Solar and storage have a large and growing role on the grid, but as new storage technologies emerge that are economically suited to longer durations than the lithium ion battery incumbent, it is not clear how tightly-coupled systems can best be designed to meet the needs of the grid. Variations in cost structures, performance characteristics, and design options drive differing optimal coupling options (AC or DC) and configurations with solar photovoltaic systems, and changing grid needs demand different plant power export profiles. This study simulates the operation of solar + storage plants in a wide variety of configurations to search for those that result in the best combinations of cost and performance, as measured at the plant point of interconnection. This study acknowledges the need for low-cost electricity generation and thus rates these hybrid configurations on their levelized cost of electricity (LCOE). These plants support the grid in other ways, like provisioning resource adequacy, so this study also rates configurations on their ability to meet target power export profiles using a self-sufficiency ratio (SSR) metric. Factors including the solar-to-interconnection power capacity ratio, solar DC:AC ratio, storage power capacity (charging and discharging), and storage energy capacity are varied in the scenarios. Lithium ion batteries, organic flow batteries, nickel-hydrogen batteries, and zinc-bromine batteries are tested alongside adiabatic compressed-air energy storage, compressed CO2 energy storage, and pumped thermal energy storage.
The results show that compressed CO2 energy storage, under the cost and performance assumptions in this study, represents a competitive option with lithium ion and organic flow batteries not far behind, even at longer durations (up to 12 hours in this study). Uncertainties in the future cost and performance of lower-technology readiness level (TRL) technologies relative to lithium ion batteries, which are commonly-deployed today, make designing optimal plant configurations sensitive to the trajectory of storage companies over the coming years.