Water from subterranean heat sources is very corrosive. This is a major problem for “hydrothermal” (i.e., open loop) generation. But closed loop systems avoid bringing that subsurface water to the surface. You still have your heat exchanger pipes buried in the ground that are subject to the corrosive environment.

The problem is that closed-loop systems are much more expensive than open-loop and cannot produce competitive electric power.

See NREL’s report “Numerical Investigation of Closed-Loop Geothermal Systems in Deep Geothermal Reservoirs.” The work was performed by Pacific Northwest National Laboratory, Sandia National Laboratories, Idaho National Laboratory, Stanford University and Pennsylvania State University.

For closed-loop systems, “Our results indicate competitive Levelized Cost of Heat (LCOH) can be achieved; however, competitive Levelized Cost of Electricity (LCOE) cannot be achieved without significant reductions in drilling costs…reservoir temperatures greater than 200 degrees C, achieved by going to greater depths (~3-4 km), may significantly enhance power production.

That’s 2 to 3 miles below the surface! Those aren’t cheap holes to drill, especially in hot zones.