What is Microhydro Power?

Hydro-electricity is fundamentally the combination of water flow and vertical drop (commonly called “head”). Vertical drop creates pressure, and the continuous flow of water in a hydro system gives us an ongoing source of pressurized liquid energy. Pressurized, flowing water is a very dense resource, and hydro-electric systems convert a very large percentage of the available energy into electricity because the resource is captive in a pipe or flume.

People have been tapping the energy in flowing water for centuries, first for mechanical power, and, in the last hundred years, for electricity. Early applications included milling, pumping, and driving machinery. Unlike wind and sun, the right hydro resource can be available 24 hours a day, 365 days a year. This allowed pioneers to run irrigation pumps and grain mills, and allows people today to make clean, renewable electricity at a reasonable cost.

A simple formula can give you a rough idea of how much capacity your stream might have. Take the head in feet, multiply it by the flow in gallons per minute (gpm), and divide by a factor of about 12. This will give you the potential wattage of a reasonably efficient, small system. For example, if you have 30 gpm available and 40 feet of head, you will be able to generate something in the range of 100 watts [(30 × 40) ÷ 12 = 100). Over the course of an entire day, the generation would be 2,400 watt-hours or 2.4 kWh (24 hours/day x 100 W).

Within this formula is the understanding that systems with low vertical drop (head) need more flow to generate the same amount of energy. Typically, low-head systems will have high flow, and high-head systems will have lower flow. Adapting the example above, if we have 400 hundred feet of head, we only need 3 gpm to generate the same 100 W.

There are a wide range of small hydro turbine types to suit the head and flow of the site. Large wooden overshot and undershot wheels tend to be less efficient for generating electricity, though they may be appropriate for mechanical work. For electricity generation, systems can be divided into “low head” and “high head.”

Low-head systems may have less than 5 feet of vertical drop—sometimes they may have only 10 or 20 inches. In this case, most or all of the water in a small stream will run through the turbine to maximize output. The runner (the part of the turbine that receives the water and turns its energy into rotation in a shaft) for low-head turbines may be a Turgo or Francis type. These systems typically have short pipelines or sluiceways that then allow the water to drop through the runner.

High-head systems may be defined as any site with more than 10 feet of head. Common runners are Turgos on the low end, and the most common, Pelton, for medium to high heads. These systems may have hundreds of feet of pipeline to develop the head (pressure), with the water delivered to the runner via multiple nozzles.

The basic components of a small hydro-electric system, running from “water to wire” are:

  • Diversion and intake screen—Directs water from the stream or river into the pipe or channel
  • Penstock (pipeline)—Carries the water to the turbine
  • Turbine—Generates electricity (includes nozzles, runner, and generator)
  • Electronics and batteries (if used)—Regulates turbine and stores energy
  • Dump load—Absorbs surplus energy
  • Transmission and distribution—Delivers the energy to its end use


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