4.4 Geothermal energy exploration and exploitation
5 min read•august 14, 2024
Geothermal energy taps into Earth's heat for power and heating. It's a key part of understanding our planet's thermal structure. From hot springs to power plants, geothermal systems offer a glimpse into Earth's inner workings.
Exploring and using geothermal energy involves geology, chemistry, and physics. We'll look at how scientists find these hot spots and turn them into clean energy sources. It's a fascinating blend of Earth science and sustainable technology.
Geothermal Systems and Characteristics
Types of Geothermal Systems
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- Geothermal systems are classified based on their geological setting, temperature, and fluid characteristics into three main types: , (EGS), and sedimentary basin geothermal systems
- Hydrothermal systems are the most common and commercially viable type characterized by the presence of naturally occurring hot water or steam in permeable rocks (Larderello field in Italy, The Geysers in California)
- Vapor-dominated systems produce dry steam and are rare but highly productive with temperatures typically exceeding 240°C
- Liquid-dominated systems produce hot water with temperatures ranging from 90°C to 240°C and are exploited for both electricity generation and direct heat applications (Wairakei field in New Zealand, Salton Sea field in California)
- Enhanced Geothermal Systems (EGS) involve the creation of artificial geothermal reservoirs by fracturing hot, dry rock at depths of 3-10 km and circulating fluid through the fractures to extract heat, potentially expanding the geographic range of geothermal energy utilization
- Sedimentary basin geothermal systems are found in deep sedimentary basins with high heat flow where hot water is trapped in permeable sedimentary layers, typically having lower temperatures (30-150°C) compared to hydrothermal systems but can be used for direct heat applications and, in some cases, electricity generation using binary cycle power plants
Characteristics of Geothermal Systems
- Geothermal systems require a heat source, fluid, and permeable pathways to facilitate the circulation of geothermal fluids
- Heat sources can be magmatic intrusions, radioactive decay of minerals, or high geothermal gradients in tectonically active areas
- Geothermal fluids are typically meteoric water that has percolated into the Earth's crust and been heated by the heat source, often containing dissolved minerals and gases (CO2, H2S)
- Permeable pathways, such as faults, fractures, and porous rock layers, allow the heated fluids to circulate and rise towards the surface
- The temperature, pressure, and fluid composition of geothermal systems determine their potential for energy production and the appropriate technology for exploitation
Geothermal Exploration Methods
Geological and Geochemical Methods
- Geological mapping and structural analysis identify favorable geologic settings, such as recent volcanic activity, faults, and fractures that can act as fluid pathways, while remote sensing techniques (satellite imagery, aerial photography) aid in regional-scale mapping and the identification of surface manifestations (hot springs, fumaroles)
- Geochemical surveys analyze the composition and temperature of geothermal fluids (hot springs, fumaroles) and gases (CO2, H2S) to estimate reservoir temperatures and fluid origin, with isotope geochemistry (oxygen and hydrogen isotopes) helping to determine the source and age of the geothermal fluids
Geophysical Surveys
- Geophysical surveys are crucial for characterizing subsurface properties and include a range of methods:
- Gravity surveys identify density variations associated with geothermal reservoirs and can delineate the extent of the reservoir and the presence of faults
- Magnetic surveys detect variations in the Earth's magnetic field caused by the presence of magnetic minerals, which can be altered by geothermal activity
- Electrical and electromagnetic methods, such as (MT) and controlled-source electromagnetics (CSEM), map the electrical conductivity of the subsurface, which is influenced by the presence of geothermal fluids and clay minerals
- Seismic surveys, including reflection and refraction methods, provide information on subsurface structure, stratigraphy, and the presence of faults or fractures that can act as fluid pathways
- Well logging involves the measurement of physical properties (temperature, pressure, electrical conductivity, gamma radiation) in geothermal wells using specialized tools to characterize reservoir properties, such as temperature gradient, permeability, and fluid composition, and to guide further exploration and development decisions
Geothermal Energy Production
Electricity Generation
- Electricity generation from high-temperature (>150°C) geothermal resources typically involves the use of steam turbines in conventional power plants, with vapor-dominated systems using dry steam directly to drive the turbines and liquid-dominated systems using a mixture of hot water and steam, separating the steam to drive the turbines
- Binary cycle power plants are used for lower-temperature (90-150°C) geothermal resources, where the heat from the geothermal fluid is transferred to a secondary working fluid (isobutane or pentane) with a lower boiling point, which vaporizes and drives a turbine before being condensed and reused in a closed loop
Direct Use and Heat Pumps
