Using Gravity & Air to create heat

[john_doe]

I'm not into power stations though I've seen a few and have attempted to sell them more efficient devices to bring down costs.

I noticed however they use "steam" to drive turbines under very high pressures.

Can anyone think of another way to extract heat from high pressure?

The earth's magnetic field is created by lots of material being compressed into a liquid and motion. Why don't we just build a really big upside down piston? Like the one that is in a car? So we fill this "Upside Down Piston" chamber with air and once it's pressurised just let the "Piston" drop to apply pressure to it.
Prior to doing this we just need to run a few pipes of water through the "Chamber" and voila, free'ish, clean energy. 
(I'm summarising quite a bit: Air heats up under pressure (About 600psi is very hot) and the chamber will need to be submerged into the ground but hey it works for cars soooooooooo???)

[john_doe]

Well after some math it appears the answer for 100c air temp is actually 2500odd psi.
15000psi is easily achievable with hydraulics that once reached would "hold" pressure. Unsure why anyone else hasn't thought of it?

[vasik041]

What about second cycle ? Who will lift piston back ?