Investigation of Geothermal Heat Extraction Using Supercritical Carbon Dioxide (sCO2) and Its Utilization in sCO2-based Power Cycles and Organic Rankine Cycles - A Thermodynamic & Economic Perspective

Abstract

CO2 capture and sequestration in deep saline aquifers is widely considered to be a leading option for controlling greenhouse gas emissions. One such possibility involves injection of supercritical carbon dioxide (sCO2) into a high-permeability geothermal reservoir. In addition to the benefit of sequestering the CO2 in reservoirs, the CO? can be used to mine geothermal heat for utilization above ground. This study adopts TOUGH2-T2Well/ECO2N multi-phase flow solver which has the capability to model fully coupled geothermal wellbores and reservoir to obtain desirable sCO? production flow rates, temperatures and pressures for power generations.As geothermal energy is widely recognized as a low grade heat source, power cycles with capabilities to convert low grade energy into electricity, such as Organic Rankine Cycle (ORC), have been considered. Additionally, sCO2-based power cycles have also been investigated comprehensively, since the similar temperature profiles between produced sCO2 from geothermal reservoirs and working fluid sCO2 potentially offer the advantages of avoiding pinch points to achieve better cycle performance. Moreover, the unique physical properties near critical point of CO2 are significant contributors for the considerably low compression work leading to higher net power output. Regarding thermodynamic optimization analyses, the maximum net power output is selected as the objective of optimizing cycle performance both for sCO2-based power cycles and ORCs.Possible cycle improvement methods have been taken into consideration and different configurations of sCO2-based power cycles have been analyzed thermodynamically. The direct turbine expansion, sCO2 Brayton cycle with pre-compression and inter-cooling, and transcritical sCO2 cycle have been chosen to perform cost estimation and optimization analyses. On the other hand, working fluid selection criteria have been proposed for ORCs to find out the most suitable working fluids using hot produced sCO2 from geothermal reservoirs. Considering cycle performance, working fluid physical property, operating pressure level and design complexity, a subcritical ORC with R245fa as working fluid is selected as the most competent ORC. Accordingly, to compare with the conventional geothermal power generation system, the cost estimation and optimization analyses have been accomplished by finding the minimum levelized cost of electricity (LCOE) for a nominal power plant capacity of 30 MWe. The plant capital cost, well cost, and operations and maintenance (O&M) cost are taken into account. The optimal results indicate that the LCOEs of selected four promising power generation technologies range from $0.276/kWh to$0.316/kWh in which more than half the portion is contributed by the O&M costs of cooling loads of rejecting heat for power cycles and geothermal loop CO? re-injections.Analyses have been performed to investigate the effects of reducing the cooling O&M cost and counting CO2 credit of sequestration on the LCOEs. It has been found if the cooling O&M cost is reduced to one quarter of the original value, the LCOEs can decrease 43 – 48% without counting the CO2 credit. On the other hand, if the counted CO2 credit is over $2/t for all new proposed power generation options where the largest breakeven point occurs, then they are all competitive compared to conventional geothermal power plants. Furthermore, improvements for reducing LCOEs of power generation options have been discussed and suggested, such as performing more detailed geothermal heat mining simulations, and adopting different cooling techniques to reduce O&M costs