Geothermal EnergyMatt Arnold9/17/96Physics 009Professor ArnsThe human population is currently using up its fossil fuel supplies atstaggering rates. Before long we will be forced to turn somewhere else forenergy. There are many possibilities such as hydroelectric energy, nuclearenergy, wind energy, solar energy and geothermal energy to name a few. Each oneof these choices has its pros and cons.
Hydroelectric power tends to upset theecosystems in rivers and lakes. It affects the fish and wild life population. Nuclear energy is a very controversial subject. Although it produces highquantities of power with relative efficiency, it is very hard to dispose of thewaste. While wind and solar power have no waste products, they require enormousamounts of land to produce any large amounts of energy. I believe thatgeothermal energy may be an alternative source of energy in the future.
Thereare many things that we must take into consideration before geothermal energycan be a possibility for a human resource. I will be discussing some of theseissues, questions, and problems. In the beginning when the solar system was young, the earth was stillforming, things were very different. A great mass of elements swirled around adense core in the middle.
As time went on the accumulation elements withsimilar physical properties into hot bodies caused a slow formation of acrystalline barrier around the denser core. Hot bodies consisting of iron wereattracted to the core with greater force because they were more dense. Thesehot bodies sunk into and became part of the constantly growing core. Less denseelements were pushed towards the surface and began to form the crust.
The earlycrust or crystalline barrier consisted of ultra basic, basic, calc-alkaline, andgranite. The early crust was very thin because the core was extremely hot. Itis estimated that the mantel e 200 to 300 degrees Celsius warmer than it istoday. 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 outerliquid core, the mantel and the lithosphere and crust. The density of thelayers gets greater the closer to the center of the earth that one gets. Theinner core is approximately 16% of the planet’s volume. It is made up of ironand nickel compounds. Nobody knows for sure but the outer core is thought toconsist of sulfur, iron, phosphorus, carbon and nitrogen, and silicon. Themantel is said to be made of metasilicate and perovskite.
The continental crustconsists of igneous and sedimentary rocks. The oceanic crust consists of thesame with a substantial layer of sediments above the rock. The crust covers the outer ridged layer of the earth called thelithosphere. The lithosphere is divided into seven main continental plates. These continental plates are constantly moving on a viscous base.
The viscosityof this base is a function of the temperature. The study of shiftingcontinental plates is called Plate Tectonics. Plate Tectonics allows scientiststo locate regions of geothermal heat emission. Shifting continental platescause weak spots or gaps between plates where geothermal heat is more likely toseep through the crust.
These gaps are called Subduction Zones. Heat emissionfrom subduction zones can take many forms, such as volcanoes, geysers and hotsprings. When lateral plate movement induced gaps occur between plates,collisions occur between other plates. This results in partial platedestruction. This causes mass amounts of heat to be produced due to frictionalforces and the rise of magma from the mantle through propagating lithospherefractures and thermal plumes sometimes resulting in volcanism. During platemovement, continental plates are constantly being consumed and produced changingplate boundaries.
When collisions between plates occur, the crust is pushed upsometimes forming ranges of mountains. This is the way that most Midoceanicranges were formed. Continental plates sometimes move at rates of severalcentimeters per year. Currently the Atlantic ocean is growing and the Pacificocean is shrinking due to continental plate movement. In Rome people first used geothermal resources to heat public bathhouses that were used for bathing or balneology.
The mineral water was thoughtto be therapeutic. The minerals in the water have been used since the beginningof time. Through out the years geothermal heated water or steam has been usedin many different systems from heating houses and baths to being a source ofboric acids and salts. Today geothermal fluids provide energy for electricityproduction and mechanical work. Boric acid is still extracted and sold. Otherbyproducts of geothermal heated liquid are carbon dioxide, potassium salts, andsilica.
The first 250 kilowatt geothermal power plant began operation in 1913 inItaly. By 1923 the United States had drilled its first geothermal wells inCalifornia. In 1925 Japan built a 1 kilowatt experimental power plant. Thefirst power plants constructed in Italy were destroyed in WWII, then rebuiltbigger and more efficient. Mexico built a 3. 5 megawatt unit in 1959.
In theUnited States an 11 megawatt system at the geysers in California was constructedin 1960. Japan then installed a 22 megawatt plant in 1966. Geothermal energyhas been used for things other than energy production, such as geothermal space-heating systems, horticulture, aquaculture, animal husbandry, soil heating andthe first industrial operation of paper mills in New Zealand. Large scalegeothermal space-heating systems were constructed in Iceland in 1930.
