Environmental Engineering Reference
In-Depth Information
Figure 7.2
The Geothermal Heat Pump Cycle
Materials
acquisition
Manufacturing
Installation
Waste disposal
Decommissioning
Operation
The choice of which design to use is determined by characteristics of a local geothermal resource.
If water comes out of a well as steam, it can be used directly, as in the dry steam plant design. If
it is hot water of a high enough temperature, a flash steam plant can be used; otherwise it must
go through a heat exchanger in a binary-system plant. Since there are more hot water resources
than pure steam or high-temperature water sources, there is more growth potential in the binary-
system design (UCS 2009).
The technologies discussed above use only a tiny fraction of the total geothermal resource.
Several miles everywhere beneath the earth's surface, hot, dry rock is continuously being heated
by the molten magma directly below it. Technology is being developed to drill into this rock,
inject cold water down a well, circulate it through the hot, fractured rock, and draw off the heated
water from another well (USDOE 2011).
Environmental Costs of Using Geothermal Energy
Because of the diversity of technologies used with geothermal energy, the geothermal energy “fuel
cycle” is really two fuel cycles, depending on which technologies one is discussing. A geother-
mal heat pump fuel cycle is rather similar to the simple solar fuel cycle, involving acquisition of
materials, manufacture and installation of geothermal heat pump equipment, operation of geo-
thermal heat pumps, and disposal or recycling of waste materials from manufacturing processes
and decommissioning, as illustrated in
Figure 7.2.
Manufacturing of geothermal heat pumps
involves industrial-scale aluminum and sheet metal fabrication of heat pumps, with installation
of plumbing, compressors, and electrical components, depending on the specific design utilized.
Installation involves some excavation of trenches or drilling of wells near the point of operation,
and disposal or recycling of waste materials must be done in licensed facilities as is the case with
other industrial manufacturing. Actual operation of geothermal heat pumps entails very low or
no environmental costs.
A second, geothermal power plant fuel cycle is rather like the oil and gas fuel cycle, at least in
its first and last stages, including exploration, geothermal field development, power plant develop-
ment, production and transmission of electricity, utilization, reclamation, and disposal of waste
by-products, as illustrated in
Figure 7.3.
The basic components of geothermal power generation
facilities include wells to access steam and superheated groundwater, steam turbines, generators,
condensers, cooling towers, reinjection pumps, and electrical grid interconnection equipment (IFC
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