For the seawater of salinity of about 35,000 mg/L on one side of the membrane (Ocean water) and drinking water of salinity of 500 mg/L on the other side of the membrane, the osmotic pressure created on the membrane is 24 bars [8]. RO can remove many types of dissolved and suspended substances as well as biological ones (principally bacteria) from the water and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side  
Figure 1: The desalination process. of the membrane and the pure solvent (H\({}_{2}\)O) can pass to the other side. To be \"selective\", this membrane should not allow large molecules or ions through the pores (holes) but should allow smaller components of the solution (such as solvent molecules, i.e., H\({}_{2}\)O) to pass freely. Since RO membranes can be clogged very easily by suspended solids and mineral scaling compounds, RO systems require a special treatment which is not used in thermal desalination systems to process the source seawater [9,10]. Desalinated water then goes through the post-treatment, such as pH regulation and disinfection, to make it potable. About 80% of the electrical energy requirement in RO systems refers to the high-pressure pumps [11,12]. In RO systems (especially seawater RO) energy from the pressurized concentrate can be recovered by energy exchanger systems, reducing the required pumping energy by about 30 to 50% [13]. A major advantage of RO is its modularity, allowing the design of any plant capacity starting from less than 1 m\({}^{3}\)/d to giant plants such as Taweelah, UAE--909,200 m\({}^{3}\)/d [14]. The special attraction of RO for small size systems is indicated by a global market share exceeding 90%, considering systems with capacities of the order of 1000 m\({}^{3}\)/d [15]. RO systems are attributed to a high potential for directly applying renewable energy (RE) supply such as wind energy or photovoltaics (PV). RO membrane suppliers recommend continuous operation and smooth shutdown and start-up procedures regarding pressure and flow [16]. Furthermore, flushing of the RO module with desalinated water is recommended when the system is shutdown. Thus, RO systems require energy storages (e.g., electrical or mechanical) to account for changes in energy supply when combined with fluctuating RE sources. Research in intermittent operation as well as variable operation has taken place for over 20 years. Intermittent operation essentially refers to the desalination plant not operating constantly throughout the year. This can happen, for example, when a desalination plant is powered by a PV system and the design choice is made not to operate constantly. This has been investigated both for brackish water [17], as well as sea water desalination plants [18]. Further to that, investigation has taken place in order to operate the desalination plant at variable pressures to better couple the desalination plant with an intermittent RE system [19] and to harvest the decreased specific power consumption albeit with decreased water production [20]. This research is coupled with research in understanding the coupling of PVs [21] and wind turbines [22,23] with desalination systems. As the artificial intelligence applications in the renewable energy sector were expanding in management and control [24] and RE production and load forecast [25, 26, 27], such applications were deployed also for desalination systems [28]. While intermittent operation is currently employed even in real world systems [29], the variable pressure operation is still under investigation regarding the effect on the membranes, but results show that there is potential for use in commercial applications as well [30,31], allowing the implementation of more advanced operation schemes for RE based RO desalination systems [32]. A special mention ought to be made to shallow depth low enthalpy geothermal resources (<150 \({}^{\circ}\)C), since they can be used for electricity generation depths [33].