Thermal efficiency gains enabled by using CO₂ mixtures in supercritical power cycles

The present paper explores the utilisation of dopants to increase the critical temperature of Carbon Dioxide (sCO₂) as a solution towards maintaining the high thermal efficiencies of sCO₂ cycles even when ambient temperatures compromise their feasibility. To this end, the impact of adopting CO₂-based mixtures on the performance of power blocks representative of Concentrated Solar Power plants is explored, considering two possible dopants: hexafluorobenzene (C₆F₆) and titanium tetrachloride (TiCl₄). The analysis is applied to a well-known cycle -Recuperated Rankine- and a less common layout -Precompression-. The latter is found capable of fully exploiting the interesting features of these non-conventional working fluids, enabling thermal efficiencies up to 2.3% higher than the simple recuperative configuration. Different scenarios for maximum cycle pressure (250–300 bar), turbine inlet temperature (550–700 ^°C) and working fluid composition (10–25% molar fraction of dopant) are considered. The results in this work show that CO₂-blends with 15–25%(v) of the cited dopants enable efficiencies well in excess of 50% for minimum cycle temperatures as high as 50 ^°C. To verify this potential gain, the most representative pure sCO₂ cycles have been optimised at two minimum cycle temperatures (32 ^°C and 50^°C), proving the superiority of the proposed blended technology in high ambient temperature applications.