## 3 Results
### Evolution of the Pressure Drop During the Discharge of the TES Systems  
In addition to the thermal performance of the TES systems, the pressure drop required to pump the HTF through the system is also a relevant parameter. In this sense, the evolution of the air pressure drop was measured during the discharge of each TES system analyzed. The evolution of the pressure drop of the TES system based on granular material was previously analyzed in detail [28]. The steepest drop in pressure was registered for the confined-bed TES system, for which the pressure drop decreased linearly as the discharge process progressed due to the deceleration of the HTF when the bed cooled down [28]. The value of the pressure drop in the confined-bed TES system was similar to that in a packed bed of alumina particles of 15 mm diameter reported previously [49]. The pressure drop in the granular material TES system operated as a fluidized bed was uniform and close to the hydrostatic pressure of the bed [28], showing high variability due to the intense bubbling inside the bed during the discharge process [50]. In contrast to the steep drop in HTF pressure obtained for the granular material TES systems, the air pressure drop in the alumina blocks was much lower, specifically, four orders of magnitude less. The trend of the pressure drop in the alumina block TES system, shown in Figure 5, was similar to that in the confined bed, obtaining a linearly decreasing value during the discharge process, which was caused by the reduction in the air velocity with the progressive cooling of the bed.  
In terms of the HTF pressure drop, the alumina block TES system is optimal, since its very low air pressure drop (less than 10 Pa, see Figure 5) results in much lower pumping consumption to circulate the HTF through the system, compared with the granular material TES systems. However, it should be noted that in this study, these pressure drops corresponded to the lab-scale systems analyzed. For a real application of these TES systems in a CSP plant, the systems should be scaled up to an industrial size. Considering the  
Figure 5: Time evolution of the pressure drop in the alumina block TES system during the discharge process. A detailed analysis of the pressure drop evolution in the TES system based on granular material can be found in [28].  
temperature distributions shown in Figures 3 and 4 and the evolution of the HTF pressure drop, the scaling up of the various TES systems analyzed will be different. The steep drop in HTF pressure characteristic of the granular material TES systems suggests that these systems can be scaled up by increasing the cross-section of the bed instead of its height. Proceeding like this will limit the HTF pressure drop in industrial-scale granular material TES systems while keeping the favorable temperature distribution in the case of the confined-bed configuration. In contrast, the alumina block TES system can be scaled up by increasing both the height and the cross-section of the blocks, due to the reduced HTF pressure drop associated with this alternative. An increase in the height of the alumina block TES system for industrial application would lead to a greater drop in HTF pressure; however, the limited value measured in the lab-scale prototype proves that the pressure drop associated with this option is not a problem. However, all these TES systems could also be scaled up modularly by connecting different units in parallel [51].