Hydropower

This article is about the general concept of hydropower. For the use of hydropower for electricity generation, see hydroelectricity.

The Three Gorges Dam in China; the hydroelectric dam is the world's largest power station by installed capacity.

Saint Anthony Falls, United States.

Hydropower or water power (from Greek: ὕδωρ, "water") is power derived from the energy of falling or fast-running water, which may be harnessed for useful purposes. Since ancient times, hydropower from many kinds of watermills has been used as a renewable energy source for irrigation and the operation of various mechanical devices, such as gristmills, sawmills, textile mills, trip hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance.

In the late 19th century, hydropower became a source for generating electricity. Cragside in Northumberland was the first house powered by hydroelectricity in 1878 and the first commercial hydroelectric power plant was built at Niagara Falls in 1879. In 1881, street lamps in the city of Niagara Falls were powered by hydropower.

Since the early 20th century, the term has been used almost exclusively in conjunction with the modern development of hydroelectric power. International institutions such as the World Bank view hydropower as a means for economic development without adding substantial amounts of carbon to the atmosphere, but dams can have significant negative social and environmental impacts.

History

Ancient Near East

Clay tablet
Water clock calculations by Nabû-apla-iddina in Mesopotamia (Iraq).
Size	H: 8.2 cm (3.2 in) W: 11.8 cm (4.6 in) D: 2.5 cm (0.98 in)
Writing	Cuneiform, Akkadian
Created	600–500 BC
Present location	Room 55, British Museum
Identification	29371

Water power was used for irrigation machines in ancient Near-Eastern civilizations such as Mesopotamia (Iraq) and ancient Egypt. Early uses of water power date back to Mesopotamia and Egypt, where irrigation has been used since the 6th millennium BC and water clocks had been used since the early 2nd millennium BC. Another early example of water power was the Qanat water system in ancient Persia.

The water wheel was an early example of hydropower, but it was initially driven by either humans or animals to raise water before later using hydropower. Water wheels and watermills date to the ancient Near East in the 4th century BC. The fundamentals of hydropower were known in the Near East during the Hellenistic period.

Waterwheels were used for sawing marble, such as the Hierapolis sawmill of the late 3rd century AD. Such sawmills had a waterwheel that drove two crank-and-connecting rods to power two saws. It also appears in two 6th century sawmills excavated at Ephesus, Syria and Gerasa, Asia Minor (Turkey) respectively. The crank and connecting rod mechanism of these watermills converted the rotary motion of the waterwheel into the linear movement of the saw blades.

Far East

A Northern Song era (960–1127) water-powered mill for dehusking grain with a horizontal wheel.

The water wheel emerged independently in ancient China. In China, watermills were widely used since the Han Dynasty. Another early example of water power was the Turpan water system in ancient China.

Water-powered trip hammers and bellows in China, during the Han dynasty (202 BC – 220 AD), were initially thought to be powered by water scoops. 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 their blast furnace bellows. Many texts describe the Hun waterwheel; some of the earliest ones are the Jijiupian dictionary of 40 BC, Yang Xiong's text known as the Fangyan of 15 BC, as well as Xin Lun, written by Huan Tan about 20 AD. It was also during this time that the engineer Du Shi (c. AD 31) applied the power of waterwheels to piston-bellows in forging cast iron.

In China and the rest of the Far East, hydraulically operated "pot wheel" pumps raised water into irrigation canals. In the 1830s, at the peak of the canal-building era, hydropower was used to transport barge traffic up and down steep hills using inclined plane railroads. Direct mechanical power transmission required that industries using hydropower had to locate near the waterfall. For example, during the last half of the 19th century, many grist mills were built at Saint Anthony Falls, utilizing the 50-foot (15 m) drop in the Mississippi River. The mills contributed to the growth of Minneapolis.

