GE 231 - Geothermal Systems,
Resources, & Technology
Group 1
Vapor-dominated and
Liquid-dominated Geothermal Systems
Members:
Maria Annabel Jamito
Fretchie Rose Husayan
Jellisa Catipay
Francisco Pineda Jr.
Adviser:
Christ C. Quinicot
There are two types of geothermal field. The first is the wet (or "liquid dominated") field which
produces water under pressure at temperatures over 100°C. On reaching the surface, the pressure
is reduced, and part of the water is "flashed" to steam, leaving a larger fraction as boiling water. The
second is the dry (or "vapor dominated") field, which produces dry saturated, or superheated,
steam at pressures higher than that of the atmosphere.
Vapor-dominated Geothermal Systems
Introduction:
Geothermal energy is a promising renewable energy source that has the potential to significantly
contribute to world energy demand. Because of their great energy potential and efficiency, vapordominated geothermal systems are particularly important among the various types of geothermal
systems.
Vapor-dominated geothermal systems are an important renewable energy source that has the
potential to contribute to global energy sustainability. While concerns like as resource sustainability
and operational complications remain, continued technological improvements and increased
research efforts are projected to pave the road for vapor-dominated geothermal systems to be
adopted and integrated into the global energy mix. Careful planning, long-term management, and
ongoing innovation will be required to fully realize the future potential of this profitable and
environmentally benign energy source. This report aims provide a thorough review of vapordominated geothermal systems, including their characteristics, application, fundamentals,
challenges, and potential for the future.
When we say the vapor-dominated reservoir has a specific volume of fluid (i.e. combined vapor and
liquid) greater than the critical volume of water, whereas the liquid-dominated reservoir has a
lower specific volume. A vapor-dominated reservoir is not necessarily a superheated steam
reservoir; it can have both vapor and liquid.
Geothermal Power Stations Conventional steam-cycle plants are used to produce energy
from vapor-dominated reservoirs. As is shown in this photo, steam is extracted from the wells,
cleaned to remove entrained solids, and piped directly to a steam turbine. This is a well-developed,
commercially available technology, with typical unit sizes in the capacity range 35–120 MWe.
Geothermal energy comes in either vapor-dominated or liquid-dominated forms. Larderello and
The Geysers are vapor-dominated. Vapor-dominated sites offer temperatures from 240 to 300 °C
that produce superheated steam.
The majority of currently exploited geothermal sites contain water at high pressures and
temperatures in excess of 100 ˚C. When this water is brought to the Earth’s surface, the pressure is
greatly reduced, resulting in large amounts of steam and a mixture of saturated steam and water.
The steam-to-water ratio changes depending on location. Cerro Prieto (Mexico), Wairakei (New
Zealand), Reykjavik (Iceland), Salton Sea (U.S.A.), and Otake (Japan) are examples of well-known
geothermal fields. These fields are known as wet steam fields because they create steam along with
water.
How does vapor dominated system work?
Vapour-dominated systems generally have limited natural thermal output on the ground surface
compared with low and medium-enthalpy two-phase systems. These natural features are strictly
steam heated (e.g. steaming ground, mud and acid pools) with no deep (chloride) waters
discharging at the ground surface.
The basic requirements of the vapor-dominated geothermal fields, whether of dry steam or of wet
steam type, include adequate supplies of water in addition to the three prerequisites mentioned
earlier.
•
Dry steam - all water molecules are in the gaseous state
•
Wet steam - a portion of the water molecules have lost their energy - latent heat - and
condensed to tiny water droplets
Heat source
The fact that vapor-dominated geothermal fields are located in areas of recent (MioceneQuaternary) volcanism, with some of them located on or near volcanoes, has confirmed that magma
is their source. High-temperature (500-1000 ˚C) magma intrusions to depths of a few to several
kilometers below the Earth's surface allow the requisite heat to be stored in economically large
quantities. Faulting in hard compact rocks may create a pathway for magma to reach the surface.
