Water Energy and Its Uses
3 min read · Energy Atlas Editorial
The amount of available water energy, kinetic and potential, depends on hydrologic potential, mostly on hydraulic head and flow rate.
The Potential Energy of Water
Within the Earth's field of gravity, water always has some potential (positional) energy. This energy can be converted into another type of energy, e.g. kinetic or pressure energy. Potential energy is utilised primarily by reaction turbines.
Kinetic Energy of Water
The kinetic (motion) energy of water depends on its mass and the speed of the water flow. Hydraulic head can be converted into kinetic energy as water accelerates. Kinetic energy drives impulse turbines.
Available Water Energy
The amount of electrical energy generated in a hydroelectric power plant primarily depends on the hydraulic head and the rate of flow through the turbine. In the course of a year, the head may remain relatively stable, while the rate of flow can vary considerably.
A hydroelectric power plant harnesses the energy of water flowing through a given hydraulic head, using turbines to convert it into mechanical energy and generators to convert this mechanical energy into electricity. The approximate output of a hydroelectric power plant can be easily calculated using the following formula:
P = ρ × g × Q × H × η
where:
P · is the power output in watts (W), ρ · is the water density (kg/m3), g · is the gravitational acceleration (9.81 m/s2), Q · is the volumetric flow rate (m3/s), H · is the hydraulic head (m), η · is the overall efficiency ranging from 0 to 1.
The annual production of electric energy also depends on fluctuations in the rate of flow throughout the year.
Hydropower Potential
The theoretical hydropower potential of a geographic region represents the total amount of electrical energy that could theoretically be produced by harnessing its water resources.
An estimate of the theoretical hydropower potential is based mainly on the topography and water availability of a region. It represents an idealised upper limit of the energy that could be obtained from water flowing from higher elevations towards sea level.
Technical Potential
Technical potential takes into account the part of the theoretical hydropower potential that can be exploited using currently available technology. However, not all technically feasible projects are economically viable. Therefore, economic potential represents the part of the technical potential that can be exploited economically.
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The Physical Properties of Water
Water is a chemical compound made from one oxygen and two hydrogen atoms. At normal conditions, it is a colourless, clear, and odourless liquid.
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The Origin of Water Energy
The Sun warms surface water and evaporates it. After condensation, the water falls on the land with exploitable potential energy relative to ocean level.
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History of Water Energy Utilization
The energy of water was used since ancient times for grinding grain or irrigation. Later it powered mills or hammers, and finally a turbine was invented.
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Dams and Reservoirs
The dams create water reservoirs for hydro power plants, to prevent floods or to collect drinking water. Typical dams are gravity or concrete arch dams.
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The Highest Dams and Largest Reservoirs
The highest dam is the 305 m high, concrete arch-shaped Jinping-I dam in China. The largest reservoir is Lake Kariba in Zambezi, with a volume of 180 km3.
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The Segner Wheel
The Segner wheel was an important step on the way to modern turbines. Its principle is today used in rotating irrigation sprinkler systems.
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Francis Turbine
The Francis turbine has a radial-axial design, variable pitch guide vanes, and fixed runner blades. The largest one has a diameter of 10 m.
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Kaplan Turbine
The Kaplan reaction water turbine with adjustable blades, invented in 1918, is used in places with minimal heads and relatively high, variable flow rates.
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Pelton Turbine
The Pelton turbine is an isobaric impulse action turbine. The water jet is directed tangentially at the spoon-shaped buckets on the runner.
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Choosing a turbine (Turbine selection graph)
The turbine selection graph helps to select the best turbine type for the given hydraulic head and rate of flow.
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Hydroelectric Power Plant Operating Principles
In hydroelectric power plants, the water propels the turbine blades, and the generator transforms the energy of a rotating turbine shaft into electricity.
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Types of Hydroelectric Power Plants
The hydroelectric power plants can be divided into accumulation ones with a reservoir, run-of-the river without a dam, derivational, and pumped-storage.
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The Largest Hydroelectric Power Plants in the World
The largest hydroelectric dams in the world are the "Three Gorges Dam" in China, Itaipú Dam and Guri Dam in South America.
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Tidal Energy and Sea Wave Power
Tidal power plants use ebb-tide energy from turbines in the dam enclosing a tidal basin, while tidal stream generators are turbines anchored to the seabed.
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Marine Current Power and Ocean Thermal Energy
Energy contained in ocean currents can be exploited by various underwater turbines or by an OTEC system that uses ocean temperature differences.
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Environmental Impact of Hydropower
The construction of a hydroelectric power plant affects a large area ecosystem and often leads to people's relocation. Then a new balance is established.
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