## 3 Results
### Evolution of Temperatures During the Discharge Process of the TES Systems  
The temperatures registered by the sets of thermocouples installed in each experimental model was analyzed to study the evolution of the temperature distribution in the TES systems during the discharge process. For the granular material TES system, the high convection heat transfer coefficient of both the fluidized [32] and packed [33] beds resulted in a negligible local variation of temperature between the granular material and the HTF. Thus, the local temperature measured by each thermocouple of the TES system based on granular material operating as a fluidized or confined bed was assumed to be representative of the temperature of both the silica sand particles and the air at that location. The evolution of the temperature distribution of the granular material TES system during the discharge process when operating as a fluidized or confined bed is depicted in Figure 3 in the form of colormaps. These colormaps were constructed considering the temperatures measured by the 19 thermocouples located at the axis of the bed and distributed along its height as representative of the temperature of the whole cross-section. Furthermore, the temperatures measured by the thermocouples placed 5 cm apart have been interpolated to improve the visualization of the colormaps. A total of nine colormaps are included in Figure 3 for the discharge of each TES system, for specific time steps of 7 min. Figure 2: Evolution of the pressure drop in the TES system based on alumina blocks, at room temperature. A detailed analysis of the pressure drop in the TES system based on granular material can be found in [28]. The colormaps of the discharge of the granular material TES system operating as a fluidized bed show a very homogeneous temperature distribution along the bed height. In fact, the maximum instantaneous temperature difference between the 19 thermocouples located along the height of the fluidized bed during the complete discharge process was only 2.4 \({}^{\circ}\)C. This temperature uniformity within the fluidized bed was caused by the vigorous bubbling of the bed [34], which induced very high mixing ratios of the granular material [35; 36]. The homogeneous temperature distribution of fluidized beds used as TES systems has already been reported in the specific literature on sensible heat [24], phase-change [37; 38], and thermochemical storage systems [39; 40]. In contrast to the uniform temperature distribution of the granular material TES system obtained during discharge when operating as a fluidized bed, the mechanical confinement caused by the top distributor prevented the presence of bubbles in the confined bed and the consequent motion of the granular material. The static nature of the granular material in the confined bed during the discharge of the confined-bed TES system resulted in a temperature  
Figure 3: Evolution of temperature distribution during the discharge of each TES system. A detailed analysis of the temperature distribution in the TES system based on granular material can be found in [28]. distribution similar to that of a fixed or packed bed, i.e., with thermal segregation in the bed. The colormaps of the discharge of the granular material TES system operating as a confined bed, presented in Figure 3, show two clearly distinguishable zones: a high-temperature zone at the top of the confined bed and a low-temperature zone at the bottom, separated by a thin layer known as a thermocline. As the discharge process of the confined bed progressed, the thermocline evolved from the bottom to the top of the bed, cooling the confined bed progressively from bottom to top. The behavior of the confined bed during the discharge process, showing the segregation of temperature and the evolution of the thermocline, is typical of fixed or packed beds in which the solids remain static [24; 41; 42].