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

Abstract

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.