Energy storage is a topic of increasing interest for purposes of decarbonization of the electric power system, and in particular to address integration of increasing quantities of variable energy resources, such as wind and solar PV. A growing focus of decarbonization analyses is the need for new long-duration storage and the potential change in operations of existing pumped storage hydropower. A limitation of much of the analysis on this topic to date has been the limited consideration of how the economic value of long-duration storage will be affected by the large deployments of short-duration energy storage, primarily lithium-ion BESS, which are currently receiving state policy support (through mandates or financial incentives) or are otherwise expected to enter the market based upon increasingly favorable economics.. The contribution of this article is to review the literature on this topic, update some simulation results, and help to advance the methodological questions which need to be addressed in subsequent simulation studies. The article examines each of the major components of potential long-duration storage value, including energy time-shift, different ancillary services, and Resource Adequacy (RA) capacity. It also draws attention to how state policies, resource planning methods, and market structure and regulatory factors can influence selection of different types of energy storage, whether short- or long-duration.
