Enhanced solar-to-hydrogen energy conversion utilizing microtubular solid oxide electrolysis cells as a volumetric solar absorber

Driving solid oxide electrolytic cells by solar power is a renewable way to produce green fuel. However, separating the solar heat collection process from the electrochemical process inevitably increases resistance of energy conversion. Herein we propose a concept of an integrated solar reactor that utilizes microtubule solid oxide electrolysis cell (SOEC) as volumetric solar absorber. This design concept not only shortens the transfer path of heat energy to chemical energy but also significantly increases the reaction density. Performance analysis demonstrates that the maximum energy efficiency can reach 65.11 % owing to the cavity effect. Assuming a typical photovoltaic efficiency of 20 %, the proposed reactor has a maximum solar-to-hydrogen efficiency of 17.18 %, and this value can increase to 20.64 % when integrated with the waste heat recovery system. Additionally, transient analysis reveals that since the external electric power is the main driving force of the reaction and can be easily controlled, the steam conversion ratio can be maintained continuously and stably at 88 % even under fluctuating solar input on a cloudy day. This stable characteristic of hydrogen production can significantly enhance the time-average efficiency of solar fuel conversion, demonstrating a substantial value for industrial applications.