GENERATION OF ELECTRICITY USINGGEOTHERMAL ENERGY

GEOTHERMAL ENERGY

GENERATION OF ELECTRICITY USING

GEOTHERMAL ENERGY

Name:- Md. Shadab Martur

USN:- 2KE08EE014

Seminar Guide:- Prof. Manjunath B. Ranadev

K.L.E.SOCIETY’S K.L.E.INSTITUTE OF TECHNOLOGY, HUBLI-30

(Affiliated to VTU Belgaum) Department of Electrical and Electronics

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GEOTHERMAL ENERGY

ACKNOWLEDGEMENTS

I take immense pleasure in thanking Vinoda madam, Head of the Department for

having permitted me to to present this seminar.

I wish to express my deep sense of gratitude to my Guide, Prof. Manjunath B.

Ranadev for his able guidance and useful suggestions, which helped me in presenting the

seminar and completing the report, in time.

Finally, yet importantly, I would like to express my heartfelt thanks to my beloved

parents for their blessings, my friends/classmates for their help and wishes for the successful

presentation of this seminar.

Md. Shadab Martur

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GEOTHERMAL ENERGY

CONTENTS:

(i) Introduction

(ii) Origin and Distribution of Geothermal energy

(iii) Types of Geothermal resources

(iv) Types of prime movers for geothermal energy conversions

(v) Advantages

(vi) Disadvantages

(vii) Conclusion

(viii) References

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GEOTHERMAL ENERGY

INTRODUCTION Geothermal energy originates from the earth’s interior in the form of

heat, volcanoes, geysers, hot springs and boiling mud pots are visible evidence of the great

reservoirs of heat that lie beneath the earth. Although the amount of thermal energy within

the earth is very large, useful geothermal energy is limited to certain sites only, as it is not

feasible to access and extract heat from a very deep location. Where it is available near the

surface and is relatively more concentrated, its extraction and use may be considered feasible.

These sites are known as geothermal fields.

As the second-largest country of the world, undergoing explosive

growth, India represents a unique and little-tapped source of expertise on multifarious issues

of economic and social development. Among many other problems of concern, the

development of alternative sources of energy is crucial to the future wellbeing of India

indeed, of the globe. The increasing demand of the energy has forced the mankind to find a

way out which will be efficiently and abundantly available source of energy. This made the

human race to think of the beneficiaries of non-conventional energy development in India.

As per US geological survey, the entire heat content of the earth’s crust up to a deep of 10 km

above 15 C is defined as geothermal resource. As such, geothermal resource of earth is

estimated to be more than 2.11*10 J, which is equivalent to 10 MTOE (million tones of oil

equivalent). This is huge amount of energy, enough to supply our energy needs at current

rates for 3,50,000 years. Thus, it is considered as an inexhaustible and renewable source.

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GEOTHERMAL ENERGY

However, it is a low grade thermal energy form and its economic recovery is not feasible

everywhere on the surface of the earth.

Among the various new and renewable energy sources, geothermal energy is

known to be one of the clean energy without smoke and also without environmental hazards.

Although it’s importance is realised long back in other countries, it’s exploitation is still far

away in our country mainly due to lack of knowledge on the deep subsurface structure and

deep drilling technology in high pressure, high temperature regions. Indian geothermal

provinces have the capacity to produce 10,600 MW of power- a figure which is five time

greater than the combined power being produced from non-conventional energy sources such

as wind, solar and biomass. But yet geothermal power projects have not seen the sunlight due

the availability of 192 billion tones of recoverable coal reserves. With escalating

environmental problems with coal based projects, Indian  has to depend on clean, cheap, rural

based and eco-friendly geothermal power in future. Due to technical and logistic problems

with other non-conventional energy sources, present industrialists mood is upbeat and IPPs

are showing keen interest in developing geothermal based power projects. With the existing

open economic policies of the Govt., and large incentives given to non-conventional energy

sectors,  the future of geothermal energy sector in India appears to be bright.

Concerted efforts are made in identifying these resources in different parts

of our country for possible exploitation of the energy source. In the present paper, the details

of geothermal energy, it’s importance and usage in other countries are discussed with

estimated potential in our country.

