DOE study probes the benefits of closed-loop pumped storage hydropower systems

Published on August 22, 2023 by Dave Kovaleski

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An analysis conducted at the U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) has found that closed-loop pumped storage hydropower systems have the lowest potential to add to global warming for energy storage.

These findings, reported in the journal Environmental Science and Technology, provide new insight into how closed-loop pumped storage hydropower compares to other grid-scale storage technologies.

The paper, “Life Cycle Assessment of Closed-Loop Pumped Storage Hydropower in the United States,” was written by Daniel Inman, Gregory Avery, Rebecca Hanes, Dylan Hettinger, and Garvin Heath, all of whom are with NREL’s Strategic Energy Analysis Center.

The researchers analyzed the global warming potential (GWP) of various energy storage technologies. Energy storage can help increase the grid’s ability to accommodate renewables such as wind and solar.

Pumped storage hydropower is an established technology, but limited information is available about greenhouse gas emissions associated with its use. A closed-loop pumped hydropower system relies on water flowing between two reservoirs to generate and store power. The NREL study provides a life cycle assessment of new closed-loop pump storage hydropower in the United States and assesses its GWP.

Pumped storage hydropower is compared against four other technologies: compressed-air energy storage (CAES), utility-scale lithium-ion batteries (LIBs), utility-scale lead-acid (PbAc) batteries, and vanadium redox flow batteries (VRFBs). Pumped-storage hydropower and CAES are designed for long-duration storage, while batteries are intended to be used for a shorter time frame.

“Closed-loop pumped storage hydropower is shown to be the smallest emitter of greenhouse gases,” Inman said. “Not all energy storage technologies provide the same services,” Inman said. “We looked at compressed-air energy storage, which allows for grid-scale energy storage and provides services like grid inertia and resilience. But pumped storage hydropower is about a quarter of the greenhouse gas emissions compared to compressed air.”

In looking at pumped storage hydropower, the researchers based their finding on 39 preliminary designs from 35 proposed sites in the United States. The average closed-loop pump storage hydropower facility was assumed to have storage capacity of 835 megawatts and an average estimated 2,060 GWh of stored energy delivered annually. The scenario also assumed the electricity mix would entirely come from renewable technologies.

The researchers discovered that GWP attributed to 1 kWh of stored electricity delivered to the nearest grid substation connection point. They estimated the GWP for pumped storage hydropower ranges from the equivalent of 58 to 502 grams of carbon dioxide per kWh. Hydropower offered the lowest GWP on a functional unit basis, followed by LIBs, VRFBs, CAES, and PbAc. Further, they found that certain decisions, like building on a brownfield rather than a greenfield site, can reduce the GWP by 20 percent.