Geothermal energy is the power obtained by the earth’s natural heat. It is the heat contained below the earth’s crust. Geothermal means earth’s heat. Boreholes are needed to reach this heat. Boreholes heat increases by one degree centigrade every thirty six metres. This heat is brought to the surface as steam or hot water, created when water flows through heated, permeable rock. This heat is used directly for space heating in homes and buildings or it is converted to electricity. Thermal regions are found in many parts of the world include Mexico, Japan, Iceland, Italy, New Zealand, and Turkey. These are indicated by geysers.

It is estimated that the temperature in the core of the earth is at least 6650 ^o C. Yet, the earth is cooling down very slowly: at the rate of 300 to 350 ^o C within three billion years. There is 42X10¹² W of heat in the earth, of which 2% is in the crust and 98% in the mantle and core. With today's technology, it would be impossible to access the energy that reside too deep within the earth, but the 840 000 000 000 W (2%) of geothermal energy that is accessible is sufficient for humans to use for a long period of time.
Areas around the plate margins make for the best geothermal energy extraction sites - that is because the crusts in those areas are much thinner.

The temperature of the earth's crust rises as the depth from the surface increases, all over the world. In some places the geothermal gradient or rate of increase in temperature, is higher than in others. These areas tend to be located in regions that are geologically active, where sections of the earth's crust are either colliding or moving apart. Due to this fact, the most promising geothermal resources are located in areas of volcanic activity. The higher the geothermal gradient, the less expensive it is to extract heat from the earth, due to drilling and pumping costs. In the ultimate case, the gradient may be so high that naturally occurring surface waters have been heated to a useful temperature. This is the case with hot springs and geysers.

Under the earth's crust, there is a layer of hot and molten rock called magma. Heat is continually produced there, mostly from the decay of naturally radioactive materials like uranium and potassium. The amount of heat within 10,000 meters (about 33,000 feet) of the surface contains 50,000 times more energy than all the oil and natural gas resources in the world.

The areas with highest underground temperatures are in regions with active or geologically young volcanoes. These "hot spots" occur at plate boundaries or at places where the crust is thin enough to let the heat through. The Pacific Rim, called the "ring of fire" for all of its volcanoes, has many hot spots, including some in Alaska, California, and Oregon. Nevada has hundreds of hot spots, covering most of the northern part of the state.

There are two main methods of utilizing geothermal energy: direct heat usage and electricity generation. Direct heat usage is the most commonly used form because it is the simplest. A reasonable comparison would be boiling eggs in hot springs. Direct heat usage is very commonly used in some high latitude places on plate boundaries like Iceland and Japan. People consume the energy by exploiting wells deep into the earth with water pipelines. The hot water from the pipelines is then used for many purposes like melting ice on the road, greenhouse warming, drying clothes, or maintaining the heating system in residential areas. Generating electricity with geothermal energy is similar to the direct use method. The only difference is that the temperature requirement is a lot higher (> 150 ^o C) so that the steam can push the turbine to produce electricity.

In California and Nevada, and in other places around the world, geothermal energy produces electricity in large power plants. Geothermal energy provides about 5 percent of California's electricity, and a third of El Salvador's. In Idaho and Iceland, geothermal heat is used to warm buildings and for other applications. In thousands of homes and buildings across the United States, geothermal heat pumps use the steady temperatures just underground to heat and cool buildings, cleanly and inexpensively.

Hot dry rock—This energy consists of dry, impermeable rock. To use this energy, water must be pumped into the rock at high pressures to widen existing fissures and create an underground reservoir of steam or hot water.

Magma—Magma is the molten or partially molten rock found below the Earth's crust. Magma reaches temperatures up to 1200°C (2192°F). While some magma bodies exist at accessible depths, a practical way to extract magma energy has yet to be developed.

Geopressured brines—These brines are hot, pressurized waters containing dissolved methane. Both the heat and methane can be used for power generation.

ADVANTAGES OF GEOTHERMAL ENERGY

Geothermal energy is localized and no fuel is required. This makes geothermal energy economical and eliminates the chance of damaging the environment during transportation, storage and usage. Also, the supply of geothermal energy is continuous and will not be fluctuated by any political or economical factors.

Geothermal energy offers an environmentally benign source of electricity. Geothermal power plants meet the most stringent environmental regulations and release little, if any, carbon dioxide, a greenhouse gas suspected of contributing to global warming.

Geothermal power plants are highly reliably and can operate 24 hours a day. Most power plants operate more than 95 percent of the time.

Geothermal energy offers a large source of secure, domestic energy. In addition, sales of geothermal technology enhance U.S. trade and stimulate the economy.

Geothermal technologies release little or no air emissions. Geothermal power production produces much lower air emissions than conventional energy technologies.

Tapping that heat is a relatively clean and sustainable way to reduce fossil fuel use.

DISADVANTAGES OF GEOTHERMAL ENERGY

The utilization of geothermal energy has yet to be perfected even though it has been there for more than a century. Many environmental impacts take place during the building of a geothermal plant.

At the beginning of each project, roads and working platforms have to be built so that exploratory and production equipment can be brought in. These changes to the environment can damage local plants and wildlife.

The main environmental concern with geothermal energy is the result of natural contaminants dissolved in the water or brine extracted from the ground. Silica, sulfates, sulfides, carbonates, silicates and halides present in geothermal fluids present problems for both equipment and the environment.

Hydrothermal and geopressurized water and brines are often very corrosive. This complicates the choice of materials for pipes, pumps and turbines.

Dissolved compounds also tend to precipitate out of solution when these fluids are flashed into steam, clogging up the system.

The drilling process damages the environment as the deep well will unavoidably pass through some underground water bodies, which the drilling fluid can contaminate.

The increased temperature of the area can kill life forms in the water. Moreover, the disposal of mud bored out with the drill is a potential environmental problem.

Problems can continue to occur when the plant is in operation. The geothermal fluid usually contains gases and dissolved substances that can pollute the environment. Some of them are carbon dioxide, hydrogen sulfide, methane, sodium chloride (salt), boron, arsenic and mercury.

If the plant is not maintained properly, hot wastewater can leak into surrounding areas and damage the ecosystem, giving rise to thermal pollution.