456 CHAPTER 18 Renewable Energy Resources
- Describe geothermal energy, ground-effect
heat pumps, and tidal energy.
G
eothermal energy and tidal energy are re-
newable energy sources that are not derived
from solar energy. Geothermal energy is the
naturally occurring heat within Earth. This
heat is used for space heating and to generate electricity.
Ground-effect heat pumps use stable temperatures just below
Earth’s surface to heat buildings in winter and cool them in
summer. Tidal energy, caused by the changes in water level
between high and low tides, is exploited to generate elec-
tricity on a limited scale.
Geothermal Energy
Earth’s subsurface can be accessed for energy in two
very different ways. Geothermal energy relies on high-
temperature heat from Earth’s interior. Ground-effect
heat pumps, discussed later, take advantage of the con-
stant but much lower temperature a few meters below
Earth’s surface. Geothermal energy, the natural heat
within Earth, arises from Earth’s core, from friction along
continental plate boundaries,
and from the decay of radioactive
elements. The amount of geo-
thermal energy is enormous. Sci-
entists estimate that just 1 percent
of the heat contained in the up-
permost 10 km (6 mi) of Earth’s
crust is equivalent to 500 times the energy contained in
all of Earth’s oil and natural gas resources.
Geothermal energy is typically associated with volca-
nism. Large underground reservoirs of heat exist in areas
of geologically recent volcanism. As groundwater in these
areas travels downward and is heated, it becomes buoy-
ant and then rises until it is trapped by an impermeable
layer in Earth’s crust, forming a hydrothermal reservoir.
Hydrothermal reservoirs contain hot water and possibly
steam, depending on the temperature and pressure of
the fluid. Some of the hot water or steam may escape to
the surface, creating hot springs or geysers. Hot springs
have been used for thousands of years for bathing, cook-
ing, and heating buildings. Drilling a well brings the hot
fluid from a hydrothermal reservoir to the surface, where
a power station may use it to supply heat directly to con-
sumers or to generate electricity (Figure 18.12a). The
electricity these power stations generate is inexpensive
and reliable.
The United States is the world’s largest producer
of geothermal electricity. Electric power is cur-
rently produced at 17 different geothermal fields in
California, Nevada, Utah, and Hawaii. The world’s larg-
est geothermal power plant—The Geysers in northern
California— provides electricity for 1.7 million homes.
Other important producers of geothermal energy in-
clude the Philippines, Italy, Japan, Mexico, Indonesia,
and Iceland (Figure 18.12b).
Iceland, a country with minimal oil and natural gas
resources, is located on the mid-Atlantic ridge, a bound-
ary between two continental plates. Iceland is therefore
an island of intense volcanic activity with considerable
geothermal resources. Iceland uses geothermal energy
to generate electricity to heat two-thirds of its homes. In
addition, most of the fruits and vegetables required by
the people of Iceland are grown in geothermally heated
greenhouses.
Is geothermal energy renewable? As a source of heat
for geothermal energy, the planet is inexhaustible on a
human timescale. However, the water used to transfer the
heat to the surface isn’t inexhaustible. Some geothermal
applications recirculate all the water back into the under-
ground reservoir, ensuring many decades of heat extrac-
tion from a given reservoir.
Geothermal energy is considered environmentally
benign because it emits only a fraction of the air pollut-
ants released by conventional fossil fuel–based energy
technologies. The most common environmental hazard is
the emission of hydrogen sulfide (H 2 S) gas, which comes
from the very low levels of dissolved minerals and salts
found in the steam or hot water. Hydrogen sulfide smells
like rotten eggs and is toxic to humans in high concentra-
tions. A lesser concern is that the surrounding land may
subside, or sink, as the water from hot springs and their
connecting underground reservoirs is removed.
Scientists are studying how to economically extract
some of the vast amount of geothermal energy stored in
hot, dry rock. Such a technology could greatly expand
the extent and use of geothermal resources.Other Renewable Energy Sources
LEARNING OBJECTIVE
geothermal
energy Energy from
Earth’s hot interior,
used for space
heating or generation
of electricity.