Hydro Design Considerations

Beginner

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

If you are blessed with water flowing downhill on your property, you are fortunate indeed. Small-scale hydro-electricity can be the most cost-effective and reliable form of renewable energy (RE) for your home. But tapping this resource responsibly requires careful planning and implementation.

Assessing Head & Flow

Hydropower is the result of two basic characteristics inherent in the stream you tap. The first is the vertical drop, commonly called “head.” This correlates directly with the pressure available, since every 2.31 feet of vertical drop equals 1 psi. The other factor is flow, and specifically, the amount of the stream’s flow that you are comfortable with or allowed to take. For home-scale systems, this is typically measured in gallons per minute (gpm). For background on hydro system basics and measurements, see articles in HP103, 104, 105, and 117.

When measuring the head and flow at your hydro site, it’s a good idea to plot your results on a map of your property. Perhaps you have a site with 220 feet of total head over a projected pipe run of 1,200 feet. The cost of 1,200 feet of pipe is high, and it may not be cost-effective to tap all of the head. Mapping the head in segments will give you the information needed to make the best decision about where your intake and hydro plant should be.

Perhaps you gain 150 feet of head in 400 feet of run, but gaining the other 70 feet of head requires 800 more feet of pipe. In this case, you might decide not to tap all of your head. In a more extreme case, you might have a 50-foot waterfall where you gain 50 feet of head with 60 feet of pipe. You might have another 20 feet of head on your property, requiring several hundred feet of pipe to tap. Often the decision is clear—tap the head that is cheapest or easiest. In other cases, the head may be gradual; with low-flow situations especially, you may be pushed toward tapping every foot of head available, from property line to property line. 

Flow can vary across a property, too. Tributaries join streams as they move downhill. Water can be lost to seepage, and whole streams can go underground for portions of their course. It’s important to take flow measurements at the top of each segment you’re mapping, so you can calculate power (watts) and energy (kilowatt-hours) for each scenario. Factors you should consider are:

  • Head gained per 100 feet of pipe
  • Flow
  • Pipe cost
  • Energy needed
  • Location of loads relative to the turbine

Choice of Intake & Location

In a hydro system, the “intake” is where water is diverted from the stream. It consists of some sort of screen to remove debris, and also helps remove suspended air bubbles from the water. The preferred intake options are simple, durable, self-cleaning, and safe for stream life.

Too often, microhydro intakes are scaled-down versions of utility-scale hydro intakes, with a dam, a “stilling pond” or reservoir, perhaps a trash rack, a diversion channel, and then a final screening. These civil works are very expensive, intrusive, and often unnecessary. When possible, find a spot in the stream to incorporate a modest self-cleaning screen that will only minimally disturb the natural flow of the watercourse. For instance, 1 square foot of the Hydro-Shear screen, which can accommodate flows up to 200 gpm according to the manufacturer, is adequate for most home-scale hydro systems. Often, existing rocks can be incorporated into the structure—I’ve seen intakes that only use a few cubic feet of concrete to cement in a screened intake at a narrow, natural drop in the stream.

Look for a place where the stream drops naturally and you can slip a screen underneath the falling water. Ideally, the stream will be narrow and stable at this point, so it will be unlikely to change course with winter floods. If there is some nonturbulent water upstream from the intake, all the better—this will allow the sediment to settle. One practical consideration for installation is selecting a spot where stream water can be temporarily diverted while you construct the intake.

In addition to the screened intake itself, you need to consider the penstock (pipeline) and, especially, how to get it out of the streambed. Though a tough screen cemented in at an appropriate angle can withstand heavy flooding and battering from logs and boulders, a penstock exiting the screen box may be more vulnerable. Have it exit the stream and stream bed as quickly as possible, being sure to keep it lower than the level of the intake. If the pipe is not kept below the level of the intake, the flow will be impeded and a siphon will need to be created. Protect the pipe with rock and concrete where it leaves the intake and as far down as it may be subjected to damage. For more information on hydro intakes, see the article in HP124 and the letter in HP125.

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