At present most of our energy demand is met by conventional fossil fuels such as coal, oil and natural gas. But these are available in limited quantities. It is estimated that we could consume all the oil in about 40 years and the natural gas soon after. However, coal is available in plenty, but its extraction is not as per demand. The other problem with fossil fuels is environmental. After combustion, they emit carbon dioxide to the atmosphere, which causes global warming. They also release many other harmful by-products that pollute the environment.
Apart from fossil fuels, we get energy from hydropower and nuclear power. But their share is less. Day-by-day, demand of energy is increasing in all the countries and there is always a shortfall. Scientists and engineers have developed some alternative sources of energy to reduce the gap between demand and availability. Geothermal energy is one of them; others being wind power, solar energy and bio-energy etc.
Geothermal is a combination of two Greek words- geo which means ‘earth’ and thermos, which means ‘heat’. Thus, geothermal energy is the heat energy from the earth. Where does the heat come from the earth? To know this, we have to know the creation of earth. Earth and other planets were formed from primordial clouds of hot gases. It was too hot at the time of creation. Gradually, it was cooled and the solid crust was formed. But below the earth’s crust, it is still in hot liquid state. This layer of hot and molten rocks is called magma. Heat is continuously produced there, mostly from the decay of radioactive materials such as uranium. Heat flows upwards from the earth’s interior. Normally the crust of the earth insulates us from the heat, but at some weaker points such as hot geyser, the heat comes to the surface of the earth.
Direct Uses of Geothermal Heat
Geothermal reservoirs of hot water can be used to provide heat directly. Direct use of geothermal energy is very old method. People in different parts of the world used hot springs for bathing, cooking food, space heating and other day-to-day heating purposes. Now, modern systems are being used for direct use where a well is drilled into a geothermal reservoir to provide a steady stream of hot water. This water is brought up through the well and with the help of mechanical systems such as pipes, heat exchanges etc., the heat is delivered directly for its intended use.
Geothermal heat can also be used for air-conditioning and refrigeration applications through a geothermal heat pump. Geothermal heat pump system consists of a ground heat exchanger, a heat pump unit and an air delivery system. This is being used directly for heating in at least 78 countries.
The most common application of geothermal energy in agriculture is in greenhouse heating, which has been developed on a large scale in many countries. The cultivation of vegetables and flowers out-of-season, or in an unnatural climate, can now draw on a widely experimented technology. Various solutions are available for achieving optimum growth conditions, based on the optimum growth temperature of each plant, the quantity of light, the CO2 concentration in the greenhouse environment, the humidity of the soil and air, and air movement.
Exploitation of geothermal heat energy in greenhouse heating can considerably reduce their operating costs, which in some cases account for 35% of the product costs (vegetables, flowers, house-plants and tree seedlings).
Electricity Generation from Geothermal Energy
All geothermal plants work by producing steam to turn a turbine and generator. However, several modifications have improved the technology, making it more suitable for mainstream usage.
Dry Steam Power Plants
Dry steam power plants were the first type of geothermal plants. This type of geothermal power plant was named dry steam since steam is extracted from the underground reservoirs in place of hot water. Geothermal steam of at least 150°C is extracted from the reservoirs through the production wells and is sent directly to the turbine, which drives a generator to produce electricity. These plants only emit excess steam and minor amounts of gases.
Geothermal reservoirs that can be exploited by geothermal dry steam power plants are rare. The first one was constructed in Larderello, Italy, in 1904. The Geysers, 22 geothermal power plants located in California, is the only example of geothermal dry steam power plants in the United States.
Figure 1: How a dry steam power plants works?
Flash Steam Power Plants
Flash steam power plants use water with temperatures greater than 200°C pumped at high pressures to the surface, where the pressure is suddenly dropped, causing the hot water to ‘flash’ into steam. The steam is then used to power a turbine and generator. Any leftover water is pumped back into the reservoir, or into a second tank where it can be flashed again to generate more steam. The only by-products of this process are excess steam and trace gases.
