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Energy Recovery by Benign Hydrothermal Processes
building is heated with hot spring water. A convenient way to tap geothermal energy is
to use heat pump, which supplies heating and cooling using geothermal energy. In this
method, either air or antifreeze liquid is pumped through pipes that are buried underground and reentered into the buildings. In the summer, the liquid moves heat from the
building into the ground, and in the winter, the opposite process takes place providing
air and water to the heating system of the building. The system can be simple, in which
heating and cooling using the ground source can occur by tubes running from outside
the air, under the ground, and into the house ventilation system, or more complex,
in which compressor and pumps are used as an electric air-conditioning system to optimize the heat transfer. These ground source heating and cooling systems are the most
environmentally clean systems. The Department of Energy has pointed out that heat
pumps operated by geothermal energy are more efficient and save more money than
any other electrical systems. Currently, more than 600,000 homes in the United States
use geothermal energy-driven heat pumps, and this number is increasing at the rate of
60,000 homes per year, with the largest growth in rural areas [6,14,19,20].
3.3.1 enhAnCed geoThermAl SySTemS
While geothermal heat can be obtained anywhere under the surface of the Earth, the
conditions that make water circulate to the surface are found only in <10% of the
Earth’s surface [5,9,13,18,20]. A method to capture geothermal heat from dry areas is
known as enhanced geothermal system (EGS) or “hot dry rock.” The systematic steps
demonstrating how EGSs work are graphically illustrated in Figure 3.4 [5,11,14]. As
shown in this figure, the hot dry rock reservoirs, typically at greater depths below
the Earth’s surface than conventional sources, are first broken up by pumping highpressure water through them. Once the rock is perforated (by the hydraulic fracturing
process), additional water not only expands perforations in the rock but also captures
heat from the open rock. This steam is collected by a production well and brought to
the surface, and it powers turbine to generate electricity. Finally, the cooled water is
returned to the reservoir by injection wells to complete the circulation loop. The system can further be optimized by employing carefully designed multiple production
wells. Plants that use a closed loop binary cycle described earlier release no fluids
or heat-trapping emissions other than water vapor, which may be used for cooling.
As indicated in the figure, water and steam play a key role in recovering geothermal
energy from deep dry rocks.
The EGS process does carry some risk as hydraulic horizontal fracturing
(fracking) used in the recovery of unconventional gas such as “shale gas” allows
permeation of carbon dioxide or “fracking fluid” to water aquifers. The EGS can
induce seismic activity that might occur from hot dry rock drilling and development, although the likelihood of this occurrence is low, when projects are located
at an appropriate distance away from the major fault lines and properly monitored.
Appropriate site selection, assessment, and monitoring of rock fracturing and seismic activity during and after construction are very critical. The EGS can produce
a continuous power and it is feasible anywhere in the world, depending on the economic limits of the drilling depth. Good locations are over deep granite covered
by a 3–5 km layer of insulating sediments that reduce the heat loss [6,7]. The EGS