8.5 Geothermal energy

Geothermal energy taps into Earth's internal heat to generate power and provide heating. This renewable resource offers a low-carbon alternative to fossil fuels, harnessing heat from underground reservoirs formed by water seeping into hot rock layers.

Geothermal systems vary in temperature and type, from high-temperature electricity generation to low-temperature direct heating applications. Exploration, drilling, and energy conversion technologies are key to harnessing this sustainable energy source effectively.

Geothermal energy basics

• Geothermal energy harnesses heat from within the Earth's crust to generate electricity or provide direct heating
• Geothermal resources are renewable and sustainable, as heat is continuously generated by radioactive decay and residual heat from Earth's formation
• Geothermal energy has a low carbon footprint compared to fossil fuels and can provide baseload power

Formation of geothermal reservoirs

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• Geothermal reservoirs form when water seeps into hot, permeable rock layers deep underground
• Heat source is typically a magma chamber or hot rock layer, which heats the water and creates a pressurized reservoir
• Reservoirs are often found in areas with active or recent volcanic activity, tectonic plate boundaries, or thin crust

Types of geothermal systems

• Hydrothermal systems: naturally occurring hot water or steam reservoirs (most common)
• Geopressured systems: deep, pressurized reservoirs containing hot water with dissolved methane
• Hot dry rock systems: lack water but can be fractured to create an artificial reservoir
• Magma systems: molten rock as a direct heat source (experimental)

High vs low temperature resources

• High temperature resources (>150°C) are suitable for electricity generation
• Low temperature resources (<150°C) are used for direct heating applications (space heating, greenhouses, aquaculture)
• Temperature determines the type of power plant and energy conversion technology used

Harnessing geothermal energy

• Geothermal energy is extracted by drilling wells into the reservoir and pumping hot water or steam to the surface
• Extracted fluid is used to generate electricity or provide direct heating before being reinjected into the reservoir
• Proper management ensures the sustainability and longevity of geothermal resources

Exploration and site selection

• Geological, geophysical, and geochemical surveys identify potential geothermal sites
• Surveys assess reservoir temperature, depth, permeability, and fluid chemistry
• Promising sites undergo exploratory drilling to confirm resource quality and quantity

Well drilling and completion

• Production wells are drilled into the reservoir to extract hot water or steam
• Injection wells are drilled to reinject cooled fluid back into the reservoir
• Wells are lined with steel casings and cemented to prevent contamination and maintain well integrity

Energy conversion technologies

• Flash steam: high-temperature water is flashed into steam to drive a turbine
• Dry steam: steam is directly used to drive a turbine
• Binary cycle: hot water heats a secondary working fluid with a lower boiling point to drive a turbine
• Direct use: heat exchangers transfer heat from geothermal fluid to a secondary fluid for heating applications

Direct use vs electricity generation

• Direct use applications include space heating, greenhouses, aquaculture, and industrial processes
• Electricity generation involves converting geothermal energy into electricity using a power plant
• Direct use is more efficient but requires the end-user to be located near the geothermal site
• Electricity can be transported over long distances to reach consumers

Geothermal power plants

• Geothermal power plants convert heat energy from geothermal fluid into electricity
• Plant design depends on the temperature, pressure, and chemistry of the geothermal resource
• Geothermal plants have a high capacity factor and can provide baseload power

Flash steam plants

• Used for high-temperature (>180°C) liquid-dominated reservoirs
• Hot water is pumped to the surface and flashed into steam in a separator
• Steam drives a turbine to generate electricity, and remaining water is reinjected

Dry steam plants

• Used for vapor-dominated reservoirs where steam is directly extracted
• Steam is piped from production wells to drive a turbine, then condensed and reinjected
• The Geysers in California is the largest dry steam field in the world

Binary cycle plants

• Used for low to moderate temperature (100-180°C) resources
• Hot water heats a secondary working fluid (e.g., isobutane) with a lower boiling point in a heat exchanger
• The working fluid vaporizes and drives a turbine, then is condensed and reused in a closed loop

Enhanced geothermal systems (EGS)

• Artificially created reservoirs in hot, dry rock formations
• Wells are drilled and water is injected to create fractures and improve permeability
• The injected water is heated by the rock and extracted to generate electricity
• EGS can significantly expand geothermal potential but is still in the development stage

Environmental impacts

• Geothermal energy is a clean and renewable resource with lower environmental impacts compared to fossil fuels
• Proper management and monitoring are essential to minimize potential negative impacts
• Environmental regulations and best practices guide geothermal development and operation

Greenhouse gas emissions

• Geothermal plants emit lower levels of greenhouse gases compared to fossil fuel plants
• Some geothermal fluids contain dissolved gases (CO2, H2S) that are released during operation
• Emissions can be mitigated through abatement systems and reinjection of gases

