Geothermics 34 (2005) 286–296
CO2 emissions from geothermal power plants and
natural geothermal activity in Iceland
Halldór Ármannsson ∗, Thráinn Fridriksson, Bjarni Reyr Kristjánsson
ISOR, Iceland GeoSurvey, Grensásvegur 9, IS-108 Reykjavı́k, Iceland
Received 17 May 2004; accepted 16 November 2004
Available online 5 February 2005
Abstract
The ratio of CO2 emissions from power plants to natural emissions is a measure of the environmental
impact associated with geothermal power production. Emissions from Icelandic geothermal power
plants amounted to 1.6 × 108 kg year−1 in 2002. Two independent estimates of natural CO2 emissions
range between 1 × 108 and 2 × 109 kg year−1 . Thus, power plant emissions are significant compared to
estimated total emissions (i.e., not less than 8–16%). However, direct CO2 flux measurements from four
of the approximately 40 geothermal/volcanic systems in the country amounted to 3 × 108 kg year−1 ,
indicating that these estimates of the total natural flux may be too low.
© 2004 CNR. Published by Elsevier Ltd. All rights reserved.
Keywords: Carbon dioxide; Geothermal power plants; Natural geothermal activity; Volcanic systems; Greenhouse
gases; Iceland
1. Introduction
Geothermal energy is generally considered a relatively benign energy source as regards environmental impact. Release of the greenhouse gas CO2 to the atmosphere is commonly considered to be one of the negative environmental effects of geothermal power
production, even though it has been shown to be considerably less than from fossil fuel
power plants (Fig. 1). Recent studies of CO2 emissions from geothermal/volcanic systems
∗
Corresponding author. Tel.: +354 528 1534; fax: +354 528 1699.
E-mail address: [email protected] (H. Ármannsson).
0375-6505/$30.00 © 2004 CNR. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.geothermics.2004.11.005
H. Ármannsson et al. / Geothermics 34 (2005) 286–296
287
Fig. 1. CO2 emissions from various types of power plants (based on Hunt, 2000).
have demonstrated that vast quantities of CO2 are released naturally and that, in many
cases, natural emissions far exceed emissions from geothermal power production (e.g.,
Seaward and Kerrick, 1996; Delgado et al., 1998; Bertani and Thain, 2002). Consequently,
doubts have recently been raised as to the validity of considering CO2 emissions as a
negative environmental impact of geothermal power production in systems in which anthropogenic emissions are negligible in comparison to natural emissions (Bertani and Thain,
2002).
In this paper we evaluate the relative magnitudes of CO2 emissions from geothermal
power production and natural CO2 emissions from geothermal systems in Iceland. We
present the emission data for Icelandic power plants and review the data available on natural
CO2 emissions from geothermal/volcanic systems in this country. We also present previous
estimates of the total CO2 emissions from all Icelandic geothermal systems; finally, we use
geological observations to constrain the upper limit for long-term average CO2 output from
Icelandic geothermal systems.
2. Background
Bertani and Thain (2002) described the results of a survey of CO2 emissions from
geothermal power plants with the purpose of demonstrating the environmental advantage
of using geothermal energy to mitigate rising atmospheric CO2 levels. The results are
presented in Table 1 in terms of emitted CO2 per electric energy output (g kWh−1 ). The
emissions from geothermal plants range between 4 and 740 g kWh−1 , with a weighted
average of 122 g kWh−1 . The authors suggested that the natural pre-development emission
rate be subtracted from that released by the geothermal operation, citing the Larderello
(Italy) geothermal project as an example of a recorded decrease in the natural release of
CO2 ; they suggest that this decrease is a result of field development.
Geothermal systems are often located in volcanic terrains or other areas characterized by
high CO2 fluxes of magmatic origin or derived from metamorphism of carbonate rocks at
depth. Large CO2 fluxes through shallow-depth layers are often observed; some of the
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Table 1
CO2 emissions and total running capacity of geothermal power plants divided into nine emission categories
(Bertani and Thain, 2002)
Emission category (g CO2 kWh−1 )a
Running capacity (MWe )b
Weighted average (g CO2 kWh−1 )
>500
400–499
300–399
250–299
200–249
150–199
100–149
50–99
<50
197
81
207
782
346
176
658
1867
2334
603
419
330
283
216
159
121
71
24
a
Refers to Bertani and Thain’s (2002) division of geothermal plants into nine categories according to the range
of CO2 emissions per energy unit.
b The total installed capacity of plants emitting CO per energy unit in the range shown.
