GEOTHERMAL POWER
Geothermal energy is a naturally occurring, semirenewable source of
thermal energy. Thermal energy within the earth approaches the surface
in many different geologic formations. Volcanic eruptions, geysers, fumaroles,
hot springs, and mud pots are visual indications of geothermal
energy.
Significant geothermal reserves exist in many parts of the world. The
U.S. Geological Survey, in Circ. 790, has estimated that in the United
States alone there is the potential for 23,000 megawatts (MW) of electric
power generation for 30 years from recoverable hydrothermal (liquid-
or steam-dominated) geothermal energy. Undiscovered reserves
may add significantly to this total. Many of the known resources can be
developed using current technology to generate electric power and for
various direct uses. For other reserves, technical breakthroughs are necessary
before this energy source can be fully developed.
Power generation from geothermal energy is cost-competitive with
most combustion-based power generation technologies. In a broader
picture, geothermal power generation offers additional benefits to society
by producing significantly less carbon dioxide and other pollutants
per kilowatt-hour than combustion-based technologies.
Electric power was first generated from geothermal energy in 1904.
Active worldwide development of geothermal resources began in earnest
in 1960 and continues. In 1993, the capacity of geothermal power
plants worldwide exceeded 6,000 MW. Table 9.1.7 lists the installed
capacity by country cently’’
active volcanoes and continuing seismic activity. In 1994 and
1995, significant additional geothermal power generation facilities were
installed in Indonesia and the Philippines. Many countries not included
in Table 9.1.7 also have significant geothermal resources that are not yet
developed.
Although geothermal energy is a renewable resource, economic development
of geothermal resources usually extracts energy from the
reservoir at a much higher rate than natural recharge can replenish it.
Therefore, facilities that use geothermal energy should be designed for
high efficiency to obtain maximum benefit from the resource.
Geothermal Resources Geothermal resources may be described as
hydrothermal, hot dry rock, geopressured, or magma.
Hydrothermal resources contain hot water, steam, or a mixture of
water and steam. These fluids transport thermal energy from the reservoir
to the surface. Reservoir pressures are usually sufficient to deliver
the fluids to the surface at useful pressures, although some liquiddominated
resources require downhole pumps for fluid production. Hydrothermal
resources may be geologically closed or open systems. In a
closed system, the reservoir fluids are contained within an essentially
impermeable boundary. Communication and fluid transport within the
reservoir occur through fractures in the reservoir rock. There is little, if
any, natural replenishment of fluids from outside the reservoir boundary.
An open system allows influx of cold subsurface fluids into the
reservoir as the reservoir pressure decreases. Hydrothermal reservoirs
have been found at depths ranging from 400 ft (122 m) to over
10,000 ft (3,050 m).
Hot dry rock resources are geologic formations that have high heat
content but do not contain meteoric or magmatic waters to transport
thermal energy. Thus water must be injected to carry the energy to the
surface. The difficulty in recovering a sufficient percentage of the injected
water and the limited thermal conductivity of rock have hindered
development of hot dry rock resources. Because of the vast amount of
energy in these resources, additional research and development is justified
to evaluate whether it is technically exploitable. In 1994 research
and development of hot dry rock resources was proceeding in Australia,
France, Japan, and the United States.
Geopressured resources are liquid-dominated resources at unusually
high pressure. They occur between 5,000 and 20,000 ft (1,500 and
6,100 m), contain water that varies widely in salinity and dissolved
minerals, and usually contain a significant amount of dissolved gas.
Pressures in such reservoirs vary from about 3,000 to 14,000 lb/in2 gage
(21 to 96 MPa) with temperatures between 140°F (60°C) and 360°F
(182°C). The largest geopressured zones in the United States exist beneath
the continental shelf in the Gulf of Mexico, near the Texas, Louisiana,
and Mississippi coasts. Other zones of lesser extent are scattered
throughout the United States.
