The Geothermal Energy-Water Nexus

Abstract

Energy and environmental analyses are critical to developing a robust set of geothermal energy technologies that meet future energy demand. Assessments of the sustainability of geothermal power generation that have focused on resource management and associated environmental impacts during plant operations have shown that overall emissions, water consumption, and land use for geothermal electricity production have a smaller impact than traditional base-load electricity generation technologies. There is a need to improve our understanding of the environmental impacts across the life cycle of geothermal electricity systems. This paper presents an assessment of life cycle freshwater requirements of various geothermal power-generating systems. Systems evaluated included hydrothermal binary (air-cooled), hydrothermal flash, binary enhanced geothermal system (EGS) (air-cooled), flash EGS, and binary geopressured (air-cooled). On a per-well basis and a per-kilowatt-hour lifetime energy output basis, higher resource temperatures result in lower water consumption for the same technology. However, moving from binary systems that typically operate at lower temperatures to flash systems that operate at higher temperatures increases the aboveground operational loss of geofluid. In most hydrothermal systems this additional loss of geofluid is not replaced. This does not increase water consumption, but does have long-term impacts on the sustainability of the reservoir. However, in EGSs, this lost geofluid will more than likely need to be replaced to maintain reservoir pressure. The use of alternative, lower quality water sources will be important for EGSs because of the high water requirements relative to competing electricity generation systems. Finally, an analysis of relevant laws and policies was conducted as a means of identifying gaps where more information could result in better, more informed decision making with regard to geothermal energy development A new platform developed with funding from the U.S. Department of Energy’s Geothermal Technologies Office — the National Geothermal Data System — is explored as an option to expand the collection and use of these kinds of data and to address these gaps.