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.
.jpg)
0 Comments