Generating Power at Night Using a Thermoradiative Diode, How is this Possible?

Conventional photovoltaic solar power conversion relies on extracting free energy from the flow of thermal radiation from a hot emitter, the sun, to a cold absorber, the PV cell. A PV cell can thus be described as a heat engine. A much less well studied configuration is to place the engine on the hot side of this problem, so for example extract free energy from the flow of thermal radiation from the Earth into the very cold void of outer space. This proposition has recently been studied by a increasingly large number of research groups, showing a semiconductor thermoradiative diode operating in the fourth IV quadrant with negative voltage and positive current will generate power. The limits to this process will be presented together with an assessment of the power density that could be achieved by either radiating into the night sky, or as a means of heat recovery.