  A thermostat senses the temperature of a physical system and switches heating
or cooling devices on or off, regulating the flow of heat to maintain the
system's temperature near a desired setpoint. Taking advantage of recent
advances in radiative heat transfer technologies, here we propose a passive
radiative "thermostat" based on phase-change photonic nanostructures for
thermal regulation at room temperature. By self-adjusting their visible to
mid-IR absorptivity and emissivity responses depending on the ambient
temperature, the proposed devices use the sky to passively cool or heat during
day-time using the phase-change transition temperature as the setpoint, while
at night-time temperature is maintained at or below ambient. We simulate the
performance of a passive nanophotonic thermostat design based on vanadium
dioxide thin films, showing daytime passive cooling (heating) with respect to
ambient in hot (cold) days, maintaining an equilibrium temperature
approximately locked within the phase transition region. Passive radiative
thermostats can potentially enable novel thermal management technologies, e.g.
to moderate diurnal temperature in regions with extreme annual thermal swings.
