## 3 System Layout  
Processing water, either for transportation or treatment, is an energy-intensive process. This challenge makes the coupling of accessing clean and fresh water with solar energy a promising field of development, with the potential to improve its sustainability by minimizing or altogether eliminating the dependence on fossil fuels and significantly reducing the operational costs.  
In the optimization of an off-grid photovoltaic operated water desalination unit, the performance and cost of a PV-operated water desalination system will be investigated. The reason behind this is to find the best solution for either submersible or surface applications to desalinate brackish water or sea water. The performance of the energy storage will be intensely discussed to set out beneficial suggestions to improve and enhance the efficiency of the whole system.  
Reverse osmosis is the technique that will be implemented in this research. This technique uses semi-permeable membranes. On the feed side of the membranes, applied pressure is used to induce water permeation through membranes while rejecting salts. Depending on the applied pressure, the concentration of dissolved salts in the permeate water can be controlled. The higher the applied pressure on the feed side of the membrane, the lower the salinity percentage of the permeate water and the higher the permeate flow rate.  
Since the storage of energy for a PV operated water desalination plant has a high initial cost, a relatively short lifespan, and requires ample space and a controlled environment, the advantage of the correlation between the applied pressure and the salt's concentration in the permeate water must be taken into consideration, in order to minimize the capacity of the storage system as much as possible. Such a plant requires an energy profile that matches the profile of solar radiation, so that when the solar energy increases, the applied pressure will be improved, and thus the percentage of dissolved salts will decrease (and vice versa).  
A mixture of different salinity rates is required to extract a specific volume of water with a specific salinity percentage. This strategy will minimize the capacity of energy storage to a certain level.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Value** & **Unit** \\ \hline pH & 7.35 & \\ Electrical conductivity EC & 3750 & \(\upmu\)S/cm \\ Turbidity & 1.1 & NTU \\ Mg++ & 40.8 & mg/L \\ Ca++ & 69 & mg/L \\ Total Hardness & 336 & mg/L as CaCO\({}_{3}\) \\ K+ & 47.5 & mg/L \\ Na+ & 267.1 & mg/L \\ Total dissolved solids (TDS) & 2590 & mg/L \\ Total suspended solids (TSS) & 1068 & mg/L \\ Total solids (TS) & 3124 & mg/L \\ SO\({}_{4}\)— & 249.4 & mg/L \\ PO\({}_{4}\)— & 0 & mg/L \\ Cl\(-\) & 92.5 & mg/L \\ NO\({}_{3}\)\(-\) & 60 & mg/L \\ COD & 40.4 & mg/L \\ \hline \end{tabular}
\end{table}
Table 1: Water specification parameters.  
Such a system would use the water itself to store the excess energy generated by the PV system. Thus, the higher the excess of energy generated by the solar system is, the lower the salinity level, and the higher the volume of the desalinated water (and vice versa). Figure 4 is a schematic diagram of the designed system.