Experimental study on the cyclic behavior of thermal energy storage in an air-alumina packed bed

Thermal energy storage (TES) in a packed bed exemplifies important technology for concentrated solar thermal (CST) applications such as electricity production, desalination, enhanced oil recovery, fuel production and chemical processing. In this study, the cyclic charge-discharge behavior of packed bed TES was studied experimentally using alumina beads as packing material. Air was used as heat transfer fluid (HTF) with an inlet temperature of 150 °C. This paper shows the effect of flow rates, partial charge-discharge cycling, and storage hold time on the exergetic efficiencies. The results indicate that the exergy efficiency increases from 35.7% to 55.4% with increasing flow rate from 0.0020 to 0.0061 m3/s. The exergy decays for multiple cycles before reaching a steady state. Over partial charge-discharge cycles at flow rates of 0.0020, 0.0034, 0.0048, and 0.0061 m3/s, the exergetic efficiency decays from 59.8% to 50.2%, 72.5% to 61.2%, 79.0% to 66.2%, and 83.1% to 69.2%, respectively. Heat losses and axial thermal dispersion are two important variables that affect the exergy efficiency, and the individual contributions were estimated via a model for the partial cycles. Heat losses were considered for three durations of holding: no hold, 30 min hold and 120 min hold. The exergy efficiency decays from 53.2% to 31.0% from no hold to 120 min hold due to the heat lost to the ambient.