How Does Geothermal Plant Works?

Geothermal energy is thermal energy generated and stored in the Earth. It is a clean, renewable source of energy that utilizes the natural heat within the Earth’s crust to produce electricity and provide direct heating and cooling.

At geothermal power plants, wells are drilled into underground reservoirs to tap into hot water or steam. This geothermal fluid is brought to the surface and piped into a power plant. The high-temperature fluid converts a secondary fluid into steam, which then drives a turbine that activates a generator and produces electricity. The fluid is then recycled back into the reservoir.

Geothermal energy offers several key benefits:

• It is sustainable and available 24/7, as thermal energy is continuously produced inside the Earth.
• It has a small environmental footprint. Geothermal plants release very low emissions of carbon dioxide, particulate matter, and other pollutants.
• It provides reliable baseload power that complements intermittent renewable sources like wind and solar.
• The land above geothermal reservoirs can still be used for other purposes like agriculture or grazing.

Geology

Geothermal energy comes from the heat within the Earth. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface. The Earth’s internal thermal energy flows to the surface by conduction at a rate of 44.2 terawatts and is replenished by radioactive decay of minerals and residual heat from the planet’s formation.

There are two types of geothermal resources that can be used to generate electricity:

High Temperature Resources – These resources are associated with magma conduits along plate boundaries and hot spots. They exist where there is thin lithosphere or mantle upwelling that allows hot magma to rise close to the surface. This type of geothermal system allows retrieval of high temperature fluids (>200°C) that can be used to drive electric generators.

Low Temperature Resources – These resources are associated with normal high geothermal gradients, usually found along plate boundaries. They exist where there is a thick lithosphere that acts as an insulator, trapping geothermal heat below it. This type of geothermal system produces lower temperature fluids (100-200°C) that require binary power plant technology to generate electricity.

Resource Detection

Before a geothermal power plant can be built, developers must locate and assess geothermal resources. This is done through a combination of geological surveys, geochemical analysis, geophysical exploration, and test drilling.

Geological surveys involve studying the area’s rock formations, mineral deposits, fault lines and hot springs. This provides clues about potential subsurface heat sources. Geologists can map out likely geothermal reservoirs based on the surrounding geology.

Geochemical analysis involves sampling and studying gases and fluids from hot springs or boreholes. Chemical composition provides information about subsurface temperatures and permeability. For example, higher concentrations of silica could indicate the presence of a geothermal system.

Geophysical exploration uses techniques like seismic surveying, gravity surveys, magnetic surveys and electrical surveys to image the earth’s surface and detect subsurface geological structures. This helps locate potential geothermal reservoirs and fluid pathways.

Test drilling involves drilling shallow temperature-gradient wells to directly measure subsurface temperatures. Higher than normal temperatures are a positive indicator of geothermal resources. Exploratory and production wells are then drilled to further evaluate the resource.

Drilling Production Wells

To create geothermal power, wells must be drilled into the reservoirs to extract the hot water or steam. Specialized drilling rigs are used that can handle the high temperatures of geothermal reservoirs, which can reach over 700°F at depth. The drilling process is similar to drilling for oil and gas.

First, a large drill bit bores a hole deep into the earth, often a mile or more. As drilling progresses, steel well casing pipe sections are inserted into the hole. Cement is pumped down to fill the gap between the casing and the surrounding rock, sealing and stabilizing the well walls. This also prevents fluids from mixing between geologic layers.

Once the target depth is reached, production tubing is installed within the casing. This production tubing extends to the bottom of the well, allowing the geothermal fluids to flow freely to the surface. At the wellhead, valves control the rate of fluid extraction. Special wellhead fittings connect the well to the pipelines that carry the geothermal fluids either to the power plant or to a direct use facility.

Often, wells are drilled in pairs with two wells producing fluids from the reservoir. The hot geothermal fluids are extracted from one well, called the production well. After losing some heat in the power plant, the spent geothermal fluids are returned to the reservoir through the second well, called the injection well. This design provides a closed-loop system that recharges the reservoir.

Steam and Fluid Extraction

After the production wells are drilled, the geothermal fluids are brought to the surface. This fluid is a mixture of steam and hot water. The exact composition depends on the geological conditions of the reservoir. Generally, wells that are drilled into a vapor-dominated reservoir will produce mostly steam, whereas wells in liquid-dominated reservoirs produce hot water.

Pipelines transport the geothermal fluids from the wellheads to the power plant. Separators are then used to separate the steam and water phases. The steam is sent to the plant’s turbines to generate electricity. The hot water and condensed steam may be injected back into the reservoir to replenish it in a process known as reinjection.

Proper pipeline construction and separators are critical components of a geothermal power plant. The pipelines must withstand high temperatures and pressures. The separators enable optimal use of the steam resource by routing steam to the turbines while excess water can be reused.

Careful fluid extraction, separation, and reinjection helps ensure the sustainability of the geothermal reservoir. With good reservoir management practices, geothermal power plants can operate for decades.

