Nuclear, hydroelectric, etc. Lots of places do not get a majority of their electricity from fossil fuels. Washington (and Canada) use a ton of hydro power. https://www.eia.gov/state/?sid=WA
Washington has not only lots of mountains and streams... it has excess water. Excess that can be used for energy storage.
In contrast, the Colorado River / Hoover Dam may generate a ton of electricity. But if you slurp up water in reverse, you might have a riot. The Colorado River water rights are oversold, there's not enough water to satisfy everybody.
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Nuclear cannot spin up or down economically. Well... okay. It can shut itself off and spin down, but its... not economical. When you spend $Billions on a plant, and it shuts down, you lose a lot of energy in practice.
Nuclear Plant economics are about running the plant at 100% as much as possible, in the hopes of making back the money you spent to build the darn thing in the first place.
If you're shutting down Nuclear Plants 10-months of the year, and only spinning them up for the 2-months where solar is inadequate, the nuclear power plants will go out of business rather quickly. They're just too expensive to build, and the fuel costs are basically nil (so you might as well run nuclear for all 12-months of the year).
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Natural Gas peakers are the opposite. The fuel is very expensive, but the plants are very cheap. You can in fact, run a peaker the minority of the time. Maybe only 20% or 30% of the year, and still make plenty of money.
So when we are realistically talking about "Oh, my energy needs work for 10 months, but just gotta figure out the last 2 months of the year", the solution is always going to be natural gas.
Furthermore, water restrictions (especially in the West) force the dams to release water even when electricity isn't needed. Because people need water to drink. A dam can serve multiple masters... but one of those masters will take priority. In most cases, water-rights will take priority over energy storage.
That is to say: we can't store energy reliably in dams that are being used for water rights management. We can release water _and_ generate electricity, but the "choice" as to water rights management vs energy storage is contradictory. Storing energy longer means "not releasing water". While serving the water/drinking needs of the downstream communities means "releasing the water".
The Colorado River hasn't touched the ocean in decades man. Its been fully consumed for a long, long time.
https://en.wikipedia.org/wiki/Colorado_River_Delta#After_dam...
Hoover Dam will probably not be a pumped-hydro plant in our lifetime. Its not about technology, its about politics and water rights management.