Enhanced geothermal involves fracturing rocks and pumping down liquids to create artificial reservoirs. The hot rocks directly heat the liquids, which return to the surface to make steam. This approach is relatively more efficient at extracting heat from the ground, but it can also raise the risk of inducing earthquakes or affecting groundwater — though experts say that’s unlikely to happen in well-managed projects. In places that ban fracking, like Germany, closed-loop systems can still move forward.
But the closed-loop design has trade-offs of its own, said Jeff Tester, a professor of sustainable energy systems at Cornell University and the principal scientist for Cornell’s Earth Source Heat project. Namely, the pipes can limit the transfer of heat from the underground rocks to the fluids inside the pipe, which in turn limits how much energy a system can produce.
“While companies developing closed-loop systems can make them work, the main challenge they face is for fluid temperatures and flow rates to be high enough to pay off economically,” Tester said. “You can get energy out of the ground; it’s just, how much can you sustainably and affordably produce from a single closed-loop well connection?”