Geothermal energy

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 ...

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