🌋Geothermal Systems Engineering Unit 5 – Geothermal Power Plant Technologies

Geothermal Basics

• Geothermal energy originates from the Earth's interior heat, which is generated by the decay of radioactive elements and residual heat from planetary formation
• Geothermal resources are classified based on their temperature and enthalpy (heat content), ranging from low-temperature (<90°C) to high-temperature (>150°C) systems
  • Low-temperature resources are suitable for direct use applications (space heating, greenhouses, aquaculture)
  • Medium-temperature resources (90-150°C) can be used for both direct use and electricity generation
  • High-temperature resources are primarily used for electricity generation
• Geothermal systems require three key components: a heat source, a permeable reservoir rock, and a fluid (water or steam) to transfer the heat
• Geothermal gradients, which measure the increase in temperature with depth, vary depending on the geological setting and can range from 10-50°C/km
• Plate boundaries, particularly in regions with active volcanism or tectonic activity (Ring of Fire), often have high geothermal potential due to increased heat flow
• Geothermal energy is considered a renewable resource as the heat is continuously generated within the Earth, although individual reservoirs may experience depletion over time
• The sustainability of geothermal systems can be enhanced through proper reservoir management, such as reinjection of spent fluids to maintain pressure and heat recovery

Types of Geothermal Power Plants

• Dry steam power plants utilize high-temperature (>235°C) steam from vapor-dominated reservoirs, where the steam is directly fed into turbines for electricity generation (Larderello, Italy; The Geysers, California)
• Flash steam power plants are the most common type, using high-temperature (>180°C) water-dominated reservoirs
  • The hot water is flashed into steam in a separator, and the steam is used to drive turbines
  • Single-flash plants have one separation stage, while double-flash plants have two stages to improve efficiency (Cerro Prieto, Mexico; Hellisheiði, Iceland)
• Binary cycle power plants operate with lower-temperature (100-180°C) water-dominated reservoirs, using a secondary working fluid (typically an organic compound like isobutane or pentane) with a lower boiling point than water
  • The geothermal fluid heats the working fluid through a heat exchanger, causing it to vaporize and drive a turbine (Raft River, Idaho; Dora-II, Turkey)
• Combined cycle power plants integrate a flash steam plant with a binary cycle plant to maximize energy extraction and improve overall efficiency (Puna, Hawaii)
• Enhanced Geothermal Systems (EGS) involve creating artificial reservoirs in hot, dry rock formations by hydraulic fracturing and circulating fluids to extract heat (Soultz-sous-Forêts, France; Habanero, Australia)
• Hybrid power plants combine geothermal energy with other renewable sources, such as solar or biomass, to provide a more stable and dispatchable power supply (Stillwater, Nevada)

Geothermal Resource Assessment

• Geothermal resource assessment involves characterizing the subsurface conditions, including temperature, permeability, and fluid chemistry, to determine the feasibility and potential of a geothermal system
• Geological surveys, including mapping of surface manifestations (hot springs, fumaroles, and altered rocks), provide initial indications of geothermal activity
• Geophysical methods, such as seismic surveys, gravity measurements, and electrical resistivity imaging, help delineate subsurface structures, fault zones, and fluid pathways
• Geochemical analyses of surface and subsurface fluids (water and gas samples) provide information on reservoir temperature, fluid origin, and potential corrosion or scaling issues
  • Geothermometers, based on the concentration of specific chemical species (silica, Na-K-Ca), estimate reservoir temperature
  • Isotope analyses (oxygen, hydrogen, carbon) help identify the source and age of geothermal fluids
• Exploratory drilling and well testing are crucial for directly measuring subsurface temperatures, permeability, and fluid properties
  • Temperature and pressure logs, core sampling, and well flow tests provide valuable data for resource characterization
• Conceptual models, integrating geological, geophysical, and geochemical data, are developed to understand the geothermal system's structure, heat source, and fluid circulation patterns
• Numerical reservoir modeling, using software tools (TOUGH2, CMG STARS), simulates the geothermal system's behavior and predicts its performance under different production scenarios
• Geothermal resource classification schemes, such as the United States Geological Survey (USGS) Resource Assessment Protocol, categorize resources based on their level of characterization and development potential

