One hydroelectric dam or transfer station is a concrete structure built directly into the current of a natural watercourse to serve as a reservoir of potential energy. It also has the mission of conveying large quantities of water to turbines which in turn run generators that produce electricity. How does this type of dam work? What is its environmental impact?

How does a hydroelectric power plant produce electricity?

It is not enough for water to accumulate in a hydroelectric dam for it to be transformed into electricity. On the contrary, it is a whole transformation process which takes place in several stages.

The lake of accumulation

Depending on its power, a natural water stream (river, lake) can release a certain amount of potential energy. The latter is generally in the form ofmechanical energy whose flow is concentrated on a specific path by gravity.

Built across a watercourse, a hydroelectric dam not only regulates its flow, but also stores its water in an artificial lake called an “accumulation lake”. At the same time, it imprisons its potential energy. The speed and depth of water passing through the dam pipe create a hydraulic load that makes the turbine in the hydroelectric plant turn.

Turbine start-up

Turbines are composed of mechanical blades of various shapes and sizes. Large reaction turbines such as the Francis turbines (found on the Three Gorges Dam in China) are equipped with large blades that need very high voltages and big motors to run.

For example, to rotate, the Francis turbine relies on hydraulic load. The latter is created by the hydrostatic pressure arising from the potential energy of the dam and increases with the speed of the water.

In other words, the hydraulic load of a hydroelectric dam depends exclusively on its height and on the flow of the water. The rotation of the turbine transforms the hydraulic load into kinetic energy, which eliminates speed and hydrostatic pressure of the fluid. From that moment on, water flows peacefully to the base of the dam.

The transformation into electricity

It is the rotating movement of the turbine that transforms the kinetic energy of water into electrical energy. A hydraulic turbine can be coupled directly to an electrical generator or through a transmission or gearbox that drives the shaft as well as the generator frame. The brooms and the collector, on the other hand, capture the flow of electricity generated by the rotating frame of the generator.

Hydroelectric plants use very powerful generators capable of creating a significant amount of resistance to mechanical rotation (reluctance). To overcome this reluctance, the rotation of the turbine must generate significant torque forces. Thanks to a system of transmission similar to that of a car, the gyratory movement of the turbine is converted into several torque-speed ratios.

The transport of electric current

The electricity produced in this way is converted into voltage and then transported to consumers via high and very high voltage power lines. Concretely, hydraulic dam elevators use the output voltages of low-current generators to generate tensions that can be directly used through homes. This is the case in particular for Round-the-river power plants.

Hydroelectric power plants also generate higher voltages intended for long-distance electrical transmissions. Moreover, the lower the electric current, the more effective its voltages are. For example, thanks to the use in electric transport ofA transmission of 500 kV, the Glen Canyon dam in Paige, Arizona, can provide electricity to households located about 1,500 km north of Nebraska.

The different types of hydroelectric dams

In order not to collapse under water pressure, a hydraulic dam must adapt perfectly to the geology of the soil where it is installed. This is the reason why there are different types of hydroelectric dams.

The weight dam

Imposing in mass, the gravity dam is designed in earth, concrete and rock and then compacted with a BCR. Its resistance to water pressure is given to it by the weight of its reinforced concrete structure.

In addition, he is equipped with a wall with a flat surface that allows water to be distributed over the entire surface. The gravity dam only rests on the ground. It is therefore essential that it is really very solid. This is why this type of dam is generally built in large valleys.

The arch dam

The arch dam owes its name to its curved shape. It is found exclusively in narrow valleys with rocky shores. It is built to close the valley so as to form an accumulation lake and a waterfall.

The arch dam is oriented towards the lake, this allows it to channel the force of the water push to the rocky shores via a diversion channel. Since the power of the fluid is directed towards the sides of the dam, the dam's natural walls have every interest in being very solid.

The dam at the foothills

Also built of reinforced concrete, the buttress dam still requires fewer materials than the others. Installed at the back of the dam and firmly anchored to the rocky ground, the foothills serve to support a flat wall.

The whole is oriented towards the accumulation lake. The sides of the structure are also anchored to the rocky ground. The latter must therefore be resistant to allow the dam to weather resistant.

Hydroelectric power: one of the most promising renewable energies

Hydroelectric dams require very large spaces. As a result, during their construction, many populations are displaced and several agricultural lands are flooded. However, their environmental impact is negligible compared to the enormous potential represented by the exploitation of Hydraulic power.

In fact, it should be noted that in addition to the potential energy of watercourses, it is also possible toHarnessing tidal energy. This is the case, for example, of the Rance dam, which feeds The Rance tidal power plant and which draws energy from the power of the tide.

Dam and tidal power plant in La Rance (Ille-et-Vilaine)/Source: Telegram

Better still, hydroelectric power is constantly evolving. Today, China has the second largest hydroelectric power plant in the world (located in Baihetan, 289 meters high), a structure intended to eventually cover more than 2.6% of the country's electrical energy needs. This is due toimprovement of production systems and the construction of new dams and new tidal power plants. In addition, turbine efficiency is now close to 100% thanks to the modeling of fluid dynamics and the reduction of manufacturing thresholds.

In addition, advances in lubricating materials science, gearbox design, driver efficiency, etc. All of these factors make hydroelectric power rightly considered to be renewable energy The most important of the world.

To go further, do not hesitate to consult our article to understand everything about controllable energy.