Geothermal Energy Note

(1) Explain flash steam steam system for liquid dominated dominated system of geothermal energy. (Oct/Nov. 2009, Nov/Dec. 2004, Nov/Dec. 2002) (2) Advantages and Disadvantages of Geothermal Energy. Energy. (Oct/Nov. 2009) (3) (3) How How are are geot geothe herm rmal al fiel fields ds clas classi sifi fied ed?? Stat Statee vari variou ouss geot geothe herm rmal al reso resour urce ces. s. (May/June 2008, May/June 2007, Nov/Dec. 2005) (4) Explain Vapour dominated system to utilize geothermal Energy. Energy. (May/June 2008, Nov/Dec. 2008, May/June 2007, Nov/Dec. 2007, Nov/Dec. 2005) (5) Define and classify geothermal resources? resources? (May/June (May/June 2009) (6) With With neat neat sketch sketch explain explain how heat is extracte extracted d from from hot dry rocks. rocks. (May/Ju (May/June ne 2007, Nov/Dec. 2005, Nov/Dec. 2003) (7) Explain Explain Principle Principle of Total Flow Concept. (Nov/Dec. (Nov/Dec. 2004) (8) Describe in brief brief heat extraction from petrothermal fields. fields. (Nov/Dec. 2004) (9) Explain Explain operational operational and environment environmental al problems of utilizing utilizing geothermal geothermal energy and and desc descri ribe be bina binary ry flui fluid d geot geothe herm rmal al plan plantt for for liqui liquid d domin dominat ated ed syst system em.. ( Nov/Dec. 2001, Nov/Dec. 2000, Nov/Dec. 1999)

(1) Advantages Advantages and Disadvantages Disadvantages of Geothermal Energy. Energy. (Oct/Nov. 2009)

Geothermal energy in the form of thermal energy is a form of renewable source of energy having high energy density. It is also called energy obtained from earth crust. Advantages of Geothermal Energy: It is reliable source of energy which is available continuously throughout • the year It is independent of weather conditions cond itions • No thermal storage facility if required • Capital and generation cost is low compared to conventional power plant • No Solid Pollutants • Needs very small land area • Disadvantages of Geothermal Energy: It is low grade heat energy since the temperature are limited to 150 C • Parts of plant may corrode due to presence of salt • Geothermal fluids also bring in dissolved gases like Hydrogen sulphide, • carbon dioxide, ammonia etc. which may cause air pollution. Life of plant is low compared with conventional power plant • Continuous extraction of fluid may cause settlement of land • It also produces noise pollution due to exhaust, separators and blow down • etc.

(2) How are geothe geotherma rmall fields fields classi classifi fied? ed? State State variou variouss geother geothermal mal resour resources ces.. (May/June 2008, May/June 2007, Nov/Dec. 2005) •

Goethermal energy means the energy obtained from earth crust.

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Below the earth crust is the molten mass called magma which is still in the process of cooling. The earth’s treamors, caused the magma to come closure to earth surface in certain places and the crust fissures were opened up. The hot magma near the earth

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surface thus causes active volcanos, hot springs and geysers where water exists. It also causes steam to vent through fumaroles. Fig.1. shows the typical geothermal field. The hot magma near the surface (1) solidifies into igneous rock (2). The heat of magma is conducted upwards into this rock. The ground water that finds its way down to this rock through fissures in it will be by the heat of the rock by mixing it with hot gases and steam emanating from the magma. The heated water will then rise upwards by convection into porous and permeable reservoir (3) above the igneous rock. (2). It is covered by the layer of solid impermeable rock (4). It traps the hot water in the reservoir called hydrothermal field. However, the solid rock (4) has the fissures (5). It acts as vents of this giant hydrot hydrother hermal mal field field which which acts acts as undergr underground ound boiler boiler.. The vents vents show show up as geysers fumaroles (6) or hot springs (7). A well made by drilling drilling hole in the upper crust crest in the region of geothermal field can be used to trap steam for use in a conventional power plant. The steam originating from magma is called meteoritic steam which is the largest source of geothermal steam.





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Magma

(3) Define and classify geothermal resources? (May/June 2009) There are four types of geothermal energy resources ca lled: Hydrothermal • Geopressurized • Hot dry rock • Magma • Hydrothermal energy resources:

Fig.2. These are deposits of hot water and steam at lesser depths. Hot water or steam can be extracted by means of production well. These systems are further sub divided into vapour dominated system and liquid dominated system. High temperature water and steam is used used for for gener generat atio ion n of elec electr tric icit ity, y, othe otherw rwis isee it is used used for for spac spacee heati heating. ng. Vapo Vapour ur dominated systems produces steam with very small amount of water or with no water. The liquid dominated system produces a mixture of steam and hot water. Fig.2 shows the hydrothermal energy resources. Geopressurized Resources:





high pressure due to overlaying formation of rock layers and restricted space available in the rocks. Geopressurized system in texas, the pressure are from 680 to 980 atm. And temperature of 160 to 200 C at depth of 4 to 5 Km Petrothermal energy resources:

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Fig.3 shows the petrothermal resources Second name of such resources are called hot dry rocks. In petrogeothermal resources, the heat is extracted from the hot dry rocks which is heated by magma. The temperature of hot dry rocks at about 3 km is 150 C to 290 C water does not exists and only heat is available is hot dry rocks (HDR).





