Off-eutectic Binary Salt Finite Volume Method

Non-linear phase diagrams for binary compositions are coupled with Thétis, a finite-volume solver, to calculate charge/discharge solutions for an off-eutectic salt thermal energy storage candidate. Previous methodologies often include linearized versions of these phase diagrams that cease to give accurate solutions when applied to materials with highly non-linear diagrams, such as the off-eutectic salt mixture NaCl-NaNO3. Consequently, large discrepancies in the solid fraction calculation and subsequently in the heat storage capacity calculation of the storage candidate may be found. While the choice of using a linear or a non-linear phase diagram will mainly affect the solid fraction calculation and thus the latent heat calculation, the thermal properties will typically have a larger effect on the sensible heat calculation. Usually, thermal properties are taken as global averages over space and time, but this may not be an adequate approximation, especially when dealing with large temperature ranges, macrosegregation, and partial melting/solidification. Instead, local thermal properties should be calculated and applied for such cases.A methodology is presented to include non-linear phase diagrams and locally calculated thermal properties in Thétis, starting with validation of the original methodology with published results for the widely studied alloy Pb-18%Sn. The original methodology is then modified to account for non-linear phase diagrams and variable local properties. Finally, the methodology is applied to a fully non-linear off-eutectic salt NaCl-55%NaNO3 (a potential candidate for thermal energy storage) in order to evaluate the importance of using the more elaborate current methodology for charge/discharge solutions. The heat storage for a particular case using the current methodology is found to be 24% lower than for the original methodology, confirming the benefit of using the current methodology.