## 3 Results and Discussion
### System Performance Comparisons of Batteries and SCs  
The final important result can be drawn from the comparisons of the system performance when equipped with batteries or SCs, as indicated in Figure 7. It is worth noting that these two system setups have the same PV power rating (500 W), but the PV voltage settings are different due to the voltage constraints between the SCs and batteries (discussed in Section 2.6). The power consumption of the motor pump with batteries remained constant around 350 W (black curve in Figure 7A), as in the previous experiments discussed above. The pump with batteries worked at a constant voltage of 48 V\({}_{\text{dc}}\) (see Figure 7B), and the PV provided the photocurrent (see Figure 7D) to the pump as required. Meanwhile, the batteries were capable of providing a continuous current to the pump during the entire day (see Figure 7E) due to high energy storage capacity (2.4 kWh), whereas the motor power with SCs largely followed the changes in the SI trend (plotted earlier in Figure 2). This is due to the fact that the pump with SCs operated in terms of the PV voltage (controlled by preset voltage thresholds, as discussed in Section 2.6; see grey curves in Figure 7C) and mainly extracted the current from the PV, hence resulting in a higher power consumption of the pump. In addition, a PTC lamp was connected in a series with the pump to increase the inner resistance, which buffered the sudden changes caused by the built-in MPPT between the SCs and SAS (with the resistance values throughout the day provided in Figure 7). However, this was at a cost of an average power loss of \(\sim\)50 W on this solar day. As indicated in Figure 7E, the SCs were discharged promptly at the beginning of the day. During the periods of 11:00 to 12:30, when large fluctuations occurred, the SCs started discharging to the pump for energy buffering, with the SOC dropping to \(\sim\)85%, which was limited by the preset voltage threshold settings (V\({}_{\text{charging\_off}}\)) to prevent deep discharging of the SCs, and hence, a big voltage drop of the pump can be avoided. This was implemented on the charge controller settings, as the pump always extracts the maximum power from the power sources (both PV and SCs); consequently, the SCs cannot step in for energy buffering when encountering the next large fluctuation. As a result, the pump with SCs worked continuously (no shutdown events), despite the occurrence of large fluctuations. The benefits of eliminating the shutdown events were reducing the potential damage to the pump and RO membrane [79], as well as improving the permeate water quality and quantity [30]. The desalination performance of the PV-membrane system was determined by the SOC of the energy storage components, which were dependent on the availability of the power from the PV. The TMP\({}_{\text{RO}}\) (Figure 7C), which was the driving force, determined the RO flux (Figure 7E) and permeate EC (Figure 7F). Furthermore, as the permeate production with SCs approached the same value as the batteries (see Figure 7G), it indicated that the system with SCs produced much higher permeate for most of the day. The SEC with batteries was much lower than the value with SCs, suggesting that less energy was required to produce a unit of clean water. When compared to the directly coupled system without SCs (see Figure A6), the use of SCs to buffer those fluctuations resulted in a 9% increase in water production and 13% improvement in the SEC. As discussed in Section 3.1, the improvement of the batteries was due to the improved power quality supplied to the membrane system as a result of providing energy and constant power over the entire day. The water quantity increased by 16% with the Li-ion batteries (energy capacity of 2.4 kWh) when compared to the reference. It can be anticipated that there is a region where the Li-ion batteries are not affected by the presence of the battery.