Hydropower is an attractive alternative tofossil fuels as it does not directly producecarbon dioxide or otheratmospheric pollutants and it provides a relatively consistent source of power. Nonetheless, it has economic, sociological, and environmental downsides and requires a sufficiently energetic source of water, such as ariver or elevatedlake.[2] International institutions such as theWorld Bank view hydropower as a low-carbon means foreconomic development.[3]
A hydropower resource can be evaluated by its availablepower. Power is a function of thehydraulic head andvolumetric flow rate. The head is the energy per unit weight (or unit mass) of water.[5] The static head is proportional to the difference in height through which the water falls. Dynamic head is related to the velocity of moving water. Each unit of water can do an amount of work equal to its weight times the head.
The power available from falling water can be calculated from the flow rate and density of water, the height of fall, and the local acceleration due to gravity:
where
(work flow rate out) is the useful power output (SI unit:watts)
("Delta h") is the difference in height between the outlet and inlet (SI unit: metres)
To illustrate, the power output of a turbine that is 85% efficient, with a flow rate of 80 cubic metres per second (2800 cubic feet per second) and a head of 145 metres (476 feet), is 97 megawatts:[note 1]
Operators of hydroelectric stations compare the total electrical energy produced with the theoretical potential energy of the water passing through the turbine to calculate efficiency. Procedures and definitions for calculation of efficiency are given in test codes such asASME PTC 18 andIEC 60041. Field testing of turbines is used to validate the manufacturer's efficiency guarantee. Detailed calculation of the efficiency of a hydropower turbine accounts for the head lost due to flow friction in the power canal or penstock, rise in tailwater level due to flow, the location of the station and effect of varying gravity, the air temperature and barometric pressure, the density of the water at ambient temperature, and the relative altitudes of the forebay and tailbay. For precise calculations, errors due to rounding and the number ofsignificant digits of constants must be considered.[6]
Some hydropower systems such aswater wheels can draw power from the flow of a body of water without necessarily changing its height. In this case, the available power is thekinetic energy of the flowing water. Over-shot water wheels can efficiently capture both types of energy.[7] The flow in a stream can vary widely from season to season. The development of a hydropower site requires analysis offlow records, sometimes spanning decades, to assess the reliable annual energy supply. Dams and reservoirs provide a more dependable source of power by smoothing seasonal changes in water flow. However, reservoirs have a significantenvironmental impact, as does alteration of naturally occurring streamflow. Dam design must account for the worst-case, "probable maximum flood" that can be expected at the site; aspillway is often included to route flood flows around the dam. A computermodel of the hydraulic basin and rainfall and snowfall records are used to predict the maximum flood.[citation needed]
Some disadvantages of hydropower have been identified.Dam failures can have catastrophic effects, including loss of life, property and pollution of land.
Dams andreservoirs can have major negative impacts on riverecosystems such as preventing some animals traveling upstream, cooling and de-oxygenating of water released downstream, and loss of nutrients due to settling of particulates.[8] River sediment builds river deltas and dams prevent them from restoring what is lost from erosion.[9][10] Furthermore, studies found that the construction of dams and reservoirs can result in habitat loss for some aquatic species.[11]
A hydropower scheme which harnesses the power of the water which pours down from the Brecon Beacons mountains,Wales; 2017
Large and deep dam and reservoir plants cover large areas of land which causesgreenhouse gas emissions from underwater rotting vegetation. Furthermore, although at lower levels than otherrenewable energy sources,[citation needed] it was found that hydropower producesmethane (CH4) equivalent to almost a billion tonnes of CO2 greenhouse gas a year.[12] This occurs when organic matters accumulate at the bottom of the reservoir because of thedeoxygenation of water which triggersanaerobic digestion.[13]
People who live near ahydro plant site are displaced during construction or whenreservoir banks become unstable.[11] Another potential disadvantage is cultural orreligious sites may block construction.[11][note 2]
One major way to classify watermills is by wheel orientation (vertical or horizontal), one powered by a vertical waterwheel through agear mechanism, and the other equipped with a horizontal waterwheel without such a mechanism. The former type can be further subdivided, depending on where the water hits the wheel paddles, into undershot, overshot, breastshot and pitchback (backshot or reverse shot) waterwheel mills. Another way to classify water mills is by an essential trait about their location:tide mills use the movement of the tide;ship mills are water mills onboard (and constituting) a ship.
