# Experimental Study on Two PCM Macro-Encapsulation Designs in a Thermal Energy Storage Tank
## 3 Results and Discussion  
### Temperature Evolution during the Charging Process  
Figure 5 shows the charging temperature profile of all sensors placed at the surface of the slabs for the two PCM encapsulation design at different flow rates. To analyze the effect of the encapsulation design in the charging duration, both slabs types were compared at  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Units** & **Sensor** & **Accuracy** \\ \hline Temperature & \({}^{\circ}\)C & Pt-100 1/5 DIN class & \(\pm 0.3+0.005\cdot\mathrm{T}\) \\  & & B IEC 60751 & \(\pm 0.3+0.005\cdot\mathrm{T}\) \\ Temperature & \({}^{\circ}\)C & Pt-100 1/5 DIN class & \(\pm 0.15+0.002\cdot\mathrm{T}\) \\  & & A IEC 60751 & \\ Flow rate & L/min & Badger meter type & \\  & & ModMAG M1000 & \(\pm 0.25\%\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Uncertainties of the different parameters involved in the analyses of the present study.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test** & **Density** & **Specific Heat** & **Heat Transfer** & **Accumulated** \\  & [\(\pm\)kg/m\({}^{3}\)] & [\(\pm\)kJ/kg \({}^{\circ}\)C] & **[\(\pm\)kW]** & **Energy [\(\pm\)kJ]** \\ \hline C\_ThinICE\_2L & \(\pm 1.28\) & \(\pm 2.44\cdot 10^{-2}\) & \(\pm 0.039\) & \(\pm 19\) \\ C\_ThinICE\_4L & \(\pm 1.28\) & \(\pm 2.42\cdot 10^{-2}\) & \(\pm 0.055\) & \(\pm 19\) \\ C\_FlatICE\_2L & \(\pm 1.27\) & \(\pm 2.43\cdot 10^{-2}\) & \(\pm 0.038\) & \(\pm 23\) \\ C\_FlatICE\_4L & \(\pm 1.28\) & \(\pm 2.45\cdot 10^{-2}\) & \(\pm 0.054\) & \(\pm 24\) \\ D\_ThinICE\_2L & \(\pm 5.5\cdot 10^{-2}\) & \(\pm 2.95\cdot 10^{-3}\) & \(\pm 0.039\) & \(\pm 19\) \\ D\_ThinICE\_4L & \(\pm 5.6\cdot 10^{-2}\) & \(\pm 3.05\cdot 10^{-3}\) & \(\pm 0.055\) & \(\pm 20\) \\ D\_FlatICE\_2L & \(\pm 5.3\cdot 10^{-2}\) & \(\pm 2.77\cdot 10^{-3}\) & \(\pm 0.039\) & \(\pm 23\) \\ D\_FlatICE\_4L & \(\pm 5.6\cdot 10^{-2}\) & \(\pm 3.21\cdot 10^{-3}\) & \(\pm 0.055\) & \(\pm 23\) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Estimated uncertainties of the HTF thermophysical properties, heat transfer rate, and cumulated energy.  
the same flow rate. Due to the higher heat transfer surface and the reduced amount of PCM (30% less according to Table 3) when using ThinICE slabs, at both flow rates, the experiment with the ThinICE slabs reached full charge (T_out = 25 \({}^{\circ}\)C) 14% faster than with FlatICE. Furthermore, when analyzing the impact of the flow rate, in both slab designs the experiments show that at 4 L/min the full charge is reached 60% faster than at 2 L/min.  
When comparing temperature distribution inside the tank (Figure 5), a constant stratification profile between the top, middle, and bottom slabs was observed in all the experiments. This effect was more pronounced in the tests performed with FlatICE slabs in which the lower part of the tank takes longer to charge, obtaining a temperature gradient up to 15 K between the coldest and hottest regions of the tank. This can be explained by the fact that the tank with FlatICE fits a lower number of slabs, as well as presenting smaller HTF channels compared with the tank with the ThinICE design (Figure 3, Table 2). Therefore, in this tank, the opposition to the HTF flow is higher, enhancing the distribution of the latter towards the regions of the tank where the density is more similar to the HTF inlet one (i.e., upper and middle region of the tank).