The word “geothermal,” refers to the thermal energy of the planetaryinterior and it is usually associated with the concept of systems in which thereis a large reservoir of heat to comprise energy sources. Geothermal systemsare classified and defined depending on their geological, hydrogelogical andheat transfer characteristics. Most geothermal heat is trapped or stored inrocks. A liquid or gas is usually required to transfer the heat from the rocks. Heat is transferred in three different ways, convection, conduction, andradiation.
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 onesubstance to another through a working moving medium, such as water. The mediumusually transfers the energy in an upward direction. Radiation is the transferof energy out of a substance through the excitement of gas molecules surroundinga substance. Radiation is dependent upon two things the object emitting theheat and the surrounding’s ability to absorb heat. Convective geothermalsystems are characterized by the natural circulation of a working fluid or water.
The heated water tends to rise and the cool to sink continually circulatingwater throughout the ground. The majority of the heat transfer is done throughconvection and conduction, radiation hardly ever effects heat flow. Whengeothermal heated water collects into a reservoir one form of a geothermalresource is created. One can approximate the amount of thermal energy presentin a geothermal resource by comparing the average heat content of the surfacerocks with the enthalpy of saturated steam. Enthalpy is energy in the form ofheat released during a specific reaction or the energy contained in a systemwith certain volume under certain pressure. It is generally accepted that belowa depth of ten meters, the temperature of the ground increases one degreeCelsius for every thirty or forty meters.
At a depth of ten meters annualtemperature changes no longer affect the temperature or the earth. The most common geothermal resources used for the production of humanconsumed energy are hydrothermal. Hydrothermal systems are characterized byhigh permeability by liquids. There are two basic types of hydrothermal systems,vapor and liquid dominated systems. In a liquid based system, pumps must beplaced very deep in the well where only the liquid phase is present. By keepingthe liquid under pressure it is possible to keep the liquid at a much highertemperature than the liquids normal boiling point.
If the liquid is not keptunder pressure, it will flash. Flashing is the process of vaporization. Itrequires 540 calories per gram of heat to vaporize water. The super heatedpressurized water is pumped up a long shaft into the plant. When it reaches theplant, controlled amounts of the pressurized water is allowed to flash orvaporize. The rapidly expanding gas pushes or turns the turbine.
A power plantmay have numerous flash cycles and turbines. The more flash cycles the higherthe efficiency of the power plant. Once the heated liquid has been used to thepoint where it has cooled to an unusable temperature it is reinjected into theground in hopes that it will replenish the geothermal well. Vapor systems workin much of the same way. The super heated gas flows through surface reboilersthat remove all of the non-condensable gases from the mixture of gases.
The gasis pumped into pressurization tanks where extreme pressure causes the gas tocondense. The super heated liquid is then allowed to flash. The rapidlyexpanding gas turns the turbine. Specific examples and sites of electricalenergy production will be discussed later. Conductive geothermal systems consistof heat being transferred through rocks and eventually being transmitted to thesurface. The amount of heat transferred in a conductive geothermal isconsiderably less than the heat transferred in a convective system.
Conductivegeothermal systems lack the water to efficiently transfer the heat, so watermust be artificially injected around the hot rocks. The heated water is thenpumped from the underground reservoir to the surface. This system is not aseffective as others because the temperature that the heated water reaches is notvery great. Geopressured geothermal systems are similar to hydrothermalsystems. The only difference is the pressure of the high temperature reservoir.
Geopressured geothermal systems may be associated with geysers. Somegeopressured geothermal systems reach pressures of fifty to one hundredmegapascals (MPa) at depths of several thousand meters. These systems provideenergy in the form of heat and water pressure making them more powerful anduseful. Currently most electricity producing geopressured geothermal systemsare only experimental. There are many factors in this type of system that arevery hard to predict such as the reservoirs potential energy. It is very hardto predict the force at which the water will be projected from the well sincethe pressure of the high temperature is constantly changing.
The salinity ofthe liquid projected is also very high. In some instances the liquid consistsof twenty to two hundred grams of impurities per liter. Today with the depletion of many other natural resources usinggeothermal resources in more important than ever. Hot springs are naturaldevices that bring geothermal heated water to the surface of the earth.
