A heat pipe acts like a high conductance thermal conductor. Due to its thermal-physical properties, its heat transfer rate is thousand's times greater than that of the best solid heat conductor of the same dimensions. The basic heat pipe is a closed container consisting of a capillary wick structure and a small amount of vaporizable fluid.
A heat pipe employs an evaporating-condensing cycle, which accepts heat from an external source, uses this heat to evaporate the liquid (latent heat) and then releases latent heat by reverse transformation (condensation) at a heat sink region. This process is repeated continuously by a return feed mechanism of the condensed fluid back to the heat zone.
In the solar collector, the condensation zone is at a higher level than the evaporation zone. The transport medium condensed (in the condensation zone) returns to the evaporation zone under the influence of the gravity. Then, there is no need of capillary wick structure.
The maximum operating temperature of a heat pipe is the critical temperature of the used heat transfer medium. Since no evaporation/condensation above the critical temperature is possible, the thermodynamic cycle interrupts when the temperature of the evaporator exceeds the critical temperature.