Thermal efficiency gains enabled by using supercritical CO2 mixtures in Concentrated Solar Power applications

Supercritical Carbon Dioxide (sCO2) power cycles have been proposed for Concentrated Solar Power (CSP) applications as a means to increase the performance and reduce the cost of state-of-the-art CSP systems. Nevertheless, the sensitivity of sCO2 systems to the usually hot ambient temperatures found in solar sites compromises the actual thermodynamic and economic gains that were originally anticipated by researchers of this innovative power cycle. In order to exploit the actual potential of sCO2 cycles, the utilization of dopants to shift the (pseudo)critical temperature of the working fluid to higher values is proposed here as a solution towards enabling exactly the same features of supercritical CO2 cycles even when ambient temperatures compromise the feasibility of the latter technology. To this end, this work explores the impact of adopting a CO2-based working mixture on the performance of a CSP power block, considering hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4) as possible dopants. Different cycle options and operating conditions are studied (250-300 bar and 550-700oC) as well as molar fractions ranging between 10 and 25%. The results in this work confirm that CO2 blends with 1525%(v) of the cited dopants enable efficiencies that are well in excess of 50% for minimum cycle temperatures as high as 50 or even 55oC. It is also confirmed that, for these cycles, turbine inlet temperature and pressure hardly have any effect on the characteristics of the cycle that yields the best performance possible. In this regard, the last part of this work also shows that cycle layout should be an additional degree of freedom in the optimisation process inasmuch as the best performing layout changes depending on boundary conditions. INTRODUCTION FROM THE FIRST STEPS TO SUCCESSFUL DEMONSTRATION The sCO2 power cycle technology has triggered a growing interest in the scientific community in the last ten years. It currently stands out as the strongest alternative to steam turbines in the next generation of Concentrated Solar Power plants. Such interest is brought about by the higher thermal efficiency of the cycle and arguably smaller footprint of the equipment, features that were already recognised in the pioneering works by Angelino and Feher in the late sixties. The former author identified 650oC as the breakeven turbine inlet temperature (TIT) beyond which sCO2 power cycles attain better thermodynamic performance than both air Brayton and water/steam Rankine cycles [1]. At the same time, Feher discussed the benefits of compressing sCO2 in the vicinity of the critical point, taking advantage of the low compressibility factor in that region [2]. After a long period without interest in the technology, a large amount of research works have been published in the last fifteen years. In 2004, Vaclav Dostal’s Ph.D thesis put sCO2 in the spotlight again, identifying the Recompression and Partial Cooling sCO2 cycles as the best candidates for IV Generation Nuclear reactors, capable of taking thermal efficiency close to 50% for a turbine inlet temperature of 650oC [3]. A little later, the work carried out by the National Renewable Energy Laboratory explored advanced configurations of the same Recompression layout for CSP applications, adding intercooling and reheat and indeed confirming the potential of the cited cycle in this application [4,5]. This theoretical work received a crucial experimental support by SANDIA National Lab, whose first-ofa-kind experimental loop proved the benefits enabled by sCO2 condensation in terms of turbomachinery design and operational flexibility [6]. This has later been confirmed by experimental activities carried out by other institutions at the small scale [7,8].