Environmental Engineering Reference
In-Depth Information
Figure 4.52. Characteristics of a hydrothermal site: an aquifer (e.g., permeable
limestone) that receives a constant supply of rainwater is confined between impermeable
rocks (e.g., clay sediments on top and granite at the bottom). The heat source (i.e.,
magma) forces hot water and steam upward through rock fissures, producing fumaroles,
geysers, and hot springs.
Another promising, but not yet commercially viable, approach involves non-permeable
dry rocks, where fractures are absent. In such cases hot water cannot be retrieved from an
underground reservoir. Instead, holes are drilled into the rocks and pressurized water or a
water-chemical mixture is injected, similarly to hydraulic fracturing (fracking) for shale oil
and gas. The hot rocks act as a kind of giant underground hot plate around which water
circulates (see Figure 4.53 ). This artificial circuit is known as an enhanced (or engineered)
geothermal system (EGS).
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