I mean the simplest one is gravity; the most scalable version of that is pumping water up into a reservoir behind a hydroelectric power plant. There's others that propose heavy trains going uphill or weights down a hole, but that doesn't scale very well and has additional maintenance costs.
But yeah, that would be electrical into mechanical energy (pumps), the cost would be flowing water up against gravity. Of course, it also requires a source of water downhill and a reservoir uphill. I was going to quip about Lake Mead being empty, but that would be huge distances to cover. Maybe something close to sea, or else a purpose built closed loop system.
Pumped hydro is great where it works, but we have used most of the best places for it. We also know it is an environmental disaster for the local area which needs to be balanced against the gain.
But yeah, that would be electrical into mechanical energy (pumps), the cost would be flowing water up against gravity. Of course, it also requires a source of water downhill and a reservoir uphill. I was going to quip about Lake Mead being empty, but that would be huge distances to cover. Maybe something close to sea, or else a purpose built closed loop system.