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Environmental
Impacts
Geothermal power plants do have some environmental
impacts. However, these impacts should be
balanced against geothermal energy’s
advantages over conventional power sources
when conducting assessments of power plant
project environmental impacts. The primary
impacts of geothermal plant construction and
energy production are gaseous emissions, land
use, noise, and potential ground subsidence.
Gaseous
Emissions
Geothermal fluids contain dissolved gases,
mainly carbon dioxide (CO2) and
hydrogen sulfide (H2S), small amounts
of ammonia, hydrogen, nitrogen, methane and
radon, and minor quantities of volatile species
of boron, arsenic, and mercury. Geothermal
power provides significant environmental advantage
over fossil fuel power sources in terms of
air emissions because geothermal energy production
releases no nitrogen oxides (NOx),
no sulfur dioxide (SO2), and much
less carbon CO2 than fossil-fueled
power. The reduction in nitrogen and sulfur
emissions reduces local and regional impacts
of acid rain, and reduction in carbon-dioxide
emissions reduce contributions to potential
global climate change. Geothermal power plant
CO2 emissions can vary from plant
to plant depending on both the characteristics
of the reservoir fluid and the type of power
generation plant. Binary plants have no CO2
emissions, while dry steam and flash steam
plants have CO2 emissions on the
order of 0.2 lb/kWh, less than one tenth of
the CO2 emissions of coal-fired
generation (see Table 9). According to the
Geothermal Energy Association, improved and
increased injection to sustain geothermal
reservoirs has helped reduce CO2
emissions from geothermal power plants.
Table 9. Comparison
of CO2 Emissions by Power Source44
Power
Source |
CO2
Emissions (lb/kWh) |
Geothermal |
0.20 |
Natural gas |
1.321 |
Oil |
1.969 |
Coal |
2.095 |
Hydrogen sulfide emissions do not contribute
to acid rain or global climate change but
does create a sulfur smell that some people
find objectionable. The range of H2S
emissions from geothermal plants is 0.03–6.4
g/kWh.45 Hydrogen sulfide emissions
can vary significantly from field to field,
depending on the amount of hydrogen sulfide
contained in the geothermal fluid and the
type of plant used to exploit the reservoir.
The removal of H2S from geothermal steam
is mandatory in the United States. The most
common process is the Stretford process,
which produces pure sulfur and is capable
of reducing H2S emissions by
more than 90%.46 More recently
developed techniques include burning the
hydrogen sulfide to produce sulfur dioxide,
which can be dissolved, converted to sulfuric
acid and sold to provide income.
Landscape
Impacts and Land Use
Geothermal power plants require
relatively little land. Geothermal installations
don’t require damming of rivers or harvesting
of forests, and there are no mineshafts,
tunnels, open pits, waste heaps or oil spills.
An entire geothermal field uses only1–8
acres per MW versus 5–10 acres per MW for
nuclear plants and 19 acres per MW for coal
plants.47
Table 10 compares acreage
requirements by technology. Geothermal power
plants are clean because they neither burn
fossil fuels nor produce nuclear waste.
Geothermal plants can be sited in farmland
and forests and can share land with cattle
and local wildlife. For example, the Hell’s
Gate National Park in Kenya was established
around an existing 45-MWe geothermal power
station, Olkaria I. Land uses in the park
include livestock grazing, growing of foodstuffs
and flowers, and conservation of wildlife
and birds within the Park. After extensive
environmental impact analysis, a second
geothermal plant, Olkaria II, was approved
for installation in the park in 1994, and
an additional power station is under consideration.48
Table 10. Comparison of Land Requirement
for Baseload
Power Genreation
| Power Source |
Land Requirement (Acre/MW) |
| Geothermal |
1–8 |
| Nuclear |
5–10 |
| Coal |
19 |
Geothermal plants are also benign with respect
to water pollution. Production and injection
wells are lined with steel casing and cement
to isolate fluids from the environment.
Spent thermal waters are injected back into
the reservoirs from which the fluids were
derived. This practice neatly solves the
water-disposal problem while helping to
bolster reservoir pressure and prolong the
resource’s productive existence.49
Noise
Noise occurs during exploration drilling
and construction phases. Table 11 (next
page) shows noise levels from these operations
can range from 45 to 120 decibels (dBa).
For comparison, noise levels in quiet suburban
residences are on the order of 50 dBa, noise
levels in noisy urban environments are typically
80–90 dBa, and the threshold of pain is
120 dBa at 2,000–4,000 Hz.50 Site workers
can be protected by wearing ear mufflers.
With best practices, noise levels can be
kept to below 65 dBa, and construction noise
should be practically indistinguishable
from other background noises at distances
of one kilometer.
