Techno-Economic Performance of Closed-Loop Geothermal Systems for Heat Production and Electricity Generation

Abstract

• Closed-loop geothermal systems evaluated for heat and electricity production • Slow heat transfer in rock and limited heat transfer area pose challenges • Targeting high temperatures, existing wells, and multi-laterals beneficial • Relatively high levelized costs for greenfields, unless drilling at low cost Closed-loop geothermal systems, recently referred to as advanced geothermal systems (AGS), have received renewed interest for geothermal heat and power production. These systems consist of a co-axial, U-loop, or other configuration in which the heat transfer or working fluid does not permeate the reservoir but remains within a closed-loop subsurface heat exchanger. Advocates indicate its potential for developing geothermal energy anywhere, independent of site-specific geologic uncertainties, and with limited risk of induced seismicity. Here, we present a technical and economic analysis of closed-loop geothermal systems using a Slender-Body Theory (SBT) model, COMSOL Multiphysics simulator, and the GEOPHIRES analysis tool. We consider a number of different scenarios and evaluate the influence of variations in reservoir temperature (100 to 500℃), well termination depth (2 to 4 km), mass flow rate (10 to 40 kg/s), injection temperature (10 to 40℃), fluid type (liquid water vs. supercritical carbon dioxide), design configuration (co-axial vs. U-loop), and degree of reservoir convection (natural, forced or conduction-only). The resulting average heat production rates range from about 2 to 15 GWh per year for cases considering a co-axial design and from 9 to 67 GWh per year for cases with a U-loop design. Assuming generous economic and operating conditions, estimates of levelized cost of heat range from ∼$20 – $110 per MWh (∼$6 – 32/MMBtu) and ∼$10 – $70 per MWh (∼$3 – $20/MMBtu) for greenfield co-axial and U-loop cases, respectively. In the scenarios in which electricity generation is considered, annual electricity production ranged between 0.12 and 7.5 GWh per year at a levelized cost of electricity from roughly $83 to $2,200 per MWh. In all scenarios, the results exhibit a large rapid drop in production temperature after initiation of operations that levels off to a steady value significantly below the initial reservoir temperature. Operating at lower flow rate increases the production temperature but also lowers the total heat production. The consistently low production temperatures hinder efficient electricity generation in most cases considered. Natural or forced convection can increase thermal output but requires sufficiently high reservoir permeability or formation fluid flow. As expected, overall system costs are heavily dependent on drilling costs; hence, repurposing existing wells could significantly lower capital and levelized costs. In comparison with other types of geothermal systems, our results for closed-loop geothermal systems predict long-term production temperatures considerably below the initial reservoir temperature, and relatively high levelized costs for greenfield closed-loop geothermal systems, particularly for electricity production, unless significant reductions in drilling costs are obtained.