Multi-criteria selection of additives for CO2-based mixtures in solar-driven supercritical CO2 Brayton cycles: A critical review of performance, compatibility, and trade-offs

The supercritical carbon dioxide (sCO 2 ) Brayton cycle offers high efficiency and compactness for power generation. However, its integration with Concentrated Solar Power (CSP) in hot, arid regions is hindered by pure CO 2 's low critical temperature (31°C), which compromises compressor performance. Doping CO 2 with high-critical-temperature additives is a promising solution, yet the selection of optimal additives remains a complex, multifaceted challenge. This review critically evaluates the selection criteria for CO 2 -based mixture additives in CSP- sCO 2 systems, moving beyond a singular focus on critical temperature to establish a comprehensive, multi-criteria decision framework. We first elucidate the fundamental thermodynamic and molecular mechanisms by which additives regulate mixture critical properties. Subsequently, we synthesize and critically assess a wide range of candidate additives including hydrocarbons, sulfur-based compounds, and inorganic compounds, against a multi-criteria matrix encompassing thermal efficiency gain, critical parameter shift, thermal stability (up to 700°C), safety (toxicity, flammability), and environmental impact (GWP,ODP). Our analysis reveals a fundamental trade-off: while sulfur-based additives such as SO 2 and H 2 S offer superior thermodynamic performance and thermal stability, they pose significant safety and corrosion challenges, whereas hydrocarbons, despite moderate efficiency gain are thermally limited. Crucially, we demonstrate that additive performance is highly layout-dependent, with molecular complexity governing the efficacy of recompression versus simple recuperated architectures. Furthermore, we identify critical research gaps, including the underexplored off-design behaviour of mixtures, their long-term compatibility with structural alloys, and the urgent need for experimentally validated thermal stability data. Based on this synthesis, we propose a hierarchical, multi-stage guideline for additive selection, providing a strategic roadmap for the future design and optimisation of high-performance, reliable CSP- sCO 2 systems.