Table 11. Geothermal
Exploration and construction Noise Levels
by Operation51
Operation |
Noise
Level (dBa) |
Air drilling |
85–120 |
Mud drilling |
80 |
Discharging wells
after drilling (to remove drilling
debris) |
Up to 120 |
| Well testing |
70–110 |
| Diesel engines (to operate compressors
and provide electricity) |
45–55 |
| Heavy machinery (e.g., for earth moving
during construction) |
Up to 90 |
Ground
Subsidence
In the early stages of a geothermal development,
geothermal fluids are withdrawn from a reservoir
at a rate greater than the natural inflow
into the reservoir. This net outflow causes
rock formations at the site to compact, particularly
in the case of clays and sediments, leading
to ground subsidence at the surface. Key factors
causing subsidence include:
-
A pressure drop in the reservoir as a result
of fluid withdrawal
- The presence of a highly
compressible geological rock formation above
or in the upper part of a shallow reservoir
- The presence of high-permeability paths
between the reservoir and the formation,
and between the reservoir and the ground
surface
If all of these conditions are present,
ground subsidence is likely to occur. In
general, subsidence is greater in liquid-dominated
fields because of the geological characteristics
typically associated with each type of field.
Ground subsidence can affect the stability
of pipelines, drains, and well casings.
It can also cause the formation of ponds
and cracks in the ground and, if the site
is close to a populated area, it can lead
to instability of buildings.
The largest recorded subsidence in a geothermal
field was at Wairakei in New Zealand. Here
the ground subsided as much as 13 meters.
Monitoring has shown that a maximum subsidence
rate of 45 cm/year occurred in a small region,
outside the production area, with subsidence
of at least 2 cm/year occurring all over
the production field.52 Effects of the subsidence
in the Wairakei region included:
- The creation of a pond about 1 km in length
and 6 m in depth in what was originally
a fast-flowing stream,
- Cracking of both a nearby highway and
the main waste water drain on the site,
- Compressive buckling and tensile fracturing
of steam pipelines, and
- Fissures in surroundings fields.
Although Wairakei presents an extreme example,
little is currently known about how to prevent
or mitigate subsidence effects. The only action
is to try to maintain pressure in the reservoir.53
Fluid re-injection can help to reduce pressure
drop and hence subsidence, but its effectiveness
depends on where the fluid is re-injected and
the permeability conditions in the field. Typically,
re-injection is done at some distance from the
production well to avoid the cooler rejected
waste fluid from lowering the temperature of
the production fluid and may not help prevent
subsidence.54 Endnotes
44. Bloomfield, K., Moore, J.N., and R.M.
Neilson Jr. (2003). Geothermal Energy
Reduces Greenhouse Gases. Geothermal Research
Council. GRC Bulletin, April
2003. Note: the value of 0.2 lbs CO2/kWh
is the weighted average of U.S. geothermal
generation. Nonemitting binary plants account
for 14% of this generation. All emissions
values based on 2002 EIA data.
45. International Energy Agency http://www.iea.org/pubs/studies/files/benign/pubs/append3g.pdf;
accessed Oct 30, 2002.
46. DiPippo, R, (1991). Geothermal Energy:
Electricity Generation and Environmental Impact,
Energy Policy, Vol 19 (8), pp 798–807.
47. Geothermal Energy Program: Environmental
and Economic Impacts http://www.eren.doe.gov/geothermal/geoimpacts.html,
accessed Oct 7, 2002.
48. The World Bank Group. Geothermal Energy
Environmental Impact Case Studies.
http://www.worldbank.org/html/fpd/energy/geothermal/case_studies.htm,
accessed January 28, 2003.
49. Energy& Geoscience Institute at the
University of Utah http://www.egi.utah.edu/geothermal/GeothermalBrochure.pdf,
accessed Oct 7, 2002.
50. DiPippo, R, (1991). “Geothermal
Energy: Electricity Generation and Environmental
Impact”, Energy Policy,
Vol 19 (8), pp 798–807. Armannsson,
H, and Kristmannsdottir, H, (1992). “Geothermal
Environmental Impact”, Geothermics,
Vol. 21, No 5/6, pp 869–880.
51. International Energy Agency. http://www.iea.org/pubs/studies/files/benign/pubs/appedn3g.pdf,
accessed Oct 30, 2002.
52. Hunt, T, and Brown, K, (1996). “
Environmental Effects of Geothermal Development
and Countermeasures”, in Proceedings
of Asia-Pacific Economic Co-operation (APEC)
Energy R&D and Technology Transfer and
Renewable Energy Resource Assessment Seminar,
Beijing, China, pp 243–255, 6–9
February, 1996.
53. Ibid.
54. International Energy Agency. Appendices
to Report on Benign Energy: The Environmental
Implications of Renewables. Appendix
G Geothermal http://www.iea.org/pubs/studies/files/benign/pubs/append3g.pdf,
accessed Nov 25, 2002.
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