"Geothermal Energy" means energy in the form of heat, inside the earth. As we move deeper and deeper down the earth's surface, the pressure increases. This leads to an increase in the temperature of gases trapped under the earth's crust. Also, as we move deeper, we come closer and closer to the earth's inner layers - the mantle and the core. These layers are at very high temperatures (The mantle is liquid, and the core solid). Both these factors point towards one thing - there is a big reservoir of energy under the earth's surface, waiting to be tapped.
Hot Springs are a very striking example of this pool of energy. Any water body consists of water trapped inside a depression of earth. If this depression is deep enough, or if the geothermally heated rocks under the earth's surface are somewhat closer to the crust, the water coming in contact with these rocks will be heated. This heat is what makes the hot springs hot.
Geothermal energy has both direct and indirect applications. Directly, it can be used for temperature control of buildings or providing hot water. Indirectly, it can be used to produce electricity.
Direct methods usually use a heat exchange system. Water (or any other fluid) is pumped deep enough under the earth's surface. When it gets heated up, it is recirculated back to the surface. Here, the water exchanges its heat with a fluid (which depends upon the user's demand). It can either be used to hear air (like an air conditioner) and heat up the house, or it can be used to heat water, which can be used for various domestic purposes.
The initial steps of Geothermal Electricity production are pretty much the same as that of the direct methods. fluid (liquid or gas) is pumped under the earth's surface. If it is a gas, it will undergo an increase in its pressure (due to the increase in temperature). If it is a liquid, it will undergo a phase change. Either way, the fluid that comes up to the surface has a high enthalpy, both in terms of its temperate and its kinetic energy. This fluid at high pressure can be used to drive turbines to produce electricity. At present, however, the electricity generation potential of Geothermal Energy is pretty low, and most of the applications utilize this useful form of energy as a means of domestic heating.
