THE RENEWABILITY OF GEOTHERMAL ENERGY
Valgardur Stefansson
Orkustofnun, Grensasvegur 9, Reykjavik – Iceland
Key Words:
renewability, geothermal energy,
sustainable exploitation,
ABSTRACT
International organisations have classified
geothermal energy as a renewable energy source,
but it is sometimes stated that this is not the case.
The meaning of the concepts “renewable energy
sources” and “sustainable energy production” is
discussed in this paper. Comparison is made with
the resources of hydropower and fish stocks in order
to clarify the meaning of the two concepts. It is
found, that the rate of energy recharge to
geothermal systems is the most critical aspect for
the classification of geothermal energy as a
renewable energy source. In the exploitation of
natural geothermal systems, the recharge of energy
takes place by advection of thermal water at the
same time scale as the production from the resource.
This justifies the classification of geothermal energy
as a renewable energy source. In the case of hot dry
rock, and eventually some of the hot water aquifers
in sedimentary basins, the energy recharge is only
by thermal conduction and, due to the slowness of
this process, hot dry rock and some sedimentary
reservoirs should be considered as finite energy
sources.
1. INTRODUCTION
International organisations have classified
geothermal energy as a renewable energy source.
This classification has been in use for a very long
time, but occasionally it is stated that thermal
depletion of geothermal reservoirs would require
such a long time for recovery, that geothermal
energy is not, strictly speaking, a renewable energy
source on the human time scale (Ledingham, 1998).
These conflicting messages might easily create
confusion in the energy debate, where it is of
importance that there is a common agreement on the
basic concepts of the energy resources.
The present paper deals with the renewability of
geothermal energy and links the discussion with the
concept of sustainable development. Common
properties of geothermal energy and hydropower are
used to demonstrate that both energy sources are
renewable. It is found that the transportation
process of heat within the crust determines whether
the geothermal energy should be considered
renewable, or not. All natural geothermal systems
are renewable on the human time scale, whereas hot
dry rock “reservoirs” can hardly be classified as
renewable on the same time scale.
2. CLASSIFICATION OF ENERGY
RESOURCES
Figure 1 shows the classification of energy
resources applied by the International Standards
Organization (ISO). The figure shows that the use
of renewable energy sources in the world is now
about 22% of the total, but depleting energy sources
contribute 78% of the world energy use.
The share in the energy mix is given in the figure
for each energy source. Furthermore, the expected
reserves (in years) at the 1996 exploitation rate are
given for the different depleting energy resources.
For the renewable energy sources, the reserves are
considered to provide continuous (unlimited)
contribution to the exploitation.
The share of the finite energy resources is at present
much larger than the contribution from the
renewable energy sources and the known reserves
of these sources are estimated to last for several
decades or centuries at the present exploitation rate.
However, the balance between renewable and
depleting energy sources is bound to shift towards
increased use of the renewable energy sources. In
this situation it is of importance to realise the
restrictions on the renewable energy sources and
analyse how the renewable energy sources can in
the best way contribute to a sustainable
development. At present, geothermal energy
contributes some 0.1% to the total use of energy in
the world. Therefore, there seem to be abundant
possibilities to increase the use of geothermal
energy and hopefully also to increase its share in the
energy mix.
3. DEFINITION OF CONCEPTS
Two concepts,
renewable
and
sustainable
are of
importance in this discussion. As there seems to
prevail some confusion about the meaning of these
concepts, it is appropriate to clarify the author’s
understanding of these concepts.
The concepts
renewable
and
sustainable
are not
comparative. Renewable describes a property of the
energy resource, whereas sustainable describes how
the resource is utilised. For an Icelander, the
comparison with the fish stocks is natural. The fish
stocks are certainly renewable resources but the
883
Proceedings World Geothermal Congress 2000
Kyushu - Tohoku, Japan, May 28 - June 10, 2000
Stefánsson
exploitation (fishing) can be done in different ways.
