N-p junction to enhance solar thermionic energy conversion: From a thermodynamic point of view

Abstract

Solar thermionics, owing to its silent operation and potential effectiveness in converting solar energy into electricity, is becoming a promising advanced technology for concentrated solar power. However, conventional solar thermionic emission and associated solar-to-electricity conversion are weakened because a portion of the cathode’s energy is transmitted to the anode in the form of thermal radiation and eventually dissipated into waste heat. In this paper, a n-p junction is employed as the cathode to couple thermoradiative and thermionic emission into a single process, lowering thermal radiation in the interelectrode while generating additional electricity. An intricate thermodynamic model is developed to investigate the effects of various system factors, such as concentration ratio and cathode electron affinity, on solar cathode-thermoradiative enhanced thermionic (C-TET) conversion. The findings reveal that the increasing induced potential barrier of thermoradiatives raises the cathode temperature, compensating for decreased thermionic current at elevated thermionic voltages. When the concentration ratio is 1000, solar C-TET surpass conventional metal thermionics in terms of solar-to-electricity efficiency by 27.5 % at a cathode bandgap of 0.15 eV and electron affinity of 1.8 eV. The n-p junction boosts thermionic exergy efficiency by 13.9 % and contributes extra thermoradiative exergy of 3.45 %, resulting in a 25.8 % increase in overall power generation over metal thermionics. Temperature-entropy diagram is also established to identify the irreversibility and principal performance bottleneck of solar C-TET conversion. Conduction band electrons transit to the valence band with a modest entropy production of 17.29 W/K·m−2 at n-p junction, while thermionic emission produces 101.91 W/K·m−2.