Enabling high conductance and high energy density in supercritical fluids for thermal storage applications

Supercritical fluids have recently been proposed as a sensible media for thermal energy storage. Given the importance of natural convection to sensible storage, the present work explores the existence of optimal operating pressures in supercritical fluids relying on natural convection as the heat transfer mechanism. A theory-based formulation for estimating the optimal operating conditions is presented and validated with experimental data reported in the literature. The heat transfer coefficient is shown to be related to the thermodynamic state of the fluid, which strongly affects its thermophysical properties. In particular, the optimal operating conditions are shown to lie in a thermodynamic region where properties present large variations within small temperature and pressure intervals. Finally, the relationship between the optimal operating pressures for maximum natural convection heat transfer, resulting in maximal global conductance, and maximum energy density in supercritical fluids is explored. Here, it is demonstrated that these desirable properties are strongly correlated, suggesting that a supercritical thermal energy storage system can take advantage of both enhanced heat transfer, as well as high energy density.