With a proper management of the fishing, a
continuous sustainable yield can be obtained, but
overexploitation can result in the collapse of the
stock and depletion of the resource.
Similar examples of energy resources are perhaps
not as obvious, but the experience from the Geysers
in the USA might be of similar nature. There,
power plants for electricity generation of some 2000
MW capacity were installed some decades ago.
Operation of these plants for some time revealed
that the geothermal system could not sustain this
production for a long time. At present, the
production is limited to some 1500 MW and
reservoir studies indicate that sustainable utilisation
(continuous utilisation at the same rate for a long
time) is probably not more than about 1000 MW. In
this case, the exploitation has not destroyed the
resource (as is frequently the case for biological
resources), but there are limits to the yield which
can be extracted in a sustainable way from the
resource.
From these examples it is evident that in order to
obtain sustainable exploitation of an energy
resource, the resource has to be renewable.
Sustainable operation is characterised by some kind
of equilibrium. For a long time operation, it is not
possible to extract more energy out of the system
than the amount of energy entering into the system.
Therefore, sustainable exploitation can only be
obtained from renewable energy resources.
Renewable energy sources are in one way or
another linked to some continuous energy processes
in nature. The conditions must be such, that the
action of extracting energy from the natural process
will not influence on the process or energy
circulation in nature. Construction of a power plant
in a river will not influence the rate of the
precipitation, which is the source of the flow of
water in the river.
A simplified description of renewability could be
that the energy extracted from a resource is always
replaced by additional amount of energy.
Furthermore, we require that the replacement takes
place on a similar time scale as that of the
extraction. It could be argued that oil and gas are
renewable energy sources on a geological time
scale. For the human time scale this time is so long
that there is a common agreement to classify oil and
gas as finite energy sources.
Sustainable exploitation of the fish stocks in the
ocean around Iceland is of a fundamental
importance for the Icelandic society. This requires
a proper management of the resource. It is therefore
quite natural for Icelanders to assume that
sustainable exploitation should be applied to energy
resources also.
4. RENEWABILITY OF GEOTHERMAL
ENERGY AND HYDROPOWER
In the geological environment of Iceland it is quite
obvious that the geothermal energy has two
components: The energy current from below and the
energy (heat) stored in the bedrock of the country.
A description and estimate of the energy current
was presented by Bodvarsson (1982), and the
estimate of the heat stored in the rocks (geothermal
assessment) was carried out by Palmason et al.
(1985).
At the surface the terrestrial energy current is
observed as:
♦
volcanic activity
♦
geothermal energy
♦
heat conduction
Bodvarsson (1982) estimated that the terrestrial
energy current through the crust of Iceland is some
30 GW. At the surface about 7 GW occur as
volcanism, 8 GW as advection of geothermal water,
and 15 GW as thermal conduction.
Comparison of geothermal energy with hydropower
is quite appropriate to show the similarities and the
dissimilarities of these two renewable energy
sources. Figures 2 and 3 show schematically the
properties of geothermal energy and hydropower in
Iceland (Stefansson and Eliasson, 1997).
The precipitation on the mountainous country
makes up the energy current for the hydropower in
Iceland (fig. 2). The mean power of the
precipitation has been estimated at 285 TWh/a
(Tomasson, 1982). The unit TWh/a is preferred to
GW in order to underline that the value given is a
mean value over one year. The precipitation varies
from day to day but the flow of water is regulated
considerably by the passage through soil and other
geological formations. The flow of rivers is much
smoother than the precipitation and dams
furthermore regulate seasonal variation in the flow
of rivers so that a constant energy production in
power plants can be obtained. For hydropower, the
time constant of one year is appropriate to describe
the renewability of the energy resource. If a short
time period (one day) during the dry season was
selected, hydro might have similar properties as a
finite energy resource. Figure 2 shows also how the
energy current is distributed in nature among
evaporation, glaciers, ground water and other
components. The bottom line is that the technically
exploitable part of the hydropower is about 64
TWh/a.
