This is an invention for harvesting energy from a small temperature difference, with some assistance from gravity.
There are two heat exchangers, a heated one at a lower altitude, a cooled one at a higher altitude.
The bottom of the top heat exchanger is connected to a pipe, which runs down to
a hydroelectric generator, whose outlet is the bottom of the lower heat exchanger.
The top of the lower heat exchanger is connected to the top of the upper heat exchanger via a second pipe.
The whole system is vacuum purged, then filled with enough refrigerant that one heat exchanger, and one vertical pipe, could be filled with liquid refrigerant, with the rest of the system filled with gaseous refrigerant.
To maximize efficiency, the density difference between liquid and gaseous refrigerant, divided by the latent heat of vaporization, should be maximized.
Mercury has such a low latent heat of vaporization, and such a high density as a liquid, that it might be a good choice... provided we can be absolutely certain that it won't leak.