GreenFire Energy has developed a geothermal power cycle that uses a closed-loop design with different working fluids. An experimental demonstration of this system has been performed at the Coso Geothermal Field with supercritical carbon dioxide (sCO2) recirculated in a downhole closed-loop heat exchanger installed into an existing well. In this demonstration, sCO2 was circulated through a 330-meter, tube-in-tube heat exchanger, hung from the wellhead. During testing, the well coproduced steam at four different flow rates (including zero flow), combined with five different flow settings of circulated sCO2. The development of a thermosiphon provides the motive force to circulate sCO2; that is, there was no pumping of the sCO2 required for operation. The resulting experimental data were analyzed using a 1-D model developed by GreenFire Energy based on the conservation of mass and energy, considering heat transfer, friction, and the thermodynamic properties of the geothermal brine and circulating sCO2. A finite-volume steady-state solution method was applied in the upflowing and downflowing directions, including heat transfer between flows, which required iteration to solve. Additionally, coupling of 1-D governing equations model and statistical techniques were applied to understand the experimental space domain, and different scenarios were simulated, which allowed insight into how to optimize power production as a function of the principal operation variables and experimental design parameters. Experimental results show that a strong thermosiphon was easily achieved and could be varied by using the pressure control valve to either have peak pressure increase (with low flow) or high flow (with low pressure increase). Power potential was calculated using an isentropic efficiency assumption and it was found that peak power potential occurred in the middle range as a balance of circulating sCO2 flow rate and produced pressure rise by the thermosiphon. Other design variables, including sCO2 injection temperature and length of closed-loop heat exchange, were simulated and analyzed using a Monte Carlo method. The primary goal of this demonstration was to verify GreenFire Energy’s supercritical CO2 process modeling assumptions and to illustrate the power production potential of closed-loop geothermal technology at different conditions.

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