Firstly, I'll mention that I've used this idea in the two most recent ideas I've posted, and thought that it merit's it's own page.
The idea is relatively simple, but I haven't found anything very much like it on google.
There are two heat exchangers (HXs), one at at some high location, and other
at a low location.
At the bottom of the lower HX is a liquid sensor, and a pump (probably hermetically sealed, like the pump in an air conditioner system). Whenever liquid is sensed, the pump turns on.
The top of the upper HX is connected via a pipe to the top of the lower HX, and allows gas to flow from the upper HX to the lower HX.
The whole system is vacuum purged, and filled with a refrigerant. The amount of refrigerant is enough so that the pump, the pipe from the pump to the upper HX, and the upper HX, can all be filled with liquid refrigerant, with the rest of the system filled with gaseous refrigerant.
When heat is added to the upper HX, the refrigerant boils there, raises the pressure in the system, and causes some of the gaseous refrigerant in the lower HX to condense, giving off heat. The condensate moves (by gravity) down to bottom of the lower HX, triggers the liquid sensor, and activates the pump. The pump of course moves the liquid back to the upper HX.
As with a regular heat pipe, heat is moved by evaporation and condensation. The only real difference is that condensate is moved from one end to the other by a pump. The pump is only working against gravity, and is not doing any compression, so little energy is needed to run it.
Efficiency can be maximized by selecting a refrigerant such that the latent heat of vaporization, divided by the difference in density between liquid and gaseous refrigerant, is maximized.
Water has a fairly high latent heat of vaporization, so it's probably best for this application.