It doesn't list the advantages over water, which seems the most common in https://en.wikipedia.org/wiki/Thermal_energy_storage systems.
You'd think water would be easier to exchange heat with since it can slosh around the heat exchanger elements in the tank more easily. Which should translate to lower costs since you don't need as many exchanger structures in the medium.
Any guesses for the motivation in using sand? Maybe it's that you can heat it over 100C? But then big heat differences to the environment mean high conductive/radiation losses or heavier insulation requirements.
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