How Things Work: Geothermal energy

Desiree Xu Feb 3, 2013

Pollution and global warming, caused by burning oils and fossil fuels, are the most talked about issues in the world of energy. However, scientists have found a new source of energy that lies right under our feet: geothermal energy.

Geothermal energy is not only clean, renewable, and efficient, but it is also more cost effective and readily accessible. It releases a fraction of the amount of carbon dioxide that fossil fuels emit, and it produces smaller amounts of nitrous oxide and sulfur gases. Geothermal plants are also more reliable, running 24 hours a day, 365 days in a year. It’s no wonder that the world consumes 7,000 megawatts of geothermal energy a year. The United States produces 2,700 of that total, according to the Geothermal Education Office.

Still, before geothermal energy is given too much praise, it’s important to find out what exactly geothermal energy is, where it comes from, and how it’s used.

Geothermal energy literally means energy from the Earth. It is heat — the energy of steam and hot water heated by the Earth’s core, which lies about 4,000 miles beneath Earth’s surface and can reach temperatures of 7,600˚F or higher. Such heat can melt surrounding rocks, turning them into magma that rise toward the Earth’s surface (considering they are less dense than the surrounding rocks.) After reaching the surface, the magma heats nearby rocks and water that is trapped within them. As a result, some of the water escapes through the surface to form geysers and hot springs. The remaining water becomes geothermal reservoirs, which are pools of water under the Earth’s surface.

There are several ways to acquire this energy according to www.howstuffworks.com; these include accessing it directly from the ground and using a geothermal pump to heat and convert the energy to electricity. To access the heat directly, a building merely needs to be built near the source — hot springs and reservoirs ­­— so that hot water can be piped directly through a heat exchanger that transfers the heat from the water into the building’s heat system. Afterward, the used water is returned to the reservoir via a well to be reused again.

The geothermal pump uses the heated soil and water near the source differently. Pipes connecting to a building are built beneath or around the heated soil and water; fluid circulates these pipes and picks up the heat from the surrounding source. The heat within the pipe is then sucked out by an electric compressor and heat exchanger, which sends the heat through a duct system throughout the building. At hotter temperatures, the opposite process occurs; The pipes pull the heat out of the building and sends it to the ground or water outside.

The geothermal power plant generates electricity by piping hot water and steam throughout underground wells. There are three kinds of plants: dry steam plants, flash steam plants, and binary cycle plants. In dry steam plants, hot steam is piped directly from geothermal reservoirs to spin turbines located in the generators to produce electricity. In flash steam plants, wells bring up water between 300˚F and 700˚F. Some of this water turns into steam to drive turbines. The steam then cools, condenses, and goes back into the ground. In binary cycle plants, moderately hot geothermal water passes through a heat exchanger to transfer its heat into a liquid that boils at a lower temperature than water. After the heated fluid becomes steam, it spins turbines to generate electricity.

Geothermal energy can be expensive at first: Drilling wells can cost between $1–4 million per drill, and installing home pumps can cost up to $30,000. However, the home geothermal pumps can decrease energy bills by 30 to 40 percent and will pay for themselves within five to 10 years. After all, geothermal energy perpetually replaces heat, as removed water can be returned to the ground after the heat is used.

Geothermal is an energy source that is clean and renewable. Some countries like Iceland utilize geothermal energy as one of their main sources of power, with the capital city of Reykjavik getting 95 percent of its energy from the source. The U.S. may soon follow in their footsteps.