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• A comprehensive transient axial-symmetric model was developed for the solar thermochemical reactor. • The electrochemical oxygen pump (EOP) with the lowest energy penalty was employed to reduce the oxygen partial pressure. • Reactor performance for EOP scheme and other two traditional schemes (i.e., SG scheme and VP scheme) was compared in detail for different DNI conditions. • Two control strategies were proposed to increase the solar-to-fuel efficiency. Solar thermochemical water splitting provides an efficient route for converting solar energy into renewable fuels. The operation of solar thermochemical reactor under real on-sun condition requires in-depth understanding of its dynamic behaivor under fluctuating direct normal irradiation (DNI). In this work, we developed a 2D axial-symmetric multi-physical model to assess the reactor's dynamic behavior with three oxygen removing technologies electrochemical oxygen pump (EOP), sweep gas (SG), and vacuum pump (VP), under varying solar irradiations. To alleviate the negative effect of decreased DNI, two control strategies (CS) including varying operational conditions for oxygen removing technologies (CS1) and replacing the reduction step with the oxidation step (CS2) were conducted. We found that CS1 was only effective when DNI reduction was larger than 75 % over the whole reduction step (case 2). In general, CS2 can be implemented to alleviate the efficiency decrease for all DNI variations expect for the case 2 in this study. Especially, CS2 significantly increased the solar-to-fuel efficiency compared to the case without control strategy for the reduced DNI from 1000 W/m2 to 500 W/m2 during the whole reduction step. The reactor dynamic behavior in response to the daily DNI variations is also reported which shows that the efficiency profiles are highly in line with the DNI profile. This modeling work as well as proposed control strategies can be used to guide the design and control of solar thermochemical reactors under realistic conditions and hence for better assessing the performance of the reactor in practice. [ABSTRACT FROM AUTHOR]