Power Plant Components

• Production wells deliver hot geothermal fluids (steam or water) from the reservoir to the power plant
  • Well casings, made of steel or fiberglass, provide structural support and prevent fluid loss
  • Wellhead assemblies control the flow and pressure of the geothermal fluids
• Pipelines transport the geothermal fluids from the production wells to the power plant, often insulated to minimize heat loss
• Separators (cyclone or gravity type) separate the steam and liquid phases of the geothermal fluid in flash steam plants
• Turbines convert the kinetic energy of the steam into mechanical energy, driving the generator to produce electricity
  • Impulse turbines (Pelton wheel) are used for dry steam and high-pressure flash steam applications
  • Reaction turbines (single or double flow) are used for lower-pressure flash steam and binary cycle plants
• Generators, typically synchronous type, convert the mechanical energy from the turbine into electrical energy
• Condensers cool and condense the steam exiting the turbine, creating a vacuum that improves turbine efficiency
  • Direct contact condensers mix the steam with cooling water, while surface condensers use a heat exchanger to keep the fluids separate
• Cooling towers or systems dissipate the waste heat from the condenser, either through evaporation (wet cooling) or air-cooled heat exchangers (dry cooling)
• Injection wells return the cooled geothermal fluids back into the reservoir to maintain pressure and sustainably extract heat
• Pumps are used to circulate the geothermal fluids (production and injection wells) and the working fluid in binary cycle plants
• Heat exchangers transfer heat from the geothermal fluid to the working fluid in binary cycle plants, typically using plate or shell-and-tube designs

Drilling and Well Construction

• Geothermal wells are drilled using rotary drilling rigs, similar to those used in the oil and gas industry
  • Drill bits, made of tungsten carbide or polycrystalline diamond compact (PDC), crush and cut the rock as they rotate
  • Drill strings, composed of drill pipes and drill collars, transmit torque and weight to the drill bit
• Drilling fluids (mud) are circulated through the drill string and annulus to cool and lubricate the drill bit, remove rock cuttings, and maintain well stability
  • Water-based muds are commonly used in geothermal drilling, with additives to control density, viscosity, and filtration properties
  • Air or foam drilling may be used in hard, fractured, or lost circulation zones to minimize fluid loss and improve penetration rates
• Well casing is installed in stages to provide structural support, prevent fluid loss, and isolate different zones of the reservoir
  • Conductor casing is set at shallow depths to prevent surface cave-ins and provide a stable foundation for the wellhead
  • Surface casing extends below the groundwater table to protect freshwater aquifers from contamination
  • Intermediate casing is installed to isolate weak or unstable formations and maintain well integrity
  • Production casing, extending to the target reservoir depth, is perforated or slotted to allow geothermal fluid flow
• Cement is pumped into the annulus between the casing and the formation to provide a seal and prevent fluid migration
• Well completion techniques, such as acid stimulation or hydraulic fracturing, may be used to improve well productivity by enhancing permeability near the wellbore
• Directional or deviated drilling allows multiple wells to be drilled from a single pad, reducing surface disturbance and improving access to the reservoir
• Well testing, including temperature and pressure surveys, flow tests, and injection tests, is conducted to evaluate well performance and reservoir properties
• Corrosion-resistant materials, such as stainless steel or titanium, are used in well components exposed to high-temperature, corrosive geothermal fluids

Power Generation Processes

• In dry steam power plants, the high-pressure steam from the reservoir is directly fed into the turbine, which drives the generator to produce electricity
  • The steam exiting the turbine is condensed in a condenser, creating a vacuum that improves turbine efficiency
  • The condensate is then reinjected back into the reservoir or used for cooling tower makeup water
• Flash steam power plants use a separator to flash the high-temperature geothermal water into steam
  • In single-flash plants, the steam from the separator is fed into the turbine, while the remaining liquid (brine) is sent to injection wells
  • Double-flash plants have a second separator at a lower pressure to extract additional steam from the brine, improving overall plant efficiency
• Binary cycle power plants use a secondary working fluid (organic Rankine cycle) to generate electricity from lower-temperature geothermal resources
  • The geothermal fluid heats the working fluid (isobutane or pentane) in a heat exchanger, causing it to vaporize and drive the turbine
  • The working fluid is then condensed and pumped back to the heat exchanger, forming a closed loop
  • The cooled geothermal fluid is reinjected into the reservoir without coming into contact with the working fluid or the atmosphere
• Combined cycle power plants integrate a flash steam plant (topping cycle) with a binary cycle plant (bottoming cycle) to maximize energy extraction
  • The high-temperature geothermal fluid is first used in the flash steam plant, and the remaining heat in the brine is then utilized in the binary cycle plant
• Cogeneration or combined heat and power (CHP) systems utilize the waste heat from the power plant for direct use applications, such as space heating or industrial processes, improving overall energy efficiency
• Load following and dispatchability are important considerations for geothermal power plants to meet varying electricity demand
  • Binary cycle plants can adjust the working fluid flow rate to match the load, while flash steam plants may use steam bypass or turbine inlet valve control
• Geothermal power plants typically have high capacity factors (>90%) and provide stable, baseload electricity generation, as they are not dependent on intermittent sources like wind or solar

