GEOTHERMAL ENERGY
The human population is currently using up its fossil fuel supplies at staggering rates.
Before long we will be forced to turn somewhere else for energy. There are many
possibilities such as hydroelectric energy, nuclear energy, wind energy, solar energy and
geothermal energy to name a few. Each one of these choices has its pros and cons.
Hydroelectric power tends to upset the ecosystems in rivers and lakes. It affects the
fish and wild life population. Nuclear energy is a very controversial subject. Although
it produces high quantities of power with relative efficiency, it is very hard to dispose
of the waste. While wind and solar power have no waste products, they require enormous
amounts of land to produce any large amounts of energy. I believe that geothermal energy
may be an alternative source of energy in the future. There are many things that we must
take into consideration before geothermal energy can be a possibility for a human
resource. I will be discussing some of these issues, questions, and problems.
In the beginning when the solar system was young, the earth was still forming, things
were very different. A great mass of elements swirled around a dense core in the middle.
As time went on the accumulation elements with similar physical properties into hot
bodies caused a slow formation of a crystalline barrier around the denser core. Hot
bodies consisting of iron were attracted to the core with greater force because they were
more dense. These hot bodies sunk into and became part of the constantly growing core.
Less dense elements were pushed towards the surface and began to form the crust. The
early crust or crystalline barrier consisted of ultra basic, basic, calc-alkaline, and
granite. The early crust was very thin because the core was extremely hot. It is
estimated that the mantel e
200 to 300 degrees Celsius warmer than it is today. As the core cooled through volcanism
the crust became thicker and cooler.
The earth is made up of four basic layers, the inner solid core, the outer liquid core,
the mantel and the lithosphere and crust. The density of the layers gets greater the
closer to the center of the earth that one gets. The inner core is approximately 16% of
the planet's volume. It is made up of iron and nickel compounds. Nobody knows for sure
but the outer core is thought to consist of sulfur, iron, phosphorus, carbon and
nitrogen, and silicon. The mantel is said to be made of metasilicate and perovskite.
The continental crust consists of igneous and sedimentary rocks. The oceanic crust
consists of the same with a substantial layer of sediments above the rock.
The crust covers the outer ridged layer of the earth called the lithosphere. The
lithosphere is divided into seven main continental plates. These continental plates are
constantly moving on a viscous base. The viscosity of this base is a function of the
temperature. The study of shifting continental plates is called Plate Tectonics. Plate
Tectonics allows scientists to locate regions of geothermal heat emission. Shifting
continental plates cause weak spots or gaps between plates where geothermal heat is more
likely to seep through the crust. These gaps are called Subduction Zones. Heat emission
from subduction zones can take many forms, such as volcanoes, geysers and hot springs.
When lateral plate movement induced gaps occur between plates, collisions occur between
other plates. This results in partial plate destruction. This causes mass amounts of
heat to be produced due to frictional forces and the rise of magma from the mantle
through propagating lithosphere fractures and thermal plumes sometimes resulting in
volcanism. During plate movement, continental plates are constantly being consumed and
produced changing plate boundaries. When collisions between plates occur, the crust is
pushed up sometimes forming ranges of mountains. This is the way that most Midoceanic
ranges were formed. Continental plates sometimes move at rates of several centimeters
per year. Currently the Atlantic ocean is growing and the Pacific ocean is shrinking
due to continental plate movement.
In Rome people first used geothermal resources to heat public bath houses that were used
for bathing or balneology. The mineral water was thought to be therapeutic. The
minerals in the water have been used since the beginning of time. Through out the years
geothermal heated water or steam has been used in many different systems from heating
houses and baths to being a source of boric acids and salts. Today geothermal fluids
provide energy for electricity production and mechanical work. Boric acid is still
extracted and sold. Other byproducts of geothermal heated liquid are carbon dioxide,
potassium salts, and silica.
