Environmental Engineering Reference
In-Depth Information
stimulation methods, such as acid injection or hydraulic fracturing, may become
very important when exploiting these kinds of reservoirs. In many cases the situa-
tion may not differ very much from the situation in the crystalline basement,
where large artificial fracture surfaces will have to be created in order to achieve
the desired flow rates. Experiments performed in Hot-Dry-Rock projects have
shown that the waterfrac-technique is most suitable and possibly the only possibil-
ity for this purpose.
On the basis of the above mentioned framework conditions, the following ap-
proaches of geothermal power generation are distinguished.
−
Power generation by open systems. Within open systems the heat carrier is
circulated within an open circuit (i.e. the heat transfer medium is pumped into
the underground, mixes up with potentially available geothermal fluid, and is
produced again). In this respect, the following kinds of reservoirs are distin-
guished.
•
Hot water aquifers (i.e. fissured porous reservoirs). By means of a two or
more well system, aquifers containing a hot geothermal fluid can be tapped
in sedimentary basins. Assuming that the aquifer has a sufficiently high tem-
perature and that a sufficient production rate is either naturally available or
produced by stimulation, such a two or more well system can either serve for
combined heat and power (CHP) generation, or exclusively for power gen-
eration.
•
Faults. Faults are potential flow paths of waters and are tapped e.g. by two
wells similar to the hot water aquifers mentioned above, provided that suffi-
cient permeability is available. As they generally reach deep into the under-
ground, they allow achievement of very high temperatures.
•
Crystalline rocks (i.e. Hot-Dry-Rock (HDR) or Hot-Fractured-Rock (HFR)).
By fracturing new or enlarging already existing small faults respectively the
existing network of fissures within the basement rocks, the Hot-Dry-Rock
(HDR) or Hot-Fractured-Rock (HFR) technologies artificially create a new
heat exchanger in the underground. If this heat exchanger is connected with
the surface by means of e.g. two wells, water can circulate and heat up. It is
thus available for geothermal heat and/or power generation.
−
Power generation by closed systems. Within closed systems the heat carrier is
circulated within a closed circuit. Hence, the heat transfer medium that is
pumped into the underground is entirely separated from any fluids possibly
available in the ground. Two types of closed systems are distinguished.
•
One-way system. The underground is opened by means of a flow-through
system, where a heat transfer medium is pumped one-way within a well into
the underground and produced within the same well some kilometres away as
a feed for heat and/or electricity generation.
•
Two-way system. Energy can also be withdrawn from the underground by
means of a deep coaxial well, through which the heat transfer medium circu-
lates as discussed in Chapter 10.2.
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