Abstract
<jats:p>The increasing demand for sustainable and reliable electricity in isolated and off-grid areas has accelerated the deployment of standalone Hybrid Renewable Energy Systems (HRESs). These systems typically integrate Photovoltaic (PV) panels, Wind Turbines (WT), Diesel Generators (DG), and energy storage technologies to enhance system reliability, operational flexibility, and resilience against the intermittent nature of renewable generation. This study presents a systematic, comprehensive, and critical review of optimization techniques and energy management strategies for standalone HRESs, with particular emphasis on the role of energy storage under uncertainty. Existing studies are analyzed using a structured framework covering system configurations, storage technologies, optimization objectives, solution methods, uncertainty modeling, and energy management approaches. Various energy storage technologies, including electrochemical, mechanical, electrical, thermal, and chemical storage, are compared in terms of performance, cost, and suitability for hybrid systems. Classical optimization methods and recent metaheuristic algorithms are critically evaluated for optimal system sizing and operational planning under uncertain conditions. The review also highlights the growing adoption of Hybrid Energy Storage Systems (HESSs), where complementary storage technologies improve overall system performance. Current research gaps include the lack of integrated optimization frameworks for multi-storage systems and limited real-time energy management under uncertainty. A standalone PV–WT–FC–DG–Battery (FC indicates fuel cell) microgrid in Abu Minqar, Egypt, is presented as a case study. The proposed Gray Langurs Optimizer (GLO) achieved the best performance, yielding a cost of energy of 0.2066 USD/kWh, a net present cost of USD 4.99 million, a renewable fraction of 90.0%, and 194.01 t CO2/year emissions, outperforming GWO and WOA in optimization accuracy and robustness. Future research directions toward intelligent and flexible HRESs with integrated energy storage are also discussed.</jats:p>