The Stefan-Boltzmann Law (sometimes called Stefan’s Law), states that the energy radiated by a blackbody is directly proportional to the fourth power of the body’s absolute temperature. Given the apparent inviolability of Stefan’s Law, schemes that attempt to harness weak radiation from relatively cool bodies (the earth) will always prove economically inferior to schemes that attempt to harness energy from much warmer bodies (the sun). Trying to capture the earth’s weak thermal radiation (only a couple of Watts per square meter contrasted with the sun’s some 1300 Watts of radiation) just sounds goofy to me. One might just as well try to capture the Moon’s infrared radiation. One minute of direct sunlight supplies roughly as much energy per unit of collector surface as ten hours of Earth’s radiation. No scheme can capture energy that just isn’t there. So large surface area collectors would be needed to capture relatively small amounts of energy from the earth’s thermal radiation. If we are going to have large surface area collectors, why not aim them at the sun and obtain 250 times as much energy per unit of area? The whole concept of rectifying thermal noise to create electrical potential, what an ideal gadget, has been studied exhaustively, and found to be non-competitive. There are good reason NASA powers its innerplanet spacecraft with photovoltaic panels and not radioisotope thermoelectric generators. The pure physics argues much higher conversion efficiency per unit of weight in photovoltaics. But at great distances from the sun, there is little solar radition to harvest, so the radioisotope thermoelectric generators becomes the most practical choice.
To obtain useful energy near the Earth’s orbit by harvesting radiation, it’s the Sun’s radiation, not the Earth’s, that by far makes the most sense.
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