Magma resources occur as formations of molten rock that have temperatures
as high as 1,300°F (700°C). In most regions in the continental
United States, such resources occur at depths of 100,000 ft (30,500 m)
or more. However, in the vicinity of current or recent volcanic activity,
magma chambers are believed to be within 20,000 ft (6,100 m) of the
surface. The Department of Energy initiated a magma energy research
program in 1975, and an exploratory well was drilled in the Long Valley
Caldera in California in 1989. The drilling program was planned in four
phases to reach a final depth of 20,000 ft (6,100 m) or a temperature of
900°F (500°C), whichever is reached first. The first phases have been
completed, but the deep, and most significant, drilling remains to be
done.
Exploration Technology Geothermal sites historically have been
identified from obvious surface manifestations such as hot springs, fumaroles,
and geysers. Some discoveries have been made accidentally
during exploring or drilling for other natural resources. This approach
has been replaced by more scientific prospecting methods that appraise
the extent, as well as the physical and thermodynamic properties, of the
reservoir. Modern methods include geological studies involving aerial,
surface, and subsurface investigations (including remote infrared sensing)
and geochemical analyses which provide a guide for selecting specific
drilling sites. Geophysical methods include drilling, measuring the
temperature gradient in the drill hole, and measuring the thermal conductivity
of rock samples taken at various depths.
Resource Development Extraction of fluids from a geothermal resource
entails drilling large-diameter production wells into the reservoir
formation. Bottom hole temperatures in hydrothermal wells and hot dry
rock formations can exceed 450°F (232°C). Geopressured resources
have lower temperatures but offer energy in the form of fluids at unusually
high pressures that frequently contain significant amounts of dissolved
combustible gases. Although research and development projects
continue to seek ways to efficiently extract and use the energy contained
in hot dry rock, geopressured, and magma resources, virtually all
current geothermal power plants operate on hydrothermal resources.
Production Facilities For most projects, a number of wells drilled
into different regions of the reservoir are connected to an aboveground
piping system. This system delivers the geothermal fluid to the power
plant. As with any fluid flow system, the geothermal reservoir, wells,
and production facilities operate with a specific flow vs. pressure relationship.
Fig. 9.1.18 shows a typical steam deliverability curve for a
110-MW geothermal power plant.
Resource permeability; the number, depth, and size of the wells; and
the surface equipment and piping arrangement all contribute to make
the deliverability curve different for each power plant. Production of
geothermal fluids over time results in declining deliverability. For onehalf
or more of the operating life of a reservoir, the deliverability can
usually be held constant by drilling additional production wells into
other regions of the resource. As the resource matures, this technique
ceases to provide additional production. The deliverability curve begins
to change shape and slope as deliverability declines. The power plant
design must be matched to the deliverability curve if maximum generation
from the resource is to be achieved.
Geothermal Power Plants A steam-cycle geothermal power plant
is very much like a conventional fossil-fueled power plant, but without a
boiler. There are, however, significant differences. The turbines, condensers,
noncondensable gas removal systems, and materials used to
fabricate the equipment are designed for the specific geothermal application.
With geothermal steam delivered to the power plant at approximately
100 lb/in2 gage (689 kPa), only the low-pressure sections of a
conventional turbine generator are used. Additionally, the geothermal
turbine must operate with steam that is far from pure. Chemicals and
compounds in solid, liquid, and gaseous phases are transported with the
steam to the power plant. At the power plant, the steam passes through a
separator that removes water droplets and particulates before it is delivered
to the turbine. Geothermal turbines are of conventional design with
special materials and design enhancements to improve reliability in
geothermal service. Turbine rotors, blades, and diaphragms operate in a
wet, corrosive, and erosive environment. High-alloy steels, stainless steels, and titanium provide improved durability and reliability. Still,
frequent overhauls are necessary to maintain reliability and performance.
The high moisture content and the corrosive nature of the condensed
steam require effective moisture removal techniques in the later
(low-pressure) stages of the turbine. Scale formation on rotating and
stationary parts of the turbine occurs frequently. Water washing of the
turbine at low-load operation is sometimes used between major overhauls
to remove scale.
