Geothermal Energy (College Board AP® Environmental Science): Revision Note

Using geothermal energy

• Geothermal energy is heat energy stored beneath the Earth's surface

• This heat is generated by the natural radioactive decay of elements like uranium, thorium, and potassium, as well as residual heat from the Earth's formation and tectonic activity

How geothermal energy is harnessed

Geothermal reservoirs

• Hot water and steam are trapped in permeable rocks or in underground reservoirs

• These reservoirs are usually near volcanoes or tectonic plate boundaries

Energy extraction

• Direct use: hot water from geothermal reservoirs is used directly for heating buildings, greenhouses, or industrial processes by drilling into the reservoir

• Electricity generation: hot water and steam from underground reservoirs are piped to the surface, where they drive turbines connected to generators

Geothermal power station

Ground-source heat pumps

• Ground-source heat pumps (GSHPs) are a small-scale way to use geothermal energy in individual homes and buildings

• A few metres below the surface, ground temperature stays roughly constant year-round (around 10–15°C)

  • This is warmer than winter air and cooler than summer air

• A network of pipes buried in the ground circulates a fluid that exchanges heat with the soil

• In winter, the fluid absorbs heat from the ground and a pump transfers it indoors to warm the building

• In summer, the cycle reverses: heat from the building is transferred into the cooler ground

• GSHPs use far less electricity than conventional heating or air conditioning because they move heat rather than generating it

Advantages & disadvantages of geothermal energy

Advantages of geothermal energy

• It is a renewable resource: providing the Earth generates heat, geothermal energy will be available

• Low-carbon energy source: it produces far less greenhouse gas than fossil-fuel power plants

• Reliable and consistent: it provides a constant energy supply regardless of weather conditions, unlike solar or wind power

• They are an efficient use of land as they take up less land space than other renewable power plants

• Geothermal has diverse applications and can be used for generating electricity and direct heating

Disadvantages of geothermal energy

• Geothermal energy depends on tectonic activity, so there are few suitable locations on Earth

• Geothermal power stations are expensive to build

• Geothermal power stations are small so they are not suitable for large-scale electricity production

• Geothermal power stations are not very efficient (around 12%)

• Overuse of reservoirs can lead to depletion if the heat extraction rate exceeds natural replenishment

• There are environmental risks if harmful gases like hydrogen sulfide is released

• Mineral-laden wastewater can contaminate nearby water sources

• Drilling and water reinjection can sometimes trigger small earthquakes