New Knowledge on the Performance of Supercritical Brayton Cycle with CO2-Based Mixtures

Abstract

As one of the promising technologies to meet the increasing demand for electricity, supercritical CO2 (S-CO2) Brayton cycle has the characteristics of high efficiency, economic structure, and compact turbomachinery. These characteristics are closely related to the thermodynamic properties of working fluid. When CO2 is mixed with other gas, cycle parameters are determined by the constituent and the mass fraction of CO2. Therefore, in this contribution, a thermodynamic model is developed and validated for the recompression cycle. Seven types of CO2-based mixtures, namely CO2-Xe, CO2-Kr, CO2-O2, CO2-Ar, CO2-N2, CO2-Ne, and CO2-He, are employed. At different CO2 mass fractions, cycle parameters are determined under a fixed compressor inlet temperature, based on the maximization of cycle efficiency. Cycle performance and recuperators’ parameters are comprehensively compared for different CO2-based mixtures. Furthermore, in order to investigate the effect of compressor inlet temperature, cycle parameters of CO2-N2 are obtained under four different temperatures. From the obtained results, it can be concluded that, as the mass fraction of CO2 increases, different mixtures show different variations of cycle performance and recuperators’ parameters. In generally, the performance order of mixtures coincides with the descending or ascending order of corresponding critical temperatures. Performance curves of these considered mixtures locate between the curves of CO2-Xe and CO2-He. Meanwhile, the curves of CO2-O2 and CO2-N2 are always closed to each other at high CO2 mass fractions. In addition, with the increase of compressor inlet temperature, cycle performance decreases, and more heat transfer occurs in the recuperators.

Performances of Transcritical Power Cycles with CO2-Based Mixtures for the Waste Heat Recovery of ICE

“…In addition, the CO 2 cycle pressure is relatively high, thus resulting into a high manufacturing cost and a great safety risk during the practical application. To solve the above issues effectively, CO 2 is proposed to mix with the organic fluid, thus forming the CO 2 -based mixture [17]. In comparison with the pure CO 2 , the CO 2 -based mixture can increase the critical temperature to extend the condensation temperature scope of the transcritical system.…”

Supercritical CO2 Binary Mixtures for Recompression Brayton s-CO2 Power Cycles Coupled to Solar Thermal Energy Plants

“…Currently, research has focused mainly on making mixtures with other gases. In this way, the critical point of s-CO2 can be adjusted change the lowest or highest operating condition of the Brayton cycle [15]. The direction and range of the critical point of supercritical CO2 mixture depend on the added substance and its quantity (mole fraction).…”

“…Valencia et al (2020), in their study on the influence of mixtures, concludes that there are two groups: mixtures that decrease the critical temperature (s-CO2/He, s-CO2/Kr, s-CO2/CH4 y s-CO2/C2H6) and mixtures that increase the critical temperature (s-CO2/COS, s-CO2/H2S, s-CO2/NH3, s-CO2/SO2, s-CO2/C5H10, s-CO2/C5H12), among others. So far, several studies have been conducted to discuss the feasibility and performance of the CO2-based supercritical mixtures power cycle [13][14][15][16][17][18][19][20].…”