ChemMatters - December 2015

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Figure 3. How a geothermal heat pump works. A fluid moves through a network of pipes that are buried underground. In the winter, the energy absorbed from the ground is transferred to a refrigerant in a heat exchanger, called an evaporator. In it, the refrigerant boils and turns into gas. Then, the gas goes through a compressor, where its pressure is increased, which raises its temperature. The hot gas goes through the second heat exchanger, or condenser, where the heat is transferred to the building’s distribution system (radiator or blown air). On a hot day, the system runs in reverse. Hot air inside a building transfers its heat to the refrigerant in the condenser, producing cooled air that is recirculated through the building.

The warmer refrigerant evaporates in the condenser, and the gas goes through the compressor, where its pressure is increased. Then, the hot gas goes through the evaporator, which transfers the heat to a fluid that goes through the underground pipes.

Energy cannot be created or destroyed. However, it can be trans- ferred from one place or substance to another, it can be converted from one form to another, and it can be “stored” in the molecules of substances we call “fuels.” Coal, oil, and natural gas are types of fuel that store this chemical energy. When these fuels burn, reacting with oxygen, chemical energy is released because the products of the chemical reaction, carbon dioxide and water, are more stable, that is, they have lower energy, than the reactants—the fuel and oxygen.

In a traditional power plant, coal, oil, or natural gas is burned, and the generated heat is used to boil water into steam. The chemical energy that was present in the molecules that make up coal, oil, or gas is converted to kinetic energy of the steam molecules.

The gaseous water molecules in steam are very hot and have lots of kinetic energy, so they are moving rapidly.

The fast-moving molecules of steam collide with a large fan, called a turbine. The collisions transfer energy from the molecules to the fan, which is set in motion, converting the steam molecules’ kinetic energy to mechanical energy. The turbine turns an axle connected to an electric generator. The axle is part of the rotor, a moving part in the generator. The rotor turns a loop of wire in a strong magnetic field, which generates an electric current. So the mechanical energy in the rotor is converted into electrical energy. The electric current then goes through an electric grid, which delivers electricity to homes and commercial buildings.

Geothermal power plants work in the same way, except that the energy required to boil the water comes from the interior of the Earth instead of a burning fuel.

—Chris Eboch

Growing potential

While geothermal power has room to grow, it does not work everywhere. The United States has only a few basins with the three factors needed to tap geothermal energy: high temperatures, enough water underground, and the ability of water to easily pass through rocks. These limitations mean that while geothermal power may work in some locations, in other areas, wind or solar energy might be better forms of alternative sources of energy.

Currently, geothermal power produces less than 1% of the electricity in the United States.

Still, the U.S. government estimates that conventional geothermal energy could provide 10 times as much power as it currently does.

Also, binary-cycle systems and geothermal heat pumps allow more access to the Earth’s energy. With these advances, geothermal power has the potential to be an even more important energy source, in the United States and abroad. Also, such advances allow us to take advantage of a cost-effective, environmentally friendly, and renewable resource provided directly by the Earth.

SELECTED REFERENCES

Renewable–Geothermal. Energy Kids, U.S. Energy Information Administration: http://www.eia. gov/KIDS/ energy.cfm?page=geothermal_home-basics [accessed Sept 2015].

Geothermal Energy. National Geographic: http://environment.nationalgeographic.com/ environment/global-warming/geothermal-profile/ [accessed Sept 2015].

How a Ground Source Heat Pump Works. Kensa Heat Pumps Ltd. Dec 8, 2011: https://www.you- tube.com/watch?v=KE3SvNRmwcQ [accessed Sept 2015]

Chris Eboch is a science writer who lives in Socorro, N.M. Her most recent ChemMatters article, “An Explosion of Diamonds,” appeared in the February/March 2014 issue.