Overview of fundamental aspects of natural circulation loops

Natural Circulation Loops (NCLs) offer a passive method for heat transfer and eliminate the need for mechanical pumps. They play a critical role in energy applications such as solar heaters, geothermal power plants, emergency cooling systems, heat-driven refrigerators, heat exchangers, advanced nuclear reactors, and oscillating heat pumps. Technological advancements have opened opportunities to enhance NCL performance, focusing on cost reduction, energy efficiency, minimizing oscillations, and improving system reliability. Special attention is directed toward the dynamics of single-phase and two-phase flows, with investigations into the thermal–hydraulic performance of NCLs using various working fluids and configurations. This review evaluates experimental setups and simulation techniques, including the lattice Boltzmann method (LBM), computational fluid dynamics (CFD), and system codes, identifying the strengths and limitations of current modeling approaches. A particular focus is given to supercritical CO2, heavy liquid metals, and molten salts as working fluids, underscoring their potential to enhance heat transfer efficiency while reducing operational risks compared to traditional fluids like water. Key trends and challenges in NCL design are explored, particularly concerning the improvement of safety and efficiency of next-generation passive thermal energy storage and cooling systems in solar power plants and advanced modular reactors. The insights from this review aim to guide future research in addressing the limitations of current technologies and furthering the development of robust and efficient passive safety systems.