I'm pretty sure I'm close to the right answer, let me know what you think!
Article regarding heat storage for sand, clay, concrete, and water:
https://www.researchgate.net/publication/321651354_Comparison_Between_the_Concrete_Sand_Clay_Water_and_the_Paraffin_like_Materials_of_Heat_Storage_for_the_Solar_Water_Heaters
Tour of Patrick's greenhouse with sand battery: https://youtu.be/0x-1hLmAmBQ]
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