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 (sCO(2)) as a solution towards maintaining the high thermal efficiencies of sCO(2) 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 (250-300 bar), turbine inlet temperature (550-700 degrees 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 degrees C. To verify this potential gain, the most representative pure sCO(2) cycles have been optimised at two minimum cycle temperatures (32 degrees C and 50 degrees C), proving the superiority of the proposed blended technology in high ambient temperature applications. (C) 2021 The Author(s). Published by Elsevier Ltd.

Automated summary

This paper investigates the use of dopants to increase the critical temperature of Carbon Dioxide (sCO(2)), aiming to maintain high thermal efficiencies of sCO(2) cycles. It explores the impact of CO2-based mixtures on power block performance and considers hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4) as possible dopants. The results show that CO2-blends with specific dopant percentages enable efficiencies exceeding 50% even at high ambient temperatures.