Watermills impact the river dynamics of the watercourses where they are installed. During the time watermills operate channels tend tosedimentate, particularlybackwater.[14] Also in the backwater area,inundation events and sedimentation of adjacentfloodplains increase. Over time however these effects are cancelled by river banks becoming higher.[14] Where mills have been removed,river incision increases andchannels deepen.[14]
A plentiful head of water can be made to generatecompressed air directly without moving parts. In these designs, a falling column of water is deliberately mixed with air bubbles generated through turbulence or aventuri pressure reducer at the high-level intake. This allows it to fall down a shaft into a subterranean, high-roofed chamber where the now-compressed air separates from the water and becomes trapped. The height of the falling water column maintains compression of the air in the top of the chamber, while an outlet, submerged below the water level in the chamber allows water to flow back to the surface at a lower level than the intake. A separate outlet in the roof of the chamber supplies the compressed air. A facility on this principle was built on theMontreal River at Ragged Shutes nearCobalt, Ontario, in 1910 and supplied 5,000 horsepower to nearby mines.[15]
Share of electricity production from hydropower, 2023[16]
Hydroelectricity is the biggest hydropower application. Hydroelectricity generates about 15% of global electricity and provides at least 50% of the total electricity supply for more than 35 countries.[17] In 2021, global installed hydropower electrical capacity reached almost 1400 GW, the highest among all renewable energy technologies.[18]
Hydroelectric power plants vary in terms of the way they harvest energy. One type involves a dam and areservoir. The water in the reservoir is available on demand to be used to generate electricity by passing through channels that connect the dam to the reservoir. The water spins a turbine, which is connected to the generator that produces electricity.[19]
The other type is called a run-of-river plant. In this case, a barrage is built to control the flow of water, absent areservoir. The run-of river power plant needs continuous water flow and therefore has less ability to provide power on demand. The kinetic energy of flowing water is the main source of energy.[19]
Both designs have limitations. For example, dam construction can result in discomfort to nearby residents. The dam and reservoirs occupy a relatively large amount of space that may be opposed by nearby communities.[20] Moreover, reservoirs can potentially have major environmental consequences such as harming downstream habitats.[19] On the other hand, the limitation of the run-of-river project is the decreased efficiency of electricity generation because the process depends on the speed of the seasonal river flow. This means that the rainy season increases electricity generation compared to the dry season.[21]
The size of hydroelectric plants can vary from small plants calledmicro hydro, to large plants that supply power to a whole country. As of 2019, thefive largest power stations in the world are conventional hydroelectric power stations with dams.[22]
Hydroelectricity can also be used to store energy in the form ofpotential energy between two reservoirs at different heights withpumped-storage. Water is pumped uphill into reservoirs during periods of low demand to be released for generation when demand is high or system generation is low.[23]
Other forms of electricity generation with hydropower includetidal stream generators using energy fromtidal power generated from oceans, rivers, and human-made canal systems to generating electricity.[19]
Aconventional dammed-hydro facility (hydroelectric dam) is the most common type of hydroelectric power generation.
