Geothermal Energy - World Future Society

© 2012 World Future Society • 7910 Woodmont Ave., Suite 450, Bethesda, MD 20814, U.S.A. • www.wfs.org • All rights reserved.
Geothermal Energy
By Gioietta Kuo
Of all the energy sources used for electricity
generation, renewable or not, geothermal energy
is probably the most neglected and has so far commanded
the least public attention. It deserves consideration,
however. Geothermal energy is a sustainable
fuel—in fact, an almost limitless resource.
Geothermal energy is energy from the heat
of the earth. Resources range from hot water and
hot rocks at or near the earth’s surface, to water
and rock at high temperature several kilometers
deep. In theory, it is possible to tap into this resource
almost everywhere, even accessing the extremely
hot temperature of the magma, the earth’s
molten rock core. It is particularly easy to tap this
energy in countries where there is existing volcanic
activity, such as the hot springs in New Zealand
and Iceland.
It is also very clean. The emission intensity
of existing geothermal plants—most of which
process hydrogen sulphide—is on average 122-
400 kg of CO 2
per megawatt-hour (MWh) of electricity
generated. 3, 4 This is a fraction of the emission
intensity of a conventional coal-fired plant,
which is estimated to be around 1,000 kg per
MWh.
In addition, geothermal plants require minimal
land and fresh water. 3 On average, a geothermal
plant uses 404 square meters per GWh versus
the 3,643 square meters that a coal facility uses
and the 2,335 square meters that a wind farm uses.
Furthermore, a geothermal plant uses only 20 liters
of freshwater to produce each MWh of elec-
tricity, whereas a nuclear, coal, and oil plant each
uses 1,000 liters of water per MWh.
Global estimates geothermal energy’s electricity-generating
potential currently vary from
35 to 2,000 GW. 1 The amount of heat within
10,000 meters of earth’s surface contains 50,000
times more energy than all the oil and natural gas
resources in the world combined—enough to supply
the energy needs of the whole world. 2 The current
installed capacity of geothermal facilities
worldwide is 10,715 MW. These facilities, generated
0.07 trillion kWh of electricity in 20103. This
geothermal power is produced in 24 countries.
The US leads the field producing 3 GW, followed
by the Philippines, Iceland, and El Salvador, each
of whom produce more than 25% of their country’s
electricity needs from geothermal sources. 3
Yet another great advantage of geothermal
generating plants is that they can generate energy
24 hours a day, something otherwise possible only
for nuclear and fossil fuel plants. In this sense,
they are far superior to wind and solar energy systems,
which both suffer from relatively low efficiency
and unpredictable down times.
Like with oil, the most expensive part of geothermal
electricity generation is the drilling. Drilling
a typical well doublet to produce 4.5 MW of
electricity may cost $10 million. In total, electrical
plant construction and doublet well drilling
could cost $3 million-$8 million per MW of electrical
capacity. How does this compare with nuclear
generation plants, which are also capital-in-
Gioietta Kuo is a research physicist specializing in energy problems. She has published articles in more than 70
top professional journals, as well as in The People’s Daily and other widely read publications in China. She can be
reached at kuopet@comcast.net.
World Future Review Spring 2012 5

tensive but cost relatively little to run A standard
nuclear reactor could have a capital cost of $3 billion
for a 1000 MW plant, while an enhanced geothermal
system (EGS) may have a capital cost of
$4 million per MW.
As for running costs, most of the energy costs
for coal, gas, and renewables are in the range of
$0.03-0.09/kWh 4 with nuclear energy toward the
low end of the scale and wind and solar costing
out at the high end. Geothermal EGS have an average
cost of about $0.06 per kWh. However, geothermal
power has the advantage that it is very
scalable. A small plant might cost-effectively supply
all the electricity needed to power a rural village,
while nuclear and coal generation plants are
only economical in larger-scale facilities producing
1,000 MW or more.
A development which has great potential is
the direct use of the heat from
shallow ground two or more
meters deep which the earth
maintains at a constant temperature
of about 10-16° C. 5 A
geothermal heat pump can tap
into this resource to heat and
cool buildings. The system
consists of heat pump, an air
delivery system (ductwork),
and a heat exchanger—a system
of pipes buried in the shallow
ground near the building.
In the winter, the heat pump
removes heat from the heat exchanger
and pumps it into the
indoor air-delivery system. In
the summer, the process is reversed: The heat
pump moves heat from the indoor air into the
heat exchanger.
The heat removed from the indoor air can
also be used to provide a free source of hot water.
There are many other possible uses for this direct
gentle heat, such as for greenhouses, drying crops,
warming the pond water in fish farms, and in in-
Figure 1. Dry Steam
Power Plant
Generator
Turbine
Ground Surface
Wellhead
dustrial processes such as pasteurizing milk. Since
these direct heat applications can use much shallower
wells and operate at lower temperatures,
smaller systems with lower costs and risks are feasible.
Residential geothermal heat pumps with a
capacity of 10 kW are routinely installed for
around $1,000-$3,000 per kW. 6 There are three
basic type of geothermal power station for electricity
generation depending on the depth of the
well and temperature of the steam used for driving
the turbines.
1. Dry Steam Power Plant
This is the most direct plant using steam of
150° C to turn turbines. Figure 1 shows a simple
system. The pipe carries cold water, which is
pumped into the ground, where it is heated by
running through hot rock (which has sometimes
been cracked by explosives or
Condenser
Wellhead
Explanation
Water
Steam
Subsurface
Injection
highly pressurized water to facilitate
the release of heat). It
converts to steam and is
pumped back up to the surface
to drive a steam turbine. A
condenser then produces water,
which is returned to the
ground.
2. Flash Steam Power Plant
Water inside a well is
heated to temperatures greater
than 180° C and then pumped
via high pressure to generation
equipment at the surface. Once
the hot water reaches the generation
equipment, pressure suddenly decreases,
which allows some of the hot water to flash-convert
to steam. This steam is then powers the turbine/generator
to produce electricity. The remaining
hot water is condensed from the steam and
pumped back into the reservoir. The largest geothermal
system in operation today is a steam
driven plant located in Geysers, California.
6 World Future Review Spring 2012

