Energy storage as a transmission asset: Definitions and use cases

There is growing interest in deploying energy storage as a transmission asset (SATA), as evidenced by an evolving body of supportive policies and regulations and an expanding body of literature on the topic. Despite nearly two decades of evolution, however, transmission planning processes in the United States rarely consider storage alternatives and only a handful of projects have been selected. Regulators have assigned the responsibility of proposing non-transmission alternatives to non-utility participants in the planning process, so the sparse record of SATA projects may therefore result from a lack of specificity about the use cases for SATA and uncertainty about when it should be proposed. Extensive literature has been devoted to the topic of SATA, but much of it relies on definitions that are inconsistent with regulatory precedent and that exacerbate confusion. This paper reviews regulatory proceedings to define three types of energy storage assets than can interact with the transmission system: storage as a transmission asset, storage in place of a transmission asset (SIPTA), and dual-use energy storage. It then provides an inventory of use cases for SATA and SIPTA projects, using brief case studies to illustrate each use case. By clarifying the definition of SATA and its roles in transmission system planning, this work may be of use to transmission system planners and other participants in transmission planning processes to identify situations in which SATA alternatives merit consideration.