> In this scenario, since your input energy is basically free, more storage capacity = more money, and switching to clay could triple your effective capacity.

That's all fine and dandy, but the whole point is that projects are evaluated in terms of money and not joules. This means that if a material like clay is , as you claim, 3 times more efficient but ends up costing twice or more to replace and more to maintain, you are in reality looking at a prospective best-case scenario that lies somewhere significant losses or irrelevant gains.

Meanwhile, building the exact same sand-based prototype right next to the original one would literally double the capacity at a far smaller cost.

People want to get warm, not discuss whether clay or sand pack as much heat. Half the world could not care less about what goes into AA batteries.

Projects are evaluated in financial and economical terms. It matters nothing if there's a theoretical possibility that some other material or design is more efficient, if it's savings are irrelevant. The world rides Volkswagens, not Formula 1 cars.