Comparative assessment of pumped hydro and battery storage in a hybrid renewable energy system with excess-energy-driven desalination

Off grid renewable systems on remote coasts lean on storage to bridge long spells of weak solar and wind, yet mechanical and electrochemical storage are rarely tested side by side under the same conditions. This work assesses seawater pumped hydro storage (PHS) and lithium iron phosphate (LFP) batteries inside a standalone system for Rangabali, Bangladesh, and routes surplus renewable energy that would otherwise be dumped into a reverse osmosis plant treated as a priority-dispatched, shiftable load. Both options were sized for the same community and water loads under cycle charging and load following dispatch using NSGA-II, NSGA-III, MOGWO, and MOSSOA, then ranked by a two stage Fuzzy TOPSIS framework on cost, lifecycle emissions, reliability, excess energy utilization, and job creation. The PV/WT/DG/PHS configuration under load following dispatch ranked first with a cost of energy of 0.2452 $/kWh and a loss of power supply probability of 4.928% while supplying 2821.6 MWh of electricity per year. Holding the diesel generator to 894 h/year kept lifecycle emissions at 0.1632 × 10 6 kg of CO₂ equivalent per year, a 97% cut against a conventional baseline, and 272.24 MWh/year of surplus produced 62,154.82 m 3 of freshwater; the RO plant and storage together used 57.77% of all surplus generation (excess energy utilization factor). The LFP bank recovered energy at 95% round trip efficiency against 70% for PHS, but its 15-year life and heavy embodied carbon left its own storage-related emissions nearly 50 times above PHS's, even though whole-system lifecycle emissions differed by less than a factor of two between the top-ranked configuration of each technology. A Monte Carlo analysis of 10,000 runs returned a mean cost of energy of 0.2031 $/kWh with narrow dispersion when demand was sampled 10% to 50% above the design load with capacity held fixed.