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GEOTHERMAL ENERGY

ORIGIN AND DISTRIBUTION OF GEOTHERMAL ENERGY

Geothermal energy is the heat that originates from the core of the earth,

where temperatures are about 4000°C. The heat occurs from a combination of two sources:

(i) the original heat produced from the formation of the earth by gravitational collapse, and

(ii) the heat produced by the radioactive decay of various isotopes. As the thermal

conductivity of earth is very low, it is taking billions of years for the earth to cool down. The

average geothermal heat dissipation from the land and ocean surface is about 0.06W/m2,

which is negligible compared to other sources. A section through the earth is shown in figure.

The core is surrounded by a region known as mantle. Mantle consists of

a semi fluid material called the magma. The mantle is finally covered by the outermost layer

known as crust, which has an average thickness of 30km. The temperature in the crust

increases with depth at a rate of 30°C/km. The temperature at the base of the crust is about

1000°C and then increases slowly into the core of the earth.

Most of the world’s geothermal sites today are located near the edges of pacific

plate, the so called ‘ring of fire’. This belt rings the entire Pacific ocean, including New

Zealand, Indonesia, Japan, Western North America, central America, Peru, Chile and

Argentina. An extension also penetrates through Asia into the Mediterranean area. Hot crustal

material also occurs at the mid ocean ridges and interior continental rifts.

Geothermal energy is a domestic energy resource with cost, reliability and

environmental advantages over conventional energy sources. It contributes both to energy

supply, with electrical power generation and direct-heat uses.

For generation of electricity, hot water is brought from the underground reservoir to the

surface through production wells, and is flashed to steam in special vessels by release of

pressure. The steam is separated from the liquid and fed to a turbine engine, which turns a

generator. Spent geothermal fluid is injected back into peripheral parts of the reservoir to help

maintain reservoir pressure.  In the absence of steam, heat from hot water is extracted through

a secondary fluid and the high pressure vapour from the secondary fluid is utilized to run the

turbine.

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GEOTHERMAL ENERGY

If the reservoir is to be used for direct-heat application, the geothermal

water is usually fed to a heat  exchanger and the heat thus extracted is used for home heating,

greenhouse, vegetable drying and a wide variety of other small scale industries. Hot water at

temperatures less than 120 o C can be used for this purpose.  Further, the spent hot water, after

generating electricity can also be used for direct application.

As a result of today's geothermal production, consumption of exhaustible fossil fuels is offset,

along with the release of acid-rain and greenhouse gases that are caused by fossil-fuel use.

Systems for use of geothermal energy have proven to be extremely reliable and flexible.

Geothermal electric power plants are on line  97% of the time, whereas nuclear plants

average only 65% and coal plants only 75% on-line time. Geothermal plants are modular, and

can be installed in increments as needed. Because they are modular, then can be transported

conveniently to any site. Both baseline and peaking power can be generated. Construction

time can be as little as 6 months for plants in the range 0.5 to 10 MW and as little as 2 years

for clusters of plants totalling 250 MW or more.

The competing goals of increased energy production for worldwide social development and

of mitigating release of atmosphere-polluting gases are not compatible using today's fuel mix,

which relies heavily on coal and petroleum. Development of geothermal energy has a large

net positive impact on the environment compared with development of conventional energy

sources. Geothermal power plants have sulphur-emissions rates that average only a few

percent of those from fossil-fuel alternatives. The newest generation of geothermal power

plants emits only ~135 gm of carbon (as carbon dioxide) per megawatt-hour (MW-hr) of

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GEOTHERMAL ENERGY

electricity generated. This figure compares with 128 kg /MW-hr of carbon for a plant

operating on natural gas (methane) and 225 kg/MW-hr of carbon for a plant using bituminous

coal. Nitrogen oxide emissions are much lower in geothermal power plants than in fossil

power plants. Nitrogen-oxides combine with hydrocarbon vapours in the atmosphere to

produce ground-level ozone, a gas that causes adverse health effects and crop losses as well

as smog. There are other environmental advantages to geothermal energy. Geothermal power

plants require very little land, taking up only a fraction of that needed by other energy

sources.  Thus  emission of  CO2 and  SO2 by geothermal power plants is far less compared

with conventional fossil fuel based power plants.

The geothermal heat pump, also known as the ground source heat pump, is a

highly efficient renewable energy technology that is gaining wide acceptance for both

residential and commercial buildings. Geothermal heat pumps are used for space heating and

cooling, as well as water heating. Its great advantage is that it works by concentrating

naturally existing heat, rather than by producing heat through combustion of fossil fuels.

