Hydroelectric Power

Introduction

   As introduction, a quote from Life Magazine's Book on Water:

“If man had been content to leave water where Nature put it, his civilization might have stopped growing long ago. Instead, he has stored water in reservoirs, converted its energy into electricity, channeled it into canals, moved it great distances to irrigate farms, employed it in industrial processes, and pushed it back with dikes to gain more land. The process of controlling the earth's water has driven man to undertake the largest of all his engineering projects. The scope of water control engineering is constantly expanding to meet the needs of growing population. Every month another major dam is completed somewhere in the world.

“Dams are designed to serve a number of purposes. They back up reservoirs for irrigation, store municipal water, produce electric power, improve navigation and protect against floods. Most dams hold back water by their sheer weight. Known as gravity dams they can be made of earth, rock or concrete. Another type of dam used especially in narrow gorges employs the structural strength of an arch in addition to weight to hold back rivers.

Energy from Water

   Hydropower comes from gravity and solar power. Water evaporates from rivers, lakes, and oceans, and forms clouds that float to higher elevations. Rain and snow fall from the clouds and flow downhill. Falling water is caught and used to power turbines. Rotating turbines create electricity.

   Water stored at rest above a certain level possesses potential energy and power. Next time you are sitting in a boat on a reservoir fishing (and the fish aren't biting) think about the potential energy and power you are sitting on. The water backed up behind a dam possesses energy called potential energy. Although lying quietly behind the dam, it is capable of producing motion when released.

   When the stored water is released, it starts to move. Moving water possesses energy, called kinetic energy, because of its motion. Potential energy and kinetic energy are interchangeable; as water is pumped uphill it gains potential energy through the energy supplied by the pump; as the water runs downhill the potential energy is transformed into kinetic energy. But some of the energy is lost due to friction, which changes the energy of motion to heat, yet another form of energy.

   The kinetic energy of water can be converted into electrical energy through the use of suitable machinery. Although one form of energy can be changed to another form in countless ways, it has been shown that through all such changes, the amount of energy remains unchanged. The energy is changed from one form to another, but neither increases nor decreases. This is one of the basic principles of science known as the “Law of Conservation of Energy.”

Balancing our Energy Resources

   Hydropower provides about ten percent of United States electricity. Hydropower provides the energy equivalent of burning nearly 1.8 billion barrels of oil annually. Each year, hydropower helps avoid millions of tons of sulfur dioxide, nitrogen oxide, and carbon dioxide emissions.

   About 80 percent of "renewable" electricity comes from hydropower. Burning wood and wastes accounts for 15 percent. Geothermal, wind, and solar power generate the remaining five percent.

Hydropower helps prevent blackouts and system failures because, like a faucet, it can be turned on instantly to shore up temporary power shortages.

Is Hydropower Renewable?

   There is a lively debate about whether hydropower should "count" as a renewable resource. Some argue that hydropower has serious environmental impacts on rivers and isn't "green" enough to be considered renewable. Others argue that hydropower is "green" because it prevents air pollution. The Energy Information Administration of the Dpartment of Energy takes a different approach, defining a renewable resource as follows; "An energy source that is regenerative or virtually inexhaustible. Typical examples are wind, geothermal, and water power."

What Hydropower Looks Like

    There are three kinds of hydropower projects: storage, run-of-the-river, and pumped storage. A lake held up by a dam is what most people think of when they think of hydropower. Lakes catch water from available stream flow as it flows downhill. The water is released as needed to power turbines, prevent floods, or meet downstream irrigation, environmental, in-stream flow, and municipal water requirements. Most SLCA/IP hydroelectric powerplants fall into this category.

   Run-of-the-river projects generate electicity from river water as it flows through the facility. They use a canal or big pipe to channel water to the turbines, sometimes without using a dam. Run-of-the-river projects typically have little if any water storage capability. Some smaller SLCA/IP hydroelectric powerplants, such as Towaoc Canal, and Upper and Lower Molina, fall into this category.

   Pumped storage projects normally cycle river water back and forth, typically between an upper and lower reservoir. When electricity demand is high, they release water from the upper reservoir into turbines. When demand is low, they pump the water back up using cheaper off-peak electricity. Water running through pumped storage systems often cycles through closed loops, sometimes underground. The systems make few continuing demands on rivers. There are no SLCA/IP pumped storage power plants.

Hydroelectric Turbines

   Dams use water, or hydraulic, turbines to generate electricity. Water strikes a series of blades or buckets that rotates a shaft. This, in turn, drives the rotor of the generator, creating electricity. The three most common hydraulic turbines are the Pelton Wheel, the Francis Turbine, and the Kaplan Turbine.

   The Pelton impulse wheel works best when there is a head 50 feet or more and water flows are low. When water is forced through a nozzle onto the wheel's cupped buckets, anywhere from 80 to 95 percent of the original energy is preserved. This efficiency allows the turbine to rapidly generate electricity without the need for expensive gearing.

   The Francis Turbine, developed by James Francis, is more versatile. It can operate at a variety of heads, some as low as 4 feet. Water is directed onto the side of the turbine, through its blades and out the bottom. This full-flow configuration has made it the prime turbine used in powerplants. Drawbacks include high parts costs and damage from pitting and grit.

