## 1 Introduction
Only a few medium-size applications exist in the MEAN region [8]. Much effort has been dedicated to developing, enhancing and optimizing systems for RO desalination, which uses various renewable energy technologies where the focus is on PV systems, wind turbines, PV-Wind hybrid systems and energy storage systems (battery banks). The primary goal of optimizing such systems is to achieve an environmentally friendly, cost-effective, reliable and renewable source of power. To achieve this aim, optimal sizing is essential. [PERSON], [PERSON] et al. performed a study modeling the effect of different parameters on seawater desalination using an RO unit [9]. The model discussed the effect of pressure fluctuations related to the variations in the power produced from a PV system; pressure values were between 32 and 47 bars. This variation affects the water flux within a range between \(0.3\times 10^{-3}\) kg/m\({}^{2}\) s and \(5.9\times 10^{-3}\) kg/m\({}^{2}\)-s. Water conductivity and concentration polarization are affected as well. [PERSON] et al. studied the performance of a water desalination RO unit powered by PV panels. The research discussed the effect of PV panels' inclination angles on the output flow rates, TDS, and PH values. The achieved results show that the performance increases from 15% to 25% when using a one-axis tracking system. The discussion also included the importance of integrating PV panels as a renewable energy source for clean water supply; however, the effect of pressure variation is not discussed as it kept as a constant value [10]. [PERSON] et al. discussed the possibility of reducing consumed energy by adding an energy recovery device for a medium scale RO unit powered from the electrical grid. The pressure increment effect was investigated to see the RO unit's performance. Three energy recovery efficiencies were tested, 80%, 85%, and 90%. It was concluded that energy consumption is reduced by 6% for the different pressure values when applying the mentioned efficiencies [11]. [PERSON] et al. evaluated the effect of long-term pressure decrement on RO performance. After the development of a model, pressure percentage was reduced from 24.4% to 17.8% after two years of operation. Therefore, a substantial quantity of low pressure RO elements (83.3%) is required to be replaced [12]. Performance and sizing optimization iterations applied to RO systems in the literature are intended to enhance the output quality and quantity versus reducing the input energy demand. Deployment of  
Figure 2: Global return flow ratio, Reproduced with permission from [PERSON], [PERSON] et al., published by city, country, 2005 [7]. RO desalination units in remote areas where a connection to the grid is not feasible are still limited. Thus, research interest is tuned to motivating the off-grid PV-powered RO desalination systems at small and medium scales. Off-grid PV solutions are hindered by the need to use high-energy-density battery banks. The batteries used to back the off-grid systems are expensive, bulky, sensitive to environmental conditions, have a short lifetime, require continuous monitoring and maintenance, are hazardous and are difficult to precisely control. Minimizing or eliminating the energy storage component in a fully off-grid PV powered RO system remains a priority for researchers. This work addresses this point in an effort to change the need for energy storage in batteries. Rather, it introduces the idea of storing the excess energy generated by the PV system in the salinity of the water produced by the RO unit. The concept of variable-pressure operation of RO systems is introduced. The relation between the applied pressure and the salinity of the output water is investigated. Inverse proportionality suggests that when there is excessive energy generation by the installed PV system, higher pressure is applied, and, thus, lower salinity levels are achieved (and vice versa).