The Closed Loop Geothermal Working Group is a collaborative study, funded by the United States Department of Energy (DOE), Geothermal Technologies Office (GTO) to understand the potential and limitations of producing thermal and mechanical energy from geothermal reservoirs with marginal working fluid losses. In this study, teams of scientists and engineers from four national laboratories, plus expert panel members are applying numerical simulators and analytical tools to model heat recovery from closed-loop geothermal systems and then subsequently using outlet temperature and pressure versus time from these models to forecast two economic indicators: 1) Levelized Cost Of Heating (LCOH) and 2) Levelized Cost Of Electricity (LCOE), over a range of drilling costs. Numerical simulators and analytical tools applied in the study, including those for technical and economic analyses, were those developed by the participating institute, yielding independent calculations of energy production and economic forecasts, increased confidence in the analysis. The study was designed to investigate an array of system configurations, working fluids, geothermal reservoir characteristics, operational periods, and heat transfer enhancements. During the opening year of the study the focus was on water as the working fluid in the closed-loop systems that either had a u-shaped or coaxial configuration. The principal objectives during the opening year were to determine upper limits for thermal and mechanical energy recovery and optimal operational and configuration parameters for each scenario, and to understand the limiting factors to system performance. One important outcome from the first year of the study was that simulation results from models using simple discretizations in a radial direction (i.e., slender-body tool (SBT)) to the borehole compared favorably with more conventional numerical simulations with fine discretizations around the borehole and embedded borehole modeling approaches. Moreover, the SBTs compared well with available field observations and analytical models and were shown to be numerically efficient. During the second year of the study, a database comprising 2.4 million simulation scenarios was created of closed-loop system performance in terms of production temperature and pressure versus time, across nine scenario parameters: 1) water and supercritical CO2 (scCO2) working fluids, 2) u-shape and coaxial configurations, 3) mass flow rate, 4) thermal conductivity, 5) geothermal gradient, 6) vertical depth, 7) horizontal extent, 8) inlet temperature, and 9) borehole diameter. LCOH and LCOE were then computed for each of the 2.4 million scenarios across a range of drilling costs. For the LCOE, electricity generation was computed using an Organic Rankine cycle (for water) or direct turbine expansion cycle (for scCO2). The database is stored in a Hierarchical Data Format (HDF5) file structure that will be available on the Geothermal Data Repository (GDR). A companion paper describes methods for extracting information from the database via Python scrpts and the execution of economic analysis. An alternative to working with the database is a reduced-order model that reproduces the database results. This paper provides an overview of the Closed-loop Working Group study, including key outcomes from first and second years and discussion of optimal configurations with respect to LCOH and LCOE over a range of drilling costs.

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