> Not sure about the math here, but it is surely cheaper to build a bigger tank with crappy sand than a smaller one with more expensive clay.

It's also interesting that the sand is heated with energy generated from wind turbines and solar panels, which means that as long as enough energy is being thrown into the thermal capacitor, it will heat up with little cost. From this point onward, it's an economics problem. Either money is thrown at the efficiency of these heating capacitors, which brings at most small gains, or money is thrown in building additional storage capacity for cheap along with increasing how much energy is fed into the system. Would it be more cost effective in replacing sand with more efficient materials, or spend the money in, say, another cheap thermal energy source?