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The Generation of Renewable Electricity Project: Micro-Hydro

On July 17th, Al Gore issued a bold challenge to our nation to commit to producing 100% of our electricity from cheap, clean renewable energy sources within 10 years. It may sound like a daunting task, but the good news is that we know how to do it!

Since 1984 Chelsea Green has been publishing books that detail many various methods for transitioning our homes, cars, and lives off oil and coal. To honor Gore’s Challenge, we’re launching “The Generation of Renewable Electricity Project”—or The G.O.R.E. Project, for short. This series will consist of tutorials from some of our books explaining how to begin generating your own renewable electricity.

The first tutorial in this series explains a simple micro-hydro generation system that can be put to use on any property with a river, stream, or brook—with minimal disruption of the ecosystem.

The following is from Mat Stein’s When Technology Fails: A Manual for Self-Reliance, Sustainability, and Surviving the Long Emergency.

Micro-Hydropower Overview

If your property contains a creek with a significant drop to it, micro-hydropower could provide you with a terrific source of electricity for an investment far less than an equivalent-sized PV system. The vertical distance that your water supply drops from the source to the turbine is known as the pressure “head.” Available water power is a function of the volume flow rate (how much water is flowing) multiplied by head feet (head relates directly to available water pressure). Micro-hydro turbines can operate with as little as 2 feet of head if you have a high flow rate, but are usually operated with heads of 25 feet or more. High water flow rate can compensate for low head, but head tends to be the more important factor.

(This diagram shows a small dam at top left. However, damming is not necessary and not recommended.)

Micro-hydro, like most PV systems, usually generates electricity at modest rates that are far below most residential peak demands, but because it generates electricity day and night, it adds up to quite a bit of energy every day. This energy is accumulated in batteries, where it is available to drive high loads when required.

Micro-Hydro Considerations

• Head. What is the vertical drop (head) from your water source to your micro-hydro generator? This is the most important factor in determining whether micro-hydro is feasible and how much power you will be able to extract. If you have a fast-flowing creek with less than 2 feet of head, you will not be able to use a micro-hydro turbine, but you may get a worthwhile amount of energy from your creek using a propeller-type turbine from Jack Rabbit Energy Systems.

• Flow rate. How many gallons per minute can you provide to your micro-hydro turbine? This factor combined with head determines your potential available power. For small flows, time the flow of your supply into a 5-gallon bucket to estimate flow rate. For larger flows, consult civil and mechanical engineering books or Microhydro: Clean Power from Water, which will show you how to build a simple “weir” to accurately estimate stream flow rates.

• Available power. A good rule of thumb for determining available power (in watts) is to multiply the flow rate (in gpm, gallons per minute) times the head (in feet), and then divide by 10. The maximum theoretical power is actually divided by only 5.3, but a factor of 10 accounts for efficiency losses in the turbine, the alternator, and some in the piping. For example, a head of 25 feet with a flow rate of 100 gpm would provide you with somewhere around 250 watts of continuous power (100 × 25/10). In general, a value of 1,000 or more for the flow rate (gpm) multiplied by the head (feet) is the point where micro-hydro turbines become worthwhile.

• Pipe head loss. The flow rate, length, size, and type of pipe will determine how much pressure (and power) are lost due to the friction of water flowing through your pipe. Friction through a pipe decreases with a cube of the diameter, so a pipe that is twice as big has one-eighth the friction loss. Fire hoses are big fat hoses because you can’t squirt a lot of water through a small hose. The same is true for your microhydro supply piping. Water pipe sizing charts will help you to size your piping by estimating how many feet of pressure head are lost flowing through the length of pipe. Losses are typically given in feet of head per 100 feet of pipe length, so you must factor in the length of your pipeline. For high flow/low head systems, a friction loss of more than 1 or 2 feet of head may be unacceptable.

• Distance to batteries. Squeezing electrical current through wires is a lot like squeezing water through pipes. Long distances between a power source and your battery bank can result in excessive power losses. Thicker wires and/or higher micro-hydro output voltages (24, 48, or more volts) can help reduce power transmission losses between generator and batteries. To keep wire thickness and wiring costs reasonable, most battery systems are currently designed to supply power to inverters at 48 volts, rather than the old standard of 12 volts. (See the “Energy, Power, and Electricity Primer” section at the end of this chapter for more information.)

• Custom systems. Micro-hydro alternators and water jets are custom matched for each application. See your RE dealer for assistance in evaluating your site and choosing among different micro-hydro options.

More Micro-Hydro Resources:

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