1. INTRODUCTION:

Geothermal energy is primarily energy from the earth’s own interior. It is classified as renewable energy because the earth’s interior is and continue in the process of heating for the indefinite future. The United States generates an average of 15 billion kilowatt hours of power per year, comparable to burning close to 25 million barrels of oil or 6 million short tons of coal per year. Among the other renewable energy, Geothermal has a higher capacity factor. As compared to solar and wind resources, which are mainly dependent upon climate changes and weather fluctuations, geothermal resources are available 24 hours a day. The carrier medium for geothermal electricity (water) must be properly managed to use maximum amount of the Earth’s heat available indefinitely. The interior of the Earth is expected to remain extremely hot for billions of year to come, ensuring an essentially limitless flow of heat.

Geothermal power plants capture this available heat and convert it to energy in the form of electricity. As depth into the Earth’s crust increases, temperature increases as well. In this system the heat energy continuously flows to the Earth’s surface from its interior, where central temperatures of about 6000°C exist. The predominant source of the Earth’s heat is the gradual decay of long-lived radioactive isotopes (40K, 232Th, 235U and 238U)[1]. The outward transfer of heat occurs by means of conductive heat flow and convective flows of molten mantle beneath the Earth’s crust. This results in a mean heat flux at the Earth’s surface of 80kW/km2 approximately. This heat flux, however, is not distributed uniformly over the Earth’s surface; rather, it is concentrated along active tectonic plate boundaries where volcanic activity transports high temperature molten material to the near surface. Although volcanoes erupt small portions of this molten rock that feeds them, the vast majority of it remains at depths of 5 to 20 km, where it is in the form of liquid or solidifying magma bodies that release heat to surrounding rock. Under the right conditions, water can penetrate into these hot rock zones, resulting in the formation of high temperature geothermal systems containing hot water, water and steam, or steam, at depths of 500 m to >3,000 m. Geothermal energy is an enormous, underused heat and power resource that is clean (emits little or no greenhouse gases), reliable (average system availability of 95%), and homegrown (making us less dependent on foreign oil). Geothermal resources range from shallow ground to hot water and rock several miles below the Earth's surface, and even farther down to the extremely hot molten rock called magma. Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and very hot water that can be brought to the surface for use in a variety of applications. Geothermal energy can be used very effectively in both on- and off-grid developments, and is especially useful in rural electrification schemes. Its use spans a large range from power generation to direct heat uses, the latter possible using both low temperature resources and “cascade” methods.[2] Cascade methods utilize the hot water remaining from higher temperature applications (e.g., electricity generation) in successively lower temperature processes, which may include binary systems to generate further power and direct heat uses (bathing and swimming; space heating, including district heating; greenhouse and open ground heating; industrial process heat; aquaculture pond and raceway heating; agricultural drying; etc.)

This paper is organized as follows. The Geothermal energy scenario is discussed in Section 2. In section 3, classification of plant, cost and challenges are discussed in Section 4. Environmental effects are discussed in Section 5 and concluded in Section 6.

