ASK THE EXPERTS: Variable Hydro

Pelton wheel hydro plant
Pelton wheel hydro plant

I read Ben Root’s “Microhydro Myths & Misconceptions” article in HP146 with great interest. I wonder if you could point me in the right direction with the following question.

Every time I look into hydropower generation, the key calculation involves flow and head. Well, head is pretty static, usually, but flow can change dramatically. That’s the case on our Washington property: We might have a 100-foot head, but the 8-inch-diameter pipe that carries the water might be 3/4 inch full on a rainy day in the winter, or have no flow in the summer. Can you direct me to microhydro plants that can deal with such flow variations?

Theo Eicher • Seattle, Washington

Hydropower is indeed all about head and flow. If you multiply head (in feet) by flow (in gallons per minute; gpm) and divide by a factor of about 12, you’ll get a rough approximation of the power (in watts) potential. This formula tells us that head and flow are equal factors in the energy output of a hydro system. If we double the head, we double the potential power. If we double the flow, we also double the potential power. We need a combination of head and flow that will give us the power and, therefore, energy we need—or at least enough to make installing the equipment worthwhile.

Both head and flow need to be measured, not guessed. And with the seasonal flow variation you describe, it’s wise to measure the flow at least several times during the year, if not monthly or more. With accurate head and flow numbers, you can make accurate power and energy calculations. And you need to remember that a year’s data is not necessarily enough to predict energy for decades, since you may be measuring in a year that happens to be very dry or very wet.

Since head is half of the hydropower equation, maximizing the head on your property—or even on neighbors’ properties, with permission—will give you the most power with whatever flow is available. I often see prospective and installed hydro systems that could be giving their owners better energy and financial return if they would simply tap the full head they have available. Of course, the longer the length of pipe, the more it will cost, and the larger the pipe diameter needs to be to avoid flow restrictions—so any additional head needs to be balanced with the extra costs involved.

A hydro turbine is ideally designed for a specific head and flow. The characteristics of the runner—diameter, bucket size and spacing, etc.—can maximize the output for a specific head and flow. If you give a runner more or less head or flow than it is designed for, you’ll get lower efficiency—less power for the same potential. That said, most turbines can function over a range of heads and flows, though not optimally across the whole range. I suggest that you seek out the turbine manufacturers’ websites and literature and look at their predictions for production of their products in heads and flows that will work for your site. And talk with them directly once you have specific measurements.

If your flow is variable, one strategy could be to have two or more smaller turbines, if you are willing to manage the system as the flow varies. Single-turbine system owners can also do this by varying the number and size of nozzles they use at a given time. Having multiple turbines and/or nozzles allows optimum use of most or all of the changing flows.

Again, with both head and flow, it’s very important to get accurate measurements in advance. Without this, your design is a guess. Mapping the head across the property will show you how much you need to spend on piping for how much vertical drop. Measuring flow in various seasons will allow you to calculate the power available in various flows, and decide whether a high but rare flow is worth designing for or not. Most often, maximizing head and designing for some middle ground in flow will be best (see “Microhydro Design Considerations” in HP132). We’re fortunate in the home-scale hydro industry to have communicative and generous turbine manufacturers who can help you sort out the best specific plan once you have good data to work with.

Ian WoofendenHome Power senior editor

Comments (2)

sluiceman's picture

Since a Ram Pump can deliver considerably more storage than a household could possibly use why not cycle the excess water through the Pelton Wheel Hydro Plant from a standing tower tank for free electricity?

Michael Welch's picture

Two considerations come to mind. First, a ram pump generally provides a slow, but steady flow of water. It would not take very long to empty a tank with a hydro generator, which uses a much greater flow of water to turn the turbine.

I think you would be much better off ratcheting down the flow of domestic water from the ram pump, and then using any additional flow in the creek to run the hydro plant directly. Cheaper and more efficient.

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