04-08-2012, 02:39 PM
Geothermal energy
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Introduction
Geothermal energy is the earth’s natural heat available inside the earth. This thermal energy contained in the rock and fluid that filled up fractures and pores in the earth’s crust can profitably be used for various purposes. Heat from the Earth, or geothermal — Geo (Earth) + thermal (heat) — energy can be and is accessed by drilling water or steam wells in a process similar to drilling for oil. 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.
ADVANTAGES OF GEOTHERMAL ENERGY
1.1.1. Geothermal energy is a domestic energy resource with cost, reliability and environmental advantages over conventional energy sources. It contributes both to energy supply, with electrical power generation and direct-heat uses.
1.1.2. For generation of electricity, hot water is brought from the underground reservoir to the surface through production wells, and is flashed to steam in special vessels by release of pressure. The steam is separated from the liquid and fed to a turbine engine, which turns a generator. Spent geothermal fluid is injected back into peripheral parts of the reservoir to help maintain reservoir pressure. In the absence of steam, heat from hot water is extracted through a secondary fluid and the high pressure vapour from the secondary fluid is utilized to run the turbine.
1.1.3. If the reservoir is to be used for direct-heat application, the geothermal water is usually fed to a heat exchanger and the heat thus extracted is used for home heating, greenhouse, vegetable drying and a wide variety of other small scale industries. Hot water at temperatures less than 120 o C can be used for this purpose. Further, the spent hot water, after generating electricity can also be used for direct application.
1.1.4. As a result of today's geothermal production, consumption of exhaustible fossil fuels is offset, along with the release of acid-rain and greenhouse gases that are caused by fossil-fuel use. Systems for use of geothermal energy have proven to be extremely reliable and flexible. Geothermal electric power plants are on line 97% of the time, whereas nuclear plants average only 65% and coal plants only 75% on-line time. Geothermal plants are modular, and can be installed in increments as needed. Because they are modular, then can be transported conveniently to any site. Both baseline and peaking power can be generated. Construction time can be as little as 6 months for plants in the range 0.5 to 10 MW and as little as 2 years for clusters of plants totaling 250 MW or more.
The competing goals of increased energy production for worldwide social development and of mitigating release of atmosphere-polluting gases are not compatible using today's fuel mix, which relies heavily on coal and petroleum. Development of geothermal energy has a large net positive impact on the environment compared with development of conventional energy sources. Geothermal power plants have sulphur-emissions rates that average only a few percent of those from fossil-fuel alternatives. The newest generation of geothermal power plants emits only ~135 gm of carbon (as carbon dioxide) per megawatt-hour (MW-hr) of electricity generated. This figure compares with 128 kg /MW-hr of carbon for a plant operating on natural gas (methane) and 225 kg/MW-hr of carbon for a plant using bituminous coal. Nitrogen oxide emissions are much lower in geothermal power plants than in fossil power plants. Nitrogen-oxides combine with hydrocarbon vapours in the atmosphere to produce ground-level ozone, a gas that causes adverse health effects and crop losses as well as smog. There are other environmental advantages to geothermal energy. Geothermal power plants require very little land, taking up only a fraction of that needed by other energy sources. Thus emission of CO2 and SO2 by geothermal power plants is far less compared with conventional fossil fuel based power plants
. CLASSIFICATION OF GEOTHERMAL RESOURCES
The Geothermal Resources can be classified into three categories, viz
a) hydrothermal ,
b) geopressure
c) Hot dry rocks.
Hydrothermal resources
Hydrothermal resources are those that are associated with natural convection systems. Their thermal reservoirs consist of porous or fractured rocks, containing hot water or steam, which in the natural state, transported towards the surface by density-driven thermo-artesian flow. All the currently identified geothermal resources are hydrothermal in origin. Hydrothermal system may be further divided into liquid-dominated or vapour-dominated, depending on the ratio of water to steam in the reservoir. Liquid dominated hydrothermal systems may be of low, moderate or high-temperature type and are the most common kind of geothermal system being exploited commercially today. Vapour dominated reservoir is much less common but includes Larderello in Italy and Geyser in USA.