The word geothermal comes from the Greek words geo (earth) and therme (heat), and means the heat of the earth.
It is related to the thermal energy of Earth’s interior. On a large scale, the intensity of this thermal energy increases with depth, that is, the temperature of the Earth increases as we travel closer to its centre. Earth contains an incredibly vast amount of thermal energy.
The geothermal gradient is the rate of change of temperature (ΔT) with depth (ΔZ), in the earth. Units of measurement are °C/km (SI). In the geosciences, the measurement of T is strongly associated with heat flow, Q, by the simple relation: Q=KΔT/ΔZ, where K is the thermal conductivity of the rock.
Temperatures at the surface of the earth are controlled by the Sun and the atmosphere, except for areas such as hot springs and lava flows. From shallow depths to about 61 m below the surface, the temperature is constant at about 11°C. In a zone between the near surface and about 122 m, the gradient is variable because it is affected by atmospheric changes and circulating groundwater. Below that zone, temperature almost always increases with depth. However, the rate of increase with depth (geothermal gradient) varies considerably with both tectonic setting and the thermal properties of the rock.
High gradients (up to 200°C/km) are observed along the oceanic spreading centers and along island arcs. The high rates are due to molten volcanic rock (magma) rising to the surface. Low gradients are observed in tectonic subduction zones because of thrusting of cold, water-filled sediments beneath an existing crust. The tectonically stable shield areas and sedimentary basins have average gradients that typically vary from 15–30°C/km.
Geothermal energy has two primary sources, primordial heat, and radioactive decay. Primordial heat is what resulted from the creation of Earth 4.5 billion years ago, when the energy and mass from colliding cosmic matter made Earth a large, hot piece of space debris. As Earth’s outside cooled, it then acted as an insulator for the heat in the middle, which is why Earth is still cool and hospitable on the outside, and hot-rock and metal at its core.
In some regions where the earth's crust is thin or fractured, or where magma bodies are close to the surface, there are high temperature gradients. Deep faults, rock fractures and pores allow groundwater to percolate towards the heat source and become heated to high temperatures. Some of this hot geothermal water travels back to the surface through buoyancy effects to appear as hot springs, mud pools, geysers, or fumaroles. If the ascending hot water meets an extensively fractured or permeable rock zone, the heated water will fill pores and fractures and form a geothermal reservoir. These reservoirs are much hotter than surface hot springs, reaching temperatures of more than 350°C, and are potentially an accessible source of energy.
Geothermal resources can be classified into three categories:
- High temperature, usually magmatic-related resources. These have temperatures of 200- 350°C at economically-drillable depth. They are of limited occurrence, and form individual convective geothermal systems of up to 50 sq km in area. Technologies may be developed in the future to exploit even deeper, hotter resources.
- Moderate to low temperature resources, of non-magmatic origin, usually associated with deep faults. Maximum temperatures at drillable depth do not exceed 140°C, and are often less. These are more widespread than the high temperature resources, but the individual systems are no larger. The distinction between these and the first kind is not entirely clear-cut, as cooled outflows from hotter resources can also fall into the same temperature range.
- Very low temperature resources, which are widespread but close to ambient temperature.
- Geothermal energy can be used for electricity production, for commercial, industrial, and residential direct heating purposes, and for efficient home heating and cooling through geothermal heat pumps.
Geothermal in Georgia
Georgia has a high potential of geothermal resources, some have been in use since ancient times. The major areas of utilization are balneology resorts, local heating systems, processing industry and greenhouse. Searching and boring researches carried out in the ‘70s of the last century and conducted up-to-date, revealed that Georgia abounds in geothermal resources, concentrated in 44 deposits (thermal waters of Georgia). According to preliminary estimations, their heat power is 420 megawatts, and elaboration of thermal energy is maximum 2.7 million megawatt/hour/year. However, the most of the existing 50 geothermal wells in Georgia are of medium depth and supply water at temperatures ranging between 40 to 60 °C. It also should be noted that most of these wells are non-operational. None of the wells are used for power generation.
Owing to the high geothermal potential in the South Caucasus and particularly in Georgia, a confirmed total reserves of 90.000 m3/day, corresponding to a heat potential of 500,000 tones of equivalent fuel annually, has been recorded. The amount of thermal flow for the main parts of Georgia can be listed as follows: 1) The South flank of Caucasus Mountains - 100 mWm-2; 2) Plate of Georgia; a) The West zone 40 mWm-2 b) The East zone 30mWm-2; 3) Adjara-Trialeti folded system a) Central part 90 mWm-2 b) The East zone 50 mWm-2; 4) Artvin- Bolnisi platform 60 mWm -2. The main geothermal fields in Western Georgia where the reservoir formations are fractured karstic limestones of the Upper Cretaceous in the sedimentary trough and at the South-East where the reservoir formations are volcanic and sandstones of Paleocene-Middle Eocene in the fold system. Thus we see the following pattern in the distribution of heat flow: the maximum heat flow is observed for the central zone of folded part of Georgia and the minimum for the plate. As to heat flow for Adjara-trialeti folded system, is characterized by the middle range. The temperature condition of Paleocene - middle Eocene thermal water bearing complex is better investigated for Tbilisi region. This investigation revealed that temperature condition of this complex is influenced by depth of lay of high thermal resistivity upper Eocene rocks as well as their thickness. From the surface of Volcanic-sediment formation of middle Eocene the temperature of rocks increases to all direction of dipping from 20 0 C till 1000C. To the North-East the increase of temperature is less then to other direction because of nearness of the plate. On the contact of Cretaceous - Eocene temperature has remarkable variation: to the farthest West, where upper Cretaceous is raised till 500 m. we have temperature variation from 100 till 160 0C, when to the North and East, where the Cretaceous dips till 600 m we have temperature about 240 0C.