How Much Power Is Available From a Stream?
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. 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. It may be difficult to find a turbine that will operate efficiently under very low head and low flow.
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.
Environmental and climatic factors, as well as human activities in the watershed, determine the amount and characteristics of stream flow on a day-to-day 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, U.S. Army Corps of Engineers, U.S. Department of Agriculture, 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 long-term precipitation data for your area, or flow data from nearby rivers, to get an estimate of long-term, 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 any plans for development or changes in land use upstream from your site. Activities such as logging can greatly alter stream flows.
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).
- 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 to 70 percent. A well-designed 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.
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 (gpm)
1 inch = 2.54 centimeters
1 foot = .3048 meters
1 meter = 3.28 feet
1 cf = .028 cubic meters (m3)
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 (m3/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
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 cost-effective. 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.
Regulation of Hydropower Projects
Access to water and the use, control and diversion of water flows is subject to federal and state regulation. Other regulations apply to any physical alteration of a stream channel or bank that may affect water quality or wildlife habitat. This is true regardless of whether or not the stream is on private property. If your project will have minimal physical impact, and you are not planning to sell power to a utility, there is a good chance that the legal process will not be too complex.
There are many local, state, and federal regulations that govern, or will affect, the construction and operation of a hydroelectric power plant. The larger the system, the more complicated, drawn out, and expensive the permitting and approval process will be. Penalties for not having the permits or necessary approvals can be severe. You will not escape the consequences by pleading ignorance. Although the legal process may seem burdensome, the intention of the laws is to protect all users of the resource, including the plant, fish, and animal communities that use the water.
When planning a hydroelectric system, your first point of contact should be the county engineer. He or she will be the most informed about what restrictions govern the development and/or control of water resources in your area.
The two primary federal agencies that you will need to deal with are the Federal Energy Regulatory Commission and the U.S. Army Corps of Engineers.
FERC is responsible for licensing all non-federal government hydroelectric projects under its jurisdiction. A hydroelectric project is within the jurisdiction of FERC if any of the following conditions apply:
- the project is on a navigable waterway;
- the project will affect interstate commerce (i.e., if the system is to be connected to a regional electric transmission grid);
- the project uses federal land; or
- the project will use surplus water or waterpower from a federal dam.
You will need to consult with FERC in order to determine whether or not your project falls under its jurisdiction. If it does, then you will need to apply for a license or exemption from FERC. The FERC application process will require contacting and consulting other federal, state and local government agencies, and providing evidence that you have done so.
You will also need to determine whether, and to what extent, you can divert water from the stream channel, and what restrictions apply to construction and operation of the system.
In Oregon, the Water Resources Department regulates water rights. You may need a separate water right to produce power, even if you already have a water right for any other use.
You should also consult with the Oregon Department of Fish and Wildlife in determining the ultimate design and operation of your system.
The Oregon Division of State Lands regulates the permits needed for removal or fill of wetland areas.
Other federal government agencies that may require permits include: U.S. Fish and Wildlife Service; Federal Aviation Administration (if a power line will be constructed near an airport); and U.S. Forest Service or Bureau of Land Management (if the project will use land administered by these agencies).
Only a few companies make micro-hydroelectric turbines. Most available turbines are high-head turbines. Low-head, low-flow turbines may be difficult to find and may have to be custom made. You may be able to find and refurbish old but operable turbines at abandoned hydro/mill sites. Commercially available turbines and generators are usually sold as a package. Do-it-yourself systems require careful matching of a generator with the turbine horsepower and speed.
Western manufacturers of in-pipe, micro and small-scale hydropower systems include:
- Scott Power – Washington-based micro-hydropower cross-flow turbine
- Alternative Power and Machine (APM Hydro) — Grants Pass-based micro-hydropower system for battery charging
- Canyon Hydro — Washington-based custom designed small hydropower systems
- Harris Hydroelectric — Northern California-based micro-hydropower battery-charging system
- Hydrovolts — Washington-based small hydropower, canal or industrial waterfall applications
- Lucid Energy — Portland-based in-pipe, lift-based vertical axis spherical turbine
- Natel Energy — California-based low-head hydropower system
The Oregon Department of Energy provides a list for your convenience, and does not make any warranty, expressed or implied, concerning these systems or the work performed by anyone installing these systems.
Oregon also has high environmental standards that require very low impacts in your design. Consider these innovative systems:
- Farmers Conservation Alliance — Farmers Conservation Alliance sells the Farmers Screen, a horizontal fish screen technology that protects fish, manages debris and provides reliable water for diverters.
- “Fish-friendly” turbine development — A design that reduces mortality or strike of fish in larger hydropower installations. Instead of 5 to 18 fast-spinning blades, separated by gaps that can trap fish, the new design (Alden) has three blades, no gaps, is bigger and rotates more slowly.
There are two ways to use the power generated from your system: for your own electricity load on-site under a net metering agreement or for sale to a utility.
Oregon's net-metering law allows renewable energy systems to be installed behind your power meter and its generation deducted from your monthly power use. All utilities must accept systems that are less than 25 kilowatts. All customer classes (residential, commercial, industrial) are eligible. If you generate more than you use, the excess generation may roll over to the next month or you may be paid a bill credit at avoided cost. Contact your utility to learn more about net metering.
The Public Utility Regulatory Policies Act of 1978 requires electric utilities to purchase power from independent power producers and pay for electricity at “avoided cost” rates. You will need to contact your local utility and/or public utility commission to determine what these technical and operating requirements are and the price that the utility will pay you for the electricity you generate. The utility will require that you insure the system. However, the interconnect requirements and insurance premiums may cost more than what you earn from selling the power.
The following list is provided to assist you with your project. We provide the list for your convenience. The Oregon Department of Energy does not make any warranty, expressed or implied, concerning these hydropower advisors.
Those interested being listed as a hydro advisor should contact Rebecca O’Neil at the Oregon Department of Energy 503-373-2295.
Public sector contacts:
Oregon State University
Private sector contacts:
Kenneth D. Hostler
Hydro 911 - Hydro Consulting & Maintenance Services
Mead & Hunt
Jim Fuller P.E.
Fuller Morris Engineering
Cornell Pump Company
References and Publications
Additional references related to micro-hydropower are available upon request.
Case Study: Juniper Ridge and Irrigation Hydropower
Irrigation districts are leading the way on building and operating low-impact hydropower developments, which can help a district offset system maintenance expenses and create revenue with a limited environmental footprint. One leader is the Central Oregon Irrigation District, which owns two successful low-impact hydroelectric developments located outside of Bend. The newest project, Juniper Ridge, in operation since late 2010, produces slightly more than three megawatts of electricity, with a capacity of 5 megawatts. Read the Juniper Ridge case study
Does Small Hydropower Need Direct Development Assistance?
In February 2012, the Oregon Department of Energy sought comments on whether the state of Oregon should consider offering direct development assistance to hydropower developers. Specifically, the department asked whether a Colorado state program model would work well for Oregon’s stakeholders, governments and resources.
The department received comments from the National Hydropower Association, Hydropower Reform Coalition, Northwest Hydroelectric Association, Oregon Water Resources Congress and the Energy Trust of Oregon.
The comments told the department that while direct assistance would be valuable, there are many challenges facing developers and many considerations in selecting the right projects to assist. The department will consider these comments carefully in its policy initiatives. Should funding opportunities arise the department will consult with stakeholders again on how to proceed.