The design of CO2-based working fluids for high-temperature heat source power cycles.

The application of CO 2 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 CO 2 (about 31°C), CO 2 is in fact compressed in the supercritical vapor phase rather than in the liquid phase, thus increasing energetic demand for compression. One of the solutions envisaged to overcome this problem is the addition of one or more chemicals that allow having a mixture with a higher critical temperature than the one of pure CO 2 . This 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 addition to CO 2 of a properly selected chemical component enables to increase the critical temperature up to 45°C with relevant improvements of cycle efficiency with respect to pure-CO 2 power cycles. In particular, it summarizes the most relevant criteria to be accounted for when selecting CO 2 -additives. Moreover, the paper warns of the thermodynamic effects deriving from adding to CO 2 a second characterized by a much more high critical temperature, such as the occurrence of infinite-pressure critical points and multiple-phase liquid-liquid and vapor-liquid critical points. Finally, the paper analyses the thermodynamic properties of a high-critical temperature CO 2 -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 CO 2 or the novel designed higher temperature CO 2 -based mixture. [ABSTRACT FROM AUTHOR]