[cyberax]

I ran the numbers on that, and it just doesn't work. Stone has rather lousy specific heat capacity (less than 1kJ/kg/K, compared to 4.2kJ for water).

A typical house in Midwest needs around 22,000kWh (7.913×10^10 J) over the winter (75 million BTU - https://www.eia.gov/todayinenergy/detail.php?id=57321 ).

If we assume the delta of 550 degrees (600 down to 50), you'll need: 7.913×10^10 J / (550K * 1000Jkg^-1K^-1) = 143,872,727 kg of material in your pile. This is a ridiculously stupid number. And I don't see any obvious mistakes?

[pfdietz]

Your decimal point slipped three places in that last calculation; the result is too high by a factor of 1000.

A more worthy criticism is that the pile for just a single house is too small and would cool off too quickly.

[cyberax]

I don't believe it did? Delta of 550 degrees Kelvin multiplied by 1000J per kg per Kelvin.

[pfdietz]

7.913e10 / ( 5.5e2 * 1.0e3 ) = 1.438e5, not 1.438e8

When doing calculations like this I just fire up a lisp and enter the thing to be calculated as lisp form.

[kragen]

I use units(1), which also helps me avoid dimensional errors (dividing when I should have multiplied, etc.):

   You have: 7.913e10 J / 550K / (1J/g/K)
   You want: kg
     * 143872.73
     / 6.9505876e-06

maxerickson says, "Still big number," and 144 tonnes would typically be an unwieldy quantity of material if you had to buy it. But Standard Thermal's intention is not to buy dirt, just pile up already-on-site dirt with a bulldozer or excavator. If we assume 1.3 tonnes/m³, that's 110m³, or, in medieval units, 144 cubic yards. https://www.eaglepowerandequipment.com/blog/2022/03/how-much... tells us:

> An excavator could be used to dig anywhere from 350 to 1,000 cubic yards per day, depending on a number of factors including bucket capacity, type of ground, operator skill and efficiency level, and more. (...)

> One of the biggest factors that impact how much an excavator can dig in one day is the unit’s bucket size, which typically ranges from 0.5 to 1.5 cubic yards of bucket capacity. Most common regular-size excavators have a 1 cubic yard bucket capacity, and mini excavators are closer to the 0.5 cubic yard capacity.

So, with this number, we're talking about a few hours of work for a "mini excavator". https://www.bigrentz.com/rental-locations/pennsylvania/pitts... tells us that a "4,000 lb. mini excavator" rents for US$197 per day. So the expense of moving the dirt is not really significant, compared to other household projects such as replacing the roof, insulating the walls, or repainting the exterior.

Standard Thermal mentions that they are in effect firing the clay in the ground, that they've had significant trouble with resistance-heater reliability, and that their objective is to power steam-turbine power stations with the stored heat. These three facts lead me to believe that they're targeting a temperature closer to 1000° than to 600°.

[pfdietz]

600 C is about what a coal fired power plant would use. And 600 C is around the maximum that you want if you're using cheap steel for the pipes. Much beyond that and creep becomes a problem. So I don't think 1000 C is their target.

[kragen]

Hmm! Interesting! I would have thought that 600° would be close to the minimum for producing supercritical steam, so any energy stored up to 600° would be "overhead" that couldn't be effectively recovered—only the heating above that. And I assumed they would have to use cheap ceramic for the pipes, because oxidation is usually a problem for cheap steel even below 600°.

[maxerickson]

You have 8*10^10 in the numerator and 5*10^5 in the denominator, so the result should be roughly 8/5*10^5.

Still big number.