Most geothermal power plants use direct-contact condensers. Only
when control of hydrogen sulfide emissions has been required or anticipated
have surface condensers been used. Surface condensers in geothermal
service are subject to fouling on both sides of the tubes. Power
plants in The Geysers in northern California use conservative cleanliness
factors to account for the expected tube-side and shell-side fouling.
Some plants have installed on-line tube-cleaning systems to combat
tube-side fouling on a continuous basis, whereas other plants mechanically
clean the condenser tubes to restore lost performance.
Noncondensable gas is transported with the steam from the geothermal
resource. The gas is primarily carbon dioxide but contains lesser
amounts of hydrogen sulfide, ammonia, methane, nitrogen, and other
gases. Noncondensable gas content can range from 0.1 percent to more
than 5 percent of the steam. The makeup and quantity of noncondensable
gas vary not only from resource to resource but also from well to
well within a resource. The noncondensable gas removal system for a
geothermal power plant is substantially larger than the same system for
a conventional power plant. The equipment that removes and compresses
the noncondensable gas from the condenser is one of the largest
consumers of auxiliary power in the facility, requiring up to 15 percent
of the thermal energy delivered to the power plant. A typical system
uses two stages of compression. The first stage is a steam jet ejector.
The second stage may be another steam jet ejector, a liquid ring vacuum
pump, or a centrifugal compressor. The choice of equipment selected
for the second stage is influenced by project economics and the amount
of gas to be compressed.
The chemicals and compounds in geothermal fluids are highly corrosive
to the materials normally used for power plant equipment and
facilities. The chemical content of geothermal fluids is unique to each
resource; therefore, each resource must be evaluated separately to determine
suitable materials for system components. Carbon steel usually
will degrade at alarmingly high rates when exposed to geothermal
fluids. Corrosion-resistant materials such as stainless steel may perform
satisfactorily, but may experience rapid, unpredictable local failures
depending upon the composition of the geothermal fluid. Based on
experience with a number of geothermal resources:
Carbon steel with a corrosion allowance is usually suitable for transporting
dry geothermal steam.
Geothermal condensate and cooling water usually require corrosionresistant
piping and equipment.
Because noncondensable gas is also corrosive, special materials are
usually required.
Copper is extremely vulnerable to attack from the atmosphere
surrounding a geothermal power plant. Therefore, copper wire and electrical
components should be protected with tin plating and isolated from
the corrosive atmosphere.
Within the context of these generalities, the fluids at each resource must
be evaluated before construction materials are chosen.
The steam Rankine cycle used in fossil-fueled power plants is also
used in geothermal power plants. In addition, a number of plants operate
with binary cycles. Combined cycles also find application in geothermal
power plants. The basic cycles are shown in Fig. 9.1.19.
The direct steam cycle shown in Fig. 9.1.19a is typical of power
plants at The Geysers in northern California, the world’s largest geothermal
field. Steam from geothermal production wells is delivered
to power plants through steam-gathering pipelines. The wells are up
to 1 mi or more from the power plant. The number of wells required to
supply steam to the power plant varies with the geothermal resource as
well as the size of the power plant. The 55-MW power plants in The
Geysers receive steam from between 8 and 23 production wells.
A flash steam cycle for a liquid-dominated resource is shown in Fig. 9.1.19b. Geothermal brine or a mixture of brine and steam is delivered
to a flash vessel at the power plant by either natural circulation or
pumps in the production wells. At the entrance to the flash vessel, the
pressure is reduced to produce flash steam, which then is delivered to
the turbine. This cycle has been used at power plants in California,
Nevada, Utah, and many other locations around the world. Increased
thermal efficiency is available from the use of a second, lower-pressure
flash to extract more energy from the geothermal fluid. However, this
technique must be approached carefully as dissolved solids in the geothermal
fluids will concentrate and may precipitate as more steam is
flashed from the fluid. The solids also tend to form scale at lower
temperatures, resulting in clogged turbine nozzles and rapid buildup in
equipment and piping to unacceptable levels.
