Water is great mainly because we have so much equipment already invented for handling it automatically. Imagine if you had to invent pumps and pipes before you could demonstrate your idea.
Cables, pulleys, and winches are another mature technology. If you can provide a large constant force on a cable over long distance, reversibly, without need to build anything that long, you might have a winner. You should be able to share the expensive bit, the motor/generator/winch, with lots of cable reels. And, it is better if the winch and reel are off-the-shelf items.
So, put a float with hundreds of tons of buoyancy on a cable through a pulley screwed into the sea floor, thence to a reel and winch on shore. You can share the expensive motor/generator/winch among as many reels as you like. The float might be a sealed balloon full of lithium-saturated liquid ammonia, which is half as dense as the water it displaces.
Air handling is a little less mature. Pump air into a big, light tank well anchored to the sea floor, displacing water. Let the air out through a turbine. People invent elaborate systems to avoid losing heat, but when marginal cost of generation is zero, efficiency doesn't matter so much. The tank only has to be strong enough to hold back the buoyancy of the air. You can add as many tanks as you like. Or, use natural cavities beneath the water table. Maybe pump hydrogen down there, instead. Or hydrogen in one, air in another.
There will be zillions of storage schemes, that all work, until we settle on a few we like best for their secondary benefits. Certainly pumped hydro will remain important.
Cables, pulleys, and winches are another mature technology. If you can provide a large constant force on a cable over long distance, reversibly, without need to build anything that long, you might have a winner. You should be able to share the expensive bit, the motor/generator/winch, with lots of cable reels. And, it is better if the winch and reel are off-the-shelf items.
So, put a float with hundreds of tons of buoyancy on a cable through a pulley screwed into the sea floor, thence to a reel and winch on shore. You can share the expensive motor/generator/winch among as many reels as you like. The float might be a sealed balloon full of lithium-saturated liquid ammonia, which is half as dense as the water it displaces.
Air handling is a little less mature. Pump air into a big, light tank well anchored to the sea floor, displacing water. Let the air out through a turbine. People invent elaborate systems to avoid losing heat, but when marginal cost of generation is zero, efficiency doesn't matter so much. The tank only has to be strong enough to hold back the buoyancy of the air. You can add as many tanks as you like. Or, use natural cavities beneath the water table. Maybe pump hydrogen down there, instead. Or hydrogen in one, air in another.
There will be zillions of storage schemes, that all work, until we settle on a few we like best for their secondary benefits. Certainly pumped hydro will remain important.