The energy current from the interior of the earth is
the primary source of geothermal energy in Iceland
as shown in fig. 3. The energy transport within the
crust takes place by three processes:
♦
advection of magma
♦
advection of geothermal fluid
♦
thermal conduction
Energy (heat) transport with the advection of
magma and thermal water is a relatively fast
process. Time constants in the range of days or
884
Stefánsson
months are suitable to describe these processes. On
the other hand, thermal conduction is a relatively
slow process where a time constant of the order of
hundreds of years is needed to characterise the
process. The utilisation of geothermal energy from
natural geothermal systems is primarily governed by
the advection of thermal fluid in the crust.
Therefore, one year is also an appropriate time
constant for geothermal energy.
Most of the thermal energy entering the crust
beneath Iceland is in the form of advection of
magma (fig. 3). On its way to the surface, there is a
continuous interaction between the three energy
transport processes. When the energy reaches the
surface, about half of the energy current occurs as
conduction, whereas volcanic activity and
geothermal energy are the main manifestations of
the other half. It should be pointed out, however,
that although the energy current occurs in three
forms at the surface, these manifestations are a part
of a single energy current through the crust. About
1/3 of the magma entering the crust from below
reaches the surface in the form of volcanic
eruptions. The other 2/3 parts of the magma end up
as intrusions in the crust. The heat (energy) in the
intrusions consequently migrates to surface, either
as thermal fluid or by conduction.
It is of interest to note that in Iceland, the energy
current from below is of the same size as the energy
current from above and that the technically usable
hydropower is, within the limits of error, equal to
the technically usable current of geothermal energy.
In addition to the energy current from above and
from below, energy is stored at certain places in the
two natural systems. Glaciers, lakes, and ground
water reservoirs are examples of stored hydropower,
whereas the heat stored in the bedrock is a huge
storage for the use of geothermal energy. In order
to underline the importance of the energy storage
for both hydro and geothermal energy it should be
noted that the energy storage in the glaciers (7600
TWh) corresponds to the current of the precipitation
for 27 years and that the usable heat in the bedrock
(1 000 000 TWh) corresponds to the energy current
from below for 3800 years. All heat (usable and
non-usable) in the crust corresponds to the current
from below for 100 000 years.
The energy storage in the glaciers of Iceland
contributes some 10% to the variation in the flow of
rivers in the country. During some periods, the
glaciers add more flow to the rivers than if there
were no glaciers present while in other periods the
flow is reduced because of accumulation of ice in
the glaciers.
The energy storage in the bedrock is a somewhat
more complex issue depending on the three energy
transport processes. The advection of water (and
magma) is such a fast energy transport process that
geothermal energy meets all requirements being a
renewable energy source, viz. energy is replaced on
the same time scale as for the energy extraction. If
the energy transport is only by thermal conduction
on the other hand, it is hardly possible to talk about
“renewable” energy sources because the time
constant of the energy replacement is much longer
than the time constant of the exploitation.
All conventional exploitation of geothermal energy
is based on energy extraction from natural
geothermal systems where water transports the
energy within and towards the systems and water
also transports the energy to the surface where the
utilisation takes place. Production causes a pressure
decline in the geothermal system, which results in
increased recharge of water and energy to the
system under exploitation. These conditions are
typical for renewable energy sources where the
replacement of energy takes place on similar time
scale as the extraction.
The exception from this rule is hot dry rock. In this
case the idea is to create an artificial geothermal
system in impermeable rocks by injecting water in
one well and extracting the heat stored in the rocks
through another well. This production method is
still at the experimental stage and continuous
production has only been possible for some months.
It is not known whether this production method will
be economically feasible within the near future, but
the energy transport towards the “heat exchanger”
of the hot dry rock system must be in the form of
heat conduction alone. Due to the long time
constant of the heat conduction process, the hot dry
rock method can not be classified as renewable
energy source. Hot dry rock is a finite energy
source, whereas natural geothermal systems are
renewable energy sources.
