Choose a micro-hydro turbine by head: a Pelton wheel above 20 meters, a crossflow turbine from 3 to 20 meters, a propeller turbine from 1 to 5 meters, and an overshot waterwheel when there are more than 2 meters of head and a slow mechanical shaft is wanted. Power is 9.81 times flow in liters per second times head in meters times overall efficiency, so a site with little head needs a very large flow to make the same watts.
Part of Micro-Hydro Turbine
A water turbine converts the pressure or speed of falling water into a spinning shaft; four designs cover every head from a river weir to a mountain stream.
Head Picks The Machine And Flow Only Sizes It
Every design below does the same job: take the energy that water loses between the intake and the tailrace and turn it into shaft power. What differs is the kind of water it likes. A high-head stream is thin and fast, so it is turned into a jet. A low-head river is thick and slow, so it needs a wide runner that a large volume can pass through.
| Type | Head band | Efficiency | Shaft speed | Best use |
|---|---|---|---|---|
| Pelton wheel | above 20 m | 80-90% | 500-1,500 RPM | steep, small streams, direct generator drive |
| Crossflow (Banki) | 3-20 m | 65-80% | 200-500 RPM | the forgiving all-rounder, welds up from plate |
| Propeller / Kaplan | 1-5 m | 70-85% | set by runner size and head | wide, slow rivers, but blade angles are critical |
| Overshot waterwheel | above 2 m | 60-85% | 5-15 RPM | grain mill, saw, hammer; needs gearing for a generator |
These bands come from the carrier article, Micro-Hydro Turbine. Other pages in this corpus quote different bands: the Crossflow Turbine page says its design works from 1.5 to 200 meters, Turbine Design puts the practical minimum for a Pelton at 15 meters, and Head Measurement moves the Pelton threshold to 30 meters. Do not average them. They overlap, and near a boundary the deciding facts are your flow, the metal you can get and the shaft speed your generator wants.
The Same 350 Watts Can Need 1 L/s Or 50 L/s
The carrier’s rule of thumb is watts equal to flow in liters per second times head in meters times 7; the factor 7 is 9.81 multiplied by an overall efficiency of about 0.71 (see Power Calculation). Fix the output at 350 watts and the product of flow and head is fixed:
- Flow × head = 350 ÷ 7 = 50 liters-per-second-meters.
- At 50 m of head: 50 ÷ 50 = 1 L/s, a trickle through a Pelton nozzle.
- At 10 m: 50 ÷ 10 = 5 L/s, the carrier’s own example.
- At 2 m: 50 ÷ 2 = 25 L/s, a stream you can see at a glance.
- At 1 m: 50 ÷ 1 = 50 L/s, the flow of a small river, for the same 350 watts.
That is why the low-head machines are large and the high-head ones are small. It is also why measuring head is cheap insurance: a site with five times the head needs one fifth of the water.
The Shaft Speed Decides The Drive
Most small alternators want somewhere around 500 to 1,500 RPM. Dividing 1,500 RPM by the turbine speed gives the step-up you must build:
- Overshot wheel at 10 RPM: 1,500 ÷ 10 = 150:1, which is two or three belt or gear stages.
- Crossflow at 300 RPM: 1,500 ÷ 300 = 5:1, one belt.
- Pelton at 1,000 RPM: 1,500 ÷ 1,000 = 1.5:1, close to direct drive.
Each stage costs power and adds a bearing that can fail. Pairing a slow wheel with a generator is possible, but it is the main reason the carrier recommends the wheel for mechanical drive rather than electricity. Coupling choices are in Generator Coupling (Belt vs Direct).
Hydro Power Is Linear; Wind Power Is Cubic
Do not carry a wind rule into a hydro design
Hydro power is proportional to flow and proportional to head. Raise the flow 44% and the power rises 44%. Wind power is proportional to the cube of wind speed: raise the wind speed 44% and the power rises by 1.44 × 1.44 × 1.44 = 2.99, about three times. A stream also changes slowly over a season, while wind changes hour by hour. The wind law is in Wind Resource Assessment.
A Ram Pump Is Not A Turbine
A hydraulic ram pump uses the same head and flow but lifts a small part of the water to a greater height; it makes no shaft power and cannot run a generator. If water supply rather than electricity is the goal, read Ram Pump instead.
Common Mistakes
- Picking by the machine you can already build — a Pelton cup wheel on a 3-meter drop wastes the site; the flow is there but the jet is not.
- Ignoring shaft speed — an overshot wheel will turn a millstone happily and a car alternator not at all.
- Quoting one head band as law — the bands above are guidance, and neighbouring pages disagree near the edges.
Safety
Pressure and spinning runners
A penstock holds 1 bar for every 10.2 meters of head, so a 20-meter line is about 2 bar before any surge. A turbine runner and its belt cannot be stopped by hand once spinning. Close the intake gate, which is the stop, and let the runner coast down before reaching near it.
What’s Next
- Learn next: Pelton Wheel (High Head) for the highest heads
- Combine with: Civil Works for the intake, penstock and powerhouse the turbine needs