## 3 Results and Discussion
### Effect of Coil Pitch  
Throughout the charging procedure, the ice formation exists around the TCHC, during which heat is transmitted to cold HTF from water. During all experiments, the temperatures \begin{table}
\begin{tabular}{l c c} \hline \hline
**Property** & **Water at 0 \({}^{\circ}\)C** & **Ice at 0 \({}^{\circ}\)C** \\ \hline Density, kg/m\({}^{3}\) & \(\rho_{\mathrm{w}}=1000\) & \(\rho_{\mathrm{i}}=920\) \\ \hline Specific heat, J/kg.\({}^{\circ}\)C & \(c_{\mathrm{w}}=4230\) & \(c_{\mathrm{i}}=2040\) \\ \hline Fusion latent heat, J/kg & \(\triangle H_{\mathrm{fus}}=333.7\times 10^{3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Properties of the distilled water (PCM) from Incropera 1996 [23]. \begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Uncertainty (\(\omega\))** \\ \hline \(\frac{\mathrm{m}_{\mathrm{a}}}{\mathrm{m}_{\mathrm{o}}}\) or \(\frac{\mathrm{m}_{\mathrm{a}}}{\mathrm{m}_{\mathrm{o}}}\), (-) & \(\pm 2.7\%\) \\ \hline \(\mathrm{Q}_{\mathrm{st}}\) or \(\mathrm{Q}_{\mathrm{reg}}\), (\%) & \(\pm 4.1\%\) \\ \hline \(\dot{\mathrm{Q}}_{\mathrm{ch}}\) or \(\dot{\mathrm{Q}}_{\mathrm{dis}}\), (W) & \(\pm 8.6\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The main parameters’ average uncertainties. are instantaneously measured in the radiating path at mid-plan of the TCHC; then, the solidified mass fraction and percentage of energy stored are computed instantaneously across the storage tank. This is employed to imply the twin coil thermal performance throughout the charging process for all studied geometrical/operating parameters. Figure 3 demonstrates the percentage of energy stored during charging against time for various pitches at three tube diameters. The figure indicates that the energy saved percentage trend is the same for all studied pitches, with a nearly linear curve for the first 50% of the charging process. It is noted that, as the pitch diminishes, the energy stored percentage curve becomes linear, which implies that the charging process becomes quicker. Furthermore, at pitch (P = 30 mm), the completed charging process takes half the needed time for pitch (P = 50 mm) with a large tube diameter. Therefore, it is advised that the chiller be turned off as soon as possible to save energy. Supercooling phenomena has no significant effect in the current study because the water is contained in a relatively large storage tank and the inlet coolant temperature is lower than the water nucleation temperature. The temperature profiles for temperature variation of the PCM in the storage tank with time during the charging process show an insignificant effect for supercooling in our study [16]. The effect of coil pitch on the energy regained percentage at a given tube diameter during the discharging process is shown graphically in Figure 4. The graph displays a profile similar to that seen during charging. However, this time, the detection rate is lower and the duration longer. This is because, as was said before, the water conductance layer with a lesser influence from natural convection in the contained water layer formed around  
Figure 3: Percentage of energy stored with time for different coil pitches with coil tube diameters during the charging process. the TCHC. With a decrease in tube diameter of 9.52 mm, the discharge duration rises by roughly 79% when the coil pitch goes from 30 to 50 mm (60%). However, when the tube diameter is just 15.88 mm, the discharge time is increased by 2. The impact of coil pitch on the average charging amount against time for the three tube diameters is illustrated in Figure 5. The figure indicates that the average charging rate declines rapidly with time for the whole studied coil pitches. Furthermore, the small coil pitch with the highest coil tube diameter indicates the shortest total charging time, and its average charging rate is nearly linear. Furthermore, as the coil pitch decreases, the average charging rate decreases rapidly for a fixed tube diameter. The charging rates are noted to be higher, which reduces the charging times.