2
gas dissolves into groundwater, where present, reaching saturation if the flux is sufficiently large. The estimated output from several volcanic and geothermal areas is shown in
Table 2.
The most thorough investigation of the amount of CO2 emitted through diverse conduits
was conducted at Pantelleria Island, Italy, by Favara et al. (2001). Estimates of the CO2
emitted through groundwater and soil have also been made for Mammoth Mountain, USA
(Sorey et al., 1998; Gerlach et al., 2001; Evans et al., 2002) and for Furnas, Azores (Cruz et
al., 1999). The results, shown as percentages, for these three areas are reported in Table 3.
The most striking feature of these results is the large proportion emitted through soil.
Table 2
CO2 output from some volcanic and geothermal areas
Area
Megaton (109 kg per year)
Reference
Pantelleria Island, Italy
Vulcano, Italy
Solfatara, Italy
Ustica Island, Italy
Popocatepetl, Mexico
Yellowstone
Mammoth Mountain, USA
0.39
0.13
0.048
0.26
14.5–36.5
10–22a
0.055–0.2
White Island, New Zealand
Mt. Erebus, Antarctica
Taupo Volcanic Zone, New Zealand
Furnas, Azores, Portugal
Mid-Ocean Volcanic System
0.95
0.66
0.44
0.01
30–1000
Favara et al. (2001)
Baubron et al. (1991)
Chiodini et al. (1998)
Etiope et al. (1999)
Delgado et al. (1998)
Werner and Brantley (2003)
Sorey et al. (1998), Evans et al. (2002),
Gerlach et al. (2001)
Wardell and Kyle (1998)
Wardell and Kyle (1998)
Seaward and Kerrick (1996)
Cruz et al. (1999)
Gerlach (1991), Marty and Tolstikhin (1998)
Total
200–1000
a
Diffuse degassing only.
Mörner and Etiope (2002), Kerrick (2001),
Delgado et al. (1998), Marty and Tolstikhin
(1998)
H. Ármannsson et al. / Geothermics 34 (2005) 286–296
289
Table 3
Relative CO2 emissions through different conduits from three volcanic areas (Sorey et al., 1998; Cruz et al., 1999;
Favara et al., 2001; Gerlach et al., 2001; Evans et al., 2002)
Soil (%)
Focussed degassing (%)
Fumarole (%)
Bubbles (%)
Groundwater (%)
a
Pantelleria Island
Furnas volcano
Mammoth Mountain
81
7
0.0004
3
9
49a
63–90a
51
10–37
Total flow directly to atmosphere.
3. CO2 emissions from geothermal plants in Iceland
Three major geothermal power plants are in operation in Iceland, at Krafla, Svartsengi, and Nesjavellir. The Svartsengi and Nesjavellir plants produce both electricity and
hot water for space-heating, whereas the Krafla plant generates electricity only. The total installed capacity of these three power plants is 195 MW and they produce about
17% of the total electricity used in the country. The CO2 emissions from Icelandic
geothermal plants have been recorded since the early 1980s when it was 48,000 tons
per year; by 2002 it was 159,000 tons. In the early years power production was very
low but the relatively high CO2 emission at that time was a result of a gas pulse in
Krafla associated with the volcanic events known as the “Krafla fires” (Ármannsson et al.,
1982).
In 1984, a well in Svartsengi started producing dry steam. It contained orders of magnitude more CO2 than steam produced previously from that and other liquid-dominated
geothermal systems in the region. A steam cap had formed in the north-eastern part of the
Svartsengi field, and in 1993 another well was drilled specifically to produce from the steam
cap. Two more wells were drilled in 1999 and 2001 to produce from this cap. Fig. 2 shows the
annual emissions of CO2 from Svartsengi during the period 1977–2002 (Verkfrædistofan
Vatnaskil et al., 2003). The sharp increases in the CO2 output in 1984, 1993, 1999, and
2001 illustrate the effects of the steam-cap wells on the total emission from the system.
