🌍Geophysics Unit 6 – Heat Flow and Geothermal Energy

Key Concepts and Definitions

• Heat flow quantifies the transfer of thermal energy from the Earth's interior to the surface
• Geothermal energy harnesses heat from the Earth's interior for various applications (electricity generation, heating)
• Conduction, convection, and radiation are the three primary mechanisms of heat transfer
  • Conduction involves the transfer of heat through direct contact between particles
  • Convection involves the transfer of heat through the movement of fluids or gases
  • Radiation involves the transfer of heat through electromagnetic waves
• Geothermal gradient describes the rate of temperature increase with depth in the Earth's crust
• Heat flux measures the amount of heat flowing through a unit area per unit time, typically expressed in mW/m²
• Geothermal reservoirs are subsurface areas containing hot water or steam that can be extracted for energy production
• Enhanced Geothermal Systems (EGS) involve creating artificial geothermal reservoirs by fracturing hot dry rock

Fundamentals of Heat Transfer

• Fourier's law of heat conduction states that the rate of heat transfer through a material is proportional to the negative temperature gradient and the area perpendicular to the gradient
  • The equation for Fourier's law is $q = -k \frac{dT}{dx}$, where $q$ is heat flux, $k$ is thermal conductivity, and $\frac{dT}{dx}$ is the temperature gradient
• Thermal conductivity is a material property that determines its ability to conduct heat
  • Rocks with higher thermal conductivity (quartzite, granite) allow heat to flow more easily compared to those with lower thermal conductivity (shale, limestone)
• Convective heat transfer occurs due to the movement of fluids or gases, driven by density differences caused by temperature variations
  • Mantle convection plays a crucial role in transferring heat from the Earth's interior to the surface
• Radiative heat transfer involves the emission and absorption of electromagnetic waves
  • The Earth's surface emits infrared radiation, which contributes to the overall heat balance of the planet

Earth's Internal Heat Structure

• The Earth's interior is divided into layers: crust, mantle, outer core, and inner core
  • The crust is the outermost layer, with an average thickness of 30-50 km in continental regions and 5-10 km in oceanic regions
  • The mantle extends from the base of the crust to a depth of about 2,900 km and is primarily composed of silicate rocks
  • The outer core is a liquid layer rich in iron and nickel, extending from 2,900 km to 5,150 km depth
  • The inner core is a solid layer, primarily composed of iron and nickel, with a radius of about 1,220 km
• The Earth's internal heat is generated by two primary sources: primordial heat and radiogenic heat
  • Primordial heat is the residual heat from the formation of the Earth, which has been gradually cooling over billions of years
  • Radiogenic heat is produced by the decay of radioactive isotopes (uranium, thorium, potassium) within the Earth's interior
• The mantle is the largest reservoir of heat within the Earth, accounting for approximately 80% of the total heat content
• Mantle plumes are localized upwellings of hot material from the deep mantle, which can lead to increased heat flow and volcanic activity at the surface (Hawaii, Iceland)

Geothermal Gradients and Heat Flow

• The geothermal gradient varies depending on the geological setting and tectonic environment
  • Continental regions typically have geothermal gradients ranging from 20 to 30°C/km
  • Oceanic regions have higher geothermal gradients, often exceeding 100°C/km near mid-ocean ridges
• Factors influencing the geothermal gradient include thermal conductivity of rocks, heat production from radioactive decay, and convective heat transfer
• Heat flow measurements provide insights into the thermal structure and dynamics of the Earth's interior
  • Heat flow is typically higher in tectonically active regions (subduction zones, rift valleys) compared to stable continental regions
• The global average heat flow is approximately 87 mW/m², but local values can vary significantly depending on the geological setting
• Geothermal maps depict the spatial distribution of heat flow and geothermal gradients, helping to identify potential geothermal resources

