A 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. Its mission is also to convey large quantities of water to turbines which in turn spin generators that produce electricity. How does this type of barrier work? What is its environmental impact?
How does a hydroelectric 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 process of transformation that takes place in several stages.
The accumulation lake
Depending on its power, a natural water current (river, river, lake) can release a certain amount of potential energy. The latter is generally in the form of mechanical energy whose flow is concentrated in 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 traps its potential energy. The speed and depth of the water passing through the dam pipe creates a hydraulic head that spins the turbine of the hydroelectric plant.
Starting the turbine
Turbines are made up of mechanical blades of various shapes and sizes. Large reaction turbines like the Francis turbines (found on the Three Gorges Dam in China) have large blades that require very high voltages and large motors to spin.
For example, to turn, the Francis turbine relies on the hydraulic head. The latter is created by the hydrostatic pressure resulting from the potential energy of the dam and increases with the speed of the water.
In other words, the hydraulic head of a hydroelectric dam depends exclusively on its height and the water flow. The rotation of the turbine transforms the hydraulic head into kinetic energy, which eliminates the speed and hydrostatic pressure of the fluid. From this point on, the water flows peacefully at the base of the dam.
Transformation into electricity
It is the rotating motion of the turbine that transforms the kinetic energy of the water into electrical energy. A hydraulic turbine can be coupled directly to an electric generator or thanks to a transmission or a gearbox which drives the shaft as well as the armature of the generator. The brushes and commutator, on the other hand, capture the flow of electricity generated by the rotating armature 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 large torque forces. Thanks to a transmission system 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 thus produced is converted into voltage and then transported to consumers via high and very high voltage power lines. In concrete terms, the step-up generators of hydraulic dams use the output voltages of low-intensity generators to generate voltages that can be used directly by households. This is particularly the case for run-of-river power plants.
Hydroelectric plants also generate higher voltages for long-distance electrical transmissions. In addition, the lower the electric current, the more effective its voltages. For example, through the use of 500 kV transmission in electric transmission, the Glen Canyon Dam in Paige, Arizona can supply electricity to households located approximately 1500 km north of Nebraska.
The different types of hydroelectric dams
In order not to collapse under the pressure of the water, a hydraulic dam must adapt perfectly with the geology of the ground where it is installed. This is the reason why there are different types of hydroelectric dams.
The gravity dam
With an imposing mass, the designed gravity dam is made of earth, concrete and rock then compacted with RCC. Its resistance to water pressure is given to it by the weight of its reinforced concrete structure.
In addition, it has a wall with a flat surface which allows water to be distributed over the entire surface. The gravity dam rests only on the ground. It is therefore essential that it is really very solid. This is why this type of dam is usually 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 banks. 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 thrust of the water towards the rocky shores via a diversion channel. The power of the fluid being directed towards the sides of the dam, the natural walls of the latter have every interest in being of great solidity.
The buttress dam
Also constructed of reinforced concrete, the buttress dam still requires less material than the others. Installed at the back of the dam and firmly anchored on the rocky ground, the buttresses serve as support for 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 withstand bad weather.
Hydroelectric energy: 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 hydropower.
It should indeed be specified that apart from the potential energy of rivers, it is also possible to exploit the energy of the tides. This is the case, for example, of the Rance dam which supplies the Rance tidal power plant and which draws energy from the power of the tide. Rance dam and tidal power plant
Rance dam and tidal power plant (Ille-et-Vilaine) / Source: Télégramme
Better, the hydroelectric power is in constant evolution. Today, China has the second largest hydroelectric power station in the world (located in Baihetan 289 meters high), a structure intended to cover more than 2.6% of the country’s electrical energy needs in the long term.
This is due to the improvement of production systems and the construction of new dams and new tidal power stations. In addition, the efficiency of the turbines is now close to 100% thanks to the modeling of fluid dynamics and the reduction of manufacturing thresholds.
Added to this are advances in lubrication materials science, gearbox design, driver efficiency, and more. All of these factors make hydroelectric power rightly considered the most important renewable energy in the world.