## 3 Results and Discussion
### Effect of Operating Parameters
#### 3.2.2 Influence of HTF Inlet Temperature  
The effect of the HTF inlet temperature on the charging and discharging behavior of the bio-inspired TES tank was explored at three different mass flow rates of the HTF. Accordingly, the operating temperature window was reduced from 15-45 \({}^{\circ}\)C to 20-35 \({}^{\circ}\)C for comparison. The bio-inspired TES tank was charged with two different HTF inlet temperatures, 45 \({}^{\circ}\)C and 35 \({}^{\circ}\)C, and discharged with two different HTF inlet temperatures, 15 \({}^{\circ}\)C and 20 \({}^{\circ}\)C. Figure 9 shows that for the TES system supplied by the higher/lower HTF inlet temperature, there was a significant increase in the rate of PCM melting during the charging cycle and solidification during the discharging cycle, respectively. In those cases, the PCM temperature changed rapidly during the charging and discharging cycles because of the larger temperature difference between the HTF and PCM temperatures. The PCM melted and solidified more quickly during the charging and discharging processes, respectively, because of the increased heat transfer rate brought about by the higher/lower operating temperature.  
Figure 8: Variations in power, PCM temperature, and HTF inlet and outlet temperatures with respect to time at three distinct HTF flow rates, (**a**) 1.5 kg/min, (**b**) 3 kg/min, and (**c**) 6 kg/min during the discharging cycle.  
Figure 10 shows a comparison of the mean power for different HTF flow rates and HTF operating temperatures for both charging and discharging cycles. It is evident that the mean power transferred by the TES tank is more significantly impacted by the inlet temperature of the HTF.  
Figure 10 also indicates that with an increase in HTF flow rate, the mean power produced by the bio-inspired TES tank increased in both operating temperature ranges. It was additionally found that the bio-inspired TES tank with the HTF flowing at 6 kg/min had the highest mean power transferred in both operational temperature ranges during the charging and discharging cycles. The highest mean power transferred by the bio-inspired TES is, however, proportional to the temperature at which the HTF transports its thermal energy. During the charging and discharging cycles, HTF flowing at 6 kg/min with the wider operating range had 133.6% and 101.5% higher mean power transferred, respectively, compared to the lower operating temperature range.  
Figure 9: Effect of HTF inlet temperature on the temperature vs. time profile of the PCM during (**a**) the charging cycle and (**b**) the discharging cycle.  
As a consequence, the change in HITF inlet temperature has a strong influence on the charging and discharging time of the PCM. When the HTF temperature was raised from 35 \({}^{\circ}\)C to 45 \({}^{\circ}\)C, the PCM melting time was significantly shortened from 1.29 h to 0.47 h.  
The Ragone plot (Figure 11) makes it clearly apparent that the bio-inspired TES tank performed better than the conventional shell-and-tube TES tank design. In particular, the bio-inspired TES tank operated optimally in the operational temperature range of 15 \({}^{\circ}\)C to 45 \({}^{\circ}\)C and at an HTF flow rate of 6 kg/min.  
Figure 11: Performance comparison between a conventional shell-and-tube TES tank and the bio-inspired TES tank.  
Figure 10: Comparison of mean power vs. flow rate for the bio-inspired TES during (**a**) the charging cycle and (**b**) the discharging cycle.