The technology relies on the fact that the Earth (beneath the surface) remains at

a relatively constant temperature throughout the year, warmer than the air above it during the

winter and cooler in the summer, very much like a cave. The geothermal heat pump takes

advantage of this by transferring heat stored in the Earth or in ground water into a building

during the winter, and transferring it out of the building and back into the ground during the

summer. The ground, in other words, acts as a heat source in winter and a heat sink in

summer.

Geothermal energy is energy derived from the natural heat of the Earth.

Geothermal resources are typically underground reservoirs of hot water or steam created by

heat from the Earth, but also include subsurface areas of dry hot rock. Geothermal energy is

considered a renewable resource because the heat emanating from the interior of the Earth is

essentially limitless.

Electricity generated from geothermal energy is sent to users through a transmission system

consisting of electric transmission lines , towers, substations , and other components (see

Energy Transmission section to learn more). The integration of geothermal energy into a

transmission system requires careful planning to balance the mix of geothermal energy with

other sources of energy generation

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GEOTHERMAL ENERGY

TYPES OF GEOTHERMAL RESOURCES

There are four types of geothermal resources:

(i) Hydrothermal

(ii) Geopressured

(iii) Hot dry rock(HDR)

(iv) Magma

At present, the technology for economic recovery of energy is

available for hydrothermal resources only. Thus this is commercially used resource at

present. Other resources are going through a development phase and have not become

commercial so far.

HYDROTHERMAL RESOURCES

There is more than one type of geothermal energy, but only one kind is

widely used to make electricity. It is called hydrothermal energy. Hydrothermal resources

have two common ingredients: water (hydro) and heat (thermal). Depending on the

temperature of the hydrothermal resource, the heat energy can either be used for making

electricity or for heating.

Low Temperature Resources: Heating

Hydrothermal resources at low temperatures (50 to 300 degrees Fahrenheit)

are located everywhere in the United States, just a few feet below the ground. This low

temperature geothermal energy is used for heating homes and buildings, growing crops, and

drying lumber, fruits, and vegetables.

In the U.S., geothermal heat pumps are used to heat and cool homes and

public buildings. In fact, approximately 750,000 geothermal exchange systems are installed

in the U.S. Almost 90 percent of the homes and businesses in Iceland use geothermal energy

for space heating.

The hydrothermal resources are located at shallow to moderate

depths(from approximately 100m to 4500m). Temperatures for hydrothermal reserves used

for electricity generation range from 90°C to 350°C but roughly two-thirds are estimated to

be in the moderate temperature range(150°C to 200°C).

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GEOTHERMAL ENERGY

Hydrothermal resources occur when underground water has access to high

temperature porous rocks, capped by a layer of solid impervious rock. Thus. Water is trapped

in the underground reservoir and is heated by surrounding rocks. Heat is supplied by magma

by upward conduction through solid rocks below the reservoir. Thus it forms a giant

underground boiler. Under high pressure temperature can reach as high as 350°C. The hot

water often escapes through the fissures in the rock, thus forming hot springs or geysers.

Sometimes steam escapes through the cracks in the surface. These are called fumaroles. In

order to utilize the hydrothermal energy, wells are drilled either to intercept a fissure or more

commonly into the hydrothermal reservoir.

High Temperature Resources: Electricity

Hydrothermal resources at high temperatures (300 to 700 degrees

Fahrenheit) can be used to make electricity.

These high-temperature resources may come from either dry steam wells or hot water wells.

We can use these resources by drilling wells into the Earth and piping the steam or hot water

to the surface. Geothermal wells are one to two miles deep.

In a dry steam power plant, the steam from the geothermal reservoir is

piped directly from a well to a turbine generator to make electricity. In a hot water plant,

some of the hot water is turned into steam. The steam powers a turbine generator just like a

dry steam plant. When the steam cools, it condenses to water and is injected back into the

ground to be used over and over again.

Geothermal energy produces only a small percentage of U.S. electricity. Today, it produces

about 15 billion kilowatt-hours, or less than one percent of the electricity produced in this

country.

