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Publication Title | PROCESS MODELING OF A CLOSED-LOOP SCO2 GEOTHERMAL POWER CYCLE

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The 5th International Supercritical CO2 Power Cycles Symposium March 29 - 31, 2016, San Antonio, Texas

PROCESS MODELING OF A CLOSED-LOOP SCO2 GEOTHERMAL POWER CYCLE

Brian S. Higgins† Mark P. Muir Alan D. Eastman

GreenFire Energy 4300 Horton Ave, Unit 15 Emeryville, CA 94608

ABSTRACT

Curtis M. Oldenburg Lehua Pan

Energy Geosciences Division 74-316C Lawrence Berkeley National Laboratory Berkeley, CA 94720

A closed-loop geothermal power cycle has been designed and optimized for power production using supercritical CO2 as the working fluid. Since it is closed loop, heat extraction from the resource to the well is by conduction. The sCO2 that is returned to the surface is used to directly produce power, then cooled and reinjected to complete the cycle. This paper reviews the process modeling, including a simple 1D explicit solution of the mass and energy conservation equations, utilizing a semi-empirical conduction relationship to capture the time-resolved depletion of the geothermal resource. A second 3D model is used containing mixed convective- conductive fluid-flow modeling with the T2Well/TOUGH2 wellbore flow model to investigate the critical factors that control closed-loop geothermal energy recovery. T2Well solves a mixed explicit-implicit set of momentum equations for flow in the pipe with full coupling to the implicit 3D integral finite difference equations for Darcy flow in the porous medium.

As a result of these modeling studies, we find that for each resource and well geometry, there is an optimum tradeoff between power produced and the mass flow of sCO2. At a moderately low flow of sCO2, a strong thermosiphon develops and the production temperature is highest, however the power production is relatively low. As the sCO2 flow rate is increased, the thermosiphon weakens due to friction in the well and the production temperature decreases because the rate of heat extraction is limited by heat transfer from the resource and not by the extraction potential of the sCO2. Through parametric analysis using these models, it is possible to determine the process conditions that maximize power output. The modeling results suggest that the thermosiphon accounts for a large portion of this maximized power output, thermal degradation is modest, and that moderate levels permeability and convection do not substantially increase power production, but very high levels of permeability do provide for a substantial power increase.

† Brian.Higgins@GreenFireEnergy.com

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