The first 250 kilowatt geothermal power plant began operation in 1913 in Italy. By 1923
the United States had drilled its first geothermal wells in California. In 1925 Japan
built a 1 kilowatt experimental power plant. The first power plants constructed in Italy
were destroyed in WWII, then rebuilt bigger and more efficient. Mexico built a 3.5
megawatt unit in 1959. In the United States an 11 megawatt system at the geysers in
California was constructed in 1960. Japan then installed a 22 megawatt plant in 1966.
Geothermal energy has been used for things other than energy production, such as
geothermal space-heating systems, horticulture, aquaculture, animal husbandry, soil
heating and the first industrial operation of paper mills in New Zealand. Large scale
geothermal space-heating systems were constructed in Iceland in 1930.
The word "geothermal," refers to the thermal energy of the planetary interior and it is
usually associated with the concept of systems in which there is a large reservoir of
heat to comprise energy sources. Geothermal systems are classified and defined
depending on their geological, hydrogelogical and heat transfer characteristics. Most
geothermal heat is trapped or stored in rocks. A liquid or gas is usually required to
transfer the heat from the rocks. Heat is transferred in three different ways,
convection, conduction, and radiation. Conduction is the transfer of energy from one
substance to another, through a body that may be solid. Convection is the transfer of
energy from one substance to another through a working moving medium, such as water. The
medium usually transfers the energy in an upward direction. Radiation is the transfer of
energy out of a substance through the excitement of gas molecules surrounding a
substance. Radiation is dependent upon two things the object emitting the heat and the
surrounding's ability to absorb heat. Convective geothermal systems are characterized
by the natural circulation of a working fluid or water. The heated water tends to rise
and the cool to sink continually circulating water throughout the ground. The majority
of the heat transfer is done through convection and conduction, radiation hardly ever
effects heat flow. When geothermal heated water collects into a reservoir one form of a
geothermal resource is created. One can approximate the amount of thermal energy present
in a geothermal resource by comparing the average heat content of the surface rocks with
the enthalpy of saturated steam. Enthalpy is energy in the form of heat released during
a specific reaction or the energy contained in a system with certain volume under certain
pressure. It is generally accepted that below a depth of ten meters, the temperature of
the ground increases one degree Celsius for every thirty or forty meters. At a depth of
ten meters annual temperature changes no longer affect the temperature or the earth.
The most common geothermal resources used for the production of human consumed energy
are hydrothermal. Hydrothermal systems are characterized by high permeability by
liquids. There are two basic types of hydrothermal systems, vapor and liquid dominated
systems.
In a liquid based system, pumps must be placed very deep in the well where only the
liquid phase is present. By keeping the liquid under pressure it is possible to keep the
liquid at a much higher temperature than the liquid's normal boiling point. If the liquid
is not kept under pressure, it will flash. Flashing is the process of vaporization. It
requires 540 calories per gram of heat to vaporize water. The super heated pressurized
water is pumped up a long shaft into the plant. When it reaches the plant, controlled
amounts of the pressurized water is allowed to flash or vaporize. The rapidly expanding
gas pushes or turns the turbine. A power plant may have numerous flash cycles and
turbines. The more flash cycles the higher the efficiency of the power plant. Once the
heated liquid has been used to the point where it has cooled to an unusable temperature
it is reinjected into the ground in hopes that it will replenish the geothermal well.
Vapor systems work in much of the same way. The super heated gas flows through surface
reboilers that remove all of the non-condensable gases from the mixture of gases. The
gas is pumped into pressurization tanks where extreme pressure causes the gas to
condense. The super heated liquid is then allowed to flash. The rapidly expanding gas
turns the turbine. Specific examples and sites of electrical energy production will be
discussed later.
Conductive geothermal systems consist of heat being transferred through rocks and
eventually being transmitted to the surface. The amount of heat transferred in a
conductive geothermal is considerably less than the heat transferred in a convective
system. Conductive geothermal systems lack the water to efficiently transfer the heat,
so water must be artificially injected around the hot rocks. The heated water is then
pumped from the underground reservoir to the surface. This system is not as effective as
others because the temperature that the heated water reaches is not very great.