For practical purposes, hydrothermal resources are further subdivided into

(i) Vapour dominated (dry steam fields)

(ii) Liquid dominated (wet steam fields)

(iii) Hot water resources

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GEOTHERMAL ENERGY

(i) Vapour-Dominated (Dry steam) system:

Dry steam fields occur when the pressure is not much above the atmospheric

pressure and the temperature is high. Water boils underground and generates

steam at temperatures of about 165°C and a pressure of about 7atm.

Steam is extracted from the well, cleaned in a centrifugal

separator to remove solid matter and then piped directly to a turbine. The exhaust

steam of the turbine is condensed in a direct contact condenser, in which the steam

is condensed by direct contact with cooling water. The resulting warm water is

circulated and cooled in a cooling tower and returned to the condenser. The

condensation of steam continuously increases the volume of the cooling water.

Excess water is reinjected at some distance deep into the ground for disposal. The

non condensable gases are removed from the condenser by steam jet ejection.

Conventional steam-cycle plants are used to produce energy

from vapor-dominated reservoirs. As is shown in Figure 6, steam is extracted from

the wells, cleaned to remove entrained solids and piped directly to a steam turbine.

This is a well-developed, commercially available technology, with typical unit

sizes in the 35-120 MWe capacity range. Recently, in some places, a new trend of

installing modular standard generating units of 20 MWe has been adopted. In

Italy, smaller units in the 15 to 20 MWe range have been introduced.

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GEOTHERMAL ENERGY

(ii) Liquid-Dominated (wet steam) system:

Steam plants offer the most cost effective technology when the

resource temperature is above 175°C. In high temperature liquid dominated

reservoir, the water temperature is above 175°C. However it is under high

pressure and remains in liquid state. The most developed such system is found in

New Zealand, where the reservoir temperature and pressure are 230°C and 40 atm,

and depths are 600m to 1400m.

When water is brought to the surface and pressure is reduced, rapid boiling

occurs and it flashes into steam and hot water. The steam is separated and used to

generate power in usual manner. The remaining saline hot water can be used for

direct heat and then re-injected into the ground. In dual-flash systems, the steam

is flashed a second time from the remaining hot fluid of the first stage, separated

and fed into the dual inlet turbine or into two separate turbines. The efficiency of

such a plant is around 8%.

Hot-water or wet-steam hydrothermal resources are much more commonly found

than dry-steam deposits. Hot-water systems are often associated with a hot spring

that discharges at the surface. When wet steam deposits occur at considerable

depths, the resource temperature is often well above the normal boiling point of

water at atmospheric pressures. These temperatures are known to range from 100-

700°F at pressures of 50-150 psig. When such resources penetrate to the surface,

either through wells or through natural geologic anomalies, the water often

flashes into steam.

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GEOTHERMAL ENERGY

(iii) Hot water system:

Hydrothermal reservoirs of low to moderate temperatures can

be used to provide direct heat for residential and industrial uses. The hot water is

brought to the surface where a heat exchanger system transfers heat to another

fluid although the resource can be used directly if the salt and solid content is low.

The geothermal fluid is re-injected into the ground after the extraction of heat.

Flash steam plants pull deep, high-pressure hot water into lower-

pressure tanks and use the resulting flashed steam to drive turbines. They

require fluid temperatures of at least 180°C, usually more. This is the most

common type of plant in operation today.

Most geothermal areas contain moderate-temperature water (below

400°F). Energy is extracted from these fluids in binary-cycle power plants. Hot

geothermal fluid and a secondary (hence, "binary") fluid with a much lower

boiling point than water pass through a heat exchanger. Heat from the

geothermal fluid causes the secondary fluid to flash to vapor, which then drives

the turbines. Because this is a closed-loop system, virtually nothing is emitted to

the atmosphere. Moderate-temperature water is by far the more common

geothermal resource, and most geothermal power plants in the future will be

binary-cycle plants.

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HOT DRY ROCK RESOURCES In some areas of the western United States, geologic anomalies such as tectonic

plate movement and volcanic activity have created pockets of impermeable rocks covering a

magma chamber within six miles of the surface. The temperature in these pockets increases

with depth and proximity to the magma chamber, but, because of their impermeable nature,

they lack a water aquifer. They are often referred to as hot dry rock (HDR) deposits. Several

schemes for useful energy production from HDR resources have been proposed, but all

basically involve creation of an artificial aquifer will be used to bring heat to the surface. The

concept is being tested by the U.S. Department of Energy at Fenton Hill near Los Alamos,

New Mexico, and is also being studied in England. The research so far indicates that it is

technologically feasible to fracture a hot impermeable system though hydraulic fracturing

from a deep well. A typical two-well HDR system is shown in Fig. 50.2. Water is injected at

high pressure through the first well to the reservoir and returns to the surface through the

second well at approximately the temperature of the reservoir. The water (steam) is used to

generate electric power and is then recirculated through the first well. The critical parameters

affecting the ultimate commercial feasibility of HDR resources are the geothermal gradient

and the achievable well flow rate.

