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.
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.
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:
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-
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.
Refrigerant absorbs heat
from ground loop fluid
and evaporates into a gas.
Gas is compressed
and its temperature
Incoming cold air
picks up heat from
Pressure of liquid
as it cools.
from ground loop
Geothermal Heat Pump