What is Geothermal Energy?

Geothermal energy is an important non-conventional source of energy. Geothermal energy is heat from within the earth. The origin of the word 'geothermal' is from Greek words geo (earth) and therme (heat). Geothermal energy is generated in the earth's core. The core is very hot at about 4000°C. The double-layered core is made of very hot molten rock called magma which surrounds a solid metallic center. Outer to the core there is another layer called mantle. It is about 2900 km thick and consists of hot, dense silicate rock. The outermost layer, which is about 30 km thick, is called the crust of the earth.

It is possible to make use of the heat energy of the earth from volcanoes, hot springs, fumaroles geysers. In some countries it is possible to pump out the water from underground hot water deposits and use it to heat houses in cold areas.

It is also possible to generate electricity from geothermal energy through transfer of heat energy to a working fluid which may operate the power cycle. Hot water or steam from geothermal sites may be used to operate the turbines directly generating electricity.

ORIGIN AND NATURE OF GEOTHERMAL ENERGY

Geothermal energy is the heat energy that has originated in the core of the earth at a temperature of 4000°C. The heat is due to the decay of radioactive materials. The center of the core of the earth is at a depth of 6,370 km. The core consists of two regions: the inner core and the outer core. The outer core is surrounded by a region called mantle which consists of hot molten rock known as magma. The outermost layer above the mantle is called crust which has an average thickness of about 30 km. The temperature in the crust increases with depth at a rate of 30°C per km.

The earth's crust consists of six major plates called tectonic plates. The world's main geothermal sites are located near the pacific tectonic plate also called the 'ring of fire'. Most of the volcanic activity takes place in the junctions of tectonic plates.

At locations where the crust is fractured, water percolates downwards and gets heated. It then pushes upward in the form of hot springs, some impermeable rocks block the upward fluid and do not allow heat to flow towards earth's surface. In these locations water is heated and converted into steam. This steam is released in the form of geysers. It is estimated that 1 km2 of surface area of the crust has enough energy to supply 5.5% of the world's total annual consumption of energy. There is a vast scope of use of geothermal energy for power generation and other heat utilizations.

Geothermal Field

All the heat stored in the earth's crust more than 15°C to a depth of 10 km beneath the surface is called the geothermal source.

The sites having geothermal sources are called geothermal fields. These fields require an underground source of water, an impermeable layer that traps water and allows formation of steam and a large mass of hot rock near the water system.

The heat is supplied by magma by upward conduction through solid rocks below the reservoir. Water is trapped in the underground reservoir also called aquefier and is heated by surrounding rocks. Thus it acts like a giant underground boiler and can reach a temperature as high as 350°C. The hot water may escape through fissures in the rocks and form a hot spring or geyser.

Sometimes steam escapes through cracks on the surface called "fumaroles". Whenever wells are drilled on such sites steam and hot water comes out of drilled holes and become a source of geothermal energy for use in power plants.

PRESENT UTILIZATION OF GEOTHERMAL ENERGY

The geothermal energy utilization in India is mostly for power generation by pilot power projects and for other purposes as described below: 

Power Generation

(a) Manikaran geothermal field: Manikaran geothermal field is situated near Kullu in Himachal Pradesh at an altitude of 1700 meter above mean sea level. The main geothermal activity is in the form of hot springs spread over a distance of about 1.25 km on the right bank river and 450 m on the left bank of the Parbati river. The temperature of hot springs are in the range of 34°C to 96°C on the right bank and 28°C to 37°C on the left bank of the river. A 5 kW pilot project of geothermal energy has been installed at Manikaran by Geological Survey of India (GSI) and National Aeronautical Laboratory (NAL), Bangalore.

(b) Tattapani geothermal fields: Tattapani geothermal field is located in the district of Balrampur in Chhattisgarh. Geothermal activity is very intense in an area of 0.05 km² having several hot spots, hot water pools and marshy land. The temperature of hot water varies from 50°C to 98°C. Several boreholes have been drilled to a depth of 100 m to 500 m and the temperature of geothermal fluid is about 112°C at a pressure of 4 kg/cm² and discharge 1600 liter/minute.