- Direct use of geothermal energy involves the utilization of low-to-moderate temperature (30-150°C) geothermal resources for heating and cooling applications, such as space heating, greenhouse heating, aquaculture, and industrial processes, using to transfer the heat from the geothermal fluid to the target application
- Geothermal heat pumps (GHPs) use the relatively constant temperature of the shallow subsurface (< 100 m depth) to provide heating and cooling for buildings by circulating a fluid through a closed loop of pipes buried in the ground, extracting heat in the winter and rejecting heat in the summer
Enhanced Geothermal Systems (EGS)
- Enhanced Geothermal Systems (EGS) involve the creation of artificial geothermal reservoirs by fracturing hot, dry rock and circulating a fluid (typically water) through the fractures to extract heat
- EGS technologies are still in the development stage but have the potential to greatly expand the geographic range of geothermal energy utilization by enabling the exploitation of geothermal resources in areas without naturally occurring hydrothermal systems
Environmental and Economic Aspects of Geothermal Energy
Environmental Considerations
- Geothermal energy is considered a renewable and sustainable energy source, as the heat extracted from the Earth is continuously replenished by natural processes, but the rate of replenishment is slow compared to the rate of extraction, so geothermal resources must be carefully managed to ensure long-term sustainability
- Geothermal energy has a low carbon footprint compared to fossil fuels, as it does not involve the combustion of hydrocarbons, but geothermal fluids can contain dissolved gases (CO2, H2S) that can be released into the atmosphere during energy production, which can be mitigated by advanced technologies, such as CO2 capture and storage
- Geothermal energy production can have local environmental impacts, such as due to fluid withdrawal, induced seismicity from fluid injection or reservoir stimulation, and the release of geothermal fluids containing heavy metals or other contaminants, but careful site selection, monitoring, and management practices can help minimize these impacts
Economic Aspects
- The economic viability of geothermal energy projects depends on factors such as the resource temperature, depth, and permeability, as well as the proximity to energy markets and infrastructure
- High upfront costs for exploration and drilling can be a barrier to development, but geothermal energy can provide a stable, baseload power source with low operational costs over the long term
- Geothermal energy can contribute to energy security and diversification, particularly in countries with significant geothermal resources (Iceland, New Zealand), and provide local economic benefits, such as job creation and increased tax revenues
- Policy support, such as feed-in tariffs, tax incentives, and research and development funding, can help promote the growth of the geothermal energy industry and make it more competitive with other energy sources
Key Terms to Review (18)
Dr. Bill McCabe: Dr. Bill McCabe is a prominent figure in the field of geothermal energy, known for his significant contributions to the exploration and exploitation of geothermal resources. His work encompasses various aspects of geothermal systems, including resource assessment, development strategies, and environmental considerations associated with harnessing geothermal energy. Through his research and expertise, he has advanced the understanding of geothermal technologies and their applications in sustainable energy production.
Emissions reduction: Emissions reduction refers to the efforts and strategies implemented to decrease the release of greenhouse gases and other pollutants into the atmosphere. This is critical in mitigating climate change and promoting sustainable practices, especially through energy production methods such as geothermal energy. By harnessing the natural heat from the Earth, geothermal energy systems can significantly lower carbon emissions compared to fossil fuels, leading to a cleaner environment and a more sustainable energy future.
Enhanced geothermal systems: Enhanced geothermal systems (EGS) are engineered reservoirs created to extract geothermal energy from hot, dry rock formations that lack sufficient natural permeability and fluid content. By injecting water into these formations to create fractures, EGS allows for the circulation of fluid that can be heated by the Earth's natural heat and then extracted for energy production. This method significantly expands the potential for geothermal energy exploitation in regions that were previously deemed unsuitable.
Feed-in tariff: A feed-in tariff is a policy mechanism designed to encourage the adoption of renewable energy by guaranteeing fixed payments to producers for the energy they generate and feed into the electrical grid. This system provides financial certainty and stability for investors and developers in renewable energy projects, including geothermal energy, which relies on consistent revenue streams to cover upfront costs and operating expenses.
Geothermal heating: Geothermal heating refers to the use of heat from the Earth's interior for various applications, such as residential heating, industrial processes, and electricity generation. This sustainable energy source harnesses the natural heat stored beneath the Earth's surface, making it an essential component in the exploration and exploitation of geothermal energy. As a renewable resource, geothermal heating contributes to reducing greenhouse gas emissions and reliance on fossil fuels.
