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Hydroelectric plants

From water to renewable energy: the functioning and types of hydroelectric power plants.

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Hydroelectric power plants transform the hydraulic energy of a river, either natural or artificial, into renewable electrical energy. The process takes place in several steps and starts with the transformation of the potential energy in masses of water that are placed at higher altitudes than where the plant’s turbines are.

How it works

In general, the process involves a barrier – a dam or a crossbeam – that interrupts the water flow, creating a body of water that can be a reservoir or a hydroelectric basin. Through adduction and diversion channels and tunnels, the water is conveyed into charge basins and is directed through penstocks toward hydroelectric turbines. Here, injection valves (safety) and flow regulation devices (distributors) are used, according to energy demand.

The water activates the turbines, generating mechanical energy, and ends up in a spillway, which returns it to the waterway. A rotating electric generator (alternator) is directly connected to the turbine and turns the mechanical energy from the turbine into electrical energy. This electricity has to be transformed in order to transmit it long distances: before being conveyed into transmission lines, the electrical energy passes through a transformer, which lowers the intensity of the current produced by the rotating electric generator, while raising its voltage.

Once it reaches the place it will be used, the energy passes through a transformer again, which raises the intensity of the current again and lowers the voltage, making it suitable for industrial, commercial and domestic uses. 

Hydroelectric plants: 1. Pullinque (Chile) | 2. Rock Creek (USA) | 3. Moyopampa (Peru)


Hydropower plants can be divided into three macro categories, according to the type of system used: run-of-river power stations, conventional hydropower stations and pumped storage power stations.

In run-of-river power stations, the natural flow of a river is used, on two different levels. The water is conveyed through a diversion channel without the aid of penstocks and reaches the turbines. The plant’s power mainly depends on the speed of the water through the passage from one level to the other, the so-called jump, and on the flow of the river.

In conventional hydropower stations, an upstream basin is used, the surge chamber, that can be natural – like a lake – or created with a dam. Penstocks convey the water from the dam toward hydraulic turbines, which generate mechanical energy by turning. This energy is then converted into electrical energy by a rotating electric generator.

There is a pool downstream, where the turbulent water that just went through the turbines is calmed before being returned to the normal flow of the river. The existence of an upstream reservoir, unlike run-of-river systems, allows for the control of water flows and therefore of the linked renewable electrical production.

Pumped storage power stations have two reservoirs at different altitudes, one upstream and one downstream, the second of which serves as an energy reserve. During times of lower energy demand, the water is brought up from the downstream basin to the upstream basin through a pumping station, allowing the system to deal with higher energy demand safely. In some plants, it’s possible to use the reversibility of Francis turbines to convert them into pumps and return the water to the upstream basin. 

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