Other fluctuations in CO2 emission are a result of variations in the production regime (such
as cycling, plant outages, etc.) from the area. Natural steam vents have formed in the field
above the steam cap and direct CO2 discharge to the atmosphere has apparently increased.
Since the formation of the steam cap the concentration of CO2 in the steam produced from
Table 4
CO2 and sulfur (expressed as SO2 ) emissions per kWh from Iceland’s major geothermal power plants in 2000
(Ármannsson, 2002)
Plant
From electricity generation only
CO2
Krafla
Svartsengi
Nesjavellir
152
181
26
(g kWh−1 )
S as SO2
23
5
21
From electricity and heat production
(g kWh−1 )
CO2 (g kWh−1 )
S as SO2 (g kWh−1 )
152
74
10
23
2
8
290
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Fig. 2. CO2 emissions from the Svartsengi geothermal plant, 1977–2002 (Verkfrædistofan Vatnaskil et al., 2003).
the steam cap has decreased gradually and is now about half what it was in 1984. Continued production from the steam cap will result in a further decrease in the concentration
of CO2 in the steam, eventually levelling out as the CO2 in the liquid resource reaches
equilibrium.
Table 4 reports the CO2 and sulfur (expressed as SO2 ) emissions from Krafla, Svartsengi
and Nesjavellir in grams per kWh of electric energy, and in grams per kWh of total energy
production (the sum of electric energy and thermal energy used in direct applications).
The CO2 emissions from Krafla and Svartsengi are slightly above the world average when
only electricity generation is considered, but a very small amount of CO2 is emitted from
Nesjavellir. The value for Svartsengi is much improved when space heating is accounted
for, illustrating the mitigating effect on the environment of cascading the use of geothermal
fluids.
4. Natural CO2 emissions from Icelandic geothermal and volcanic systems
Geothermal systems can be considered as geochemical reservoirs of CO2 . Carbon isotope ratios of CO2 in Icelandic geothermal fluids indicate that degassing of mantle-derived
magma is the sole source of CO2 in these systems (Ármannsson, 1998; Fig. 3); elsewhere
metamorphic decarbonation of marine limestone and decomposition of organic sediments
are also important sources of CO2 in some geothermal systems. CO2 sinks in geothermal
systems include calcite precipitation, CO2 discharge to the atmosphere and release of CO2
to enveloping groundwater systems. In Sections 4.1 and 4.2 we review the available direct
measurements of CO2 emissions from some geothermal and volcanic systems in Iceland,
and different attempts to estimate the total geothermal CO2 discharge to the atmosphere.
H. Ármannsson et al. / Geothermics 34 (2005) 286–296
291
Fig. 3. Carbon-13 in CH4 and CO2 from some geothermal gas emissions from Icelandic systems (Ármannsson,
1998).
In Section 4.3 we use geological observations to place constraints on the total CO2 output
from Icelandic geothermal systems.
4.1. Direct measurements
Direct measurements of total CO2 discharge are available from only 2 of the
35–40 volcanic/geothermal systems in Iceland. Ágústsdóttir and Brantley (1994) estimated
the long-term average CO2 flux from the Grı́msvötn central volcano and Gı́slason (2000)
presented measurements of CO2 release rates from the Eyjafjallajökull caldera during the
1993–2000 period. In both cases the CO2 release occurs in subglacial calderas where the
CO2 is dissolved in glacial meltwater and the release can be determined by analyzing the
total carbonate content and flow-rate of the meltwater. Grı́msvötn is one of the most active volcanic systems in Iceland with at least five known eruptions in the 20th century
(Sigmarsson et al., 2000). Ágústsdóttir and Brantley (1994) found that the average flux of
CO2 from this system over the previous 39 years was 1.9 × 108 kg year−1 . The observed
CO2 flux from the much less active Eyjafjallajökull (two known eruptions in the last 1100
years; Sturkell et al., 2003) is one to two orders of magnitude smaller than from Grı́msvötn,
or between 2.6 × 106 and 2.6 × 107 kg year−1 (Gı́slason, 2000).