Thisprocesses is very efficient, little heat is lost during the transportation ofthe water to the surface. The heat is brought to the surface via watercirculation in either the liquid or gaseous form. Geothermal hot springs are agood source of energy because it is probable that they will never be exhaustedas long as water is not pumped from the spring faster than it naturallyreplenishes itself. A simplified version of a vapor run geothermal electricplant might operate under the following conditions.
Holes are drilled deep intothe ground and fitted with pipes that resist corrosion. When the hole is firstopened, steam escapes into the atmosphere. Once the pipes are inserted into theholes the steam expansion becomes adiabatic. An adiabatic system is a systemin which there is little or no heat loss. Next the pipe is connected to thecentral power station.
No condensation takes place because the steam issuperheated. Many drill holes are connected to the central power station whichresults in mass quantities of superheated water vapor pushing the turbine. Themore drill holes that are connected to the power station the greater thepressure of the gas flowing through the turbine. The greater the pressure ofthe gas the faster the turbine turns and the more electricity produced. In somepower plants the water vapor itself is not used to turn the turbines but only toheat another purer substance.
This method is less efficient but does notcorrode the machinery. Most superheated gas from geothermal resources is notpure water but a mixture of gases. Some of these gases can be extremelycorrosive so using purer non-corrosive materials has its advantages. Somecommon gases used are ethyl chloride, butane, propane, freon, ammonia.
Theefficiency of these generators is limited by the second law of thermodynamics. The second law of thermodynamics states that a thermal engine will do work whenheat entering the engine from a high temperature reservoir is at a differenttemperature than the exhaust reservoir. The thermal engine must take heat fromthe high temperature reservoir convert some of that heat to work and exhaust theremaining heat into a low temperature reservoir. The difference between theheat put into the engine and the heat deposited as waste energy is transformedby the engine into mechanical work. The maximum possible efficiency of a heatengine is called its Carnot efficiency. Carnot efficiency is never reached andthe actual efficiency is always lower than the Carnot efficiency.
The greaterthe difference in temperature between the superheated gas and the lowtemperature exhaust reservoir the higher the efficiency of the power plant. Theaverage actual efficiency for a geothermal power plant ranges from the singledigits to about twenty percent. The average actual efficiency for a fossil fuelburning electrical power plant is approximately thirty percent. While othermethods of electricity production may have slightly better efficiency than ageothermal power plant, the less destructive environmental impacts of geothermalpower plants offset the importance of the a higher efficiency.
Direct use ofgeothermal heat for heating purposes can result in actual efficiencies of up toninety percent. Fossil fuel powered heat systems can generally only reachactual efficiencies of seventy to eighty percent. As well as being used for electricity, geothermal energy is currentlybeing used for space heating. Geothermal heated fluid used for space heating iswidespread in Iceland, Japan, New Zealand, Hungary and the United States. In ageothermal space heating system, electrically powered pumps push heated fluidthrough pipes that circulate the fluid through out the structure.
Geothermalheated fluid is also being used to heat greenhouses, livestock barns, fish farmponds. Some industries use geothermal energy for distillation and dehydration. Although there are many pluses to using geothermal energy thereare also some problems. It was generally assumed that geothermal resources wereinfinite or they could never be completely depleted. In reality the exactopposite is true.
As water or steam is pumped out of the well the pressure maydecrease or the well may go dry. Although the pressure and fluid willeventually return it may not do so fast enough to be useful. Drillinggeothermal wells is very expensive. It is generally figured that a geothermalwell should last 30 years in order to pay for itself. Another factor to takeinto consideration is the disposal of the waste water.
Some geothermal fluidconsists of several toxic materials such as arsenic, salt, dissolved silicaparticles. These materials can pollute drinking water and lakes. When thewaste water is reinjected back into the earth the previously dissolved silicaparticles precipitate out of the liquid and can block up the pores in thereinjection well. The cool water can also create new passages through the rocksand create unstable ground above. There are three main problems that can plaguea power plant when it is operated using geothermal energy, silting, scaling andcorrosion. Scaling is caused by silting or when suspended particles build up onthe 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 theliquid to try to neutralize the liquid. Silting is when the particles that weredissolved in the hot fluid precipitate out when the fluid cools. This generallyoccurs in the pipes and can cause considerable damage to the pipes ifsignificant pressure builds. This problem can be solved by using simple filtersthat are periodically changed in the pipes.
Corrosion occurs because of acidicsubstances incorporated in the geothermal fluid. Usually geothermal fluidcontains some boric acid. Using pipes that are not affected by these liquidgenerally takes care of corrosion. Unfortunately most metals that are non-corrosive are very expensive. Most types of wildlife can not live in or consumesaline water.