A 300 kW pilot power plant based on a binary cycle has been installed by a joint venture of ONGC, GSI and the state government. (e) Puga geothermal field: It is located in Ladakh region of Jammu and Kashmir across the great Himalayan range at an altitude of 4400 meter above mean sea level. The geothermal activity occurs in the form of hot springs, hot pool sulfur condensates, and borax evaporation spread over a distance of 4 km.

The temperature of hot springs is in the range of 30°C to 84°C. There are several boreholes with depths of 28 m to 385 m have been drilled in the Puga valley. The geothermal fluid is a mixture of water and steam at a temperature of 140°C and pressure of 3 kg/cm² and total discharge of 190 tonnes per hour. The Puga geothermal field has a potential to generate 3 MW electrical power, considering a depth level of 500 m.

Space Heating

(a) Tapovan geothermal field: It is located in Dhauliganga river valley at an altitude of 1800 m above mean sea level in Chamoli district of Uttrakhand. The geothermal activity is in the form of five hot springs spread over a distance of 1 km along the hill slope on the left bank of the Dhauliganga river.

There are over 2500 people living in the nearby villages where temperature varies from 0°C to 30°C and snowfall occurs during the winter. These people need space heating for comfortable living and other heating purposes. The space heating is also required for poultry farming, cattle farming, biogas fermentation, aquaculture, growing of mushrooms, etc.

The main project at Tapovan consists of transportation of geothermal water through insulated pipe from the geothermal site to the Tapovan village. In the village erection is proposed for space heating, poultry farming, etc.

The temperature of hot water is 65° C at a pressure of 4 kg/cm² at a discharge rate of 500 liter/minute. The energy available is 1400 kWh.

(b) Puga geothermal field: The puga village is located at high altitude and has experienced a temperature as low as - 35°C during winter. Space heating has been successfully implemented by utilizing heat of geothermal water. This project provides space heating to a hut of 5m x 5m x 2m dimensions with the help of geothermal hot water and maintains the ambient temperature of the hut at 20°C. The hot water obtained from hot springs has a temperature in the range of 30°C to 84°C with a total discharge of about 300 liters/minute. In addition to room heating geothermal energy is also used for poultry farming, growth of mushrooms.

(c) Refrigeration: A geothermal energy based cold storage plant of 7.5 ton capacity is working at Manikaran in Kullu district of Himachal Pradesh. This plant uses ammonia as refrigerant and geothermal water at 90°C.

(d) Other uses of geothermal energy: In additional to the power generation space heating and refrigeration, geothermal energy is also used for extraction and refining of borax and sulfur at Puga valley and greenhouse heating suitable for cultivation at a temperature of 20°C to 25°C at Chumathang in Jammu and Kashmir.

The geothermal energy is also utilized for aquaculture, pond heating, agricultural drying, industrial heating, heating for bathing, swimming, snow melting.

POTENTIAL OF GEOTHERMAL ENERGY

India has a great potential for geothermal energy. The estimated thermal energy stored in geothermal sites is more than 4 x1016 kilocalories. Its coal equivalent is 6 billion tonnes, oil equivalent is 28 billion barrels and electrical power equivalent is 10,600 MW.

There are several geothermal provinces in India.

They are as follows:

• the Himalayas
• Sohna
• West coast
• Cambay
• Son-Narmada-Tapi (SONATA)
• Mahanadi valley
• Godavari valley
• Damodar valley
• Naga-Lushai

Of these geothermal provinces, the important potential geothermal sites have been identified as given below in the descending order of potential:

• Tattapani in Chhattisgarh
• Puga in Ladakh
• Cambay Graben in Gujarat
• Manikaran in Himachal Pradesh 
• Surajkund in Jharkhand.
• Chhumathang in Jammu and Kashmir 

Apart from the above, other potential sites in India are Godavari basin, Bakreshwar (West Bengal), Tuwa (Gujarat), Unai and Jalgaon (Maharashtra), Tapovan (Uttarakhand).

Global Potential of Geothermal Energy

The main sources of geothermal energy are the heat flow from the core and the mantle of earth which constitutes 40% and heat generated by gradual decay of radioactive materials in the earth's crust which constitutes 60%. The world's geothermal heat resources are enormous but it is difficult to determine global geothermal energy potential accurately due to their hidden nature.

