Geothermal Energy

1.            SCIENTIFIC AND TECHNICAL INFORMATION

1.1        Basic Principles of Geothermal Energy

The term geothermal comes from the Greek words "gea" meaning earth and "thermo" meaning hot. Geothermal energy thus stands for the natural heat of the earth, representing the inner warmth of rocks that form the solid crust of the Earth. The heat that originates from the earth’s interior persistently flows towards the surface through the mantle and crust, providing a virtually inexhaustible source of energy.

A small part of this heat flow is gained from the potential and kinetic energy of shrinking substances, which is the aftermath of gravity that was set into motion by the birth of our planet. The greater part - according to some theories - comes from the radioactive decay of elements as a result of the nuclear fission operating in the core of the Earth.

The heat generated by the Earth's core is conducted to the surrounding layer of rocks, i.e. to the mantle. When both temperature and pressure grow high enough, some mantle rocks melt and become magma. Then, because it is less dense than the surrounding rocks, the magma rises, moving slowly up towards the earth's crust, carrying the heat from below.

If hot magma happens to reach the surface, mostly as an accompanying phenomenon of volcanic eruptions, it is called lava. However usually magma stays below the earth's crust and heats the nearby rocks as well as the underlying waterbeds of rainwater that has seeped deep into the earth. Sometimes this hot water finding its way through faults and cracks reaches the earth's surface in the form of hot springs or geysers, yet its overwhelming part stays deep underground, trapped in cracks and porous rock. This natural collection of hot water is called a geothermal reservoir.

Today geothermal energy utilization exploits the heat that has been accumulated for thousands of years in the solid mass of rocks and geothermal reservoirs. In order to deploy the potential thermal energy of the Earth either regions with hotter heat flow than average must be explored, or we must apply a “mining” technology that extracts heat from rock faster than it emits it on its own.

The simplest way to exploit geothermal energy is the direct use of hot springs (e.g. in Iceland hot water circulates in the heating system of houses). Another possibility is the utilization of geo-pressured systems harnessing the internal pressure of the crust. In this case hot water, which is located in a sedimentary basin that has descended relatively quickly, can only be mined at a high temperature and under high pressure. In general, the fluid is excavated from the reservoir through one drilling and re-injected into it through another.

Although geothermal energy is present all around the world, its dimensions and therefore its potential for utilization greatly depend on local conditions. One reason for this is the rate of heat flux, which varies from region to region, as we move towards the crust.

Miners have long been aware that whenever a shaft or borehole is sunk, a temperature rise occurs. In deep mines this effect makes intensive ventilation necessary, at the same time setting a limit to which it is practical to drive galleries and shafts. Later on their observations were confirmed and given a scientific basis.

Indicators that characterise the regional and on-depth differences of the heat flow include:

At various points on the earth's surface, and particularly in volcanic regions, hot water or steam, and volcanic gases make their appearance. These thermal springs, geysers, fumaroles, mofettes etc. are evidence of the high temperatures prevailing in the deep-lying strata of the earth’s interior.

The continuous flow of heat from the earth's core through the mantle and crust is increased by radioactive decay in the crust. The average dissipation per second and per square centimetre is 6.7 μ Joule. This is the quantity of heat required to cause a 0.5-cm-thick layer of ice to melt in one year.

The temperature rise brought about by this flow of heat is 10,000 times smaller than that produced by solar irradiation, and amounts to 0.01 – 0.02 °C or 58 MW/km². Therefore direct utilization of all geothermal energy is out of the question. Only depths down to 5,000 m are likely to be technically exploitable.

Based on the temperature available, geothermal reservoirs are generally classified as being either low temperature (under 150°C) or high temperature (over 150°C) ), though other classifications stand as well. It is the high temperature reservoirs that are suitable for commercial production of electricity, while lower temperature fluids provide hot water for space-heating purposes, for greenhouses and industrial uses, or supply resort spas with hot or warm water.