Indian subcontinent

Ancient Indian texts dating back to the 4th century BC refer to the term cakkavattaka (turning wheel), which commentaries explain as arahatta-ghati-yanta (machine with wheel-pots attached), however whether this is water or hand powered is disputed by scholars India received Roman water mills and baths in the early 4th century AD when a certain according to Greek sources. Dams, spillways, reservoirs, channels, and water balance would develop in India during the Mauryan, Gupta and Chola empires.

In the Indian subcontinent, water wheels and watermills were built.

Roman Empire

In the Roman Empire, water-powered mills produced flour from grain, and were also used for sawing timber and stone.

Islamic world

The Norias of Hama on the Orontes River.

The Islamic Empire spanned a large region, mainly in Asia and Africa, along with other surrounding areas. During the Islamic Golden Age and the Arab Agricultural Revolution (8th–13th centuries), hydropower was widely used and developed. Early uses of tidal power emerged along with large hydraulic factory complexes. A wide range of water-powered industrial mills were used in the region including fulling mills, gristmills, paper mills, hullers, sawmills, ship mills, stamp mills, steel mills, sugar mills, and tide mills. By the 11th century, every province throughout the Islamic Empire had these industrial mills in operation, from Al-Andalus and North Africa to the Middle East and Central Asia. Muslim engineers also used water turbines while employing gears in watermills and water-raising machines. They also pioneered the use of dams as a source of water power, used to provide additional power to watermills and water-raising machines. Islamic irriguation techniques including Persian Wheels would be introduced to India, and would be combined with local methods, during the Delhi Sultanate and the Mughal Empire.

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-powered shadoof (a crane-like irrigation tool), and a reciprocating device with hinged valves.

Medieval Europe

File:Braine-le-Château JPG02.jpg

Watermill of Braine-le-Château, Belgium (12th century)

The power of a wave of water released from a tank was used for extraction of metal ores in a method known as hushing. Hushing was widely used in Britain in the Medieval and later periods to extract lead and tin ores. It later evolved into hydraulic mining when used during the California gold rush.

Hydraulic power pipes

Saint Anthony Falls, United States; hydropower was used here to mill flour.

Hydraulic power networks also existed, using pipes carrying pressurized liquid to transmit mechanical power from a power source, such as a pump, to end users. These were extensive in Victorian cities in the United Kingdom. A hydraulic power network was also in use in Geneva, Switzerland. The world famous Jet d'Eau was originally only the over pressure valve of this network.

Calculating the amount of available power

A hydropower resource can be evaluated by its available power. Power is a function of the hydraulic head and volumetric flow rate. The head is the energy per unit weight (or unit mass) of water. 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:

$\dot{W}_{out} =-\eta \ (\dot{m} g \ \Delta h) =-\eta \ ((\rho \dot{V}) \ g \ \Delta h)$

where

• $\dot{W}_{out}$ (work flow rate out) is the useful power output (in watts)
• $\eta$ ("eta") is the efficiency of the turbine (dimensionless)
• $\dot{m}$ is the mass flow rate (in kilograms per second)
• $\rho$ ("rho") is the density of water (in kilograms per cubic metre)
• $\dot{V}$ is the volumetric flow rate (in cubic metres per second)
• $g$ is the acceleration due to gravity (in metres per second per second)
• $\Delta h$ ("Delta h") is the difference in height between the outlet and inlet (in 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 (480 feet), is 97 Megawatts:

$\dot{W}_{out} = 0.85\times 1000 \ (\text{kg}/\text{m}^3) \times 80 \ (\text{m}^3/\text{s}) \times 9.81 \ (\text{m}/\text{s}^2) \times 145 \ \text{m} = 97 \times 10^6 \ (\text{kg}\ \text{m}^2/\text{s}^3) = 97 \ \text{MW}$

Operators of hydroelectric stations will 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 as ASME PTC 18 and IEC 60041. Field testing of turbines is used to validate the manufacturer's guaranteed efficiency. Detailed calculation of the efficiency of a hydropower turbine will account for the head lost due to flow friction in the power canal or penstock, rise in tail water level due to flow, the location of the station and effect of varying gravity, the temperature and barometric pressure of the air, the density of the water at ambient temperature, and the altitudes above sea level of the forebay and tailbay. For precise calculations, errors due to rounding and the number of significant digits of constants must be considered.

