A heat pipe can carry heat from a hot component to a cooler surface without a motor-driven pump. Inside its sealed shell is a small amount of working fluid and often a wick along the wall.
A two-phase loop inside a tube
At the hot end, liquid evaporates and absorbs energy. Vapor moves toward the cooler end, condenses and releases that energy. Capillary action in the wick returns the liquid to the hot end so the cycle can repeat. Spacecraft use heat pipes to move heat toward radiators; similar principles appear in electronics cooling.
The limits of passive transport
The pipe does not destroy heat: the cooler end still needs a place to reject it. Fluid choice, orientation, temperature and the wick’s ability to return liquid set the operating range. If the hot region dries out or the cold end cannot cool, performance falls.
Related: how phase-change materials store heat.