The heat energy stored under the earth’s surface is called Geothermal Energy. This heat energy exists due to continuous decay of radioactive material and also due to continual heat loss from Earth’s formation. The earth’s core is at a very high temperature of around 4000⁰-7000⁰Celsius and hence there is a huge difference between the temperature on the surface and at the core of the earth. The phenomenon of transfer of heat from a surface at higher temperature to a surface at lower temperature is called convection. Due to this phenomenon heat travels from the core towards the surface.
This energy can be used for generation of electricity and as of 2013, 11,700 MW of geothermal power is generated. Although this form of energy is reliable and environment friendly, it cannot be easily extracted as drilling for geothermal reservoirs is generally expensive. Some technological advancement is made in this field and extraction has become relatively easier now as it was a decade ago. Although Geothermal wells release greenhouse gases as well but the emissions per unit energy are relatively lower than those released by fossil fuels hence geothermal energy can be potentially used in place of fossil fuels.
Ways through which Geothermal energy can be extracted –
- Flash steam power plants–Flash plants are used to generate electricity from Liquid-Dominated water sources. These sources have water with temperature more than 180⁰C and are predominantly found near young volcanoes, rift zones, mantle hotspots surrounding the Pacific Ocean. Water by its own pressuregoes up through wells in the ground, so it can be discharged without pumping. As it flows upward the pressure decreases and hence it starts converting into steam. This steam is used to power the turbines and hence electricity is generated. The remaining water and steam are sent back to the reservoir to increase the efficiency of the process.
These reservoirs are most commonly found and hence these are used more commonly to generate electricity.The Cerro Prieto geothermal field in Mexicois the largest developed liquid-dominated system and has measured water temperatures up to 350°C.
- Binary Cycle Power Plants – These plants work with water at lower temperatures at about 107°-182°C, compared to flash steam plants that use very hot water. These systems have a very simple design and are where the water is heated through deep circulation along faults located in Turkey or the western parts of the United States. Water must first be pumped in order to be accessed. After being pumped up, the water is boiled and gets converted into a working fluid. The working fluid is typically a low boiling organic compound. A heat exchanger is used to convert this working fluid into vapor form. These vapors are then used to rotate the turbine due to which electricity is produced. This process produces little or no emissions and hence is environment friendly. Most new geothermal plants are binary cycle power plants.
- Dry-steam Power Plants – These plants use the underground vapor resources. The steam is directly brought to the surface of the earth and is used to run the turbines which generate electricity. Such vapor resources are very rare and very few of them have been found yet.
The volcanically active area in Larderello, Italy houses the first commercial Dry-steam power plant which was installed in 1913. This plant is one of the largest geothermal electricity producer in the world. The plant generates 594 megawatts of electricity, which is enough to supply power to approximately 594,000 households.
Sometimes a hybrid power plant is also used because in many cases a reservoir produces both steam as well as hot water and to use such resources effectively and efficiently power plants which are hybrids of above mention power plants are used.
- Enhanced Geothermal Systems – These systems are engineering techniques to artificially create geothermal resources. The limitation in using naturally occurring geothermal reservoirs is that they are rarely at a convenient location or of a desirable size. EGS mitigates these constraints by allowing for the creation of hydrothermal reservoirs in deep, hot but naturally dry geological formations.EGS techniques can also be used to extend the lifespan of naturally occurring hydrothermal resources. Although these systems are currently very expensive and very limited research is being conducted to develop such systems, EGS remains a very feasible way of extracting geothermal energy.
INDIA & GEOTHERMAL ENERGY
- It is estimated that India has 10,000 MW of stored geothermal energy which can be used. More than 300 hot spring locations have been found to exist in India by Geological survey of India (Thussu, 2000). The surface temperature of these hot springs varies from 35⁰ C to as much as 98⁰ Based on their occurrence these hot springs have been grouped together and termed as different geothermal provinces in specific geotectonic regions, geological and structural regions such as occurrence in orogenic belt regions, structural grabens, deep fault zones, active volcanic regions etc.
- Puga Valley (J&K)
- Tatapani (Chhattisgarh)
- Godavari Basin Manikaran (Himachal Pradesh)
- Bakreshwar (West Bengal)
- Tuwa (Gujarat)
- Unai (Maharashtra)
- Jalgaon (Maharashtra)
Indian organizations working in geothermal energy:
- Central Electricity Authority
- Geological Survey of India
- Indian Institute of Technology, Mumbai
- Regional Research Laboratory, Jammu
- National Geophysical Research Institute, Hyderabad
- Oil and Natural Gas Corporation, Dehradun
Currently there are no Geothermal power plants operating in our country, however work is underway on two projects.
Ongoing Projects in India:
- Magneto-telluric investigations in Tattapani geothermal area in Madhya Pradesh
- Magneto-telluric investigations in Puga geothermal area in Ladakh region, Jammu & Kashmir.