When soft or plastic rocks are present, they can flow and block the fault space, causing magma to
spread at the interface of the soft and hard rocks. Surface manifestations of recent volcanic activity
include active volcanoes, fumaroles, hot springs, and geysers. In addition, certain geological
environments, such as regions of Quaternary uplift and regions of Late Tertiary and Quaternary
subsidence, are indicative of shallow magmatic intrusions.
Reservoir
The anomalous magmatic intrusion must come into contact with porous and permeable, waterfilled rock strata in order to form a heat reservoir. Convection currents of hot water and/or steam
are set up within the reservoir, allowing for good heat exchange, and the temperature difference
between the top and bottom of the reservoir is not significant. A wide range of rocks have been
discovered to be suitable reservoirs. Good reservoirs can also form at geological unconformities and
formation boundaries if they are permeable and have adequate hydraulic continuity and water
supply. The origin of geothermal fluids has long been debated. Geothermal fluids have been
suggested to have magmatic and juvenile origins in addition to a meteoric origin.
However, recent isotopic studies in geothermal fields have revealed that at least 90% of geothermal
water is meteoric in origin. For rainwater to be available in continuous supply, the permeable
aquifers forming the reservoir must have hydraulic continuity with large recharge areas. The newly
supplied water is heated conductively at the reservoir's impermeable base. The withdrawal of
heated reservoir fluid through boreholes, as well as its upward movement through vents and
fissures, disturbs the hydrological balance.
This is restored, fully or partially, by the inflow of new water. An idea of the amount of the inflow
can be had from the fact that a natural steam field operating a 100 MW power plant lets out
between 1,000 and 2,000 tons of water every hour. Some of the geothermal fields, such as the
Larderello in ltaly, have easily identifiable recharge areas. At Larderello, the permeable reservoir
terrain, consisting of Mesozoic limestones and dolomites, outcrops thereby providing an easy access
to surficial water.
Cap rock - the barrier
An impermeable cap rock, or a cap rock with low permeability, overlying the reservoir, is necessary
to prevent the escape of hot reservoir fluids through convection. The heat loss through conduction
is not prevented by the cap rock. However, the amount of heat conducted is much smaller than that
which could be lost through possible convection. Since volcanism is associated with tectonic
movements causing fissures, ideal unfissured impermeable cap rock is nowhere to be found. The
geochemical processes associated with geothermal fields, i.e., hydrothermal alteration of rocks and
mineral deposition, are helpful in sealing off the fissures.
Typical examples of cap rocks rendered impermeable through chemical action and deposition are
seen at The Geysers and Otake geothermal fields. At The Geysers, calcite- and silica-filled fractures,
up to 1 in. wide, are commonly seen. Evidence of hydrothermal alteration is presented by the
bleaching of graywacke as well as by the absence of vegetation in patches. The geochemical and
hydrothermal processes are complicated and vary from place to place.
At many other steam-producing fields, original impervious rocks constitute the cap rock. Examples
are the lacustrine Huka Formation at Wairakei (New Zealand), the deltaic clay at Cerro Prieto
(Mexico) and Salton Sea (California) and the Flysch Formation at Larderello (Italy).
Key components of Vapor-dominated geothermal systems
1. Reservoir: The core of the vapor-dominated geothermal system is the high-temperature
reservoir, usually consisting of fractured rocks or porous formations containing hot water
and steam.
2. Production Wells: Wells are drilled to tap into the reservoir and allow the extraction of
steam, which is subsequently used to drive turbines for electricity generation.
3. Steam Separation and Power Generation Units: Steam from the production wells is
separated from any liquid water and directed to turbines, where the high-pressure steam
drives the turbines to generate electricity.
4. Environmental Considerations: Vapor-dominated geothermal systems have the potential to
produce clean and sustainable energy, but they also require careful environmental
considerations. Proper management of geothermal fluids, including reinjection of used
fluids, is critical to maintaining the long-term sustainability of these systems.
Fig. 4.1. Conceptual model of a steam geothermal system.
Challenges and Limitations:
1. Resource Sustainability: Sustainable management of the reservoir is essential to prevent
resource depletion and ensure the long-term viability of the geothermal power plant.