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Before injecting the water, the production wells and injections wells are drilled by drilling operations. The cold water gets heated by the hot dry rocks. The hot water and steam can be pumped out from the production well.

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Below the solid crust, there is a molten mass ca lled magma It possesses very high temperature. It solidifies into igneous rock through which heat is co nducted by conduction. In some countries in foreign, use magma as a source of energy.

(4) (4) With With neat neat sket sketch ch expl explai ain n how how heat heat is extr extrac acte ted d from from hot hot dry dry rock rocks. s. (May/June 2007, Nov/Dec. 2005, Nov/Dec. 2003) OR Describe in brief heat extraction from petrothermal fields. (Nov/Dec. 2004)





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Fig.3 shows energy extraction from Hot dry rocks. Hot dry rocks are called petrogeothermal resources.. In petrogeothermal resources, the heat is extracted from the hot dry rocks which are heated by magma. The temperature of hot dry rocks at about 3 km is 150 C to 290 C water does not exists and only heat is available is hot dry d ry rocks (HDR). The cold water is injected to the cavity of hot dry rocks through the injection well. The cavity is produced by the high pressure injected water or by ground chemical explosion. Before injecting the water, the production wells and injections wells are drilled by drilling operations. After taking the heat from the hot dry rocks by the water, steam is formed, and then steam expands in the turbine so power is developed. After turbine, exhaust steam is condensed in the condenser and condensate water is pumped into the heat exchanger and cycle is repeated. The cold water gets heated by the hot dry rocks. The hot water and steam can be pumped out from the production well.

(5) Expla Explain in flash flash steam steam system system for liquid liquid domin dominate ated d syste system m of geothe geotherm rmal al energy. (Oct/Nov. 2009, Nov/Dec. 2004, Nov/Dec. 2002)







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When When the the reso resour urce ce temp temper erat atur uree is belo below w 180 180 C, it is call called ed high high temperature system. It is suitable for power generation. Fig. 5 shows the wet steam or liquid dominated system used in Cerro Prierol (maxico) and Octake (Japan) where the hot water with steam at high pressure and high temperature steam issued by deep wells drilled into the ground The hot water from the underground water water from a depth of 1 Km at point 1 is at about 40 bar reaches the well heat at point 2 at lower pressure. Process (1-2) is essentially a throttling process which results into two





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Vapour dominated fields deliver steam with little or no water. The pressure and temperature of the steam from such resources reaching to the surface limited to 8 bar and pressure of 200 C. The sites available in the world are very few. Some of the important sites are in California, italy and Japan. Fig.7 shows the vapour dominated system.

Fig.7. Dry steam extracte extracted d from from the geotherma geothermall field field may contai contain n water water and solid solid particles. These are removed in a centrifugal separator. Pure dry steam is then directly supplied to the steam turbine. The exhaust steam from the turbine is condens condensed ed in the condens condenser er with with the help help of coolin cooling g water water circul circulate ated. d. The resultant hot water is returned to the cooling tower. The condensate is either sent to chemical recovery or it is re-injected into hot field with the help of condensate pump. The problems associated with these plants are the presence of corrosive gases and abrasive materials which cause the environmental problems.





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In total flow concept, both kinetic energy as well as heat energy of steam-liquid mixture produced by flashing the geothermal brine is used. The geothermal brine is passed through special turbines. Radial impact turbine, helical screw expander turbine and bladeless turbines are generally used. The hot brine from geothermal well is admitted to the special turbines where it expands and drive the turbine which is coupled with the generator. The geothermal fluid from the turbine is discharged into condenser. The condensate fluid is reinjected into ground by reinjection well. Fig. 9 shows the total flow concept and an d fig.10 show the T-S diagram

(10) (10) Expl Explai ain n oper operat atio iona nall and and envi enviro ronm nmen enta tall prob proble lems ms of of uti utili lizi zing ng geo geoth ther erma mall energy and describe binary fluid geothermal plant for liquid dominated system.





A large volume of flash steam escaping into the atmosphere could cause dense fog to occur. All geothermal site, some harmful substances may escape into the air. • These may contain radioactive materials also. Systematic monitoring is advisable. Geothe Geotherma rmall water water contai contains ns dissol dissolved ved solids solids.. The amount amount of dissol dissolved ved • solids are in the rang of 300 – 1500 ppm. •

Binary Cycle : The hydrothermal resources are available in the temperature range of 90 C to 170 • C. This temperature is not sufficient for production of two phase mixture of steam. In order to utilize this geothermal heat, an organic compound of low boiling temperature like isobutene is used under pressure in the primary heat exchange. The geothermal fluid is re injected after extracted heat in to the ground. This system is shown in fig.9

Fig.9





Geothermal Energy Note

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