GEOPRESSURED RESOURCES Near the Gulf Coast of the United States are a number of deep sedimentary

basins that are geologically very young, that is, less than 60 million years. In such regions,

fluid located in subsurface rock formations carry a part of the overburden load, thereby

increasing the pressure within the formation. Such formations are referred to as geopressured

and are judged by some geologists to be promising sources of energy in the coming decades.

Geopressured basins exist in several areas within the United States, but those of current

interest are located in the Texas—Louisiana coast. These are of particular interest because

they are very large in terms of both areal extent and thickness, and the geopressured liquids

appear to have a great deal of dissolved methane. In past investigations of the Gulf Coast, a

number of "geopressured fairways" were identified; these are thick sandstone bodies expected

to contain geopressured fluids of at least

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PRIME MOVERS FOR GEOTHERMAL ENERGY CONVERSION

The prime movers can be classified as:

(i) Impulse reaction turbine

(ii) Positive displacement machines

(i) IMPULSE REACTION MACHINES

An impulse turbine has fixed nozzles that orient the steam flow into

high speed jets. These jets contain significant kinetic energy, which the rotor

blades, shaped like buckets, convert into shaft rotation as the steam jet changes

direction. A pressure drop occurs across only the stationary blades, with a net

increase in steam velocity across the stage.

As the steam flows through the nozzle its pressure falls from inlet pressure to

the exit pressure (atmospheric pressure, or more usually, the condenser

vacuum). Due to this higher ratio of expansion of steam in the nozzle the

steam leaves the nozzle with a very high velocity. The steam leaving the

moving blades has a large portion of the maximum velocity of the steam when

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leaving the nozzle. The loss of energy due to this higher exit velocity is

commonly called the carry over velocity or leaving loss.

In the reaction turbine, the rotor blades themselves are arranged to form

convergent nozzles. This type of turbine makes use of the reaction force

produced as the steam accelerates through the nozzles formed by the rotor.

Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the

stator as a jet that fills the entire circumference of the rotor. The steam then

changes direction and increases its speed relative to the speed of the blades. A

pressure drop occurs across both the stator and the rotor, with steam

accelerating through the stator and decelerating through the rotor, with no net

change in steam velocity across the stage but with a decrease in both pressure

and temperature, reflecting the work performed in the driving of the rotor.

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(ii) POSITIVE DISPLACEMENT MACHINES

A positive displacement pump causes a fluid to

move by trapping a fixed amount of it and then forcing (displacing) that

trapped volume into the discharge pipe.

Some positive displacement pumps work using an expanding cavity on the

suction side and a decreasing cavity on the discharge side. Liquid flows into

the pump as the cavity on the suction side expands and the liquid flows out of

the discharge as the cavity collapses. The volume is constant given each cycle

of operation.

Positive displacement pumps, unlike centrifugal or

roto-dynamic pumps, will in theory produce the same flow at a given speed

(RPM) no matter what the discharge pressure. Thus, positive displacement

pumps are "constant flow machines". However due to a slight increase in

internal leakage as the pressure increases, a truly constant flow rate cannot be

achieved.

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A positive displacement pump must not be operated against a

closed valve on the discharge side of the pump, because it has no shut-off head

like centrifugal pumps. A positive displacement pump operating against a

closed discharge valve will continue to produce flow and the pressure in the

discharge line will increase, until the line bursts or the pump is severely

damaged, or both.

A relief or safety valve on the discharge side of the positive displacement

pump is therefore necessary. The relief valve can be internal or external. The

pump manufacturer normally has the option to supply internal relief or safety

valves. The internal valve should in general only be used as a safety

precaution, an external relief valve installed in the discharge line with a return

line back to the suction line or supply tank is recommended.

