This report is one in a series of the National Renewable Energy Laboratory's Storage FuturesStudy(SFS) publications. The SFS is a multiyear research project that explores the role andimpact of energy storage in the evolution and operation of the U.S. power sector.The SFS isdesigned to examine the potential impact of energy storage technology advancement on thedeployment of utility-scale storage and the adoption of distributed storage, and the implicationsfor future power system infrastructure investment and operations. The research findings andsupporting data will be published as a series of publications. The table on the next page lists theplanned publications and specific research topics they will examine under the SFS.
This document explores the definition of “long duration”as applied to energy storage. Given thegrowing use of this term, a uniform definition could aid in communication and consistency among various stakeholders. There is large and growing use of the Advanced Research ProjectsAgency-Energy(ARPA-E)definition of greater than 10 hours. However, the term “long-duration energy storage”is often used as shorthand for storage with sufficient duration to providefirm capacity and support gaid resource adequacy.The actual duration needed for this applicationvaries significantly from as little as a few hours to potentially multiple days. This dual use of theterm means that there cannot be a simple, uniform, and static definition of long-duration storagethat captures its ability to provide firm capacity and also aids consistent communication.Toaddress this issue, the National Renewable Energy Laboratory recommends that qualitativedescriptions of long-duration energy storage always be accompanied by quantitative
descriptions, and that power sector stakeholders be deliberate in how they choose to define long-duration energy storage technologies.
