| Geothermal Energy Essay, Research Paper
GEOTHERMAL ENERGY
By Sean Griffiths
Physics 200
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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