Geopressured geothermal systems are similar to hydrothermal systems. The only difference
is the pressure of the high temperature reservoir. Geopressured geothermal systems may
be associated with geysers. Some geopressured geothermal systems reach pressures of
fifty to one hundred megapascals (MPa) at depths of several thousand meters. These
systems provide energy in the form of heat and water pressure making them more powerful
and useful. Currently most electricity producing geopressured geothermal systems are
only experimental. There are many factors in this type of system that are very hard to
predict such as the reservoirs potential energy. It is very hard to predict the force at
which the water will be projected from the well since the pressure of the high
temperature is constantly changing. The salinity of the liquid projected is also very
high. In some instances the liquid consists of twenty to two hundred grams of impurities
per liter.
Today with the depletion of many other natural resources using geothermal resources in
more important than ever. Hot springs are natural devices that bring geothermal heated
water to the surface of the earth. This processes is very efficient, little heat is lost
during the transportation of the water to the surface. The heat is brought to the
surface via water circulation in either the liquid or gaseous form. Geothermal hot
springs are a good source of energy because it is probable that they will never be
exhausted as long as water is not pumped from the spring faster than it naturally
replenishes itself.
A simplified version of a vapor run geothermal electric plant might operate under the
following conditions. Holes are drilled deep into the ground and fitted with pipes that
resist corrosion. When the hole is first opened, steam escapes into the atmosphere.
Once the pipes are inserted into the holes the steam expansion becomes adiabatic. An
adiabatic system is a system in which there is little or no heat loss. Next the pipe is
connected to the central power station. No condensation takes place because the steam is
superheated. Many drill holes are connected to the central power station which results
in mass quantities of superheated water vapor pushing the turbine. The more drill holes
that are connected to the power station the greater the pressure of the gas flowing
through the turbine. The greater the pressure of the gas the faster the turbine turns
and the more electricity produced. In some power plants the water vapor itself is not
used to turn the turbines but only to heat another purer substance. This method is less
efficient but does not corrode the machinery. Most superheated gas from geothermal
resources is not pure water but a mixture of gases. Some of these gases can be extremely
corrosive so using purer non-corrosive materials has its advantages. Some common gases
used are ethyl chloride, butane, propane, freon, ammonia. The efficiency of these
generators is limited by the second law of thermodynamics.
The second law of thermodynamics states that a thermal engine will do work when heat
entering the engine from a high temperature reservoir is at a different temperature than
the exhaust reservoir. The thermal engine must take heat from the high temperature
reservoir convert some of that heat to work and exhaust the remaining heat into a low
temperature reservoir. The difference between the heat put into the engine and the heat
deposited as waste energy is transformed by the engine into mechanical work. The maximum
possible efficiency of a heat engine is called its Carnot efficiency. Carnot efficiency
is never reached and the actual efficiency is always lower than the Carnot efficiency.
The greater the difference in temperature between the superheated gas and the low
temperature exhaust reservoir the higher the efficiency of the power plant. The average
actual efficiency for a geothermal power plant ranges from the single digits to about
twenty percent. The average actual efficiency for a fossil fuel burning electrical power
plant is approximately thirty percent. While other methods of electricity production
may have slightly better efficiency than a geothermal power plant, the less destructive
environmental impacts of geothermal power plants offset the importance of the a higher
efficiency. Direct use of geothermal heat for heating purposes can result in actual
efficiencies of up to ninety percent. Fossil fuel powered heat systems can generally
only reach actual efficiencies of seventy to eighty percent.
