Microhydro: Measure First to Reap the Bounty

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Measuring stream flow with a bucket and a stopwatch
Measuring stream flow with a bucket and a stopwatch. If it’s not possible to measure the full stream flow, measuring part of it gives you some data to work with. It may even be enough data, since typical systems only use a portion of the stream flow.

Very few residential properties have a hydro resource to tap. But if you have it, hydro can be a cost-effective and reliable renewable electricity source with a modest investment.

Hydropower comes from a combination of vertical drop (head) and water flow. It takes both in reasonable quantities to make meaningful amounts of energy. If your stream only has a little head, you’ll need a lot of flow. If your stream only has a little flow, you’ll need a lot of head.

Head

Head is the vertical drop from the intake, where water enters the pipeline (penstock) to where the water hits the “runner”—the wheel that converts the energy in the water jet into rotational force. Head is measured in feet, meters, or pounds per square inch (psi). Vertical drop and pressure are the same thing—every 2.31 feet of head results in 1 psi.

Smart hydro designers consider all possible head on a site, and choose what is needed and most economical to tap. Not taking advantage of available head will shortchange your available energy—double the system’s head and you double your potential energy.

Flow

Flow is the amount of water that runs through the penstock and nozzle to hit the runner. It is typically measured in gallons per minute (gpm), or cubic feet per second in larger systems.

The penstock need not follow the stream, but should be as short as reasonably possible, follow a route that is secure from physical damage and frost, and preferably runs continuously downhill. Pipe sizing is crucial to good hydro system design, to minimize friction loss. Undersizing the diameter of a penstock means losing some of the pressure to friction—and that means losing some of the power potential. Friction-loss tables and calculators can help make the right pipe size and type choices. Choosing the appropriate type and pressure rating of pipe is also important.

Math

A rough formula for estimating your site’s hydro potential says that the flow (in gpm) times the head (in feet) divided by 12 (or thereabouts) will give you the potential power (in watts). For example, if your stream has a flow of 22 gpm and a head of 120 feet, it might provide about 220 watts, or about 5 kilowatt-hours per day (22 × 120 ÷ 12 × 24 hrs./day).

Once you’ve generated the electricity with a hydro turbine, you need to transmit it to where you want to use it. This may entail using low-voltage (12 to 48 V) lines if the distances are short, or using higher-voltage (120+ V) lines for longer distances. Exact system electrical details will depend on whether you are charging batteries on- or off-grid, or connecting directly to the utility grid.

Whatever you end up powering, the energy available comes back to how much head and flow you have. So measure both carefully, including seasonal flow variations. Without reliable measurements, any microhydro dreams you have may not come true. Get the numbers, see if you have the resource, and then work on making those dreams a reality.

Ian Woofenden

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