I started writing this, and it started sounding like some lecture.
I really don't mean it as such, and I hope folks can wade through my writing it for the information...
There are things that work in a lab, and then there's the real world. For some reason, stupid little details can cause something to not work, when it theory it should. For example, pure water. How long does water stay pure, before corrosion from some metal connection contaminates the theory?
Probably the main problem with water is that it also freezes really easily. Are you willing to risk your motor controller? My dad risked his solar water heater to his preventive maintenance in Idaho; I doubt things would work out much better for a pure water cooled motor controller in Boston.
I just got back from a trip to Idaho, with -10F temperatures. I drove my Biodiesel powered truck and one of the big lessons I learned is that some thermal overkill - low and high side - is a good thing. Trying to start a diesel that's not plugged into a block heater and has gelled biodiesel (using 50% dino-diesel) in the fuel system is not fun. At those temperatures 15W-40 engine oil looks like honey.
I think if anyone is going to use a water-based coolant for electronics, it would be very wise to make sure that water leaks
absolutely cannot get to the circuits. I have a dead PC that is pretty good testimony to what tiny little condensation-sized dribbles can do. If you think that this can only kill low pwer electronics, my dad's washing machine is testimony against that. Fortunately the circuit board was pretty old school, and we were able to trace the blown relay that controlled the motor's field windings and thus, the spin cycle. A $3.50 relay for a $700 washing machine. Simply sealing the wires, like a standard automotive harness would have prevented the problem.
I do plan to use water based cooling in my motor controller. For this reason, and because of the amount of water a real world vehicle is subjected to, I plan to seal the motor controller completely, like an OE device. Even though pure water is probably the best coolant from a conductivity/thermal mass perspective and I live in California, I will still sacrifice the ideal performance to a level of thermal safety.
-E*clipse