Hydropower plants

Pumped-storage hydropower plants; how do they work?

Its main mission is storing huge amounts of water when the demand is low. At the same time, the plant takes advantage of it to generate energy during peak hours. The largest one in Europe is La Muela II, launched by Iberdrola on the Júcar River, in the municipality of Cortes de Pallás, Valencia. Nowadays, the company also develops the ambitious Támega project in northern Portugal.


Pumping hydroelectric technology is currently the most efficient system for storing energy on a large scale. It is more profitable and provides stability, safety and sustainability to the electrical system. It also generates a big amount of energy at a very fast response time; all of this without releasing any type of emissions into the atmosphere.

Iberdrola is the leader in energy storage with a capacity of 4,400 MW by means of pumping technology. It also presents a much higher yield than the best batteries in the market. By 2022, the company aims to reach 90-gigawatt hours (GWh) of storage capacity. That would represent an increase of 20 GWh compared to 2018.

Inside a pumped-storage hydropower plant

In the off-peak hours, usually during the night on weekdays and on weekends, excess energy is used – which also has a lower cost in the market at that time – to raise the water contained in the reservoir located at the lowest level to the upper tank. This is achieved by a hydraulic pump that makes the water rise through a forced pipe and ino the driving gallery. The upper reservoir then acts as a storage tank.

The pumping station works as a conventional hydroelectric plant during the peak hours; that is, during the day. The water accumulated in the upper reservoir closed by a dam is sent through the driving gallery to the lower reservoir. In this jump, the water goes through the forced pipe. The water gains kinetic energy that is then transformed into rotating mechanical energy in the hydraulic turbine. Later, this is converted into electrical energy of medium voltage and high intensity in the generator. In order to regulate the water pressures between the previous pipes, sometimes an equilibrium chimney is built.

The next step is the transformers. They send the electricity produced in the power plant through high voltage transmission lines until reaching homes and industries on the electricity network that consume it.

When the electricity is generated, the water goes through the drainage channel to the lower reservoir, where it is stored again.

Because of all of these factors, pumped-storage hydropower plants are efficient in energy storage. They also represent a long-term solution, favour the integration of renewables in the system and with remarkable profitability.


Great examples of pumped-storage hydropower plants

The largest pumping hydroelectric plant in Europe is La Muela II, in the Cortes de Pallàs dam, on the bank of the Júcar river. Its annual output is around 800-gigawatt hours (GWh), enough to meet the electricity consumption of almost 200,000 households. It doubles the generation of the Cortes-La Muela complex to 1,625 GWh – where the annual demand is almost 400,000 households. The plant has four groups of reversible turbines inside a cavern that allow it to take advantage of the difference in height of 500 meters between the artificial deposit of La Muela and the Cortes de Pallás dam to produce electricity.

Another of the major pumping initiatives of the Spanish group is the Támega project. It will require the construction of three new plants on the homonymous river. The Támega is a tributary of the Duero located in the north of Portugal, near Porto. The three plants will add an installed capacity of 1,158 MW. This will mean an increase of 6% of the total installed electric power in the country. The complex will be able to produce 1,766 GWh per year, enough to meet the energy needs of neighbouring towns and the cities of Braga and Guimarães (with an approximate demand for 440,000 homes).