Experimental Analysis of a Confined Bed of Granular Material As Thermal Energy Storage System

Thermal Energy Storage (TES) systems contribute to the uniform and efficient production of green energy in concentrated solar power plants. Storage of sensible heat in a granular material is widely used, among the different alternatives of TES systems, due to its simple operation. In this work, a lab-scale TES system is experimentally analyzed. The setup consists of a cylindrical vessel of stainless steel with 20 cm in diameter and 1 m in height, filled with sand as granular material, using air as working fluid. A perforated plate distributor prevents the sand particles to fall in the plenum chamber at the bottom of the vessel. The vessel walls are surrounded by 4 electric heaters to heat up the system to the desired initial temperature. The system is equipped with a pressure sensor at the plenum chamber to measure the pressure drop of the bed, and with thermocouples in the plenum chamber and at the outlet of the bed to determine the inlet and outlet temperature of air during the discharge process. Thermocouples are also distributed along the bed height in intervals of 5 cm to measure the evolution, with time and bed height, of the temperature at the center of the bed. The discharging process of the system is experimentally characterized using air velocities below and above the minimum fluidization velocity of sand, operating the system in fixed and fluidized bed regimes. For the case of velocity higher than the minimum fluidization velocity, another perforated plate distributor was used at the top of the vessel to mechanically confine the bed, preventing the motion and thus the fluidization of the particles. The distribution of the bed temperature for the fixed, fluidized, and confined beds are compared in terms of the air exergy availability at the outlet of the TES system.