Power Generation

There are three main types of geothermal power plants that are used to generate electricity from geothermal energy:

Dry Steam Power Plants

Dry steam power plants use steam from a geothermal reservoir to directly turn turbines and generate electricity. The steam goes directly through the turbine without being condensed into water first. After passing through the turbine, the steam is cooled and condensed into water, which is then injected back into the reservoir to be reheated and recycled. Dry steam power plants are the oldest and simplest type of geothermal power plants.

Flash Steam Power Plants

Flash steam power plants use water at temperatures over 360°F that is pumped under high pressure from deep inside hot, liquid-dominated reservoirs. As this very hot water flows up through the production well, the lowering pressure causes it to rapidly vaporize into steam. The steam is then separated from the liquid and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir.

Binary Cycle Power Plants

Binary cycle power plants operate on water at lower temperatures of about 225-360°F. These plants use the hot water from the reservoir to boil a working fluid that has a much lower boiling point than water. This working fluid, usually an organic compound like isobutane or isopentane, turns to vapor and drives the turbines. The water and working fluid are kept in separate closed loops, so there are no emissions. After the working fluid vapor turns the turbines, it is condensed back into a liquid and recycled back around the loop.

Power Plant Components

Geothermal power plants have several key components that work together to convert geothermal energy into electricity.

Condensers

Condensers are heat exchangers that cool the steam exiting the turbine back into water so that it can be reused. The steam passes over a series of tubes with cool water running through them, which causes the steam to condense into water. This water is then pumped back into the geothermal reservoir to be reheated into steam.

Cooling Towers

Cooling towers are structures used to cool the water that was heated up in the condenser. Cooling towers allow the water to cool through evaporation and by using fans to blow air through the falling water. Cooling towers allow the water to be reused again and again in the condenser.

Transformers

Transformers are used to increase the voltage of the electricity generated by the turbines to the high voltages needed for transmission on the electric grid. This allows the electricity to be efficiently transmitted over long distances.

Substation

The substation acts as the interchange between the power plant and the transmission grid. This is where the generated electricity is synchronized and connected to the high voltage transmission lines that carry it to customers.

Direct Use Applications

Geothermal resources can be harnessed directly for purposes other than electricity generation. Some of the most common direct use applications include:

Greenhouses

The heat from geothermal sources can be used to heat greenhouses year-round. This allows growing seasons to be extended and exotic plants to be cultivated in colder climates. Geothermal heating reduces greenhouse operation costs and provides optimal growing conditions.

Aquaculture

Aquaculture facilities can use geothermal heating to maintain optimal water temperatures for raising fish, shellfish, and plants. The heat stimulates growth and allows high-density cultivation. Geothermal aquaculture is sustainable and environmentally-friendly.

District Heating

District heating systems distribute heat generated from a centralized location to residential and commercial buildings via a network of insulated pipes. Geothermal district heating provides a constant, reliable heat source that doesn’t depend on variable energy costs. It can heat entire communities cleanly and efficiently.

Environmental Impacts

The environmental impacts of geothermal energy plants vary depending on the type and location of the facility. Some key impacts include:

Land Use

Geothermal power plants require large areas of land for production wells, pipelines, power plants, cooling towers, roads, and transmission lines. This can fragment habitat and disturb natural ecosystems. However, the land use footprint of geothermal is smaller than coal and comparable to solar PV.

Emissions

Geothermal plants release greenhouse gases trapped in reservoirs, including carbon dioxide, hydrogen sulfide, methane, ammonia, and boron. Emissions are generally much lower than fossil fuels. Air pollutants like sulfur dioxide and particulate matter can also be released from some plants.

Induced Seismicity

Injecting water into geothermal reservoirs or withdrawing fluids can trigger small earthquakes, ranging from imperceptible to high enough magnitude to damage buildings in rare cases. Careful reservoir management using microseismic monitoring can reduce induced seismic risks.

Future of Geothermal

The future of geothermal energy looks promising, with innovations that can unlock even greater capacity. One key area is enhanced geothermal systems (EGS). These utilize advanced technology to access geothermal resources that were previously unreachable. Drilling deeper wells and hydraulic fracturing creates artificial reservoirs that can produce renewable baseload power. EGS has the potential to vastly expand usable geothermal resources globally.

Combining geothermal with carbon capture, utilization and storage (CCUS) is another emerging opportunity. CCUS can mitigate the CO2 emissions from geothermal plants. It involves capturing CO2 from the geothermal fluid stream and then sequestering it underground. This further improves the sustainability of geothermal power generation.

With its abundant renewable energy, low carbon footprint and ability to provide constant reliable electricity, geothermal is poised for major growth worldwide. The installed global geothermal capacity is projected to nearly double to 17 gigawatts by 2030. With technology improvements like EGS and CCUS, the long-term outlook for geothermal is even brighter.