Analytical Investigation of the Heat-Transfer Limits of a Novel Solar Loop-Heat Pipe Employing a Mini-Channel Evaporator

This paper presents an analytical investigation of heat-transfer limits of a novel solar
loop-heat pipe developed for space heating and domestic hot water use. In the loop-heat pipe, the
condensate liquid returns to the evaporator via small specially designed holes, using a mini-channel
evaporator. The study considered the commonly known heat-transfer limits of loop-heat pipes,
namely, the viscous, sonic, entrainment, boiling and heat-transfer limits due to the two-phase pressure
drop in the loop. The analysis considered the main factors that affect the limits in the mini-channel
evaporator: the operating temperature, mini-channel aspect ratio, evaporator length, evaporator
inclination angle, evaporator-to-condenser height difference and the dimension of the holes. It was
found that the entrainment is the main governing limit of the system operation. With the specified
loop design and operational conditions, the solar loop-heat pipe can achieve a heat-transport capacity
of 725 W. The analytical model presented in this study can be used to optimise the heat-transfer
capacity of the novel solar loop-heat pipe.