Temperature control using variable conductance closed two-phase heat pipe

The concept of using variable conductance heat pipes (VCHP) for controlling the temperature of solar collectors is introduced. This closed system does not need any external force, is self-controllable and therefore ensures high reliability in thermal control. A copper/water heat pipe equipped with a cold reservoir and buffered with air as non-condensable gas (NCG) has been tested for temperatures from 25 degrees C up to 85 degrees C. Using thermodynamic water properties, the operation of the VCHP is analysed. Temperature distributions along the VCHP are presented and a simplified model for calculation of the required mass of NCG is included. The model predictions agree well with experimental data. It was found that there is no hysteresis associated with heating and cooling of VCHP. The effect of mass diffusion at the vapour and non-condensable gas interface has been analysed. Test results show that the starting point of operation for VCHP is mainly dependent on the amount of NCG present in the pipe and on the superheat required for boiling.