Hydrogen-based hybrid energy system: A review of technologies, optimization approaches, objectives, constraints, applications, and outstanding issues

As the global energy sector transitions toward sustainability, hydrogen-based hybrid energy systems (HESs) have emerged as a viable solution for integrating renewable energy with long-duration storage, emissions reduction, and grid resilience. However, their widespread adoption is constrained by several critical challenges, including system complexity, technology maturity gaps, competing optimization objectives, and operational constraints. This review critically examines the state-of-the-art in hydrogen-based HES, offering a multi-dimensional assessment of technologies, optimization methods, objectives, constraints, applications, and outstanding challenges. Firstly, this study categorizes and evaluates key renewable energy components, including hydrogen production technologies (fossil fuel-based and renewable-based), and storage mechanisms. Comparative analyses are presented for techno-economic parameters, including levelized cost of hydrogen (LCOH), capital expenditures, technology readiness levels, and associated CO 2 emissions. In addition, we classify and compare various optimization approaches, including the stochastic approach, probabilistic approach, rule-based approach, mathematical approach, artificial intelligence-based approach, and software tool-based approach, to solve complex objective problems in system design, operation, and energy management. This research also explores several optimization objectives, including efficiency maximization, power flow improvement, cost minimization, and emission reduction, alongside various system constraints related to system reliability, technical limitations, and economic factors. Moreover, sector-specific applications in transport, power generation, industrial processing, and off-grid electrification are analyzed, supported by real-world case studies and deployment metrics. Furthermore, this review distinguishes itself from prior studies by integrating technical, economic, environmental, and regulatory dimensions, while identifying critical knowledge gaps in system modeling, power electronics, and safety. Challenges are prioritized, and future research directions are proposed to guide the commercial deployment of hydrogen-based HESs. By integrating fragmented insights, this study provides a unified reference for researchers, practitioners, and policymakers working toward the development of sustainable and decarbonized energy systems. • This study provides an overview of hydrogen-based HES in a comparative manner. • Hydrogen-based HES system optimization approaches, objectives, and constraints are highlighted. • An in-depth analysis of diverse real-world applications of hydrogen-based HES is presented. • Factors, issues, and challenges linked to the hydrogen-based HES are outlined. • Recommendations are provided for hydrogen-based HES to facilitate secure operation.