How Does Geothermal Power Plant Generate Electricity Step By Step?

Geothermal energy is thermal energy generated and stored beneath the Earth’s surface. It arises from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. Geothermal energy is a renewable energy source that can be utilized for electricity generation as well as direct heating and cooling applications.

A geothermal power plant is used to generate electricity from geothermal energy. It requires high temperature hydrothermal resources that can be extracted from at least 3 km beneath the Earth’s surface. Geothermal power plants harness the Earth’s natural heat in the form of hot water or steam that rises to the surface through geothermal reservoirs. The steam rotates a turbine which activates a generator to produce electricity. Geothermal power plants have average availabilities of 90-95% and can provide reliable base-load power with a small carbon footprint. There are three main types of geothermal power plants: flash steam, dry steam, and binary cycle.

Geothermal Reservoir

A geothermal reservoir is an area deep underground where heat from the earth’s core is stored in hot rocks, water or steam. There are three main types of geothermal reservoirs that can be used for geothermal power generation:

• Hydrothermal reservoirs contain hot water or steam in cracks and porous rocks. They are the most common type used for geothermal power plants.
• Hot dry rock reservoirs have high temperatures but little water. Water is pumped down injection wells to transfer heat to the surface.
• Magma reservoirs are molten rock at extremely high temperatures. Technology to harness magma is still in development.

The temperature and pressure conditions of the reservoir determine its potential for electricity generation. Hydrothermal reservoirs typically range from 150-350°C. Higher temperature reservoirs above 225°C are best suited for power generation. Pressure in the reservoir also affects the boiling point – high pressures allow for higher temperature liquid before boiling occurs.

Drilling Wells

Drilling geothermal wells to tap into hot underground reservoirs is a crucial step in harnessing geothermal energy. These specialized wells can be over a mile deep and must withstand high temperatures, pressures, and caustic fluids.

Rotary drilling rigs with diamond-impregnated drill bits are used to bore through hot hard rock to reach geothermal reservoirs. The wells are encased in steel and cement casing to protect groundwater aquifers from contamination during drilling and geothermal fluid extraction.

Production wells bring geothermal fluids to the surface. Injection wells pump depleted geothermal fluids back underground to replenish the reservoir. The number and configuration of wells depends on the geothermal field and power plant design.

Drilling geothermal wells is an expensive endeavor, but it enables access to the earth’s vast heat below the surface. Proper casing and sealing off non-production zones ensures the resource can be utilized in an environmentally responsible manner.

Steam and Hot Water Extraction

Once the geothermal reservoir is located and wells are drilled, the next step is bringing the geothermal fluids to the surface. This is done through production wells that are drilled into the reservoir. The geothermal fluids, which can be steam, hot water, or a combination, rise up through the production wells under their own pressure.

If both steam and hot water are produced, they are separated in surface facilities called separators. The steam is sent to the power plant while the hot water is often reinjected back into the reservoir through injection wells. This reheating and reinjecting of the fluid allows the geothermal reservoir to be sustained for long-term use.

For reservoirs that lack natural steam and produce mainly hot water, a process called flashing can be used to generate steam. The hot water is pumped up the production well and its pressure suddenly drops as it reaches the surface. This pressure drop causes the water to vaporize or “flash” into steam.

Reinjecting the geothermal fluids also helps maintain reservoir pressures and prevent land subsidence. The reinjection wells send the fluids back underground, preventing the fluids from being released at the surface. This makes geothermal power a sustainable and environmentally-friendly energy source.

Geothermal Power Plant

The geothermal power plant is where the electricity is generated from the geothermal energy. It contains several key components:

Heat Exchangers: The hot geothermal fluid from the wells is passed through heat exchangers, which transfer the heat to a separate working fluid that has a much lower boiling point than water. This causes the working fluid to vaporize.

Turbines: The vaporized working fluid is used to turn turbines. As the vapor moves through the turbine blades, it causes the turbine shaft to spin. The spinning turbine shaft turns an electrical generator, which converts the mechanical energy into electrical energy through electromagnetic induction.

Condenser: After passing through the turbine, the working fluid is cooled and condensed back into a liquid so it can be reused. Cooling water from the cooling towers is used to condense the working fluid.

Generator: The generator converts the mechanical spinning motion of the turbine shaft into electrical energy. The electricity can then be transmitted via power lines to homes and businesses.

