6.10 - Geothermal Energy

Learning Objectives

I. Geothermal Basics and Electricity Generation

A. Geothermal Energy Source

• Geothermal energy is heat from Earth's interior. About half of that heat comes from the slow radioactive decay of elements like uranium, thorium, and potassium in the crust and mantle, and the rest is heat left over from when Earth formed.

• Temperature rises with depth: in most places it climbs about 25°C for every kilometer you drill down.

• The mantle is mostly solid rock that flows very slowly, not a layer of magma. Rock melts into magma only in certain places, mainly at plate boundaries and hot spots.

• Where magma or very hot rock sits close to the surface, like Iceland, the Pacific Ring of Fire (Indonesia, the Philippines, Japan, New Zealand), and the western U.S., geothermal energy is easy to reach.

B. Electricity Generation Process

• In certain places, deep drilling can reach naturally hot water deposits heated by nearby magma.

• Alternatively, water can be piped down into those hot rock layers to be heated by the magma’s proximity.

• Important: drilling does not go into the magma itself, just into rock layers close enough to be extremely hot.

• The superheated water gets piped back up to the surface to generate steam.

• The steam turns a turbine, which powers a generator to produce electricity.

• Like nuclear power, geothermal uses a cooling tower. Some water evaporates, but water should be piped back into the ground to be recycled and reused.

II. Sustainability and Availability

• Geothermal is renewable because the heat from the Earth’s core is not going to run out.

• It is sustainable as long as the used water keeps getting returned to the ground. If it is not returned, the groundwater may run out.

• Availability drawback: Geothermal power isn't available everywhere. It works best where hot rock or hot water sits within a few kilometers of the surface (most geothermal power wells are about 1 to 3 km deep), mostly near plate boundaries and volcanic areas. Elsewhere you'd have to drill much deeper, which costs far more.

• Scale: True geothermal heating usually happens at a municipal scale, for a city or industrial complex, and requires drilling down thousands of meters.
• Real examples: The United States has the most geothermal power capacity of any country (about 3,950 MW at the end of 2025), much of it at The Geysers in northern California, the largest geothermal field in the world. Iceland heats about 90% of its homes with geothermal hot water.

III. Ground Source Heat Pumps (Common Misconception)

• A ground source heat pump is often called geothermal, but it is not technically true geothermal energy.

• Source of heat: A heat pump uses the steady ground temperature found about 10 feet (3 m) down, around 50 to 60°F in much of the U.S. That temperature comes from the sun warming Earth's surface, not from heat deep in Earth's interior.

• Function: That consistent temperature gets used strategically to either heat homes in winter or cool them in summer.
• Cooling (summer): Liquid absorbs heat from the house, transfers it into the cooler ground, and returns cool liquid to the house.
• Heating (winter): The warmer ground passes heat into the liquid, which transfers that heat into the colder air of the house.

IV. Environmental Benefits and Drawbacks

A. Benefits

• Reduced $CO_2$ emissions: Far, far less carbon dioxide emission than using fossil fuels for electricity.

• Fewer air pollutants: Geothermal releases far fewer air pollutants because nothing is being burned.
• No particulate matter, no SOx, no NOx, and no carbon monoxide released during the heating process.

B. Drawbacks

• Release of hydrogen sulfide ($H_2S$): Geothermal plants can release hydrogen sulfide, an air pollutant that is lethal to humans and other animals and can kill people if it builds up to high concentrations.

• High initial cost: The upfront cost is very high because extensive testing, surveying, and drilling thousands of meters deep are needed to find the right location and reach the heat source. That can make a project financially prohibitive.