An Evolving Dictionary for an Evolving Grid: Defining Long-Duration Energy Storage

New Thought Piece Describes the Challenges of Defining Long-Duration Energy Storage To Reflect Both Duration and Application

Nov. 8, 2021

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Our understanding of the energy system is ever changing, and so is the language used to describe it. Photo courtesy of iStock

What do you think when you hear the term "long-duration energy storage"?

There is no single definition for long-duration energy storage, or LDES, in the energy community. For some, it refers to storage systems that can provide at least 10 hours of stored energy. For others, it refers to storage systems that have enough stored energy to provide firm capacity to the grid.

Our understanding of the energy system is ever changing. Our energy language, and more importantly, the meaning behind that language, is also changing—but defining terms is a bit like landing a plane on a moving airstrip.

In a new thought piece by the National Renewable Energy Laboratory (NREL), analysts describe the challenge of creating a uniform definition for LDES that reflects how it is used to support the grid. This work is part of NREL's multiyear Storage Futures Study supported by the U.S. Department of Energy to explore the potentially fundamental role of energy storage in the evolving U.S. power system.

The Importance of a Common Definition

As the share of U.S. power generation from variable renewable energy grows, LDES has gained increasing attention as a possible way to enable affordable, reliable electricity and to support power system decarbonization.

"Long-duration energy storage has become a larger part of stakeholder conversations that seek to understand the needs of the current and future grids and establish policy and market rules," said Paul Denholm, lead author of the paper. "Therefore, varying definitions can have important consequences."

In the paper, NREL contends that a consistent definition for LDES would create common language and shared understanding among stakeholders about the role of LDES in future grids, including those that approach 100% decarbonization and rely primarily on renewable energy.

A Duration-Application Conundrum

Currently, LDES is typically defined by duration, or the length of time that a system can sustain maximum power output. NREL found in a literature review that LDES can refer to duration ranging from 4 hours to multiple days, with 10-plus hours being cited most frequently (consistent with ARPA-E's definition).

While duration may be the most straightforward way to define LDES for communication purposes, NREL argues it does not indicate how the stored energy is used or the value it provides to the grid.

Energy storage is increasingly deployed to provide firm capacity, or the ability to help keep the power system running despite outages, extreme weather, low generation from variable renewable technologies like wind and solar photovoltaics, etc. The duration needed to provide firm capacity varies significantly and can range from a few hours to multiple days depending on many factors, like the share of variable renewable generation or the electricity load the system serves.

Currently, in many locations, a 4-hour storage system can provide significant firm capacity, but that is not consistent with an LDES definition based on 10-hour-plus duration—highlighting the conundrum of trying to uniformly define LDES to reflect both duration and application.

The Search for a Definition Continues

NREL has yet to determine a single uniform definition for LDES to describe both duration and application and concludes it may not be possible to reconcile the two. The team suggests any use of the term LDES should be accompanied with a short qualitative description to clarify the intent of using the term. And if a duration must be used, the team recommends using ARPA-E's definition of 10–100 hours but with the appropriate caveats.

"This may be somewhat unsatisfying," Denholm said. "But it reflects the growing complexity of our grid. Our language reflects how we see and understand the grid, which is ever-changing, so our terminology needs to be flexible, too. The role of storage of varying durations will ultimately be determined by the costs and benefits of providing firm capacity and other services, which we're still understanding."

The NREL team encourages researchers, analysts, and others working in the energy storage space to carefully consider how they define LDES to collectively advance the understanding of how storage fits into future power systems.

Learn more about NREL's energy analysis research and the Storage Futures Study.