Development of Helmholtz-Energy based Multi-Parameter Property-Models for New Binary and Multinary Working-Fluid Mixtures

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Due to characteristics of the involved heat storage, the inverse design of Carnot-Batteries will likely identify new binary and multinary zoetrope mixtures as ideal working fluids. And very likely only few data will be available in literature for thermodynamic properties of these previously not considered mixtures. Property calculations in inverse design approaches usually rely on physically based equations of state with limited accuracy. However, in a second step accurate property data are required to validate the results of the inverse design. For applications in energy technologies, accurate calculations of thermodynamic property data usually rely on empirical multiparameter equations of state in terms of the reduced Helmholtz energy. The need to describe new working fluids, for which only restricted data sets are available in literature, resulted in large progress with regard to fitting such empirical equations of state for pure fluids to small data sets. The methodological developments, which are crucial for this progress, were: - The reduction of intercorrelations between parameters of the equations; - The consideration of ideal curves as criteria for reasonable extrapolation; - The consideration of a multitude of particularly sensitive properties in order to check physically reasonable behavior; - The introduction of complex constraints in completely non-linear optimization and fit algorithms; - The introduction of hybrid data-sets consisting of experimental data and of simulation results for derivatives of the Helmholtz energy. Thermodynamic properties of mixtures relevant in energy technologies are described in high accuracy by empirical multiparameter equations of state formulated in terms of the reduced Helmholtz energy as well. However, fitting such models to mixture properties results in additional degrees of freedom, which are safely mastered only for well measured systems to date. The goal of the project proposed here is to transfer the progress that has been made for pure fluids to the development of multiparameter mixture models. However, due to the more complex structure of both the models and the thermodynamic-property surface, findings for pure fluids cannot be simply transferred to mixtures. Instead, new approaches have to be developed, which enable analogous solutions. Ideally in cooperation with other projects, which can contribute suitable experimental data and results of molecular simulations and which allow for an exemplary application of results, the proposed project can deliver the thermodynamic property basis required to achieve the goals of the SPP. The methodological approaches developed in this project will allow for the development of improved property models for a multitude of technical and scientific applications in relatively short time.