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quote: "I think if you pull out some numbers on it you will find ....
Let's say you have 50kg salt at 100c and it stores twice the heat of steel etc."
PCM (salt hydrates / eutectics) - storage density: 93 kWh/m3
Water (as just sensible heat) - storage density: ~8 kWh/m3
And Iron fairs much worse than Water by weight (water is actually very good)
Water has a Specific Heat of 4190 J/kg.degC
Cast Iron has a Specific Heat of 540 J/kg.degC
Remember the great thing with PCM salts, etc is that they can tap the large amount of energy needed for phase change (The latent heat).
So from above 200kg of iron holds about the heat of 25.7kg of water.
So we'll be generous and say a biggish engine block with water has a heat capacity of equivalent to 30kg of water. And the PCM can hold over eleven times as much heat once molten.
Unless I'm mistaken we find that just 2.6 kilos of our pcm can hold the equivalent heat of your entire engine.
Also, the PCM salt can give up very near to its entire heat as it solidifies/freezes (as most of the thermal capacity is the latent heat) providing a salt is chosen that melts just below the ideal coolant temp.
So whether you're trying to keep coolant heat stored for fast warm up later on or overnight (as you're idea is focused on), or you're trying to cut aerodynamic overheads required by a large radiator and opening (as is my starting point), phase-change materials are the way to go.
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