Hydropower

• 1. Introduction

  ›

  Hydropower is the use of falling or fast-running water to produce electricity or to power machines. The word borrows from the Ancient Greek for water, and the principle is simple. Water has energy because it moves or because it falls, and that energy can be captured. A single unit of water can do work equal to its weight times the head, the distance it drops. From that one idea comes a sprawling story. It runs from ancient gristmills to dams that power entire countries. Along the way it answers a few questions. How do you turn a river into watts. Why do nations fight over a single dam. And why did a technology praised as clean also become one of the most contested forms of energy on earth. Consider one number to begin. Hydroelectricity now generates about 15% of the world's electricity, and it does so without directly producing carbon dioxide.

• 2. The Arithmetic Of Falling Water

  ›

  Power is a function of the hydraulic head and the volumetric flow rate. Head is the energy per unit weight of water, and it splits into two kinds. Static head is proportional to the difference in height through which the water falls. Dynamic head relates to the velocity of the moving water. To turn those quantities into a number, engineers combine the flow rate, the density of water, the height of the fall, and the local acceleration due to gravity.

  The efficiency of the turbine, written as the Greek letter eta, scales the result. A worked example shows the scale involved. A turbine that is 85% efficient, fed a flow rate of 80 cubic metres per second over a head of 145 metres, produces about 97 megawatts. That single calculation captures why head and flow are the two levers that matter.

  Real stations refine that figure far past the textbook. Operators compare the total electrical energy produced against the theoretical potential energy of the water passing through the turbine. Test codes such as ASME PTC 18 and IEC 60041 define the procedures, and field testing validates a manufacturer's efficiency guarantee. A detailed calculation accounts for head lost to friction in the penstock, the rise in tailwater, the effect of varying gravity, the air temperature, the barometric pressure, and the relative altitudes of forebay and tailbay.

  Not every machine bothers with height at all. Some systems, such as water wheels, draw power purely from the kinetic energy of flowing water without changing its level. Over-shot water wheels can capture both the kinetic and the potential energy at once. But a stream's flow varies widely from season to season, which is why developing a site requires analysis of flow records that sometimes span decades.

• 3. The Bargain Of The Reservoir

  ›

  Dams and reservoirs smooth the seasonal swings that make raw rivers unreliable, providing a more dependable source of power. The price of that dependability is steep. Dam design must account for the worst case, the probable maximum flood expected at the site. A spillway is often included to route flood flows around the dam, and a computer model of the hydraulic basin, fed by rainfall and snowfall records, predicts how high that flood might rise.

  The environmental ledger runs long. Dams can prevent some animals from traveling upstream, cool and de-oxygenate the water released downstream, and strip nutrients as particulates settle out. River sediment normally builds deltas, but a dam holds it back so the delta cannot replace what erosion takes. Studies have linked dam and reservoir construction to habitat loss for some aquatic species.

  The water itself can become a source of emissions. Large and deep reservoirs cover wide areas of land, and underwater vegetation rots beneath them. Organic matter accumulates at the bottom, the water deoxygenates, and anaerobic digestion begins. The result is methane. Hydropower produces methane equivalent to almost a billion tonnes of carbon dioxide a year, lower than other renewables but far from nothing.

  The human cost lands closest to home. People who live near a plant are displaced during construction or when reservoir banks become unstable. Cultural or religious sites can block construction entirely. A dam failure carries the gravest risk of all, with potential loss of life, loss of property, and pollution of the land.

• 4. Mills, Forges And Empires

  ›

  Evidence suggests the fundamentals of hydropower date to ancient Greek civilization, while the waterwheel appears to have emerged independently in China around the same time. Water wheels and watermills trace to the ancient Near East in the 4th century BC, and irrigation machines served civilizations such as Sumer and Babylonia. Studies suggest the water wheel was the initial form of water power.

