How Much Power is Available
Flow and Head
The first step in assessing the feasibility of any hydroelectric system is to determine the amount of power that you can obtain from the stream at the site. The power available at any instant is primarily a product of the flow volume and "head." Flow volume is typically measured in cubic feet per second (cfs) or gallons per minute (gpm). Higher flow means more available power.
Head is a measure of the pressure of falling water, and is a function of the vertical distance that water drops and the characteristics of the channel, or pipe, through which it flows. Dams are constructed to impound water and create a pressure head for turbines to operate. Higher head means more available power. The higher the head the better, because less water is needed to produce a given amount of power. If less water is needed, then smaller, more efficient, and less costly turbines and piping can be used.
Hydroelectric sites are broadly categorized as low or high head sites. "Low head" typically refers to a change in elevation of less than 10 feet (3 meters). A vertical drop of less than 2 feet (0.61 meters) will probably make a hydroelectric system unfeasible. A high flow rate can compensate for low head, but a larger and more costly turbine will be necessary in an impunded situation. New kinetic turbines are coming into the marketplace which do not require an impoundment and are designed to operate in open channel flow. It may be difficult to find a turbine that will operate efficiently under very low head and low flow.
Determining Head
When determining head, you must consider both gross or "static" head, and net or "dynamic" head. Gross head is the vertical distance between the top of the penstock (the piping that conveys water, under pressure, to the turbine) and the point where the water discharges from the turbine. Net head is gross head minus the pressure or head losses due to friction and turbulence in the penstock. These head losses depend on the type, diameter, and length of the penstock piping, and the number of bends or elbows. You can use gross head to approximate power availability and determine general feasibility, but you must use net head to calculate the actual power available.
Determining Flow
Environmental and climatic factors, as well as human activities in the watershed, determine the amount and characteristics of stream flow on a daytoday and seasonal basis. A storage reservoir can control flow, but unless a dam already exists, building one can greatly increase cost and legal complications.
You may be able to obtain stream flow data from the local offices of the U.S. Geological Survey, the U.S. Army Corps of Engineers, FEMA flood maps, the U.S. Department of Agriculture, the county engineer, or local water supply or flood control authorities. If you cannot obtain existing flow data for your stream, you will need to do a site survey. Generally, unless you are considering a storage reservoir, you should use the lowest average flow of the year as the basis of the system design. Alternatively, you can use the average flow during the period of highest expected electricity demand. This may or may not coincide with lowest flows.
There may be legal restrictions on the amount of water that you can divert from a stream at certain times of the year. In such a case, you will have to use this amount of available flow as the basis of design. There are a variety of techniques for measuring stream flow. For more information on these methods, consult the references below or your local library for books that cover hydroelectric systems, surveying, or civil engineering.
You may be able to correlate your survey data with longterm precipitation data for your area, or flow data from nearby rivers, to get an estimate of longterm, seasonal low, high, and average flows for your stream. Remember that no matter what the volume of the flow is at any one time, you may be able to legally divert only a certain amount or percentage of the flow. Also, try to determine if there are any plans for development or changes in land use upstream from your site. Activities such as logging can greatly alter stream flows.
Determining Power
Once you have the flow and head figures, you can roughly estimate the potential power available, in kilowatts (kW), with the following formula:
Gross Head x Flow x System Efficiency (in decimal equivalent) x C = Power (kW)
C is a constant (the value is different in English and metric units).
Examples:
20 feet x 2 cfs x 0.55 x 0.085 = 1.9 kW or: 6 meters x 0.05 cms x 0.55 x 9.81 = 1.62 kW
50 feet x 0.8 cfs x 0.55 x 0.085 = 1.9 kW or: 15 meters x 0.02 cms x 0.55 x 9.81 = 1.62 kW
Note that in the two examples, much less flow is needed at a higher head to produce the same amount of power. Turbine and generator efficiencies depend on make and operating conditions (head and flow). Generally, low head, low speed water wheels are less efficient than high head, high speed turbines.
The overall efficiency of a system will range between 40 percent and 70 percent.
A welldesigned system will achieve an average efficiency of 55 percent.
Turbine manufacturers should be able to provide a close estimate of potential power output for their turbine, given the head and flow conditions at your site. There will also be line losses in any power lines used to transmit the electricity from the generator to the site of use.
Conversion Factors
Here are some of the conversion factors you may need to assess your site’s feasibility
1 cubic foot (cf) = 7.48 gallons
1 cubic foot per second (cfs) = 448.8 gallons per minute
1 inch = 2.54 centimeters
1 foot = .3048 meters
1 meter = 3.28 feet
1 cf = .028 cubic meters (cm)
1 cm = 35.3 cf
1 gallon = 3.785 liters
1 cf = 28.31 liters
1 cfs = 1,698.7 liters per minute
1 cubic meter per second (cm/s) = 15,842 gpm
1 pound per square inch (psi) of pressure = 2.31 feet (head) of water
1 pound (lb) = .454 kilograms (kg)
1 kg = 2.205 lbs
1 kilowatt (kW) = 1.34 horsepower (hp)
1 hp = 746 Watts
Other Considerations
Many other factors will determine whether developing the site is practical. Penstock routing and placement is important. You will need to inspect and clean the penstock intake regularly. Freezing weather, livestock, and vandals can damage exposed piping, but burying it may not be practical or costeffective. The piping must have adequate support to keep it from breaking apart or moving under the weight and pressure of the water. The turbine/generator should be above the stream’s flood stage. A power line from the generator could be expensive.
