What is Geothermal Energy?
Geothermal energy is the thermal energy stored in the Earth’s crust and mantle, originating primarily from:
- Primordial heat from planetary formation (~20%)
- Radioactive decay of U-238, Th-232, and K-40 (~80%)
The Earth's core temperature is ≈ 5700 °C; temperature increases ≈ 25–30 °C per km depth in continental crust (geothermal gradient).
How Does Geothermal Energy Work?
- Heat Sources: The Earth's interior contains immense heat generated by radioactive decay and residual heat from planetary formation.
- Reservoirs: Hot rocks and underground reservoirs of steam or hot water are accessed through drilling.
- Extraction: Wells are drilled into geothermal reservoirs to bring hot water or steam to the surface.
- Energy Conversion:
- Electricity Generation: The steam drives turbines connected to generators.
- Direct Use: Hot water or steam is used directly for heating buildings, greenhouses, or industrial processes.
- Heat Pumps: Geothermal heat pumps utilize the stable ground temperature for heating and cooling.
Types of Geothermal Resources
| Type | Temperature Range | Typical Use |
|---|---|---|
| High-temperature (>180 °C) | 180–350+ °C | Electricity generation |
| Medium-temperature (100–180 °C) | 100–180 °C | Electricity (binary cycle) + direct use |
| Low-temperature (<100 °C) | 20–100 °C | Direct heating, heat pumps |
Geothermal Power Plant Technologies
1. Dry Steam Power Plants
Steam directly from wells → turbine → generator
Example: The Geysers (California), Larderello (Italy)
2. Flash Steam Power Plants (most common)
Hot water (>180 °C) under pressure is flashed to steam in separator
Used in Iceland, New Zealand, Indonesia
3. Binary Cycle Power Plants
Hot water (even 57 °C) heats a secondary working fluid (isobutane, pentane) with low boiling point → vapor drives turbine
Most suitable for lower-temperature resources
4. Enhanced Geothermal Systems (EGS)
Hot dry rock is fractured, water injected → creates artificial reservoir
Still largely in demonstration phase (e.g., Soultz-sous-Forêts, France; FORGE Utah)
Global Installed Geothermal Electricity Capacity (2024–2025)
| Rank | Country | Installed Capacity (MWe) |
|---|---|---|
| 1 | United States | ≈ 3,900 |
| 2 | Indonesia | ≈ 2,400 |
| 3 | Philippines | ≈ 1,900 |
| 4 | Turkey | ≈ 1,700 |
| 5 | New Zealand | ≈ 1,000 |
| 6 | Iceland | ≈ 755 |
| 7 | Kenya | ≈ 950 |
Total global ≈ 16–17 GW (2025)
Direct Use of Geothermal Energy
- District heating (Iceland: 90% of homes heated geothermally)
- Greenhouse heating, aquaculture
- Industrial processes (pulp/paper, food drying)
- Balneology (hot springs & spas)
- Ground-source heat pumps (worldwide millions installed)
Advantages of Geothermal Energy
- Base-load power (capacity factor 80–95%)
- Very low CO₂ emissions (10–50 g/kWh vs. coal 800–1000 g/kWh)
- Small land footprint
- No fuel cost
- Long plant life (30–60+ years)
Challenges & Limitations
- High upfront drilling & exploration cost
- Resource location-specific (not everywhere viable)
- Risk of induced seismicity (EGS & reinjection)
- Potential subsidence and thermal drawdown
- Corrosion & scaling from geothermal fluids
Future Potential
US DOE estimates EGS could provide >500 GW in the USA alone.
Global technical potential: several thousand GW of electricity + enormous direct-use potential.
One-Sentence Summary
Geothermal energy is a reliable, low-carbon, base-load renewable energy source that harnesses heat from the Earth’s interior for electricity generation and direct heating applications worldwide.