Multiphysics simulation of thermal energy storage and extraction in microwave air heaters

Materials with a high loss tangent, such as activated carbon (AC), can serve as microwave (MW) absorbers for thermal energy storage. The stored energy can be recovered using a microwave-assisted air heater (MAH), in which thermal energy is transferred from the hot AC to the air stream within a helical coil. The MAH system is compact, efficient and emission-free compared to traditional heating methods such as solar and biomass heaters. Experimental demonstrations of the MAH have been conducted. This study aims to further understand the factors that affect the overall efficiency of the MAH. A new coupled multi-physics model was developed and validated using published experimental data. The effects of helical coil geometry (axial pitch length and major diameter), reactor diameter, and AC reactor porosity on overall efficiency were examined. The simulation results show that axial pitch length has a minimal effect on the generation and extraction of MW heat rate. The major diameter of the helical coil affects the relative position between the hot spot and the helical coil, thereby affecting extraction efficiency. The simulation results predict an overall MAH efficiency of 0.64–0.78. The findings suggest that increasing the MW heat rate requires a larger AC reactor but reduces the extraction efficiency. Hence, further research on multi-objective optimisation is required to optimise the overall efficiency of the MAH. • A new model was developed to simulate the heat extraction efficiency of a microwave (MW)-assisted air heater. • Heat rate extraction efficiency increases with the temperature difference between the MW absorber and airflow. • A larger reactor size favors MW energy absorption but decreases the heat extraction efficiency. • The optimum thermal power extraction requires the trade-off between MW absorption and heat extraction efficiency.