Enhanced Geothermal Systems (EGS) present a unique challenge for a future energy
source: they could provide CO2-free cogeneration. Theoretically, cooling down a 1km3
rock volume at 200°C by 20°C corresponds to an electric power generating capacity of
10MWe over 20 years. EGS power potential estimates for Germany yield 300,000TWh,
equivalent to 600 times the annual power consumption. Numerical simulations show that
sustainable production is achievable over many decades.
Operational numbers, necessary for a technically feasible and economically viable EGS
system call for heat exchange surfaces >2.106m² in a volume >2.108m3 and production
flowrates of 50-100l/s with 150-200°C, at flow impedance <0.1MPa/l/s, water losses
<10%. So far, such numbers have not yet been demonstrated; presently there is no
power generation from EGS systems.
Here it should be noted that power generation can already be achieved, albeit at
lower conversion efficiency, with fluid temperatures of 120-150°C. Attempts to
produce this, from shallower depths, could be less risky. In other words there is
still room for optimisation of the approach.
Numerous problems must be solved to reach the goals and many unknowns need to be
clarified: irregularities of the temperature field at depth, favourable stress field
conditions, long-term effects of rock-water interaction, possible short-circuiting,
environmental impacts like man made-seismicity, to name only a few.
A key issue is the creation, characterization and management of the central (and
crucial) system component: an extended, sufficiently permeable fracture network at
several km depth, with suitable heat exchange surfaces. To achieve this, no direct
observation/ manipulation is possible; it must be accomplished by a kind of
remote-sensing and control.
Promising developments to provide the tools needed here are underway (e.g. the HEX-B
and HEX-S software of GEOWATT).
What will really be needed is the planning and establishment of successful EGS
systems in several contrasting geological settings. Joining forces by a broad,
internationally based interdisciplinary effort like ENGINE is an important step
towards the ambitious goals. The EGS adventure resembles an Alpine tour: the
difficulties and struggles underway are numerous and major, the prospect however
("the view from the top") is rewarding.
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