Billions of litres of rainwater can fall, which can generate huge amounts of electrical energy if used in the right way.[24] Research is being done into the different methods of generating power from rain, such as by using the energy in the impact of raindrops. This is in its very early stages, with new and emerging technologies being tested, prototyped and created. Such power has been called rain power.[25][26] One way in which this has been tried is by using hybrid solar panels called "all-weather solar panels" that can generate electricity from both the sun and the rain.[27]
According to zoologist and science and technology educator, Luis Villazon, a 2008 French study estimated that you could usepiezoelectric devices, which generate power when they move, to extract 12 milliwatts from a raindrop.[clarification needed][An individual raindrop is not a continuous process, so its electrical output must be measured in joules, not watts.] Over a year, this would amount to less than 1 Wh per square metre – enough to power a remote sensor. Villazon suggested a better application would be to collect the water from fallen rain and use it to drive a turbine, with an estimated energy generation of 3 kWh of energy per year for a 185 m2 roof.[28] A microturbine-based system created by three students from the Technological University of Mexico has been used to generate electricity. The Pluvia system "uses the stream of rainwater runoff from houses' rooftop rain gutters to spin a microturbine in a cylindrical housing. Electricity generated by that turbine is used to charge 12-volt batteries."[29]
The term rain power has also been applied to hydropower systems which include the process of capturing the rain.[24][28]
A water piston from theNongshu byWang Zhen (fl. 1290–1333)Saint Anthony Falls,United States; hydropower was used here to mill flour.Directly water-powered ore mill, late nineteenth century
Evidence suggests that the fundamentals of hydropower date toancient Greek civilization.[30] Other evidence indicates that the waterwheel independently emerged in China around the same period.[30] Evidence ofwater wheels andwatermills date to theancient Near East in the 4th century BC.[31] Moreover, evidence indicates the use of hydropower using irrigation machines to ancient civilizations such asSumer andBabylonia.[11] Studies suggest that the water wheel was the initial form of water power.[11]
In theRoman Empire, water-powered mills were described byVitruvius by the first century BC.[32] TheBarbegal mill, located in modern-day France, had 16 water wheels processing up to 28 tons of grain per day.[4] Roman waterwheels were also used for sawing marble such as theHierapolis sawmill of the late 3rd century AD.[33] Such sawmills had a waterwheel that drove two crank-and-connecting rods to power two saws. It also appears in two 6th centuryEastern Romansawmills excavated atEphesus andGerasa respectively. Thecrank and connecting rod mechanism of theseRoman watermills converted the rotary motion of the waterwheel into the linear movement of the saw blades.[34]
Water-powered trip hammers and bellows in China, during theHan dynasty (202 BC – 220 AD), were initially thought to be powered bywater scoops.[35] However, some historians suggested that they were powered by waterwheels. This is since it was theorized that water scoops would not have had the motive force to operate theirblast furnace bellows.[36] Many texts describe the Hun waterwheel; some of the earliest ones are theJijiupian dictionary of 40 BC,Yang Xiong's text known as theFangyan of 15 BC, as well asXin Lun, written byHuan Tan about 20 AD.[37] It was also during this time that the engineerDu Shi (c. AD 31) applied the power ofwaterwheels topiston-bellows in forging cast iron.[37]
Ancient Indian texts dating back to the 4th century BC refer to the termcakkavattaka (turning wheel), which commentaries explain asarahatta-ghati-yanta (machine with wheel-pots attached), however whether this is water or hand powered is disputed by scholars[31] On this basis,Joseph Needham suggested that the machine was anoria. Terry S. Reynolds, however, argues that the "term used in Indian texts is ambiguous and does not clearly indicate a water-powered device."[This quote needs a citation] Thorkild Schiøler argued that it is "more likely that these passages refer to some type of tread- or hand-operated water-lifting device, instead of a water-powered water-lifting wheel."[This quote needs a citation]
India received Roman water mills and baths in the early 4th century AD when a certain according to Greek sources.[38] Dams, spillways, reservoirs, channels, and water balance would develop in India during theMauryan,Gupta andChola empires.[39][40][41]
