piping (Penstock) design

The piping used with microhydro systems is usually either four-inch PVC pipe or smaller 1.5-2 inch polyethylene pipe.  The polyethylene pipe is usually only used in systems where it is possible to use low flow volumes.  The advantage of this type of piping is that it is very flexible and can be purchased in very long roles.  Sometimes a single roll may be sufficient to run from the water intake all the way to the turbine, particularly if there is a steep slope.

In most installations the slope is not extremely steep and so in order to get a hundred feet of head (vertical distance) you might have to run a thousand feet or more of pipe.  Consequently the cost of running the pipe can be significant.  Fortunately PVC pipe is not particularly expensive.  Most PVC pipe is sole in 10 or 20 foot sections and the pipe ends can be glued together to form a continuous run. 

If you live in a cold climate you will probably need to bury the piping below the frost line to keep it from freezing.  Burying the pipe is usually a good idea in any event since exposed pipe on the surface can easily be damaged by human traffic or by events such as a falling tree. Also, PVC pipe tends to deteriorate when exposed to sunlight so burying the pipe protects it from sun exposure as well.

Some terrain, particularly that close to mountain streams, is extremely rocky and in these situations burying the pipe is not a practical option without significantly disturbing the environment.  In these situations you probably want to consider surrounding the exposed piping with rubber or foam insulation. The good news is that water moving through a pipe is a bit less susceptible to freezing than standing water so as long as the flow is steady the chances of the exposed pipe freezing are greatly reduced.