Thermal stress numerical study in granular packed bed storage tank
Thermal energy storage (TES) systems are central elements of various types of power plants operated using renewable energy sources. Packed bed TES can be considered as a cost-effective solution in concentrated solar power plants. Such a device is made up of a tank filled with a granular bed through which a heat-transfer fluid circulates. However, in such devices, the tank might be subjected to an accumulation of thermal stresses during cycles of loading and unloading due to the differential dilatation between the filler and the tank walls. The evolution of tank wall stresses over thermal cycles, taking into account both thermal and mechanical loads, is studied here using a numerical model based on the discrete element method. Simulations were performed for two different thermal configurations: (i) the tank is heated homogeneously along its height or (ii) with a vertical gradient of temperature. Then, the stresses resulting from the two different loadings applied on the tank are compared as well the kinematic response of the internal granular material. The kinematics of the granular material are analyzed at the particles scale (i.e. discrete elements), with a focus on the effect of particle/particle and wall/particle friction. Results show that a faster rearrangement of the packing occur when a thermal gradient is moving along the tank, leading to higher values of stresses applied on the tank walls. In addition to this, the behavior of the packed bed is dependent on the friction levels in the tank, whether the friction between particles themselves or the friction at the contact of particles with the shell. The influence of the slenderness ratio of the tank is investigated as well. Moreover, a reduction of 20% of thermal applied stresses can be obtained when inclined wall boundaries are used. The combination of an homogeneous configuration with low levels of friction (using lubricants) in thermocline storage tanks with inclined fixed boundaries can decrease significantly the induced stresses applied on the wall.