How They Work: Pumped-Storage Power Plants

Pumped-storage power plants are reversible hydroelectric facilities where water is pumped uphill into a reservoir. The force of the water flowing back down the hill is then harnessed to produce electricity in the same way as conventional hydroelectric plants. Their ability to store electricity makes them an effective tool to overcome the intermittent nature of wind and solar power.

How They Work: Pumped-Storage Power Plants
Turbines in the Grand'Maison hydropower plant in the French Alps, the most powerful of its kind in France. The two reservoirs, an upper and a lower, together form a pumped-storage power system. ©JEAN-PIERRE CLATOT / AFP


Pumped-storage powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... plants are structured around two bodies of water, an upper and a lower reservoir1 (see the diagram below).

At times of very high electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... consumption on the grid, the water from the upper reservoir, carried downhill by a penstock, drives a turbine and a generator to produce electricity, which is used to meet the increased demand.

When demand is low, electricity is taken from the grid to power a pump that sends water from the lower reservoir back up to the upper reservoir, where it can be discharged again to drive the turbine. In this way, the water in the upper reservoir constitutes a stock of gravitational potential energyEnergy contained in an object or physical system that has the potential to be converted into kinetic energy... , ready to be used when needed.

The stored energy is proportional to the volume of water and the height from which it falls.

Schema explaining how Pumped-Storage Power Plants work

A Partial Solution to Intermittency

Pumped-storage power plants were first developed in the 1970s to improve the way major thermal and nuclear power plants dealt with widely fluctuating demand for electricity at different times of the day.

There is now fresh enthusiasm for them worldwide due to growing use of intermittent renewable energyEnergy sources that are naturally replenished so quickly that they can be considered inexhaustible on a human time scale... such as wind and solar power. Depending on wind strength and light conditions, electricity production can at certain times exceed grid demand. The surplus electricity can then be used to power the pumps.

As with any system, the key is to strike the right economic balance: the cost of pumping during off-peak periods should be significantly lower than the sale price at peak times, especially since some energy is lost during the process2.

Pumped-Storage Power Plants Around the World

Pumped-storage power plants store electricity using water from dams.

The new model for using the plants in combination with renewable energy has led to a revival of the technology in China, the United States, Europe (particularly Germany and Norway), the United Arab Emirates, Morocco and other countries. In 2000, there were around 30 pumped­storage power plants with a capacity of more than 1,000 megawatts worldwide. In 2020, that number will have more than doubled. There are currently 170 units in Europe, but generally with a capacity of a few hundred megawatts.

The largest power plants need to pump every 20 to 40 hours. Their form of storage now represents 95% of all electricity storage worldwide even though the proportion from high­capacity electrochemical batteries is steadily increasing. However, these plants are still only useful on a small scale and cannot cover wide seasonal variations in demand. In France, where annual electricity consumption is around 500 terawatt-hours, the country's six pumped­storage power plants3 produce on average five terawatt-hours per year.

Future Prospects

Two barriers are preventing more pumped-storage power plants from being set up – first, the significant financial investment required, and second, the impacts on the environment and the landscape.

Pumped-storage power plants are generally built in the mountains, but coastal power plants using seawater are now emerging as a new model. These can be useful for recovering electricity from large offshoreRefers to sea-based oil exploration and production operations, as in "offshore license" or "offshore drilling". wind farms, or for ensuring that islands are energy independent. For the system to work, the upper basin, located on the top of a cliff or behind a dam, must be at least 100 meters higher than the sea, which acts as the lower basin.

One such plant has been set up on the Canary Island of El Hierro, for example4 (see photo). It uses the excess energy produced by five wind turbines to pump water up to a reservoir 700 meters above sea level. EDF is also considering projects on Réunion Island, in Guadeloupe and in Martinique.

Many companies are working on designs for micro pumped-storage power plants with small tanks on two levels, which could provide energy for remote villages in the mountains, for example.


Station STEP de l’ile d’El Hierro

The pumped-storage power plant on the island of El Hierro, with an upper basin connected to the sea through a pump-turbine system. In the background are the wind turbines that power the pumps. ©DESIREE MARTIN / AFP 




 (1) More information from EDF (in French only)

(2) For the turbine to produce 1 megawatt-hour of power, an average of about 1.25 megawatt-hours has to be consumed to pump the water up to the upper reservoir.

(3) Grand’Maison (1,790 megawatts) and Le Cheylas (460 megawatts) in Isère, Montézic (910 megawatts) in Aveyron, Revin (800 megawatts) in Ardennes, and Super-Bissorte (730 megawatts) and La Coche (330 megawatts) in Savoie.

(4) Sciences et Avenir (in French only)