As well as being used for electricity, geothermal energy is currently being used for
space heating. Geothermal heated fluid used for space heating is widespread in Iceland,
Japan, New Zealand, Hungary and the United States. In a geothermal space heating
system, electrically powered pumps push heated fluid through pipes that circulate the
fluid through out the structure. Geothermal heated fluid is also being used to heat
greenhouses, livestock barns, fish farm ponds. Some industries use geothermal energy for
distillation and dehydration. Although there are many pluses to using
geothermal energy there are also some problems. It was generally assumed that geothermal
resources were infinite or they could never be completely depleted. In reality the exact
opposite is true. As water or steam is pumped out of the well the pressure may decrease
or the well may go dry. Although the pressure and fluid will eventually return it may
not do so fast enough to be useful. Drilling geothermal wells is very expensive. It is
generally figured that a geothermal well should last 30 years in order to pay for itself.
Another factor to take into consideration is the disposal of the waste water. Some
geothermal fluid consists of several toxic materials such as arsenic, salt, dissolved
silica particles. These materials can pollute drinking water and lakes. When the waste
water is reinjected back into the earth the previously dissolved silica particles
precipitate out of the liquid and can block up the pores in the reinjection well. The
cool water can also create new passages through the rocks and create unstable ground
above. There are three main problems that can plague a power plant when it is operated
using geothermal energy, silting, scaling and corrosion. Scaling is caused by silting or
when suspended particles build up on the insides of the pipes. Scaling is directly
related to the pH of the liquid. In some cases chemicals or other additives such as HCl
have been added to the liquid to try to neutralize the liquid. Silting is when the
particles that were dissolved in the hot fluid precipitate out when the fluid cools.
This generally occurs in the pipes and can cause considerable damage to the pipes if
significant pressure builds. This problem can be solved by using simple filters that are
periodically changed in the pipes. Corrosion occurs because of acidic substances
incorporated in the geothermal fluid. Usually geothermal fluid contains some boric acid.
Using pipes that are not affected by these liquid generally takes care of corrosion.
Unfortunately most metals that are non-corrosive are very expensive. Most types of
wildlife can not live in or consume saline water. If the cooled fluid containing
dissolved toxins and salt contaminates lakes or streams the environmental effects can be
disastrous. Air pollution from geothermal resources is also significant. The most
common type of air pollution is the release of hydrogen sulfate gas into the air. At the
geysers in California an estimated 50 tons per day of hydrogen sulfite is released into
the atmosphere. Iron catalysts have been added to try to offset the effects of
pollution but have failed because moisture and carbon dioxide reduce the efficiently of
the catalysts so much that it is not effective. Noise pollution is another consideration
that must be taken into account. When the steam and water escape from the system it
makes a relatively loud noise. If the wells are located near any residential areas it
can raise problems and discontentment within the community. Some geothermal power plants
have installed cylindrical towers where the water vapor and water is swirled around. The
friction created by the movement of the gas or fluid decreases the overall kinetic energy
of the gas or fluid causing the internal energy to decrease. When the internal energy is
decreased the noise of gas escaping is also decreased. Geothermal resources do produce
pollution but the pollution would be there even if we did not exploit the resource.
Other energy producing systems used today produce and emit pollution that otherwise would
not be introduced into the environment. I feel that the benefits of using geothermal
resources as a source of energy for electricity and mechanical work production out weigh
the downfalls.
The world has many different geothermal regions that are exploited for the production of
electricity and other things. The United States is one of the leaders in manufacturing
geothermal produced electricity. One of the most productive regions in the U.S. is the
Pacific Region. Most geothermal regions contain mostly heated water. Geysers produce
very large amounts of water vapor and other gases. Geysers have the potential to produce
electricity relatively efficiently.
In 1979 The Geyser power plants had a rating of 600 megawatts of electricity(MWe). Today
they are rated for over 2000MWe. Most of the geysers are located on the side of a
mountain near Big Sulfur Creek, on the California coast west of Sacramento. William Bell
Elliott was the first to see this natural wonder in 1947 while surveying, exploring and
looking for grizzly bears. The earth around the Geysers geothermal site consists of
highly permeable fractured shale's and basalt's created during Jurassic age. The ground
above the wells consists of graywake sandstone. This form of sand stone is very hard to
penetrated. Scientists believe that the large geothermal reservoir was created when an
earthquake caused fault and shear zones. Steam temperatures in the geothermal wells
range from 260 to 290 . Pressures deep in the wells range from 450psig to 480psig
(3.1MPa to 3.3) . Some wells are 3000 meters deep and produce almost 175 tonnes of steam
per hour.
