Download Recent heat flow in the Alps – Geothermal energy

Recent heat flow in the Alps –
Geothermal energy
Thimo Philipp, 287098
MSc. Angewandte Geowissenschaften
Fieldcourse Alps (2013)
Table of content
Abstract ................................................................................. Fehler! Textmarke nicht definiert.
1.
Physical basics of Geothermics ............................................................................................ 3
1.1.
Relevant parameters ..................................................................................................... 3
1.2.
Types of heat transfer through the lithosphere .............................................................. 4
2.
Sources of heat in the Alps ................................................................................................... 4
3.
Acquisition of heat flow data ................................................................................................. 5
4.
Heat flow in the Alps and the surroundings........................................................................... 6
5.
Geothermal energy use in the Alps .................................................................................... 11
References ................................................................................................................................ 14
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1. Physical basics of Geothermics
1.1.
Relevant parameters
Heat flow (q), also known as heat transmission, is defined as the energy, in the form of heat,
being transferred from one substance to another in a unit of time. In SI units, heat flow is
given in milliwatt per squaremeter (Werner, 1977) but is sometimes also expressed in heat
flow units (HFU) where
Fourier’s Law describes the heat flow as the product of thermal conductivity λ (given as a
tensor) and the geothermal gradient ΔT/Δx (given as a vector) (Clauser, 2013):
According to the equation, the heat flow is solely dependent on the two parameters thermal
conductivity and temperature gradient which will be introduced briefly in the following. The
average approximate heat flow value in the earth’s crust is 65 mW/m2 (Vedova, 1991).
The geothermal gradient is the rate of increasing temperature with respect to increasing
depth (δT/δx). It is mostly measured along vertical profiles in boreholes. The average value
is approx. 30 °C/km within the continental crust, and much lower in the mantle (see fig 1).
Causes of elevated gradients within the lithosphere will be explained in chapter 2 (sources of
heat in the Alps).
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Figure 1:Temperature within the earth plotted against depth (Clauser, 2013)
The thermal conductivity λ is the ability of a material to conduct heat, which is routinely
measured on rock samples in the laboratory. For magmatic rocks it is assumed to be
isotropic which allows an ideal vertical heat flow. This does not hold for many sedimentary
and metamorphic rocks where the thermal conductivity can be anisotropic so that lateral heat
flow is significant and has to be considered (Clauser, 2013). Consequently, the type of rock
and its structure determines the potential for transferring heat through the lithosphere.
1.2.
Types of heat transfer through the lithosphere
In general there are three main mechanisms of heat transfer: Conduction, convection and
radiation.
The dominant process in the earth’s crust is conduction (Clauser, 2013). Heat is conducted
from deep regions through the lithosphere and is diffused into the atmosphere and the
oceans. In the oceanic crust this conductive heat flow is higher because the crust is thinner.
The heat is transported in the form of energy through material contacts. No physical
movement of solids or fluids is taking place.
Sometimes, when fluids flow through the underground, also convection contributes to the
heat flow. Here the heat is transported within hot fluids that are ascending in the lithosphere.
This can be the case near mid-ocean ridges, where magma is upwelling, above hot spots in
volcanic regions or simply in in sedimentary basins where heat is redistributed by regional or
focused groundwater flow through faults and cleats (Clauser, 2013). In those regions of
convective influence, thermal anomalies of elevated heat flow can be found. Economic
production of geothermal energy is facilitated, but not restricted to regions of elevated heat
flow. Various types of geothermal energy extraction schemes are available, suitable for both
power production and direct thermal use, which can be implemented in high and low
temperature regimes, respectively (Clauser, 2013).
2. Sources of heat in the Alps
The previous chapters introduced how the heat is transported within the lithosphere. But
what are actually the sources of this heat within the Alps? One of them is the so called
original heat of the earth. During the accretion of the earth a great amount of potential
energy was released and stored in the form of heat inside of the earth. Since then the earth
is cooling with decreasing speed. Today’s contribution to the total global heat flow is of minor
relevance (Clauser, 2013). In regions of crustal extension and uplift of the Mohorovičić
discontinuity, transport of the mantle heat to the surface is elevated due to a higher
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temperature gradient. A potential source of heat which comes into mind when dealing with
the Alps, is heat through orogeny of this young mountain range. However, several studies
have shown that the thermal effects of alpine orogeny are negligible today because the
overthrusting was too slow. Models that assume a overthrusting for 10 Ma, starting 50 Ma
ago show that the heating effects vanished already after 20 Ma (Rybach, 1976). The most
important heat source in the earth is radioactive decay. Heat is released during the
radioactive decay of the isotopes 238U, 235U, 232Th and 40K. Those are especially
abundant in acidic intrusions. They are rarer in sediments and ultrabasic rocks (Rybach,
1976). Consequently the distribution of radioactive heat sources governs the temperature
field in the continental crust. Figure 2 shows heat production values of some characteristic
rock types in the Alps.
