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Geothermal
Energy

Explore the Earth's internal heat as a renewable energy source: how it works, where it's found, and why it matters for a sustainable future.

AP Environmental Science

What Is Geothermal Energy?

Geothermal energy is thermal energy generated and stored within the Earth. The word comes from the Greek geo (earth) and thermos (heat). This heat originates from the formation of the planet and from radioactive decay of isotopes like \( ^{238}\text{U} \), \( ^{232}\text{Th} \), and \( ^{40}\text{K} \) in the mantle and crust.

Earth's Internal Structure

Inner Core Outer Core Mantle Crust ~5,200 °C ~4,000 °C ~1,000 °C ~15 °C Temperature Gradient

Why Geothermal Matters

Renewable
Earth's heat is virtually inexhaustible on human timescales, continuously replenished by radioactive decay.
Low Emissions
Produces roughly \( 45\,\text{g CO}_2/\text{kWh} \), compared to \( 820\,\text{g} \) for coal-fired plants.
Baseload Power
Unlike solar or wind, geothermal provides consistent 24/7 power with capacity factors above 90%.

The Geothermal Gradient

Temperature (°C) Depth (km) 0 500 1000 1500 0 10 20 30 Normal (~25 °C/km) Plate boundary (~75 °C/km)

Types of Geothermal Resources

Hydrothermal
Hot water or steam trapped in permeable rock near the surface. Used in conventional geothermal plants. Requires heat, water, and permeability.
Enhanced (EGS)
Hot dry rock at depth. Water is injected to create an artificial reservoir. Expands geothermal potential to non-volcanic regions.
Ground-Source
Uses the shallow ground's stable temperature (~10-16 °C) for heating and cooling via heat pumps. Available nearly everywhere.

Hydrothermal vs. Enhanced Systems

Hydrothermal Enhanced (EGS)
Water Source Natural underground reservoir Injected from surface
Rock Permeability Naturally permeable Artificially fractured
Location Plate boundaries, volcanic zones Potentially anywhere
Depth 1-3 km 3-10 km
Technology Mature, commercial Emerging, R&D stage
Risk Reservoir depletion Induced seismicity

Explain why heat alone does not guarantee an accessible hydrothermal resource.

Use this illustration to explore the concept described in its caption.

Two tactile geological cutaways show rounded interconnected pores at left and angular engineered fractures at right. Cyan arrows point down the injection wells; pale gold pathways connect through each reservoir and rise up the production wells.
Conventional hydrothermal resources have naturally connected fluid-bearing permeability. Enhanced geothermal systems engineer connected fractures in suitable hot rock. Downward injection, connected reservoir paths and upward production are authored on the preserved geological cutaways. These are conceptual paths, not actual pore scale or a detailed surface power cycle; resource performance and possible induced seismicity depend on site conditions.

How a Geothermal Plant Works

Geothermal Reservoir (~150-350 °C) Production Well Turbine + Generator Steam Electricity Condenser Injection Well Cooled water

Explain local reservoir management and finite extraction rate.

Use this illustration to explore the concept described in its caption.

A rich rock and metal equipment cutaway shows production upward and reinjection downward, with thermal colors explicitly illustrative. A separate editable diagram traces two nonmixing fluid circuits through a heat exchanger. The heat-budget view compares 100 and 85 remaining synthetic heat units after equal input and different extraction.
The reservoir contains a finite heat store. Returning fluid can help manage fluid supply but does not guarantee that heat replenishment matches extraction. The primary material image shows separate equipment context with schematic thermal colors. The exact companion traces a reservoir-fluid production and reinjection path on one heat-exchanger side and a distinct turbine–condenser–pump circuit on the other; only heat crosses the dividing wall. A second companion uses a common zero-based scale for explicitly synthetic final heat stores: 100 + 20 − 20 = 100 and 100 + 20 − 35 = 85.
Two separate closed routes show reservoir production and reinjection, and a secondary exchanger–turbine–condenser–pump cycle.
The reservoir-fluid and turbine-fluid circuits do not mix; only heat crosses the heat-exchanger wall.
Common-scale bars compare 100 + 20 − 20 = 100 with 100 + 20 − 35 = 85 synthetic heat units.
Equal initial heat stores and equal input give different final stores when extraction changes; reinjection alone does not ensure heat sustainability.

Reservoir management and fluid circuits in geothermal generation

A geothermal plant with production and injection wells and a surface heat exchanger.In a binary geothermal plant, brine transfers heat to a separate working fluid and is reinjected. The working fluid drives a turbine, condenses and returns through a pump. Returning fluid does not instantly replace extracted reservoir heat.

Binary geothermal: two separate fluids

Binary geothermal: two separate fluids. In a binary geothermal plant, brine transfers heat to a separate working fluid and is reinjected. The working fluid drives a turbine, condenses and returns through a pump. Returning fluid does not instantly replace extracted reservoir heat.In a binary geothermal plant, brine transfers heat to a separate working fluid and is reinjected. The working fluid drives a turbine, condenses and returns through a pump. Returning fluid does not instantly replace extracted reservoir heat.