It is thought that the center of the magma or the heat source at The Geysers geothermal
site lies under Mt. Hannah. Geologists are led to believe that there is a large mass of
magma cooling under the geysers and power plants that is the source of all the heat.
This assumption is proven when seismic waves caused by earth quakes are slowed when they
pass through the mountain. A fairly large fractured steam reservoir rests above the
cooling molten.
In 1967, the Union Oil Company in partnership with Magma Power Corporation and Thermal
Power Company began producing electricity from the Geysers Geothermal region and selling
it to the Pacific Gas and Electric Company. The turbines in the power plant were designed
to operate under intake pressures of 80psig to 100psig. At first the plant operated at
maximum efficiency but as the years went by the geothermal resource was slowly depleted.
The depleted heat source did not produce the constant pressure that was required for
maximum efficiency so the efficiency decreased. There are two methods of drilling wells,
mud drilling and air drilling. Mud drilling tends to clog up the porous rock but it is
easier on the drilling machinery. Air drilling leaves the porous rock free for water
and steam flow but it is very hard on machinery due to abrasion and heating. Air
drilling is therefore very expensive. Geothermal wells do not always maintain constant
pressure. New wells must be drilled to continually maintain constant pressure on the
turbine. The system built at The Geysers geothermal field delivers of super heated
steam. The steam produced by the wells is not pure water but consists of 1%
non-condensable gases along with dust particles. If not cleaned off, the dust can
accumulate on the inside of the turbine blade shrouds and cause turbine failure. This
problem was virtually eliminated when heavy duty blades and shrouds replaced the faulty
ones. It was thought that by the time the steam made it to the turbine very little of it
was still superheated, so special non-corrosive metal was not required in the
construction of the upper level piping and the turbine. Normal carbon piping was used in
the original construction. This proved not to be the case, after a while the pipes began
to corrode. As steam condenses non-condensable gases become more of a problem. They
become more concentrated, more corrosive and can form sulfuric acid. This new problem
was solved by replacing the carbon steel used in the original construction with
austenitic stainless steel. Electrical connections and wires were also effected by
concentrations of sulfuric acid. They were replaced with aluminum and stainless steel.
The steam generated from the wells and geysers has a constant enthalpy of 1200-1500 Btu
per lb. The use of condensing steam turbines that exhausted waste water below
atmospheric pressure increased the efficiency of the plant. There were no rivers or
streams in the immediate area that were sufficiently cool enough to be used as a cooling
mechanism, so cooling towers were constructed. Incorporating the cooling towers into the
system allowed the waste water to be discharged at a cooler temperature f 18 therefore
increasing the possible efficiency of the system.
Carnot Efficiency of The Geysers Power Plant
Carnot Efficiency =
=18
=290
Carnot Efficiency =
Carnot Efficiency = .4831
or
48%
This is a relatively efficient cycle. It certainly can compete with other modern day
types of electricity production. Unfortunately carnot efficiencies can never be reached.
A large amount of energy is lost in the condensers and turbines. I feel that while the
efficiency of this geothermal power plant might not be overwhelmingly better than other
modern day methods of electricity, the lack of pollution makes up for the loss in
efficiency. Even though The Geysers power plant is relatively efficient, it does not
even come close to taking advantage of all the emitted heat. Only 2% of the emitted heat
from the source is used to heat water for electricity production. This geothermal
resource will not last for ever though.
Heat Content of the Entire Geysers Geothermal Site
-The Geysers geothermal site covers approximately .
-Heat is only recovered from the top 2km of the earth at The Geysers site.
-The average temperature in this top 2km of earth is 240 .
-The average air temp at The Geysers site is 15 .
