Performance evaluation of a sand energy storage unit using response surface methodology

Abstract

The utilization of affordable and cost-effective storage materials is a crucial factor in the development of such systems. In this study, the influence of coil pitch, inlet fluid temperature and hot fluid velocity on sand based thermal energy storage (TES) unit is investigated, using experimental results and theoretical models. The experimental segment of this study focuses on measuring the thermophysical properties of two sand samples obtained from different locations within the United Arab Emirates. A conjugated heat transfer model is developed to predict TES using the experimentally measured sand properties. A regression model utilizing response surface methodology (RSM) approach is developed to represent the energy stored per kilogram of sand as a function of the input factors. Furthermore, an optimization algorithm is employed to determine the optimal values of input factors that maximize the energy storage density. The results reveal that the three factors (fluid inlet temperature, velocity, and number of coil turns) significantly affect the stored thermal energy. The RSM analysis illustrates that maintaining high levels of both inlet temperature and fluid velocity maximizes the energy stored. Similarly, keeping inlet temperature and coil turns at the high level maximizes the energy stored. The optimized sand energy storage unit mass reaches 6.348 kJ/kg after an 8-h charging period, with an associated pressure drop of 71.4 Pa for the currently designed unit.