Thermal Ratcheting Modelling in Packed Bed Thermal Energy Storage
Thermocline packed bed storage system is considered as a cheaper alternative to the common two-tank molten salt approach. A packed bed TES consists of storage material elements in various shapes and sizes and the use of a heat transfer fluid (HTF) that flows between these elements. The TES unit exploits the heat capacity of the particulate material in order to store energy. Generally, in operation the HTF flows in one direction during the addition of heat and in the opposite direction during the discharge phase. Then, the TES charging and discharging consists in shifting hot and cold regions in the tank by circulating HTF through the solid TES medium.
However, the intrinsic thermal cyclic behavior may lead to a catastrophic structural failure known as thermal ratcheting. This phenomenon affects the structural integrity of the storage unit. Particularly, it might happen when the TES tank is filled with packing material and cyclically heated and cooled; this will lead to thermal expansion and contraction in both TES filler material and tank. If the wall material has a greater thermal expansion than the packed bed material, a radial gap is generated between wall and particles during heating, thus some of the particles will settle down filling this gap. Then, during the subsequent cooling phase, the tank will not be able to contract completely. This will result in additional thermal stresses in the tank unit, as well as in the packing material, that may cause plastic deformation of the tank and cracking of the pebbles. When this phenomenon is repeated for large numbers of cycles fatigue and rupture of the TES unit may occur.
Thermal ratcheting is one of the main concern that stops the implementation of large scale packed bed TES. Therefore, of special interest, in this sense, is the modelling of this phenomenon in different TES configuration and considering different materials. The aim of this thesis is two-fold: firstly, to develop flexible CFD or DEM models to evaluate the packed bed behavior and interaction with the tank unit; secondly, to evaluate different TES design and potential improvements to limit the thermal ratcheting. The modelling work is performed primarily in Matlab, COMSOL, YADE open-source or similar software.
The main deliverables of the project include:
• Final project report and presentation comprising description of project, literature review packed bed and thermal ratcheting, implementation of models, comparative analysis of different designs and final suggestions.
• Flexible model in COMSOL (or similar): model scripts and user guidelines / instructions.
The project should start in January 2020 the latest, and should not extend for more than 6 months.
Specific earlier starting date to be discussed.
Location: KTH - Energy Department.
PhD, Researcher, Heat and Power Division
Contact person, MSc, PhD Student, Heat and Power Division
Examiner, Professor and Head of Energy Department