Numerical modeling of large-scale finned tube latent thermal energy storage systems

Abstract

In latent thermal energy storage, heat is transferred between a single- or two-phase heat transfer fluid and a solid/liquid phase change material. Due to the low thermal conductivity of most suitable storage materials, heat exchangers with highly thermally conductive fin structures are used to obtain feasible heat transfer rates. To study and optimize the performance of latent thermal energy storage systems, simulation models applicable on a large-scale level are required. However, the modeling of such a system in detail requires high computational effort. To overcome the current limitations of such models, a simplified and fast model for large-scale industrial storage systems is proposed in this work. The model was implemented with a self-tailored MATLAB code and consists of two coupled parts: the heat transfer fluid region and the storage region, which includes the phase change material and the heat exchanger. The heat transfer fluid region is modeled with quasi-stationary onedimensional single- or two-phase flow models and constitutive equations for pressure drop and heat transfer. Material properties of liquid thermal oil and two-phase water/steam are used. The heat transfer fluid model was verified using simulation results obtained with the commercial software Apros. The storage region was modeled based on the transient energy equation and a phase change model on a structured cylindrical geometry. To efficiently include the effect of the heat exchanger, an effective fin model for the mixture of the storage and the fin materials was developed and implemented. The effective model was first adjusted and verified using detailed reference simulations of the discretized fin structure with ANSYS Fluent. Finally, simulations with the coupled large-scale models of two reference finned tube storage systems were performed: The first one used single-phase oil as heat transfer fluid and radially oriented plate fins in the storage material. The second one used a two-phase water/steam heat transfer fluid and axially extruded fins in the storage material. The effective fin model could be verified by comparison with detailed models of the same storage systems that discretized the fin structures. The proposed modelling approach proved to be accurate and enables a more efficient design and optimization process for latent thermal energy storage systems.