A numerical study of supercritical carbon dioxide as a medium for thermal energy storage applications under natural convection

This numerical study investigates supercritical carbon dioxide (s-CO2) as a medium for thermal energy storage (TES) systems operating under natural convection regarding heat transfer coefficient and energy density. The numerical domain comprises a horizontal heated cylinder surrounded by the supercritical fluid. The modeling employs a 2-D formulation due to the axial symmetry while the constitutive equations consider that the thermophysical properties vary spatially with temperature and pressure. The results revealed thermodynamic states that maximize the heat transfer coefficient and energy density for constant-pressure operation. Besides, considering the numerous studies on correlations for the natural convection heat transfer coefficient for s-CO2, the analysis compares simulation data with correlations based on constant and integrated-averaged fluid properties. The results suggested that the constant-property formulation fails in the thermodynamic region where properties change significantly. Thus, this work proposes a criterion for deciding when to consider integrated-averaged properties regarding the behavior of the isobaric thermal expansion coefficient. Finally, the study extended the analysis to air, helium, and nitrogen while employing operational conditions suitable for power cycles using such fluids. The comparisons revealed that, among these competitors, helium presents the highest heat transfer coefficient, but s-CO2 has the highest energy density.