Another example of the early use of hydropower is seen inhushing, a historic method of mining that uses flood or torrent of water to reveal mineral veins. The method was first used at theDolaucothi Gold Mines inWales from 75 AD onwards. This method was further developed in Spain in mines such asLas Médulas. Hushing was also widely used inBritain in theMedieval and later periods to extractlead andtin ores. It later evolved intohydraulic mining when used during theCalifornia Gold Rush in the 19th century.[42]
Furthermore, in his book,The Book of Knowledge of Ingenious Mechanical Devices, the Muslim mechanical engineer,Al-Jazari (1136–1206) described designs for 50 devices. Many of these devices were water-powered, including clocks, a device to serve wine, and five devices to lift water from rivers or pools, where three of them are animal-powered and one can be powered by animal or water. Moreover, they included an endless belt with jugs attached, a cow-poweredshadoof (a crane-like irrigation tool), and a reciprocating device with hinged valves.[48]
Benoît Fourneyron, the French engineer who developed the first hydropower turbine
In the 19th century, French engineerBenoît Fourneyron developed the first hydropower turbine. This device was implemented in the commercial plant ofNiagara Falls in 1895 and it is still operating.[11] In the early 20th century, English engineerWilliam Armstrong built and operated the first private electrical power station which was located in his house inCragside inNorthumberland, England.[11] In 1753, the French engineerBernard Forest de Bélidor published his book,Architecture Hydraulique, which described vertical-axis and horizontal-axis hydraulic machines.[49]
The growing demand for theIndustrial Revolution would drive development as well.[50] At the beginning of the Industrial Revolution in Britain, water was the main power source for new inventions such asRichard Arkwright'swater frame.[51] Although water power gave way to steam power in many of the larger mills and factories, it was still used during the 18th and 19th centuries for many smaller operations, such as driving the bellows in smallblast furnaces (e.g. theDyfi Furnace) andgristmills, such as those built atSaint Anthony Falls, which uses the 50-foot (15 m) drop in theMississippi River.[52][51]
Technological advances moved the open water wheel into an enclosedturbine orwater motor. In 1848, the British-American engineerJames B. Francis, head engineer of Lowell's Locks and Canals company, improved on these designs to create a turbine with 90% efficiency.[53] He applied scientific principles and testing methods to the problem of turbine design. His mathematical and graphical calculation methods allowed the confident design of high-efficiency turbines to exactly match a site's specific flow conditions. TheFrancis reaction turbine is still in use. In the 1870s, deriving from uses in the California mining industry,Lester Allan Pelton developed the high-efficiencyPelton wheel impulse turbine, which used hydropower from the high head streams characteristic of theSierra Nevada.[citation needed]
The modern history of hydropower begins in the 1900s, with large dams built not simply to power neighboring mills or factories[54] but provide extensive electricity for increasingly distant groups of people. Competition drove much of the global hydroelectric craze: Europe competed amongst itself to electrify first, and the United States' hydroelectric plants inNiagara Falls and theSierra Nevada inspired bigger and bolder creations across the globe.[55] American and USSR financers and hydropower experts also spread the gospel of dams and hydroelectricity across the globe during theCold War, contributing to projects such as theThree Gorges Dam and theAswan High Dam.[56] Feeding desire for large scale electrification with water inherently required large dams across powerful rivers,[57] which impacted public and private interests downstream and in flood zones.[58] Inevitably smaller communities and marginalized groups suffered. They were unable to successfully resist companies flooding them out of their homes or blocking traditionalsalmon passages.[59] The stagnant water created by hydroelectric dams provides breeding ground for pests andpathogens, leading to localepidemics.[60] However, in some cases, a mutual need for hydropower could lead to cooperation between otherwise adversarial nations.[61]
Hydropower technology and attitude began to shift in the second half of the 20th century. While countries had largely abandoned their small hydropower systems by the 1930s, the smaller hydropower plants began to make a comeback in the 1970s, boosted by government subsidies and a push for more independent energy producers.[57] Some politicians who once advocated for large hydropower projects in the first half of the 20th century began to speak out against them, and citizen groups organizing against dam projects increased.[62]