Environmental Considerations

• Geothermal energy is considered a clean and renewable energy source, as it emits minimal greenhouse gases compared to fossil fuel-based power generation
  • However, geothermal fluids may contain trace amounts of non-condensable gases (NCGs), such as carbon dioxide (CO2) and hydrogen sulfide (H2S), which should be properly managed
  • Abatement systems, such as Stretford or LO-CAT processes, can be used to remove H2S from the NCGs before venting to the atmosphere
• Geothermal power plants have a relatively small land footprint compared to other energy sources, as the majority of the infrastructure is located underground
  • However, the construction of access roads, well pads, and pipelines may impact local ecosystems and wildlife habitats
  • Directional drilling and multi-well pads can help minimize surface disturbance and fragmentation
• Geothermal fluid withdrawal and reinjection can cause subsidence or uplift of the ground surface, depending on the net fluid balance and reservoir properties
  • Careful monitoring of subsidence using techniques like InSAR (Interferometric Synthetic Aperture Radar) and GPS (Global Positioning System) is essential to ensure sustainable reservoir management
• Induced seismicity, or the triggering of small earthquakes, may occur due to changes in stress and fluid pressure during geothermal operations
  • Proper site selection, monitoring, and injection management strategies can help mitigate the risk of induced seismicity
  • Traffic light systems, which adjust operations based on seismic activity levels, are used to minimize the potential impact on local communities
• Geothermal fluids may contain dissolved minerals, such as silica, calcium carbonate, or heavy metals, which can cause scaling or corrosion in well casings, pipelines, and plant equipment
  • Scale inhibitors, pH control, and material selection are used to manage scaling and corrosion issues
  • Proper handling and disposal of geothermal brines and solid wastes are necessary to prevent contamination of soil, surface water, or groundwater
• Noise pollution from drilling operations, well testing, and power plant equipment may impact nearby residents and wildlife
  • Noise mitigation measures, such as sound barriers, mufflers, and enclosed buildings, can help reduce noise levels
  • Engaging with local communities and addressing their concerns is crucial for the social acceptance and sustainable development of geothermal projects

Efficiency and Performance Metrics

• Thermal efficiency measures the percentage of heat input that is converted into electrical energy in a geothermal power plant
  • For flash steam plants, thermal efficiency typically ranges from 10-20%, depending on the resource temperature and plant design
  • Binary cycle plants have lower thermal efficiencies (8-15%) due to the lower temperature of the heat source, but they can utilize a wider range of geothermal resources
• Exergy efficiency considers the maximum theoretical work that can be extracted from a system, taking into account the quality (temperature) of the heat source
  • Exergy analysis helps identify the main sources of irreversibility and potential areas for improvement in geothermal power plants
  • Second law efficiency, or the ratio of actual work output to the maximum theoretical work, is a measure of exergy utilization
• Specific steam consumption (SSC) is the amount of steam required to generate one unit of electricity (kg/kWh)
  • Lower SSC values indicate higher plant efficiency, as less steam is needed to produce the same amount of power
  • Double-flash plants have lower SSC values compared to single-flash plants, as they extract more energy from the geothermal fluid
• Capacity factor is the ratio of the actual energy generated by a power plant to its maximum possible output over a given period (usually a year)
  • Geothermal power plants typically have high capacity factors (>90%), as they can operate continuously and are not dependent on intermittent sources
  • Higher capacity factors result in better utilization of the capital investment and lower levelized cost of electricity (LCOE)
• Availability factor measures the percentage of time a power plant is available to generate electricity, excluding planned and unplanned outages
  • Geothermal power plants generally have high availability factors (>95%), as they have fewer moving parts and lower maintenance requirements compared to other thermal power plants
• Specific capital cost ($/kW) and specific operating cost ($/kWh) are important economic metrics for evaluating the feasibility and competitiveness of geothermal projects
  • Specific capital costs for geothermal power plants vary depending on the resource type, location, and plant design, but they are generally higher than those for fossil fuel-based plants
  • Specific operating costs are relatively low for geothermal power plants, as the fuel (geothermal heat) is essentially free and not subject to market price fluctuations
• Levelized cost of electricity (LCOE) represents the average cost per unit of electricity generated over the lifetime of a power plant, taking into account capital costs, operating costs, and financing parameters
  • LCOE allows for the comparison of different energy sources on a consistent basis
  • Geothermal power plants have competitive LCOE values, particularly for high-temperature resources and in regions with favorable geological conditions