Water use and pollution

• Geothermal plants require water for drilling, cooling, and reservoir recharge
• Water consumption is lower than other thermal power plants and can be minimized through recycling and efficient design
• Geothermal fluids may contain dissolved minerals and chemicals that can contaminate surface or groundwater if not properly managed

Land use and subsidence

• Geothermal plants have a smaller land footprint compared to other renewable energy sources
• Subsidence (gradual sinking of land) can occur due to fluid withdrawal and pressure changes in the reservoir
• Subsidence is monitored and managed through proper reinjection and reservoir management practices

Induced seismicity risks

• Geothermal drilling and fluid injection can potentially trigger small earthquakes
• Induced seismicity is closely monitored and managed through proper site selection, injection rates, and pressure control
• The risk of induced seismicity is lower than other human activities (e.g., oil and gas extraction, wastewater injection)

Economic considerations

• Geothermal energy is a cost-competitive renewable energy source with potential for long-term economic benefits
• Geothermal projects have high upfront costs but low operational costs and long lifetimes
• Economic viability depends on resource quality, location, and market conditions

Cost of geothermal energy

• Geothermal power plants have high capital costs for exploration, drilling, and construction
• Operational costs are relatively low due to the lack of fuel costs and low maintenance requirements
• Levelized cost of electricity (LCOE) for geothermal is competitive with other baseload power sources

Comparison to other renewables

• Geothermal provides baseload power, unlike intermittent sources like solar and wind
• Geothermal has a higher capacity factor (>90%) compared to solar (20-30%) and wind (30-40%)
• Geothermal plants have a smaller land footprint and lower visual impact compared to solar and wind farms

Incentives and policies

• Government incentives (tax credits, grants, feed-in tariffs) support geothermal development
• Renewable portfolio standards (RPS) and carbon pricing mechanisms encourage geothermal adoption
• International cooperation and technology transfer promote global geothermal growth

Barriers to development

• High upfront costs and financial risks associated with exploration and drilling
• Limited geothermal resources in some regions and difficulty in identifying viable sites
• Lack of awareness and understanding of geothermal energy among policymakers and the public
• Regulatory and permitting challenges, particularly for cross-border projects

Global geothermal resources

• Geothermal resources are distributed across the world, with the highest potential in regions with active tectonic and volcanic activity
• Geothermal energy is being harnessed in over 20 countries, with significant untapped potential in many regions
• Technological advancements and increased investment are driving the growth of the global geothermal industry

Leading geothermal countries

• United States: the world's largest geothermal electricity producer, with a capacity of over 3.7 GW
• Indonesia: the second-largest producer, with a geothermal capacity of over 2.1 GW
• Philippines: the third-largest producer, with a geothermal capacity of over 1.9 GW
• Other notable countries include Turkey, New Zealand, Mexico, Italy, and Iceland

Untapped geothermal potential

• East African Rift System: a promising region with high geothermal potential, particularly in Kenya and Ethiopia
• South America: significant untapped resources in countries like Chile, Argentina, and Bolivia
• Southeast Asia: vast geothermal potential in countries like Indonesia, Philippines, and Japan
• Europe: untapped resources in countries like Hungary, Croatia, and Serbia

Future growth projections

• Global geothermal capacity is expected to reach 18.4 GW by 2030, a significant increase from the current 15.4 GW
• The geothermal heat pump market is projected to grow at a CAGR of 8.2% from 2020 to 2027
• Increased adoption of geothermal energy in developing countries and the development of EGS technology will drive future growth

Advantages and challenges

• Geothermal energy offers several advantages as a clean, renewable, and reliable energy source
• However, geothermal development also faces challenges that need to be addressed to realize its full potential
• Overcoming these challenges requires technological innovation, policy support, and increased public awareness

Reliability and consistency

• Geothermal energy provides consistent, baseload power, independent of weather conditions or time of day
• Geothermal plants have high capacity factors (>90%) and long operational lifetimes (30-50 years)
• Geothermal resources are less subject to seasonal or annual fluctuations compared to hydro or biomass

Scalability and flexibility

• Geothermal plants can be built in a modular fashion, allowing for incremental capacity additions
• Geothermal energy can be used for both electricity generation and direct heating applications
• Geothermal resources can be cascaded to maximize efficiency, using high-temperature resources for electricity and lower-temperature resources for heating

Environmental sustainability

• Geothermal energy has a low carbon footprint and minimal air pollutant emissions
• Geothermal plants have a small land footprint and can coexist with other land uses (agriculture, tourism)
• Proper management and monitoring minimize the environmental impacts of geothermal development

Technological and financial hurdles

• Identifying and characterizing geothermal resources requires advanced exploration techniques and data analysis
• Drilling deep, high-temperature wells is technologically challenging and expensive
• High upfront costs and financial risks can deter investors and hinder geothermal development
• Continued research and development are needed to improve the efficiency and cost-effectiveness of geothermal technologies