Experimental evaluation of the CO2-based mixture CO2/C6F6 in a recuperated transcritical cycle

Zeotropic CO 2 -based mixtures as working fluids in the power block have the potential to enhance concentrated solar power (CSP) plants and other high-temperature heat source applications. One promising working fluid is the CO 2 / C 6 F 6 mixture, which enables condensation at 50°C – a necessity when dry cooling with ambient air. Given the many theoretical studies on topics such as potential, optimized performance, or economic assessments, an experimental validation and a reality-check in a facility of significant size is required to vindicate further research. The experimental campaign was performed on pure CO 2 and the CO 2 / C 6 F 6 mixture in two compositions in a test facility (recuperated transcritical cycle). The long-term test (170h) revealed no operational issues, including no signs of thermal degradation. However, a composition shift - an effect previously regarded as an issue in closed cycles with zeotropic mixtures - affected the conditions at the vapor-liquid-equilibrium in the systems tank but also self-stabilizes the system to remain condensing, even at higher ambient air temperatures. The successful proof-of-concept at cycle temperatures of up to 500°C – significantly higher than earlier studies on mixtures reported (<300°C) – justifies further research in this area. • The CO 2 / C 6 F 6 mixture experienced no issues during prolonged operation at T<500°C. • VLE prediction via a fitted Peng-Robinson EoS is sufficient. • A composition shift is confirmed experimentally.