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Long Duration Energy Storage Essential to Grid Decarbonization: Pacific Northwest Lab

Long term energy storage will be a necessary ingredient in the transition to a decarbonized grid, according to a paper by the Pacific Northwest National Laboratory.

The report, Defining Long Duration Energy Storage, published in the Journal of Energy Storage, explores how the growth of renewable energy generation will require long duration energy storage to fill the gap between variable energy generation and customer demands.

The report also describes the barriers in current energy planning and procurement processes that prevent planners from recognizing those needs.

“This isn’t something we are going to be able to flex out of like we have in the past,” Jeremy Twitchell, senior energy analyst at PNNL, said in a statement. “Things like energy efficiency, demand response, and overbuilding generation capacity will all be part of the solution. But when you look at the sheer size of these mismatches between when energy is produced in a decarbonized grid and when it is consumed, we simply can’t deploy those things at the required scale,” he said.

Long duration energy storage is currently loosely defined as the ability to store energy from 10 hours to up to 100 hours, but the PNNL paper proposes that the industry energy adopt two classes long duration energy storage: diurnal storage that lasts up to 20 hours and is used to reconcile daily cycles of surplus and deficits in generation and seasonal energy storage that is used to reconcile seasonal periods of surplus power generation with seasonal periods of insufficient generation.

“By thinking of long-duration energy in two different ways, utility and grid planners can look at historical load data and better pinpoint the length of ‘stop gap’ energy that will be needed,” the paper’s authors wrote.

The PNNL paper used load and generation data from the California Independent System Operator to create an illustrative model to show energy shortfalls. The model showed the highest energy consumption in the morning between 5 a.m. and 8 a.m. and in the evening after 5 p.m. Solar power generation throughout the day offsets customer demand, but when the highest demand is reached at about 6 p.m. solar generation output has already begun to decline, creating a shortfall. The data from PNNL’s model showed a 30 gigawatt capacity deficit in one day, approximately half of California’s total daily generation. “To make up that deficit, the state would need 15 Hoover Dams producing energy over that same period," Twitchell said.

The PNNL paper did not discuss long term energy storage technologies, except for pumped hydroelectric and compressed air storage, as those other technologies are “not yet mature,” the authors said. Nor did the paper address the charging schemes of long term energy storage but, the authors noted, “total generation will have to increase to supply adequate energy to fully charge” long term energy storage resources and said that the economics of long duration versus multiple short-duration storage systems would also need further consideration.

In the meantime, the authors said, “it is important that grid planning processes begin to conduct more detailed modeling of decarbonized grid operations identifying the type, scale, and timing” of long duration energy storage needs. “Planning processes can play an important role in the development of those technologies by sending a market signal for research and development,” they added.

“The motivation for this paper and research was to make the case that LDES will be an absolute necessity in a decarbonized grid,” Twitchell said. “If we want it to be available when we need it, then we need to start sending clear market signals that will support research and development of LDES technologies now. These technologies take years to develop, but our energy planning and procurement processes work on short (less than three-year) cycles. By the time our processes identify LDES needs, there won’t be time to develop and deploy them.”

Funding for the PNNL paper came from the Department of Energy’s Office of Electricity and supports its efforts for the Long Duration Storage Shot, the Office of Electricity Storage Program, and the department’s Net Zero Labs Pilot Initiative.