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

Abstract

Zeotropic CO₂-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 CO₂/C₆F₆ 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 CO₂ and the CO₂/C₆F₆ 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.