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Page:Popular Science Monthly Volume 90.djvu/781

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Water Power From Small Streams

How you may estimate fairly accur- ately the capabilities of any stream

��tiic capauiiiLics oi any ;

By H. B. Richmond

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��kHE very stream to run my lighting generator!" How many times have you thought that, when unex- pectedly you came upon a little stream in the woods? But have you really any idea of just how much power can be obtained from a stream? Let us consider the situa- tion, so that the next time you see a stream you will be able to estimate instead of specu- late how much work it is capable of doing. We will not bother about the refinements of stream gaging, or the intricacies of the design of turbine blades. Just enough of the fundamental principles to enable us to arrive at the desired result will be sufficient. That result is power, and the two items which make it up are the quantity of water flowing and the distance it falls. Quantity is measured in gallons per second, cubic feet per second, or other convenient units. The distance through which the water falls is known as the head and is measured in feet. Power may then be said to be the product of the quantity Q by the head H. If we accept the cubic foot per second as the measure for Q, the foot as our measure for H, and the horsepower as our measure for power we have the equation P = ^ This assumes that all the water power may

��be converted into work, which unfortu- nately is not true. In large installations it is not uncommon to utilize 80 per cent of the water, but for small streams it would not be wise for us to count on much over one-half. Taking allowance for this factor our equation would become, say P = ^.

Now all this is very fine if we know Q and H. Fortunately it is not very difficult to get an estimate of these quantities. If the water is coming over a single fall it is not at all hard to estimate or to actually measure the height in feet. If there are a series of falls to be utilized the sum of the heights of the several falls would be the height to use. This, of course, implies that the stream would be dammed at the last fall so that the water would back up to the level of the first fall, or that the dam would be placed at the first fall and then the stream carried on a level through a pen- stock to a point above the last fall.

It is not such an easy matter to determine the quantity of water flowing. A very simple way to get an estimate is to drop a small piece of wood in the stream a little distance above the falls and note the speed with which it is carried down the stream. I dropped a chip in a little woodland

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