2. GEOTHERMAL ENERGY SCENARIO:

As per the report publish by 2010, Geothermal power plants operated in at least 24 countries, and geothermal energy was used directly for heat in at least 78 countries. These countries currently have geothermal power plants with a total capacity of 10.7 GW, but 88% of it is generated in just seven countries: the United States, the Philippines, Indonesia, Mexico, Italy, New Zealand, and Iceland. The most significant capacity increases since 2004 were seen in Iceland and Turkey. Both countries doubled their capacity. Iceland has the largest share of geothermal power contributing to electricity supply (25%), followed by the Philippines (18%).[3] The number of countries utilizing geothermal energy to generate electricity has more than doubled since 1975, increasing from 10 in 1975 to 24 in 2004. In 2003, total geothermal energy supply was 20 MToE (metric Tonne Oil Equivalent), accounting for 0.4% of total primary energy supply in IEA member countries. The share of geothermal in total renewable energy supply was 7.1%. Over the last 20 years, capital costs for geothermal power systems decreased by a significant 50%. Such large cost reductions are often the result of solving the “easier” problems associated with science and technology improvement in the early years of development. Although geothermal power development slowed in 2010, with global capacity reaching just over 11 GW, a significant acceleration in the rate of deployment is expected as advanced technologies allow for development in new countries. Heat output from geothermal sources increased by an average rate of almost 9% annually over the past decade, due mainly to rapid growth in the use of ground-source heat pumps. Use of geothermal energy for combined heat and power is also on the rise. India has reasonably good potential for geothermal; the potential geothermal provinces can produce 10,600 MW of power (but experts are confident only to the extent of 100 MW). But yet geothermal power projects has not been exploited at all, owing to a variety of reasons, the chief being the availability of plentiful coal at cheap costs.[4] However, with increasing environmental problems with coal based projects, India will need to start depending on clean and eco-friendly energy sources in future; one of which could be geothermal. It has been estimated from geological, geochemical, shallow geophysical and shallow drilling data it is estimated that India has about 10,000 MWe of geothermal power potential that can be harnessed for various purposes. Rocks covered on the surface of India ranging in age from more than 4500 million years to the present day and distributed in different geographical units. The rocks comprise of Archean, Proterozoic, the marine and continental Palaeozoic, Mesozoic, Teritary, Quaternary etc., More than 300 hot spring locations have been identified by Geological survey of India (Thussu, 2000). The surface temperature of the hot springs ranges from 35 C to as much as 98 C. These hot springs have been grouped together and termed as different geothermal provinces based on their occurrence in specific geotectonic regions, geological and strutural regions such as occurrence in orogenic belt regions, structural grabens, deep fault zones, active volcanic regions etc., Different orogenic regions are – Himalayan geothermal province, Naga-Lushai geothermal province, Andaman-Nicobar Islands geothermal province and non-orogenic regions are – Cambay graben, Son-Narmada-Tapi graben, west coast, Damodar valley, Mahanadi valley, Godavari valley etc.[5]

3. CLASSIFICATION OF PLANT:

In general four types of power plants are operating today:

Flashed steam plant

The force of high pressure steam (termed as flashed steam) released from drill holes is used to rotate turbines. The steam after striking the turbines passes through condenser gets condensed and is converted into water again, which is returned to the reservoir. Flashed steam plants are widely distributed throughout the world.

Fig. 1 Flashed steam plant

Dry steam plant

Generally geysers are the main source of dry steam. Those geothermal reservoirs which mostly produce steam and little water are used in electricity production systems. As steam from the reservoir shoots out, it is used to rotate a turbine, after sending the steam through a rock-catcher. The rock-catcher protects the turbine from rocks which come along with the steam.

Fig. 2 Dry steam power plant

Binary power plant

In this type of power plant, the geothermal water is passed through a heat exchanger where its heat is transferred to a secondary liquid, namely isobutene, iso-pentane or ammonia–water mixture present in an adjacent, separate pipe. Due to this double-liquid heat exchanger system, it is called a binary power plant. The secondary liquid which is also called as working fluid should have lower boiling point than water. It turns into vapor on getting required heat from the hot water. The vapor from the working fluid is used to rotate turbines. The binary system is therefore useful in geothermal reservoirs which are relatively low in temperature gradient. Since the system is a completely closed one, there is minimum chance of heat loss. Hot water is immediately recycled back into the reservoir. The working fluid is also condensed back to the liquid and used over and over again.

Fig. 3 Binary power plant

Hybrid power plant

Some geothermal fields produce boiling water as well as steam, which are also used in power generation. In this system of power generation, the flashed and binary systems are combined to make use of both steam and hot water. Efficiency of hybrid power plants is however less than that of the dry steam plants.

Fig. 4 Hybrid power plant

Enhanced geothermal system

The term enhanced geothermal systems (EGS), also known as engineered geothermal systems (formerly hot dry rock geothermal), refers to a variety of engineering techniques used to artificially create hydrothermal resources (underground steam and hot water) that can be used to generate electricity. Traditional geothermal plants exploit naturally occurring hydrothermal reservoirs and are limited by the size and location of such natural reservoirs. EGS reduces these constraints by allowing for the creation of hydrothermal reservoirs in deep, hot but naturally dry geological formations.EGS techniques can also extend the lifespan of naturally occurring hydrothermal resources. Given the costs and limited full-scale system research to date, EGS remains in its infancy, with only a few research and pilot projects existing around the world and no commercial-scale EGS plants to date. The technology is so promising, however, that a number of studies have found that EGS could quickly become widespread.

