(1) Explain flash steam system for liquid dominated system of geothermal energy. (Oct/Nov. 2009, Nov/Dec. 2004, Nov/Dec. 2002)(2) Advantages and Disadvantages of Geothermal Energy. (Oct/Nov. 2009)(3) How are geothermal fields classified? State various geothermal resources.

(May/June 2008, May/June 2007, Nov/Dec. 2005)(4) Explain Vapour dominated system to utilize geothermal Energy. (May/June 2008,

Nov/Dec. 2008, May/June 2007, Nov/Dec. 2007, Nov/Dec. 2005)(5) Define and classify geothermal resources? (May/June 2009)(6) With neat sketch explain how heat is extracted from hot dry rocks. (May/June

2007, Nov/Dec. 2005, Nov/Dec. 2003)(7) Explain Principle of Total Flow Concept. (Nov/Dec. 2004)(8) Describe in brief heat extraction from petrothermal fields. (Nov/Dec. 2004)(9) Explain operational and environmental problems of utilizing geothermal energy

and describe binary fluid geothermal plant for liquid dominated system. ( Nov/Dec. 2001, Nov/Dec. 2000, Nov/Dec. 1999)

(1) Advantages and Disadvantages of Geothermal 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 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 geothermal fields classified? State various geothermal resources. (May/June 2008, May/June 2007, Nov/Dec. 2005)

Goethermal energy means the energy obtained from earth crust.

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

Fig.1.

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 hydrothermal field which acts as underground boiler. The vents show up as geysers fumaroles (6) or hot springs (7).

A well made by 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.

There are four types of geothermal energy resources called: Hydrothermal Geopressurized Hot dry rock Magma

(3) Define and classify geothermal resources? (May/June 2009)

There are four types of geothermal energy resources called: 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 for generation of electricity, otherwise it is used for space heating. Vapour 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:

These resources are located at greater depths of 3 km to 10 km with abnormally high pressure of water up to 1400 atm. Because of abnormal high pressure in deepest region, it is known as geopressurized resources. The water is at relatively low temperature and very 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:

Fig.3.

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).

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.

The cold water gets heated by the hot dry rocks.

The hot water and steam can be pumped out from the production well.

Magma:

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

(4) With neat sketch explain how heat is extracted from hot dry rocks. (May/June 2007, Nov/Dec. 2005, Nov/Dec. 2003)

ORDescribe in brief heat extraction from petrothermal fields. (Nov/Dec. 2004)

Fig.4.

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 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) Explain flash steam system for liquid dominated system of geothermal energy. (Oct/Nov. 2009, Nov/Dec. 2004, Nov/Dec. 2002)

Fig.6.

When the resource temperature is below 180 C, it is called 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 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 phase mixture having the steam of low quality.

The mixture is further throttled in the flash chamber cum brine separator. It results into dry steam at 3 and the separated brine is collected from bottom.

The dry steam is supplied to steam turbine of conventional steam power plant as shown in fig.

The hot brine is re-injected into the ground along with the steam condensate from the condenser.

Fig.6. shows the T-S Diagram of cycle.

(6) Explain Vapour dominated system to utilize geothermal Energy. (May/June 2008, Nov/Dec. 2008, May/June 2007, Nov/Dec. 2007, Nov/Dec. 2005)

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 extracted from the geothermal field may contain water and 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 condensed in the condenser with the help of cooling water circulated. 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.

Fig.8 shows the T-S diagram.

(7) Explain Principle of Total Flow Concept. (Nov/Dec. 2004)

Fig.8.

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 fig.10 show the T-S diagram

(10) Explain operational and environmental problems of utilizing geothermal energy and describe binary fluid geothermal plant for liquid dominated system. ( Nov/Dec. 2001, Nov/Dec. 2000, Nov/Dec. 1999)

Geothermal energy is not completely pollution free energy. At geothermal site, the air pollution is major problem because of emission of poisonous gases such as Hydrogen sulphide, ammonia, methane, carbon dioxide etc.

The major adverse environmental effects are noise pollution, thermal pollution, surface disturbance, physical effects caused by fluid withdrawal.

The extraction of energy from hot dry rocks, it is necessary to force water down boreholes as a working fluid and return it to surface to use in turbine. The environmental problems is that a large quantity of water is required. If the underground reservoir is highly permeable, there is no way to know how much water will be need to be injected before a useful amount of steam or hot water is returned to the surface.

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.

Geothermal water contains dissolved solids. The amount of dissolved 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

The isobutene Vapour is so generated is passed through a turbine where it expands. The mechanical power of turbine is converted into electrical power by a generator.The exhausts of the turbine are passed through a regenerator where it is cooled and then condensed in the condenser. The returned condensate is heated in the regenerator by exhaust Vapour of the turbine.

Advantages of this system are:

Scaling and corrosion problems are avoided since geothermal fluid does not come in contact with plant.No problem of geothermal fluid is reinjected into ground and it is circulated in a heat exchanger or regenerator.Efficiency is high.