Evaluation of the property methods for pure and mixture of CO₂ for power cycles analysis

The use of a binary cycle coupled to the complete reinjection of non-condensable gases could provide a valid answer to the improvement of the sustainability of geothermal plants. In recent years, the interest in the use of CO₂ as a working fluid in transcritical cycles has increased. However, the low critical temperature of carbon dioxide (≈30 °C) requires the cooling cycle, temperatures below 15 °C, which are not always available. In this work, to overcome this limitation and obtain a higher critical temperature and a lower maximum pressure for more flexible applications of transcritical binary cycles, the possibility of using a second component, mixed with CO₂, has been evaluated. For this purpose, the following fluids have been proposed: R1234yf, R1234ze(E), n-butane, n-hexane, n-pentane, and propane, with a minimum considered carbon dioxide molar content of 60%. To carry out a cycle analysis, the knowledge of the thermodynamic properties of CO₂-mixtures is fundamental; however, suitable equations of state under the appropriate conditions for these blends have not been clearly defined yet. Therefore, in the first part of this paper, different EoS for predicting thermodynamic properties of pure CO₂ and CO₂-based mixtures are analyzed and compared with reference data obtained from works published in the literature. However, because of the lack of experimental data of the selected blends, the values of density, enthalpy and entropy, obtained with the selected EoS, are compared with NIST REFPROP results. The EoS involved in the evaluation are cubic-type (PR, PR-Twu, PRSV, RK, SRK, GCEOS), Virial-type (LKP, BWRS), Helmholtz-type (SW), and SAFT-type (PC-SAFT). In a power cycle, the fluid works under different conditions, involving several possible states across the components. So the influence of the different EoS on each power cycle's key component for pure CO₂ and two selected CO₂-based mixtures has been investigated. The thermodynamic results show that the CO₂-based mixtures, in a transcritical configuration, can achieve efficiencies higher than the sCO₂ power cycle.