In the 1980s and 90s the international anti-dam movement had made finding government or private investors for new large hydropower projects incredibly difficult, and given rise to NGOs devoted to fighting dams.[63] Additionally, while the cost of other energy sources fell, the cost of building new hydroelectric dams increased 4% annually between 1965 and 1990, due both to the increasing costs of construction and to the decrease in high quality building sites.[64] In the 1990s, only 18% of the world's electricity came from hydropower.[65]Tidal power production also emerged in the 1960s as a burgeoning alternative hydropower system, though still has not taken hold as a strong energy contender.[66]
Especially at the start of the American hydropower experiment, engineers and politicians began major hydroelectricity projects to solve a problem of 'wasted potential' rather than to power a population that needed the electricity. When theNiagara Falls Power Company began looking into damming Niagara, the first major hydroelectric project in the United States, in the 1890s they struggled to transport electricity from the falls far enough away to actually reach enough people and justify installation. The project succeeded in large part due toNikola Tesla's invention of thealternating current motor.[67][68] On the other side of the country,San Francisco engineers, theSierra Club, and the federal government fought over acceptable use of theHetch Hetchy Valley. Despite ostensible protection within a national park, city engineers successfully won the rights to both water and power in the Hetch Hetchy Valley in 1913. After their victory they delivered Hetch Hetchy hydropower and water to San Francisco a decade later and at twice the promised cost, selling power toPG&E which resold to San Francisco residents at a profit.[69][70][71]
The American West, with its mountain rivers and lack of coal, turned to hydropower early and often, especially along theColumbia River and its tributaries. TheBureau of Reclamation built theHoover Dam in 1931, symbolically linking the job creation and economic growth priorities of theNew Deal.[72] The federal government quickly followed Hoover with theShasta Dam andGrand Coulee Dam. Power demand inOregon did not justify damming the Columbia untilWWI revealed the weaknesses of a coal-based energy economy. The federal government then began prioritizing interconnected power—and lots of it.[73] Electricity from all three dams poured into war production duringWWII.[74]
After the war, theGrand Coulee Dam and accompanying hydroelectric projects electrified almost all of the ruralColumbia Basin, but failed to improve the lives of those living and farming there the way its boosters had promised and also damaged the river ecosystem and migrating salmon populations. In the 1940s as well, the federal government took advantage of the sheer amount of unused power and flowing water from the Grand Coulee to build anuclear site placed on the banks of the Columbia. The nuclear site leaked radioactive matter into the river, contaminating the entire area.[75]
Post-WWII Americans, especially engineers from theTennessee Valley Authority, refocused from simply building domestic dams to promoting hydropower abroad.[76][77] While domestic dam building continued well into the 1970s, with the Reclamation Bureau andArmy Corps of Engineers building more than 150 new dams across the American West,[76] organized opposition to hydroelectric dams sparked up in the 1950s and 60s based on environmental concerns. Environmental movements successfully shut down proposed hydropower dams inDinosaur National Monument and theGrand Canyon, and gained more hydropower-fighting tools with 1970s environmental legislation. As nuclear and fossil fuels grew in the 70s and 80s and environmental activists push for river restoration, hydropower gradually faded in American importance.[78]
Foreign powers andIGOs have frequently used hydropower projects in Africa as a tool to interfere in the economic development of African countries, such as theWorld Bank with theKariba andAkosombo Dams, and theSoviet Union with theAswan Dam.[79] TheNile River especially has borne the consequences of countries both along the Nile and distant foreign actors using the river to expand their economic power or national force. After the British occupation ofEgypt in 1882, the British worked with Egypt to construct the first Aswan Dam,[80] which they heightened in 1912 and 1934 to try to hold back the Nile floods. Egyptian engineerAdriano Daninos developed a plan for the Aswan High Dam, inspired by the Tennessee Valley Authority's multipurpose dam.