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(iii) IMPULSE MACHINES

In 1882 De Laval introduced his concept of an impulse steam

turbine and in 1887 built a small steam turbine to demonstrate that such

devices could be constructed on that scale. In 1890 Laval developed a nozzle

to increase the steam jet to supersonic speed, working off the kinetic energy of

the steam, rather than its pressure. The nozzle, now known as a de Laval

nozzle, is used in modern rocket engine nozzles. De Laval turbines can run at

up to 30,000 rpm. The turbine wheel was mounted on a long flexible shaft, its

two bearings spaced far apart on either side. Since the wheel could not be

perfectly balanced, this allowed it to spin slightly out of true, without breaking

the bearings.

The higher speed of the turbine demanded that he also designed

new approaches to reduction gearing, which are still in use today. Since the

materials available at the time were not strong enough for the immense

centrifugal forces, the output from the turbine was limited and large scale

electric steam generators were dominated by designs using the alternative

compound steam turbine approach of Charles Parsons. Using high pressure

steam in a turbine that had oil-fed bearings meant that some of the steam

contaminated the lube-oil, and as a result, perfecting commercial steam-

turbines required that he also develop an effective oil/water separator.

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After trying several methods, he concluded that a centrifugal

separator was the most affordable and effective method. He developed several

types, and their success established the centrifugal separator as a useful device

in a variety of applications.

De Laval also made important contributions to the dairy industry, including

the first centrifugal milk-cream separator and early milking machines, the first

of which he patented in 1894. It was not until after his death, however, that the

company he founded marketed the first commercially practical milking

machine, in 1918. Together with Oscar Lamm, de Laval founded the company

Alfa Laval in 1883, which was known as AB Separator until 1963 when the

present name was introduced.

In 1991, Alfa Laval Agri, a company producing dairy and farming machinery

was split from Alfa Laval when it was bought by the Tetra Pak Group. When

Alfa Laval was sold, Alfa Laval Agri remained a part of the Tetra Pak group

and was renamed DeLaval, after the company's founder.

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ADVANTAGES:

(i) Significant Cost Saving : Geothermal energy generally involves low running costs

since it saves 80% costs over fossil fuels and no fuel is used to generate the power.

(ii) Reduce Reliance on Fossil Fuels : Dependence on fossil fuels decreases with the

increase in the use of geothermal energy. With the sky-rocketing prices of oil, many

countries are pushing companies to adopt these clean sources of energy.

(iii) Environmental Benefits : Being the renewable source of energy, geothermal

energy has helped in reducing global warming and pollution. Moreover, Geothermal

systems does not create any pollution as it releases some gases from deep within the

earth which are not very harmful to the environment.

(iv)Direct Use : Since ancient times, people having been using this source of energy for

taking bath, heating homes, preparing food and today this is also used for direct

heating of homes and offices.

(v) Job Creation and Economic Benefits : Geothermal energy on the other hand has

created many jobs for the local people.

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DISADVANTAGES:

(i) Not Widespread Source of Energy : Since, this type of energy is not widely used

therefore the unavailability of equipment, staff, infrastructure, training pose hindrance

to the installation of geothermal plants across the globe.

(ii) High Installation Costs : To get geothermal energy, requires installation of power

plants, to get steam from deep within the earth and this require huge one time

investment and require to hire a certified installer and skilled staff needs to be

recruited and relocated to plant location. Moreover, electricity towers, stations need to

set up to move the power from geothermal plant to consumer.

(iii) Can Run Out Of Steam : Geothermal sites can run out of steam over a period

of time due to drop in temperature or if too much water is injected to cool the rocks

and this may result huge loss for the companies which have invested heavily in these

plants.

(iv)Suited To Particular Region : It is only suitable for regions where temperature

below the earth are quite low and can produce steam over a long period of time. For

this great research is required which is done by the companies before setting up the

plant.

(v) May Release Harmful Gases : Geothermal sites may contain some poisonous gases

and they can escape deep within the earth, through the holes drilled by the

constructors.

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REFERENCES

(i) Non-Conventional energy resources by B H Khan

(ii) Non conventional Energy sources by G.D.Rai

(iii) en.wikipedia.org/wiki/Geothermal_energy - 

(iv) inlportal.inl.gov › Renewable Energy Home › Geothermal

(v) www1.eere.energy.gov/geothermal/ - 

(vi) www.eai.in/ref/ae/geo/geo.html - 

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