Send your mining robot below. It cools itself with phase change liquids, but also opens high pressure tanks to the 90 bar atmosphere. When you hoist it back, just plug those pressure tanks in to turbines connected to generators. That wouldn't be a huge amount of power, but maybe wildcat prospectors could operate like that in the early days.
Would it be enough energy to compensate for the work you'd need to do to lift the robot back up? Presumably there's some energy gained from sending it down but not enough to bring it back, especially since it is coming back heavier. I just don't know how to work the math on that.
Why not use liquid oxygen as the phase change liquid? Then the gas could be caught in a bladder tethered to the ground. When it comes time to ascend, use it as a balloon?
There must be some way to exploit the temperature difference between the surface and 50km up, without mega-engineering.
