Molten Tin can hold 2 times the BTUs/lb/hr of heat, and its thermal conductivity is well beyond lead. Molten Zinc is also way ahead of lead by about 4 times its performance. Tin is very non reactive, very easy to handle, and is not poisonous or hazardous in most “Normal” ways. Tins performance as a thermal medium is very acceptable and would stabilise the steam cycle to a nice 450 degree F at a rather stable steam pressure of 430 lb/sq in. Tins drawback is its expense. Zinc would be a very acceptable substitute for Al because it is very cheap.
Zinc is also somewhat reactive to steels and nickel alloys in a similar manner to Al, necessitating the use of non-reactive plating. Zinc would very readily produce high quality superheated steam above critical pressure at a temperature of roughly 790 degrees F at pressures somewhere in the vicinity of 3500 lb/sq in. Steam perfect for running a Tesla disk turbine!! it would require four times the mass of Aluminum in zinc, because it only has 43 BTU/lb/hr as opposed to Aluminums 170 BTU/lb/hr. Zinc may still be easier to use than Aluminum because it is less reactive and readily obtainable on the open market. I would always use a vacuum in the space above the metal because it is easier to maintain as opposed to an inert gas atmosphere. Again, is there anyone who knows what metal coating can be used on steels that maintains a high rate of thermal conductivity while in contact with a reactive molten metal like Aluminum or Zinc?
I would suggest that you look at commercial refractory materials. Take a look at what they use for crucibles. The stuff is a little pricey, but failure here is not an option. Are you aware of what happens when molten aluminum mixes with moisture? The results are not pretty. I knew of a hapless soul that put a cold piece of aluminum into a almost full crucible of the molten metal. The problem was the moisture that condensed on the cold metal. When that moisture hit the molten aluminum there was an instant explosion which sprayed the hot stuff all over his shop. By some miracle he was not injured but he never did that again.
I have seen studies on thermal batteries that posited a molten sodium storage material. In mcmaster-carr they sell bismuth alloys than melt at temperatures as low as 115F. I know that old linotype machines used cast type and kept the metal molten. IIRC it is a combination of lead, bismuth, tin, antimony. I don’t think it is very reactiveand should be workable for a storage medium.
I don’t think there’s any bismuth, or if so, not much. I’ve used a lot of linotype for casting bullets, it’s primarily lead w/ just a tad of tin and antimony for hardness. Certainly doesn’t react with castiron or steel. The fumes are pretty nasty tho — you have to keep checking your gums to see if you’ve got too much.
Have you thought of Titanium? 1) It is very high temperature tolerant. 2) It is used as the main metal in marine aquarium chillers coils. 3) It does not react with most metals except at extream temperatures ( I have no idea how it would react with aluminum ). 4) It is already available in various tubing sizes, on the comericalmarket.