> [Sand] Can store a lot more heat by volume.

A quick caveat/clarification: It's only true if you're pushing the system over the 100°C mark. Otherwise a volume of liquid water--with its greater latent heat-capacity--will outclass the same volume of sand.

Water's heat-capacity is 4.186 J/g°C, while estimates for sand run towards ~0.830 J/g°C. If we also assume the sand is 1.6x denser, then our below-boiling water still comes out ahead at ~3.15x the joules per volume.

There are hints [0] this system tops out around 600°C.

[0] https://polarnightenergy.fi/sand-battery

I think the original plan was to convert the heat back into electricity with a turbine. So the higher temperature of sand would greatly improve thermodynamic efficiency.

>I think the original plan was to convert the heat back into electricity with a turbine.

Is that just speculation or did you read it somewhere? IIRC the original motivation of PNE was a bunch of engineers at uni speculating on how to build the perfect building for engineers, and making it self-sufficient would require handling its own heating, which they originally thought would be best done with a big hot-water tank to store the heat. No turbine was suggested, IIRC.

Yeah see that but i imagine the safety engineering is a fair bit easier due to not having to worry about the pressure of super heated water.