The estimated potential of global geothermal resources is about 6.5 x 106 MW. Out of the total potential, presently identified hydrothermal resources capable for generation of electricity is about 0.2 x 106 MW and the remaining are useful mainly for direct heat and other applications.

GEOTHERMAL ENERGY EXTRACTION

There are mainly three types of geothermal sources which are used for extraction of energy depending on the temperature and quality of the source. If the source temperature is above 150°C it is useful for generating electricity, source temperature below 150°C is useful for direct heating purposes.

1. Hydrothermal source: These sources have no water and steam reservoir. Energy can be extracted from these sources by drilling into the source to obtain heat energy useful for generation of electricity. These sources are of two types: Hot water type and Wet steam type. Hot water fields have temperature below 100°C. Wet steam fields have both water and steam having temperatures of more than 100°C and 350°C respectively. 

2. Vapor-dominated source: It is also called dry steam source. These sources have saturated steam at high temperature at about 350°C. It is mainly dominated by steam and water has less content.

3. Hot dry rock source: The temperature of hot rock is 650°C but contains no water. Energy can be extracted from this source by injecting water to create an artificial reservoir. To get hot water and steam another well is to be drilled into this reservoir which will be used to generate electricity.

Flash Steam Power Plants

In these power plants the temperature of water is above 175°C and obtained at a depth of 600 m to 1400 m. Water is brought to the surface and flashed into steam and hot water due to pressure reduction. The steam is separated by a flash separator and used to run a steam turbine. The turbine drives the generator and produces electricity. The steam at the turbine outlet is condensed and the hot water is sent back to the reinjection well.

Binary Cycle Power Plant

In this power plant the temperature of water available in the geothermal source is less than 100°C. This temperature is not sufficient to produce steam. So heat of the geothermal fluid is used to vaporize an organic fluid (isobutane or freon) as working fluid. The boiling point of isobutane is 10°C so it can be easily vaporized by a low temperature source.

Working of the binary cycle power plant is based on two steps. In the first step, the geothermal fluid is fed to a heat exchanger where its heat energy is utilized to vaporize the working fluid, after that it is sent back into reinjection well. This completes one cycle. In the second step, the vaporized working fluid obtained is fed to a turbine. The turbine drives the generator and produces electrical power. The working fluid is condensed at the turbine outlet and again fed into the heat exchanger and this completes the cycle. The process is a closed cycle and is repeated again and again to generate electricity. There are two separate closed cycles so this plant is called a binary cycle power plant.

Vapor Dominated (Dry Steam) Power Plant

The dry steam geothermal fields have a temperature of about 200°C at a pressure of about 7 kg/cm². Steam is extracted from the production well and is fed to a centrifugal separator. Here the solid particles are separated and dry steam is fed to the turbine. The turbine runs the generator which produces electrical power. Steam is condensed at the turbine outlet and the hot brine obtained is sent back into the reinjection well.

ADVANTAGES AND DISADVANTAGES OF GEOTHERMAL ENERGY

Advantages of Geothermal Energy

1. It is independent of weather conditions. 2. As the tremendous energy in the form of heat is trapped inside the earth, no extra storage facilities are required. 

3. Geothermal power plants require less area.

4. Geothermal energy is a free and renewable energy source. 

5. This source can be utilized for electricity generation as well as for direct heating purposes.

6. Emission of pollutant gasses like carbon dioxide (CO₂), sulfur dioxide (SO₂) is much less compared to fossil fuel-based conventional power plants.

7. Geothermal energy is cheaper than energy obtained from fossil fuel based plants.

Disadvantages of Geothermal Energy

1. Energy obtained from geothermal fields is of low grade heat. 

2. Noise pollution due to drilling work during exploration. 

3. The geothermal fluids are corrosive and abrasive in nature. This adversely affects the life of a plant. 

4. Geothermal energy is available at limited sites. 

5. Geothermal fluids have dissolved gasses like carbon dioxide (CO₂), Hydrogen sulfide (H₂S), ammonia (NH3), methane (CH₂). nitrogen (N₂) and hydrogen (H₂). They lead to air pollution. 

