That problem was solved almost 50 years ago.
California's San Luis dam.
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...
Grand Coulee Dam in Washington utilizes this design.
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.