## 3 Results and Discussion  
### Operation Carried out on the \"Partly Cloudy Day\" (With and without Fully Charged Batteries)  
To demonstrate the effects of adding one day's worth of energy storage to the water production and SEC to the PV-membrane system, comparisons of the system performance on the \"partly cloudy day\" were performed as shown in Figure 3. When batteries were used, the motor power consumption remained constant around 350 W throughout the entire period (Figure 3A, black curve). This is ~20 W higher than the motor power when no batteries were implemented in the system during the middle of the day (Figure 3A, grey curve). This occurred, because the pump was always seeking to extract the desired current from the power source (both PV and batteries). Hence, the discharging current of the batteries was added to the PV current to supply the pump. As a result, the batteries were discharged continuously throughout the day, resulting in the drop of the SOC from 100% to 20% (Figure 3B). Comparing the maximum PV output power (light blue in Figure 3A) and the pump power, this resulted in ~50 W power losses in the additional electronics--namely, the batteries (efficiency of 96% [65]), DC/DC converter (efficiency of 88% [69]) and charge controller (efficiency of 98% [70]). This resulted in a total efficiency of power delivery to the pump motor of 83%. A further reason for high power consumption is that the motor is supplied with a constant voltage of 48 V\({}_{\rm dc}\) when connected to batteries. This relatively low voltage limits the ability of the pump motor to start [71] and also draws a higher current, which, in turn, reduces the motor efficiency further and results in greater resistive losses. As a result, the low system efficiency leads to a low TMP\({}_{\text{RO}}\) and, hence, a RO flux (black curve in Figure 3C,D). In comparison, the TMP\({}_{\text{RO}}\) and RO flux (grey curve in Figure 3C,D) followed the same trend as the changes in the SI when the system was operated without batteries. This is due to the driving force that change with the variations of the SI for the desalination process by providing the hydraulic pressure needed to overcome the osmotic pressure of the feedwater, hence affecting the recovery and retention that are controlled by a mass transfer at low pressure (grey curve in Figure 3E) and ultimately result in fluctuations in the permeate EC (grey curve in Figure 3F) and SEC (grey curve in Figure 3H). Further, it can be seen from the weather conditions on the \"partly cloudy day\" that the motor power (grey curve in Figure 3A) directly followed the changes in SI when no batteries were deployed. It dropped to 0 W several times during periods of fluctuations and intermittency due to the lack of energy from the PV panels, resulting in system shutdowns. The impact of adding batteries was clearly reflected in the TMP and flux (black curve in Figure 3C,D), where the operation was very constant due to the fact that power could be  
Figure 3: Performance of the PV-membrane system shown, firstly, with fully charged battery storage (100% initial state-of-charge (SOC), black curves) and, secondly, the reference system without energy storage (grey curves) on the “partly cloudy day”: (**A**) power, (**B**) SOC, (**C**) transmembrane pressure of UF membrane (TMP\({}_{\text{UF}}\)), (**D**) transmembrane pressure of RO membrane (TMP\({}_{\text{RO}}\)) (**E**) flux of RO membrane (flux\({}_{\text{RO}}\)), (**F**) flux of UF membrane (flux\({}_{\text{UF}}\)), (**G**) retention/recovery, (**H**) permeate electrical conductivity (EC), (**I**) production and (**J**) specific energy consumption (SEC). drawn from the batteries continuously throughout the day. Overall, the recovery of \(\sim\)30% (Figure 3G) resulted in an average permeate EC of 294 uS/cm (Figure 3H (World Health Organization guideline value of 1000 mg/L (1700 uS/cm) [72]) and SEC of \(\sim\)4 kWh/m\({}^{3}\) (Figure 3)).