Which fluid reaches the turbine?

Which fluid reaches the turbine?. Dry-steam plants send reservoir steam to a turbine. Flash plants reduce the pressure of hot water and separate the resulting steam, while residual brine returns for reinjection.Dry-steam plants send reservoir steam to a turbine. Flash plants reduce the pressure of hot water and separate the resulting steam, while residual brine returns for reinjection.

Three Plant Types

Dry Steam
Uses steam directly from the reservoir. Oldest type (since 1904). Requires rare dry steam fields like The Geysers, California.
Flash Steam
Hot water (>180 °C) flashes to steam at reduced pressure. Most common type worldwide. Water is reinjected.
Binary Cycle
Uses a secondary fluid with a lower boiling point. Works with lower temperatures (100-180 °C). Fastest-growing type.

Evaluating a Geothermal Site

1
Geological survey: Identify volcanic activity, tectonic setting, and surface manifestations (hot springs, fumaroles).
2
Measure the geothermal gradient: Drill shallow wells to confirm \( G \geq 40\,{}^\circ\text{C/km} \) for viable power generation.
3
Assess the reservoir: Determine rock permeability, porosity, and fluid volume using seismic surveys and test wells.
4
Select plant type: Match reservoir temperature to the appropriate technology: dry steam, flash, or binary cycle.
5
Environmental impact assessment: Evaluate potential land subsidence, water chemistry impacts, and induced seismicity risks.

Example: Temperature at Depth

Problem: A geothermal site has a surface temperature of \( 15\,{}^\circ\text{C} \) and a geothermal gradient of \( 30\,{}^\circ\text{C/km} \). What is the temperature at a depth of \( 4\,\text{km} \)?

Example: Capacity Factor

Problem: A geothermal plant has a rated capacity of \( 50\,\text{MW} \). Over one year, it produces \( 394{,}200\,\text{MWh} \). Calculate its capacity factor.

Example: \( \text{CO}_2 \) Reduction

Problem: A 50 MW geothermal plant replaces a coal plant. Geothermal emits \( 45\,\text{g CO}_2/\text{kWh} \); coal emits \( 820\,\text{g CO}_2/\text{kWh} \). How much \( \text{CO}_2 \) is avoided per year at 90% capacity factor?

Global Geothermal Hotspots

Ring of Fire
Indonesia, Philippines, Japan, New Zealand: subduction zone volcanism creates ideal hydrothermal reservoirs.
Rift Zones
Iceland, East Africa (Kenya): divergent boundaries with thin crust and high heat flow. Iceland gets 25% of electricity from geothermal.
Hot Spots
Hawaii, Yellowstone: mantle plumes bring heat close to the surface. The Geysers (California) is the world's largest geothermal field.

Environmental Benefits & Concerns

✓ Benefits
Low \( \text{CO}_2 \) emissions, small land footprint (~1-8 acres/MW vs. 5-10 for solar), minimal water consumption in binary plants, reliable baseload power.
✕ Concerns
Dissolved minerals (\( \text{H}_2\text{S} \), mercury) in geofluids, induced seismicity from EGS, thermal pollution of water bodies, land subsidence near extraction sites.

Common Mistakes

✕ "Zero emissions"
Geothermal plants do emit some \( \text{CO}_2 \) and \( \text{H}_2\text{S} \) from geofluids. It is low emission, not zero emission.
✕ Available everywhere
Conventional hydrothermal is limited to tectonic boundaries. Only ground-source heat pumps work nearly everywhere.
✕ Confusing types
Students often confuse geothermal power plants (electricity) with ground-source heat pumps (heating/cooling). They are fundamentally different technologies.

Geothermal vs. Other Renewables

Feature Geothermal Solar PV Wind
Capacity Factor ~90% 15-25% 25-45%
Baseload? Yes No No
\( \text{CO}_2 \) (g/kWh) ~45 ~40 ~11
Land Use (acres/MW) 1-8 5-10 30-60
Location Constraint High Low Moderate
Intermittent? No Yes Yes

Question 1 of 5

The primary source of Earth's internal heat that drives geothermal energy is:

Question 2 of 5

A site has a surface temperature of \( 20\,{}^\circ\text{C} \) and a geothermal gradient of \( 40\,{}^\circ\text{C/km} \). At what depth would the temperature first exceed \( 180\,{}^\circ\text{C} \), the minimum for a flash steam plant?

Question 3 of 5

Which type of geothermal power plant is most suitable for a reservoir at \( 130\,{}^\circ\text{C} \)?

Question 4 of 5

The main advantage of geothermal energy over solar and wind energy is that geothermal:

Question 5 of 5

A potential environmental concern specific to Enhanced Geothermal Systems (EGS) that does not apply to conventional hydrothermal plants is: