Generous Geothermals

Geothermal (heat energy) is the most abundant source of available energy on Earth. In fact, Earth's crust, which is about 6 miles thick, contains 50,000 times as much thermal energy as it does energy stored in coal and gas.

Humans have used geothermal energy for thousands of years. Ten thousand years ago, Native Americans used hot springs for cooking. The Romans built reservoirs around springs to create heated bathing complexes. But not until the turn of the 20th century did we realize how to extract Earth's heat energy.

Here's how it works: In conventional geothermal tapping, a drill bores 1,000 to 10,000 feet deep into Earth's crust—into a subterranean water source whose surrounding rocks are heated by molten lava below. (The temperature of the rocks found in association with these regions can run from several hundred degrees Fahrenheit to several thousand.) Once the hole is drilled, there are several ways to tap the heat energy.

The simplest—and oldest—way is to tap steam right from the source and run it directly into a turbine, which, as it spins, creates a constant flow of electricity.

Another way is to draw hot water (about 400 degrees F)—which is also under high pressure—from deep within Earth and pump it into low-pressure tanks. In the process, some of the water instantly vaporizes. This residual steam can be used to run turbines. If the subterranean water source is cooler (in the 300 degrees F range), the water can be piped through a heat exchanger, where it is used to vaporize a second fluid that boils at a lower temperature. This is called a “binary process.”

Today, geothermal energy is a mainstay in many nations around the world. There are more than 40 power plants producing electricity from geothermal energy in the United States alone. In the Philippines, 16 percent of the power used on the island of Luzon, which is home to more than 40 million people, comes from geothermal power plants. And Reykjavik, Iceland, receives almost all its heat energy from geothermal sources. (Iceland, overall, receives 49.5 percent of its energy from geothermal sources.)

Using geothermal power plants to generate electricity produces less than 1/1000 of the air pollution of equivalent coal plants. But conventional methods (especially the hot water ones) have some drawbacks. Geothermal plants can produce much more wastewater than a coal-fired or atomic power plant. Without proper management, extremely hot wastewater can be discharged into local streams, lakes, or salt water and disrupt the local ecology. The wastewater can also contain toxic chemicals (such as arsenic, lead, and mercury).

Scientists and engineers are working to develop new methods of tapping this energy reserve that would minimize some of these problems.

Vocabulary

binary:
Characterized by or consisting of two parts or components; twofold.

subterranean:
Underground.

Back to Top

Activity

  1. What provides the heat source for subterranean water?
    [anno: Rocks that are heated by molten lava provide the heat source for subterranean water.]
  2. When water under high pressure is taken from Earth and placed into a lower-pressure tank, what happens? Do you think the liquid water has more kinetic energy when it is under higher pressure or when it is under lower-pressure? Why?
    [anno: When the water is taken from Earth and put into a lower-pressure tank, some of the water vaporizes into steam. The liquid water has more kinetic energy at higher pressure than it does at lower-pressure because some of the liquid water in a lower-pressure tank is lost to steam.]
  3. If the water pumped from the ground is around 300°F, what might be done with it at a geothermal power plant?
    [anno: The water might be piped through a heat exchanger. When it passes through the heat exchanger, it can be used to vaporize a second fluid with a lower boiling point.]
  4. Why do you think this binary process is used for water that is around 300°F? Think about how vapor is used to power turbines.
    [anno: Answers may vary but could include that more energy in the form of vapor might come from the liquid that has a lower boiling point than would come from the amount of vapor produced by water that is around 300°F.]