within 3 km of Earth’s surface. These plants operate 90–98 percent of the time, providing baseload electricity. Most hydrothermal plants are binary cycle, converting lower-temperature geothermal water (90–175°C) to electricity by routing it through a closed-loop heat exchanger, where a low-boiling-point hydrocarbon (such as isobutane or isopentane) is evaporated to drive a Rankine power cycle. The other possibilities are steam plants, which use steam directly from the source, or flash plants, which depressurize hot water from the source (175–300°C) to produce steam. These technologies are well developed but limited to particular geological areas—in the United States, the Southwest (WGA, 2006).
A more challenging possibility for tapping Earth’s internal heat are enhanced geothermal systems. As discussed in the “Resource Base” section above in the chapter, this approach mines the heat stored deep below Earth’s surface (typically beyond 3 km and down to about 10 km) in hot and low-permeability rock by artificially creating porous or fractured reservoirs. To access the stored thermal energy, the hot rock must first be “stimulated” by drilling a well to reach the rock, and high-pressure water is then used to form a fractured rock region. Injection and production (extraction) wells can then be drilled into the fractured region; water is circulated in the injection well to extract the stored heat.
A fractured EGS reservoir would cool significantly during heat-mining operations. MIT (2006) estimated that a normal project life would be some 20–30 years before reservoir temperatures fell by 10–15°C and abandonment occurred. The MIT study also found that production wells would need to be redrilled every 4–8 years during that project life.
EGS technology is not currently in large-scale operation. Significant challenges include a general lack of experience in drilling to depths approaching 10 km, even in oil and natural gas exploration, and the need to enhance heat-transfer performance for lower-temperature fluids in power production. Another challenge is to improve reservoir-stimulation techniques so that sufficient connectivity within the fractured rock can be achieved; in that way, the injection and production well system may realize commercially feasible and sustainable production rates (MIT, 2006). Genter et al. (2009) reviews the progress on the EGS plant at Soultz, France, a project that has been under way for more than two decades. Progress is ongoing at this site, with a recently added 1.5 MW power plant currently beginning a 3-year scientific and technical monitoring phase. EGS activity is also ongoing in the United States, Germany, and Australia.