What is a pumped-storage hydroelectric power plant?

How does a pumped-storage plant work?

Pumped-storage plants have two reservoirs at different elevations, connected by pipes and reversible turbines. Their operation involves two main phases:

This cycle allows excess renewable energy to be harnessed and stored for later use, maximising its integration in the electricity system.

Technical components of a reversible pumped-storage plant

These plants rely on a range of infrastructure elements to operate effectively:

• Upper reservoir (1) and lower reservoir or river (11): act as water storage basins. The upper reservoir stores water when there is surplus energy, while the lower reservoir collects it after it passes through the turbines.

• Dam (2): holds water in the upper reservoir and regulates flow toward the intake tunnel.

• Intake tunnel (3): underground channel that directs water from the upper reservoir to the penstock.

• Surge shaft (4): a safety feature that absorbs pressure surges during sudden changes in water flow.

• Penstock (5): a pressurised channel that carries water to the turbines, converting potential energy into kinetic energy.

• Turbines (6): hydraulic machinery that works in two modes—generating electricity when water flows down and pumping water back up using electrical energy.

• Generator (7): converts mechanical energy from turbine movement into electricity.

• Transformers (8): adjust the voltage of the electricity for transmission.

• Outlets (9): channels that return water to the lower reservoir or river.

• Electricity transmission lines (10): transport the generated electricity to consumption points.

A key feature of these plants is their rapid response capability—they can inject electricity into the grid in less than two minutes, significantly faster than other storage technologies.

Advantages of pumped-storage hydroelectric plants

Their ability to store large volumes of energy enables better management of demand peaks. They help ensure a stable supply and optimise the use of renewable energy sources.

Their storage capability prevents excess wind or solar energy from going to waste while facilitating a smoother integration of renewables into the electricity system.

They also offer longer lifespans and lower operational costs compared to other storage technologies. Plus, they help reduce CO₂ emissions. 

Examples of pumped-storage hydroelectric plants

Endesa operates several reversible pumped-storage plants in Spain, which play a key role in energy storage and grid management, such as:

Tajo de la Encantada H.P.P. (Malaga)

• Installed capacity: 360 MW.

• Two reservoirs connected by a system of pressurised pipes.

• Key to grid regulation in southern Spain.

• Stores surplus renewable energy and releases it during critical periods.

Other types of hydroelectric plants

Besides pumped-storage plants, there are other types of hydroelectric power plants:

Run-of-river plants use the natural flow of the river to generate a very consistent supply of electricity. Since they don’t rely on reservoirs, their output depends on the river’s flow. This makes them highly efficient where flow is stable.

Regulating plants have reservoirs that allow for water storage and energy generation control, giving them the flexibility to adapt to demand variations and maintain supply even during droughts.

Reversible pumped-storage plants, with their two-reservoir system at different elevations, can store excess energy for use during peak demand and operate in both turbine and pumping modes.