23 JANUARY 2024 WorldWide Drilling Resource® New Tool for Modeling the Viability of Closed-Loop Systems Adapted from Information by Sandia National Laboratories Although geothermal power has a lot of promise as a renewable energy source, there are challenges to implementing it across the country. One of the key issues is the limited number of locations in the U.S. which naturally have the right conditions: hot rock relatively close to the surface and plentiful groundwater to heat up. Sandia National Laboratories researchers used computer models of closedloop geothermal systems to determine if they would be economically viable sources of renewable energy. In the end, they found the cost of drilling would need to decrease significantly to hit cost targets. Although closed-loop geothermal systems were dismissed in the 1980s for being too inefficient, a team of experts at several national laboratories just finished a two-year effort to computationally model closed-loop geothermal systems and the efficiency of those systems. One of the challenges of a closed-loop method is building a system capable of extracting enough heat to be cost-effective, according to Mario Martinez, a mechanical engineer and the principal investigator for the project at Sandia National Laboratories. “The subsurface, the rock, becomes hotter the deeper you go, so it is beneficial to go deep,” he explained. Sandia led the computational modeling of the belowground system, while the National Renewable Energy Laboratory used the numerical results to estimate the economic viability of the system through their aboveground power plant and economic model. The Sandia team looked at two basic setups for closed-loop geothermal systems. A U-tube system pumps cool water down a deep vertical pipe, which then extends horizontally for a certain distance, at a depth where the rock is hot, and then comes up in a different location. The tube-in-a-tube, or coaxial design, pumps cool water down the outer layer of a pipe to a certain depth, then the pipe takes a 90-degree turn and extends a horizontal distance. When the hot water hits the end of the pipe, it’s then pushed into the inner pipe, and back up the way it came. Using a simplified numerical model instead of a full 3D representation, and running the computations on Sandia’s high-performance computing clusters, allowed researchers to model millions of sets of parameters including various depths and circulation fluids. They also studied the temperature of the fluid, how fast the fluid was being pumped, how well the rock transferred heat, the size of the pipes, and other factors. Additionally, Sandia researchers modeled geothermal systems in permeable rock with groundwater, where the additional convective heat transfer would produce a more rapid and sustained transfer of heat from the rock to the circulating fluid. They found this increased heat transfer and improved the economic viability of a closed-loop geothermal system. Several possible enhancements to the system were examined, such as coating the well with a highly thermal conductive cement. They found it would be better to just make the pipe larger. They also discovered their model could approximate the efficiency of a multipronged or “spider” geothermal configuration by merely setting the horizontal extent in the tool to the total extent of all the legs. Yaro Vasyliv, a Sandia computer scientist said, “We asked the question, ‘what is the drilling cost required to meet [the Department of Energy’s] 2035 target for the levelized cost of electricity for enhanced geothermal systems?’ This target is $45 per megawatthour. We found that to achieve this goal using closed-loop systems in hot, dry rock, there would need to be a very aggressive reduction in the cost of drilling.” Illustration by Ray Johnson. GEO For more information call: (270) 786-3010 or visit us online: www.geothermalsupply.com All New! Atlantis-Pro Vault • Traffic-Rated Capable • Simple installation • Trouble-free operation
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