Geothermal Energy
Introduction
Geothermal energy is energy that exists in nature as a result of heat from the Earth's core. The
Earth's core lies nearly 6000 km below the surface and holds temperatures that near 5000ºC.
These extreme temperatures are enough to heat the rock surrounding the core (called the mantle)
and cause the rock to melt. Melted rock in the mantle is called magma. The liquid magma has a
lower density than the solid rock around it, so it tends to move upwards towards the Earth's
surface. The majority of the time, the magma stays underneath the Earth's surface and heats up
the rock and pockets of water that it comes in contact with. Sometimes the magma actually
finds its way through the Earth's crust and vents through volcanoes as lava. Most often, humans
have not used the geothermal energy from the magma directly; humans have spent much time
exploiting the water tables that are heated by the magma. These water sources are very often
vented to the surface by cracks in the crust called fissures. Once the heated water has reached
the surface, humans have put it to a variety of uses. A couple of wonderful introductions to
geothermal energy can be found at The Geothermal Education Office and the Energy Web
Directory.
Classes of Geothermal Energy Sources
There are three main classes of geothermal energy sources. The first is called direct usage. The
water that is heated by the magma beneath the Earth's surface can be pumped to buildings and
used in heat exchanging systems. The second way that geothermal energy is harnessed is
through using the steam that comes from superheated water. If the steam vents are under
sufficient pressure, then they can be used to turn turbines. The third class of geothermal energy
is called dry steam. An outside water source (naturally or otherwise) is applied to fractured rock
that has been heated to high temperatures, and then the steam that arises can be used to turn
turbines. For a look at various geothermal sources (especially geysers), World Geyser Links has
a huge list of links that will show you many of the world's most active geothermal areas. A beta
version of a world map showing the locations of geothermal sites worldwide is also available at
the International Geothermal Association. Many of these areas are the center of work in
geothermal technologies.
History of Geothermal Energy
Humans have used geothermal energy for a variety of uses in a variety of time periods. The
Romans used geothermally heated water in their bathhouses for centuries. The Romans also
used the water to treat illnesses and heat homes. In Iceland and New Zealand, many people
cooked their food using geothermal heat. Some North American native tribes also used
geothermal vents for both comfort heat and cooking temperatures. Most of these early uses of
the Earth's heat were through the exploitation of geothermal vents. Some other historical uses of
geothermal energy can be found at The Geothermal Education Office, and WSU Environmental
Geology.
Modern Uses of Geothermal Energy
The most common utilization of geothermal energy is through hydrothermal means.
Hydrothermal energy is energy derived from heated water. Water that is heated by the Earth can
be used directly to supply heat to human homes. The steam generated by heated water can be
used to move turbines if under sufficient pressure. A third way of harnessing geothermal energy
is through dry steam, which can also turn turbines. One of the finest sites for surveying the uses
of geothermal energy is the U.S. Department of Energy Geothermal Technical Site.
Applications of Geothermal Energy Sources
Heating and cooling buildings using geothermal energy is the primary use of the Earth's heat
energy. Much energy is placed into the moderation of temperature inside buildings, especially
during times of extreme cold or heat. Using geothermal energy as a way of maintaining
temperatures in buildings is one way to continue to provide that comfort but reduce the use of
energy sources that are more polluting to the Earth's atmosphere. Geothermal energy can also be
used to create electricity and supplement the conventional sources available. An interesting
proposal on geothermal space conditioning can be found at Enviro$ense. According to the article
on Enviro$ense, using geothermal energy to heat homes could save between 20%-50% in total
emissions, and reduce the load of utilities and appliances (many of which are refrigeration or
heating units) on the electrical power grid by 75%. Not only would the use of geothermal energy
reduce emissions, it could also reduce energy costs over time. According to the Energy Web
Directory, much of the cost of energy in developed countries is hidden. Geothermal energy
provides a source of energy that has little subsidiary costs (such as processing, pollution control,
and transportation of raw fuels) compared to other fuel sources. The dependency of geothermal
energy as an alternate fuel source is also very high. The University of Erlangen Geology
Department says that geothermal plants can be on-line an average of 97% of the time. In
contrast to nuclear (65% average on-line time) and coal (75% average on-line time), geothermal
sources score quite high. A more economical approach to geothermal harnessing is found at
Renewable Energy: Geothermal Energy. Some notable statistics from the report are as follows.
Geothermal energy in the U.S. currently costs a very competitive $0.03-$0.08 per kilowatt hour.
As well, the known sources of geothermal energy in the U.S. could replace nearly 1.2 trillion
cubic feet of natural gas (a popular fuel source spreading throughout the U.S.) every year for 30
years. This amounts to over 60 times the amount of natural gas used each year. Geothermal
plants that are placed near active geothermal areas (such as at The Geysers in California's Lake
County), are capable of producing the same amount of energy as more typical fossil fuel plants,
but produce only 1% of the sulfur dioxide and 5% of the carbon dioxide of a fossil fuel plant.
As mentioned in Being Creative, geothermal systems also take up very little land and are a very
low hazard for accidents that will effect the environment significantly.
