Influence of the operational conditions on the transient charging performance of a thermal storage system using supercritical carbon dioxide

Building upon the available literature, the present study investigates how the operational conditions of supercritical carbon dioxide (s-CO2) acting as a storage medium influence the performance of a thermal energy storage (TES) system. The geometry is represented by a rigid-wall cylindrical reservoir filled with s-CO2 and surrounded by an axially flowing heat transfer fluid (HTF) (not modeled in the present analysis). A 2D axisymmetric, transient formulation is developed for the reservoir and s-CO2 assembly while relying on the SST k - omega turbulence model. The numerical solution uses finite volume-based commercial software to solve the conservation equations for both domains. The heat transfer mechanism within the s-CO2 is exclusively based on natural convection and mimics the thermal charging process of the supercritical TES medium. The parametric analysis considers the s-CO2 pressure and the charging temperature of the HTF as independent variables. A total of three discrete values were assumed for the charging temperature of the HTF (i.e., 325 K, 335 K, and 345 K) and four discrete initial pressure values for the CO2 for each temperature, which ranged between 9 MPa and 19 MPa, totaling 12 different initial combinations. The results show that, for a given temperature, there is a pressure that maximizes the heat transfer coefficient. However, the volume-specific energy stored (i.e., kWh/m(3)) within the supercritical TES medium increases with the pressure since higher pressures imply more s-CO2 within the storage system as the domain encapsulating the fluid has a fixed volume. Conversely, the results show that the mass-specific energy stored (i.e., kWh/kg) presents an optimal pressure value for a given temperature.