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Part VIII: Wind, Water, and BiofuelsChapter 12. Small Wind and Micro-Hydro

Micro-Hydro Basics: Head, Flow, and Power

The two measurements that set the power of a stream, and a first estimate you can do with a bucket.

Difficulty
Intermediate
Cost
Planning only
published

A stream makes power from two facts: how much water, and how far that water falls. Everything else is a machine for turning those two facts into electricity, and a permit for taking the water at all. This page estimates the watts. It does not size a pipe, build a dam, or grant a water right.

Flow is volume per time. Head is the vertical drop from the water surface at the intake to the turbine, along a route a pipe could actually follow. A pretty waterfall you cannot reach with a pipe is not head. Friction in a real pipe spends some of the head before the turbine sees it, so the sketch below is the optimistic number.

Other people's hydro projects are in the links at the bottom. Read them there.

Difficulty

Level
Intermediate
Time
A dry-season afternoon with a bucket, and a second visit in a wet month
Skills
Measuring a drop and a flow without calling either one a permit

Cost estimate

Planning only

A bucket, a watch, and a way to estimate elevation are enough for the first number. A machine, a penstock, and the legal right to divert water are a different project and are not priced here.

Materials

ItemQtyNotes
A bucket of known gallons and a watch1For a small flow you can catch. A river you cannot bucket needs a different method and usually a hydrologist.
A way to estimate vertical drop1A survey level is better than a phone. The phone is a start if you treat it as rough.
The name of your state water agency1Before any diversion. Fish, other users, and the dry month are their subject.

The estimate

Gallons divided by the seconds it took to fill them, times 60, is gallons per minute. Do that in the season you would need the power, and again when the stream is low. One June afternoon after rain is how people imagine a plant the creek cannot support in August.

Watts in the sketch are efficiency times flow in gallons per minute times head in feet times 0.1886. Half is a planning efficiency for a small turbine, not a rating. Many real small machines do worse once you include the pipe, the drive, and the generator. Use the sketch to learn whether you are holding a light bulb or a house. Then stop and talk to the water agency and someone who designs this equipment.

Head is the drop, flow is the waterA stream at the upper left, a pipe descending to a turbine, with head marked as the vertical gap.IntakeTurbineHeadMeasure the drop you can pipe, and the flow in the dry month.
Head is the vertical drop you can put a pipe on. Flow is the water in that pipe, including the dry month.

Watts from flow and head

Electrical watts are efficiency times gallons per minute times head in feet times 0.1886. That factor is water, gravity, and the unit change from gallons and feet. The 50 gallon, 20 foot, 50 percent starters are a small stream, not a site. Head here is the vertical drop you can actually put a pipe on, before friction.

94 W

About 2.3 kWh if this flow and head held for 24 hours. A creek usually does not. A small turbine is often well under the efficiency you might hope for. This is not a pipe size and not a water right.

A result of a few dozen watts can still water a battery for a light, if the flow is truly year-round and the battery page is where you put it. It will not run a shop. A result that looks like a house is a reason to hire a designer, not a reason to bury a homemade penstock. High head is stored energy. A plastic pipe that is not rated for that pressure can burst, and the joints can walk out of the ground. Material, thrust blocks, and screens are a pressure-pipe design. This page does not give diameters.

Water rights and the stream below you

Diverting a stream changes habitat downstream. Leaving a share of the flow in the channel is part of a lawful project, not a courtesy. Fish screens, timing, and the right to use the water are state rules, and they differ. Some water is already spoken for by a downstream user. "It crosses my land" is not the end of the question. Damming is a larger version of the same question and is not a weekend masonry project.

Floods move rocks and plug intakes. The plant you designed for a gentle flow has to survive the flow that fills the banks, or it has to be out of that water. That is another reason the sketch is not a construction set.

Composite: the June bucket

An invented creek, not a named river. Someone times a bucket in a wet week and gets a cheerful flow, then multiplies by the height of a falls they can see but not pipe. The August visit, which they skip, would have shown a trickle. The falls height was never available as head. The composite plant exists only on the notepad. Take both measurements in the poor month, on the route a pipe could follow. Round on purpose. The further-reading links are where real sites are described, by the people who built them.

  1. Ask the water agency before you design a diversion

    Rights, fish, and minimum flow. A no here ends the project cleanly.

  2. Measure flow more than once

    Bucket a small stream. Gallons over seconds, times 60. Include the dry month.

  3. Measure head on a real route

    Vertical drop from intake surface to turbine, on a path you can lay a pipe. Subtract the hope.

  4. Run the sketch, then stop

    Efficiency near one half is a planning stand-in. A house-sized number means hire a designer. A pipe size is not on this page.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.