AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/6/6-10)
Unit 6 · Topic 6.10
6.10 Geothermal Energy
Geothermal energy uses heat from inside the earth to make electricity or to heat buildings directly. It runs day and night with very low emissions, but large-scale geothermal power only works where hot rock is near the surface, and it can release hydrogen sulfide gas.
Key terms
- geothermal energy
- magma
- steam turbine
- hydrogen sulfide
- ground-source heat pump
Where the heat comes from
The earth's interior is hot partly because of radioactive decay of elements such as uranium, thorium and potassium in rock deep underground, and partly because of heat left over from when the planet formed. That heat flows slowly outward.
In most places, hot rock is far too deep to reach cheaply. But near tectonic plate boundaries and volcanic hot spots, magma (melted rock underground) sits closer to the surface and heats groundwater. Iceland, the western United States, New Zealand, Indonesia, the Philippines and Kenya all use this heat.
Geothermal power plants
Wells are drilled into underground reservoirs of hot water and steam. The steam, or steam flashed from very hot water as its pressure drops, spins a turbine and generator. The cooled water is usually pumped back underground to be reheated, which keeps the reservoir from running dry.
Geothermal power is steady. Unlike solar and wind, it runs around the clock, so it can provide constant baseload power.
Ground-source heat pumps
A ground-source heat pump works almost anywhere. A few meters below the surface, the ground stays at a fairly constant temperature year-round, cooler than the air in summer and warmer than the air in winter. Pipes buried in the ground circulate a fluid. In winter the system pulls heat from the ground into the house; in summer it moves heat from the house into the ground. It uses electricity to run the pump, but far less than a conventional furnace or air conditioner, because it moves heat rather than creating it.
Can a geothermal field run out?
Earth's internal heat will last billions of years, so geothermal energy counts as renewable. A single geothermal field is a different matter. If a plant pulls out hot water and steam faster than rain and underground flow replace it, pressure drops and output falls.
This happened at The Geysers in northern California, the largest geothermal complex in the world. Its output peaked in the late 1980s and then declined as the steam reservoir was drawn down. Operators responded by piping in treated wastewater from nearby communities and injecting it underground to be reheated, which helped steady production. The lesson: geothermal stays renewable only when water is reinjected and extraction is matched to what the field can supply.
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| Renewable and available 24 hours a day | Power plants only work where hot rock is near the surface |
| Very low CO₂ emissions compared with fossil fuels | Can release hydrogen sulfide (H₂S), a toxic gas that smells like rotten eggs |
| Small land footprint | High upfront cost to drill and explore |
| Heat pumps cut heating and cooling energy almost anywhere | A site can cool down or lose water if heat and water are removed faster than they are replaced |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Choosing between geothermal options
A school in Ohio, far from any plate boundary, wants to use geothermal energy. A town in Iceland wants to do the same. Which type of geothermal system suits each, and why?
Show the solutionHide the solution
- Step 1: Ohio is in the middle of a tectonic plate, so there is no hot rock near the surface to run a geothermal power plant.
- Step 2: But the ground a few meters down stays at a steady temperature anywhere, so a ground-source heat pump can heat the school in winter and cool it in summer.
- Step 3: Iceland sits on the Mid-Atlantic Ridge, a plate boundary, so hot water and steam are close to the surface. It can run geothermal power plants and pipe hot water directly to heat buildings.
Answer: Ohio: a ground-source heat pump (works anywhere). Iceland: geothermal power plants and direct heating (hot rock near the surface at a plate boundary).
Common mistakes
- Saying geothermal power plants can be built anywhere. Only heat pumps work almost everywhere; power plants need hot rock near the surface.
- Forgetting hydrogen sulfide as the signature pollutant of geothermal plants.
- Describing a ground-source heat pump as tapping magma. It only uses the shallow ground's steady temperature.
On the exam
- A common question asks for one disadvantage of geothermal energy. 'Limited to areas near plate boundaries' and 'releases hydrogen sulfide' are both accepted, specific answers.
- Link geothermal locations to plate tectonics; questions may show a map of plate boundaries and ask where geothermal plants are likely.
Connected topics
Videos
Check yourself
4 questions on 6.10 Geothermal Energy. Pick an answer to see if you got it, and why.
Iceland produces much of its electricity and home heating from geothermal energy. Which characteristic of Iceland best explains this?
A family in Ohio, far from any volcano or hot spring, installs a ground-source heat pump. Pipes buried a few meters deep carry fluid that warms the house in winter and cools it in summer. Why does this work almost anywhere?
People living near a geothermal power plant notice a rotten-egg smell on some days. Which gas released from underground is the most likely cause?
At a geothermal power field, steam output fell steadily for years as more wells pulled hot water and steam from underground. Output leveled off after operators began piping treated wastewater back down into the hot rock. Which conclusion is best supported?
0 of 4 answered