NREL outlines four-phase framework for energy storage development

by Ethan Howland
APPA News
February 11, 2021

With energy storage deployments growing, Department of Energy researchers have developed a four-phase framework to help utilities and others understand the technology’s possible evolution on the grid.

The National Renewable Energy Laboratory researchers expect their report — The Four Phases of Storage Deployment: A Framework for the Expanding Role of Storage in the U.S. Power System — will help utilities, regulators and other stakeholders evaluate different pathways for storage and other sources of grid flexibility.

The report released late last month is the first publication to come out of NREL’s multi-year Storage Futures Study, which will explore energy storage technologies across a range of potential future cost and performance scenarios through 2050.

There are about 24,000 megawatts of energy storage on the U.S. grid, mainly in the form of pumped hydroelectric facilities that pump water from a lower reservoir to an upper reservoir when electric demand is low and then run the water through turbines back to the lower reservoir when power is needed.

Looking ahead, the NREL researches expect energy storage to develop in four phases, with the storage’s duration increasing in length in each successive phase.

The boundaries between the phases will be indistinct and the transition between phases will vary between regions, driven partly by how much wind and solar are added in each region, according to the researchers.

The first phase started around 2011 and is characterized by energy storage with no more than one-hour duration that can provide operating reserves, according to the report.

The potential deployment of short-duration storage is limited by the overall need for operating reserves, which is less than 30,000 MW in the United States, the researchers said.

The second phase, which has started in some areas, centers on storage with two to six hours of discharge duration to provide peaking capacity, according to the NREL framework.

Energy storage in the second phase gets most of its value from replacing traditional peaking resources, mainly natural gas-fired combustion turbines, the researchers said.

The opportunities for storage in the second phase are tied to the local or regional length of the peak demand period, the NREL researchers said, noting the phase could support at least 40,000 MW of storage.

The second phase is characterized in part by the positive feedback between solar photovoltaics increasing the value of storage by boosting its ability to provide capacity and storage increasing the value of solar by augmenting its energy value by shifting its output to periods of greater demand, the researchers said.

“Thus, greater deployment of solar PV could extend the storage potential of Phase 2 to more than 100 GW in the United States in scenarios where 25% of the nation’s electricity is derived from solar,” the researchers said.

The third phase of the storage framework is characterized by lower costs and technology improvements that enable storage to be cost-competitive while serving longer-duration peaks that last four to 12 hours, according to the report.

“Deployment in Phase 3 could include a variety of new technologies and could also see a reemergence of pumped storage, taking advantage of new technologies that reduce costs and siting constraints while exploiting the 8+ hour durations typical of many pumped storage facilities,” the NREL researchers said.

Technology options for the third phase include next-generation compressed air and various thermal or mechanical-based storage technologies, according to the report.

The researchers said storage in the third phase might provide additional sources of value, such as transmission deferral and additional time-shifting of solar and wind generation to address diurnal mismatches of supply and demand.

There is at least 100,000 MW of new storage opportunities in the third phase, according to the report.

The final phase is the most uncertain and is characterized by storage with durations lasting from days to months that could help achieve very high levels of renewable energy in the power sector, or as part of multi-sector decarbonization, the researchers said.

Potential phase-four technologies include production of liquid and gas fuels that can be stored in underground formations for a long time with very low loss rates, according to the report. There could be roughly 250,000 MW of storage in the fourth phase.

Upcoming reports from the Storage Futures Study will cover the economic potential of diurnal storage, the implications of widespread storage deployment and other topics.