## 3 Results and Discussion
### Effect of Pressure Difference Derived by Vacuum Pump  
In the previous section, the effect of the heat pump set-point temperature on the system performance and water production was examined. However, the pressure difference applied on the fast tank, which is derived from the vacuum pump, also has a significant impact on the system's performance. Figure 10 provides the water production for two pressure differences at two different heat pump set-points (75 and 80\({}^{\circ}\)C), as the recirculation ratio increases. For the case of 75 \({}^{\circ}\)C, it can be observed that, when the recirculation ratio  
Figure 8: Coefficient of performance (COP) variation with recirculation ratio for the set-points of the heat pump at 75, 80 and 85 \({}^{\circ}\)C.  
Figure 9: Specific energy consumption as the recirculation ratio increases for the set-points of 75, 80, 85 \({}^{\circ}\)C.  
is up to 81%, the highest pressure difference (\(\Delta\)p = 0.8, i.e. vacuum at 0.21 bar) presents higher water production, while for the higher recirculation ratio, a lower vacuum leads to slightly higher water production, up to 0.13 m\({}^{3}\)/h. For the case of 80 \({}^{\circ}\)C, the pressure difference of 0.75 bar (vacuum 0.26 bar) presents much higher water production, up to 0.2 m\({}^{3}\)/h, than the pressure difference of 0.7 bar (vacuum 0.31 bar). Figure 11 presents the salinity of the outlet brine flowing out of the flash tank. It is observed that, for a recirculation ratio of more than 95%, the salinity increases significantly for all the cases. Figure 12 presents the power consumption as the recirculation ratio increases to produce the corresponding quantity of water. Figure 12a shows the vacuum pump, the heat pump and the total power consumption for the pressure difference of 0.75 bar (vacuum  
Figure 11: Salinity of the liquid outlet brine as the recirculation ratio increases, for two set-points at \(\Delta\)p = 0.7, 0.75 and 0.8 bar. Figure 10: Water production as the recirculation ratio increases for two set-points at pressure differences, \(\Delta\)p = 0.7, 0.75 and 0.8 bar. 0.26 bar) and 0.8 bar (vacuum 0.31 bar) at the heat pump's set-point at 75 \({}^{\circ}\)C. It is observed that the case with \(\Delta\)p = 0.8 bar presents much higher total power consumption, up to 51 kW, as the recirculation ratio increases up to 93%, as the vacuum pump has a larger load and the heat pump operates to reach the set temperature. Figure 12b depicts the same parameters for the pressure difference of 0.75 bar (vacuum 0.26 bar) and 0.7 bar (vacuum 0.31 bar) for the heat pump's set-point at 80 \({}^{\circ}\)C. The behavior for the two scenarios is similar to the case presented in Figure 12a. Figure 13 depicts the COP of the heat pump as the recirculation ratio increases for the two different set-points of the heat pump and the pressure differences derived from the vacuum pump. For the set-point of 75 \({}^{\circ}\)C, two pressure differences are presented, namely \(\Delta\)p = 0.75 bar (vacuum 0.26 bar) and \(\Delta\)p = 0.8 bar (0.21 bar), showing that the COP decreases for every case when the recirculation ratio is lower than 80%, but after this point, the COP increases with the recirculation ratio, up to 4.15 for \(\Delta\)p = 0.8 bar and 3.66 for \(\Delta\)p = 0.75 bar. This can be explained by the heat pump energy consumption presented in Figure 10, which decreases after the recirculation ratio is 80%, since the brine reaches the set-point temperature, and its compressor reduces the power consumption. For the case of the heat pump's set-point of 80 \({}^{\circ}\)C, the COP reduces with the increase in the recirculation ratio, and drops below 3 when the recirculation ratio equals 85% for \(\Delta\)p = 0.7 bar and 93% for \(\Delta\)p = 0.75 bar, while after this point, the COP increases again up to 3.1 and 3.25, respectively.