Density MgO 3.58
Density C 3.52
Kind of close but might be enough.
Energy can be solar, wind, geothermal, hydro, tidal, perhaps nuclear.
I don't think we can afford cooling. Don't want to lose energy in the MgCO3⇌CO2+MgO reaction. Want to recover energy from 2Mg+CO2⇌2MgO+C for electrolysis of...