The biggest expense will be the pipework inside that pile. That you won't be able to service easily. And you'll also need a LOT of insulation around it to prevent heat from leaking out too quickly.
The biggest expense will be the pipework inside that pile. That you won't be able to service easily. And you'll also need a LOT of insulation around it to prevent heat from leaking out too quickly.
Thermal storage benefits from the square/cube scaling, so the amount of insulation goes down as you scale up the storage.
The insulation is just more dirt.
Homework: compute the heat flux from a 40 meter radius sphere, given the surface temperature 600C and the ambient temperature 0C. Now compute the amount of insulation needed to keep its temperature above 500C for 3 months, given that dry sand has an R value of 0.25 W/mK.