Experimental Performance of a Completely Passive Thermosyphon Cooling System Rejecting Heat by Natural Convection Using the Working Fluids R1234ze, R1234yf, and R134a

The present paper proposes a proof of concept of a completely passive thermosyphon for cooling of power electronics. This thermosyphon is composed of an evaporator to cool down a four-heater pseudo-transistor module and a natural air-cooled condenser to reject the heat into the environment. R1234ze, R1234yf, and R134a are used as the working fluids with charges of 524, 517, and 566 g, respectively, for the low charge tests, and 720, 695, and 715 g for the high charge tests. It has been demonstrated that the refrigerant R1234ze with a low charge is not a good solution for the cooling system proposed here since low evaporator performance and fluid instability have been detected at moderate heat fluxes. In fact, R1234ze needed a larger charge of refrigerant to be safely used, reaching a transistor temperature of 53 °C at a heat load of 65 W. R1234yf and R134a, on the other hand, showed good results for both the low and the high charge cases. The maximum temperatures measured, respectively, were 52 °C and 48 °C at 65 W for the low charge case and 55 °C and 47 °C at 62 W for the high charge case. The corresponding values of overall thermal resistances of the thermosyphon for the working fluids R1234yf and R134a at the maximum heat load are very similar, being in the range of 0.44−0.46 K/W.