As with time resourses of petroleum, coal etc are decreasing. So due this Geothermal energy is getting attention. People are awaring about the advantages and cost of enrgy production. At present there is no operational geothermal plants in India.[6]

4. COST AND CHALLENGES:

Unlike traditional power plants that run on fuel that must be purchased over the life of the plant, geothermal power plants use a renewable resource that is not susceptible to price fluctuations. New geothermal plants currently are generating electricity from 0.05$ to 0.08$ per kilowatt hour (kwh).Once capital costs .Once the capital costs have been recovered price of power can decrease below 0.05$ per kwh. The price of geothermal is within range of other electricity choices available today when the costs of the lifetime of the plant are considered. Most of the costs related to geothermal power plants are related to resource exploration and plant construction. Like oil and gas exploration, it is expensive and because only one in five wells yield a reservoir suitable for development .Geothermal developers must prove that they have reliable resource before they can secure millions of dollar required to develop geothermal resources.

5. ENVIRONMENTAL EFFECTS:

Noise Pollution: Normal geothermal power plant operation typically produces less noise than the equivalent produced by other non renewable sources and thus is not considered an issue of concern.

Water Use: Geothermal plants use 5 gallons of freshwater per megawatt hour, while binary air-cooled plants use no fresh water.

Water Quality: To prevent cross-contamination of brines with groundwater systems, geothermal system fluids used for electricity are injected back into geothermal reservoirs using wells with thick casing instead t0 release into surface waterways.

Land Use: Geothermal power plants uses 403 square meters of land per gigawatt hour as compared to a coal facility that uses 3630 square meters per gigawatt hour.

6. CONCLUSION:

From the above review it is clear that Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect. The power stations do not take up much room, so there is not much impact on the environment. No fuel is needed. Once you've built a geothermal power station, the energy is almost free. It may need a little energy to run a pump, but this can be taken from the energy being generated. The big problem is that there are not many places where you can build a geothermal power station. You need hot rocks of a suitable type, at a depth where we can drill down to them. The type of rock above is also important, it must be of a type that we can easily drill through. Harmful gases can escape from deep within the earth, through the holes drilled by the constructors. The plant must be able to contain any leaked gases, but disposing of the gas can be very tricky to do safely.

7. REFERENCES:

[1] Chandrasekharam, D. 2000a. Geothermal Energy Resources of India- Facts. Proced. Geothermal Power Asia 2000 Conference, Manila, Feb. 2000, 12-19

[2] Chandrasekharam, D. 2000b. Geothermal Energy Resources of India- Country Update. Proced. World Geothermal Congress 2000, May 29-June 11, 2000, Japan, 365-376

[3] Ravi Shanker, 1996. Development of geothermal energy resources in India: Possibilities and constraints. Geol. Surv. India Sp. Pub., 45, 1-5

[4] Giggenbach, W.F. 1991. Chemical techniques in geothermal exploration. In Applications of geochemistry in geothermal reservoir development, (Ed) D'Amore, United Nations Institute for Training and Research, USA, Pub., 1991, 119-144

[5] Minissale, A., Vaselli, O., Chandrasekharam, D., Magro, G., Tassi, F. and Casiglia, A. 2000. Origin and evolution of 'intracratonic' thermal fluids from central-western peninsular India. Earth. Planet. Sci. Lett., 181, 377-398

[6] World Geothermal Congress, 2000. Proceedings of the International World Geothermal Congress, Beppu-Morioka, Japan. May 28-June 11, 2000.

Received on 24.10.2016 Accepted on 25.10.2016

Research J. Engineering and Tech. 2016; 7(4): 175-178.

DOI: 10.5958/2321-581X.2016.00030.1