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Hydroelectric power stations

1.What is a hydroelectric power station?

A hydroelectric power station is an installation allowing taking advantage of moving water masses flowing in the rivers to transform them into electricity. Depending on the power installed, the hydroelectric power stations can beGreat power hydraulic power stations(more than 10MW of electric power), Hydraulic mini central power stations: (beetween 1MW and 10MW) and Hydroelectric micro power stations (less than 1MW of power) 

2.Types of hydroelectric power stations 

There are many types of hydroelectric power stations, as the characteristics of the terrain where the power station is located largely condition its design. We could have a classification of three basic models:

  • Moving Water Power Stations: In this case there is no reservoir, the terrain does not have a much of a slope and it is necessary for the river flow to be constant enough in order to guarantee a determined level of power throughout the year. During the rainy season they generate their maximum power and have to release excess water. However, in the dry season the power decreases depending on the flow, in some rivers being almost zero during the summer.

  • Reservoir Power Stations: By means of the construction of one or more dams making up artificial lakes, a considerable volume of water is stored upstream of the turbines. The reservoir allows regulating the amount of water going through the turbines. With a reservoir electricity can be produced throughout all the year even if the river fully dries during some months; this would not be possible with a moving water power station. These power stations generally require a highest investment than moving water stations. Within this type there are two possibilities:

  • Power Stations built in the dam itself: in a section of a river with a noticeable difference in the level of the water a dam is built at a certain height. The turbine room is located after the dam.

  • Diverted water Power Stations:the river water is diverted by means of a small dam and are driven using a canal, with a minimum difference in the water level, to a small deposit called a forebay or pressure chamber. From here there is a forced pipe that goes to the turbine room. Then, the water is returned downstream using a discharge canal. Greater differences in level are achieved than in the power stations on the dam itself. 

  • Pumping or reversible power stations: This is a special type of power station that makes a more rational use of the hydraulic resources. They have two reservoirs located at different level. When the daily demand of electric energy is maximum these power stations work as a conventional hydroelectric power station: the water falls from the top reservoir making the turbines turn and then it is stored in the lower reservoir. During the hours with less demand, the water is pumped back to the top reservoir to start the production cycle again.

3. Advantages and disadvantages of the hydroelectric power stations

The advantages of the hydroelectric power stations are the following:

  • They do not require fuel and are clean.

  • On many occasions the reservoirs of the power stations have multiple and important uses: irrigation, protection against flooding or supply of water to nearby towns and cities.

  • They have low exploitation and maintenance costs.

  • Las obras de ingeniería que se realizan para construir la central son de larga duración. 

  • The hydraulic turbines are easy to control and have low maintenance costs .  

On the other hand, the following disadvantages can be listed:

  • The investment cost per installed kilowatt is high.

  • In general, they are located in places at a great distance from the consumption points, and therefore, the investment costs in transport infrastructure can be high.

  • The construction time is, in general, longer than that of the other types of electric power stations.

  • The generation of electric energy is influenced by the meteorological conditions and can vary from season to season.

  

4. Main components of a hydroelectric power station

  • The dam: is the first element we find in a hydroelectric power station. It retains the river water and stores it in a reservoir. With the construction of a dam a certain difference in height of the water is achieved, which is used to generate energy. The dam is an essential element and mainly depends upon the orography of the terrain and of the water course where it is to be located. The dams can be classified depending on the material used in their construction, in earth dams and in concrete dams. The concrete dams are the most resistant and the most used. There are three types of concrete dams depending on their structure: 

  • Gravity dams: They are triangular concrete dams with a wide base that is narrower at the top. They are long-duration construction and do not need maintenance. The height of this type of dams is limited by the resistance of the terrain.           

  • Arch dams: In this type of dams the wall is curved. The pressure caused by the water is fully transmitted to the valley walls due to the arch effect. When the conditions are favourable, the structure needs less concrete than a gravity dam, but it is difficult to find places to build them.

  • Buttress dams: They have a wall supporting the water and a series of triangular shaped buttresses or pillars, holding the wall and transmitting the water load to the base. In general unstable terrains are not used, and they are very uneconomical.

  • Spillways: which allow the flow of part of the water retained without it going through the machine room.

  • Energy destructors: which are used to prevent the energy from the water falling from the dam discharges which are at a height to create considerable erosion in the terrain when reaching the ground. Basically we have two types of energy destroyers:

  • The Teeth or concrete prisms which cause an increase in the turbulence and in eddying.

