It is an interesting these technologies you compare. Yes, a sand battery is in potential capable of storing higher temperatures if the source can generate these temperatures. We also have to look at the heat transfer that will seperate both energy buffers if seen from an application point of view. The heat transfer in sand is very low and this intrinsic insulation of sand begins to be very interesting when larger volumes are used. Water has a problem that it needs an extra insulation layer and larger volumes would be less interesting in comparison. However water is faster in exchange and is interesting as smaller buffer with shorter bursts and intake of heat.
I like aluminium powder idea. And use the metal bar as heatpipe is a good idea. I would not see temperature as problem as most materials you mention can handle 800 deg. The idea is that you can draw energy from it thus cooling it. I think a molten salt chamber uses this combination of fast transfer and high temperature
Unfortunately that would negate the high storage temperature of sand (up to 800 degree c) as water will turn into steam after 100deg. So it is either low temperature sand or water with lower energy density.
Also if these are not load bearing beams ( they have been almost cut throughto allow the pipe going through), make the gap wider and put some isolation around the pipe.
But only if the pipe is the case of the water problem
Sand Batteries vs Water Batteries
I can't remember if I saw the argument here or on Reddit, but this is my preferred platform so it's going here....
Is this discoloration a problem? ( photos.app.goo.gl )
Bathroom renovation revealed this black staining in the ceiling lumber next to a vent pipe. Is it a problem? (The PVC pipe is new.)...