This paper documents the approach taken to characterize and represent an updated assessment of U.S. geothermal supply for use in forecasting the penetration of geothermal electrical generation in the National Energy Modeling System (NEMS). This work is motivated by several factors: The supply characterization used as the basis of several recent Department of Energy (DOE) forecasts of geothermal capacity is outdated; additional geothermal resource assessments have been published; and a new costing tool that incorporates current technology, engineering practices, and associated costs has been released.

The approach has two main components:

-- Characterization - estimates the development and operating costs for specific geothermal reservoir volumes in the subsurface identified in existing databases and source publications.
-- Representation - groups and generalizes the costs in a manner suitable for input into NEMS using the Geothermal-Electric Sub-module (GES) input file format.

While the geothermal resource can be considered as a single resource with geographically variable properties and designated by reservoir or system, for convenience, the resulting supply representation incorporates five specific resource types: hydrothermal flash, hydrothermal binary, geothermal fluids coproduced with oil and gas, and two types of Enhanced Geothermal Systems (EGS) resource - convective EGS associated with hydrothermal resources at depths less than 3 kilometers, and conductive EGS potential for depths between 3 and 10 kilometers. This updated supply representation extends supply beyond the traditional three Western regions considered by DOE in the past by including cost and capacity estimates for three midcontinent regions. This representation is based on recently available updated supply estimates for hydrothermal and convective EGS resources, and new estimates for coproduced and conductive EGS resources completed as part of the MIT study of EGS resource supply and costs.

This new representation comprises 126 GW of resource potential nationally: 89 GW across all resource types in the Western regions and 37 GW mostly from coproduced potential in the non-Western regions. The total represented capacity is nearly the same for the Western regions as used in previous recent DOE Office of Energy Efficiency and Renewable Energy (EERE) Government Performance and Results Act (GPRA) benefits assessments. However, the Western region mix among specific resources is different. The updated supply features significantly lower levelized cost of energy (LCOE) for hydrothermal resources and somewhat lower LCOEs for EGS than used previously. Further, the inclusion of a significant amount of relatively low-cost coproduced resource further accentuates these cost differences and contributes to a significant increase in the total amount of geothermal resource that is likely to be technologically and economically accessible in the near through midterm period.

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