Weekly to Seasonal Energy Storage

Various electrochemical battery energy storage technologies will work to mitigate renewable variability, up to a point. Where that point is depends on the climate when the mitigation is needed, and the amount of mitigation (MWh) required in a given event. A large majority of large BESS procured by investor-owned utilities in California are lithium-ion batteries with a 4-hour run-time at their rated output.

Without going into gory details about this, where I live (Northern California), these battery energy storage systems (BESS) work well to mitigate most of the daily variability that comes with photovoltaic arrays. However wind-power has variability that is much less predictable and sometimes has low-output durations of several days to over a week. Currently this does not cause a major problem because there is still enough gas-fired generation to offset the wind-variability, but as this is retired in the future years, it would be good to have more long-term storage options.