Heating and Cooling Systems
The GeoExchange Information Center has some thorough descriptions of both residential and
commercial geothermal systems. Commandaire Geothermal Overview also has a nice graphical
representation of a geothermal heating system. Basically, they work as follows. Heating and
refrigeration units work by moving heat from one source to another via a gradient system. For
example, an air conditioner takes the hot air inside a building and exchanges its heat to the
outside air. Generally, such units need large surface areas to dispense an adequate amount of
heat to the environment. The same thing can be done with the gradient directed towards the
Earth. Using a system of valves (often called a ground loop) that are connected to the cooler
rock in the Earth, the heat from the air (or a water medium, like in water heaters) can be
exchanged into the crust. This cooling process can be done nearly anywhere in the world, and in
temperate climates can sufficiently heat a building the majority of the year. For places that are
near sites which are geothermally heated to reasonably high temperatures, then this set-up can
also be used to heat a building in cold climates. The nice aspect of a geothermal type system is
that the system is reversible; it can be used to heat or cool. An interesting discussion of
designing a building that is created using geothermal methods in mind is the topic of Getting
Started in the Geothermal Business. Geothermal systems are often hidden in the ground, so
unsightly and noisy heating and cooling systems are no longer encountered on a building with a
geothermal system. They also take up less physical space when designing a building, and
generally provide very low maintenance costs.
Another usage of geothermal energy is on a more industrial level. The heat from geothermal
water sources can be used on large scales to heat buildings in a way that is similar to the above
discussion. Geoexchange looks at geothermal heating systems capable of supplying large
amounts of energy. Geothermal plants tend to be expensive to produce because of the ground
loop that is used in heat exchange beneath the surface. However, these plants also have very low
operating costs and can recover the capital investment within a few years. A 3 ton system
(capable of heating a house) generally costs approximately $2000 American. However, this
price can quadruple in parts of the U.S. in which not many geothermal projects exist. This
increase in cost is generally due to lack of abilities in contractors, and generally falls quickly as
more systems are placed in region, but the cost can still be a deterrent. Currently, the largest
geothermal heating plant is 1100 ton system at New Jersey's Stockton College. For a look at a
large scale geothermal project, a visit to Richard Stockton College of New Jersey Geothermal
Project will not disappoint.
Electricity Generating Systems
In a electricity generating capacity, U.S. DOE Geothermal Technologies states that 2700 MW of
power could be derived from currently used geothermal sites in the U.S. This energy is enough
to power 3.5 million homes. The U.S. DOE believes that this is a gross understatement of the
total geothermal potential in that country. Electricity is derived from geothermal sources three
ways. The first is by using steam from geothermal fissure to turn generators. The second is to
use what is called flashing. Superheated water (over 200°C under extreme pressures) from
beneath the Earth's surface is passed into a chamber of lower pressure. There, part of the water
instantaneously turns into steam. This steam can be used to turn a generator. The third type is to
use a secondary fluid to turn a generator. The secondary fluid is heated to a gaseous state by
moderate temperature geothermal water (less than 200°C) and then reconstituted to run through
the heating cycle continuously. The best place for this these types of electrical conversions is
near a site where steam occurs naturally: locations around geysers. However, locations that
exhibit superheated and moderate heated water are much more common, and therefore provide a
better energy option. As mentioned earlier, there is also the process of dry steam from which
electrical energy can be derived. Geothermally heated rock is fractured and then water is
pumped into these fractures. The steam that rises from the rocks can be used to turn generators,
as in the previous cases. Electricity produced this way was quoted to cost between $0.03 and
$0.08 per kilowatt hour.
For a much more complete and exhaustive study on industrial scale geothermal operations and
their specifics, EREN Geothermal Power Plants has a large storehouse of reports on file.
Geothermal Projects in Canada
The International Geothermal Association includes a reference to Canadian geothermal projects.
From 1974-1982 there was considerable effort put into researching a geothermal plant in Meager
Mountain, B.C. Since then, several more exploration-type missions have been made into that
area, and there is serious consideration towards creating a geothermal site there. Since the
federal government has done little to support the exploration of geothermal resources in Canada,
the Meager Mountain site is one of the only major projects to be under review. The I.G.A. site
has some references to other minor geothermal activities in Canada for those who are interested.
Disadvantages of Geothermal Energy
In our discussion thus, much of what has been said is in regard to the positive aspects of
geothermal energy. However, like all energy sources, geothermal sources have their
disadvantages. A rather concise and to the point listing of advantages and disadvantages of
alternative energy sources can be found at the University of Oregon Lecture Study Site.
Geothermal energy is generally a highly localized resource, and the processes used to extract
energy move at a much higher rate than the processes that restore energy into the geothermal
environment. In the same way that wind and hydro power rely upon certain wind speeds and
certain levels of water, geothermal energy relies upon an area having a certain level of activity.
Areas with very stable geothermal properties (or very unstable, such as near a volcano) may not
be able to support a geothermal project. In areas where geothermal activity is present but not at
high rates, exhausting the supply of energy (at least temporarily) is possible. Surprisingly (or
maybe not so surprisingly), a large number of critical viewpoints on geothermal energy are not
available on the World Wide Web as of yet. Most of the thorough geothermal sites are so
concerned with showing the positives of the technology that they do not list many of the
disadvantages of the systems. This state is bound to change as more and more viewpoints reach
the Web in the coming days.
Randy Ostridge, March 10, 1998, Written for Physics 261
University of Prince Edward Island