Experimental study of layered thermal energy storage in an air-alumina packed bed using axial pipe injections

This paper presents the experimental results of thermal behavior in an air-alumina packed bed storage system using a new technique for charging/discharging processes. A normal packed bed system, 100 cm in length, is divided into layers via pipes inserted internally along the axial length of the bed. Alumina beads were used as solid storage material and air was used as the heat transfer fluid (HTF) with an inlet temperature of 150 °C. This study analyzes the thermal behavior for full charge/discharge processes by dividing the bed domain into layers, focusing on the thermal exergetic efficiency for different charging/discharging schemes. One, two, and three layer configurations are considered along with various schemes including duration and magnitude of mass flow to each layer. In the most efficient configuration, the thermal exergetic efficiency increases with the number of layers from 53.2% to 69.6% for 0.0048 m3/s and 55.4% to 73.4% for 0.0061 m3/s, from one layer to two layers. At these same flow rates, thermal exergetic efficiencies increase to 76.8% and 80.3% for three layers. To determine the contributions of axial thermal dispersion and heat losses, a numerical model was run for a full charge/discharge cycle in adiabatic and non-adiabatic cases. The model results show that the dispersive effect is reduced by 23.2% in the best two-layer case and 25.6% in the best three-layer case for 0.0048 m3/s and by 22.8% in the best two-layer case and 26.5% in the best three-layer case for 0.0061 m3/s, resulting in these gains in exergetic efficiency.