Closed-loop pumped storage hydropower systems rank as having the lowest potential to add to the problem of global warming for energy storage when accounting for the full impacts of materials and construction, according to a new US Department of Energy (DoE) analysis.

These systems rely on water flowing between two reservoirs – upper and lower - to generate and store power.

Those findings in a paper written by scientists at DoE’s National Renewable Energy Laboratory (NREL) provide previously unknown insight into how closed-loop pumped storage hydropower—which is not connected to an outside body of water—compares to other grid-scale storage technologies.

Increasing the energy storage capacity can support a higher amount of renewable energy generation on the electric grid.

In the US, pumped storage hydro provides 94% of bulk energy storage capacity and batteries and other technologies make-up the remaining 6%.

Because renewable sources such as wind and solar do not generate electricity continuously, there is the risk of curtailment, where excess electricity is produced but cannot be used.

Storage will help this excess electricity generation and provide a buffer between supply and demand, ensuring that more renewable electricity makes its way to consumers.

Findings in the paper, Life Cycle Assessment of Closed Loop Pumped Storage Hydropower in the United States, were reported in the journal Environmental Science and Technology,

“Closed-loop pumped storage hydropower is shown to be the smallest emitter of greenhouse gases,” said Daniel Inman, who co-authored the paper.

The researchers analyzed the global warming potential (GWP) of energy storage technologies, which currently stand as a bottleneck that inhibits the end use of renewable electricity generation. Storage can help increase the grid’s ability to accommodate renewables such as wind and solar.

Pumped storage hydropower stands out as an established technology, but limited information is available about greenhouse gas emissions associated with its use. The NREL study provides a life cycle assessment of new closed-loop pump storage hydropower in the United States.

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.

“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.”