Page 253 - Geothermal Energy Renewable Energy and The Environment

242 Geothermal Energy: Renewable Energy and the Environment

For long-term sustainability of the geothermal resource, it is important to replenish the extracted
fluid. Although this will occur naturally, the time frame over which that will be accomplished can
vary from a few hours or days to many years. If the extraction rate from a geothermal resource is
very high and the rate at which water is naturally replenished is also high, it is possible that draw
down of water from other resource reservoirs can occur. Such an effect might be expressed as a drop
in the local water table, diminution of geothermal surface manifestations such as hot springs and
geysers, and other effects. If the recharge rate is very slow, it is possible that subsidence of the land
surface could occur, as well as reduction in the geothermal resource productivity. For these reasons,
it is important to balance fluid extraction by reinjection.
The loss of a large fraction of the geothermal fluid to evaporative cooling is a significant challenge
to maintaining mass balance. The Geysers site has developed an agreement with local water dis-
tricts for disposal of their wastewater by injecting it into the geothermal reservoir. This undertaking
has moved The Geysers system much closer to achieving mass balance and thus sustainability. This
solution, however, is not likely to be useful for many sites located in areas removed from significant
population centers that need to dispose of wastewater in an environmentally appropriate way. In
areas where there is an abundance of surface water, collection and injection of that water into the
reservoir can provide the mass needed to replenish the reservoir and maintain sustainability. For
sites located near coastlines or other nonpotable water, such as brackish groundwater, those water
resources can be used to make up the loss to evaporation.
Consideration is also currently being given to a hybrid approach that uses air cooling for the
coolest part of the year. Such a system would be designed for maximum efficiency at the expected
ambient air temperature range during the coolest season. As the diurnal temperature range changes
during the warmer months, air cooling would be supplemented by incremental addition of water
cooling. Such an approach allows all of the geothermal fluid to be reinjected for some of the year,
and minimizes losses to the atmosphere during the remainder of the year.
Regardless of the approached used, it is important that careful attention be given to maintaining
the long-term stability of the geothermal resource by careful management of the water cycle through-
out the power generation facility and the surrounding ecosystem. Although geothermal resources
are inherently renewable on the geological timescale, their use on human time scales requires that
they be managed in a way that astutely balances all parts of the system to assure sustainability.

land Use
Construction and operation of a geothermal facility requires land, thus impacting the local land-
scape. The construction phase, as with many development efforts, produces the largest footprint a
project may cause but, in most cases is significantly reduced once construction and testing have been
completed and operations begun. The construction phase for a geothermal facility usually consists
of two basic components, drilling a well or wells and construction of the geothermal facility. These
construction components are the same for all applications, regardless of whether one is dealing with
ground source heat pumps, direct use, or power generation.
Drilling a geothermal well requires land access for a drilling rig of sufficient size to achieve
the needed depth. This may require road building to get to the site selected for the well. At the
well site, a drilling pad must be excavated that is large enough to allow maneuvering the drilling
rig or construction of the well drilling platform. The site must also have adequate storage area for
the drill pipe and casing that will be used. Excavation will also be required for a sump or multiple
sumps, which are pits into which the drilling mud is pumped as it exits the well and is recycled.
For very deep wells in geothermal systems for power generation, it may also be necessary to have
a facility for cooling the drilling mud. The total area required to accommodate these facilities
2
2
will vary from 1000 m to 10000 m , depending upon the depth to be drilled and the equipment
to be used.