2. Corrosion and Scaling: The high-temperature and corrosive nature of geothermal fluids pose
challenges for the materials used in the power plant infrastructure, leading to corrosion and
scaling issues that require continuous maintenance.
3. Exploration and Drilling Risks: Exploration and drilling in geologically complex areas can be
challenging and expensive, with potential risks such as encountering low-permeability
formations or encountering unexpected geological barriers.
Future Prospects:
The role of vapor-dominated geothermal systems in the global energy transition is expected to
become increasingly important. Continuous advancements in drilling technologies, materials
science, and reservoir management techniques are expected to increase the efficiency and costeffectiveness of geothermal energy utilization. Furthermore, increased research and development
investments in enhanced geothermal systems (EGS) hold the promise of expanding geothermal
energy utilization's geographic reach beyond traditional volcanic regions.
Conclusion:
Vapor-dominated geothermal systems are an important renewable energy source that has the
potential to contribute to global energy sustainability. While challenges such as resource
sustainability and operational complexities remain, ongoing technological advancements and
increased research efforts are expected to pave the way for vapor-dominated geothermal systems to
be adopted and integrated into the global energy mix. Careful planning, long-term management, and
ongoing innovation will be required to fully realize the potential of this valuable and
environmentally friendly energy source.
Liquid-dominated Geothermal Systems
Introduction:
A hot water (liquid) dominated system works by utilizing a heat source to raise the temperature of
water and then distributing the hot water to various points of use or storage. In hot water
geothermal fields, water-convection currents carry the heat from the deep source to the shallow
reservoir. The bottom of the convective cell may be heated through conduction from hot rocks.
The geology of hot water geothermal fields is quite similar to that of an ordinary groundwater
system. They differ from the earlier discussed vapor-dominated geothermal fields in the fact that
the hot water geothermal fields are characterized by liquid water being the continuous pressurecontrolling fluid phase.
Typically, the temperature of hot-water reservoirs varies from 60 to 100˚C and they occur at depths
ranging from 1500 to 3000. A hot water geothermal field could develop in the absence of a cap rock,
if the thermal gradients and the depth of the aquifer are adequate to maintain a convective
circulation. When the cap rock is absent, the temperatures in the upper part of the reservoir cannot
exceed the boiling point at atmospheric pressure, since with the convective rise, the water loses
pressure and also becomes mixed with the cool groundwater.
Depending upon the temperature, chemistry and the structure of the reservoir, hot-water systems
have been classified into several subtypes (White, 1974). The following is a brief description of
various subtypes:
1. Systems characterized by low-to-moderate temperatures (say, 50–150˚C) and producing water
with a chemical composition similar to the regional surface and shallow groundwaters.
2. Systems characterized by the presence of partly non-meteoric water. Such systems usually occur
in deep sedimentary basins.
3. Systems characterized by the presence of brine of very high salinity. The chemistry can vary
considerably from one field to another. The Salton Sea (California) and the Red Sea brine pools
belong to this subtype and have very differing bulk chemistry of the sediments and the associated
rocks, probably attributable to the difference in brine composition.
4. Systems characterized by the presence of natural cap rocks. Geothermal fields at Cerro Prieto
(Mexico) and Salton Sea (California) have cap rocks constituted by fine-grained, low-permeability
sediments.
5. Systems characterized by the creation of their own self-sealing cap rocks. As explained earlier in
discussing vapor-dominated geothermal systems, these cap rocks are formed through chemical
alteration and deposition of sediments near the surface where the temperature decreases suddenly.
Wairakei (New Zealand) and Yellowstone Park (Wyoming) are typical examples.
Fig. 1. Conceptual model of a hot water geothermal system.
Here is a general overview of how such a system works:
Heat Generation:
The system begins with a heat source, which can be a boiler, water heater, geothermal system, or
any other device that generates heat. This heat source is responsible for heating the water to the
desired temperature.
Water Circulation:
The heated water is then circulated through a network of pipes and valves. This circulation can be
achieved through pumps or natural convection, depending on the design of the system. The pipes
are typically insulated to minimize heat loss during transportation.