If the cooled fluid containing dissolved toxins and saltcontaminates lakes or streams the environmental effects can be disastrous. Airpollution from geothermal resources is also significant. The most common typeof air pollution is the release of hydrogen sulfate gas into the air. At thegeysers in California an estimated 50 tons per day of hydrogen sulfite isreleased into the atmosphere. Iron catalysts have been added to try to offsetthe effects of pollution but have failed because moisture and carbon dioxidereduce the efficiently of the catalysts so much that it is not effective. Noisepollution is another consideration that must be taken into account.
When thesteam and water escape from the system it makes a relatively loud noise. If thewells are located near any residential areas it can raise problems anddiscontentment within the community. Some geothermal power plants haveinstalled cylindrical towers where the water vapor and water is swirled around. The friction created by the movement of the gas or fluid decreases the overallkinetic energy of the gas or fluid causing the internal energy to decrease. When the internal energy is decreased the noise of gas escaping is alsodecreased.
Geothermal resources do produce pollution but the pollution wouldbe there even if we did not exploit the resource. Other energy producingsystems used today produce and emit pollution that otherwise would not beintroduced into the environment. I feel that the benefits of using geothermalresources as a source of energy for electricity and mechanical work productionout weigh the downfalls. The world has many different geothermal regions that are exploited forthe production of electricity and other things.
The United States is one of theleaders in manufacturing geothermal produced electricity. One of the mostproductive regions in the U. S. is the Pacific Region. Most geothermal regionscontain mostly heated water. Geysers produce very large amounts of water vaporand other gases.
Geysers have the potential to produce electricity relativelyefficiently. In 1979 The Geyser power plants had a rating of 600 megawatts ofelectricity(MWe). Today they are rated for over 2000MWe. Most of the geysersare located on the side of a mountain near Big Sulfur Creek, on the Californiacoast west of Sacramento.
William Bell Elliott was the first to see thisnatural wonder in 1947 while surveying, exploring and looking for grizzly bears. The earth around the Geysers geothermal site consists of highly permeablefractured shales and basalts created during Jurassic age. The ground abovethe wells consists of graywake sandstone. This form of sand stone is very hardto penetrated. Scientists believe that the large geothermal reservoir wascreated when an earthquake caused fault and shear zones.
Steam temperatures inthe geothermal wells range from 260 to 290 . Pressures deep in the wells rangefrom 450psig to 480psig (3. 1MPa to 3. 3) . Some wells are 3000 meters deep andproduce almost 175 tonnes of steam per hour.
It is thought that the center of the magma or the heat source at TheGeysers geothermal site lies under Mt. Hannah. Geologists are led to believethat there is a large mass of magma cooling under the geysers and power plantsthat is the source of all the heat. This assumption is proven when seismicwaves caused by earth quakes are slowed when they pass through the mountain.
Afairly large fractured steam reservoir rests above the cooling molten. In 1967, the Union Oil Company in partnership with Magma PowerCorporation and Thermal Power Company began producing electricity from theGeysers Geothermal region and selling it to the Pacific Gas and Electric Company. The turbines in the power plant were designed to operate under intake pressuresof 80psig to 100psig. At first the plant operated at maximum efficiency but asthe years went by the geothermal resource was slowly depleted. The depletedheat source did not produce the constant pressure that was required for maximumefficiency so the efficiency decreased.
There are two methods of drilling wells,mud drilling and air drilling. Mud drilling tends to clog up the porous rockbut it is easier on the drilling machinery. Air drilling leaves the porousrock free for water and steam flow but it is very hard on machinery due toabrasion and heating. Air drilling is therefore very expensive.
Geothermalwells do not always maintain constant pressure. New wells must be drilled tocontinually maintain constant pressure on the turbine. The system built at TheGeysers geothermal field deliversof super heated steam. The steam producedby the wells is not pure water but consists of 1% non-condensable gases alongwith dust particles. If not cleaned off, the dust can accumulate on the insideof the turbine blade shrouds and cause turbine failure. This problem wasvirtually 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 ofit was still superheated, so special non-corrosive metal was not required in theconstruction of the upper level piping and the turbine. Normal carbon pipingwas used in the original construction. This proved not to be the case, after awhile the pipes began to corrode. As steam condenses non-condensable gasesbecome more of a problem. They become more concentrated, more corrosive and canform sulfuric acid. This new problem was solved by replacing the carbon steelused in the original construction with austenitic stainless steel.