Figure 2: Average heat production figures of characteristic surface rocks in the Alps (HGU=heat generation units), Rybach 1976
3. Acquisition of heat flow data
Almost all geophysical data is thermally relevant because most parameters are temperature
dependent. However, most of these Geothermometers are not very precise (Werner, 1977).
Therefore the most straightforward method to determine the heat flow is to conduct nearsurface temperature measurements (in lakes, mines, tunnels or boreholes) and to
extrapolate in greater depth, considering the respective temperature gradient and thermal
conductivity (Werner, 1977). However, this might be problematic since the near-surface
geothermal field is not a sound footing for extrapolation into great depth, because it is easily
influence by disturbing factors such as deep reaching faults. Furthermore also long-term
motions in the underground have to be considered so that a convection term has to be
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included in the thermal conduction equation (Werner, 1976). For rough temperature
estimations, results from seismic and gravity measurements can be used to derive heat
production values from great depth. While the heat production in the continental crust
decreases exponentially with depth, the seismic velocity increases (fig. 3). With the help of
the empirical relationship between seismic velocity v and heat production A,
(
)
where H is the scale factor of decrease (varies from region to region), the heat production
can be estimated if the velocity is known from seismic measurements (Werner, 1976).
Figure 3: Seismic velocity v and heat production A plotted against depth, Rybach 1976
4. Heat flow in the Alps and the surroundings
First of all it has to be stated that geothermal data of the Alps is too scarce to get a complete
overview of heat flow in the whole region. A wide zone in the eastern Alps provides
practically no data for drawing the geothermal field pattern. Scattered data from the Upper
Rhine Graben and the Alps are present from boreholes, tunnels and lakes but it is difficult to
link them together to a consistent image. Nevertheless B.D. Vedova and his team tried to do
exactly this as part of their work about heat flow along the European Geotraverse (Vedova,
1991). Figure 4 shows the attempt of drawing isolines of equal heat flow values in the alpine
region. Although the image is not complete, some general conclusions can be drawn from
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this work. Heat flow in the Upper Rhine Graben is strongly elevated (>100 mW/m2) and
values in the Molasse Basin are slightly elevated (80 mW/m2), while heat flow in the Alps
varies widely (60-80 mW/m2) but is all in all normal. Those results will be discussed and
explained in the following.
Figure 4: Isolines of equal heat flow in the alpine region. Values are strongly
elevated in the Upper Rhine Graben, slightly elevated in the Molasse BAsin and
normal (but highly variable) in the central Alps, Vedova 1991
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A representative profile that was studied in more detail is the „Swiss Geotraverse”. It runs
more or less perpendicular to the general strike direction of the alpine units and crosses the
Upper Rhine Graben, the Jura Mountains, Molasse Basin, Helvetic Nappes, Central Massifs
with their autochtonous cover, Penninic units and the basement and sedimentary units of the
southern alps (Rybach, 1976)(Figure 5).
Figure 6 shows a profile of the traverse with the position in depth of the 500°C, 1000 °C and
1500°C isotherms as well as the position of the “Mohorovičić discontinuity”. The position of
the “Moho” along the profile is known from seismic measurements. It shows two prominent
deviations from its normal depth of approx. 30 km. In the Upper Rhine Graben it is bulged up
to 25 km depth and underneath the alps it dived down to 55 km depth. Since the density
changes at the crust-mantle boundary there should be a deviation visible in gravity
measurements in this region. According to the thickened package of light crustal material in
the Alps, a negative gravity anomaly is recorded. However, a positive gravity anomaly in the
Upper Rhine Graben could not be detected.
