Numerical Analysis of the Flow Field in a Packed Bed Thermal Energy Storage and its Improvement

The integration of thermal energy storage (TES) systems is key for the commercial viability of concentrating solar power (CSP) plants. Furthermore, TES units can be integrated in different power cycles as a source of flexibility and stability for the grid, particularly in locations with high renewable penetration, and can be employed in industrial processes, to reduce the requirement and waste of thermal energy. A packed bed TES consists of storage material elements in various shapes and sizes and the use of an HTF that flows between these elements. The TES unit exploits the heat capacity of the particulate material in order to store energy. Inside a packed structure, the heat transfer area between solid and HTF is maximized, improving the heat transfer and minimizing the heat transport within the TES media (particularly when low conductivity materials, such as rocks, are used). The work presented in this thesis concerns the CFD modeling of a lab-scale pilot Thermal Energy Storage (TES) built by the members of the Energy Department of KTH – Royal Institute of Technology. The simulation software Comsol Multiphysics has been used to create a suitable model which permits to assess its performance in different conditions. The assessment consists in a first start of the TES, with a complete charge and discharge starting from uniform temperature distribution. A cycle stability analysis has been done, to evaluate the behavior of the storage and the thermocline spread during reasonable working conditions. The effect of thermal insulation and thermal dispersion during charge, discharge and stand-by has been evaluated. Once the model is verified, a series of measures have been proposed and analyzed, in order to improve its performances. The improvements influence both the fluid flow and the thermal behavior, therefore appropriate performance indicators have been chosen. The study ends with some recommendations about further possible investigations and future directions.