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 degrees 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 m(3)/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 m(3)/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.