to produce electricity, over and over again, by
injecting it into the ground to refill the reservoir. When the water reaches the reservoir, it is
reheated, re-extracted, and its steam is again
used to spin the blades of the turbine, starting
the cycle all over again.
The main difference between a fossil-fuel
power plant and a geothermal power plant is
that the former needs to burn fossil fuels to
boil water (see “Energy Conversion,” p. 12)
while the latter extracts the boiling water from
the ground. Because a geothermal
plant does not need to burn fossil fuels, it does not generate
greenhouse gases, as fossil-fuel
plants do.
Binary-cycle power
plants
In the United States, most of the geothermal
power plants are concentrated in the western
states. The reason is that in many other areas,
the underground water is not hot enough to be
used in traditional geothermal power plants.
In those regions, another type of power plant,
called a binary-cycle system, may be used.
The main idea is to use a fluid, called a binary
working fluid, that boils at a lower tempera-
ture than water. This fluid is usually an organic
compound such as isobutene or pentafluoro-
propane.
Instead of using steam from geothermal
water, a binary-cycle system uses vapor from
the binary working fluid. In this system, the
hot geothermal water is pumped through
pipes in a heat exchanger, which is a device
that allows the transfer of heat between two
fluids without their mixing or coming into
direct contact (Fig. 2). The binary fluid goes
to the heat exchanger, too, and picks up
heat from the hot water. The heat causes the
binary fluid to boil and turn into steam. This
steam then impacts on the
blades of a turbine, causing
them to move and to drive
an electric generator. Then,
the extracted geothermal
water is injected back into
the reservoir, and the cycle
starts anew.
Binary-cycle systems are
expected to be the primary
geothermal systems of the
future because they can be
used in more places.
Geothermal
heat pumps
Heat pumps are another
way to produce geothermal
energy. These devices,
Ten feet below the ground, the Earth stays
at a fairly constant temperature, between
50 °F and 60 °F. Usually, such temperatures
are warmer than the air temperature in winter
and cooler than the air temperature in the
summer. A geothermal heat pump takes
advantage of this temperature difference.
A heat pump is connected to a set of buried
underground pipes in which an energy-absorbing liquid circulates. In the winter, this
liquid from the pump is colder than surrounding rocks, so it gains heat from them; in the
summer, the liquid from the pump is warmer
than the surrounding rocks, so it dissipates its
heat and becomes cooler.
Inside the heat pump are two heat exchangers, called an evaporator and a condenser
(Fig. 3). They are connected to each other
through a reversing valve and an expansion
valve (top and bottom of Fig. 3, respectively).
In these heat exchangers, a liquid, called a
refrigerant, circulates. In the evaporator, the
refrigerant exchanges heat with the liquid that
circulates in the underground pipes. In the
condenser, the refrigerant exchanges heat with
the building’s air ventilation system.
In the winter, the liquid from the underground pipes transfers heat to the refrigerant
in the evaporator, which boils and turns into a
gas. The gas then goes through the reversing
valve and to the compressor, which increases
the pressure of the gas and causes its temperature to rise. The hot gas then goes to the
condenser, where it transfers heat to a liquid
in a radiator or to incoming cold air, which
becomes warm in an air ventilation system.
Having cooled, the refrigerant goes from gas
to liquid (it condenses), and the liquid goes to
the evaporator, and the cycle starts again.
In the summer, the system runs in
reverse. Imagine the reversing valve (Fig. 3)
turned 90 degrees, so that compressed gas
goes to the evaporator. The cycle starts as follows: Hot air inside a building heats the refrigerant inside the condenser. As a result, the
air cools down and is blown through an air-conditioning system. The warmer refrigerant
evaporates in the condenser, and the gas goes
through the compressor, where its pressure is
increased, which raises its temperature even
more. The hot gas goes through the evaporator, which transfers heat to the fluid that goes
through the underground pipes, and this heat
is then released underground. Having cooled,
the refrigerant goes from gas to liquid, and the
liquid goes to the condenser, where the cycle
starts anew.
Many families in the United States use geo-
thermal heat pumps in their homes. According
to the U.S. Environmental Protection Agency,
geothermal heat pumps are the most energy-
efficient, cost-effective, and environmentally
clean systems. People in nearly every state use
geothermal heat pumps. R S G
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Production
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Turbine Generator
Heat exchanger
with working fluid
Binary-Cycle Power Plant
Rock layers
Figure 2. A binary-cycle power plant uses a heat exchanger, in which
heat from geothermal water is transferred to another fluid which
boils at a lower temperature than water. The heat causes the binary
fluid to boil into steam, and the steam turns the turbines that power a
generator, which creates electricity.
