Solar thermochemical heat storage via the Co₃O₄/CoO looping cycle: Storage reactor modelling and experimental validation

Thermochemical energy storage (TCES) systems utilize reversible reactions to store solar energy in chemical form. The present work focuses on the cobalt/cobaltous oxide (Co₃O₄/CoO pair) based redox cycle in which the active oxide is coated on a cordierite honeycomb structure. During the redox cycle, cobalt oxide uptakes and releases oxygen from/to an air stream coming in direct contact with it. Thus air acts as a reaction medium as well as a heat transfer fluid (HTF). In this configuration, the storage material works as a heat storage medium and also a heat exchanger. A two-dimensional, axisymmetric numerical model to simulate the heat and mass transfer and the chemical reaction in the thermochemical heat storage reactor has been developed. Experimental results from a 74 kW h_th-capacity prototype reactor installed at the Solar Tower Jülich test facility, Germany, were used to validate the numerical model. The time-dependent boundary conditions in the form of inlet temperature and inlet mass flow rate from the experiments were employed in the numerical model. The temperatures of the redox material at different locations inside the prototype thermochemical storage/heat exchanger reactor were used for the numerical model validation. Total energy stored/released (sensible as well as chemical) during the experiments was also compared with the numerical model results. From this study, it is concluded that the numerical model can accurately predict charging/discharging processes for the cobalt oxide based thermochemical storage reactor system for multiple redox looping cycles. The model allows a better understanding of the complete process and helps to identify the effect of variation of boundary conditions on the system.