## 3 Results and Discussion
### Operation with Different Energy Storage Capacities  
The next task was to evaluate the impacts of different energy storage capacities--realized by varying the initial SOC of the battery bank--on the PV-membrane system performance when operated throughout the \"partly cloudy day\", with a particular focus on the SEC and permeate production. The results can be seen from Figure 5, which starts with the batteries at an initial SOC of 50%. The batteries were capable of providing full power to the pump for 7 h 20 min (see Figure 5A) before reaching the limits of their capacities. Thus, after 14:00, the pump was directly subjected to the fluctuations in the SI and repeated attempts at charging and discharging of the batteries. This was compared to a pump that was operated at full power for 9 h 20 min with fully charged batteries (see Figure 3A). As indicated in Figure 5B, a positive current value represented the PV-membrane system source current via photocurrent generation (I\({}_{\text{pv}}\)), while a negative current occurred during the discharging of the batteries. From the beginning of the day until around 14:00, the batteries were discharged continuously at a maximum current up to 20 A (see Figure 5B). This corresponded with the decline of the SOC to 0% at 14:15 (Figure 5D). Subsequently, oscillations occurred due to the batteries reaching the lower threshold of DC/DC converter (20 V). Then, the PV membrane started charging the batteries and caused the shutdown of the pump. These power fluctuations were encountered due to the charge controller not being able to power the pump and charge the batteries simultaneously, indicating a system shortcoming that needs to be improved for future research. The flux and TMP of the RO membrane followed the same pattern as the pump discussed above (see Figure 5E,F). Overall, the system produced 669-L permeate, which was comparable with the production (663 L) when the system was operated without batteries. The SEC was increased by 15.9% from 3.7 to 4.0 kWh/m\({}^{3}\). Hence, it is recommended to use batteries with an initial SOC > 50% (energy capacities > 1.2 kWh) to further enhance the water quality and quantity. The impacts of different energy storage capacities (varied SOC) on the PV-membrane system--in particular, the average SEC and water quality and quantity--are presented in Figure 6. It can be clearly seen that the water production declined with the decrease of the initial battery SOC (see Figure 6A). Compared to the reference, the increase of the water production started at an initial SOC > 50%. The average permeate EC was less affected by the SOC (see Figure 6B) due to the dense membrane with high retention (BW30) used in these experiments. The SEC (see Figure 6C) was increased with the decrease of the SOC, indicating that the lower SOC reduced the flux and increased the SEC. It is perhaps intuitive that the SEC is expected to return to the state of the reference case after the batteries are empty. However, this will only happen when the batteries are no longer coupled with the pump so that they do not have the same voltage potential, avoiding the repeatable attempts of the charging and discharging behaviors. Figure 5: The performance of the PV-membrane system equipped with batteries at a 50% SOC on the “partly cloudy day”, indicating (**A**) the pump power, (**B**) battery current, (**C**) battery voltage, (**D**) SOC, (**E**) Flux\({}_{\text{RO}}\), (**F**) TMP\({}_{\text{RO}}\), (**G**) production and (**H**) SEC. Note that the black curves on graphs (**A**), (**E**) and (**F**) are the moving average values of 10 points, while the grey curves are the original measurement data from the sensors. The remaining performance graphs of the PV-membrane system in terms of the varied initial SOC (70%, 40%, 30% and 20%) are provided in Appendix A (Figures A2-A5) for further comparisons.