Pumped-Storage Hydropower

Pumped-storage hydropower stores electricity indirectly as gravitational potential energy in water. Surplus electricity pumps water from a lower reservoir to an upper reservoir; when electricity is needed, the water flows downhill through turbines to generate power. It is therefore an energy-storage system, not a primary energy source.

Charging and discharging cycle

The system uses two reservoirs separated by an elevation difference called the hydraulic head. Plants may use reversible pump-turbines or separate pumping and generating equipment.

• During charging, electricity operates pumps that lift water to the upper reservoir, increasing its potential energy.
• During discharging, water descends through penstocks and turbines, converting potential energy into electricity.
• Pumping and generation involve round-trip energy losses, so the plant consumes more electricity than it later returns.

Role in renewable electricity balancing

Pumped storage shifts electricity across time and helps match variable renewable generation with demand. Its output duration depends mainly on stored water volume, hydraulic head and plant capacity.

• It can absorb surplus solar or wind electricity that might otherwise be curtailed.
• It can discharge during evening peaks, renewable shortfalls or sudden increases in demand.
• Its controllable pumping and generation can provide frequency regulation, operating reserves and other grid-balancing services.
• Unlike batteries, its storage medium is water, while the reservoirs and civil works generally determine its scale and operating life.

Environmental assessment and safeguards

A pumped-storage project requires site-specific assessment because reservoirs, tunnels, transmission infrastructure and construction can create significant local impacts.

• Assessment should examine land and forest diversion, habitat fragmentation, aquatic ecology, altered river flows, excavation spoil, water availability and community displacement.
• Projects using existing reservoirs or purpose-built off-stream reservoirs may reduce some impacts, but do not eliminate ecological and social costs.
• The environmental impact assessment should compare alternatives, evaluate cumulative impacts and prescribe mitigation, monitoring and dam-safety measures under applicable laws.