Pumped-storage hydropower is a large-scale energy storage system that works like a giant rechargeable battery. It uses two reservoirs at different heights, a pump-turbine, a generator, and a large pipe called a penstock. When extra electricity is available, the system pumps water uphill to store energy as gravitational potential energy.
When electricity is needed, water flows downhill and spins a turbine to generate power.
Key Facts
- Stored gravitational energy is E = mgh, where m is water mass, g is 9.8 m/s^2, and h is height difference.
- Electrical power from falling water can be estimated by P = ηρgQh, where η is efficiency, ρ is water density, Q is flow rate, and h is head.
- Pumped storage does not create energy, it shifts energy from low-demand times to high-demand times.
- Round-trip efficiency is often about 70% to 85%, so some energy is lost as heat and friction.
- Higher head or greater flow rate increases the power output of the system.
- A reversible pump-turbine can pump water uphill in storage mode and spin as a turbine in generation mode.
Vocabulary
- Upper reservoir
- The high-elevation water storage area where energy is stored as gravitational potential energy.
- Lower reservoir
- The low-elevation water storage area that receives water after it flows through the turbine.
- Penstock
- A large pipe that carries water between the reservoirs and the pump-turbine station.
- Pump-turbine
- A reversible machine that can pump water uphill or be spun by falling water to help generate electricity.
- Head
- The vertical height difference between the upper reservoir and the lower reservoir.
Common Mistakes to Avoid
- Thinking pumped storage makes free energy, which is wrong because it uses electricity to pump water uphill and only returns part of that energy later.
- Confusing head with pipe length, which is wrong because head is the vertical height difference, not the diagonal distance through the mountain.
- Ignoring efficiency, which is wrong because friction, turbulence, motor losses, and generator losses reduce the energy returned to the grid.
- Assuming more water always means more power, which is incomplete because power depends on flow rate, head, water density, gravity, and efficiency.
Practice Questions
- 1 A pumped-storage plant lifts 2.0 x 10^6 kg of water by 300 m. How much gravitational potential energy is stored? Use g = 9.8 m/s^2.
- 2 A plant has head h = 250 m, flow rate Q = 80 m^3/s, water density ρ = 1000 kg/m^3, and efficiency η = 0.80. Estimate the electrical power output using P = ηρgQh.
- 3 Explain why pumped-storage hydropower is useful for a power grid with solar and wind energy, even though it loses some energy during each storage cycle.