Do you know what pumped-storage hydropower stations are used for?

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Their main purpose is to store water during off-peak periods, using it to generate energy during peak consumption periods. La Muela II is the largest such installation in Europe commissioned by Iberdrola, currently also developing the ambitious Tâmega project in northern Portugal.

Hydroelectric pumping technology is the most efficient system that allows to store energy in a large-scale today. It is more cost-effective and provides the electrical system with stability, safety and sustainability, whilst generating large amounts of energy with fast response times without resulting in any type of emission into the atmosphere.


Pumped storage hydropower station.#RRSSOperation of a pumped-storage hydropower station.

 SEE INFOGRAPHIC: Operation of a pumped-storage hydropower station [PDF]

This kind of electric plant relies on two reservoirs at different altitudes that allow water to be stored when demand is low and then used to generate energy during peak consumption times in order to meet overall electricity demand.

During off-peak hours, typically weekday and weekend nights, excess generation capacity is used — which also has a lower market cost — to pump water from the lower level (1) reservoir to the upper one by means of a hydraulic pump that propels the water through a penstock (2) and conduction tunnel structure. The upper reservoir (3) therefore acts as a storage facility.

During peak hours, i.e. daylight hours, the pumping station functions as a conventional hydroelectric plant: water accumulated in the upper reservoir closed by a dam (4) is sent to the lower reservoir through the conduction tunnel (5). In this step the water passes through the penstock, where it acquires kinetic energy which is transformed into rotational mechanical energy in the hydraulic turbine (6). This is then turned into medium voltage, high intensity electricity in the generator (7). In some cases, a surge tank (8) is built to regulate the water pressure between the intake and discharge tunnels.

The next step sees the transformers (9) transmit the electricity produced in the plant, for consumption by the homes and industries that form part of the grid (10) via high voltage transmission lines.

Once the electricity is generated the water falls through the drainage channel (11) to the lower reservoir, where it is stored again.

Pumped hydroelectric plants thereby bring efficient energy storage, offer a long-term solution and facilitate the integration of renewable energies into the system and bring high returns.


La Muela II is the largest pumped-storage hydropower plant in Europe, located on the Cortes de Pallàs reservoir on the right bank of the Júcar river. Its annual output is around 800 gigawatt hours (GWh), enough to provide the electric consumption for close to 200,000 households, duplicating that of the Cortes-La Muela complex to reach 1.625 GWh — the annual demand of almost 400,000 families —. The plant features a hall with four groups of reversible turbines, allowing it to take advantage of the 500 metre altitude difference between the artificial La Muela dam and the Cortes de Pallás reservoir to produce electricity.

Another great pumping initiative by the Spanish group is the Gouvães plant, one of the three that will form the Tâmega hydroelectric complex in northern Portugal. Still under construction, the three power plants will have a total installed capacity of 1,158 MW and will be capable of producing 1,766 GWh per year.