  Rome turned the wheel into industry. Vitruvius described water-powered mills by the first century BC. The Barbegal mill in modern-day France ran 16 water wheels and processed up to 28 tons of grain per day. Roman waterwheels even sawed marble, as at the Hierapolis sawmill of the late 3rd century AD, where a waterwheel drove two crank-and-connecting rods to power two saws. Similar machines appear in two 6th century Eastern Roman sawmills excavated at Ephesus and Gerasa.

  China applied the same force to metal. During the Han dynasty, from 202 BC to 220 AD, water-powered trip hammers and bellows ran in the workshops, and around AD 31 the engineer Du Shi applied the power of waterwheels to piston-bellows for forging cast iron. Texts such as the Jijiupian dictionary of 40 BC and Huan Tan's Xin Lun of about 20 AD describe the waterwheel of the period.

  The Islamic Golden Age spread the technology across continents. From the 8th to the 13th centuries, water-powered fulling mills, paper mills, sawmills, steel mills, sugar mills, and tide mills operated from Al-Andalus to Central Asia, and by the 11th century every province had them. Muslim engineers pioneered dams as a source of water power. Al-Jazari, who lived from 1136 to 1206, described designs for 50 devices in The Book of Knowledge of Ingenious Mechanical Devices, many of them water-powered, including clocks, a wine-serving device, and machines to lift water from rivers and pools.

• 5. The Turbine Revolution

  ›

  In the 19th century, the French engineer Benoit Fourneyron developed the first hydropower turbine. That device was later installed in the commercial plant at Niagara Falls in 1895, and it is still operating. Decades earlier, in 1753, the French engineer Bernard Forest de Belidor had published Architecture Hydraulique, which described both vertical-axis and horizontal-axis hydraulic machines.

  The Industrial Revolution pulled the technology forward. In early industrial Britain, water was the main power source for new inventions such as Richard Arkwright's water frame. Steam later displaced water in many large mills, yet water still drove smaller operations through the 18th and 19th centuries, including the bellows of small blast furnaces such as the Dyfi Furnace and gristmills like those at Saint Anthony Falls, which used the 50 foot drop in the Mississippi River.

  Enclosing the wheel changed everything. In 1848, the British-American engineer James B. Francis, head engineer of Lowell's Locks and Canals company, designed a turbine with 90% efficiency. He applied scientific principles and testing methods to turbine design, and his mathematical and graphical methods let engineers match a turbine to a site's exact flow conditions. The Francis reaction turbine is still in use.

  The mining country produced its own machine. In the 1870s, drawing on uses in the California mining industry, Lester Allan Pelton developed the high-efficiency Pelton wheel, an impulse turbine suited to the high-head streams of the Sierra Nevada. England, meanwhile, offered a quieter milestone, when William Armstrong built and operated the first private electrical power station at his house in Cragside, Northumberland.

• 6. Dams And The Cold War

  ›

  The modern history of hydropower begins in the 1900s, when large dams were built not to power a neighboring mill but to send electricity to distant groups of people. Competition fueled the craze. Europe raced internally to electrify first, and the American plants at Niagara Falls and in the Sierra Nevada inspired bolder projects across the globe. American and Soviet financiers and experts spread the gospel of dams during the Cold War, contributing to projects such as the Three Gorges Dam and the Aswan High Dam.

  The scale carried a social cost. Feeding large-scale electrification required big dams across powerful rivers, which harmed interests downstream and in flood zones. Smaller communities and marginalized groups suffered, unable to resist companies that flooded their homes or blocked traditional salmon passages. The stagnant water behind a dam bred pests and pathogens, leading to local epidemics. Yet a shared need for power could also push otherwise adversarial nations toward cooperation.

  The technology then began to swing back. Countries had largely abandoned their small hydropower systems by the 1930s, but small plants made a comeback in the 1970s, boosted by government subsidies and a push for independent energy producers. Some politicians who once championed large projects began to speak out against them, and citizen groups organizing against dams multiplied.

  The economics turned hostile too. By the 1980s and 90s, an international anti-dam movement made investors hard to find and gave rise to NGOs devoted to fighting dams. The cost of building new hydroelectric dams rose 4% annually between 1965 and 1990, driven by construction costs and a shortage of high-quality sites. By the 1990s, only 18% of the world's electricity came from hydropower.

