Life Cycle Analysis of Geothermal Systems in Comparison to other Power Systems

Abstract

It is increasingly argued that, if we are to end our over dependence on foreign oil and reduce fossil carbon emissions, we must rely more heavily on renewable energy resources for the power delivered to American households, industry, business, and potentially powering its vehicles. Often mentioned in this context is wind, solar, and biomass energy, which figure prominently in the Renewable Portfolio Standards of many states. Clearly, these alternatives can help. In fact, the EIA projects that renewable electricity, which now represents around 8.5% of U. S. electricity generation, will increase to about 17% by 2035. Much of this increase is projected to come from additional wind turbines and biomass combustion plants. However, another alternative is geothermal electricity generation, for which a greater demand is also anticipated in the next decade or two. In fact, EIA projects that geothermal electricity generation will increase roughly 100% in that time frame. Environmental assessments are essential part of evaluating future choices in the U. S. electricity generating mix. For this purpose, the method employed here is life cycle analysis (LCA). From both a literature review and a modeling effort, an LCA study has been conducted where the plant infrastructure stage of the life cycle is considered. This stage includes the energy and materials required for plant construction and the energy needed for production of plant materials. The set life cycle metrics tracked herein are energy, carbon dioxide (CO2), water per kWh generated, and those materials per MW capacity used in significant quantities (typically steel, aluminum, and concrete) for the construction of the generating facility. On a per MW of output capacity basis, conventional thermal electric photovoltaic plants require the least steel, concrete, and cement, followed by the renewable generating technologies of geothermal, wind, hydro, and biomass. Fossil power plants are found to emit far more greenhouse gases than renewable plants. The GHG emissions for renewable systems tended to be dominated by plant construction, though flash geothermal emissions are primarily due to fugitive CO2 from the geofluid during the fuel use stage of the life cycle. Overall, with the possible exception of hydrothermal flash the geothermal systems are found to be among the lowest greenhouse gas emitters. Finally, a complete life cycle characterization of many different electric power technologies, including the infrastructure stage, is available in GREET.