Hydroelectric dams. Pump water uphill when the energy isn’t needed for something else. Hydroelectric is a good variable energy source too. Probably not very efficient, but simple.
I think pumped hydro is reasonably efficient, but requires specific terrain in order to be practical to build. Batteries are getting cheaper, and since grid storage doesn’t have the stringent weight and volume requirements that vehicles do, a wider range of chemistries (not just lithium) can be used.
I really hope to see more developments in the realm of gravity batteries. In many cases they are not geographically constrained, given that it only requires vertical space and mass. Ideally, the land already has high variation in elevations, but the storage capacity can still be increased in areas with little vertical space by increasing the mass of the weight being lifted/lowered.
I’m sure you know this, but for curious readers: pumped hydro is also a gravity battery, but it requires two bodies of water at differing elevation. This means it’s limited to coastal areas and places like dams. It’s also limited by the smaller capacity reservoir between the two bodies of water; the amount of water moved cannot exceed the maximum capacity of the upper body, nor can it exceed the extra capacity of the resevoir of the upper body.
