Pumped Storage Hydroelectricity: The Giant Water Battery Powering the Grid

Grady explains pumped storage hydroelectricity, a grid scale energy storage method that uses gravity to store energy as water in an upper reservoir and release it through turbines to generate power during peak demand. The talk covers why electricity storage is challenging, how solar creates the duck curve and spikes in the evening, and why peaking plants remain essential despite renewables. A hands on demonstration shows pumping water up at night with a low power pump and generating power from water falling back through the turbine during the day. It also discusses site limitations, efficiency differences between small scale experiments and large scale facilities, and future directions like seawater and demand management to balance generation and demand.

Overview

Electric grids must balance generation and demand in real time. Solar and wind are variable, creating duck curves where demand spikes in the evening while solar fades. Storage is a key solution, and pumped storage hydroelectricity is the dominant grid scale approach that uses gravity to store energy as water at a higher elevation and release it through turbines when needed.

How pumped storage works

Two reservoirs or pools are kept at different heights. During cheap off peak times, electricity powers pumps that lift water to the upper reservoir. When demand rises, water flows back through turbines to generate electricity. It is like a giant water battery that can respond quickly to fluctuations and provide fast ramping capability as well as emergency power. The energy stored depends on the head and the volume of water, not just the water mass.

Demonstration and measurements

In the demonstration, a small aquarium pump fills the upper reservoir on a ladder. The energy used to fill measured nearly 0.7 watt hours, while the water bucket stores roughly 0.1 watt hours of gravitational potential energy. The turbine recovery was only a few milliwatt hours, yielding an efficiency of about 0.3 percent in this tiny setup. This illustrates that at small scales, pump storage is not practical due to inefficiencies and equipment losses.

Efficiency and scale

Large pump storage facilities typically achieve efficiencies of 70 percent or higher, making them net energy consumers but economically viable when pumping costs are below the sale price of stored energy. The advantages include fast response, grid stability, and resilience in emergencies, and their value is especially pronounced on isolated grids such as islands with limited generation diversity.

Challenges and future directions

Key challenges include energy density and site requirements. Pump storage needs two pools and a large vertical separation, often requiring major civil engineering projects. Future directions include using seawater and improved demand management to shave or shift load, enabling higher overall grid efficiency as renewable capacity grows.