I&#x27;m surprised Nickel-iron batteries[0] aren&#x27;t used for this type of grid-scale storage. They are <i>extremely</i> time and cycle-durable: 20-<i>50 years</i>, almost immune to repeated deep discharges, relatively cheap and simple to produce. Cons: they need a bit of maintenance (not a big deal at grid scale), and have a less-than-stellar efficiency.<p>The problem with compressed air is that the Ideal-gas law [1] actively works against you to reduce efficiency, assuming it will be kept at ambient temperature. Inserting energy by compressing gas heats it up, which increases the pressure of the gas in the tank, which makes you work harder to put more gas in. Extracting energy cools the gas down, which reduces the pressure of the gas, which reduces the amount of energy you can extract from it. You can mitigate these issues by carefully controlling the temperature of the gas (more energy) or having a much much bigger tank so a single cycle only thermally cycles by a small amount (more $$).<p>Another option I hadn&#x27;t considered until now is to let it heat up -- and insulate it to let it stay hot. It will cycle in both pressure <i>and</i> temperature on a nearly daily basis so you may not lose all that much energy in heat. But I doubt this is an original idea.<p>[0]: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Nickel%E2%80%93iron_battery" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Nickel%E2%80%93iron_battery</a><p>[1]: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Ideal_gas_law" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Ideal_gas_law</a>