There's pipework for circulating air inside it when they want to charge/discharge it, but yes, essentially it's mostly tons of sand.

They have resistors for charging it with electricity (resistors heat the air, air is circulated in the pipes which heats the sand) when the electricity price is cheap, and then for discharging they have a air-water heat exchanger so they can pump the heat energy into the district heating network.

Why do they use air for this instead of water?