Electricalconnections 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 enthalpyof 1200-1500 Btu per lb. The use of condensing steam turbines that exhaustedwaste water below atmospheric pressure increased the efficiency of the plant. There were no rivers or streams in the immediate area that were sufficientlycool enough to be used as a cooling mechanism, so cooling towers wereconstructed. Incorporating the cooling towers into the system allowed the wastewater to be discharged at a cooler temperature f 18 therefore increasing thepossible efficiency of the system.
Carnot Efficiency of The Geysers Power PlantCarnot Efficiency ==18=290Carnot Efficiency =Carnot Efficiency = . 4831or48%This is a relatively efficient cycle. It certainly can compete withother modern day types of electricity production. Unfortunately carnotefficiencies can never be reached. A large amount of energy is lost in thecondensers and turbines. I feel that while the efficiency of this geothermalpower plant might not be overwhelmingly better than other modern day methods ofelectricity, the lack of pollution makes up for the loss in efficiency.
Eventhough The Geysers power plant is relatively efficient, it does not even comeclose to taking advantage of all the emitted heat. Only 2% of the emitted heatfrom the source is used to heat water for electricity production. Thisgeothermal resource will not last for ever though. Heat Content of the EntireGeysers Geothermal Site-The Geysers geothermal site covers approximately . -Heat is onlyrecovered 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 ofthe permeable rock that makes up most of geothermal region is . Volume xSpecific Heat x Change in Temperature = Heat ContentVol = x = SpHt== 240 – 18 = 222Q =( x )( )( )(222 )Q= Joules of Heat Content in the entire Geysers geothermal regionLife 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/yearJ/ J/year = 24. 67years. According to my calculations The Geysers geothermalresource will be depleted in 24.
67 years at the current rate of usage. Ofcourse this is not taking into account the rate at which the resource is renewedfrom heat coming from deeper in the earth. I am assuming that the rate ofdepletion is so much greater than the rate of renewal that it is not significantin 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.
Theprocess of electrical power generation used at The Geysers power plant isrelatively simple when compared to other modern day power plants. The steamthat evolves from the wells flows through pipes that lead to the turbine. Thepressure exerted by the superheated steam turns the turbine which produceselectricity. The steam then flows into the direct-contact condensers below theturbine.
Cooling water from the cooling towers is constantly circulated throughthe condensers. The condensed steam and cooling water is then pumped back intothe cooling towers. Because the evaporation rate from the towers is slower thanthe rate at which water is pumped into the towers, excess amounts of wateraccumulate in the cooling tower. This excess water is then pumped toreinjection wells where it flows down through the soil and porous rock and isreheated by the heat source. The cycle begins all over again. See the diagrambelow.
The costs of running this particular geothermal electrical plant are verycompetitive with the cost of other types of modern day plants. The operationcosts for the plant at The Geysers is almost same the as the operation costs ofan average fossil fuel powered plant and much less than the operating costs of ahydroelectric or nuclear plant. One of the greatest advantages of this and mostgeothermal systems is the relative lack of pollution. While most coal plantsgive off significant amounts of sulfur, somewhere around 93 tons per day for theaverage coal plant, geothermal plants produce no gas pollution other than thegases that would be naturally emitted from the geysers anyway.
Coal plants areby 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. Totalelectricity produced in the U.
S. during 1985 = 652000MW Percent of Geothermalenergy contributed to total U. S. production 3%3% x 652000MW = 19560MW Methods of geothermal energy productionCapital Dollars per KilowattDry Steam Flash 83%$1000/kWBinary 17%$3600kWDry 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 kWThe future of geothermal energy looks very promising. There have beenmany technological breakthroughs that have resulted in increased efficiencies ofmodern day geothermal electrical plants. I feel that with the currentenvironmental situation that the world now faces a viable method of clean upwill include the use of geothermal power plants and resources. In a world thatis suffocating from the chemicals, and particulates that are created in theproduction of electricity and other commercial industries, we have no choice butto 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 seenow will become irreversible. Using geothermal resources for other purposes suchas space heating can only help reduce pollution emission. With in the nextcentury the world will begin to feel the energy crunch. Supplies of othernatural resources such as coal, oil and other petroleum products will begin tobecome scarce.
The world today is completely electricity dependent. Withoutelectricity, the world as we know it would cease to exist. In the next centurywe must learn to be less electricity dependent or find other sources of energy.If less env