We often do not want to use all the energy we harvest right away. This is especially true as we continue to transition toward renewable energy sources that are intermittent (versus burning coal and gas, for which we can mostly control when we burn it). We want to be able to convert excess electricity into another energy form and then convert this energy back to electricity when needed, with little loss in usable energy during the process and low cost. As energy usage continues to grow and policies increasingly support renewable energy, energy storage capacity must increase greatly.
As of 2024, over 95% of all energy storage in the U.S. is in the form of pumped hydropower, which consists of two water reservoirs at different elevations. Excess electricity is used to pump water uphill to the higher reservoir. When electricity is needed, water is released and allowed to flow to the lower reservoir, converting the water’s potential energy to kinetic energy. The water moves a turbine, converting its kinetic energy to mechanical motion, and then to electrical energy through a generator. Pumped hydro will likely continue to dominate grid-scale energy storage in the coming decades.
However, a variety of energy storage systems are needed to satisfy the increasing demand for energy storage. Different systems include compressed air energy storage, flywheels, thermal energy storage using molten salt, and batteries, each of which has its unique advantages. Compared to pumped hydro, batteries offer shorter discharge times, faster installation, and higher efficiencies. In contrast to pumped hydro, batteries also do not depend on landscape and can be implemented anywhere.