Describe how pumped hydro storage helps balance electricity supply and demand.

• A. It stores energy by compressing air in underground caverns
• B. It stores energy as chemical energy in large batteries
• C. It stores water in elevated reservoirs with no release mechanism
• D. It stores excess energy by pumping water uphill to a reservoir; releases water through turbines to generate electricity when demand rises or wind/solar falter

Answer: (D)

Pumped hydro storage balances electricity supply and demand by turning excess electrical energy into gravitational potential energy and then converting it back when needed. When there’s plenty of generation or low demand, electricity is used to pump water from a lower reservoir up to a higher one, storing energy as water at height. Later, during high demand or when intermittent wind/solar drops, that stored water is released through turbines to generate electricity again. This setup provides large-scale, rapid ramping to match real-time demand and smooth out fluctuations, helping keep the grid stable.

The other ideas describe different storage approaches. Compressing air in underground caverns stores energy in a different form and isn’t how pumped hydro works. Storing energy as chemical energy in batteries is a separate technology with its own characteristics. Storing water with no release mechanism wouldn’t actually provide usable electricity. And the concept here specifically uses pumping and then releasing water through turbines to generate power.