Status, Trends and Significance of Single Phase, Single and Coupled Natural Circulation Loops in Sustainable Energy Technologies – A Comprehensive Review

Researchers have made continuous efforts to understand the behavior of the natural circulation systems, their application, and the challenges involved in implementation. A large number of numerical, analytical, and experimental studies during recent years reflect the importance of passive technology. However, there is a dearth of systematic review of such studies based on geometry, loop fluid, and the intended application. Hence, the present review extensively focuses on thermal-hydraulic parameters in single-phase single and coupled loop natural circulation systems. Performance aspects like steady-state, transient, and stability criteria, including instability restraining techniques have been discussed. Even the importance of validating the numerical codes is highlighted. Since the coupled natural circulation loop finds major application in nuclear reactors as passive cooling systems, various configurations and fluid combinations intended are illustrated. As there is a dearth of experimental studies in the coupled loop, a preliminary analysis was done, which showed the efficacy of such a system. The review further revealed the scope for experimental studies along with numerical analysis of the single and coupled system, which can be linked to sustainable and renewable energy applications.