Modeling the cross-section of finned-tube heat exchangers for Latent Heat Thermal Energy Storage Systems used in Carnot Batteries

Kurzfassung

Carnot Batteries are proposed as a solution for the dispatchability problem caused by the growth of fluctuating renewable energy sources. Here, Thermal Energy Storages are combined with thermal power cycles to store electrical energy. In Carnot Battery concepts based on steam power cycles, Latent Heat Thermal Energy Storages (LHTES) are essential components due to their ability to reach high energy densities in a narrow temperature gap thus, allowing nearly isothermal charge and discharge. In LHTES systems, usually storage materials with low thermal conductivities are applied, the identification of effective solutions to transfer heat between the storage material undergoing a phase change and the working fluid is a key challenge, therefore, the development of simulation tools for the analysis of LHTES on different scales is essential for their application in Carnot Batteries This paper analyses the charge and discharge behavior of four different longitudinal finned-tube heat-exchanger geometries with common fin fraction, as an initial step for the development of a multi-scale simulation tool for the integration of LHTES in Rankine-based Carnot Batteries. Each fin geometry is analyzed numerically using COMSOL Multiphysics on a two-dimensional model, to obtain their characteristic charge and discharge curves, specifically in terms of the Phase Change Material (PCM) liquid fraction and the heat flux over a given period. A high-melting-temperature PCM eutectic mixture of KNO3-NaNO3 with a melting temperature of 222 °C was selected. Alternative geometric and thermal performance parameters were proposed as a framework to compare the finned-tube heat exchanger designs. To determine the effectiveness of the proposed parameters to describe each geometry, a correlation between the parameters and the phase change time was made. The average minimum distance (ADmin) between the fin and PCM presented a better correlation compared to the contact perimeter between fins and PCM. Profiles with lower ADmin exhibited higher heat fluxes and lower discharge times due to better material distribution. The insights gained from this study will be applied to the development of an equivalent resistance model based on the characteristic curves of each fin geometry. This model will be experimentally validated using a currently under-construction test rig.