Hydropower generates 6.7 percent of the nation's electricity. The United States
is the one of the largest producers of hydropower in the world, second only
to Canada. In the Pacific Northwest, up to 70 percent of electricity is
generated from hydropower. Of the 75,187 existing significant dams in the United
States however, less than 3 percent are used for hydroelectric generation. The
U.S. Energy Information Administration (EIA) estimates that hydropower will provide 5.7 percent of the nation's electricity in 2040.
Total U.S. hydroelectric capacity is 98.5 gigawatts (GW) including pumped
storage projects. Non-federal, licensed conventional hydroelectric capacity
(excluding pumped storage) equals 39.6 GW at 1,617 sites in the United States.
The federal government owns another 38.5 GW at 173 sites (excluding pumped
Hydropower produces electricity by running water from a reservoir through a
hydraulic turbine that spins and drives the generator shaft. A hydroelectric
plant produces between 100 kilowatts (kW) to 500 megawatts (MW) of electricity,
depending on size. Though the emissions produced from hydropower plants are
negligible, dam construction to create water reservoirs carries other
While hydropower turbine manufacturers have incrementally improved turbine
technology to improve efficiencies, the basic design concepts have not changed
The type of hydropower turbine selected for a project is based on the height
of standing water and the flow, or volume of water, at the site. Other deciding
factors include how deep the turbine must be set, efficiency, and cost.
- Impulse Turbine
The impulse turbine generally uses the velocity of the water to move the
runner and discharges to atmospheric pressure. The water stream hits each bucket
on the runner. There is no suction on the down side of the turbine, and the
water flows out the bottom of the turbine housing after hitting the runner. An
impulse turbine is generally suitable for high head, low flow applications.
- Pelton Hydropower Turbine
A pelton wheel has one or more free jets discharging water into an aerated
space and impinging on the buckets of a runner. Draft tubes are not required for
impulse turbine since the runner must be located above the maximum tailwater to
permit operation at atmospheric pressure.
- Cross-Flow Turbine
A cross-flow turbine is drum-shaped and uses an elongated,
rectangular-section nozzle directed against curved vanes on a cylindrically
shaped runner. The cross-flow turbine allows the water to flow through the
blades twice. The first pass is when the water flows from the outside of the
blades to the inside; the second pass is from the inside back out. A guide vane
at the entrance to the turbine directs the flow to a limited portion of the
runner. The cross-flow was developed to accommodate larger water flows and lower
heads than the Pelton.
- Reaction Turbine
A reaction turbine develops power from the combined action of pressure and
moving water. The runner is placed directly in the water stream flowing over the
blades rather than striking each individually. Reaction turbines are generally
used for sites with lower head and higher flows than compared with the impulse
- Propeller Turbine
A propeller turbine generally has a runner with three to six blades in which
the water contacts all of the blades constantly. Through the pipe, the pressure
is constant. The pitch of the blades may be fixed or adjustable. The major
components besides the runner are a scroll case, wicket gates, and a draft tube.
There are several different types of propeller turbines.
- Bulb turbine: The turbine and generator are a sealed unit
placed directly in the water stream. The generator is attached directly to the
perimeter of the turbine.
- Tube turbine: The penstock bends just before or after the
runner, allowing a straight line connection to the generator.
- Kaplan: Both the blades and the wicket gates are
adjustable, allowing for a wider range of operation.
- Francis: A Francis turbine has a runner with fixed buckets
(vanes), usually nine or more. Water is introduced just above the runner and all
around it and then falls through, causing it to spin. Besides the runner, the
other major components are the scroll case, wicket gates, and draft
- Kinetic Energy Turbines
Also called free-flow turbines, the kinetic energy turbines generate
electricity from the kinetic energy present in flowing water rather than the
potential energy from the head. The systems may operate in rivers, man-made
channels, tidal waters, or ocean currents. Kinetic systems utilize the water
stream's natural pathway. They do not require the diversion of water through
manmade channels, riverbeds, or pipes, although they might have applications in
such conduits. Kinetic systems do not require large civil works; however, they
can use existing structures such as bridges, tailraces, and channels.
During the 1980s, the environmental effects of hydropower became a
significant issue impacting hydropower projects. Many hydro projects
constructed decades ago blocked fish passage and affected wildlife habitats.
Research and development is currently underway to help fishery biologists and turbine designers
better address these concerns while improving hydropower technology.
In the mid-1990s, the U.S. Department of Energy (DOE) began research into
advanced hydropower technology. The goal was to develop systems that generate
more electricity with less environmental impact. DOE funded the conceptual
designs of four turbine types: a redesigned Kaplan and Francis turbine, a
dissolved-oxygen-enhancing turbine, and a new turbine type that borrows
technology from the food processing industry.
Many of the turbine manufacturers have begun designing environmentally
friendly turbines. The hydropower industry and DOE have joined together in
developing the Advanced Hydropower Turbine System (AHTS) program, which could
improve the survival rates of fish while also improving the efficiency of