A matter of meters: state of the art in the life cycle assessment of enhanced geothermal systems†
This work presents a review of the studies applying life-cycle assessment (LCA) methodologies to evaluate the environmental performance of enhanced geothermal systems (EGSs). Due to the scarcity of commercially installed EGS power plants such studies are rare and usually represent very site-specific conditions and plant characteristics. A detailed inspection of the outcome of these studies shows that the major environmental impacts of the investigated EGS plants are caused by the drilling of geothermal wells during construction. However, recent developments in environmentally friendly drilling technologies, which up to now have only marginally been considered in LCA studies, will provide opportunities to reduce the impact by drilling. Our analysis reveals that the use of electricity from a grid instead of diesel to drive the drilling rigs can improve the environmental performance of EGS plants significantly, provided that the employed electricity is supplied by environmentally friendly technologies. The largest share of uncertainty in the LCA of EGS plants is consequently linked to the incalculable number of boreholes that need to be drilled during the life-time of the plant in order to sustain efficient power production. From the LCA perspective, however, the power needed to drill these additional boreholes can be subtracted directly from the power production of the EGS plant. Under this presumption the examination of greenhouse gas emissions of EGS plants as a function of drilled borehole meters shows that even EGS plants with a large number of deep wells can be competitive in terms of environmental effects compared to conventional energy technologies. When predictions of technological improvements in geothermal drilling and plant design are taken into account, future EGS power plants are prone to have the potential to perform environmentally better than most other renewable energy technologies.