It is generated by using the force of moving water, usually from streams, rivers, or lakes, to spin turbines that activate electrical generators.
This energy is widely used around the world for large-scale power generation.

Reservoir (or impoundment) hydroelectric power plants :
These plants are equipped with a dam and a reservoir to store water. Water is released from the reservoir through penstocks to turn the turbines and generate electricity. Reservoir power plants can be large in size and usually have a large water storage capacity, which allows them to regulate electricity production according to demand.

Run-of-river hydroelectric power plants :
Unlike reservoir power plants, run-of-river power plants do not have dams or reservoirs. They simply exploit the natural flow of streams or rivers to turn turbines and generate electricity. These plants are generally smaller in size and depend on hydrological conditions for their electricity production.

Pumped storage hydroelectric power plants :
Pumped storage power plants are designed to store energy using two tanks, an upper tank and a lower tank. During periods of low electricity demand, water is pumped from the lower reservoir to the upper reservoir to store potential energy. When electricity demand is high, water is released from the upper tank to spin the turbines and generate electricity.

Micro-hydropower plants :
Micro-hydropower plants are small hydroelectric installations generally with a capacity of less than 100 kW. They can be installed on small streams or rivers, often for local purposes, such as supplying electricity to remote communities or industrial sites.

Mini-hydro plants :
Mini-hydro plants have a slightly higher generation capacity than micro-power plants, usually up to a few megawatts. They are often used to power small towns, industries, or remote rural areas.

Gravity-fed power plants take advantage of the flow of water and a difference in level. They can be classified according to the turbine flow and their head height. There are three types of gravity-fed power plants (listed here in order of importance in the hydropower mix) :

- Run-of-river power plants use the flow of a river and provide baseload energy produced "run-of-river" and injected immediately into the grid. They require simple developments that are much less expensive than higher power plants : small diversion structures, small dams used to divert the available flow from the river to the power plant, possibly a small reservoir when the river flow is too low (emptying constant(2) less than 2 hours). They usually consist of a water intake, a tunnel or a canal, followed by a penstock and a hydroelectric plant located on the bank of the river. The low pressure drop(3) in the tunnel or canal allows the water to gain height in relation to the river and therefore to acquire potential energy;
- lock power plants in large rivers with a relatively steep slope such as the Rhine or the Rhone, dams on the river or on a canal parallel to the river cause a series of decametric waterfalls that do not disturb the valley as a whole thanks to dikes parallel to the river. The hydroelectric plants placed at the foot of the dams turbine the water of the river. Careful management of the water stored between two dams makes it possible to provide peak energy in addition to baseload;
- lake-power plants (or high-head power plants) are also associated with a water reservoir created by a dam. Their large reservoir (emptying constant of more than 200 hours) allows seasonal water storage and modulation of electricity production : lake power plants are called during the hours of highest consumption and make it possible to respond to peaks. There are many of them in France. The plant can be located at the foot of the dam or much lower. In this case, the water is transferred through tunnels in charge of the lake to the entrance of the power plant.

Pumped energy transfer stations have two basins, an upper basin (e.g. a high-altitude lake) and a lower basin (e.g. an artificial reservoir) between which is placed a reversible device that can function as a pump or turbine for the hydraulic part and as a motor or alternator for the electrical part.

The water in the upper basin is turbined during periods of high demand to produce electricity. Then, this water is pumped from the lower basin to the upper basin in periods when energy is cheap, and so on. These plants are not considered to produce energy from renewable sources since they consume electricity to bring up turbine water.
These are energy storage facilities.
They frequently intervene for short-term interventions at the request of the network and as a last resort (after other hydroelectric power plants) for longer interventions, in particular because of the cost of the water to be lifted. The efficiency between the energy produced and the energy consumed is in the order of 70% to 80%.
The operation is profitable when the difference in electricity prices between off-peak periods (buying low-cost electricity) and peak periods (selling high-priced electricity) is significant.

Hydropower plants are made up of 2 main units :

- a reservoir or a water intake (in the case of run-of-river power plants) which makes it possible to create a waterfall, usually with a storage tank so that the power plant continues to operate, even during periods of low water.

- A dug diversion channel can be used to divert excess water arriving laterally to a dam pond. A spillway allows the floods of the river to pass without danger to the structures;
the power plant, also called a factory, which allows the waterfall to be used to drive the turbines and then to drive an alternator.

By far the most frequent are dams made of earth embankment or riprap obtained in quarries by blasting. The waterproofing is central (clay or bituminous concrete) or on the upstream surface (cement concrete or bituminous concrete). This type of dam adapts to a wide variety of geologies;
gravity dams built first in masonry, then in concrete and more recently in concrete compacted with a BCR roller) which allows significant savings in time and money. The foundation rock must be of good quality;
the concrete arched dams adapted to relatively narrow valleys and whose banks are made of good quality rock. The subtlety of their shapes makes it possible to reduce the quantity of concrete and to build economical dams;
the multi-arch and buttress dams are no longer built. BCR gravity dams replace them.

The plants are equipped with turbines that transform the energy of the water flow into a mechanical rotation in order to drive alternators.

The type of turbine used depends on the height of the waterfall :
- for very low head heights (1 to 30 metres), bulb turbines can be used;
- for low headfalls (5 to 50 metres) and high flow rates, the Kaplan turbine is preferred : its blades are steerable, which makes it possible to adjust the power of the turbine to the head height while maintaining good efficiency;
- the Francis turbine is used for medium heads (40 to 600 metres) and medium flow. Water enters through the periphery of the blades and is discharged at their center;
- the Pelton turbine is suitable for high falls (200 to 1,800 metres) and low flow. It receives water under very high pressure via an injector (dynamic impact of the water on the bucket).

For small hydropower plants, low-cost (and less efficient) turbines and simple concepts facilitate the installation of small units.

Cost-effectiveness and predictability of production

The construction of dams is characterized by investments that are all the higher the height of the fall and the wider the valley.
These capital expenditures differ greatly depending on the characteristics of the development and the ancillary expenses related to social and environmental constraints, in particular the cost of the expropriated land.
The economic advantages linked to the modulation capacity of electricity production make it possible to make these investments profitable because the water resource is free of charge and maintenance costs are reduced.

Hydropower makes it possible to meet the needs of adjusting electricity production, in particular by storing water in large reservoirs by means of dams or dikes.
However, the annual fluctuations in hydropower production are significant. They are mainly related to rainfall. Production can increase by 15% in years when water resources are high and decrease by 30% in years of great drought.

Hydropower is sometimes criticized for causing population displacements, with rivers and streams being privileged places to set up housing.
For example, the Three Gorges Dam in China has displaced nearly two million people. Due to modified water regulation, ecosystems upstream and downstream of dams may be disturbed (including the migration of aquatic species) although devices such as fishways are installed.

Measurement of hydroelectric power

The power of a hydropower plant can be calculated by the following formula :

P = Q.ρ.H.g.r

With :

• P : power (expressed in W)
  


• Q : average flow measured in cubic meters per second
  


• ρ : density of the water, i.e. 1 000 kg/m3
  


• H : fall height in metres
  


• g : gravity constant, i.e. nearly 9.8 (m/s2)
  


• A : Plant efficiency (between 0.6 and 0.9)
  

Worldwide :

hydropower accounted for nearly 15.8% of global electricity production in 2018 (with an annual production of around 4,193 TWh);
a dozen countries, including four in Europe, produce more than half of their electricity from hydropower. Norway leads the way, followed by Brazil, Colombia, Iceland, Venezuela, Canada, Austria, New Zealand and Switzerland.