Distribution:
The hot water is distributed to different points of use or storage locations. This can include faucets,
showers, radiators, underfloor heating systems, or any other application that requires hot water.
Valves and controls are used to regulate the flow and temperature of the hot water at each point of
use.
Utilization:
At the points of use, the hot water is utilized for its intended purposes. For example, in domestic
settings, it can be used for bathing, washing dishes, or laundry. In heating systems, it is used to
provide warmth to the indoor space. In industrial processes, it may be used for cleaning,
sterilization, or as a heat source for various applications.
Return and Recirculation:
After the hot water has been used, it may be returned to the system for re-heating or recirculation.
This can be achieved through a return pipe or a recirculation loop, which brings the cooled water
back to the heat source for reheating or to maintain a constant temperature in the system.
Safety and Control:
The system includes safety measures such as pressure relief valves, temperature sensors, and
control devices to ensure safe operation and prevent any potential hazards. These safety features
help maintain the system within safe limits and protect against excessive pressure or temperature.
Overall, a hot water dominated system works by generating heat, circulating the hot water through
a distribution network, utilizing it at various points of use, and ensuring safety and control
throughout the process. The specific design and components of the system may vary depending on
the application and requirements.
In a hot water (liquid) dominated system, the key elements and basics include:
1. Heat Source: This is the component that provides the energy to heat the water. It can be a boiler, a
water heater, a geothermal system, or any other device that generates heat.
2. Water Supply: A constant supply of water is required for the system to function. This can be from
a municipal water source or a well.
3. Distribution System: The hot water needs to be distributed throughout the building or facility.
This is typically done through a network of pipes and valves.
4. Controls: The system requires controls to regulate the temperature of the water and ensure it is
delivered at the desired temperature. This can be achieved through thermostats, pressure
regulators, and other control devices.
5. Storage: In some cases, hot water may need to be stored for later use. This can be in the form of a
hot water tank or a storage tank.
6. Safety Measures: Safety measures such as pressure relief valves and temperature sensors are
essential to prevent accidents and ensure the system operates within safe limits.
7. Insulation: Proper insulation of pipes and storage tanks is important to minimize heat loss and
improve energy efficiency.
8. Maintenance: Regular maintenance and inspections are necessary to ensure the system operates
efficiently and to identify any potential issues or repairs needed. These are the basic elements of a
hot water (liquid) dominated system. The specific design and components may vary depending on
the application and requirements of the system.
Hot water has a wide range of uses in various systems and applications. Some of the key elements
and basics of using hot water in a liquid-dominated system include:
1. Domestic Use: Hot water is commonly used for domestic purposes such as bathing, washing
dishes, and laundry. It provides comfort and cleanliness in our daily lives.
2. Heating Systems: Hot water is a primary component in central heating systems. It is circulated
through radiators, baseboard heaters, or underfloor heating systems to provide warmth and
maintain a comfortable indoor temperature.
3. Industrial Processes: Hot water is extensively used in industrial processes such as manufacturing,
food processing, and chemical production. It is used for cleaning, sterilization, and as a heat source
in various industrial applications.
4. Power Generation: Hot water is utilized in power plants for generating electricity. In a liquiddominated geothermal system, hot water is extracted from underground reservoirs and used to
drive turbines, producing clean and renewable energy.
5. Greenhouses and Agriculture: Hot water is used in greenhouse heating systems to maintain
optimal temperatures for plant growth. It can also be used for irrigation purposes, especially in
colder climates, to prevent frost damage.
6. Spa and Wellness: Hot water is a key component in spa treatments, hydrotherapy, and relaxation
therapies. It helps in relieving muscle tension, promoting blood circulation, and providing a
soothing experience.
7. Cleaning and Sanitization: Hot water is an effective tool for cleaning and sanitizing various
surfaces, equipment, and utensils. Its high temperature helps to kill bacteria, viruses, and other
pathogens, ensuring proper hygiene. It is important to note that the uses of hot water in a liquiddominated system can vary depending on the specific requirements and applications in different
industries and settings.
