## 3 Results
### Evolution of Temperatures During the Discharge Process of the TES Systems
For the discharge of the granular material TES system operating as a confined bed, the local temperature registered by the thermocouples was quite uniform around the initial temperature of the bed until the thermocline reached the location of the thermocouple, when the temperature decreased rapidly to the colder temperature of the air at the inlet. The reduction in temperature registered by the thermocouple located at 60% of the bed height occurred sooner than that by the thermocouple placed at 95% of the height, due to the evolution of the thermocline, which travelled from bottom to top of the bed during the discharge process. The time evolution of the temperature of the confined bed during the discharge was similar to that of a regular packed bed [48; 49], confirming the appropriate mechanical confinement of the bed. The evolution of air temperature in the alumina block TES system was somewhere between that in the confined between and in the fluidized bed. The air temperature of the solid blocks decreased progressively from the beginning of the discharge process, with a slower temperature reduction at the thermocouples located closer to the top of the system. In contrast to the temperature reduction of the fluidized-bed TES system, the decrease in air temperature in the alumina block TES system was slower and followed a linear or parabolic trend instead of an inverse exponential reduction with time. The evolution of the HTF temperature at the outlet of the TES system during the discharge process is particularly relevant, since the HTF leaving the TES system is directly circulated to the power block of the power plant. Therefore, the outlet HTF temperature of the TES system can be considered as the input temperature of the turbine of the power cycle. Regarding the HTF temperature at the outlet in the different TES systems analyzed, the confined bed is optimal, since the HTF can be supplied from this TES system to the power block at a uniform and high temperature over a long time period. In contrast, taking into account the HTF temperature at the outlet, the fluidized bed is the worst alternative due to the absence of thermal segregation in the fluidized bed, which operates at a homogeneous temperature during the discharge process. This results in an inverse exponential reduction in the HTF temperature at the outlet of the fluidized-bed TES system that may induce a reduction in the efficiency of the turbine to which the outlet of the  
Figure 4: Time evolution of the temperature at 60 and 95% of the total height of the TES systems during the discharge test (CB: confined bed, FB: fluidized bed, AB: alumina block). TES system is connected, due to the significant reduction in temperature of the HTF as the discharge process progresses. The performance of the alumina block TES system was intermediate between those of the confined- and the fluidized-bed systems. Even though the HTF temperature profile at the outlet of the alumina blocks was not so uniform as that of the confined bed, the decrease in the HTF outlet temperature in the solid block TES system was lower than that in the fluidized bed, leading to a lesser effect on the performance of the power cycle turbine compared with the granular material TES system operating as a fluidized bed.