6. Geothermal fluids also contain some radioactive materials. They may cause health hazards.

7. Huge amounts of steam which escapes into the atmosphere without utilization may cause dense fog. This may lead to road traffic problems.

8. Because of low efficiency of geothermal power plants, huge amounts of heat which escapes without being fully utilized for electricity generation may cause heat pollution. That may contribute to the problem of global warming. If this unused hot fluid is flown into the rivers, it may cause damage to fisheries and other aquatic lives.

9. The efficiency of geothermal power plants is about 15% which is low as compared to conventional power plants (fossil fuel-based plants have about 30% efficiency).

LIMITATION AND CHALLENGES

1. Locations of geothermal energy are far away from the load center. This causes transmission losses.

2. The technology to explore and harness geothermal energy is in the developing stage. 

3. Geothermal energy is of low grade because the temperature of steam or hot water obtained lies between 150°C to 250°C and the pressure obtained is also low (100 psi). Whereas steam in a conventional thermal power plant has a temperature of about 550°C at high pressure (1000 psi).

4. The geothermal sources of dry steam type are very few. In wet steam systems only a part of the hot water that flashes into steam can be used for generation of electricity. The rest of unutilised hot water is thrown away into lake or river. is complex because it requires

5. Geothermal energy extraction is a wide variety of systems.

6. The geothermal power plants are smaller in sizes as compared to conventional thermal power plants.

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Small Hydro Power Plant

Hydroelectric power can be captured whenever a flow of water falls from a higher level to a lower level due to gravity. Hydraulic turbines convert the energy of flowing water into mechanical energy which in turn can be converted into electrical energy with the help of generators.

Ancient Greeks used wooden water wheels to convert kinetic energy of water into mechanical energy as far back as 2000 years ago. In Ancient Egypt, Persia and China, water wheels were used for irrigation, grinding of grains, etc.

In India the first hydroelectric power plant was built at Darjeeling in the year 1897. The second hydroelectric power plant was built at Shivasamudram in Karnataka in 1902.

There is a big gap between the demand and supply of electricity in our country. To me the challenge, small hydro power (SHP) upto 25 MW has been given top priority by the Ministry of New and Renewable Energy, Government of India as SHP has an estimated potential of more than 15000 MW in the country.

TERMS AND DEFINITIONS

1. Gestation period: Time required for completion of a project. 

2. Flow: Quantity of water falling.

3. Hydraulic machine: A device which converts hydraulic energy of a fluid into mechanical energy or vice-versa.

4. Turbine: the hydraulic machine which converts hydraulic energy into mechanical energy. It is a prime mover for electric generators.

5. Pump: The hydraulic machine which converts mechanical energy into hydraulic energy.

6. Dam: A structure constructed across a river or a channel to store water.

7. Head race: Upper water storage.

8. Tail race: Channel carrying water away from the turbine.

9. Head: Vertical difference of water level between head race and tail race.

CONSTRUCTION OF SMALL HYDRO POWER PLANT

The main components of small hydro power plants are as described below:

1. Weir: The water from the river stream is diverted to a channel through a diversion structure called weir. This helps in maintaining constant flow in the channel for variable flow of water in the river.

2. Desilting tank: A desilting tank is provided to trap the pebbles and other coarse material in the water in order to prevent the damage to the turbine blades. The silt particles of size more than 0.5 mm deposited in the tank are flushed out periodically.

3. Power channel: Power channel is a water conductor system from weir to forebay through desilting tanks. It should be designed to minimize head loss and water seepage. The power channel has a trapezoidal cross-section and longitudinal slope of 1 : 500.

4. Forebay: At the end of the power channel, a large storage tank called forebay is constructed with reinforced cement concrete (RCC) and stone masonry. The water stored in the forebay is utilized for generation of electricity. The forebay provides the minimum water head required for the turbine. To filter out trash, ice, grasses, debris, a crash rack is provided before the penstock.

5. Penstock: Penstock is a water conduit joining the forebay and the turbine inlet. They are made of RCC concrete, mild steel or PVC depending on their diameter and length.

