Hydropower

Summary: Hydropower is the most common source of renewable energy, accounting for an estimated 16 percent of global electricity production, in 2019.

Humans have used the force of water for thousands of years. The Greeks used waterwheels for grinding wheat into flour more than 2,000 years ago. In China, Tu Shih recorded the use of water power for cast-iron manufacture in 31 CE. The Roman engineer Vitruvius wrote about waterwheels in the first century BCE, and in the fourth century CE. Romans built an elaborate, sixteen-waterwheel flour mill in the south of France at Barblégal, near Arles. Similar mills have been found in Tunisia and Israel. Other uses for mechanical energy generated by hydropower were sawing wood and powering textile mills and manufacturing plants.

The technology to use falling water to create hydroelectricity has existed for a long time. The evolution of the modern hydropower turbine began in the mid-1700s when a French hydraulic and military engineer, Bernard Forest de Bélidor, wrote Architecture Hydraulique. In this four-volume work, he described using a vertical-axis versus a horizontal-axis machine. During the 1700s and 1800s, water turbine development continued. In 1880, an arc-light dynamo invented by Charles Brush was driven by a water turbine and used to provide theater and storefront lighting in Grand Rapids, Michigan; in 1881, a Brush dynamo connected to a turbine in a flour mill provided street lighting at Niagara Falls, New York.

Early applications of hydropower used direct current (DC), which limited the range of applications. The shift to alternating current (AC) came when the electric generator was coupled with the turbine, which resulted in the United States’ (and the world’s), first hydroelectric plant, located in Appleton, Wisconsin, in 1882.

From Water to Watts

Hydropower uses moving water to power machinery or generate electricity. On a global scale, water constantly moves through the hydrologic cycle, evaporating from water bodies such as lakes and oceans to form clouds, which precipitate water over land as rain or snow, which then flows back to the ocean. The energy of this water cycle, which is driven by the sun, can be harnessed for mechanical tasks, such milling grains, pressing oil-containing seeds, sawing wood, and turning a turbine to generate electricity. As hydropower uses a fuel—water from the hydrologic cycle—that is not consumed in the process, it is, in principle, a renewable form of energy. The use of hydropower can make a contribution to saving exhaustible energy sources such as fossil fuels. Each 600 kilowatt-hours of electricity generated with a hydroplant is equivalent to approximately 1 barrel of oil (assuming an efficiency of 38 percent for the conversion of oil into electricity).

The basic principle of hydropower is that if water can be piped from a certain level to a lower level, the resulting water pressure can be used to do work. If the water pressure is allowed to move a mechanical component, then that movement involves the conversion of the potential energy of the water into mechanical energy. Hydraulic turbines, or hydroturbines, convert water pressure into mechanical shaft power, which can be used to drive an electricity generator, a grinding mill, or some other useful device.

To know the power potential of water in a river, it is necessary to know the flow in the river and the available head. The flow of the river is the amount or volume of water (in cubic meters or liters) that passes in a certain amount of time a cross-section of the river. Flows are normally given in cubic meters per second (m^3/s) or in liters per second (l/s). Head is the vertical difference in level (in meters) the water falls down. The theoretical power (P) available from a given head of water is in exact proportion to the head H and the flow Q:

P = QHc

where the constant c is the product of the density of water and the acceleration due to gravity (g). If P is measured in watts, Q in m³/s, and H in meters, the gross power of the flow of water is

P = 1000 × 9.8 × Q × H

This available power will be converted by the hydroturbine into mechanical power. As a turbine has an efficiency lower than 1, the generated power will be a fraction of the available gross power.

