Comprehensive Comparison of Different Models for Large-Scale Thermal Energy Storage

Large-scale thermal energy storages (TES) are advantageous to bridge the seasonal gap between heat demand and availability of renewables. However, the high investment costs associated with large-scale TES is still seen as a major barrier. Among others, challenges are space availability and the presence of groundwater. The complexity of the processes and interactions motivate the application of simulation tools for planning such systems. For TES optimization, flexible and detailed models are required that allow to investigate different geometries, insulation levels and boundary conditions, e.g. presence of groundwater (GW). In contrast, fast and easy-to-use models are required for simulations of a TES integrated in a larger system. In this work, different TES models are compared in various simulation platforms: COMSOL Multiphysics, TRNSYS, Modelica/Dymola and MATLAB/Simulink. The paper summarizes the features of the different TES models, shows the different concepts to reduce the model complexity and compares the results with respect to thermal losses and temperature stratification. TES Types and geometries include buried tanks (cylinder, cuboid), pits (truncated cone, pyramid stump) and hybrids (cylinder with cone stump, cuboid with pyramid stump). TES can be built either completely buried or partially buried building a dam with (part of) the excavated soil. Detailed 2D and 3D FE models developed in COMSOL Multiphysics were validated against measured data from a pit TES and were used as reference for this study. Some of the models take advantage of symmetry and cylindrical coordinates in order to reduce the model to 2D (cylinder, cone). Within this work, deficiencies could be identified, models could be improved and also the influence of the user was seen. Overall, good to acceptable agreement between the tools was achieved after a review phase and after eliminating bugs and user influence.