You basically design a material that is "black" (good absorber-emitter, conversely: bad reflector) in the infrared window[0] and "white" (good reflector/bad abs

You basically design a material that is "black" (good absorber-emitter, conversely: bad reflector) in the infrared window[0] and "white" (good reflector/bad absorber-emitter) in the ranges where the bulk of the incoming radiation is. That way you maximize the amount of energy it can shed and minimize the amount it absorbs. You're not cheating thermodynamics, you're just doing better than for example titanium dioxide paint or metal foil normally do because those also reflect as much sunlight as possible but at the cost of of not shedding heat through IR radiation. [0] https://en.wikipedia.org/wiki/Infrared_window https://en.wikipedia.org/wiki/Infrared_window