WhenGamal Abdel Nasser took power in the 1950s, his government decided to undertake the High Dam project, publicizing it as an economic development project.[77] After American refusal to help fund the dam, and anti-British sentiment in Egypt and British interests in neighboringSudan combined to make the United Kingdom pull out as well, the Soviet Union funded the Aswan High Dam.[81] Between 1977 and 1990 the dam's turbines generated one third of Egypt's electricity.[82] The building of the Aswan Dam triggered a dispute between Sudan and Egypt over the sharing of the Nile, especially since the dam flooded part of Sudan and decreased the volume of water available to them.Ethiopia, also located on the Nile, took advantage of the Cold War tensions to request assistance from the United States for their own irrigation and hydropower investments in the 1960s.[83] While progress stalled due to thecoup d'état of 1974 and following 17-year-longEthiopian Civil War Ethiopia began construction on theGrand Ethiopian Renaissance Dam in 2011.[84]
Beyond the Nile, hydroelectric projects cover the rivers and lakes of Africa. TheInga powerplant on theCongo River had been discussed since Belgian colonization in the late 19th century, and was successfully built after independence.Mobutu's government failed to regularly maintain the plants and their capacity declined until the 1995 formation of theSouthern African Power Pool created a multi-national power grid and plant maintenance program.[85] States with an abundance of hydropower, such as theDemocratic Republic of the Congo andGhana, frequently sell excess power to neighboring countries.[86] Foreign actors such as Chinese hydropower companies have proposed a significant amount of new hydropower projects in Africa,[87] and already funded and consulted on many others in countries likeMozambique and Ghana.[86]
Small hydropower also played an important role in early 20th century electrification across Africa. In South Africa, small turbines powered gold mines and the first electric railway in the 1890s, and Zimbabwean farmers installed small hydropower stations in the 1930s. While interest faded as national grids improved in the second half of the century, 21st century national governments in countries including South Africa and Mozambique, as well as NGOs serving countries like Zimbabwe, have begun re-exploring small-scale hydropower to diversify power sources and improve rural electrification.[88]
In the early 20th century, two major factors motivated the expansion of hydropower in Europe: in the northern countries ofNorway andSweden, high rainfall and mountains proved exceptional resources for abundant hydropower, and in the south, coal shortages pushed governments and utility companies to seek alternative power sources.[89]
Early on,Switzerland dammed the Alpine rivers and theSwiss Rhine, creating, along withItaly andScandinavia, a Southern Europe hydropower race.[90] In Italy'sPo Valley, the main 20th-century transition was not the creation of hydropower but the transition from mechanical to electrical hydropower. 12,000 watermills churned in the Po watershed in the 1890s, but the first commercial hydroelectric plant, completed in 1898, signaled the end of the mechanical reign.[91] These new large plants moved power away from rural mountainous areas to urban centers in the lower plain. Italy prioritized early near-nationwide electrification, almost entirely from hydropower, which powered its rise as a dominant European and imperial force. However, they failed to reach any conclusive standard for determining water rights before WWI.[92][91]
Modern German hydropower dam construction was built on a history of small dams powering mines and mills in the 15th century. Some parts of the German industry relied more on waterwheels than steam until the 1870s.[93] The German government did not set out building large dams such as the prewarUrft,Mohne, andEder dams to expand hydropower: they mostly wanted to reduce flooding and improve navigation.[94] However, hydropower quickly emerged as a bonus for all these dams, especially in the coal-poor south.Bavaria even achieved a statewide power grid by damming theWalchensee in 1924, inspired in part by loss of coal reserves after WWI.[95]
Hydropower became a symbol of regional pride and distaste for northern 'coal barons', although the north also held strong enthusiasm for hydropower.[96] Dam building rapidly increased after WWII, aiming to increase hydropower.[97] However, conflict accompanied the dam building and spread of hydropower: agrarian interests suffered from decreased irrigation, small mills lost water flow, and different interest groups fought over where dams should be located, controlling who benefited and whose homes they drowned.[98]
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