-The specific heat of the permeable rock that makes up most of geothermal region is .
Volume x Specific Heat x Change in Temperature = Heat Content
Vol = x =
SpHt=
= 240 - 18 = 222
Q =( x )( )( )(222 )
Q= Joules of Heat Content in the entire Geysers geothermal region
Life of The Geysers Heat Source
-Power output of The Geysers plant =2000MW
-Fraction of the total heat used in the production of steam = 2%
-Power taken from the geothermal resource = 2,000MW/2% =
100,000 MW
-Heat content of the entire Geysers geothermal region = Joules
-Seconds in one year =
-1 Watt = 1 Joule/sec
100000MW = J/year
J/ J/year = 24.67years.
According to my calculations The Geysers geothermal resource will be depleted in 24.67
years at the current rate of usage. Of course this is not taking into account the rate
at which the resource is renewed from heat coming from deeper in the earth. I am
assuming that the rate of depletion is so much greater than the rate of renewal that it
is not significant in the calculation.
The power plant at The Geysers site is run on dry superheated gases. The power plant
now has 11 generators and has a rating of over 2000 MWe. The process of electrical
power generation used at The Geysers power plant is relatively simple when compared to
other modern day power plants. The steam that evolves from the wells flows through pipes
that lead to the turbine. The pressure exerted by the superheated steam turns the
turbine which produces electricity. The steam then flows into the direct-contact
condensers below the turbine. Cooling water from the cooling towers is constantly
circulated through the condensers. The condensed steam and cooling water is then pumped
back into the cooling towers. Because the evaporation rate from the towers is slower
than the rate at which water is pumped into the towers, excess amounts of water
accumulate in the cooling tower. This excess water is then pumped to reinjection wells
where it flows down through the soil and porous rock and is reheated by the heat source.
The cycle begins all over again. See the diagram below.
The costs of running this particular geothermal electrical plant are very competitive
with the cost of other types of modern day plants. The operation costs for the plant at
The Geysers is almost same the as the operation costs of an average fossil fuel powered
plant and much less than the operating costs of a hydroelectric or nuclear plant. One of
the greatest advantages of this and most geothermal systems is the relative lack of
pollution. While most coal plants give off significant amounts of sulfur, somewhere
around 93 tons per day for the average coal plant, geothermal plants produce no gas
pollution other than the gases that would be naturally emitted from the geysers anyway.
Coal plants are by far the worst polluters but other types of plants are not far behind.
Average Cost of Geothermal Produced Energy per Kilowatt in the U.S.
Total electricity produced in the U.S. during 1985 = 652000MW
Percent of Geothermal energy contributed to total U.S. production 3%
3% x 652000MW = 19560MW
Methods of geothermal energy production Capital Dollars per Kilowatt
Dry Steam Flash 83% $1000/kW
Binary 17% $3600kW
Dry Steam Flash = 83% x 19560MW x 1000kW/MW x $1000/kW =
Binary = 17% x 19560MW x 1000kW/MW x $3600/kW =
Total = +
total = per 19560MW
/1956MW x 1MW/1000kW = $1431.5 per kW
The future of geothermal energy looks very promising. There have been many
technological breakthroughs that have resulted in increased efficiencies of modern day
geothermal electrical plants. I feel that with the current environmental situation that
the world now faces a viable method of clean up will include the use of geothermal power
plants and resources. In a world that is suffocating from the chemicals, and
particulates that are created in the production of electricity and other commercial
industries, we have no choice but to change our ways. The earth can not support the
current rates of pollution. If we do not change reduce pollution the effects that are
beginning to be see now will become irreversible. Using geothermal resources for other
purposes such as space heating can only help reduce pollution emission. With in the next
century the world will begin to feel the energy crunch. Supplies of other natural
resources such as coal, oil and other petroleum products will begin to become scarce.
The world today is completely electricity dependent. Without electricity, the world as
we know it would cease to exist. In the next century we must learn to be less
electricity dependent or find other sources of energy.
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