## 5 Results and Discussion
### Pilot Plant Analyses  
#### 5.2.1 Temperature Evolution  
Figure 5 shows the temperature evolution of the HTF at the inlet and outlet of the tank and the temperature profiles of the mid-height control volumes of the PCM (Figure 2a) during the charging and discharging processes at a HTF mass flow rate of 1500 kg/h.  
During the charging process (Figure 5a), the PCM located at the beginning of the tubes bundle and controlled by the temperature sensor TPCM2 achieved higher temperatures with less time, since the energy supplied by the HTF was received during the first stage, and, therefore, started and finished the melting and charging processes before the rest of the PCM located along the tank. The PCM located at the end of the tube bundle and controlled by the temperature sensor TPCM5 had the opposite behavior for analogous reason. From the first seven minutes (0.12 h), the PCM started to melt at a temperature of 50 \({}^{\circ}\)C; after 100 min (1.67 h) of experimentation, when the PCM reached 59 \({}^{\circ}\)C, the melting process was finished, and after 209 min (3.5 h), the charging process was completed. During the discharging process (Figure 5b), it can be seen that the solidification started 15 min (0.25 h) after starting the process, when the PCM reached 60 \({}^{\circ}\)C, and the PCM was fully solidified after 190 min (3.16 h), when the PCM  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Property** & **Units** & **Initial 0 Cycles** & **500 Cycles** & **1000 Cycles** & **1200 Cycles** \\ \hline Phase change Temperature & (\({}^{\circ}\)C) & 58.3 & 58.8 & 58.6 & 57.3 \\ \hline Latent heat & (J/g) & 204.2 & 255.5 & 242.9 & 209.9 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Thermo-physical properties of RT58 before and after 500, 1000 and 1200 cycles.  
Figure 5: PCM and heat transfer fluid (HTF) temperature profile at an HTF mass flow rate of 1500 kg/h: (**a**) charging process and (**b**) discharging process. Shaded area indicates the melting and the solidification temperature range.  
reached 50 \({}^{\circ}\)C. The discharging process was terminated after 296 min (4.93 h). Comparing the two processes, it was observed that the discharge was 42% slower than the charge. The main reasons for these differences in time lie in the presence of natural convection during the melting process, which enhanced the charging process and the creation of a solid layer around the tubes bundle when the PCM started to solidify, which hindered the discharging process. Furthermore, the upper value of the melting temperature range of the sample analyzed at pilot plant scale lie is 1-2 \({}^{\circ}\)C lower than the value obtained from the DSC analysis, while the lower value was the same in both cases.  
Figure 6 provides the comparison between the three evaluated mass flow rates (500, 1500 and 2500 kg/h), showing the temperature profiles of two representative PCM temperature sensors (T\({}_{\text{PCM},2}\) and T\({}_{\text{PCM}11}\)) during a charging and a discharging process. Notice that in both processes, the higher the mass flow rates were, the faster the PCM reached the desired temperatures, demonstrating the influence of the mass flow rate on the PCM heating rate. The reason lies in the fact that the heat transfer from the HTF to the first layer of PCM is dominated by the convection mechanism. Although the heat transfer coefficient remains constant in the different evaluated flow rates (fully developed internal laminar flow), the temperature difference between the HTF and the PCM is the parameter that determines the influence of the flow rate on the heat transfer rate.