Figure 5: Route of the Swiss Geotraverse
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Figure 6: Profile of the Swiss Geotraverse showing the position of isotherms in depth
This can only be explained with a temperature anomaly in this region so that the temperature
dependence of the density becomes significant (Werner, 1977). Because the Upper Rhine
Graben is a zone of extensive crustal extension, temperatures are strongly elevated in the
Tertiary sediments and heat flow reaches values of 100 mW/m2 and more (Vedova, 1991).
This reduction of crustal thickness logically goes in hand with an upwards bulging of the
“Moho”/mantle, leading to a higher temperature gradient which governs the heat flow. In
addition, faults of this graben structure form a shattered zone which allows vertical water
movement. Like this convection can contribute to the local heat flow and create this
prominent thermal anomaly. However, high temperatures in the sediments alone are no clear
evidence for a deep-reaching heat anomaly underneath the graben system but there are
some arguments which promote that model (Werner, 1977). Generally the high position of
the base of the lithosphere indicates elevated temperatures in the deep subsurface. In
addition, the fact that the Moho-bulging cannot be traced in the gravity data can only be
explained by a heat anomaly, which can compensate for this gravity difference. Furthermore
magnetotelluric measurements show a zone of highly elevated electric conductivity under the
Upper Rhine Graben. This deep reaching anomaly can be geodynamically described as a
mobile zone in which a long-term continuous ascent of hot lithospheric material is going on.
The bulging of the “Moho” is a consequence of this upwelling material. Figure 6 provides a
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nice summarizing overview, showing the “Moho”-uplift and the 500°C and 1000 °C isotherms
in highly elevated positions underneath the Upper Rhine Graben (Werner, 1977).
Although average heat flow values in the Alps and in a “normal” range, the temperature
distribution in the active lithosphere of the Alps is not identical with the one in the static
“normal” Lithosphere and local heat flow values can vary widely. Those strong local heat flow
deviations are mostly caused by advection of hot groundwaters. In regions of high
topography those positive disturbances are very narrow and may reach up to 1.5-1.8 times
the basal conductive heat flow value. They concentrate in groundwater discharge areas
(valleys). On the other hand the heat flow may be quite reduced in recharge areas (ridges)
(Bodri, 1998). Nevertheless some general observations can be made regarding the
temperature distribution underneath the Alps. In the upper lithosphere/crust, temperatures
are elevated due to orogenic uplift and erosion of material with high heat generation rates
such as granites and gneisses. Intrusive rock bodies like the Bergell Granite, which was
observed in the course of the Alps excursion, were brought to the upper crust during folding
and thrusting of the Alps. In contrast to the crust, the lower lithosphere of the Alps is
relatively cool. This is also due to collision tectonics during the alpine orogeny. Originally
cool parts of lithosphere were subducted during the formation of the Alps, rest now in great
depth and cool the surroundings. Consequently the 500°C isotherm is also elevated
underneath the central Alps, while the 1000°C isotherm is considerably downshifted (Figure
6) (Werner, 1977).
The Molasse Basin roughly embodies temperature conditions of “normal” Lithosphere. It is
kind of a transition zone between Upper Rhine Graben and Alps with partly convection
related moderately positive heat anomalies of about 84 mW/m2 (Vedova, 1991). Conditions
for geothermal heat generation are excellent in the Molasse Basin. Reasons for that will be
given in chapter 5. Temperature-depth relations in the Alps and the Upper Rhine Graben are
summarized in Figure 7. N displays the temperature profile of “normal” Lithosphere. While in
the Upper Rhine Graben temperatures are elevated down to the depth of partial melting,
temperatures in the Alps are only elevated in the crust and cooler than in “normal”
lithosphere below 50 km depth (Werner, 1977).