Advantages and Disadvantages

Hydropower has several advantages. Perhaps most important, it is a long-term renewable resource. Given a reasonable head, it is also a concentrated energy source. The available energy it can produce is predictable and nonpolluting. Hydropower dams and reservoirs can assist in flood control; by the same token, if a drought occurs, a dam creates a reservoir containing a water supply. Hydroelectricity generation is 90 percent efficient, whereas fossil-fueled energy is only 30 to 40 percent efficient. Hydropower infrastructure offers a form of storing energy for other industrial, agricultural, recreational, and personal uses. It is reliable and quick in reacting to changes in demand and supply of electricity, helping to balance the intermittent character of other renewable sources, such as wind and solar power. Finally, hydropower is cost-efficient, with low operating and maintenance expenses and with a projected life span of up to seventy years for large facilities.

On the other hand, there are disadvantages to hydropower as well. It is a site-specific technology, and for large-scale production geared toward populous urban centers, the sites that are well suited to the harnessing of water power and also close to a location where the power can be economically exploited are not very common. There is always a maximum useful power output available from a given hydropower site, which limits the level of expansion of activities that make use of the power. Damage is caused by flooding above the dams. Building new large-scale facilities (dams, reservoirs, and related infrastructure) is associated with high initial construction costs, as well as the costs of relocating people because of the adjustment of water levels related to damming. Marine transportation is limited unless locks are constructed. Finally, large-scale hydropower has significant environmental impacts: Migratory travel by aquatic life such as fish is restricted with the dam structure; plant and animal habitats may be destroyed; some water areas may dry up and agricultural land can be degraded; silt buildup in reservoirs and river bottoms can cause transportation hazards and ecological damage; some essential minerals (fertilizers) do not get distributed below the dam; and archaeological artifacts, such as holy grounds, can be destroyed.

Environmental factors also impact the efficiency and availability of hydropower. For instance, Lake Mead, the reservoir that feeds the Hoover Dam, experienced record low water levels in 2022 that saw it reach just 27 percent of its capacity by the summer of that year. Both climate change and long-term drought were attributed to the record low levels. Around the same time, a spokesperson from the United States Bureau of Reclamation announced that the Hoover Dam was running at 66 percent capacity due to the lower water levels of Lake Mead.

Sizes of Hydropower

Facilities range in size from large power plants that supply many consumers with electricity to small and microhydropower plants that individuals operate for their own energy needs or to sell power to utilities. Internationally, no common definitions of hydropower sizes exist, although it is generally understood that small hydropower has a capacity of less than 10 megawatts, in line with the recommendations by the World Commission on Dams. However, some countries have set that limit at 15 megawatts (such as India) or even 25 megawatts (China) or 30 megawatts (United States). Large-scale hydropower consequently includes all plants above the upper limit of small-scale hydropower.

There are three categories of small-scale hydropower: microhydropower, minihydropower, and small hydropower. However, again no uniform standards exist. In the United States, for example, microhydropower is usually considered to include installations generating less than 100 kilowatts and minihydropower between 100 kilowatts and 1 megawatt; in China, minihydropower runs up to 500 kilowatts and small hydropower up to 25 megawatts.

The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water may be released to meet changing electricity needs, to maintain a constant reservoir level, or to provide water downstream of the dam. A diversion, sometimes called a run-of-the-river (or run-of-river) facility, channels a portion of a river through a canal or penstock. It may not require the use of a dam.

Small-scale hydropower stations combine the advantages of hydropower with those of decentralized power generation, without the disadvantages of large-scale installations. Small-scale hydropower has few disadvantages: There is no costly distribution of energy and no huge environmental costs (as with large hydropower). It is independent of imported fuels, and there is no need for expensive maintenance. Small-scale hydropower can be used in a decentralized manner, locally implemented and managed. Power generated with a small hydro station can be used for agro-processing, local lighting, water pumps, and small businesses.

The context of small hydropower can be described as follows:

• It is decentralized, with small demand for power (from small industries, farms, households, and rural communities).

• The distribution network involves only low voltages (eventually the subregional grid).

• It is owned by an individual, cooperative, or community with semiskilled workers.

• It requires only short planning horizons and construction periods, using locally available materials and skills.

