[publius911]

On March 27, a sunny day, some solar farms had to shut down because there was more power on the grid than Californians were using.

That problem was solved almost 50 years ago.
California's San Luis dam.

Pumped Storage

The secret: Pump water to a storage lake higher than the solar generating site, and as close to it as possible to minimize transmission losses.
Reverse the flow of water at night through hydro-generators to provide most of the power back into the power grid. The nighttime demand is never zero...

The only requirement is finding a nearby site for the dam/lake.

Smarter specialists than me can figure that one out...

[JmyBryan]

Grand Coulee Dam in Washington utilizes this design.

[Rio]

They’re working up a pumped storage site near me:

http://www.edf-re.com/projects/detail/swan_lake_north_pumped_storage_hydro_project/

It’s the other way around, though. They plan to use night-time power to pump, and then generate in the afternoon during peak demand.