• 7. Rivers Of Power Abroad

  ›

  In the American West, mountain rivers and a lack of coal pushed early reliance on hydropower, especially along the Columbia River. The Bureau of Reclamation built the Hoover Dam in 1931, linking the job creation and economic growth of the New Deal, and the Shasta Dam and Grand Coulee Dam soon followed. Electricity from all three poured into war production during the Second World War. After the war, the Grand Coulee electrified almost all of the rural Columbia Basin, but it failed to improve lives as promised and damaged the river ecosystem and migrating salmon.

  The Niagara project proved the concept could reach people at all. When the Niagara Falls Power Company began looking into damming Niagara in the 1890s, engineers struggled to transport electricity far enough to justify the installation. The project succeeded largely because of Nikola Tesla's invention of the alternating current motor. On the other coast, San Francisco engineers won rights to water and power in the Hetch Hetchy Valley in 1913, then delivered it to the city a decade later at twice the promised cost, selling power to PG&E.

  In Africa, foreign powers used dams as instruments of influence. The World Bank backed the Kariba and Akosombo Dams, and the Soviet Union funded the Aswan High Dam. After the United States and the United Kingdom refused to fund it, the Soviet Union stepped in, and between 1977 and 1990 the dam's turbines generated one third of Egypt's electricity. The project triggered a dispute between Sudan and Egypt over sharing the Nile, since the dam flooded part of Sudan and reduced its water. Ethiopia began construction on the Grand Ethiopian Renaissance Dam in 2011.

  Europe split along a north-south line. High rainfall and mountains gave Norway and Sweden abundant resources, while coal shortages pushed southern countries to seek alternatives. In Italy's Po Valley, 12,000 watermills churned in the 1890s, but the first commercial hydroelectric plant, completed in 1898, ended the mechanical reign. Bavaria achieved a statewide power grid by damming the Walchensee in 1924, inspired partly by the loss of coal reserves after the First World War.

• 8. Air, Tides And Rain

  ›

  A plentiful head of water can generate compressed air directly, with no moving parts. In these designs, falling water is deliberately mixed with air bubbles, then dropped down a shaft into a subterranean, high-roofed chamber. There the compressed air separates from the water and stays trapped, held under pressure by the weight of the falling column, while a separate roof outlet supplies the air. A facility built on this principle at Ragged Shutes near Cobalt, Ontario, in 1910 supplied 5,000 horsepower to nearby mines.

  Hydroelectricity remains the dominant application by far. It provides at least 50% of the total electricity supply for more than 35 countries, and in 2021 global installed hydropower capacity reached almost 1400 gigawatts, the highest among all renewable technologies. Plants split into two basic types. A dam-and-reservoir plant releases stored water on demand to spin a turbine, while a run-of-river plant uses a barrage without a reservoir and depends on continuous flow, generating more in the rainy season and less in the dry. As of 2019, the five largest power stations in the world were conventional hydroelectric stations with dams.

  The ocean offers another reservoir. Tidal stream generators draw on tidal power from oceans, rivers, and human-made canals, and tidal production emerged in the 1960s as a burgeoning alternative, though it has not yet become a strong contender. Pumped-storage works differently again, pumping water uphill into a reservoir during low demand and releasing it for generation when demand is high.

  The newest frontier falls from the sky. Researchers are testing ways to harvest energy from rain, including the impact of raindrops, a field still in its earliest stages. A 2008 French study, cited by zoologist Luis Villazon, estimated that piezoelectric devices could extract 12 milliwatts from a single raindrop, amounting over a year to less than 1 watt-hour per square metre, enough to power a remote sensor. Villazon suggested a better use would be to collect fallen rain and drive a turbine, estimating 3 kilowatt-hours per year for a 185 square metre roof. Three students from the Technological University of Mexico built one such answer, the Pluvia system, which spins a microturbine on rooftop runoff to charge 12-volt batteries.