Nitrous oxide as working fluid in enhanced geothermal systems (EGS) stars

Abstract

Geothermal energy is a renewable energy source that can be found in abundance on our planet. Only a small fraction of it is currently converted to electrical power, though in recent years installed geothermal capacity has increased considerably all over the world. Enhanced Geothermal Systems (EGS) represents a path for turning the enormous resources provided by geothermal energy into electricity for human consumption efficiently and on a large scale. Initially, a general overview of this ever-expanding technology from its origins to the current state of the art is presented. The Geodynamics plant in Habanero (Australia), which started up on 2 May 2013, is the first privately-run commercial EGS plant to produce electricity on a large scale. Thanks to the technological development of EGS in recent years, the future looks bright for such plants in the decades to come. Subsequently, a complete economic analysis of EGS is presented. Existing software packages for estimating and simulating costs, conventionally used in studying EGS facilities have been examined, focusing on EURONAUT, the top European software, and the US GEOPHIRES package. The main findings obtained with these two tools are presented and different approaches are proposed in order to obtain better results from the initial assumptions and parameters used in areas still to be explored by software packages. Finally, this doctoral dissertation presents a study into the working fluids that can be used at an Enhanced Geothermal System (EGS) plant as a way of making efficient, large-scale use of the enormous resources offered by geothermal energy. Firstly, we investigate the two working fluids most used in such plants: water (H2O) and carbon dioxide (CO2). The comparative analysis brings to light the advantages of each one, making it possible to assess their beneficial properties. This leads us to establish what properties and alternative working fluid should have. Secondly, we analyse fluids included in the database of the Engineering Equation Solver (EES) program. This entails a study of their thermodynamic properties in the working conditions established. Based on the properties of each alternative working fluid and the results obtained from the EES, we seek to determine which working fluid has the best performance. Lastly, the results obtained after the analysis leads us to conclude that single supercritical nitrous oxide (SCN2O) seems to be an alternative to the two working fluids used to date.