The design of CO2-based working fluids for high-temperature heat source power cycles
Silvia Lasala, Davide Bonalumi, Ennio Macchi, Romain Privat, Jean-Noël Jaubert
Session: Session 1A: CO2 Power System
Session starts: Wednesday 13 September, 10:30
Presentation starts: 10:50
Room: Building 27 - Lecture room 01
Silvia Lasala (Université de Lorraine, Laboratoire Réactions et Génie des Procédés (UMR CNRS 7274) 1 rue Grandville, 54000 Nancy, France)
Davide Bonalumi (Politecnico di Milano, Energy Department via Lambruschini 4, 20156 Milano, Italy)
Ennio Macchi (Politecnico di Milano, Energy Department via Lambruschini 4, 20156 Milano, Italy)
Romain Privat (Université de Lorraine, Laboratoire Réactions et Génie des Procédés (UMR CNRS 7274) 1 rue Grandville, 54000 Nancy, France)
Jean-Noël Jaubert (Université de Lorraine, Laboratoire Réactions et Génie des Procédés (UMR CNRS 7274) 1 rue Grandville, 54000 Nancy, France)
The application of CO2 power cycles is advantageous to exploit high-temperature sources (500-800°C) in the case of available low-temperature heat sinks (15-25°C). However, their efficiency is strongly reduced for higher heat sink temperatures. At these temperatures, due to the low-critical temperature of CO2 (about 31°C), CO2 is in fact compressed in the supercritical vapour phase rather than in the liquid phase, thus increasing energetic demand for compression. One of the solutions envisaged to overcome this problem is to increase the critical temperature of CO2 in order to preserve the working fluid compression in its liquid phase, even in the case of heat sinks with temperatures greater than 25°C.
This research shows that the increase of CO2 critical temperature up to 45°C, by adding to CO2 a low amount of a properly selected component, enables relevant improvements of cycle efficiency, with respect to pure-CO2 power cycles. In particular, it summarizes the most relevant criteria to be accounted for when selecting CO2-additives. Moreover, the paper warns of the thermodynamic effects deriving from adding to CO2 a second component characterized by a much more high critical temperature, such as the occurrence of infinite-pressure critical points and multiple-phase liquid-liquid and vapour-liquid critical points.
Finally, the paper analyses the thermodynamic properties of a high-critical temperature CO2-based mixture, suitable for these applications, that presents multiple phase critical points. In this regard, it is specified that the paper also aims at filling a knowledge gap in the study of thermodynamic properties of mixtures presenting how do enthalpy and specific volume change in response to pressure variations in the event of liquid-liquid and vapour-liquid critical points. Finally, we present the comparison between performances of power cycles which use, as working fluid, either pure CO2 or the novel designed higher temperature CO2-based mixture.