   The Kaplan Turbine was designed by Viktor Kaplan in 1913. It operates much like a boat propeller. Broad, swiveling blades on the turbine are spun by high-pressure water as it is released through a sluice, driving the axle of a generator.

The Pelton Wheel

   The Pelton Wheel was the brainchild of Lester A. Pelton. Born in 1831, he built the first Pelton Wheel in Comptonville, California, in 1878. After patenting his invention in 1880, he perfected the design at a foundary near Nevada City, California. In 1888, he founded the Pelton Wheel Water Company in San Francisco. Beginning in 1899, improvements were made to the wheel's nozzle and bucket designs to improve efficiency.

   The secret of the Pelton Wheel is not how much or how fast water hits the wheel's buckets. It's where the water hits. Water that hits a water wheel bucket straight on splashes out - resulting in lost energy. On the other hand, if the bucket moves and the water hits at an angle, there is greater force. But only to a point - if the stream of water overtakes the bucket, the speed of rotation decreases.

   That means the wheel not only more efficiently, it's rate of turning (and generating energy) can be varied - great for ramping up and ramping down.

Hydroelectric Head

   Water stored behind a dam that raises it above the point where it will seek normal stream bed level is said to have head. When referring to a hydroelectric powerplant one thinks of a dam, a lake or reservoir and flowing water at a certain head, used to produce the mechanical force which can be converted into electricity.

   The Bureau of Reclamation's Power Test Code for Hydraulic Prime Movers states:

“The total or gross head acting on the plant shall be taken as the difference in elevation between the equivalent still water surface before the water has passed through the trash racks, and the equivalent still water surface in the tailrace after discharge from the draft tube.”

   In less technical terms, head is the vertical distance in elevation between the still water in the reservoir, sometimes called the forebay, and the still water in the tailrace below the powerplant, sometimes called the afterbay. In powerplant operator language the elevation of the water in the reservoir is usually called the elevation of the “head water,” and the elevation of the water in the afterbay just below the discharge of the turbines and the powerplant proper is called the elevation of the “tail water.”

   Reclamation's Power Test Code also states:

“Total or gross operating head on the plant is different from the net or effective head on the turbines. Total operating head on the plant is the elevation difference of the forebay and tailrace water levels, both corrected for velocity heads. In computing the efficiency and other factors of turbine performance, the net or effective head shall be used.”

    In other words, the effective or net head is the gross head corrected for the head loss due to the water flowing through trashracks, losses due to velocity, losses due to the water flowing through the valves and penstocks, and the height of the tailwater.

   In computing turbine efficiency and other performance factors, the net or effective head is used. In some Bureau of Reclamation dams, especially earth or multiple-arch dams, the penstocks may be long and the head loss is fairly high. Other Reclamation dams have short penstocks with low head losses. Some dams built to catch and store water have long penstocks that transmit water from one elevation to another with a powerplant at the lower end.

Head of Other Hydroelectric Turbines

   So far we have discussed head as related to reaction-type turbines. Reclamation also uses impulse-type turbines known as the Pelton-type turbine. When impulse-type turbines are used, gross head is usually determined as the elevation difference of still water in the forebay and the water jets acting on the Pelton turbine buckets, rather than the still tailwater elevation. Impulse turbines have no draft tubes as such, so the drop from the center of the water jets to the tailwater surface is not used to generate power.

   Quoting Reclamation's Power Test Code again in reference to impulse turbines:

“In tests of impulse turbines, the effective head is the elevation corresponding to the pressure head in the turbine inlet immediately upstream from the nozzle, or immediately upstream from branch diverging to two or more nozzles, plus the velocity head at this point, minus the lowest elevation of the runner bucket pitch circle. The pitch circle is that circle which is tangent to the axis of the power jet. If the turbine is supplied by more than one inlet pipe, the effective head for the turbine is the average of the effective heads."

   The effective head in feet from the still water elevation in the forebay to the turbine inlet is calculated by measuring the water pressure in pounds per square inch (lb/in²) at the turbine inlet, and multiplying the reading obtained by 2.308. Likewise, head in feet may be converted to pounds per square inch by multiplying it by 0.43327.

   All that has been said above may be simply stated by saying the net head or effective head on a hydroturbine is the gross head less all hydraulic losses except those chargeable to the turbine. Some of the nonturbine losses are due to flow through trashracks, flow through the penstock intake, flow through the penstock itself, and flow through any penstock valves as well as friction and velocity losses.

   Net head varies with the load on the unit because friction and velocity losses increase approximately as the square of the water discharge from the unit. Low-head hydroelectric powerplants, with very short water passages, have a net head almost equal to gross head. However, medium-head or high-head hydroplants with long penstocks or tunnels may have a net head 90 percent or less of the gross head.

This web document contains excerpts from a Bureau of Reclamation training booklet "Hydroelectric Power", dated December 1975, as well as from an article in Western's "Closed Circuit" employee newsletter, Volume XXII, No. 20, written by Leslie Peterson.