# Numerical Design and Laboratory Testing of Encapsulated PCM Panels for PCM-Air Heat Exchangers
## 3 Results and Analysis
### Laboratory Test
#### 3.2.1 Heat Transfer and Pressure Drop of One Panel with Different Gaps between Panels  
At low Reynold numbers, the new panel performed better than the existing panel only at 8 mm gap (Table 6). Conversely, at higher Reynolds numbers (Re = 4067 and 6061), the new panel increased the heat transfer at significant values for 8 and 9 mm gaps.  
Figure 12: Head curve of panels 11, 24, 25 and 26 as well as the smooth and the existing panel with fan curves at different rotation speeds.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & **11** & **24** & **25** & **26** & **27** & **28** & **29** & **30** & **31** & **32** \\ \hline  & 13.68\% & 0.08\% & 2.63\% & 20.51\% & \(-2.56\%\) & \(-2.56\%\) & \(-2.56\%\) & \(-2.56\%\) & 3.93\% & 2.31\% \\
**Panels/module** & 7 & 9 & 8 & 6 & 6 & 6 & 5 & 4 & 8 & 9 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Volume change in comparison with existing panel.  
Figure 13: Fabricated panel.  
For Re = 6061 (airflow used during PCM-Air heat exchanger charging period), an 8 mm gap increased the heat transfer by 124%. This is important to note as the charging period occurs during a short period of time and the heat transfer needs to be as efficient as possible. When Nusselt number between the new and the existing panel is compared, 8 mm gap was shown to be the most effective.  
When the gaps between the new panel are compared, a 10 mm gap is the most effective at Re = 6061. The values can be seen in Figure 14 and the percentages in Table 7. Higher rates of heat transfer are important to ensure fast charge and response. The gap of 10 mm will also lead to a smaller pressure drop and noise. However, fewer panels can be fitted in the thermal battery as its dimensions are limited. At low Reynolds number, the opposite occurs: an 8 mm gap is the most effective. This is probably due to the attachments and detachments at the boundary layer, which is easier for small gaps and higher Reynolds number.  
Figure 14 also shows the Nusselt number for the flat panel. At Re = 4067 and 6061 for 9 and 10 mm gaps, the existing panel performs similar to the flat panel and shows inefficiency in terms of heat transfer. This increases the charging period, reduces chances to fully charge the thermal battery and requires a lower outdoor temperature during the charging period when compared to the new panel. This information can also be found in Figure 15 where the Nusselt number ratio \((Nu/Nu_{o})\) is presented. Figure 15 shows that the performance is similar at any gap when Re = 6061 and the ratio remains the same for 8 and 9 mm. At Re = 1954, gaps of 9 and 10 mm perform worse than the existing panel.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reynolds Number** & **8 mm Gap** & **9 mm Gap** & **10 mm Gap** \\ \hline
1954 & 20\% & \(-\)5\% & \(-\)26\% \\
4067 & 94\% & 36\% & 5\% \\
6061 & 124\% & 72\% & 77\% \\ \hline \hline \end{tabular}
\end{table}
Table 6: Nusselt number improvement from existing to new panel.  
Figure 14: Nusselt number for existing, new and flat panels with error bar showing the uncertainty of 5.6%.