A binary cycle is the economic choice for hydrothermal resources
with temperatures below approximately 330°F (166°C). A binary cycle
uses a secondary heat-transfer fluid instead of steam in the power generation
equipment. A typical binary cycle is shown in Fig. 9.1.19c.
Binary cycles can be used to generate electric power from resources
with temperatures as low as 250°F (121°C). The binary cycle shown in
Fig. 9.1.19c uses isobutane as the heat-transfer fluid. It is representative
of units of about 10-MW capacity. Many small modular units of 1- or
2-MW capacity use pentane as the binary fluid. Heat from geothermal
brine vaporizes the binary fluid in the brine heat exchanger. The binary
fluid vapor drives a turbine generator. The turbine exhaust vapor is
delivered to an air-cooled condenser where the vapor is condensed.
Liquid binary fluid drains to an accumulator vessel before being
pumped back to the brine heat exchangers to repeat the cycle. Binarycycle
geothermal plants are in operation in several countries. In the
United States, they are located in California, Nevada, Utah, and Hawaii.
A geothermal combined cycle is shown in Fig. 9.1.19d. Just as combustion-
based power plants have achieved improved efficiencies by
using combined cycles, geothermal combined cycles also show improved
efficiencies. Some new power plants in the Philippines use a
combination of steam and binary cycles to extract more useful energy
from the geothermal resource. Existing steam-cycle plants can be modified
with a binary bottoming cycle to improve efficiency.
Cycle optimization is critically important to maximize the power generation
potential of a geothermal resource. Selecting optimum cycle
design parameters for a geothermal power plant does not follow the
practices used for fossil-fueled power plants. While a higher turbine
inlet pressure will improve the efficiency of the power plant, a lower
turbine inlet pressure may result in increased generation over the life of
the resource. The resource deliverability curve (Fig. 9.1.18) is used with
turbine and cycle performance predictions to determine the flow and
turbine inlet pressure that will yield maximum generation. The technical
optimum must then be subjected to an economic analysis to identify the
best parameters for the power plant design. Because the shape and slope
of the deliverability curve vary from resource to resource, the optimum
turbine inlet conditions will likewise vary.
Direct Use There are substantial geothermal resources with temperatures
less than 250°F (121°C). While these resources cannot currently
be used to generate electric power economically, they can be used for
various low-temperature direct uses. Services such as district heating,
industrial process heating, greenhouse heating, food processing, and
aquaculture farming have been provided by geothermal fluids. For these
applications, corrosion and fouling of surface equipment must be addressed
in the system design.
The geothermal heat pump (GHP) is another direct use of the earth’s
thermal energy. The GHP, however, does not require a high-temperature
geothermal reservoir. The GHP uses essentially constant-temperature
groundwater as a heat source or heat sink in a conventional, reversible,
water-to-air heat pump cycle for building heating or cooling. The
ground, groundwater, and local climatic conditions must be included in
the design of a GHP for a specific location. Systems are currently available
for residential (single- and multifamily) dwellings, offices, and
small industrial buildings.
Environmental Considerations Geothermal fluids contain many
chemicals and compounds in solid, liquid, and gaseous phases. For both
environmental protection and resource conservation, spent geothermal
liquids are returned to the reservoir in injection wells. This limits the
release of compounds to the environment to a small amount of liquid
lost as drift from the cooling tower and noncondensable gases. Problems
with arsenic and boron contamination have been encountered in
the immediate vicinity of cooling towers at geothermal power plants.
The noncondensable gases, composed primarily of carbon dioxide, usually
also contain hydrogen sulfide. Along with its noxious odor, hydrogen
sulfide is hazardous to human and animal life. Although many
geothermal power plants do not currently control the release of hydrogen
sulfide, others use process systems to oxidize the hydrogen sulfide
to less toxic compounds. A number of the process systems produce 99.9
percent pure sulfur that can be sold as a by-product. Using geothermal
energy for power generation and other direct applications provides environmental
benefits. Carbon dioxide released from a geothermal power
plant is approximately 90 percent less than the amount released from a
combustion-based power plant of equal size, and they create little, if
any, liquid or solid waste.

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