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Microhydro.jpg    See also the Microhydro category.
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Hydro Power is the conversion of the energy in moving water to electrical or mechanical energy using a turbine or waterwheel. From backyard, single resident systems made from recycled car parts, to grand scale projects such as those on the Colorado river in the western United States, people are harnessing the energy of water moving down a gradient. The size and application of a Hydro Power system can vary widely. Microhydro systems are specifically those systems that are not grand in scale. Damming a river usually will not qualify as microhydro, but partially diverting a stream into a holding tank and running the water into a small hydro-electric turbine or pelton wheel is more along the lines of what the term microhydro refers to.

The following article is a brief overview of the more technical aspects and considerations needed to design and construct a microhydro system.

Hydropower Sizes[edit]

Size Power Output Typical Use
Large >10MW Usually part of a large grid.
Small 1MW-10MW Usually grid intertied. (up to 50MW in Canada[1])
Mini 100kW-1,000kW (1MW) Community and industry. Often grid intertied.
Micro 1kW-100kW Small low energy consuming community, small industry, rural high energy consuming household. Usually off-grid.
Pico <1kW Radio tower, low energy consuming household, charging station. Almost always off grid.

Please note that these are averages, many different communities classify hydropower somewhat differently.


The amount of energy \, E released by lowering an object of mass \, m by a height \, h in a gravitational field is[2]:

\, E = mgh where \, g is the acceleration due to gravity.

Converting these units, a common field equation to measure the maximum power available in a moving body of water is:

\, P_{max} = \frac{Q_{max}*H_{max}*e_{max}}{K}


  • Pmax=Maximum Power Available (kW)
  • Qmax=Flow (Volume/time)
  • Hmax=Head (Vertical drop in ft)
  • emax=Efficiency of the turbine (use a value of 1 for max power available)
  • K=Unit conversion factor (see table below for some common values)
For Q measured in K is equal to
ft3/min 708 (ft4)/(min*kW)
ft3/sec (CFS) 11.8 (ft4)/(sec*kW)
l/sec 334(l*ft)/(sec*kW)
gal/min (GPM) 5302 (gal*ft)/(min*kW)

To find the actual power you will get from that moving body of water, calculate Pnet with the following changes made.

\, P_{net} = \frac{Q_{net}*H_{net}*e_{net}}{K}


  • Pnet=The net power extracted from the river, not including loss in delivery from power station to load (kW)
  • Qnet=Flow (Volume/time) - Only take a portion of the max flow (%take). For delicate streams this may be a small percentage of the total flow.
    • Qnet=Qmax*%take
  • Hnet=Head (Vertical drop in ft) - This is the actual head that you have available due to losses from friction. Calculate friction loss using tables based on the materials you use for diversion (e.g. PVC).
    • Determine equivalent length of pipe by adding actual length of pipe and equivalent lengths of fittings based on tables using pipe size.
    • Find Frictional Pressure Loss Ratio (FPL) coefficient in ftloss/ftpipe based upon flow rate and pipe size
    • calculate Hloss=equivalent length of pipe * FPL


  • enet=Efficiency of the turbine - Always between 0 and 1, usually between .5 and .9 depending on the turbine type and flow rate. A value of 0.78 is a good guess for modern turbines in average conditions.
  • K=Unit conversion factor (see table above for some common values)

Note that these equations are static in time. You must run these equations for with a resolution high enough to cover periods of variation (e.g. monthly river data).

Water Wheels[edit]

Probably the most accessible technology for hydro power is the water wheelW. It can be built entirely from local materials. Only the generator has to be brought in. For small systems a modified motor or car alternator can be used.

  • The vertical undershot water wheel is most appropriate for relative low head situations even if it is the least efficient of all water wheels. You should avoid building an undershot wheel with straight buckets and go for either a Poncelet wheel or a Zuppinger wheel which can both double the efficiency. They have an efficiency of about 30%, but enclosed like a breast shot of up to 70%.
  • The breast shot is next when the head is large enough. Here the water enters at a height similar to the axle height. It is more complicated to build and needs a structure that encases the wheel to function with high efficiency. If done correctly it uses the weight of the water and its impulse. It can have an efficiency of about 85% if it's well-built.
  • The over shot wheel needs the most head of the water wheels. Under optimal conditions with steel buckets it can have an efficiency of up to 80%.
  • The back shot wheel can be seen as an cross between a breast shot wheel and an overshoot wheel. The water enters at the top of the wheel but the buckets are like a breast shot wheel. The direction of rotation is the same as in a breast shot wheel. The efficiency can exceed that of the breast shot.
  • Horizontal water wheels apart from museum pieces are today found mainly in the Himalaya region in the form of the ghatta. That version is a primitive version of a turbine.

Impulse Turbines[edit]

Turbine Type Flow Head
PeltonW Lowest Highest >10ft
TurgoW Intermediate Intermediate >4ft
CrossflowW Highest Lowest <4ft

The only machine that can be built without access to metal casting equipment or a 3D milling machine is the crossflow turbine. The only advanced tools needed are a cutting torch and welding equipment. Needed raw materials are sheet metal and metal pipe. Information how to build a cross-flow turbine can be found on the CD3WD ( see Resources ) web site.

To build runners for small Pelton or Turgo turbines it is necessary to have either access to a 3D milling machine or have access to a casting shop or to build a casting shop. For the latter the largest problem is probably to get information how to build a mold for a suitable runner. For more information see Casting ( Metal )


  2. Some high flow, low head situations can use hydropower systems such as a [water wheel] to convert just kinetic energy of the flowing water with very little change in the potential energy. In those cases, P = \frac{1}{2}\,\rho\,\phi\, v^2 where \, v is the velocity of the water.


  • CD3WD - Helping the 3rd World to help itself
    • A website which gathers articles from NGO's and other organizations and compiles them on CD's . The images of that CD's can be downloaded through their website. The CD's that have identified by me to contain information about hydro power are cd3wd407 and cd3wd430.
  • Power versus Pipe Diameter
    • Live spreadsheet graph of Power versus Pipe Diameter for Various Flows (0 to 150GPM). Customizable amount of run and fittings.

This topic is moderated by Lonny

Interwiki links[edit]