The word “geothermal”
comes from the Greek words geo (earth) and thermal (heat). It means the
heat of earth. The energy potential beneath our feet, in the form of
geothermal energy, is vast.
This tremendous resource amounts to
50,000 times the energy of all oil and gas resources in the world.
Geothermal energy has attracted worldwide attention as an alternate
source of energy for the last few decades. Presently this
non-conventional energy constitutes about 1% of the total global
electricity output but the scenario is changing very fast due to its
eco-friendly pollution free and renewable nature. About onethird of the
total available energy is spent on space heating, bathing, fish and
green house farming and industrial uses.
The USA, Iceland, China and New Zealand make maximum use of geothermal energy for electricity generation or as heat energy.
Together,
geothermal power plants and direct-use technologies are a winning
combination for meeting our country’s energy needs while protecting the
environment. Whether geothermal energy is used for producing electricity
or providing heat, it’s a clean alternative for the nation. And
geothermal resources are domestic resources. Keeping the wealth at home
translates to more jobs and our national economic and employment picture
improve.
Geothermal Energy Explained
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| The heat geothermal energy |
The Earth’s crust is a bountiful source of energy.
Nearly everyone is familiar with the Earth’s fossil fuels oil, gas, and
coal but fossil fuels are only part of the story. Heat, also called
thermal energy, is by far the more abundant resource. The Earth’s core,
4000 miles (6437 kilometers) below the surface, can reach temperatures
of more than 9000°F (4982°C).
The heat geothermal energy
constantly flows outward from the core, heating the surrounding area.
Nearby rock melts at high temperatures and pressure, transforming into
magma.
Magma can some times well up to the surface as lava, but
most of the time it remains below the Earth’s crust heating nearby rock.
Water seeps into the Earth and collects in fractured or porous hot
rock, forming reservoirs of steam and hot water. If those reservoirs are
tapped for their fluids, they can provide heat for many uses, including
electricity production.
Geothermal Drilling
Before
the Earth’s heat can be used for purposes such as generating
electricity or heating buildings, conduits between the geothermal
reservoir of hot water or steam and the Earth’s surface must be
provided. This is done by drilling production and injection wells, which
are often thousands of feet deep, into the reservoir. Drilling of
exploratory wells also helps collect data to define the size and
productivity of the geothermal reservoir. Construction of wells is
clearly essential, but it is also expensive, accounting for 15 to 30
percent of the total cost of a geothermal power project.
To drill
almost any well, a drill bit is mounted on the end of a long metal pipe
called the drill string, which is rotated from the surface by machinery
called a drill rig. New 30-foot lengths of pipe are added to the top of
the drill string as the bore hole gets deeper. To cool and lubricate the
drill bit and to carry away the chips of rock cut by it, a viscous
fluid called drilling mud is pumped down the drill string.
The mud
passes through holes in the drill bit and then flows back up the hole
in the space between the bore hole wall and the drill string.
Drill Bits
Drilling
costs are greatly affected by how quickly the drill bit can penetrate
the hard, abrasive, fractured rocks of a geothermal location, and by how
long it can last before the drill string needs to be taken out of the
hole to replace the bit. If both penetration rate and bit life were
doubled, drilling costs would drop an average of 15 percent.
Two kinds of bits are used:
- Roller-cone bits
- Polycrystalline diamond compact (PDC) bits
For
virtually all drilling in either geothermal or oil and gas wells
roller-cone bits and polycrystalline diamond compact (PDC) bits. Roller
cone bits have toothed cones that roll on the bottom of the hole as the
bit rotates, each tooth crushing the small area of rock beneath it.
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Polycrystalline diamond compact
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The
PDC bit uses thin layers of synthetic diamond bonded to tungsten
carbide-cobalt studs or blades. The diamond layer gives the cutter
extreme resistance to abrasive wear in the shearing action of cutting.
PDC bits are especially well suited to drilling through hot rock because
they have no moving parts, so high-temperature seals, bearings, and
lubricants are not an issue. They have gained this tremendous market
acceptance because they have consistently drilled faster and lasted
longer than roller cone bits. For geothermal drilling, however, PDC bits
do not work reliably well in rock that is more than moderately hard.
These
efforts will lead to enhance performance, extending full application of
PDC bits, with its attendant cost savings, to the hot, hard rocks of
geothermal reservoirs.
Bore hole Measurements
Measurements
in the borehole are used both to evaluate the reservoir once the well
is drilled and to provide data during drilling that will make the
process faster, cheaper, and safer. To function effectively for
geothermal drilling, this instrumentation must be adapted for slim hole
drilling and high-temperature conditions. Sandia has developed tools
that meet these temperature and size requirements, including a promising
new self contained, battery-powered, memory-storage system. Several of
these tools have been used extensively in the field and are available
for application or have been commercialized; others are in the late
stages of testing.
Baker Hughes has signed a licensing agreement
with DOE for use of down hole instrumentation, and Board Long year, a
supplier of drilling equipment for geothermal and mineral exploration,
recently commercialized core tube data logging equipment.
Electricity Production
Electricity
production using geothermal energy is based on conventional steam
turbine and generator equipment, where expanding steam powers the
turbine/ generator to produce electricity. Geothermal energy is tapped
by drilling wells into the reservoirs and piping the hot water or steam
into a power plant for electricity production.
Types or geothermal power plants:
- Dry steam
- Flash steam
- Binary cycle
Dry steam power plants
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Dry steam power plants
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The
steam is piped directly from wells to the power plant, where it is
directed into a turbine. The steam turns the turbine, which activates a
generator. The steam is then condensed and injected back into the
reservoir via a well.
