Organic Rankine Cycle
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any people have access to some form of running water and are wondering just how much Mpower, if any, can be produced from it. Almost any house site has solar electric potential (photovoltaic). Many sites also have some wind power available. But water power depends on more than the presence of water alone. A lake or well has no power potential. The water must be FLOWING. It also must flow from a high point to a low one and go through an elevation change of at least three or four feet to produce useable power. This is called the head or pressure, usually measured in feet or pounds per square inch (PSI). The flow is measured in gallons per minute (GPM) or for those blessed with larger flows, cubic feet per second (CFS).
At most sites, what is called run of river is the best mode of operation. This means that power is produced at a constant rate according to the amount of water available. Usually the power is generated as electricity and stored in batteries and can be tied to an existing PV or other system. The power can take other forms: shaft power for a saw, pump, grinder, etc.
Both head and flow are necessary to produce power. Even a few gallons per minute can be
useful if there is sufficient head.
Since power = Head x Flow, the
more you have of either, the more power is available. A simple rule
of thumb to estimate your power is Head (in feet) x Flow (in gpm)
/10 =Power (in Watts).
This will give you a
rough idea of the power
available at the
average site and
reflects an overall
efficiency of 53%. This is
a typical output for a well
designed system. For
example: if your head is
100 feet and the flow is 10
gpm, then 100 x10/10 = 100
watts. Keep in mind this is power
that is produced 24 hours a day. It is
equivalent to a PV system of 400-500 watts -
if the sun shines every day. Of course, the water
may not run year round either. So it is apparent how a combined system can supply your power needs on a continuous basis.
Determining Head & Flow
Let's start with the head since that is easier than the flow and will give you confidence to continue. The best method to determine the head is also the easiest and can be used at any site. It is also very accurate. It involves using a length of hose or pipe in the neighborhood of 1/2" diameter. You can start anywhere along the brook and proceed upstream or down. First submerge the upstream end in the water and weigh it down with a rock or something similar. With the top end fixed in place underwater
you move the rest of the pipe downstream. When you have reached the end, it is now time to start the water flow through the pipe. This may require you to suck on the end. Once flow is established and all air bubbles are removed, slowly raise the pipe upward until the flow ceases. When this point has been reached, use a tape measure to measure the distance from the
end of the pipe to the surface of the water. This reading is the head for the stretch of brook. The pipe then becomes a convenient measure of horizontal run if you use a standard length like 100 feet. If you are working with a brook longer than your length of pipe, then simply carry the pipe to the next section to measure and repeat the procedure as required, starting where you ended
It is probably best to "map" more of the brook than you intend to use. This will give you a good overall idea of your site and may reveal
Measuring flow is a little more difficult. This should probably be done in more than one place too. This is because most streams pick up water as they go.
Therefore choosing the best spot for your system requires careful consideration of several
There are several ways to measure flow; here are two. In both cases, the brook water must all pass through either a pipe or a weir. The weir system uses an opening that the water flows through and measuring the depth of water gives the flow. The first involves a technique very similar to the head measuring technique. You must divert all of the water into a short length of pipe. This will usually require the use of a dam in order to pack dirt around the intake end. Pipe size may be from 1" to 6"
Home Power #8 • December 1988/ January 1989 17
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