Black Sheep wrote:
If you have equipment that can easily and efficiently (and, most important, cost-efficiently) extract all of that thermal energy, the outside can be liquid hydrogen cold with no problem.
Right, and as I said, such equipment would violate the second law of thermodynamics.
Black Sheep wrote:
Indeed, you could have the inner surface layer extract all the light and heat, the outer surface layer being very cold, and maybe run power from themocouples, as well as from the solar ourput.
A thermocouple is one of the ways that energy can be extracted by allowing heat to flow from a hot place to a cold place. Like all the other ways it can only extract some of the energy. The rest ends up as heat in the cold place, raising its temperature.
A portion of the energy in high-temperature heat can be converted into a low-entropy form of energy such as electricity, provided that the rest of the high-temperature heat gets converted into low-temperature heat, which has higher entropy, so that the entropy increases in the system as a whole, satisfying the second law of thermodynamics. But low-temperature heat can be low-temperature only if it's dissipated into a large volume, and the higher the energy turnover is, the larger that volume must be.
If the envelope has radiators covering parts of its surface, radiating its waste heat into space, and massive amounts of insulation combined with heat pumps to keep other parts of the surface cold, then I suppose it could be possible to achieve "liquid hydrogen" temperatures (which probably means between 14 and 20 kelvins) on
parts of the surface, but its designers would have needed a really good reason to design it that way.
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