• Geothermal energy is heat within the earth. The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is a renewable energy source because heat is continuously produced inside the earth. People use geothermal heat for bathing, to heat buildings, and to generate electricity. 
• The slow decay of radioactive particles in the earth’s core, a process that happens in all rocks, produces geothermal energy.

How is it used? 

• It has been used in some countries for thousands of years for cooking and in heating systems. The underground geothermal reservoirs of steam and heated water can be used for electricity generation and other heating and cooling applications. 
• One example of heating and cooling is where a geothermal heat pump is installed around 10 feet underground. These pipes are filled with water or an antifreeze solution. The water is pumped around the closed loop of pipes. These ground source heat pump systems help to cool buildings in summer and maintain warmth in summer. This occurs by absorbing the earth’s heat as the water circulates back into the building. 
• Geothermal water has been used to help grow plants in greenhouses, for district heating in homes and businesses. It can also be piped under roads to melt snow.

How is Geothermal Energy Produced? 

• Wells of up to a mile deep or more are drilled into underground reservoirs to tap into the geothermal resources These resources can be exploited from naturally occurring heat, rock and water permeability or through enhanced geothermal systems, which enhance or create geothermal resources through a process called hydraulic stimulation. These geothermal resources, whether natural or enhanced, drive turbines linked to electricity generators. 
• The first recorded instance of geothermal heat being used for producing electricity was in Larderello, Italy in 1904. Yet, geothermal heat has been used for bathing since the Palaeolithic Age. Monkeys in Japan have also been shown to use heated water from hot springs to keep warm during winter months in mountainous regions.

How Does Geothermal Energy Work? 

• Geothermal power plants come in three different designs; dry steam, flash and binary:
• The oldest type is dry steam, which takes steam directly from fractures in the ground to drive a turbine.
• Flash plants pull high pressure hot water from underground and mix it with cooler low pressure water. This, in turn, creates steam that is used to drive a turbine. 
• Binary plants use hot water passed through a secondary fluid that has a lower boiling point than water. The secondary fluid is turned into vapour which drives a turbine. Most future geothermal power plants are expected to be binary plants.

Advantages 

• As a source of renewable energy, the main advantages of geothermal energy are environmental. It produces just onesixth of the carbon dioxide emitted by a clean natural-gas power plant.
• Geothermal is also cheaper than conventional energy, with savings of as much as 80% compared with fossil fuels. 
• Unlike other renewable energy sources, such as solar and wind, it is constantly available.

Disadvantages 

• Despite being inexpensive, sustainable and environmentally-friendly, geothermal is not without its drawbacks.
• Firstly, production is limited to areas near tectonic plate boundaries. In addition, some locations may cool down after decades of use. 
• Although it is cheaper than fossil fuels once a plant has been built, the drilling and exploration of these sites is expensive. This is in part due to the amount of wear experienced by drills and other tools in such aggressive environments. 
• Geothermal plants can release hydrogen sulphide, a gas that smells like rotten eggs. Finally, some geothermal fluids contain low levels of toxic materials which need to be disposed of.

The thermal springs of India have been classified as follows: