The rising energy demand, the depletion of fossil fuels, increasing environmental pollution, and fears of global warming have led countries worldwide to turn to clean and renewable energy sources. In this context, international protocols such as the Paris Agreement have encouraged countries to set voluntary emission reduction targets and transition to renewable energy to reduce greenhouse gas (GHG) emissions. Among renewable sources such as hydroelectric, wind, solar, biomass, and geothermal energy, geothermal energy stands out due to its continuous availability and low carbon footprint.

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Geothermal energy is a form of heat energy derived from the Earth's interior, originating from the Greek words "geo" (earth) and "therme" (heat). This energy is continuously generated by the slow decay of radioactive isotopes (such as potassium-40 and thorium-232) in the Earth's core. The Earth consists of four main layers:

• The inner core is a solid iron sphere approximately 1,500 miles in diameter.
• The outer core, composed of molten magma, is about 1,500 miles thick.
• The mantle, made of magma and rock, extends 1,800 miles.
• The crust, which forms the continents and ocean floors, ranges from 15–35 miles thick beneath continents and 3–5 miles thick under oceans.

The temperature in the core exceeds 5,000°C (approximately 10,800°F), while temperatures at the mantle boundary range between 392°F and 7,230°F. This heat rises to the surface at tectonic plate boundaries and manifests through natural formations such as volcanoes, geysers, and hot springs.

Geothermal energy is a renewable resource because the heat generated in the Earth's core has continued for billions of years and will persist in the future. Humans have utilized this energy for thousands of years for bathing, cooking, and heating, and today, geothermal power plants have been developed for electricity generation.

Operating Principles of Geothermal Power Plants

Geothermal power plants harness the Earth's internal heat to generate electricity. These plants operate similarly to coal or nuclear power plants but use the Earth's natural heat instead of a boiler or reactor. Three fundamental components are required for electricity generation:

1. Heat – A vast amount of energy stored in deep rocks.
2. Fluid – Water or steam to transport the heat to the surface.
3. Permeability – Small pathways that allow fluid to move through rocks.

Systems where these elements naturally coexist are called conventional hydrothermal resources.

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Geothermal power plants function by extracting hot water or steam from underground reservoirs. Typically, high-pressure hot water is pumped from wells about 1–2 miles deep. When the pressure drops at the surface, the water turns into steam, which drives turbines to generate electricity. The used water is usually re-injected into the reservoir, making the system sustainable. Geothermal power plants are classified into three main types: dry steam, flash steam, and binary cycle.

• Dry Steam Power Plants: Use naturally occurring underground dry steam. The steam is directly directed from wells to turbines, then condensed and re-injected into the reservoir. This is the oldest type of geothermal plant; the first example was built in Larderello, Italy, in 1904. In the U.S., The Geysers (Northern California) is the largest example of this technology, operating at approximately 1.5 GW capacity. However, due to high-temperature requirements, its application is limited to specific areas.

• Flash Steam Power Plants: The most common type, utilizing reservoirs with temperatures above 182°C (360°F). High-pressure water from underground turns into steam through a "flash" process when it reaches a low-pressure tank at the surface. This steam drives turbines while the remaining water can be used in a secondary flash tank or re-injected. This technology is widespread in volcanic regions like Iceland, providing both electricity and heating.

• Binary Cycle Power Plants: Utilize lower-temperature sources (107–182°C / 224–360°F). Hot water transfers heat to a secondary fluid with a lower boiling point (such as pentane or butane) in a heat exchanger. This secondary fluid vaporizes and drives turbines. Since the geothermal water and secondary fluid remain separate, emissions are minimal. Binary cycle technology is expected to expand in the future due to its adaptability to more locations.

In cases where natural conditions are insufficient, Enhanced Geothermal Systems (EGS) are used. EGS involves injecting water into hot but dry or low-permeability rocks to create artificial reservoirs. According to the 2019 GeoVision analysis, EGS could supply energy to 40 million households by 2050 in the U.S., while the 2023 Enhanced Geothermal Shot analysis projects this number to reach 65 million.

Geothermal Power Plants in Turkey

Turkey is among the top five countries in geothermal energy production, alongside the U.S., the Philippines, New Zealand, and Indonesia. The country’s geothermal potential arises from its location in tectonically active regions. Turkey's installed geothermal capacity grew from 1,064 MW in 2017 to 1,696 MW in 2021, with a compound annual growth rate (CAGR) of 12.3%. By 2030, the capacity is expected to reach 2,785 MW, growing at 5.61% CAGR between 2020–2030. Electricity production is projected to increase from 8,151 TWh in 2021 to 13,183 TWh in 2030. In 2023, geothermal energy accounted for 2% of Turkey’s total installed capacity and 3% of its electricity production.

Amendments to the Renewable Energy Law in 2011 introduced incentives such as fixed price guarantees, low licensing fees, priority grid access, and ease of land acquisition, accelerating geothermal plant development and deployment.

Advantages and Disadvantages

Geothermal energy produces one-sixth the CO₂ emissions of natural gas power plants and, unlike solar or wind, is continuously available.

Key Advantages

• Compact facility structures.
• Long lifespan (several decades).
• Low water consumption.

Challenges

• High initial costs.
• Risk of microseismic activity (small earthquakes).
• Potential reservoir cooling over time.

Technologies like EGS offer promising solutions to these issues, but implementation costs and further technical advancements are needed.

Geothermal power plants utilize the Earth's internal heat to provide a sustainable and clean energy source. Countries with high geothermal potential, such as Turkey, effectively use this resource for both electricity generation and direct heating. With advancements in technologies like EGS and binary cycle, geothermal energy could play a larger role in the global energy transition. However, increased investments and research are essential to fully unlock its potential.