## 3 Results
### Exergy Analysis of the Discharge Process of the TES Systems  
The evolution of the temperature of the air at the outlet of the TES system together with the HTF pressure drop was used to determine the exergy efficiency \(\eta_{ex}\), as stated in Equation (6). The time evolution of the exergy efficiency of each TES system during the discharge process is represented in Figure 6a. At the beginning of the discharge, i.e., at \(t=0\) s, the exergy efficiency of the confined-bed TES system was slightly higher than 60% due to the high initial drop in pressure of its HTF. For the fluidized-bed TES system, the lower initial drop in pressure resulted in a higher initial exergy efficiency of around 85%. For the alumina block TES system, the initial exergy efficiency was close to 100% because of the negligible HTF pressure drop in this system. The exergy efficiency of the alumina block TES system decreased parabolically from its initial value close to 100% as the discharge process progressed, as a result of the decrease in the air outlet temperature mentioned above. Similarly, the exergy efficiency of the fluidized-bed TES system followed an inverse exponential trend due to the effect of the temperature of the air at the outlet, since the pressure drop in the fluidized bed was uniform during the discharge process. In contrast, the exergy efficiency of the confined-bed TES system remained constant or even slightly increased until the thermocline arrived at the top of the bed, because of the uniform temperature of the air at the outlet and the reducing pressure drop in the confined bed throughout the discharge process. Once the thermocline reached the top of the confined bed, the temperature of the air at the outlet dropped sharply while the pressure drop was still high; thus, the exergy efficiency decreased rapidly to values that were even below zero because of the steep drop in HTF pressure in the confined bed.  
In some studies, the pressure drop in a TES system is considered as parasitic consumption of the plant [52]. In this case, the HTF pressure drop is considered part of the parasitic consumption of the plant and its value is not attributed to the calculation of the TES system's exergy efficiency. Figure 6b presents the time evolution of the exergy efficiency of the different TES systems studied considering the pressure drop as parasitic consumption of the plant, i.e., assigning a null \(\Delta p\) for each TES system to calculate the exergy efficiency via Equation (5). When the pressure drop is considered as parasitic consumption of the plant, the initial exergy efficiency of all the TES systems is 100%. From this point, the evolution of the exergy efficiency of each TES system during the discharge process differed. While the exergy efficiency of the alumina block and the fluidized-bed TES systems decreased with time parabolically and inverse exponentially, respectively, the exergy efficiency of the confined-bed TES system remained higher than 90% until the thermocline reached the top of the bed, with slight variations due to the initial temperature distribution in the system. Therefore, if the HTF pressure drop is considered as parasitic consumption of the plant, a granular material TES system operating as a confined bed is the optimal alternative. Nonetheless, the power required to pump the HTF through the confined bed, derived from its pressure drop, should be accounted for.