|
F I g u r E 2 . 6
CO
2
Emissions by Primary Energy Source in United States
Source | Fridleifsson et al. 2008.
Grams of CO
2
per Kilowatt Hour
Geothermal
Natural Gas
Oil
Coal
0
200
400
600
800
1000
91
599
893
955
P
rimar
y Ener
gy Sour
ce
The first perceptible effects on the environment come from drilling and related infrastructure. The
magnitude of these risks depends on whether the wells being drilled are shallow wells for measuring
the geothermal gradient in the study phase and whether they are exploratory or production wells.
However, in all cases, solid waste generated during well drilling, such as drilling mud and cuttings, and
other solid waste needs to be disposed of in an environmentally responsible manner; risk of ground
water aquifer contamination during well drilling needs to be controlled; and risk of a steam blowout or
of geothermal water rising to the surface and spreading during well drilling needs to be minimized.
The installation of a drilling rig and all the accessory equipment entails the construction of access
roads and a drilling pad and the management of drilling inputs (e.g., mud and water). Specific
investment to ensure appropriate casing and cementing of drilled holes is necessary to avoid aquifer
contamination, including after a well is abandoned. Ground water should not be contaminated with
geothermal reservoir fluids.
Installation of the pipelines that will transport the geothermal fluids and construction of the power plant
can also disrupt natural habitats and the surface morphology. Some of these pipelines can be buried
to reduce environmental disturbances.
Environmental impacts can also arise during plant operation. Geothermal fluids (steam or hot water)
usually contain gases, such as CO
2
, H
2
S, ammonia (NH
3
), methane (CH
4
), and trace amounts of
65
C h a p t e r 2
40
Wet-bulb temperature is, simply put, the temperature one feels when one’s skin is wet and is exposed to moving air. It is an indication of the
amount of moisture in the air.
other gases, which can contribute to global warming, acid rain or noxious smell if released into the
atmosphere. They can also contain trace amounts of toxic dissolved chemicals whose concentrations
usually increase with temperature, and which can also cause damage if released into the environment.
A number of proven technologies, often developed for other types of power generation or other
industries, are available on the market to control, filter, or chemically modify the emissions streams from
geothermal plant operation.
Geothermal power plant condensers can operate on direct (river or ocean), wet (cooling tower), or
dry cooling, depending on the availability of water, the power plant technology used and the size and
altitude of the plant. Criteria for choosing the cooling equipment are largely the same as for any other
thermal power generation technology, since the design of all these cooling systems is based on the
wet-bulb temperature
40
of the actual site.
Amongst water-cooled power plants, geothermal plants tend to use less water per unit of power
produced than other thermal solutions; water-cooled geothermal plants use only about 20 liters of
freshwater per megawatt hour generated, while binary air-cooled plants use no freshwater. This
compares, for instance, with over 3,000 liters per MWh for nuclear plants, over 2,500 liters per MWh
for coal plants (World Nuclear Association), and 1,400 liters per MWh for natural gas facilities (Kagel,
Bates, and Gawell 2007). In practice, however, the consumption of water for wet cooling purposes
per generated unit of power depends on multiple factors that affect the overall efficiency of the power
generation process.
Large water requirements can also lead to conflicts with other competing uses when water is scarce.
In addition, waste water from cooling towers has a higher temperature than ambient water, therefore
constituting a potential thermal pollutant when discharged to nearby streams or lakes. This can be
mitigated by an environmental management plan that sets authorized discharge and temperature
levels.
Discharge of waste fluids is a potential source of chemical pollution. After having passed the turbine,
geothermal fluids with high concentrations of chemicals, such as sodium chloride (NaCl), boron (B),
fluoride (Fl), or heavy metals such as mercury (Hg) and arsenic (As), should either be treated or
reinjected into the reservoir. Fluids coming from low to medium temperature geothermal fields, as used
in most direct-use applications, generally contain low levels of chemicals.
The withdrawal and/or reinjection of geothermal fluids may cause ground subsidence at the surface.
In certain areas, this may trigger or increase the frequency of micro seismic events, which are
imperceptible and can only be detected by means of instrumentation. No major seismic events
induced by the exploitation of geothermal fluids have been observed so far. The few incidents that
induced perceptible earthquakes were linked to the “fracking” process (the creation of an artificial
underground reservoir by induction of highly pressured cold water) as part of EGS projects (see the
section on Classification of Geothermal Systems for more information on EGS).
The noise associated with operating geothermal plants could be a problem in populated areas near
where the plant in question generates electricity. During the production phase, there is high-pitched
66
G e o t h e r m a l H a n d b o o k : P l a n n i n g a n d F i n a n c i n g P o w e r G e n e r a t i o n
noise from the steam travelling through pipelines and from the occasional vent discharge as well as
noise from the cooling towers. These issues can be mitigated by determining the maximum decibel
levels and investing in appropriate mitigation measures, such as sound barriers or other insulation.
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