Fan cooling is common for power electronic devices like rectifiers and inverters. Brushless DC fans have been cheap and readily available for at least 2 decades. Open frame construction is common so it is inevatable that the fan sucks dust and debris into delicate high voltage electronics causing failure. How feasable would it be to bathe a power electronics module in transformer oil? The oil is good at mains frequencies like on power distribution transformers but how would it be at say 50KHz? Would this relatively inert oil attack standard electronic components like say TO247 IGBTs, metal film caps, ceramic caps, metal film resisters?
The fluid does only provide cooling when it is flowing, transporting the heat from e.g. the power module case to e.g. a heat exchanger. So if you want to replace a heat sink plus fan with a fluid like oil (or water), you have two options:
- Natural convection, where the oil flow is simply driven by temperature gradients and gravitational field. Oil is better in "transporting" the heat than air, but viscosity (characterizing the ability to flow easily) is much higher.
- Forced convection, where the flow is driven by a pump. If you already need a fan for air-cooling, a pump is probably required because natural convection might be not enough in your application.
Applying liquids for cooling basically just transports the heat away from the lossy component. Therefore, additionally you need a heat exchanger to dissipate the heat into the environment. In the most simple case this is the converter housing wall which needs to be of large enough surface. If the surface is not large enough, you need a heat exchanger which employs a fan for air-cooling.
In transformers (and some medium-voltage power electronics) a main argument for the oil is its properties of electrical insulation.
Oil simply transfers heat energy to a radiator/convector/conductor that can move it out of the system.
It would be feasible to use it for this purpose BUT the need is largely well enough met by the use of "heat pipes" which use the large energy involved in
- the latent heat of vaporisation of the heat transfer fluid,
- vapour phase heat-energy transport and
- liquid phase flow and wicking to recycle the energy transfer compound
to together give energy transfer rates liable to be superior to that obtained from oil without substantial pumping.
Water, with among the highest latent heat of vaporisation of any substance, is a good compound for this purpose, possibly with the addition of additives to deal with corrosion and other secondary factors. Reducing pressure in the 'heat pipe' when it is sealed allows any desired transfer temperature below 100 degrees Celsius to be obtained.