In addition to these studies, partial measurements of natural CO2 discharge have been
reported for two other systems. The CO2 released from the Hekla magma chamber into the
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overlying groundwater system has been estimated to be of the order of 7 × 107 kg year−1
(Gı́slason et al., 1992), and preliminary results reported by Kristjánsson et al. (2004)
indicate that the CO2 flux through soil in the Reykjanes geothermal area is equal to
2.7 × 107 kg year−1 . It is not appropriate to infer the total CO2 flux from the 35 to 40
Icelandic geothermal/volcanic systems on the basis of these four studies only, but it is
worth noting that the sum of the total measured and/or estimated natural CO2 flux from
these four systems is 3 × 108 kg year−1 .
4.2. Earlier estimates of total emissions from Icelandic geothermal systems
Ármannsson (1991) estimated that the total CO2 emission via steam vents in Icelandic
geothermal systems was 1.5 × 108 kg year−1 . This value was obtained by extrapolating
observed steam emissions per unit area of active geothermal manifestations in Krafla to
other geothermal areas in the country, and using observed CO2 concentrations in steam
from individual systems. The steam-vent emissions at Krafla were determined by visual
comparison of steam plumes from vents with steam plumes from drillholes with known
discharge.
Arnórsson (1991), and later Arnórsson and Gı́slason (1994), used the estimated heat flux
from Icelandic geothermal areas (Pálmason et al., 1985) to reach an estimate of the total CO2
emitted from these systems: between 1.0 × 109 and 2.1 × 109 kg year−1 . Their estimate is
based on the assumption that convective flow of steam is the dominant heat transport process
in these areas. This assumption, and the observed or estimated heat flux from a given area,
allows us to evaluate the amount of steam released at the surface of a geothermal system. The
estimated steam flux can in turn be combined with the observed local average concentration
of CO2 in the steam to obtain the total CO2 flux for a given area. This methodology was
later used by Seaward and Kerrick (1996) to estimate hydrothermal CO2 emission from the
Taupo Volcanic Zone, New Zealand. Óskarsson (1996) combined the results of analyses
of fluid inclusions and tectonic modeling to obtain a total CO2 flux of 2.2 × 109 kg year−1
for Iceland. The large discrepancy between the estimate of Ármannsson (1991) on the one
hand and the estimates of Arnórsson (1991), Arnórsson and Gı́slason (1994) and Óskarsson
(1996) on the other, can possibly be explained by the fact that the latter authors include
diffuse emissions through soil and by bubbling through surface water bodies, as well as
steam-vent emissions. Ármannsson (1991), on the other hand, considered emissions from
steam vents only in his estimate.
4.3. Geologic constraints
The CO2 in Icelandic geothermal systems is, as noted above, solely derived from basaltic
magma originating from the mantle. Consequently, it is possible to use the observed rate of
magma emplacement in the Icelandic crust and estimated CO2 concentration in the magma
to place constraints on the maximum CO2 input into the geothermal systems. Basaltic
magma is intruded into the Icelandic crust along the ∼500 km long volcanic zone where
spreading occurs at a rate of 2 cm year−1 (Björnsson, 1985). The thickness of the crust
in the rift zone is of the order of 20 km (Bjarnason et al., 1993); assuming a density of
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293
2900 kg m−3 for the magma, the annual amount of magma that is emplaced in the crust is
therefore equal to 5.8 × 1011 kg year−1 .
