Geothermal energy is the heat, which the Earth consists of within itself. This name compounds from the Greek words "gea"-Earth and "termos" - heat. Geothermal energy observes, like fosil, nuclear energy. Total heat inside the Earth amounts to about 4 1030 J or 1020 toe (ton of equivalent of naphtha).Global, geothermal sources represent cumulative heat energy inside the Earth. Mostly, heat energy is transferring through heat convection.
The springs of hot waters, geysers, fumaroles, occurrences of hot mud and other mineral occurrences are witnesses of the inside heat of the Earth. All these appearances are closely linked to a heat transfer between rock masses and underground waters. These waters often bring with themselves solute minerals and gasses. Heat flow, which is generated inside the Earth, in the route of her surface, is continual over the planet, but in some places, the heat flow is bigger so, in that way, it makes hot waters and other occurrences. Zones of high flow often occur on the brinks of the earth tectonic sheet and they are linked to a volcanic activity. All underground waters, which are in contact with heated rocks, warm and their boiling point raise above normal, because of the pressure in the depth; so, in that way they come to their overheating. Hot waters tend to move towards the Earth surface, with a pressure reduction. These hot waters occur on the surface as geysers and hot springs.
Sources of geothermal energy can be divided in four groups:
Hydrogeothermal energy of hot water sources
Hydrogeothermal energy of steam sources
Hydrogeothermal energy of hot waters in a large depth
Petrothermic energy - energy of hot and dry rocks
Heat energy accumulated in underground waters is called hydrogeothermal energy. Geothermal energy is a recovering type of energy and it is all around us in a different measure. Thereby, geothermal energy is ecologically the most valid energy and from an economic point of view, it is the most favorable energy, because it is free.
Water from steam and hot water occurrences arrive in deeper layers through permeable rocks. It accumulates the heat of hot rocks and heat from deeper depth and reaches temperature of 400 0C. If water comes to the earth surface through the borehole system or natural water outflow, it is in a shape of hot or boiling water (fumaroles) or in a shape of steam (geysers).
There are earth layers, which are like impermeable bad, so waters from the surface cannot arrive to them. With a deeper depth, porosity is smaller and temperatures are bigger, so huge volumes of energy accumulated in dry rocks exist in the depth of the Earth.
Water springs in a large depth are water which came from the earth surface in a previous geological periods and was captured beneath impermeable bad. It is under the pressure, which is confirmed by the rock mass above. In Mexico, gulfs are water occurrences with a pressure of 760 bars and with solute gas CH4. Until now, technically, it neither solves the usage of energy accumulated in dry rocks, nor does the energy accumulated in waters in large depth.
View of the deep of drilling with today’s technology, which are limited to 10 km, only geothermal energy of rocks until these depths can be proper for using. Total energy of these rocks is about 3.1016 toe and 1/4 is in the rocks under the continents. That energy is about 60 000 times bigger then energy of total coal reserves. If we take larger depth, that energy is multiply bigger.