Cooling Towers: Cooling towers circulate water to condense the working fluid after it exits the turbine. This cooled water is then reused.

Electricity Generation

The hot geothermal fluid is pumped into the power plant where it turns the blades of a turbine. The turbine spins a generator that produces electricity. The generators operate similar to regular electric generators but are designed to withstand the high temperatures of geothermal fluid. The spinning turbine converts the thermal energy of the geothermal fluid into mechanical energy that spins electromagnets inside the generator. The generator contains conductive wire coils that have a magnetic field applied to them. As the magnets spin past the coils, it induces a flow of electrons, creating alternating current (AC) electricity.

The electricity generated has properties comparable to conventional power plants, allowing it to be fed into the electric grid for transmission and distribution. The geothermal power plant controls regulate and monitor electricity production, ensuring steady and reliable output. The spinning turbine and generator continue producing electricity as long as hot geothermal fluid is pumped through the plant.

Power Plant Efficiency

The efficiency of geothermal power plants is a key factor in determining their overall capacity and viability as an energy source. There are several metrics used to measure power plant efficiency:

Plant Efficiency Factor

The plant efficiency factor measures how much of the geothermal resource’s thermal energy is converted into electricity. Modern geothermal power plants can achieve plant efficiency factors between 10-20%. Binary cycle plants tend to have lower efficiency rates around 10-13%, while flash steam plants are on the higher end of the range at 14-20%.

Capacity Factor

The capacity factor measures how much electricity a geothermal power plant produces compared to its maximum possible output if it operated at full capacity continuously. Geothermal plants generally have high capacity factors of 90-98% because they can operate 24/7 with minimal downtime. This gives geothermal power the advantage of providing reliable base load capacity.

The main factors that affect efficiency and capacity factors are the geothermal reservoir’s characteristics, conversion technology used, plant maintenance, and transmission infrastructure. Optimizing these factors is key to maximizing the energy output and economic viability of geothermal power generation.

Transmission Lines

Once electricity is generated at the geothermal power plant, it needs to be transmitted to the electrical grid to be distributed for use. This requires building transmission lines to connect the power plant to the grid.

Transformers are used to step up the electricity voltage at the power plant for more efficient long-distance transmission along these lines. Once it reaches its destination in the grid, transformers will step down the voltage again for distribution and use.

New high-voltage direct current (HVDC) transmission lines and cables are often used for geothermal plants, especially in remote locations far from existing grid connections. HVDC lines have lower electricity losses over long distances compared to traditional alternating current transmission.

The transmission line routes need to be planned and approved, which can involve permitting, rights-of-way access, environmental impact assessments, and other considerations to connect the geothermal plant seamlessly to the electrical grid.

Environmental Impacts

Geothermal power plants can have various environmental impacts that need to be considered and mitigated.

Land Use

Geothermal plants require land for the power plant itself, wells, pipelines, transmission lines, and access roads. This can lead to habitat loss and fragmentation. Careful siting and land management is important to minimize the footprint.

Emissions

Geothermal sites emit gases such as carbon dioxide, hydrogen sulfide, ammonia, and methane. Newer plants capture and reinject these gases to reduce emissions. Scrubber systems can also minimize air pollution.

Groundwater

The drilling and flow testing of geothermal wells can affect groundwater quality and availability. Proper well casing and cements ensure geothermal fluids don’t contaminate groundwater supplies. Reinjecting geothermal fluids also helps reduce subsurface contamination.

Costs

The costs of geothermal power plants can vary significantly depending on the site location and resource depth. The main costs associated with geothermal power include:

Drilling Costs

Drilling geothermal wells is one of the most expensive components of a geothermal project. Wells must be drilled to depths of 1-3 miles to reach the high temperature resources. The drilling cost is highly dependent on the depth, with deeper wells costing exponentially more to drill. Drilling costs can range from $2-7 million per well.

Operation Costs

Once built, geothermal plants have relatively low operating costs compared to fossil fuel plants. Fuel is free and constant, and plants require less maintenance. However, wells must be periodically maintained, refreshed or redrilled to sustain output. Operation and maintenance costs range 1-3 cents per kWh.

Levelized Cost of Energy (LCOE)

The estimated levelized cost of energy (LCOE) for geothermal power plants ranges from $0.04 – $0.18 per kWh, making it competitive with conventional fossil fuel plants. LCOE depends heavily on the resource depth and site-specific conditions. Shallower, hotter resources provide cheaper geothermal electricity.