Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 469

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tial is tremendous they are pinheads on a map of global scale. Most areas of the
continents are lacking such favourable conditions and many of the geothermal
fields are in remote areas far from any consumer. For this reason geothermal
power generation faces two major challenges.
− Firstly, geothermal reservoirs of relatively low temperatures at relatively great
depth have to be exploited.
− Secondly, concepts need to be developed that allow to cost-effectively access
geothermal energy from hot rock formations at great depths.
Due to the relatively low thermal conductivity of rock, which is generally between
1 and 5 W/(m K), the well itself is a too small heat exchanger to withdraw heat at
rates of economic interest. Deep wells are expensive and a thermal power in the
order of magnitude of at least 10 MW is required to justify the investment. This
can only be achieved if fluid is produced from the reservoir at a flow rate exceed-
ing 100 m 3 /h per production well.
The major technical problem encountered with such a geothermal power gen-
eration is the generally low hydraulic permeability of deep rock formations. There
are only a few types of rock formation, such as highly porous sandstone, intensely
fractured rock, or karstic limestone that provides sufficient permeability to
achieve the production flow rates required for cost-efficient power generation.
This type of reservoir is referred to as hot water aquifer. Their temperatures only
rarely exceed 150 °C (i.e. only in case of a geothermal anomaly); hence, generally
temperatures between 100 and 150 °C are expected. In general, higher tempera-
tures are only achieved from depths of 5,000 m onwards. However, at this depth
rock permeability is generally very low and thus insufficient for geothermal
power generation.
There are two possibilities of tapping such tight rock formations. Firstly, tap-
ping of fault zones which reach down far below and allow for natural water
movements, and, secondly, creation of artificial heat exchanging surfaces accord-
ing to the Hot-Dry-Rock (HDR) concept. At present geothermal power production
in the regions with normal to slightly above normal temperature gradients appears
feasible only for these three types of reservoirs: hot water aquifers, fault zones,
and crystalline bedrock.
The geothermal power potential of these reservoirs is practically inexhaustible.
A recently performed study for Germany for instance /10-13/ estimated the geo-
thermal power potential for this relatively small country lacking major tempera-
ture anomalies to more than 1,100 EJ of electric energy. In many other countries
the crystalline basement was and will by far be the biggest resource. The geo-
thermal power potential of deep reaching faults of 45 EJ, and of the hot water
aquifers of 9 EJ, was much smaller but, if compared to the annual power con-
sumption of Germany of 2 EJ/a, they are nevertheless very interesting resources.
It has to be mentioned, however, that the hydraulic properties of the hot water
aquifers and of the faults at great depths are not known in wide areas and that
there is a high probability not to meet the desired productivity. For this reason