Hydroelectric power

Hydroelectric power is the most commonly used renewable energy resource. It can be stored in the form of impounded water and is relatively nonpolluting.

Supply and Transmission

Even though running water has been used by people for centuries to turn the wheels of gristmills and sawmills, it was not until the end of the nineteenth century that water power began to be employed to generate electricity. Hydroelectric projects can be as small as a waterwheel supplying energy to a single household, or as large as a system of dams and storage projects that supply electricity to many cities and millions of people. Electric energy, generated by water-powered turbines, is transported to houses, factories, mills, and other sites of consumption along high-voltage transmission lines that may extend for more than 2,000 kilometers. These transmission lines are either alternating current (AC), the type of electricity used in houses, or direct current (DC), the type of electricity used in batteries. These lines can deliver hundreds of megawatts of power. In the United States, agreements between states, regions, and Canada have created a network of transmission lines that allows the flow of electricity from one part of North America to another. This ability to transport electricity from one place to another was one of the driving forces behind the relocation of factories and mills from along rivers to locations adjacent to sources of raw materials. The mobility of electricity has also allowed for the growth of numerous cities located away from sources of energy.

The easiest way to harness a river for the purpose of generating hydroelectric power is to construct a dam across a river and funnel the water through a turbine that creates electricity. A dam with a large reservoir of water behind it is best for generating electricity because both the amount of water in the river and the demand for electricity vary throughout the year. For example, in the Columbia River system in the Pacific Northwest, the river reaches peak flow during the spring snowmelt, but demand for electricity is greatest during the late winter (for heating homes) and summer (for air conditioning). The ability to store large volumes of water behind each of the dams in the system allows electric utilities to meet summer and winter demands, “storing” the water until electricity is needed. Water from spring runoff is stored and released to generate electricity to power air conditioners in the summer and heat homes in the winter.

Large storage dams also allow a utility to increase or decrease electric generation to match the demand. Electrical demand in the morning is met by releasing extra water. At night, when the demand is less, water is either kept in the reservoir or passed through the turbines using a process called spinning. Spinning is the method in which water passes through a turbine, but no electricity is produced. In a matter of seconds, the spinning turbine can be engaged and electricity produced.

Pumped Hydro

Another form of stored hydroelectric power is pumped storage. During the period of midnight to six in the morning, when energy demand is at its lowest, hydroelectric projects must maintain a minimum outflow of water for navigation, agriculture, mining interests, recreational interests, fisheries, and water quality. A utility may choose to use the electricity produced by minimum stream flow regulations to pump water into a storage pond. During the day, when energy demand increases, the pumped water is released, and electricity is generated.

During the late 1980s, pumped hydro required 1.3 to 1.4 kilowatt-hours for every kilowatt-hour produced. That may not seem economic, but because the water is pumped using surplus energy (energy that cannot be saved), the utility is able to postpone generation until demand is present. Another consideration is the difference in the cost of energy at peak and nonpeak hours. Electricity sold during peak demand may be several times the cost of electricity sold during nonpeak hours. Some utilities have even pumped water into excavated caverns. Hydroelectric power produced by the pumped-hydro plants amounts to thousands of megawatts.

Water Flow Rate and Head

Hydroelectric power is produced by converting the potential power of natural stream flow into energy. This is commonly done by employing falling water to turn turbines, which then drive generators and produce electricity. The amount of electric power produced is dependent upon the flow rate of the water and the head.

The flow rate of water is the volume of water that moves past a point during a specific period of time. The quantity of water is commonly measured by first determining the cross-sectional area (width and depth) of the river; second, the speed of the water is measured by defining a reference length of river to monitor, dropping a float at the upper end of the reference length, and recording the length of time the float takes to travel down the reference length of the river. Then, if one calculates the volume of water (length × width × depth) divided by the time the float took to travel down the reference length of the river, the result is volume per time, or flow rate.

