Geothermal energy uses heat from inside Earth to produce useful power at the surface. This heat comes from the planet's formation, radioactive decay in rocks, and the slow movement of heat through the crust. In a geothermal power plant, engineers drill wells into hot underground regions where water or steam can carry thermal energy upward.
It matters because geothermal energy can provide steady electricity with low greenhouse gas emissions compared with burning fossil fuels.
The basic physics is energy transfer from hot rock to a working fluid, usually water. Cold water may be pumped downward through an injection well, heated by contact with deep rock, and returned upward through a production well as hot water or steam. At the surface, the steam or hot fluid spins a turbine connected to a generator, converting thermal energy into electrical energy.
After giving up energy, the cooled water is often sent back underground to continue the cycle.
Key Facts
- Heat flows from hotter rock to cooler water: Q = mcΔT.
- Power is the rate of energy transfer: P = E/t.
- Turbine generators convert mechanical energy into electrical energy by electromagnetic induction.
- Geothermal temperature usually increases with depth, called the geothermal gradient.
- A typical geothermal gradient is about 25°C to 30°C per kilometer in many crustal regions.
- Efficiency compares useful electrical output to thermal input: efficiency = useful output energy / input energy.
Vocabulary
- Geothermal energy
- Energy obtained from heat stored inside Earth.
- Production well
- A drilled well that brings hot water or steam from underground to the surface.
- Injection well
- A drilled well that sends cooled water back underground to be reheated.
- Turbine
- A rotating machine that converts the motion of steam or fluid into mechanical energy.
- Geothermal gradient
- The rate at which temperature increases with depth below Earth's surface.
Common Mistakes to Avoid
- Assuming magma must be reached directly is wrong because most geothermal plants use hot rock and underground water, not open magma.
- Confusing heat with temperature is wrong because temperature measures how hot something is, while heat is energy transferred because of a temperature difference.
- Forgetting to reinject water is wrong because many geothermal systems depend on returning cooled water underground to maintain pressure and renew the heat-transfer cycle.
- Treating geothermal power as available equally everywhere is wrong because useful systems require high heat flow, suitable rock, fluid pathways, and safe drilling conditions.
Practice Questions
- 1 A geothermal plant brings 80 kg of water from 40°C to 160°C underground. If water has c = 4180 J/(kg°C), how much thermal energy is added to the water?
- 2 A turbine-generator produces 12 MW of electrical power from 60 MW of thermal power supplied by geothermal fluid. What is the efficiency of the plant as a percent?
- 3 Explain why a geothermal plant can produce steadier electricity than a solar power plant, but still depends strongly on local geology.