Earlier quoted context omitted.
Make hydrogen, store it underground, and have enough combustion turbines available to back up the grid. A simple cycle combustion turbine power plant is about 5% of the cost of a nuclear power plant, per unit of power output.
That seems physically plausible, though it would be a lot easier if we could make methane efficiently, because we already have the infrastructure to transport and burn it.
Yet another possibility is underground storage of high temperature heat, artificial geothermal, if you will. But it would have to be extractable with cheap equipment. Perhaps it could be used to superheat hydrogen for injection into those combustion turbines.