Determining a Potential Microhydropower Site's Head
In a potential microhydropower site, head is the vertical distance that water falls. When evaluating a potential site, head is usually measured in feet, meters, or units of pressure. Head also is a function of the characteristics of the channel or pipe through which it flows.
You can use your site's head calculation along with its flow calculation to determine the site's potential power output.
Most microhydropower sites are categorized as low or high head. The higher the head the better because you'll need less water to produce a given amount of power and you can use smaller, less expensive equipment. Low head refers to a change in elevation of less than 10 feet (3 meters). A vertical drop of less than 2 feet (0.6 meters) will probably make a small-scale hydroelectric system unfeasible.
However, for extremely small power generation amounts, a flowing stream with as little as 13 inches of water can support a submersible turbine. This type of turbine was originally used to power scientific instruments towed behind oil exploration ships.
When determining head, you need to consider both gross head and net head. Gross head is the vertical distance between the top of the penstock that conveys the water under pressure and the point where the water discharges from the turbine. Net head equals gross head minus losses due to friction and turbulence in the piping.
The most accurate way to determine gross head is to have a professional survey the site. To get a rough estimate, you can use U.S. Geological Survey maps of your area or the hose-tube method.
Hose-Tube Method
The hose-tube method involves taking stream-depth measurements across the width of the stream you intend to use for your system—from the point at which you want to place the penstock to the point at which you want to place the turbine. You will need the following:
- An assistant
- A 20–30 foot (6–9 meters) length of small-diameter garden hose or other flexible tubing
- A funnel
- A yardstick or measuring tape.
Instructions:
- Stretch the hose or tubing down the stream channel from the point that is the most practical elevation for the penstock intake.
- Have your assistant hold the upstream end of the hose, with the funnel in it, underwater as near the surface as possible.
- Meanwhile, lift the downstream end until water stops flowing from it.
- Measure the vertical distance between your end of the tube and the surface of the water. This is the gross head for that section of stream.
- Have your assistant move to where you are and place the funnel at the same point where you took your measurement.
- Then walk downstream and repeat the procedure. Continue taking measurements until you reach the point where you plan to site the turbine.
The sum of these measurements will give you a rough approximation of the gross head for your site.
Note: due to the water's force into the upstream end of the hose, water may continue to move through the hose after both ends of the hose are actually level. You may wish to subtract an inch or two (2–5 centimeters) from each measurement to account for this. It is best to be conservative in these preliminary head measurements.
If your preliminary estimates look favorable, you will want to acquire more accurate measurements. As stated already, the most accurate way to determine head is to have a professional survey your site. But if you know you have an elevation drop on your site of several hundred feet, you can use an aircraft altimeter. You may be able to buy, borrow, or rent an altimeter from a small airport or flying club. A word of caution, however: while using an altimeter might be less expensive than hiring a professional surveyor, your measurement will be less accurate. In addition, you will have to account for the effects of barometric pressure and calibrate the altimeter as necessary.