CO2-TiCl4 working fluid for high-temperature heat source power cycles and solar application

Abstract

The application of CO2 power cycles has proved to be particularly advantageous to exploit high-temperature heat sources (500–800 °C) in the case of available low-temperature heat sinks (15–25 °C). Otherwise, the efficiency of these cycles is strongly reduced when cold sink temperatures are higher than 25 °C. This is the case, for example, of solar applications installed in desert areas whose cold sink is represented by available hot air. Due to these high temperatures of the cold sink, CO2 is inevitably compressed in the supercritical phase thus preventing its more efficient pressurization in the liquid phase.One of the solutions envisaged to overcome this problem consists of adding to CO2 a small amount of one or more chemicals, resulting in a mixture with a critical temperature higher than the one of pure CO2 (about 31 °C). This preserves the working fluid compression in its liquid phase, even in the case of cold sinks with temperatures greater than 25 °C.This research aims to show that the addition to CO2 of a specifically selected second component enables to increase the critical temperature up to 45 °C with relevant improvements of cycle efficiency with respect to pure-CO2 power cycles. In particular, after summarizing the most relevant criteria to be accounted for when selecting CO2-additives, the paper warns about 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 vapor-liquid critical points. Moreover, the paper specifically analyses the thermodynamic properties of CO2-TiCl4 mixtures which, depending on the content of TiCl4, may lead to a mixture characterized by the sought higher critical temperature. While studying this mixture, it has been observed that it presents multiple-phase critical points. For the sake of completeness, the paper also shows how do enthalpy and specific volume change in response to pressure variations in the event of either liquid-liquid or vapor-liquid critical points. This research finally shows the comparison between performances of power cycles which use, as working fluid, either pure CO2 or the specifically designed CO2-TiCl4 mixture. As expected, the TiCl4 addition brings about a significant efficiency gain.