3. Binary Cycle Power Plant
This is a recent development that can operate
using fluids at temperatures as low as 58° C.
It works by passing moderately hot water through
a heat exchanger with a fluid that has a much
lower boiling point than water. This secondary
fluid flash vaporizes, which in turn drives the turbine.
This is the most common type of geothermal
electricity generating plant being constructed
today, chiefly because it relies on relatively low
temperature and requires a shallower well. Hence
it has the widest potential for development.
Status of Geothermal Energy Worldwide
A new system now under development, called
Enhanced Geothermal Systems (EGS), aims to capture
the heat in “hot dry rock.” The hot rock reservoirs,
typically situated 4-10 kilometers below the
surface, are first broken up by high-pressure water
that is pumped through them. The plants then pump
cold water through the broken hot rocks. The cold
water heats up, turns into steam, and returns to the
surface, where it powers turbines. This resource has
enormous potential and can be used on a much
larger scale than other geothermal methods.
At present, two emerging technologies are attempting
to tap into the full potential of geothermal
energy. EGS is one of these. The other is coproduction
of geothermal electricity in oil and gas
wells. Although this latter technology still needs
further development, it offers a huge advantage in
that the wells themselves already exist, so there is
no need for capital-intensive additional drilling.
Considering that geothermal energy offers
practically limitless resources, why is it that the
existing world production is only around 0.07 trillion
kWh, or roughly 0.3% of global energy production
The main reason is the high cost of drilling
geothermal wells. As we know, drilling for oil
has been going on for over a century, and the process
has developed gradually with time. Like nuclear
reactors, a typical geothermal plant requires
a long lead time, taking 5-10 years to develop.
Yet the advantages of geothermal energy are
numerous. It is worthwhile to summarize them
briefly below:
• It is a limitless energy resource with no fuel
costs.
• As an energy baseload, it is capable of producing
electricity 24 hours/day.
• It produces only a small fraction of the toxic
gas emissions that coal produces.
• It uses a mere 12% of the land per GWh produced
as coal facilities.
• It uses only 20% of the water per MWh required
by nuclear, coal or oil powered plants.
• It is very scalable, a small plant can easily be built
to supply a rural village at relatively low capital cost.
• Geothermal heat is obtainable almost
everywhere on earth.
• Exploiting the co-generation from existing
oil wells would greatly reduce drilling costs.
• Heat from shallow ground has multiple industrial
uses that include heating greenhouses,
fish farms, pasteurizing milk, etc.
• Capital cost, though high, is comparable to
that required to build other energy facilities.
How does geothermal energy compare with
other renewable sources like wind, solar, or biofuels
As noted above, geothermal offers many distinct
advantages, and development costs are similar.
In sum, the potential for geothermal energy is
great, and it is high time we paid more attention to
developing this neglected energy resource.
References
1. http://iga.cnr.it/documenti/IGA/Fridleifsson_et_al_
IPCC_Geothermal_paper_ 2008.pdf
2. http://www.uxsusa.org/clean_energy
3. http://en.wikipedia.org/wiki/Geothermal_electricity
4. http://www.renewableenergyworld.com/rea/tech/
geothermal-energy
5. Knapp and Kuo, “The Future of Nuclear Energy,”
Innovation and Creativity in a Complex World, World
Future Society, 2009
6. http://en.wikipedia.org/wiki/Geothermal_energy
World Future Review Spring 2012 7