6. Surge tank: For a medium/high head plant, a surge tank is provided with the penstock near the turbine. This releases water from the penstock when excessive pressure develops in the penstock due to the water hammering effect. This happens whenever there is a sudden load throw on the generator causing the turbine inlet gates to close suddenly. Also water is taken from the surge tank into penstock at time of sudden opening of turbine inlet gates.

7. Spillway: It is an overflow arrangement provided in the forebay to discharge the surplus water into the downstream of the river.

8. Power house: It is a building constructed to house the turbine, generator, control panels, auxiliary equipment. The size of the building depends on the number of generating units.

9. Tail race: The water passing through the turbine is collected in the draft tube and discharged into a water channel called tail race. Tail race then drains out this discharged water into the river downstream.

POTENTIAL OF SMALL HYDROPOWER PLANTS

India has an estimated potential of small hydro power plants of 15,380 MW from 5,718 identified sites. Out of this about half lies in Himachal Pradesh, Uttarakhand, Jammu & Kashmir and Arunachal Pradesh. Plain regions in Maharashtra, Chattisgarh Karnataka, Punjab, Kerala, etc., have also sizable potential.

TURBINE

The selection of a suitable turbine for any particular hydro site depends on site characteristics mainly the head and the discharge available. The turbine should produce the expected power at particular speed, head and discharge.

Classification of Turbines

The turbines are classified as high head, medium head and low-head turbines on the basis of available head. They are also classified as impulse and reaction turbines based on the action of flowing water on turbine blades.

1. Impulse Turbine

The principle of operation of an impulse turbine is based on the conversion of pressure energy of water into kinetic energy when water passes through a nozzle and strikes the turbine blades with high impulse force causing rotation of the turbine. Main types of impulse turbine are as described below:

(a) Pelton wheel turbine: It consists of a wheel or runner with a series of split buckets set around its rim. The water from the penstock flows into the turbine through a jet nozzle. The high speed jet of water from the nozzle strikes the bucket attached to the wheel. This causes rotation of the turbine. The amount of water striking the vanes is controlled by forward and backward motion of a valve called a spear. A casing is provided to prevent splashing of water and to discharge the water into tail race. It is made of cast iron (CI) or fabricated steel plates.

The main feature of pelton wheel turbine are:

1. It is suitable for high water heads in the range of 60 to 700 m which can give an output power of 50 to 10,000 kW.

2. Turbine efficiency is 85-90%.

3. It is mostly used in a high head micro hydro system.

4. Its shaft is horizontally mounted.

5. Buckets have a minimum cavitation effect.

6. It is a radial flow turbine as water strikes tangentially at the wheel.

(b) Turgo Turbine: The turgo wheel is an upgraded version of pelton wheel. Here the water jet is designed to strike the plane of the runner at an angle about 20° so that the water enters the runner at one side and exits on the other. The speed of a turgo turbine is much higher than a pelton wheel. It can operate under low-flow conditions with medium or high head.

The main features of a turgo turbine are: 

1. It is suitable for water heads in the range of 30-210 m.

2. It is suitable for horizontal shaft arrangement.

(c) Ossberger cross-flow turbine: It has a drum-shaped runner with a solid disk at each end connected together by a series of curved blades. The shaft of the turbine is always horizontal. A rectangular nozzle directs a jet of water over the full length of the runner. Water enters the top of the runner and impinges on the curved blades imparting its kinetic energy. Water before falling away strikes the runner blade again on its exit. Thus there is a cross-flow of water over the runner.

The main features of Ossberger crossflow turbine are:

• Its efficiency is about 85%.
• It is suitable for water heads in the range of 1 to 200 m.
• It is free from cavitation.
• The shaft of the turbine is horizontal.

2. Reaction Turbines

The runner of a reaction turbine is completely immersed in a water filled casing. The pressure difference across runner blades imposes reaction force which causes the rotor to rotate. All reaction turbines have a draft tube below the runner through which water discharges.