• Depending on the amount of power it generates, it can have a substantial impact on local standards of living (expanding access to previously inadequate power supplies or providing power where none existed).

• As only some information is available about the potential power, often not more than 10 percent of the potential is used.

Based on the advantages and the context as described above, microhydropower and small hydropower could play an important role in the electrification of remote rural areas in developing countries.

Pumped-Storage Hydropower

Pumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower-elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps. During periods of high electrical demand, the stored water is released through turbines. Although losses from the pumping process make the plant a net consumer of energy overall, the system increases revenue by selling more electricity during periods of peak demand, when electricity prices are highest. Pumped storage is the largest-capacity form of grid energy storage now available.

At times of low demand for electricity, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine, generating electricity. Reversible turbine/generator assemblies act as pump and turbine (usually a Francis turbine design). Nearly all facilities use the height difference between two natural bodies of water or artificial reservoirs. Pure pumped-storage plants simply shift the water between reservoirs, while the “pump-back” approach is a combination of pumped storage and conventional hydroelectric plants that use natural stream flow.

Taking into account evaporation losses from the exposed water surface and conversion losses, approximately 70 to 85 percent of the electrical energy used to pump the water into the elevated reservoir can be regained. The technique is currently the most cost-effective means of storing large amounts of electric energy on an operating basis, but capital costs and the presence of appropriate geography are critical decision factors.

Tidal and Wave Power

Tidal power, also called tidal energy, is a form of hydropower that converts the energy of tides into electricity or other useful forms of power. The first large-scale tidal power plant, the Rance Tidal Power Station in France, started operation in 1966.

Small tidal “mills” were used in southern England and northern France in the Middle Ages. Tidal flows in bays and estuaries offered the potential to drive cereal-grinding apparatus in areas that were too low-lying to allow the use of conventional waterwheels. In the twentieth century, tides were seriously reexamined as potential sources of energy to power industry and commerce.

Wave power converts the energy contained in ocean waves into usable energy. Wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave power generation is not a widely employed commercial technology, although there have been attempts at using it since at least 1890. In 2008, the first experimental wave farm was opened in Portugal, at the Aguçadoura Wave Farm.

Bibliography

Calma, Justine. "What Record-Low Water Levels at the Hoover Dam Reservoir Look Like from Space." The Verge, 22 Jul. 2022, www.theverge.com/2022/7/22/23274082/lake-mead-record-low-nasa-satellite-images-hoover-dam-reservoir. Accessed 30 Jan. 2023.

Chiras, Daniel D. The Homeowner’s Guide to Renewable Energy: Achieving Energy Independence Through Solar, Wind, Biomass and Hydropower. Gabriola, BC: New Society, 2006.

Craddock, David. Renewable Energy Made Easy. Ocala, FL: Atlantic, 2008.

Douglas, J. F., J. M. Gasiorek, and J. A. Swaffield. Fluid Mechanics. Englewood Cliffs, NJ: Prentice-Hall, 2001.

Førsund, Finn R. Hydropower Economics. New York: Springer, 2007.

Haas, Greg. "Hoover Dam Power Production down 33%, Officials Say." KTSM El Paso, MSN, 26 May 2022, www.msn.com/en-us/news/us/hoover-dam-power-production-down-33-official-says/ar-AAXLLD6. Accessed 30 Jan. 2023.

Hjort, Anders. Turning Hydropower Social: Where Global Sustainability Conventions Matter. Berlin: Springer, 2008.

International Energy Agency. “Renewable Energy Essentials: Hydropower.” OECD/IEA, 2010. PDF file.

Nunez, Christina. "Hydropower, Explained." National Geographic, 13 May 2019, www.nationalgeographic.com/environment/article/hydropower. Accessed 30 Jan. 2023.

Western Area Power Administration. Harnessing Hydropower: The Earth’s Natural Resource. Lakewood, CO: Western Area Power Administration, U.S. Department of Energy, 2011.