Some hydropower systems such as water wheels can draw power from the flow of a body of water without necessarily changing its height. In this case, the available power is the kinetic energy of the flowing water. Over-shot water wheels can efficiently capture both types of energy. The water flow in a stream can vary widely from season to season. Development of a hydropower site requires analysis of flow 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 significant environmental impact, as does alteration of naturally occurring stream flow. The design of dams must also account for the worst-case, "probable maximum flood" that can be expected at the site; a spillway is often included to bypass flood flows around the dam. A computer model of the hydraulic basin and rainfall and snowfall records are used to predict the maximum flood.

Social and environmental impact of dams

Large dams can ruin river ecosystems, cover large areas of land causing green house gas emissions from underwater rotting vegetation and displace thousands of people and affect their livelihood.

Use of hydropower

File:Garwnant Hydropower Scheme, Breckon Beacons, Cymru, (Wales).webm

A hydropower scheme which harnesses the power of the water which pours down from the Brecon Beacons mountains, Wales; 2017

File:Higashiyama Botanical Garden Shishiodoshi 20170617.gif

A shishi-odoshi powered by falling water breaks the quietness of a Japanese garden with the sound of a bamboo rocker arm hitting a rock.

Mechanical power

Watermills

Compressed air hydro

See also: Trompe

Where there is a plentiful head of water it can be made to generate compressed air directly without moving parts. In these designs, a falling column of water is purposely mixed with air bubbles generated through turbulence or a venturi pressure reducer at the high-level intake. This is allowed 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 the Montreal River at Ragged Shutes near Cobalt, Ontario in 1910 and supplied 5,000 horsepower to nearby mines.

Hydroelectricity

Main article: Hydroelectricity

Hydroelectricity is the application of hydropower to generate electricity. It is the primary use of hydropower today. Hydroelectric power plants can include a reservoir (generally created by a dam) to exploit the energy of falling water, or can use the kinetic energy of water as in run-of-the-river hydroelectricity. Hydroelectric plants can vary in size from small community sized plants (micro hydro) to very large plants supplying power to a whole country. As of 2019, the five largest power stations in the world are conventional hydroelectric power stations with dams.

Hydroelectricity can also be used to store energy in the form of potential energy between two reservoirs at different heights with pumped-storage hydroelectricity. 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.

Other forms of electricity generation with hydropower include tidal stream generators using energy from tidal power generated from oceans, rivers, and human-made canal systems to generating electricity.

Hydroelectric dam.svg

A conventional dammed-hydro facility (hydroelectric dam) is the most common type of hydroelectric power generation.

Chief Joseph Dam.jpg

Chief Joseph Dam near Bridgeport, Washington, is a major run-of-the-river station without a sizeable reservoir.

Nw vietnam hydro.jpg

Micro hydro in Northwest Vietnam

Stwlan.dam.jpg

The upper reservoir and dam of the Ffestiniog Pumped Storage Scheme in Wales. The lower power station can generate 360 MW of electricity.

See also

• Deep water source cooling
• Gravitation water vortex power plant
• Hydraulic efficiency
• Hydraulic ram
• International Hydropower Association
• Low head hydro power
• Marine current power
• Marine energy
• Ocean thermal energy conversion
• Osmotic power
• Wave power

Notes

References

External links

• International Hydropower Association
• International Centre for Hydropower (ICH) hydropower portal with links to numerous organizations related to hydropower worldwide
• IEC TC 4: Hydraulic turbines (International Electrotechnical Commission – Technical Committee 4) IEC TC 4 portal with access to scope, documents and TC 4 website
• Micro-hydro power, Adam Harvey, 2004, Intermediate Technology Development Group. Retrieved 1 January 2005
• Microhydropower Systems, US Department of Energy, Energy Efficiency and Renewable Energy, 2005