Environmental Engineering Reference
In-Depth Information
is pumped back into the same aquifer via a second well—called a discharge well—located at a
suitable distance. This loop configuration is used less frequently than others, but may be employed
cost-effectively if groundwater is plentiful (GeoExchange 2011).
Standing Column Well Systems
Standing column wells have become an established technology in some regions, especially the
northeastern United States. Standing wells are typically six inches in diameter and may be 1,500
feet deep. Temperate water from the bottom of the well is withdrawn, circulated through the heat
pump's heat exchanger, and returned to the top of the water column in the same well. Usually, the
well also provides potable drinking water. However, groundwater must be plentiful for a stand-
ing well system to operate effectively. If a standing well is installed where the water table is too
deep, pumping would be prohibitively costly. Under normal circumstances, water diverted for
drinking use is replaced by constant-temperature groundwater, which makes the system act like a
true open-loop system. If well-water temperature climbs too high or drops too low, water can be
bled from the system to allow groundwater to restore well-water temperature to the normal oper-
ating range. Permitting conditions for discharging the bleed water vary from locality to locality,
but are eased by the fact that quantities are small and the water is never treated with chemicals
(GeoExchange 2011).
Fluid circulating in the loop carries ground heat to the home. An indoor geothermal heat pump
then uses electrically driven compressors and heat exchangers in a vapor compression cycle, the
same principle employed in a refrigerator, to concentrate the earth's energy and release it inside
the home at a higher temperature (GeoExchange 2011). Forced air or hot water systems distribute
the heat to various rooms.
Geothermal power generation involves harnessing high-temperature, underground reservoirs of
geothermal waters or steam and converting the thermal energy to electricity. Geothermal power
generation plants, which are typically located adjacent to sources of thermal energy to reduce
heat losses from transportation, typically require 0.5 to 3.5 hectares of land per megawatt (IFC
2007, 12). Geothermal heat occurs everywhere under the earth's surface, but the conditions that
make water circulate to the surface are found in less than 10 percent of the land area (UCS 2009).
Consequently, in the United States most geothermal reservoirs are located in the western states,
Alaska, and Hawaii, limiting availability of this energy resource to these areas.
There are two major types of geothermal resources suitable for electric power generation: dry
steam and hot water (Duffield and Sass 2003). The basic configuration for a geothermal power plant
is illustrated in
Figure 7.1.
In dry steam resources, the output of producing wells is a dry steam
that can be used directly to run turbine-generators. In hot water resources, the well discharge is
high-temperature (>180°C) water. For water resources under 180°C, power generation is possible
using a binary cycle system involving the use of a secondary fluid (IFC 2007, 12-13).
The following options for geothermal power production are available today:
s$RYSTEAMPLANTSDIRECTLYUSEGEOTHERMALSTEAMTOTURNTURBINES(IGHPRESSUREDRYSTEAM
discharged from production wells is used directly in turbines to generate electricity. After use,
lower-pressure, lower-temperature steam may be discharged to the atmosphere or (rarely)
reinjected into the geothermal reservoir.
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