A heat pipe is a device used to transfer heat from one end to the other more effectively than a rod of massive metal.

It works by a liquid boiling away at the hot end, and condensating at the cold end. The condensate is then re-transferred to the hot end by the means of a wick, or simply by gravity if that is possible.

The phase change from liquid to gas takes a lot of energy and has the additional benefit that the hot end can't get any hotter than the boiling point of the liquid in question, as long as the heat pipes specs are not crossed.

The problem with this design is that it has to be matched to the type of heat source by choosing the appropriate liquid, boiling point and amount of liquid in the device, as it will work effectively only in a narrow band of temperature/amount of energy.

In low-range temperatures (30° - 80°C), alcohol under normal or reduced pressure is often used. In higher ranges, liquid metals are the medium of choice.