5. GEOTHERMAL ENERGY IN ICELAND
All production of geothermal energy in Iceland is
from natural geothermal systems and it should
therefore be classified as utilisation of a renewable
energy source according to the description above.
At present there are about 200 geothermal systems
(small and large) in use in the country (Stefánsson
and Fridleifsson, 1998). The longest continuous
exploitation time for a single system is 70 years for
the Laugarnes area, situated within the city of
Reykjavik. In none of these cases has the
production been discontinued because the source
was depleted. On the contrary, the experience is
such that the geothermal systems appear to be able
to sustain continuous production for such a long
time, that it is appropriate to talk about sustainable
exploitation. The production from the Laugarnes
field is a good example of these conditions.
For the first 25 years of exploitation in the
Laugarnes field, the production was only by free
flow from wells, but submersible pumps were
introduced in the late fifties. The new production
method made it possible to increase the production
ten times as shown in figure 4. The response of the
system was that the pressure (water level) fell but a
new equilibrium state was reached where the water
level was on the average 120 m below the initial
885
Stefánsson
level when the production started in the year 1930.
The increased production from the field has not
caused changes in the reservoir temperature. The
geothermal system in Laugarnes is approximately in
equilibrium for the 6 Gl/a (160 l/s) production,
which has been maintained there for the last 30
years. This means that the pressure decline has
caused increased natural recharge and the rate of
recharge is, on the average, the same as the rate of
production from the system. It is quite obvious that
the present production in Laugarnes is a sustainable
exploitation and that the energy resource is
renewable.
The next step in the discussion would be to
“explain” how the geothermal energy in the
Laugarnes field is renewed, or rather to put forward
a conceptional model for the geothermal system.
Two possibilities seems to be at hand for such a
model:
♦
The lower pressure diverts a larger
part of the geothermal fluid in the region through
the production area.
♦
The increased recharge of cool
water from the surroundings makes it possible to
extract heat from a larger volume of rock than
before.
As seen in fig. 3, the energy current from below
reaches the surface in three components (volcanism,
hot water, and conduction). If increased energy
extraction at certain locations of the system makes
the energy current to go preferably through that
channel, we have obtained an indirect management
of the terrestrial energy current. For example, an
increased production of geothermal energy from a
geothermal system close to a magma chamber,
might result in more efficient cooling of the magma
chamber and that less energy needs to go through
the magma channel to surface. This effect would
lower frequency of volcanic eruptions in the same
area.
In the Krafla field, the time between the two last
volcanic eruptions was 250 years. The volume of
magma erupted during 1975-1984 was 0.35 km
3
.
The heat transported to surface during this volcanic
event was about 2*10
18
J. If this energy were
distributed evenly over the 250 years between
volcanic eruptions, it would correspond to a
continuous 250 MW heat extraction (production)
from the system. Therefore, it might not be
unrealistic that geothermal production can influence
(reduce) the frequency of volcanic eruptions.
6. GEOTHERMAL ENERGY IN THE WORLD
The discussion in this paper is based on the
conditions of geothermal energy in Iceland. The
question is then whether this discussion can be
extended to other parts of the world. It seems quite
imperative that high temperature fields all over the
world are so similar in nature that all such systems
can be classified as renewable energy sources. For
the low temperature fields on the other hand, it is
questionable whether the hot water aquifers in
sedimentary basins are renewable. One opinion is
that the geothermal gradient is responsible for the
existence of these hot water systems, whereas an
other opinion is that these aquifers have
hydrological connections over large areas, such as
other natural geothermal systems. If the energy
transport towards these aquifers is only through
conduction (geothermal gradient) these systems
should be classified as finite sources like hot dry
rock, but if there is a natural hydrological energy
recharge to the sedimentary aquifers, the resource is
renewable.
7. SUMMARY AND CONCLUSIONS
The renewability of geothermal resources is
discussed in this paper. Comparison is made with
hydropower and with the exploitation of fish stocks.
