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A temperature-doubler circuit is the functional equivalent of a voltage-doubler in the thermal domain. Effective temperature-doubler circuits could benefit energy scavenging from fluctuating thermal resources, e.g. the diurnal cycle. However, the current paradigm relies on static photonic designs of the selective solar absorber or blackbody emitter, which aims at maximizing energy harvesting from either the sun or outer space, but not from both. Furthermore, photonic and thermal optimizations have not yet been coupled to maximize the power output. Here we develop a general framework to optimize the energy acquisition and conversion simultaneously to maximize a temperature-doubler's power output under a realistic solar-thermal boundary condition. With an ideal self-adaptive absorber/emitter to fully exploit the thermodynamic potential of both the sun and outer space, the theoretical limit of the temperature-doubler circuit's average output power in a diurnal cycle is found to be 168 W m−2, a 12-fold enhancement as compared to the blackbody emitter. We provide a numerical design of such a self-adaptive absorber/emitter, which, combined with a thermoelectric generator, generate 2.3 times more power than the blackbody emitter in a synthetic "experiment". The model further reveals that, as compared to traditional thermal circuits, the key merit of the temperature-doubler is not to enhance the total power generation, but to convert the fluctuating thermodynamic input to a continuous and stable power output in a 24 h day-night cycle. [ABSTRACT FROM AUTHOR]