## 3 Results and Discussion
### Operations on Other \"Solar Days\" (With and without Fully Charged Batteries)  
The aim of this section is to investigate the effects of very different solar radiation conditions on the system performance, again both in the directly coupled configuration and with fully charged batteries. Figure 4 indicates the cumulative permeate water production and permeate EC and SEC of the PV-membrane system over the \"very cloudy\" and the \"sunny\" days. From the top two graphs of Figure 4, it can be seen that, when incorporating the batteries into the system, the motor power was maintained constantly at 350 W through \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Solar Day**} & **Initial** & **Avg.** & **Avg.** & **Avg.** & **Avg.** & **Perm.** & **Avg.** & **Full-Power** \\  & **SOC** & **Flux\({}_{\text{RO}}\)** & **TMP\({}_{\text{RO}}\)** & **Perm. EC** & **Retent.** & **Prod.** & **SEC** & **Duration** \\  & **(\%)** & **(L/m\({}^{2}\).h)** & **(bar)** & **(\%/cm)** & **(\%)** & **(L)** & **(kWh/m\({}^{3}\))** & **(hh:mm)** \\ \hline \multirow{4}{*}{Party cloudy} & 20 & 7.3 & 6.6 & 328 & 96.2 & 402 & 4.8 & 1:55 \\  & 30 & 7.4 & 6.7 & 336 & 96.0 & 443 & 4.7 & 3:02 \\ \cline{1-1}  & 40 & 9.0 & 7.2 & 335 & 95.9 & 557 & 4.5 & 5:09 \\ \cline{1-1}  & 50 & 9.9 & 8.2 & 330 & 95.9 & 669 & 4.4 & 7:22 \\ \cline{1-1}  & 70 & 11.8 & 8.9 & 287 & 96.3 & 725 & 4.1 & 8:16 \\ \cline{1-1}  & 100 & 12 & 9.4 & 274 & 96.4 & 767 & 4 & 9:20 \\ \cline{1-1}  & Ref. & 10.7 & 8.4 & 310 & 96.3 & 663 & 3.7 & – \\ \hline \multirow{2}{*}{Very cloudy} & 100 & 11.8 & 9.4 & 274 & 96.3 & 646 & 4.1 & 8:00 \\ \cline{1-1}  & Ref. & 7.3 & 6.5 & 347 & 95.9 & 396 & 4.6 & – \\ \hline \multirow{2}{*}{Sunny} & 100 & 11.3 & 9.1 & 290 & 96.4 & 892 & 4.3 & 11:00 \\ \cline{1-1}  & Ref. & 10.2 & 8.2 & 353 & 95.8 & 770 & 4.0 & – \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The overall average performance of the photovoltaic (PV)-membrane system with/without batteries over the three “solar days”. Flux\({}_{\text{RO}}\): flux of RO membrane, TMP\({}_{\text{RO}}\): transmembrane pressure of RO membrane, SOC: state-of-charge and EC: electrical conductivity. out the day, despite two different weather conditions (Figure 4A,F). This was \(\sim\)20 W higher than the motor power without batteries present in the system during the middle of the day. This is for the same reasons as discussed in the previous section, 3.1 (Figure 3A). When comparing the values at which the motor power is saturated with that from the \"partly cloudy day\", it was observed that the saturation in Figure 4A,F was more pronounced. It can be found that the saturation occurred at a SI > 800 W/m\({}^{2}\), where the shortest duration appeared on the \"partly cloudy day\" (see Figure 2B). Furthermore, a high temperature (above 40 \({}^{\circ}\)C; see Figure 2C) also resulted in a low PV voltage (\(\sim\)10 V lower when reaching saturation), hence reducing the power input to the pump (see Figure 2). On the \"very cloudy day\", the performance with batteries improved as follows: production increased by 81% from 395 to 714 L/d, the average permeate EC was improved by 27% from 347 to 274 L\(\upmu\)S/cm and the average SEC was reduced by 17 % from 4.8 to 4.1 kWh/m\({}^{3}\) as well. Again, the average parameters are summarized in Table 1 in Section 3.3. Additionally, on the \"sunny day\", as is shown in Figure 4H, it was noted that the permeate production without batteries was lower in the morning due to the low SI; then, it approached the same level as the case with the batteries at 4:00 p.m. This occurred during the time periods (9:30 to 15:15) when the TMP\({}_{\text{RO}}\) without batteries exceeded the TMP\({}_{\text{RO}}\) with batteries (see Figure 4G).