Cumulative energy, emissions, and water consumption for geothermal electric power production

Abstract

A life cycle analysis has been conducted on geothermal electricity generation. The technologies covered in the study include flash, binary, enhanced geothermal systems (EGS), and coproduced gas and electricity plants. The life cycle performance metrics quantified in the study include materials, water, and energy use, and greenhouse gas (GHG) emissions. The life cycle stages taken into account were the plant and fuel cycle stages, the latter of which includes fuel production and fuel use (operational). The plant cycle includes the construction of the plant, wells, and above ground piping and the production of the materials that comprise those systems. With the exception of geothermal flash plants, GHG emissions from the plant cycle are generally small and the only such emissions from geothermal plants. Some operational GHGs arise from flash plants, and though substantial when compared to other geothermal power plants, these are nonetheless considerably smaller than those emitted from fossil fuel fired plants. For operational geothermal emissions, an emission rate (g/kW h) distribution function vs. cumulative capacity was developed using California plant data. Substantial GHG emissions arise from coproduced facilities and two other “renewable” power plants, but these are almost totally due to the production and use of natural gas and biofuels. Nonetheless, those GHGs are still much less than those from fossil fuel fired plants. Though significant amounts of water are consumed for plant and well construction, especially for well field stimulation of EGS plants, they are small in comparison to estimated water consumed during plant operation. This also applies to air cooled plants, which nominally should consume no water during operation. Considering that geothermal operational water use data are scarce, our estimates show the lowest water consumption for flash and coproduced plants and the highest for EGS, though the latter must be considered provisional due to the absence of field data. The EGS estimate was based on binary plant data.