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 -Pre-compression-. 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 (250e300 bar), turbine inlet temperature (550e700 C) and working fluid composition (10e25% molar fraction of dopant) are considered. The results in this work show that CO2-blends with 15e25%(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.

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