The head of a stream is the vertical height through which the water falls. The head measurement of a hydroelectric project is the elevation difference between where the water enters the intake pipe, or penstock, and the turbine below. When waterwheels are employed to produce electricity, the head measurement is the total distance that water falls to the waterwheel. As with all energy conversions, friction results in some loss. The type of turbine or waterwheel utilized can also contribute to greater or lesser losses of energy. The theoretical maximum power available in kilowatts is equal to the head, measured in meters, multiplied by the flow rate, measured in cubic meters per second, multiplied by 9.8. Efficiencies (actual energy produced divided by the amount of energy available in the flowing water) for hydropower plants (turbines, waterwheels) vary from a high of 97 percent, claimed by manufacturers of large turbines, to less than 25 percent for some waterwheels.

Waterwheels

Two devices used to convert the potential energy of water to mechanical energy are the waterwheel and the water turbine. The type of waterwheel or turbine used is dependent upon the flow and head. The ideal situation is high head (more than 18 meters) and high flow, but it is feasible to produce electric energy with any combination of high head and low flow, or low head and high flow.

Waterwheels are the simplest machines employed to generate hydroelectric power. The central shaft of the waterwheel, which in the past was directly connected to a grindstone, is hooked to a generator to produce electricity. Efficiency has been claimed by waterwheel manufacturers to be around 90 percent, but the usual efficiency ranges from 60 percent down to around 20 percent. The most efficient is the overshot wheel, in which water falls onto the top of the wheel and turns it. The wheel is suspended over the tailwater (the water on the downstream side of the wheel) and is not resting in the water but suspended above the water's surface. This type of waterwheel requires at least a 2-meter head of water.

Three other types of waterwheels are able to operate at lower heads than does the overshot wheel, but all three are costly to construct. The first type, the low and high breast wheels, are turned by water striking the wheel at a point one-third to over one-half of the height of the wheel. The “low” or “high” defines the level at which the water enters the wheel. The second type, the undershot wheel, is probably the oldest style presently in use. The wheel is turned by water running under the wheel. Although this type of waterwheel has an efficiency of less than 25 percent, it can operate with less than a third of a meter head. The third low-head waterwheel is the Poncelet wheel, which is an improved undershot that rests just at the water level and depends upon the velocity of the water to turn the wheel. Because this wheel forces the water through narrow openings on the wheel, it is suitable for heads of less than 2 meters, but it is easily damaged by debris carried in the water.

Because the waterwheel rotates at a slow rate, the gear box, which transfers the rotation energy to the turbine, is a very costly, complex collection of gears. This expense is a major disadvantage. Another disadvantage is the large size of a waterwheel. Given the large amount of time and material involved and the low efficiency, the rate of monetary return is low. Overshot wheels, however, have the advantage of being able to operate with fluctuating water flows better than do water turbines. A second advantage is that once set up, and unlike low-head water turbines, an overshot wheel requires little repair and is not damaged by grit or clogged by leaves.

Turbines

There are two types of hydraulic turbines: impulse turbines, which utilize water under normal atmospheric pressure, and reaction turbines, which use water under pressure to drive them. Impulse turbines are driven solely by the impact of water against the turbine blades. The Pelton impulse wheel was designed in 1880 and is the crossover from waterwheels to turbines. The Pelton wheel is composed of a disk with buckets attached to the outside of the wheel. The wheel requires a head of at least 18 meters (roughly 1.8 times atmospheric pressure) but can operate under low flow rates because the water is forced under its own pressure through a nozzle to strike the buckets. The water striking the buckets causes the wheel to spin. Because operating efficiencies are commonly over 80 percent, this wheel is still a favorite of many small utilities in North America. The turgo impulse wheel represents an improvement over the Pelton. The water jet is aimed at the buckets at a low angle, thus allowing the stream of water to strike several buckets at once. That results in higher efficiencies with smaller wheels and lower flow rates than those needed for the Pelton. The turgo has an efficiency reported over 80 percent and is suited for use with heads greater than 10 meters. The cross-flow turbine is a drum-shaped impulse turbine with blades fixed along its outer edge. The drum design allows water to pass over the blades twice: once from the outside to the inside, then (after entering the drum) back outside again. The net result is up to 88 percent efficiency in large units and the ability to operate with heads as low as 1 meter. The cross-flow turbine is in widespread use around the world. It is simple to operate and largely self-cleaning.

