[pfdietz]

Pumped THERMAL storage. This is not pumped hydro. Rather, it involves a heat pump, for example compressing argon near-adiabatically, transferring the heat to a hot store (cooling the argon in the process), then expanding the argon back to the starting pressure. The argon is now around at mildly cryogenic temperature, and the cold is transferred to a cold store (liquid hexane, perhaps).

To recover energy, this is reversed, with the temperature difference driving the cycle in the other direction. Detailed calculations with inefficiencies show an overall round trip efficiency of 60% or better could be achieved. All the temperatures are below the creep limit of ordinary steel, so this system would require no exotic materials whatsoever.

It's also possible to design a thermal storage system without the cold store, using the ambient environment as a heat sink when running the generator. In that case, adding a backup heater (burning hydrogen, say) would make the store double as a backup generator at extremely low extra capital cost.

[sir_bearington]

Right and who can I call to install 1 GWh of thermal storage? I can't, because this technology only exists in prototypes. The only actual use of thermal storage is in solar thermal energy, which hasn't experienced the massive drop in price that photovoltaic solar has.