The entire point of a sand battery is to store more heat than liquid water can in a smaller amount of space. If you want to use water (or water/glycol) for the medium to transfer heat from one place to another, you may as well store the heat in water. Storing hot water will not get you through the winter, if you live in a cool climate.
It is my understanding that to get sand beyond 212°F (100°C), you’ll need to use electric heating elements. Another way to get heat into a sand battery would be to heat up oil like they do with parabolic solar troughs.
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Although hot air and hot water rise,
I hope to have almost none of either in my sand battery. This heat will migrate through the sand via conduction, not convection. So I should try to put my electric heating elements throughout the sand, not at the bottom. Hmm, I could put them at the top for easier accessibility and maintenance.
To get heat out of the sand battery, you would need to be able to survive in heat that exceeds 300°C, maybe as much as 600°C. Sand will not melt until 1700°C. Dammit! We’re back to oil as a medium in the pipes. Water will flash to steam at 100°C, and steam takes up 1000x the space as liquid water. Kaboom!
You could also blow air over the hot sand, but you may end up with mold issues and silica dust in the ductwork. Silica dust is microscopic glass shards that end up in your lungs — you can imagine why I’m not a fan of that plan.
I’m looking into creating a sand battery by building an insulated box under my driveway. If I can get it hotter than 100°C, the sides of the box will be made of 3 layers of Corrugated steel (Melting point 1200°C), with peaks and valleys offset to make a honeycomb structure; mineral wool (melting point over 1100°C) between the layers of corrugated steel that are screwed together.
If I cannot go higher than 100°C, I will use 2” thick Owens-Corning FOAMULAR rigid insulation panels (softens at 104°C and decomposes at 316°C).
I THINK either will maintain its structural integrity because the sand is dense enough to keep the surrounding dirt from crushing or displacing it. (Yes, I am planning to compact the sand in 1-foot lifts.). An alternative is to buy a prefabricated concrete septic tank (or something similar) but they only come in certain sizes and it would still need to be insulated.