One of the key design parameters is the thermal diffusivity of the gas. The greater it is, the deeper the stack can be, and the better it performs. Helium is ab

One of the key design parameters is the thermal diffusivity of the gas. The greater it is, the deeper the stack can be, and the better it performs. Helium is about 190, while nitrogen is about 22. Next best to helium is hydrogen, but that has a bunch of other tradeoffs in terms of use. Among other things, it's very hard to seal high pressure hydrogen, and it's notably explosive in almost all concentrations with air. As for moving the heat, since it's using water as a heat medium, it just moves the water to the hot or cold spot on the internal stack to heat or cool as appropriate.