Flash steam is today’s most common power plant type. The first geothermal power plant that used flash steam technology was the Wairakei Power station in New Zealand, which was built in 1958.
Figure 2: How a flash steam power plants works?
Binary-Cycle Power Plants
The binary cycle power plant has one major advantage over flash steam and dry steam power plants. The water-temperature can be as low as 57°C. It utilises a secondary working fluid, usually an organic fluid (typically n-pentane), that has a low boiling point and high vapour pressure at low temperatures when compared to steam. The geothermal steam or hot water yields heat to the secondary fluid through heat exchangers. As a result, the secondary fluid is heated and vapourised. The vapour drives the turbine. Then the vapour is cooled and condensed by cooling water and the cycle begins again (Fig – 3). Because the two fluids are separated during the whole process, almost nothing is emitted to the atmosphere. Since water in underground reservoirs usually has moderate temperatures, binary-cycle power plants will likely be the main geothermal technology in the future. The disadvantage of this system is that it tends to be less efficient. These power plants have a thermal efficiency rate of only 10-13%. However, geothermal binary cycle power plants enable us, through lowering temperature requirements, to harness geothermal energy from reservoirs that with a dry- or a flash steam power plant wouldn’t be possible. First successful geothermal binary cycle project took place in Russia in 1967.
Fig. – 3 : How a binary-cycle power plant works?
Advantages & Disadvantages of Geothermal Energy
Geothermal energy does not cause any pollution. It is a clean source of energy and it has no harmful by- products. It is inexpensive, as no fuel is required to produce energy and hence, running cost of power plant is less. Further, it can be used to produce electricity 24 hours a day (comparatively other renewable sources of energy such as solar and wind energy have limitations). Geothermal power plants are generally small and have little effect on the natural landscape or the nearby environment. Though geothermal energy has several advantages, it also has some disadvantages and limitations. If harnessed incorrectly, it can sometimes produce pollutants. Improper drilling into the earth can release hazardous minerals and gases deep inside the earth.
The worldwide use of geothermal energy is increasing. Today 11,772 MW of power is being generated in at least 24 countries from geothermal energy and in 2010; it generated 67,246 GWh of electricity. This is a rise of about 20% since 2005. By 2015, figure is expected to grow even more to 18,500 MW. The largest producer of this energy is USA that generates about 3,086 MW of electricity. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California of USA. The Philippines is the second highest producer, with1,904 MW of capacity online. Geothermal power makes up approximately 18% of the country’s electricity generation. Also in Indonesia 5% of overall electricity generation is from geothermal energy. The installed capacity of geothermal power plant in different countries is given in Table-1.
Status of Geothermal Energy in India
Geological Survey of India has identified about 340 geothermal hot springs in the country. Most of them are in the low surface temperature range from 370C to 900C, which is suitable for direct heat applications. Only some are suited for power generation. The potential for power generation at these sites is about 10,000 MW. Though, India has been one of the earliest countries to begin geothermal projects way back in the 1970s, but at present there are no operational geothermal plants in India. The hot springs in the country are grouped into seven geothermal provinces, i.e., Himalayan, Sahara Valley, Cambay Basin, San-Narmada –Topi lineament belt, West Coast, Godavari Basin and Mahanadi Basin.
An experimental geothermal power plant of 5 kW capacity has been set up at Manikaran in HP. A cold storage plant has also been constructed there to utilise the geothermal energy at 900C for preserving vegetables and fruits grown in that area. Some of the prominent places where a power plant can be established based on geothermal energy are Puga Valley and Chhumathang in Jammu and Kashmir, Manikaran in HP, Jalgaon in Maharashtra, Tapovan in Uttarakhand, Bakreshwar in WB, Tuwa in Gujarat and Tattapani in Chhatisgarh. MNRE is giving thrust on exploration and harnessing of India’s geothermal energy resources.
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