Modeling of energy carrier in solar-driven calcium-looping for thermochemical energy storage: Heat-mass transfer, chemical reaction and stress response

The solar-driven calcium looping process (CaL) poses a great potential for thermochemical energy storage. The calcium-based particle, a core energy carrier for CaL, however, is prone to fragmentation, significantly reducing the efficiency and stability of energy storage. In this work, a particle scale model for core–shell structured energy carrier that considers the heat transfer, hierarchical reaction, mass transport and thermal stress processes is proposed to study the characteristics of energy storage performance. The multiprocess model confirms that stress failure at the particle center is the main reason of energy carrier fragmentation, resulting from the exceedingly high radial tensile stress. To improve the energy storage performance, the operation conditions and modified particle properties of CaL are investigated. It was found that the thermal stress can be relieved with the reduction of energy carrier temperature, while energy storage efficiency is decreased. In addition, the thermal stress can also be reduced by an appropriate increase in the airflow temperature without sacrificing the efficiency. Moreover, energy carrier can yield a high energy storage efficiency and cycle stability when the radius falls into 300–500 μm.