It is argued that the natural recharge of energy to
most natural geothermal systems takes place on a
similar time scale as the exploitation of these
resources. Therefore, it is concluded as a general
rule, that geothermal energy is truly a renewable
energy source.
The exceptions from this general rule are the hot dry
rock concept and the confined hot water aquifers in
sedimentary basins. Some of the sedimentary
systems might be renewable and other finite.
ACKNOWLEDGEMENTS
The author thanks Gudni Axelsson, Sveinbjörn
Björnsson, Ingvar B. Fridleifsson, Knutur Arnason,
and an anonymous reviewer for reviewing the
manuscript and suggesting several improvements.
REFERENCES
Böðvarsson, Gunnar,1982: Terrestrial energy
currents and transfer in Iceland. In
Continental and
oceanic rifts
, ed. G.Pálmason, Geodynamic Series
Vol. 8, pp.271-282, American Geophysical Union,
Washington D.C.
Grob, Gustav R., 1998: Energy sustainability and
standards, In:,
The world directory of renewable
energy; Suppliers and services 1998
, B. Cross
editor ,pp. 47-49, James and Lames, London, 1998.
Ledingham, Peter, 1998: Geothermal energy, In:
The world directory of renewable energy; Suppliers
and services 1998
, B. Cross editor, pp. 108-110,
James and Lames, London, 1998.
Pálmason, Guðmundur, Gunnar V. Johnsen, Helgi
Torfason, Kristján Sæmundsson, Karl Ragnars,
Guðmundur Ingi Haraldsson og Gísli Karel
Halldórsson, 1985:
Mat á jarðvarma Íslands.
Orkustofnun Report OS-85076/JHD-10, 134 pages.
886
Stefánsson
Stefánsson, Valgarður and Elías B. Elíasson ,1997:
Samnýting orkulinda.
Orkustofnun Report OS-
98005, 12 pages.
Stefánsson, Valgarður and Ingvar B. Friðleifsson
1998:
Geothermal energy. European and
worldwide perspective.
Paper presented at Expert
hearing on “Assessments and Prospects for
Geothermal Energy in Europe” in the framework of
Sub-Committee on Technology Policy and Energy
of the Parlamentary Assembly of the Council of
Europe, 12 May 1998, Salle 10, Palais de l’Europe,
Strasbourg.
Tómasson, Haukur,1982:
Vattenkraft i Island och
dess hydrologiska förutsättningar.
Orkustofnun
Report OS-82059/VOD, 17 pages.
Figure 1. ISO’s classification of the energy resources (from Grob, 1998).
Figure 2. The energy current of hydropower in Iceland
Nuclear
power
260 years
4 %
NUCLEAR
ENERGY
4 %
Coal
220 years
25 %
Petroleum
derivates
40 years
32 %
Natural
gas
60 years
17 %
FOSSIL
ENERGY
74 %
FUSION
ENERGY
0 %
DEPLETING ENERGY
78 %
Clean
gases
Continous
0.01 %
Solar
power
Continous
0.01 %
Solar
heat
Continous
1 %
SOLAR
DIRECT
1 %
Hydro, waves
& tidal
Continous
6 %
Bio
energy
Continous
14 %
Wind
energy
Continous
0.1 %
SOLAR
INDIRECT
21 %
Geo &
ocean
Continous
0.1 %
Heat
pumps
Continous
0.1 %
GEO, OCEAN
& AMBIENT
0.2 %
RENEWABLE ENERGY
22 %
PRIME ENERGY
887
Stefánsson
Figure 3. The terrestrial energy current in Iceland.
Figure 4. Production and water level in the Laugarnes field in Reykjavík
30 35 40 45 50 55 60 65 70 75 80 85 90
250
200
150
100
50
0
-50
-100
Water level
Production
Production
Production
Yearly production [ Gl ]
Submersible pumps
Free flow
Water level [ m. u. s. ]
0
2
4
6
8
10
12
14
16
18
20
Year
Laugarnes
888
