Natural gas and wood chips,

> "The installation will supply heat to the Vääksy district heating network and is expected to lower fossil-based emissions by approximately 60% annually, primarily through an estimated 80% reduction in natural gas consumption and reduced reliance on wood chips."

https://www.pv-magazine.com/2025/11/25/finlands-polar-night-...

Those are the energy sources they're replacing with this tech - according to <https://reneweconomy.com.au/new-worlds-largest-sand-battery-...> it's surplus energy from renewables that will 'charge' the battery (so likely wind, hydro and solar that is produced but surplus to the grid's requirements)