Temperature drop during charging process

Figure E1 to Figure E2 shows the temperature drop that occurs at a flow rate of 7g/s (low flow) for two different inlet HTF temperatures (65°C and 70°C). The temperature drop at inlet HTF temperature of 70°C is more than at inlet HTF temperature of 65°C. Both figures show a similar pattern in the temperature–time graph; where there is an initial huge temperature difference and as the PCM melts, the temperature drop reduces towards the end of the experiment. Figure E2 shows that the melting time is shorter for an inlet HTF temperature of 70°C when compared to Figure E1 for an inlet

HTF temperature of 65°C at the same flow rate. This is due to the time it takes the PCM to attain its melting temperature as shown in Figure 9:15, Figure E18 and Figure E19 for inlet HTF temperature of 65°C and Figure 9:16, Figure E20 and Figure E21 for inlet HTF temperature of 70°C .

The temperature profiles (Figure E1 to Figure E7) shows that the temperature drop at the start of the experiment is larger than at the end of the experiment. This can be attributed to the fact that at the beginning of the experiment, the PCM starts to melt gradually as the hot fluid flows through the copper pipes. Once the PCM starts to change phase (from solid to liquid), the HTF loses more heat to the PCM, hence the difference in the temperature drop at the start and end of the experiment. It is also noticeable at inlet HTF temperature of 60°C and 65°C, the temperature drop at the end is much smaller when compared to inlet HTF temperature of 70°C.It is observed that as the temperature of the PCM increases, the temperature difference between the inlet HTF and outlet HTF decreases for all experiment at this flow.

Rate of melting was faster at higher inlet HTF temperature. The greater the temperature difference between the PCM melting temperature and inlet HTF temperature, the faster it takes the PCM to melt. Table 9:6 shows the change of temperature drop at different times during the charging process.

Table 9:6: Overview of the temperature drop for the charge process. Low Flow Charging temperature 3600sec. (1hour) 10000sec. (2.7hours) 20000sec. (5.6hours) 300000sec. (8.3hours) 40000sec. (11.1hours) 65°C 10.66°C 8.33°C 6.08°C 5.70°C 5.26°C 70°C 12.58°C 10.62°C 10.11°C 10.03°C 8.91°C Medium Flow Charging temperature 3600sec. (1hour) 10000sec. (2.7hours) 20000sec. (5.6hours) 250000sec. (6.9hours)

65°C 4.32°C 4.49°C 3.77°C 1.28°C 70°C 6.29°C 6.12°C 4.32°C 3.75°C High Flow Charging temperature 3600sec. (1hour) 10000sec. (2.7hours) 20000sec. (5.6hours) 30000sec. (8.3hours) 60°C 1.85°C 1.61°C 1.55°C 1.40°C 65°C 2.52°C 2.59°C 2.20°C 1.75°C 70°C 3.68°C 3.51°C 2.84°C Fully melted

From Table 9:6, the highest inlet HTF temperature, 70°C possess the highest temperature drop, when compared to that at 60°C and 65°C at different flow rates. The charging time is affected by the inlet HTF temperature. The higher the inlet HTF temperatures, the quicker the charging time. For example, at flow rate of 30g/s, the charging time is longer for inlet HTF temperature of 60°C when compared to the charging time at 65°C and 70°C. Figure E5 shows the charging time to be 79,200 seconds (22 hours); this was due to the time it took for the store to be fully charged as shown in the PCM temperatures in Figure 9:10, Figure E8 and Figure E9. Figure E6 shows the charging time for an inlet HTF temperature of 65°C to be 39,600 seconds (11 hours) based on the PCM temperature on Figure 9:11, Figure E10 and Figure E11. Figure E6 shows the charging time for the highest inlet HTF temperature, 70°C to be 21,600 seconds (6 hours), since the PCM temperature melts quicker as shown in Figure 9:12, Figure E12 and Figure E13. For low flow, the charging time for inlet HTF temperature of 65°C is 79,200 seconds (22 hours) based on the PCM temperature as shown in Figure 9:15, Figure E18 and Figure E19. The charging time at the same flow rate for inlet HTF temperature of 70°C is 50,400 seconds (14 hours). The PCM temperatures are presented in Figure 9:16, Figure E20 and Figure E21. The charging time for inlet HTF temperature of 65°C and 70°C at flow rate of 15g/s (medium flow) are twenty-two and seven (7) hours respectively. Figure 9:13, Figure E14, Figure E15 and Figure 9:14, Figure E16 and Figure E17 shows the PCM temperature for each inlet HTF temperature. Based on the charging time to melt the PCM, it shows that the rate

of melting is higher at higher inlet HTF temperature. Also, the temperature difference between the inlet and outlet HTF temperature at high flow is smaller when compared to low and medium flow.

Figure 9:2 and Figure 9:3 are used to compare outlet HTF temperature profile at same inlet HTF temperature, but different flow rate. The experimental result of the outlet HTF temperature from Figure E1 to Figure E7 are compared using Figure 9:2 and Figure 9:3 for two different inlet HTF temperatures (65°C and 70°C) at varying mass flow rate. It shows that the highest outlet HTF temperature is at high flow, whilst the least outlet HTF temperature is obtained at low flow for both cases.

Figure 9:2: Outlet HTF temperature at different flow rate (inlet HTF 65°C).

9.2.3 Effect of the varying inlet HTF temperature and flow rate on the