Wacky paraffin wax thermal reservoir
 

A thread on GH took an interesting turn the other day, and I figured the topic is worth bringing to you guys. We know that the Prius II uses a thermos bottle to keep engine coolant hot between cold starts in order to reduce warmup times, improving both emissions and fuel economy. We can accomplish this with an engine block heater, but folks like me can't implement that due to inaccessible A/C power (live in an apartment). One possible solution is to use a phase-change thermal reservoir, which would store tremendous amounts of heat for a longer period of time.

The concept is to run engine coolant through some kind of insulated bottle that contains ~10lb of something like paraffin wax, which picks up loads of energy from the engine coolant as it changes to a liquid. The heat is stored overnight, and rapidly warms up the engine coolant the next morning to temps that are very near nominal.

Now, I haven't crunched any numbers of my own yet so I'm just taking it on faith that this is actually feasible. But how crazy do you guys think it would be to try this as a project? The way I see it working is this: the wax would be stored inside of a large thermos bottle, with a small heater core sealed within it. Engine coolant runs through that heater core and naturally picks up or pulls heat from the wax. Some kind of valve would be necessary to close both ends of the bottle when the engine is shut down in order to reduce thermal losses while parked.

Any better ideas for a system that could be added as a retrofit? Reasons that my heater core/thermos thing would fail miserably?

I have a lot of space under my hood for such a reservior. The only concern I would have is that the piping thru the reservior would cause the wax on the surface of the piping to solidify and act as an insulator to the liquid wax.