They also state the cost per kWh(th) of storage capacity is about $0.10, or $100,000/GWh(th). This is thermal energy, which is converted to work at maybe 40% efficiency, mind you. There are costs over and above the dirt (for example, pipes laid through the dirt to carry heat in/out).
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.
Even better than that: the thermal time constant is proportional to the square of the radius. There's the square/cube effect that causes a factor of r ratio in the volume to surface area, but there's also a factor of r from the temperature gradient (which, from the center of the pile to the surface, is inversely proportional to r).
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.
so $36-72k for days of 1GW of power? Sounds pretty cheap to me ...
They also state the cost per kWh(th) of storage capacity is about $0.10, or $100,000/GWh(th). This is thermal energy, which is converted to work at maybe 40% efficiency, mind you. There are costs over and above the dirt (for example, pipes laid through the dirt to carry heat in/out).
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.
Even better than that: the thermal time constant is proportional to the square of the radius. There's the square/cube effect that causes a factor of r ratio in the volume to surface area, but there's also a factor of r from the temperature gradient (which, from the center of the pile to the surface, is inversely proportional to r).
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.
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