Interesting! Wonder if it's also due to ease / lower risk of containment. And sand doesn't expand if you accidentally let it freeze, which is again nice from an "it won't rupture" perspective.

As to cost, the article does note that they're reusing crushed soapstone from a local byproduct, so maybe that helps reduce the cost?

That said, their FAQ says it's about heat capacity: https://polarnightenergy.fi/sand-battery

> Why do you use sand?

> Many solid materials, such as sand, can be heated to temperatures well above the boiling point of water. Sand-based heat storages can store several times the amount of energy that can be stored in a water tank of a similar size; this is thanks to the large temperature range allowed by the sand. So, it saves space and it allows versatile use in many industrial applications.

So perhaps they're also specifically targeting future applications where they would need to supply > 100C heat.

Another concern is losses. Water can evaporate, can leak, and tends to corrode fittings.

Sand? If some spills just grab a skid loader and stuff it back in.

More water takes more space, and perhaps the higher temperatures make it easier to reuse the heat? And you probably don't want your water to get above 100 degrees C, because then you need to deal with pressure.

Sand is also a really good insulator if I'm not mistaken. That could also be a factor somehow.