Considered the simplest biphasic device, thermosyphons are similar to heat pipes but, instead of wick structures, they use gravity to return the liquid from the cold side to the heated side. Latent heat transports the majority part of the heat in thermosyphons. They are, therefore, efficient, being composed of only an involucre and the working fluid.
Heat is delivered to the thermosyphon in the evaporator section, reaching the working fluid and causing liquid–vapor phase change. The generated vapor, due to pressure gradients inside the thermosyphon, crosses the adiabatic region and reaches the condenser, where heat is removed. The vapor contained in the condenser region condenses, and the resulting liquid returns to the evaporator by the action of gravity forces. Therefore, the evaporator must be located in a position inferior to the condenser.