Geothermal Binary Plants Utilising an Innovative Non-Flammable Azeotropic Mixture as Working Fluid

Abstract

Several attempts have been made to use a non flammable fluid as a working fluid in geothermal binary systems. The adoption of a non-flammable fluid is in particular interesting when the geothermal plant has to be located in a densely populated area, where it can be difficult to stay away from buildings and other infrastructure. A flammable fluid involves in any case a higher cost of insurance and of safety systems. Other requirements for the working fluid are a suitable thermodynamic behaviour, chemical stability, acceptable toxicological and environmental properties, including zero-ODP (ozone depleting potential) and low GWP (global warming potential). The potential use of a mixture of two well known fluids, respectively an HFC and a Perfluoro-poli-ether, exhibiting azeotropic behaviour, is analysed in the paper from the performance point of view, adopting economically optimised heat exchange surfaces. As reference geothermal fluid a liquid-only flow was assumed, with temperatures in the range from 105 to 180 deg C. The described working fluid has been successfully adopted as a substitute for a Perfluoro-carbon in the low temperature geothermal binary plant of Altheim, Austria. The experimental results, as well as the analysis implemented in the paper, indicate that this fluid could be a very interesting alternative to the hydrocarbons widely used at present in geothermal binary plants