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.
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