  • The Deflector which dissipate the energy increasing the friction of the water with the air and by means of a collision with the water cushion encountered when free-falling.

  • Conduits: The supply of water to the turbines is carried out by means of complex channelling systems.

In the case of the channels, these can be carried out by excavating the terrain or artificially by means of concrete structures. Their distribution is always conditioned by local geographical conditions, That is why the best solution is the construction of a load tunnel, although the investment cost is higher.

The final part of the water course from the loading chamber to the turbines is carried out using a forced pipe. Steel is used for the construction of these pipes for water falls of up to 2000m and concrete for water falls of up to 500m 

  • Valves: devices allowing the control and regulation of the circulation of water along the pipes.

  • Balance chimneys: they are pressure wells of the turbines used to prevent the so-called "water hammering", which is produced when there is a sudden pressure surge due to the quick opening or closing of the valves in a hydraulic installation..

  • Machine room: Construction where the machines are installed (turbines, alternators…) as well as the regulation and control elements of the power station.

  • Turbine: The hydraulic turbines are the essential element for the exploitation of the energy in the hydraulic power stations. They transform the mechanical energy into kinetic energy (fruit of the movement) of a current of water. Its most important component is the rotor, which has a series of blades pushed by the force produced by the water in movement, making it turn. The hydraulic turbines can be classified into two groups:

  • Action turbines: They are those in which the water pressure energy is fully transformed in kinetic energy. Their main characteristic is that the water has the maximum pressure in the input and output of the roller. An example of this type are the Pelton turbines.

    • Turbine Pelton: It is also known as pressure turbine. They are suitable for high water falls and with relatively small flows. The most frequent installation is with that of a horizontal shaft layout.

  • Reaction turbines: These are the turbines in which only a part of the water pressure energy is transformed into kinetic energy. In this type of turbine, the water has less pressure at the output than at the input. An example of this type are the Kaplan turbines.

    • Francis turbine : It is known as the inward-flow reaction turbine, as the pressure is variable in the roller areas. The Francis turbines can be used in different height water falls within a wide margin of flow, but they have an optimum performance when they work in a flow between 60 and 100% of maximum flow. They can be installed with the shaft in a horizontal position or vertical position, but in general, the most usual position is that of a vertical shaft

  • Kaplan turbine: Total admission and reaction turbines. They are used in lower height water falls with medium and large flows. They are normally installed with the shaft in a vertical position, but can also be installed horizontally or sloped.
  • Alternator: Type of electricity generator destined to transform the mechanical energy into electric energy.

5. Operation of a hydroelectric power station

The dam, located in the course of a river, artificially accumulates a volume of water to make up a reservoir. This allows the water to obtain a potential energy which will be then transformed into electricity. For this, the dam is located upstream, with a valve that allows regulating the water input to the pressure gallery; prior to a forced pipe that takes the water to the turbine to the machine room in the power station.

The pressurised water of the forced pipe transforms its potential energy into kinetic (that is to say, loses force and gains speed).

Upon arriving to the machine room the water acts on the vanes of the hydraulic turbine, transforming its kinetic energy into rotation mechanical energy. The shaft of the turbine is linked to that of the electricity generator, which when turning transforms the rotation energy into medium voltage alternating current. Once it has lost its energy the water is returned to the river downstream from the power station by means of a drainage canal. 

6. Environmental impact of the hydroelectric power stations

It has always been considered that the electricity of a hydraulic origin is a clean alternative energy. Even so, there are some environmental effects due to the construction of the hydroelectric power stations and their infrastructure. The construction of dams, and by large, the creation of reservoirs cause an environmental impact that go from the upper limits of the reservoir to the coast. This impact has the following consequences, many of them irreversible:

  • Floods land, changing the territory.

  • Modifies the life cycle of the fauna.

  • Makes river navigation difficult as well as the transport of material downstream (nutrients and food, such as lime and clay).

  • Decreases the river flow, modifying the level of the phreatic layers, composition of the water in the reservoir and the microclimate.  

The environmental and social costs can be reduced or avoided to an acceptable level, if they are carefully assessed and corrective measures are implemented.

For all this, it is important that at the moment of building a new dam, the possible environmental impacts are very carefully analysed considering the creation of a new reservoir.