Numerical investigation of thermocline packed bed thermal energy storage system for CSP plants

Thermal energy storage system is becoming indispensable part of any Solar Energy Generation Systems (SEGS) because of its intermittent nature and for enhancing dispatchability of Concentrating Solar Power (CSP) plants. In this study, a two dimensional transient analysis of thermocline sensible heat storage system was carried out numerically to study the discharging characteristics of the packed bed storage system. The governing equation was discretized by finite volume method and coded in MATLAB R2014a. The model takes into account local thermal non-equilibrium between the Heat Transfer Fluid (HTF) and the solid filler bed, thermal diffusion inside HTF, various temperature dependent correlations to obtain the thermo-physical properties for the HTF and considers heat loss through the insulated tank wall henceforth making the model more reliable to design a Thermocline Thermal Energy Storage (TTES) tank. The behaviour of thermocline during discharging and various parameters affecting the performance of the system, namely – tank dimensions, inlet velocity of HTF, storage temperature difference, and cut-off temperature difference criterion were considered. The model was carefully verified based on grid refinement and time step independence studies, and validation of the reference case was done with experimental results in the literature to ascertain the consistency of the proposed model. After validation, all the further analyses were done by taking a modified reference case. It was observed that during discharging, hot region shrinks continuously and the thermocline region is moving upward accompanied by slight expansion and it reached a maximum value of 4.76m inside the tank, almost 1/3rd of the tank height; discharging time increases with height of the tank and decreases with inlet velocity of the HTF; increasing the diameter of tank improves the storage capacity of the system whereas it has no significant influence on the discharging time. It was also obtained that discharging time not only depends on the storage temperature difference, but also with the cut off temperature difference criterion. Discharge time increased from 4.52 hours to 5.32 hours when the cut-off temperature difference was changed from 20 K to 30 K.