Dry steam is the oldest type of plant first used in Italy in 1904 but it is still very effective.
The Geysers in northern California, the world’s largest single source of geothermal power, uses dry steam.
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Flash steam power plants
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Flash Steam Power Plants
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Flash steam power plants
tap into reservoirs of water with temperatures greater than 360°F
(182°C). This very hot water flows up through wells in the ground under
its own pressure. As it flows, the fluid pressure decreases and some of
the hot water boils or “flashes” into steam.
The steam is then
separated from the water once at the surface and is then used to power a
turbine/generator unit. The remaining water and condensed steam are
injected through a well and back into the reservoir.
Binary cycle power plant
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| Binary cycle power plant |
Binary
cycle power plants operate on water at lower temperatures of about 225°
to 360°F (107° to 182°C). These plants use the heat from the geothermal
water to boil a working fluid, usually an organic compound with a low
boiling point. The working fluid is vaporized in a heat exchanger and
used to turn a turbine.
The water is then injected back into the
ground to be reheated. The water and the working fluid are confined to
separate geothermal temperatures required for direct use 70° to 302°F
(21° to 150°C) are lower than those for electric power generation.
Hot
water from geothermal resources can be used directly to provide heat
for industrial processes, crop drying, or heating buildings. This
is called direct use. The consumer of direct-use geothermal energy can
count on savings of as much as 80 percent from traditional fuel costs,
depending on the application and the industry.
Direct-use systems
do require a larger capital investment compared to traditional systems,
but have lower operating costs and no need for ongoing fuel purchases.
Geothermal Heat Pumps
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Geothermal Heat Pumps
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Geothermal
heat pumps, also known as GHPs, enable the ground to serve as an energy
storage device.GHPs are similar to conventional air conditioners or
refrigerators.GHPs discharge heat to the ground during the cooling
season and extract useful heat from the ground during the heating
season.
GHPs marketed today also provide hot water. There are over
500,000 GHPs in service today in the United States, including about 600
systems at schools and colleges.
Heating
The direct use of the geothermal resource, however, evolved into a modern system that today provides space and domestic water heating
throughout the city of Boise to many homes, businesses, and government
buildings. The hot water from a geothermal well can replace the
traditional heat source often natural gas of a boiler, furnace, and hot
water heater.
Geothermal water can also heat a working fluid that
melts snow as it flows through piping installed underneath pavement.
Generally, an individual home or building only needs one geothermal well
for a heating system. In larger applications, like in Boise, a district
heating system can be used to supply heat from a central location of
one or more wells through a network of pipes to entire blocks of
buildings. percent compared to the cost of natural gas heating.
The savings are much higher when compared to electric, propane, or fuel oil heating systems.
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Geothermal District Heating System
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Agricultural
This
number continues to rise as word spreads about the benefits of direct
use in agriculture, such as lower operating costs and increased growth
rates. These can be significant competitive advantages. Many crops like
cucumbers, tomatoes, flowers, houseplants, tree seedlings, and cacti
flourish in geothermally heated greenhouses.
Several fish farms
and other aquaculture operations have found success using geothermal
water as a habitat for their livestock, making it the fastest growing
direct-use application in the country.
Industrial
Geothermal
direct use continues to show great commercial potential and competitive
advantages for a variety of industries. Industrial applications include
food dehydration, gold mining, laundries, milk pasteurizing, mushroom
culture, and sewage digestion. Geothermal direct-use resources are
especially well suited to vegetable dehydration operations, such as in
the production of dried onions or garlic. The dry climates throughout
much of the West also assist in the process.
The dehydration
process begins with geothermal water flowing through a heat exchanger,
which warms the air to temperatures ranging from 100° to 220°F (38° to
104°C).
Clean Energy from the Earth for the 21st Century
DOE
funds research to reduce the cost of geothermal components, systems,
and operations. Geothermal facilities use the natural heat in the
earth’s interior to produce electricity or to satisfy other heat energy
needs. The Program’s R&D activities closely align with its mission
and goals.
With improved exploration methods, industry will locate
and characterize new geothermal fields more accurately, reducing the
high cost and risk of development. Better technology for drilling wells
will make it possible to access deeper resources and reduce costs,
thereby expanding the economic resource base.
Advances in energy
conversion will establish air-cooled binary technology as a means of
generating competitively priced electricity from more plentiful
lower-temperature resources. These activities all contribute directly to
reducing the cost of geothermal development and enabling the
installation of more geothermal facilities. Geothermal electric
generation projects are capital-intensive enterprises, with the major
expenses being incurred before the plant begins to produce revenue. The
high-cost components of a geothermal development project include:
drilling exploration, production, and injection wells; and plant
equipment and construction.
The primary risk in a geothermal
project is confirmation of a viable reservoir, which usually requires
extensive drilling and well testing.
To help reduce the risks and costs in geothermal development, the program’s research strategy involves:
- Improving technologies for exploration, detection of fractures and permeable zones, well sitting, and fluid injection
- Decreasing the cost of drilling and completing geothermal wells
- Reducing the capital, operation, and maintenance costs of geothermal power plants.
Conclusion
Reducing
drilling costs will substantially cut the costs of geothermal
development, thus helping the domestic geothermal industry to maintain
its world-leader status and to expand its markets. Today, society uses
only a small fraction of the geothermal energy resource base. The
ultimate promise of geothermal energy is that a much larger fraction of
the total resource base can be tapped.
New and improved drilling technologies can make this happen.
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