Solar-driven ejector cooling under transient operation and Hot-Climate Constraints: A critical review and operability-based framework

Solar-driven ejector refrigeration systems (SERS) use low- to medium-grade thermal energy to enable compressor-free cooling for sustainable cooling. In principle, SERS performance should be comparable to that of mechanical systems; however, real-world performance is affected by solar transience and ambient temperature, which often drive the ejector out of its critical (double-choked) operating regime, thereby severely limiting its widespread deployment. This review proposes a transition from the study of SERS performance on a steady-state, peak-performance basis to an operationally based analysis that accounts for time-dependent, transient, and climate-dependent operating conditions. Specifically, the relationship between the system-level variables (temperatures of the generator T g , condenser T c , and evaporator T e ) and the flow regime transitions through the ejector demonstrates the importance of backpressure critical limits in ensuring entrainment stability and the feasibility of cooling. The proposed unified operability framework is based on dynamic operational metrics, such as backpressure margin, and the critical regime residence time, thereby enabling valid comparisons across studies and climatic conditions. The coupling of solar variability, condenser temperature penalties, and system trajectories in the T g − T c − T e coordinate system indicates that climatic conditions are the primary limiting factor for operational feasibility. Mitigation strategies for off-design loads, such as thermal energy storage, adaptive ejector configurations, and system-level control, are examined for their ability to extend operational parameters rather than improve peak efficiency. Finally, the working fluids used in SERS are also reviewed in this framework to emphasize that their suitability depends on how well they operate within the ejector operating limits and their robustness to fluctuations in condenser temperature. This review therefore establishes a unified operability framework linking thermodynamics, ejector flow regimes, and climatic constraints. The primary SERS challenge is not maximizing peak efficiency, but rather providing stable operation under real-world, time-varying, ambient conditions. The proposed operability-based framework provides a single basis for climate-responsive design, performance comparison, and system optimization. Because of this, the transition of SERS from laboratory-scale study into practical, reliable, real-world cooling applications will be easier to achieve.