| Authors |
Thomas A. BUSCHECK, Jeffrey M. BIELICKI, Mingjie CHEN, Yunwei SUN, Yue HAO, Thomas A. EDMUNDS, Martin O. SAAR, Jimmy B. RANDOLPH |
| Abstract |
Sedimentary geothermal resources typically have lower temperatures and energy densities than hydrothermal resources, but they often have higher permeability and larger areal extents. Consequently, spacing between injection and production wells is likely to be wider in sedimentary resources, which can result in more fluid pressure loss, increasing the parasitic cost of powering the working fluid recirculation system, compared to hydrothermal systems. For hydrostatic geothermal resources, extracting heat requires that brine be lifted up production wells, such as with submersible pumps, which can consume a large portion of the electricity generated by the power plant. CO2 is being considered as an alternative working fluid (also termed a supplemental fluid) because its advantageous thermophysical properties reduce this parasitic cost, and because of the synergistic benefit of geologic CO2 sequestration (GCS). We expand on this idea by: (1) adding the option for multiple supplemental fluids (N2 as well as CO2) and injecting these fluids to create overpressured reservoir conditions, (2) utilizing up to three working fluids: brine, CO2, and N2 for heat extraction, (3) using a well pattern designed to store supplemental fluid and pressure, and (4) time-shifting the parasitic load associated with fluid recirculation to provide ancillary services (frequency regulation , load following, and spinning reserve) and bulk energy storage (BES). Our approach uses concentric rings of horizontal wells to create a hydraulic divide to store supplemental fluid and pressure, much like a hydroelectric dam. While, as with any geothermal system, electricity production can be run as a base-load power source, production wells can alternatively be controlled like a spillway to supply power when demand is greatest. For conventional geothermal power, the parasitic power load for fluid recirculation is synchronous with gross power output. In contrast, our approach time-shifts much of this parasitic load, which is dominated by the power required to pressurize and inject brine. Thus, most of the parasitic load can be scheduled during minimum power demand or when, due to its inherent variability, there is a surplus of renewable energy on the grid. Energy storage is almost 100 percent efficient because it is achieved by time-shifting the parasitic load. Consequently, net power can nearly equal gross power during peak demand so that geothermal energy can be used as a form of high-efficiency BES at large scales. A further benefit of our approach is that production rates (per well) can exceed the capacity of submersible pumps and thereby take advantage of the productivity of horizontal wells and better leverage well costs—which often constitute a major portion of capital costs. Our vision is an efficient, dispatchable, renewable electricity system approach that facilitates deep market penetration of all renewable energy sources: wind, solar, and geothermal, while utilizing and permanently storing CO2 in a commercially viable manner. This study was funded by the U.S. Department of Energy (DOE) Geothermal Technologies Office (GTO) under grant number DE-FOA-0000336, a U.S. National Science Foundation (NSF) Sustainable Energy Pathways (SEP) grant, CHE-1230691. This work was performed under the auspices of the USDOE by Lawrence Livermore National Laboratory (LLNL) under DOE contract DE-AC52-07NA27344. |