Page 255 - Geothermal Energy Renewable Energy and The Environment

244 Geothermal Energy: Renewable Energy and the Environment

synopsIs
The environmental impact of converting energy to electricity or some other useful form inevita-
bly disturbs the environment. For this reason it is imperative that aggressive, scientifically based
monitoring, analysis, and mitigation efforts be considered an integral part of any energy develop-
ment. The importance of renewable energy resources such as geothermal are that their environ-
mental impacts can be minimal, if properly managed. The impacts from geothermal energy are
primarily from emissions, water disposal, seismicity, ground subsidence, water use, and land use.
Emissions from flash geothermal power plants (there are no emissions from binary plants) are due
to gases dissolved in the geothermal fluid. The volumetrically most significant of these is CO , with
2
occasional impacts from H S and Hg. The CO emissions are quite small, amounting to less than
2
2
1% of those from fossil-fueled power conversion technologies. Since the amount of dissolved CO
2
is determined by the local geology, which emits measurable CO as a background component to the
2
atmosphere, it is likely that, in most cases, a geothermal power plant will represent a relatively small
increment to the natural emissions at a site. The other gaseous components are routinely removed
from the output of a geothermal plant, if concentrations might exceed regulatory limits. Disposal
of geothermal water is generally by reinjection, but some components in brines can represent a
potential resource of significant economic value. Resource recovery is currently an active area of
research and development. Currently, silica, zinc, and manganese are capable of being economically
recovered. Seismicity induced by development and operations activities associated with geothermal
power development occurs in response to fluid extraction, reinjection, and reservoir enhancement.
The magnitude of ground shaking from such induced events is very low under most circumstances.
Larger amplitude events can happen only if the local stress regime and geological structures have
sufficient capability for rupture over a large area. Thorough analysis of a site can identify such
conditions and guide activities to avoid significant rupture events. Subsidence of the land surface
can result if there is inadequate replenishment of geothermal fluids in areas where the subsurface
rock has low compressive strength. As with the case involving seismicity, thorough site analysis
and monitoring can guide operational activities, resulting in greatly reduced risk. Water use in areas
where water availability is limited can impose important challenges on geothermal operations if
water cooling is involved. Planning, design, and operation of a geothermal facility must thoroughly
consider the water mass balance through the entire operations cycle in order to assure sustainable
use. Use of seawater, wastewater, brackish groundwater, and surface water, where appropriate can
significantly mitigate the impact on water budgets. Development of hybrid cooling technologies
and operational approaches are also under development to reduce water use. Finally, the land use
footprint of a technology can be an important environmental issue. Habitat disruption and displace-
ment of other uses are two important impacts that can result from developing an energy use facility.
Geothermal power, as with any application, has a land use impact. That impact, however, is the
smallest of all energy conversion technologies, per MW generated, with the exception of nuclear
power.

Problems
12.1 What studies and measurements can be done before a geothermal project is initiated to
determine the likely contaminants that would need to be dealt with?
12.2 What are the environmental emissions that must be considered for geothermal power
projects? How do they compare, quantitatively, with those from fossil-fueled power
generation systems?
12.3 What are the factors that control seismic activity? What is the rupture area that is
required to cause a magnitude 5 earthquake?
12.4 What are the reasons seismicity can occur during operations associated with geother-
mal power development and production? How can it be mitigated?
12.5 What are ways in which water is used for power production? What methods could be
employed to reduce water consumption?