Critical overview and feasibility study of two-phase immersion cooling systems for power electronics

The ever more decreasing size of power electronics is going to obtain devices always more compact and with a direct correlated consequence of higher heat fluxes obtained. After a first introduction to power electronics devices (classification, applications, market size,...) are analyzed the related heat losses causes and the failure types with the aim to start the study of thermal aspects. Indeed to stabilize the temperature below a maximum threshold of safety up to now were used different air and liquid cooling systems (indirect single and two-phase) that are in this work reviewed from the historical ones applied to electronics devices up to those used in our decade. A detailed classification of the cooling techniques for power electronics is showed with a particular attention to heat fluxes (W/cm2) that each one is able to cool and, at the end of chapter 3, are graphically compared in an histogram. All this previous work is done to compare and to have a focused vision on the two-phase immersion cooling in comparison to the traditional ones solutions. Indeed from now on, starting from a theoretical study of boiling phenomena (why a liquid boil, curve of ”pool boiling”, variation of CHF at different pressures, Rohsenow correlation, CHF varying the surface inclination,...) and condensation process, are reviewed the historical applications of two-phase cooling for electronics till nowadays; are in detail reported and compared both Papers/Scientific articles and Patents regarding prototypes of entire systems to cool power electronics immersed in two-phase fluids (semi-open bath or sealed and pressurized tanks) with emphasis on the parameters and configurations able to increase heat flux or to simply improve some characteristic of reliability, material compatibility, compactness, etc... Is done a critical overview on the kinds of fluids mostly used to cool power electronic, from the the traditional ones to the ones subject of researches in our years. The comparison is done mainly between the non-dielectric traditional fluids (Water, Ethylene glycol,...) and the new Low-Global-Warming dielectric fluids, so the ones with low environmental impact and a non-electrically conductive nature used with success in direct immersion cooling of power electronics. To conclude this thesis work is also done a review of surface types for pool boiling, tabulating the maximum CHF reached for traditional and most innovative couplings enhanced surfaces-fluids.