Reaction turbines of several types are normally used in the largest hydroelectric projects; a single unit at Grand Coulee Dam can produce 600 megawatts or up to 805 megawatts. Reaction turbines exploit not only the impact of water but also the drop in water pressure as it passes through the turbine. Reaction turbines work by placing the whole runner (which is what is left of the “wheel” and resembles all blades set into a central shaft) into the flow of water. The water is carried to the turbines from the reservoir by a long tube called a penstock. The penstock can be more than 10 meters in diameter and tens of meters long. A propeller turbine is a reaction-type turbine that resembles a boat propeller in a tube. This type of turbine may be set either horizontally or vertically, depending on the design of the system. The Kaplan turbine is a turbine with adjustable blades on the propeller to allow operation at different flow rates. The water pressure in this system must be constant or the runner will become unbalanced. Very large hydroelectric plants usually install the Francis turbine. This type of turbine is designed to be set up and adjusted for the specific site. It can be used with a head of 2 meters and has an efficiency rating greater than 80 percent. The turbine spins as water is introduced just above the runner and directed onto the blades, causing the blades to rotate. The water then falls through and out a draft tube. A complicated mechanical governor is often used to guide the water around the runner.

Costs and Benefits

Hydroelectric power is the most developed renewable energy resource. According to the International Energy Agency (IEA), hydropower supplied about 17 percent of the world’s total primary energy supply in 2020. Where available, hydroelectric power likely is less expensive than alternatives. Its primary environmental impacts come from flooding land behind a dam and blocking a river’s flow; flooding destroys habitat and displaces the population of the flooded area. Mercury levels in impounded water rise somewhat for about twenty years in a newly flooded area, but return to normal as the mercury is leached from the flooded soil. Carbon dioxide, nitrous oxide, sulfur dioxide, and methane are released to the atmosphere as newly submerged vegetation decays, which contribute significantly to greenhouse gas emissions, especially when water levels drop. A 2016 study in BioScience assessed 250 hydroelectric dams and estimated that decaying matter in hydroelectric reservoirs worldwide may be emitting as much as a billion metric tons of greenhouse gases (methane, carbon dioxide, and nitrous oxide) annually. However, the IEA reported in 2021 that greenhouse gas emissions per unit of energy generated was one of the lowest methods of energy generation available.

During the early twenty-first century, government and private industry studies were still identifying many regions in the world that could be developed with hydroelectric power plants. According to the IEA, about half of hydropower's potential remained untapped in 2020, particularly in developing nations, where hydropower has already seen significant growth and provides electricity to some 800 million people. In advanced economies like North America, however, hydroelectric generation has declined and hydropower plants are aging. The available technology allowed for construction of dams that would permit unhindered fish migration, coexistence of fish hatcheries and hydroelectric projects, and maintenance of natural fish and wildlife populations. Concerns about commercial and sport fishing populations have resulted in the close monitoring of hydroelectric power plant operation by biologists to ensure fish survival.

A major obstacle to the development of new hydroelectric power plants is the large amount of paperwork involved. The cost of environmental impact studies can exceed the cost of actual construction. Dams must be licensed by the federal government and meet hundreds of county and state regulations. The amount of water allowed to flow downstream is regulated to ensure that agriculture, sport and commercial fisheries, recreation, environmental considerations, and the water rights of Indigenous people are satisfied. These competing interests imply that no single user of a given river will determine the quantity and conditions by which water moves through the dams.

Principal Terms

electricity: a flow of subatomic charged particles called electrons used as an energy source

energy: the capacity for doing work; power (usually measured in kilowatts) multiplied by the duration (usually expressed in hours, sometimes in days)

flow rate: the amount of water that passes a reference point in a specific amount of time (liters per second)

generator: a machine that converts the mechanical energy of the turbine into electrical energy

head: the vertical height that water falls or the distance between the water level of the reservoir above and the turbine below

kilowatt: 1,000 watts; a unit of measuring electric power

megawatt: 1 million watts; a unit of measuring electric power

penstock: the tube that carries water from the reservoir to the turbine

power: the rate at which energy is transferred or produced

pumped hydro: a storage technique that utilizes surplus electricity to pump water into an elevated storage pond to be released later when more electricity is needed

turbine: a device used to convert the energy of flowing water into the spinning motion of the turbine’s shaft; it does this by directing the flowing water against the blades mounted on the rotating shaft

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