The main types of reaction turbines are described below:

(i) Francis Turbine: It consists of a spiral scrolled casing which distributes water around the perimeter of the runner. Guide vanes are fixed in the spiral casing which can be adjusted to control the flow of water into the runner. When guide vanes are opened, water is directed tangentially to the runner. When water passes over runner blades, it causes reaction force on it. This reaction force rotates the runner.

In this turbine water flows radially inwards into the runner and discharges axially out of the runner. The draft tube below the runner discharges the water into the tailrace. It is a vertically mounted type turbine.

(ii) Kaplan Turbine: Kaplan turbine is a reaction type axial flow turbine. It has adjustable blades inside a tube. The inlet guide vanes can be opened and closed to control the amount of water flow through the turbine. Fully closed guide vanes will stop the water flow completely and bring the turbine into rest. When guide vanes open water hits the turbine blades thereby causing rotation of the turbine. A draft tube is fitted below the turbine runner to extract water and discharge it into the tail race. Kaplan turbine is suitable for low head with high flow rate. Kaplan turbines are available in vertical axis, horizontal axis and inclined axis configurations. Horizontal axis turbines are slightly more efficient than vertical axis turbines. This turbine suffers cavitation of blades near the outlet edges due to silt water.

ENVIRONMENTAL IMPACT OF SMALL HYDRO POWER PLANTS

1. Construction of road, reservoir, power house switchyard, diversion tunnel in small hydro power plants adversely affect the environment. 

2. SHP may adversely affect the flora and fauna of the area.

3. Small hydro power plants with reservoirs provide habitat for mosquitoes and snails which may cause diseases like malaria, yellow fever, dengue, encephalitis and schistosomiasis.

4. The operation of small hydropower equipment may increase the noise level of the surrounding area. This adversely affects the environment in hilly and mountainous regions.

5. Alteration of water flow such as broadening of stream and reduction of current may cause adverse environmental impact. 

6. The other environmental impacts include sedimentation and deterioration of water quality.

7. The released water from SHP contains low dissolved oxygen, this may adversely affect fishery and cause mortality of sensitive species.

8. The construction activity in the SHP sites has an adverse impact on ecology (flora and fauna). 

9. The small hydro power plants may cause loss of water falls and other recreational activities.

MERITS AND DEMERITS OF SHP

Mertis

1. It is a renewable and clean source of energy. It does not require any fuel, so the dependency on imported fuel also reduces.

2. It helps in reducing emission of greenhouse gasses. 

3. Low operating and maintenance costs.

4. It benefits the people in remote areas where there is no grid connectivity.

5. It does not require large civil engineering construction. 

6. The capital cost of the small hydro power plants is quite low as compared to conventional energy sources.

7. No large scale water storage is required, so there is no problem of submergence, deforestation and rehabilitation. 

8. It is a more concentrated form of energy as compared to solar and wind energy which are dilute in nature. 

9. Few operating manpower is required in small hydropower plants.

10. Energy can be tapped whenever water flows along small streams, small rivers, and even canals also.

11. The small hydro does not require much expertise to build and operate.

Demerits

1. SHP Plants have low installed capacity. 

2. They are located in remote areas, so less chances of grid connectivity, so the surplus power cannot be utilized.

3. There are uncertainties about their potential as a reliable source of energy due to inadequate technical data.

4. SHP plants adversely affect migration of fish, fishing, boating, etc.

5. SHP technology is not fully developed.

6. Limited sites are available for SHP according to MNRE.

7. In many locations the water flow fluctuates seasonally. During the summer there will be less flow and therefore less power output. 

8. Additional equipment is required for voltage and frequency control as they are mainly operated in off-grid mode.

9. The potential sites of SHP are worthy in hilly or mountainous regions where grid connectivity will probably never reach.

10. Although small hydro power is a clean, renewable energy source, the projects require approval from the Ministry of Environment and Forests (MOEF) and other departments.

SELECTION OF SITE

Selection of site depends on the following factors :

1. Topography and geography of the site.

2. Evaluation of the water source and its generating potential.

3. Site selection and layout.

4. Hydraulic turbines, generators and their control.

5. Environmental impact assessment and mitigation measures.

6. Economic evaluation and potential of finance.

7. Institutional framework.

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