Hydro Design Considerations: Page 3 of 3


Inside this Article

Good potential hydro site
Good potential hydro site
A Complex and Expensive Intake
Complex and expensive intakes are sometimes necessary.
Self-Cleaning Intake
A simple and inexpensive, self-cleaning intake right in the stream flow does the trick.
Double-Cup Design of the Pelton Runner
The double-cup design of the Pelton runners is best for a very high head-to-flow ratio.
Turgo Runner
The design of the Turgo runner performs better with more flow than a Pelton runner, but is also a pressure-dependent design.
Four different-sized nozzles on this turbine
Four different-sized nozzles on this turbine allow 15 possible combinations of flow and power.
Four valves control flow to the four nozzles
Four valves control flow to the four nozzles, while a pressure gauge monitors system head.
Good potential hydro site
A Complex and Expensive Intake
Self-Cleaning Intake
Double-Cup Design of the Pelton Runner
Turgo Runner
Four different-sized nozzles on this turbine
Four valves control flow to the four nozzles

System Configurations

Home-scale hydro systems have four basic configuration possibilities. A stand-alone system with batteries looks very much like an off-grid PV or wind-electric system. It’s not important how the batteries get charged—hydro, wind, PV, or generator. You’ll need a controller to protect the batteries and a diversion load to accept excess energy as the batteries approach full.

A batteryless stand-alone system, or AC-direct system, is only appropriate for larger hydro plants (rated at 3 kW or more), to provide the peak power required to start motors and make sure there is enough power to run all the concurrent loads. In these systems, there is no storage, buffer, or surge capacity. To keep the turbine fully loaded all the time and keep the combined load consistent, a load-control system turns staged diversion loads on and off depending upon how heavily the home is loading the system. The fewer loads that are turned on in the home, the more dump loads will be switched on.

A battery-based grid-tie system closely resembles a stand-alone system with batteries. You’ll need the charge controller and diversion load, but these will really only come into play during grid outages. While the grid is up, the grid-tied inverter will regulate the batteries, and any surplus hydro electricity will be fed to the utility grid.

Batteryless grid-tied systems can take at least two different forms, depending on the size of the system and local connection regulations. Larger systems may use induction motors and directly tie to the grid, and they rely on controllers to divert or shut off the water flow if the grid goes down. Smaller systems will likely use a modified batteryless PV inverter, sometimes with added electronics, and also use a controller and dump load for grid outages.

Take a look at what your goals are, how close the grid is, and the scale of your system. Then build a robust system to work within those parameters. See articles on going off-grid or staying on-grid in HP128, and on basic hydro system configurations in HP117.

Choosing Nozzles

Once your system is installed, you’ll need to learn the nuances of operating it. Many systems have variable flow over the seasons and varying loads. This means that you won’t necessarily be able to run the turbine in the same way or at full output year-round. Pelton and turgo turbines offer the option of varying the number or size of nozzle to adjust the flow. If you don’t wish to take the time and effort to adjust nozzle sizes and numbers, you should find a single flow level to run the turbine so you will always have enough energy. In times of higher flow, this means you’ll be sacrificing additional available energy for the benefit of simplicity. Most streams undergo periods of flooding and times of low flow due to freezing or low precipitation. This can create a design challenge, since you want a turbine that can run at reasonable efficiency over a range of flows.

While the total head, available flow, and pipe friction affect the amount of water hitting your runner, the nozzles are the primary flow regulators. These precision-made pieces direct a jet of water at the runner’s cups through holes that can be 1/8 to 1 inch in diameter. The nozzles control the flow, and you can determine the flow through each size of nozzle from information available from your turbine manufacturer (see “Nozzle Flow Rates” table for an example).

Small turbines can have one to four nozzles, each controlled by a valve. By installing a variety of nozzle sizes on the multiple inputs, a wide variety of flow configurations are available. This minimizes nozzle changing, which can be cumbersome with some turbines and setups, and uncomfortable depending upon the season. Often, it’s useful to buy a turbine with three or four nozzles even if you will only use one or two at a time, just to be able to microadjust the flow to the runner based on the amount of water available.

Penstocks should have a pressure gauge installed just upstream of the stop valve(s) or turbine manifold, where the pipeline is divided to go into multiple nozzles. When all nozzle valves are shut, the gauge will show the static pressure. When a valve or valves are open, the gauge will show the dynamic pressure. Under normal operation, by watching the gauge, you’ll be able to record baseline pressures for various flows. Then, if you turn on too many nozzles, which causes too much water to be taken in and the pipe to empty, the decreasing dynamic pressure and loss of power will alert you that you should switch to a smaller nozzle. The dynamic pressure can also help you diagnose problems with the system: too high can mean a plugged jet, and too low can mean a clogged intake.

Hydro Design

After you’ve completed some basic research and reading (see Access), consulting with local and regional hydro dealers/installers is an excellent next step. Turbine manufacturers’ Web sites often have extensive planning information available at no charge.

Designing and operating a small hydro-electric system is not simple. A number of key decisions are involved in both design and operation. But thinking ahead can save you time and money, and help you tap your valuable resource carefully and responsibly.


Ian Woofenden can only dream of hydro electricity from his flat-island home. He plays in other people’s streams in western Washington and Central America.

Comments (3)

jzani's picture

Hi. Our school has a Lodge in the Southern Alps that is currently powered by a 5kW diesel generator. I'd love to replace this with a hydro system. The main barrier has been identified as a very dynamic mountain stream, which often rips up and buries pipes when in flood, under large boulders. A dam would fill almost immediately with rocks and sediment, but we do have ample flow and head. Would a steel screened intake be appropriate for this?

L V Hatcher's picture

I need info on a system that uses flowing water (stream) with no drop offs. Are there any out there?

Michael Welch's picture
Yes, though mostly this is a DIY kind of thing. Do an internet search for "flow of river turbine." Also search for "undershot water wheel."
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