I think rather than think about it as "pros and cons", a better approach is to think about where these various types of storage are used. I think the analogy of

I think rather than think about it as "pros and cons", a better approach is to think about where these various types of storage are used. I think the analogy of hard disk drive -> RAM -> on-chip cache for memory storage (that is cheapest/longest lived/slowest to most expensive/short term/fastest) also works well for energy storage: 1. Pumped hydro is currently best for long term-storage. The lack of other cost-competitive long-term storage options is currently one of the biggest issues with renewables and an active area of investigation. 2. Batteries are good for daily smoothing (i.e. when the sun goes down), but they have limited charge/discharge cycles, they lose charge over time, and they don't, as of yet, have storage capacities needed for long term storage. 3. As the article points out, flywheels are good at instantly changing output based on changes in the grid, so they're good for frequency regulation and onboarding/offboarding new energy sources. Their biggest downside is their limited capacity compared to other tech.