond. Can anyone tell me what RPM it
needs to turn to run one headlamp?
Thanks
Walt


The alternator, or the wheel? What sort of headlamp?

Given that most automotive alternators seem to be driven at about 2x the
crankshaft speed, and the engines idle at least 500 (probably more like
750 or 1000) rpm I'd say you need to give it at least 1000 rpm.

I'm not sure how your average automotive alternator would do without
working into a battery -- I'm sure a regulator could be built that would
allow you to run without one, but it'd be a bit of a science project.

Lessee --

(10A)(12V) = 120W = 120 Joules/sec

Assuming you can extract all the energy out of a 7' drop:
(7 ft)(0.305 m/ft)(9.8 m/sec^2)(1kg/liter) = 21 Joules/liter

which means that you need a flow of
(120 Joules/sec) / (21 Joules/liter) = 5.7 liter/sec

which in the US would be
5.7 liter/sec = 343 liter/min = 94 gallons/min.

That's assuming 100% efficiency, you'll probably be lucky to get 50%, so
say 200 gallons/min at a bare minimum.

Unless I've slipped a digit it needs to be a healthy stream, and that
wheel is either going to need to turn fast or it's going to need to be
fairly wide. I'd try to find an old book on efficient water wheel
design, to see about building efficent wheels.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com