COMPARATIVE PROCESS MODELING OF WATER AND SUPERCRITICAL FLUIDS IN CLOSED-LOOP TUBE-IN-TUBE HEAT EXCHANGERS FOR THE PRODUCTION OF POWER FROM UNPRODUCTIVE HYDROTHERMAL WELLS

ECO2G, the circulation of supercritical carbon dioxide through a closed-loop well system, has been modeled successfully, showing economically-viable power can be readily produced at hot geothermal resources. The process model includes 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. Supercritical carbon dioxide is proposed as the working fluid due to advantageous properties near the critical point; namely, large changes in density as it is heated produces a strong thermosiphon effect, responsible for as much as half of the electrical power produced. To demonstrate this effect, water is also modeled in the same well geometry. Since the vast majority of the costs in an ECO2G power installation is the cost of the well, in this study, we propose using existing unproductive geothermal wells to produce power by installing a tube-in-tube heat exchanger down the center of the perforated production liner. Our modeling shows that the electrical power that can be produced, albeit small, is sufficient to justify the costs of the installation.