Differences of Vapor-dominated systems and Liquid-dominated systems
The vapor-dominated reservoir has a specific volume of fluid (i.e. combined vapor and liquid)
greater than the critical volume of water, whereas the liquid-dominated reservoir has a lower
specific volume. A vapor-dominated reservoir is not necessarily a superheated steam reservoir; it
can have both vapor and liquid. The PVT characteristics of geothermal fluid are considered as of
pure water.
Thermodynamically, both vapor and liquid may exist in a vapor-dominated geothermal system.
Thus, the combined specific volume (of vapor and liquid) of the fluid in the reservoir becomes a
fundamental parameter to define the tendency of vapor or liquid domination. A simplified twophase flow approach is applied to calculate the combined specific volume of the fluid in the
reservoir
There is no unanimous definition of vapor- and liquid-dominated geothermal reservoirs. If
superheated steam reservoirs are vapor-dominated, then compressed liquid systems will be liquiddominated.
On the other hand, if both phases can be present together in the vapor- and liquid- dominated
reservoirs, then the proportion of the phases becomes important to distinguish between vapor- and
liquid-dominated geothermal reservoirs. Suppose a 1 l container having 400 g of total water as
vapor and liquid. The specific volume of the fluid is 2.5 cm3/g. Let the initial temperature (T2) be
25°C, as for the point J on the isotherm T2. If we heat the container, the volume of liquid will
continuously increase and the container will be filled with liquid at ~373°C (see steam table from
Henley et al., 1984). The heating process is represented by the path from J,K,L to P. On further
heating the process can be described by the constant specific volume (2.5 cm3/g) path PVí in the
liquid phase region. The slope of this path will depend on the compressibility and expansivity
properties of water.
Now, if the container has only 200 g of total water (i.e. combined vapor and liquid), the specific
volume of the fluid will be 5 cm3/g. If we heat the container, the container will be filled with vapor
at ~370°C. The heating process can be represented by the path from point J,K,L to point N in the PT
diagram. On further heating the process may be described by the constant specific volume (5
cm3/g) path PV in the vapor region.
In the third situation, when the specific volume of the fluid is equal to the critical volume of water
(3.16 cm3/g), the heating process can be described by the path J,K,L through C to Vc. There will be
no distinction between vapor and liquid beyond the critical point C.
In conclusion, the combined specific volume (vapor and liquid) of the fluid in a container can be
either larger, equal or less than the critical volume of the fluid. If the specific volume is larger than
the critical volume, then on heating all the liquid will convert to vapor. If the specific volume is less
than the critical volume, there will be only liquid in the container on heating. When the specific
volume is equal to the critical volume of the fluid, the heating path will be from point J,K,L to C and
to the constant volume (Vc) dashed path; but there will be no distinction between liquid and vapor.
Thus, a geothermal system can be classified as vapor- or liquid-dominated depending on whether
the specific volume of the fluid in the reservoir is smaller or greater than the critical volume,
respectively. If the specific volume of the reservoir fluid is smaller than the critical volume, all the
fluid will convert to liquid as it gets heated in the reservoir, and vice versa. Yet both types of
reservoirs can produce vapor only at the wellhead, depending on pressure and temperature
conditions of production and in the reservoir. It is not correct to define the type of geothermal
reservoir by the characteristics of the geothermal fluid at the wellhead. Thus, it is necessary to
calculate the deep reservoir fluid specific volume from the fluid characteristics at the wellhead to
classify the geothermal system.
References:
Hudson, R.B., Dickson, M.H., Fanelli, M. (Eds.) 1995. Electricity generation, Geothermal Energy,
Wiley, Chichester, pp. 39-72.
Gupta, H, and Roy, S., 2007. Geothermal Energy: An Alternative Resource for the 21st Century.
Oktoberiman, D.A., Ramadhan, P., Fajar, R.W., and Rizal., T.A. 2015. Identification of Geothermal
Potential Based on Fault Fracture Density (FFD), Geological Mapping and Geochemical Analysis. doi:
10.18502/ken.v2i2.369