Energy storage can accelerate the decarbonization of the electrical grid. As useful energy storage technologies are developed, investors and manufacturers want to determine the needs for storage in a wide range of scenarios. In this study, we introduce a strategy for identifying the types of storage that will be most valuable to the grid given specific generation and load profiles. This method estimates the annual minimum number of cycles for each storage, how long each holds the charge, and charging and discharging rates for an idealized system, giving insight into tomorrow's complex systems. We demonstrate the proposed hierarchical approach and quantify how many fewer times wind-driven grids cycle the storage at night compared with solar-driven grids, as well as how winter-dominant wind generation and latitude-tilt solar may reduce the need for seasonal storage. Also, we quantify how higher discharging rates are required for energy storage products that cycle most frequently.

Currently, the most difficult time of year for California to supply the demanded electricity is around sunset on very hot summer days. As California uses more renewable electricity, that challenge may shift to any time of the year depending on the supply of electricity more than on the demand. We study various scenarios for applying a 100% renewable energy grid using six years (2015-2020) of historical demand and scaled-up solar and wind generation to investigate the main function of the storage in affording adequate electricity supply at all times of the year. We identify the times of year that may be most challenging. We detect that, for a solar dominant generation profile, the ultimate challenge shifts from summer to winter. Furthermore, the critical time of the year may be shifted by one or two months depending on the amount and the mix of the renewable generation that will be built.

Today resource adequacy is most often maintained by installing natural gas plants to meet the peak load. In California, the current risk of inadequate electricity supply is highest around sunset in late summer. In a zero-carbon grid, resource adequacy will increasingly require adequate stored energy throughout the entire year. Here we seek to develop an intuition about the times of the year when resource adequacy may be most challenged for a solar-dominant system. We use a simplified approach and show that the month of the biggest challenge occurs in winter and can shift by more than two months depending on the amount of solar and storage that are built.