Thanks for the good ideas! I'm currently fabricating parts for my next attempt at a "passive" system. It does still have a valve controlled by a microcontroller based on two temperature sensors. Anyone have ideas on cheap electric valves that can take the pressure of a cooling system (1-2 bar max.)? I have some car vacuum solenoids, but I'm yet to test how they hold up to coolant and pressure. They are also quite small so they may resist flow. As for heat storage, I'm still staying with water. With no additional material in between the heat transfer is faster and this is key to the efficiency of the system. It's also cheaper to make.
