🌋Geothermal Systems Engineering Unit 1 – Geothermal Energy Basics

What is Geothermal Energy?

• Energy derived from the Earth's internal heat, which originates from the formation of the planet and the decay of radioactive elements
• Heat is continuously produced inside the Earth's core, mantle, and crust
• Temperature increases with depth, averaging a gradient of about 25-30°C/km (72-87°F/mi)
• Geothermal resources are reservoirs of hot water, steam, or hot dry rock located at depths shallow enough to be tapped for energy production
  • These reservoirs are replenished by water seeping through cracks and pores in the rock
• Geothermal energy is considered a renewable resource because the heat extraction rate is small compared to the Earth's heat content
• Can be used for electricity generation, space heating, industrial processes, and other applications
• Provides a reliable, clean, and sustainable energy source with a small environmental footprint compared to fossil fuels

How Geothermal Systems Work

• Geothermal systems harness heat from the Earth's interior to generate electricity or provide direct heating
• Three main components: heat source, reservoir, and fluid
  • Heat source is the hot rock or magma deep within the Earth
  • Reservoir is a permeable rock formation containing hot water or steam
  • Fluid (water or steam) transfers the heat from the reservoir to the surface
• Fluid is brought to the surface through production wells drilled into the reservoir
• At the surface, the fluid's thermal energy is converted into electricity using a turbine or used directly for heating
• Cooled fluid is then reinjected back into the reservoir through injection wells to maintain pressure and sustain the system
• Two main types of geothermal systems:
  • Hydrothermal systems: naturally occurring hot water or steam reservoirs
  • Enhanced Geothermal Systems (EGS): artificially created reservoirs in hot dry rock formations
• Geothermal systems can be open-loop (fluid is released after use) or closed-loop (fluid is continuously recirculated)

Types of Geothermal Resources

• Hydrothermal resources: naturally occurring reservoirs of steam or hot water
  • Convective hydrothermal resources: characterized by the circulation of water driven by differences in temperature and pressure
  • Conductive hydrothermal resources: heat is transferred through rocks without significant fluid movement
• Geopressured resources: deep, high-pressure reservoirs containing hot water with dissolved methane
• Magma resources: molten rock or partially molten rock at accessible depths
• Hot Dry Rock (HDR) resources: high-temperature rocks without significant fluid content or permeability
  • Enhanced Geothermal Systems (EGS) are created by fracturing HDR resources and circulating fluid through the fractures
• Sedimentary basin resources: permeable sedimentary rocks at depths with high enough temperatures for direct use applications
• Radiogenic resources: heat generated by the decay of radioactive elements in granite or other crystalline rocks

Geothermal Power Plants

• Convert geothermal energy into electricity using steam or binary cycle power plants
• Dry steam power plants: use steam directly from the geothermal reservoir to drive a turbine
  • The Geysers in California is the world's largest dry steam field
• Flash steam power plants: high-temperature water is pumped under pressure to the surface, where it flashes into steam to drive a turbine
  • Separated water can be flashed again at lower pressures to generate additional electricity (double or triple flash)
• Binary cycle power plants: use a secondary working fluid with a lower boiling point (e.g., pentane or butane) to drive a turbine
  • Geothermal fluid heats the working fluid through a heat exchanger, causing it to vaporize and drive the turbine
  • Suitable for lower-temperature geothermal resources (74-180°C or 165-356°F)
• Combined cycle power plants: integrate a flash steam plant with a binary cycle plant to improve efficiency
• Hybrid power plants: combine geothermal with other renewable energy sources (e.g., solar, biomass) or fossil fuels

Direct Use Applications

• Use geothermal heat directly for various purposes without converting it to electricity
• Space heating and cooling: use geothermal heat pumps to transfer heat between buildings and the shallow subsurface
  • Can provide both heating in winter and cooling in summer
• District heating: distribute geothermal heat to multiple buildings through a network of insulated pipes
• Agricultural applications: greenhouse heating, soil warming, crop drying, and aquaculture
• Industrial processes: food dehydration, pasteurization, chemical extraction, and drying
• Balneology and tourism: hot springs, spas, and swimming pools
• Snow melting and de-icing: keep pavements, bridges, and runways clear of snow and ice
• Desalination: use geothermal heat to distill freshwater from saline water sources

Environmental Impacts

• Geothermal energy has lower environmental impacts compared to fossil fuels but still poses some challenges
• Greenhouse gas emissions: geothermal fluids can contain dissolved gases (e.g., CO2, H2S, methane) that are released during power production
  • Emissions are generally lower than those from fossil fuel plants
• Water use: geothermal power plants require water for cooling and reservoir recharge
  • Water consumption can be reduced by using air-cooled systems or binary cycle plants
• Land use: geothermal facilities have a smaller land footprint compared to other energy sources
  • Directional drilling can minimize surface disturbance
• Induced seismicity: fluid injection and extraction can cause microseismic events
  • Proper monitoring and management can mitigate seismic risks
• Thermal pollution: geothermal effluents can impact surface and groundwater temperatures if not properly managed
• Noise pollution: drilling and power plant operations can produce noise, which can be mitigated through various techniques
• Visual impact: steam plumes and power plant structures can affect the visual landscape

Geothermal Energy Economics

• Geothermal projects have high upfront costs for exploration, drilling, and plant construction but low operational costs
• Levelized cost of electricity (LCOE) for geothermal is competitive with other renewable energy sources and fossil fuels
  • LCOE depends on factors such as resource temperature, well productivity, and power plant technology
• Geothermal power plants have high capacity factors (typically >90%), providing reliable baseload electricity
• Direct use applications can offer significant energy cost savings compared to conventional heating and cooling systems
• Economic benefits include job creation, energy security, and reduced exposure to fossil fuel price volatility
• Government incentives and policies (e.g., tax credits, feed-in tariffs, renewable portfolio standards) can support geothermal development
• Technological advancements (e.g., advanced drilling techniques, improved reservoir characterization) can reduce costs and improve project economics

Future of Geothermal Technology

• Enhanced Geothermal Systems (EGS) have the potential to greatly expand the geographic range and scale of geothermal energy production
  • EGS involves creating artificial reservoirs in hot dry rock formations by hydraulic stimulation
  • Challenges include improving reservoir characterization, reducing drilling costs, and managing induced seismicity
• Advanced drilling technologies (e.g., laser drilling, plasma drilling) could enable deeper and more cost-effective access to geothermal resources
• Hybrid systems combining geothermal with other renewable energy sources (e.g., solar, biomass) can improve efficiency and dispatchability
• Supercritical geothermal systems, which tap into high-temperature (>374°C) and high-pressure (>22 MPa) resources, could significantly increase power output per well
• Closed-loop geothermal systems, where a working fluid is circulated through a sealed wellbore, can reduce water consumption and environmental impacts
• Geothermal energy storage, using the subsurface as a thermal battery, can help balance intermittent renewable energy sources and meet peak demand
• Integration of geothermal with other sectors (e.g., desalination, hydrogen production, mineral extraction) can create synergies and improve project economics
• Continued research and development in reservoir characterization, power plant design, and direct use applications will drive innovation and deployment of geothermal technologies