## 1 Introduction
Significant fluctuations in osmotic pressure may occur that can affect the performance of BWRO systems. Several authors have studied RES-powered BWRO systems. [PERSON] and [PERSON] [41] studied a BWRO system powered by photovoltaic energy using real groundwater. A pilot plant was used and the study focused on cost assessment without providing operating data. [PERSON] et al. [42] used a pilot BWRO desalination plant with a \(Q_{\text{p}}\) capacity of 40 m\({}^{3}\) d\({}^{-1}\). [PERSON] et al. [43] carried out a performance analysis of a photovoltaic-powered hybrid BWRO membrane system considering variations in feedwater salinity. The RO system only had one membrane element and \(SEC\) variations of between 5.5 kWh m\({}^{-3}\) at a feed concentration of 1 g L\({}^{-1}\) salt and 26 kWh m\({}^{-3}\) at a feed concentration of 7.5 g L\({}^{-1}\) salt were reported. The same research group [44; 45] continued their study by evaluating the effect of energy fluctuations and feed salinity on the performance of a small single-stage BWRO system. The \(SEC\) and permeate quality were evaluated for different membranes along 12 h of operation. [PERSON] et al. [46] carried out an interesting work based on the optimization of a solar-powered BWRO system with a \(Q_{\text{p}}\) capacity of 0.2 m\({}^{3}\) d\({}^{-1}\) for drinking water. A synthetic solution of 6 g L\({}^{-1}\) NaCl was used as feedwater in the aforementioned small single-stage plant. Runs of 2 h were done providing operating data in terms of production, salt rejection and energy. The authors concluded that the optimized RO plant could guarantee potable water production with a \(SEC\) from 1.2 to 1.3 kWh m\({}^{-3}\). [PERSON] et al. [47] analyzed the performance of a small, single-stage photovoltaic-powered BWRO desalination plant to produce 0.5 m\({}^{3}\) d\({}^{-1}\). The variation of \(R\) and salt rejection with temperature and the increase of \(SEC\) when \(R\) decreased were reported. [PERSON] and [PERSON] [48] evaluated a hybrid photovoltaic-wind system to produce desalinated water from a BW source. The BWRO desalination plant design was based on software simulation (ROSA software from Dupont(r)). The first stage had 4 PVs, each with 4 SWMMs, and the second stage 4 PVs, each with 2 SWMMs. [PERSON] et al. [49] studied the effect of wind speed fluctuation on the performance of an RES-powered BWRO plant comprised of one 4-inch diameter SWMM element. They used synthetic solutions of NaCl (2.75 and 5.5 g L\({}^{-1}\)) as feedwater in a system with a \(Q_{\text{f}}\) capacity of 0.3 m\({}^{3}\) h\({}^{-1}\). Similarly, [PERSON] et al. [50] considered the effect of real wind fluctuation and energy buffering on the performance of a BWRO pilot plant in terms of \(Q_{\text{p}}\), \(C_{\text{p}}\) and \(SEC\). The same group continued this research line considering a small single-stage BWRO system unit, studying a safe operating window methodology using a new and old SWMM [51] and the influence of solar irradiance fluctuation on plant performance [52]. Most of the studies that have been undertaken have only considered small size (pilot-scale or lab-scale) single-stage BWRO system configurations which differ considerably from full-scale BWRO systems that commonly have at least two stages and 4 SWMMs per PV [5; 53]. The aim of this work was to evaluate through an on-off control strategy the different SOWs of a simulated BWRO system using the real feedwater fluctuation characteristics of a groundwater well that has been under study for 10 years. The BWRO system has three PVs in the first stage and 2 PVs in the second stage. Depending on the power input and \(C_{\text{f}}\) in the BWRO system, the control system established a BWRO system configuration to maximize \(Q_{\rm p}\). A computational tool validated in a previous work by the authors [53] was used to simulate the behavior of the BWRO systems under different operating conditions.