How does a pumped-storage hydroelectric plant help balance electricity supply and demand?

• A. It generates energy only when water is present.
• B. It uses batteries to store energy chemically.
• C. It stores energy by pumping water uphill during high demand.
• D. It stores excess energy by pumping water uphill during low demand and releases it through turbines to generate electricity during high demand, balancing supply.

Answer: (D)

The key idea is using stored gravitational potential energy to balance supply and demand on the grid. In a pumped-storage plant, when electricity is plentiful and demand is low, pumps push water from a lower reservoir up to an upper reservoir, storing energy as elevated water. Later, when demand spikes, the stored water is released back down through turbines to generate electricity quickly. This provides a fast, controllable source of power to match the grid to real-time needs and smooth out shortages or surges.

This mechanism relies on energy storage in water, not chemical storage like batteries, and it operates best by storing energy during low demand and releasing it during high demand. Pumping during high demand would waste energy and fail to address the peak, which is why that option isn’t correct.