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Hydro with an elevated reservoir is a (more or less) closed system. Since you are storing excess solar and/or wind, you pump water up above the hydro station to the reservoir. It sits there until you need some additional base load, and then you let some of the water down through the generators, and hold it in another reservoir; to be be pumped back up to the high reservoir when you can.
Air in place of methane should not be a problem. Once a natural gas field is depleted, it could be used to store compressed air.
Geothermal can be done in more places than where it is naturally near the surface. Deep drilling can let water be pumped down, and then capture the steam.
I think the environmental problems with oil, gas, coal and nuclear are obvious and abundant. Can you name any issues with solar PV or solar heat or wind or wave or tidal power that even approach a small fraction of the problems with the finite energy we are now using?
Oil is finite. Coal is finite. Gas is finite. Even nuclear is finite -- uranium is not unlimited, and can we accumulate spent fuel rods forever?
The Great Fusion Reactor in the Sky is close to infinite. All the other renewable energies originate with the sun, except geothermal. They all will last as long as the Earth does.
So, let's review: finite energies will run out pretty soon, they will pollute the environment we depend on for our life, they will cost more and more as they dwindle away.
Renewable energies on the other hand, will never run out as long as the Earth exists, they have no pollution, and they will cost less over time, because we can build each generation of gathering system with renewable energy.
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