Geothermal Energy Systems

• Geothermal energy systems harness heat from the Earth's interior for various applications, including electricity generation and direct heating
• Hydrothermal systems are the most common type of geothermal energy system, relying on naturally occurring hot water or steam reservoirs
  • High-temperature hydrothermal systems (>150°C) are suitable for electricity generation, while low-temperature systems (<150°C) are used for direct heating applications
• Enhanced Geothermal Systems (EGS) involve creating artificial geothermal reservoirs by fracturing hot dry rock and circulating fluid through the fractures to extract heat
  • EGS has the potential to significantly expand the geographic range of geothermal energy production
• Geothermal power plants convert heat from geothermal fluids into electricity using various technologies (dry steam, flash steam, binary cycle)
  • Dry steam plants directly use steam from the geothermal reservoir to drive turbines (Larderello, Italy)
  • Flash steam plants use high-temperature water that is flashed into steam to drive turbines (Reykjanes, Iceland)
  • Binary cycle plants use a secondary working fluid with a lower boiling point to generate electricity from lower-temperature geothermal fluids (Raft River, USA)
• Direct use applications of geothermal energy include space heating, greenhouse heating, aquaculture, and industrial processes

Exploration and Extraction Techniques

• Geothermal exploration involves identifying and characterizing potential geothermal resources using various techniques
• Geological mapping and structural analysis help identify favorable geological settings for geothermal systems (fault zones, volcanic areas)
• Geophysical methods, such as seismic surveys, gravity surveys, and electrical resistivity surveys, provide insights into subsurface structures and fluid pathways
• Geochemical analysis of hot springs and fumaroles can indicate the temperature and composition of geothermal fluids at depth
• Temperature gradient drilling involves measuring the temperature at various depths in exploratory wells to assess the geothermal potential of an area
• Well logging techniques (temperature logs, pressure logs, flow logs) provide detailed information about the properties of geothermal reservoirs
• Geothermal extraction involves drilling production wells to access and produce hot water or steam from the reservoir
  • Directional drilling techniques allow for the precise targeting of geothermal resources and the creation of multiple production zones from a single well pad
• Reinjection of cooled geothermal fluids back into the reservoir helps to maintain pressure and prolong the productive life of the geothermal system

Applications and Environmental Impact

• Geothermal energy offers a renewable and environmentally friendly alternative to fossil fuels for electricity generation and heating
• Geothermal power plants have a low carbon footprint and emit minimal greenhouse gases compared to coal or natural gas-fired plants
• Geothermal energy provides a reliable and consistent baseload power source, as it is not dependent on weather conditions like solar or wind energy
• Geothermal district heating systems can efficiently provide space heating and hot water to communities located near geothermal resources (Reykjavik, Iceland)
• Geothermal heat pumps use the stable temperature of the shallow subsurface to provide efficient heating and cooling for buildings
• Geothermal energy development can have local environmental impacts, such as land subsidence, induced seismicity, and changes in groundwater chemistry
  • Proper management practices, such as careful site selection, monitoring, and reinjection of geothermal fluids, can help mitigate these impacts
• Geothermal energy can contribute to energy security and reduce dependence on imported fossil fuels in countries with significant geothermal resources (Kenya, Philippines)

Case Studies and Real-World Examples

• The Geysers, California, USA: The largest geothermal field in the world, producing over 900 MW of electricity from dry steam resources
• Larderello, Italy: The first geothermal power plant in the world, operating since 1911 and currently producing over 700 MW of electricity
• Reykjanes, Iceland: A high-temperature geothermal field that supplies electricity and hot water to the Reykjavik area, demonstrating the successful integration of geothermal energy into a modern energy system
• Raft River, Idaho, USA: A binary cycle geothermal power plant that utilizes low-temperature geothermal fluids (150°C) to generate electricity, showcasing the potential for geothermal energy production in areas with lower-temperature resources
• Krafla, Iceland: A geothermal power plant located in a volcanic area, which has experienced challenges related to magmatic intrusions and changes in reservoir pressure, highlighting the need for adaptive management strategies in dynamic geothermal systems
• Hellisheiði, Iceland: A combined heat and power plant that produces electricity and supplies hot water for district heating in Reykjavik, demonstrating the efficient cascading use of geothermal resources
• Soultz-sous-Forêts, France: An Enhanced Geothermal System (EGS) project that has successfully created an artificial geothermal reservoir in hot dry rock, paving the way for the development of EGS technology in other locations
• Menengai, Kenya: A geothermal field under development in the East African Rift Valley, which has the potential to significantly contribute to Kenya's electricity supply and support economic growth in the region