Geothermal power plants: Geothermal power plants are facilities that convert heat from the Earth’s interior into electricity by utilizing steam or hot water. They harness the Earth's natural heat, which can be found in reservoirs beneath the surface, and convert it into usable energy, making them a sustainable and renewable source of power.
Geothermal reservoir: A geothermal reservoir is a subsurface area containing hot water or steam that can be harnessed for geothermal energy production. These reservoirs are often found in volcanic regions or tectonically active areas, where heat from the Earth's interior heats groundwater, creating a potential source of renewable energy. The exploration and exploitation of these reservoirs is critical for harnessing geothermal energy for electricity generation and direct heating applications.
Heat exchangers: Heat exchangers are devices designed to efficiently transfer heat from one medium to another, often without mixing them. These devices are essential in many applications, including geothermal energy systems, where they help capture and transfer thermal energy from the Earth to a working fluid, which can then be used for heating or electricity generation. By maximizing the efficiency of heat transfer, heat exchangers play a critical role in optimizing energy use and enhancing the sustainability of geothermal energy exploitation.
Heat extraction: Heat extraction refers to the process of removing heat from geothermal reservoirs to convert it into usable energy. This concept is fundamental in harnessing geothermal energy, where hot water or steam is drawn from the Earth's subsurface to generate electricity or provide direct heating. Effective heat extraction depends on various factors, including the temperature and pressure of the geothermal resource, the technology used, and the methods for maintaining reservoir sustainability.
Hydrothermal systems: Hydrothermal systems are geological formations where heated water circulates through the Earth's crust, usually associated with volcanic activity. These systems play a crucial role in geothermal energy exploitation, as they can produce steam or hot water that can be harnessed for energy production. Additionally, hydrothermal systems can create unique mineral deposits and influence local ecosystems.
Land subsidence: Land subsidence is the gradual sinking or lowering of the Earth's surface, which can occur due to various factors, including the extraction of groundwater, mining activities, or natural geological processes. This phenomenon is particularly relevant in the context of geothermal energy extraction, where the withdrawal of underground fluids can lead to significant surface deformation and impact surrounding ecosystems and infrastructure.
Lyndon B. Johnson: Lyndon B. Johnson was the 36th President of the United States, serving from 1963 to 1969 after the assassination of John F. Kennedy. His administration is known for significant domestic policies, including those that advanced the exploration and exploitation of geothermal energy, reflecting a broader push for energy independence and environmental stewardship during the 1960s.
Magnetotellurics: Magnetotellurics is a geophysical method used to study the Earth's subsurface by measuring the natural variations of electromagnetic fields at the surface. This technique helps reveal information about the Earth's internal structure and composition, particularly in relation to its layers, heat transfer mechanisms, geothermal energy potential, and the characteristics of the magnetic field.
Renewability: Renewability refers to the ability of a resource to be replenished naturally over time, ensuring that it can be sustainably utilized without depleting the supply. In the context of energy, renewability is critical for maintaining long-term energy production while minimizing environmental impact. This concept becomes especially relevant when assessing geothermal energy, as it relies on the Earth's internal heat, which can be continuously harnessed if managed properly.
Renewable portfolio standard: A renewable portfolio standard (RPS) is a regulatory mandate that requires electricity providers to obtain a specific percentage of their energy from renewable sources, such as wind, solar, and geothermal. This requirement aims to increase the generation of renewable energy, reduce greenhouse gas emissions, and promote sustainable energy practices. By setting clear targets, RPS helps stimulate investment in renewable technologies and drives innovation in the energy sector.
Resource Management: Resource management involves the strategic planning and utilization of natural resources to ensure sustainability and efficiency in their use. This concept is crucial when considering how geothermal energy is sourced and maintained, as it directly impacts the environment and economic viability. Effective resource management balances the demand for energy with the preservation of geological formations and ecosystems, making it essential for both geothermal gradients and energy extraction processes.
Seismic surveying: Seismic surveying is a geophysical technique that uses the propagation of seismic waves to investigate subsurface geological structures. It involves sending seismic waves into the ground and analyzing the reflected waves to create images of underground formations, which can be critical for resource exploration and risk assessment. This method plays an essential role in various applications, including energy resource evaluation and monitoring geological hazards.
Well drilling: Well drilling is the process of creating holes in the ground to access underground resources, such as water, oil, or geothermal energy. This technique involves using specialized equipment to penetrate various soil and rock layers, allowing for the extraction or monitoring of these valuable resources. In the context of geothermal energy, well drilling is essential for tapping into the Earth's heat stored beneath the surface, which can be used for electricity generation or direct heating applications.