Estimating the amount of CO2 released from the emplaced magma is less straightforward than estimating the amount of magma itself. In this study we use reported observations
from Ágústsdóttir and Brantley (1994) to constrain the concentration of “degassable” CO2 in
Icelandic magma. These authors determined the geothermal heat flux from the Grı́msvötn
system as 4250 MWt . The heat flux allows evaluation of the annual amount of basaltic
magma required to account for the observed heat flux by crystallization and cooling from
1300 to 200 ◦ C (∼8.5 × 1010 kg year−1 ; see Ágústsdóttir and Brantley, 1994). Combining
the observed CO2 flux from Grı́msvötn (1.9 × 108 kg year−1 ) with the inferrred rate of
magma emplacement into the Grı́msvötn system results in an estimate of the concentration
of “degassable” CO2 in the Grı́msvötn-system magma of about 2200 ppm. By “degassable”
CO2 concentration we mean the difference between the initial CO2 concentration of the
magma and the final concentration, after degassing. The resulting estimate of the CO2 flux
from basaltic magma emplaced in the Icelandic crust is equal to 1.3 × 109 kg year−1 . Note
that this value represents an estimate of the input into the Icelandic geothermal systems. As
noted above, this CO2 input is in the long term equal to the output. The CO2 that comes
from depth may be captured by precipitatation of carbonates (mainly calcite) dissolved in
groundwater and emitted into the atmosphere. Thus, the 1.3 × 109 kg year−1 value should be
interpreted as the maximum possible CO2 flux from Icelandic geothermal and volcanic systems to the atmosphere. This estimated value for maximum CO2 emissions from Iceland’s
30–40 active systems seems low, considering that the observed total flux from Grı́msvötn
and Eyjafjallajökull and partial discharge from Hekla and Reykjanes amount to 25% of this
value. Direct measurements are, however, too sparse to determine whether this apparent
discrepancy is a result of anomalously high CO2 fluxes from the four systems where measurements have been conducted, or our estimate for maximum CO2 emission from Icelandic
geothermal and volcanic systems is too low.
5. Discussion
Worldwide geothermal power (i.e., electricity generation and direct applications) is a
small sector of the energy industry, and CO2 emissions related to the utilization of geothermal resources are consequently negligible. In some countries, however, geothermal energy
production contributes significantly to their energy budget, so that any discussion of the
importance of CO2 emissions from geothermal power plants is extremely relevant. In one
such country, Italy, the Larderello geothermal field has been generating electricity for more
than 100 years, with the result that it has been possible to set up a database on gas emissions
covering a long period of time (Bertani and Thain, 2002). The Larderello study concluded
that all gas discharge resulting from power production is balanced by a reduction in natural
emissions, and that the resultant net change is insignificant. Furthermore, it seems that, as
a rule, geothermal power plant CO2 emissions are small compared to natural ones. On the
basis of these results, Italy decided not to consider CO2 emission from geothermal plants as
anthropogenic and does not include it in their inventory of anthropogenic airborne material
(R. Bertani, personal communication, 2003). Iceland has now followed suit temporarily in
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relation to the international conventions it is party to, that is, the Framework Convention
on Climatic Change (FCC) and the Convention on Long-Range Transboundary Air Pollution (CLRTAP) (Hallsdóttir, 2001). Using heat-flow measurements as a proxy for gas-flow
measurement, Sheppard and Mroczek (2004), on the other hand, suggest that exploitation
of the Wairakei geothermal system, New Zealand, has resulted in significantly increased
diffuse CO2 discharge from the field, i.e., of the same order as from the power station
itself.
Comparison of the observed CO2 emissions from Icelandic geothermal power plants
with the estimated natural emissions from geothermal systems is of critical importance
for evaluating the environmental effect of geothermal exploitation in Iceland. As noted
above, the total CO2 emission from the three major geothermal power plants in the country
was 1.6 × 108 g in 2002, which is essentially equal to the natural CO2 discharge from
Grı́msvötn, the most active volcano in Iceland. The emissions from power plants correspond
to more than 8–16% of the maximum annual total emissions estimated on the basis of
geologic constraints, as described above. This causes concern about the validity of the
argument that CO2 emissions from geothermal power plants in Iceland are negligible in
comparison to natural emissions, although this argument may still apply to the situation in
Italy.
Acknowledgements
Funding for this study was provided by Hitaveita Sudurnesja, Landsvirkjun, Icelandic
Research Center (Rannı́s), the National Energy Authority, and Iceland GeoSurvey.
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