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Figure 7: Temperature distribution model. A=Alps, R=Rhine Graben, N="normal" lithosphere, Werner 1977
5. Geothermal energy use in the Alps
Best conditions for geothermal energy production in the alpine region can be found in the
Molasse Basin. While geothermal projects in the central Alps are very rare because of the
heterogeneity and complexity of geological structures which do not provide large geothermal
fields which are easily detectable and accessible, the alpine foreland is hosting several large
geothermal heat generation sites and electricity power plants (fig.8). In the Molasse Basin
the heat is stored in a 1500 m to 3800 m depps limestone unit from the Malm (~150 Ma) of
up to 600m thickness. The rock is highly karstified so that flow of hot water through the
cleats is allowed. With temperatures of ~130-160 °C, the water is hot enough for electricity
production (Hüttl, 2010). The limestone layer dips towards the south under the central Alps
down to a depth of ~6000 m. Temperatures become substantially higher towards the south,
but mining these waters also gets increasingly complicated. Additionally the degree of
karstification decreases towards the south which leads to a decreasing permeability and
reservoir quality (Bundesministerium für Umwelt, 2011). Unsuccessful projects south of the
Bodensee have shown the decreasing geothermal potential when approaching the central
Alps. Beside the perfect reservoir qualities of the Karst, those structures can also entail
some risks. Karst structures are difficult to predict concerning their direction and hence the
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direction of fluid flow. Drainage areas, even if far away from each other, might draw away
water from each other and contaminated water can take unexpected flow paths (Hüttl, 2010).
In the following, the situation of geothermal energy use will be outlined for the three countries
bordering to the Molasse Basin: Germany, Switzerland and Austria.
Germany: Besides the Upper Rhine Valley and the North German Plain, the Alpine region,
especially the northern alpine Molasse Basin, is one of the most important geothermal
provinces in Germany. Especially in the area around Munich deep geothermal energy mining
is very prevalent (fig.8). At the moment 11 deep geothermal power plants (>400m) are
operating in the Bavarian Molasse, one of them for electricity production (Bundesverband
Geothermie). This is the powerplant in Unterhaching which generates produces the
geothermal waters from the 3500 m deep Malm limestone. After drilling the first borehole in
2004, long distance heating started in 2007, before in 2008 the electricity production was
initiated. In addition to the deep geothermal sites, hundreds of near surface geothermal
power plants (<400m) are installed in the Bavarian Molasse (Bundesverband Geothermie,
2013).
Switzerland: Figure 8 shows that the biggest geothermal potential in Switzerland can be
found in the very north of the country in the Molasse Basin and the Upper Rhine Graben.
This is where the most important geothermal projects of Switzerland are operating. Those
are the geothermal power plant Riehen, close to Basel, which is the oldest one in the
country, and the geothermal project St. Gallen. Until this year the project in St.Gallen was
most promising project of Switzerland because it was part of the energy revolution plan
“Energiekonzept 2050” (Informationsstelle Geothermie, 2013). Seismic and gravimetric
exploration from 2009 assumed 170°C hot geothermal waters in cleats of a 4100 m deep
Jura bed. In 2010 the inhabitants of St.Gallen voted in favor of the project in spring the 2013
the drilling began. On 20th July 2013 an Earthquake of magnitude 3.6 occurred, which was
triggered by drilling in 1km depth. The project was immediately stopped. To the date of
research for this paper no decision about the future of the project was made yet (FAZ, 2013).
Due to a similar happening in Basel in 2006, the project “Deep heat mining Basel” was
stopped after an earthquake of magnitude 3.4. Another popular way of geothermal heat
production in Switzerland is using the heat from Switzerland’s 700 road- and railway tunnels.
During the drainage of the overlying rocks through those tunnels the waters can reach
temperatures of up to 40°C, which is sufficient for heating purposes (Informationsstelle
Geothermie, 2013).
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Figure 8: Location of important geothermal powerplants in the Molasse Basin (modified after Alexander
Weltatlas 1996)
Austria: The energetic use of geothermal springs is not playing a mojor roll in the energy mix
of Austria so far. The mining for thermal waters began in the 1970s to use these for thermal
spas and bathing. Three-fourth of the 62 deep geothermal projects that were operated
between 1977 and 2004 were drilled for the purpose of thermal bathing. Deep geothermal
power plants are located mainly in the eastern Steiermark (e.g. electricity producing power
plant Bad Blumau) and in the “Innviertel” (e.g. electricity producing power plant Altheim)
(fig.8).
For the future years a strong increase of geothermal energy production is predicted for all
three countries. They all have energy revolution projects running where an extensive use of
geothermal energy is incorporated. Alone in the Molasse Basin 20 deep geothermal projects
are under construction or planned, 8 of them are electricity producing. The BEE
(Bundesverband erneuerbare Energieen) declared the aim that in 2020 3750 GWh electricity
will be produced per year from geothermal projects. The more conservative aim of the
german government is to produce 1654 GWh per year by 2020, which is still an incredible
increase from today’s annual production of 19 GWh.
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