English

Geothermal Energy is the heat from the Earth’s interior able to be converted to other forms of energy. The surface expression of this energy is manifested in volcanoes, fumaroles, steam geysers, hot springs, and boiling mud pools. Geothermal energy is based on the natural internal heat generation, the Earth’s surface heats at 82MW/m2. Comparing this figure with the daily global energy consumption for all types of energy (equivalent to 100 million barrels of oil/day) results that harvesting energy from proximately 10 km depth, all estimated energy needs for the next six million years would be solved, as the temperature within the Earth increases with 30 ˚C/km reported to the ambient temperature.

Collecting worldwide geographical data made possible the design of global heat-flow maps, indicating the highest heat zones, mainly along the active geological plate boundaries. It is pretty obvious that the geothermal energy is nourished mainly from the volcanic most active regions, even if the principle of the method can be put in place all over the world (regardless the forages, capture and conversion costs, drilling deep enough the temperature will increase with 30 ˚C/km from any point of this planet).

Geothermal Energy Historic

Geothermal Water has been used for at least 10.000 years for cooking, space heating or bathing from natural pools and hot springs. Ancient civilizations raised next these natural energy sources (Romans, Japanese, Persian, Icelanders, Central Europeans and Maori of New Zealand). Industrial Revolution coincides with the first acknowledged applications in industry using geothermal energy for extracting boric acid (early 1800s, in Italy) or developing the world’s first geothermal district heating system, which is still working in our days (in France) and a geothermal converter providing heating for up to 450 homes, since 1892 (in United States of America, at Warm Springs, Idaho)

Geothermal resources are used in two types of applications:

· Electric Power Production, using the steam that is provided by the boiling water which spins a number of electrical generators turbines. The first geothermal power station began operation in Italy, in the early 1900s. Since then similar facilities have been built in various countries, including Iceland, Japan, Mexico, New Zealand, Turkey, the Tibet Autonomous Region of China, and the United States. In most cases turbines are driven with steam separated from superheated water tapped from underground geothermal reservoirs and geysers.

· House Heating, using directly the heat of the geothermal water which is pumped into heating system of the houses. Since these first approaches, a wide variety of applications have been developed for geothermal energy. For example, public buildings, residential dwellings, and greenhouses in such areas as Reykjavík, are heated with water pumped from hot springs and geothermal wells. Hot water from such sources also is used for heating soil to increase crop production and for seasoning lumber.

Geothermal Resource Types

Convective hydrothermal resources

The Earth’s heat is lifted by convective circulation, of hot water/steam in a natural way; also it can also be obtained from some high-temperature hydrothermal resources with circulation of water at deep levels along fractures.

Vapour dominated

Has a temperature range of 240 ˚C, it produces steam from boiling saline waters at deep levels in low permeability rocks. There are a few resources of this type: The Geysers in California, Larderello in Italy and Matsukawa in Japan, used generally for producing electrical energy.

Hot-water dominated

Water dominated systems are produced by ground water; circulating to depth and ascending on permeable reservoirs have a temperature range of 20 to 350 ˚C. It appears an outflow zone in the centre of each convection cell, moving laterally from the centre and a down flow where it recharges. At surface they can manifest as hot springs, fumaroles, geysers, chemically altered rock or no manifestation at all.

Sedimentary basins

They have low thermal conductivity or high heat flow and produce resources with a higher temperature range of 20 to 150 ˚C, using a gradient > 30 ˚C/km. There are found in larger areas, like Madison Formation of North Dakota, South Dakota, Montana and the Pannonian Basin of central Europe where it has been intensively exploited in Hungary.

Geopressured resources

They are buried deeply in basin environments, found in permeable sedimentary rocks and heated in better geothermal gradient caused by the great depth that they are found, having a temperature range of 90 to 200 ˚C. The fluids are surrounded with impermeable rock and high pressure, so they became the target for drilling because they contain dissolved methane and they are easier to extract.

Radiogenic

They can be found where granitic intrusions are next to the surfaces, heating up the nearby groundwater of decomposing radioactive thorium, potassium and uranium.

Regarding the temperature range this is from about 30 to 150 ˚C, so the localized heating increases the geothermal gradient, providing hot water at a small drilling depth. This type of resource exists along the eastern United States, but having not being developed until present.

Hot rock resources

The rocks that stock heat are at nearly 10 km below the surface, so the energy as steam or hot water couldn’t be extracted in an economic way although they have a temperature range of 90 to 650 ˚C. Some experimental projects have artificially fractured the rocks with hydraulic pressure, and sent cold water to extract the heat from the rock, by circulating in a closed system.

Molten rock (magma)

It has the higher temperature range > 600 ˚C, the system includes a heat exchanger built on the surface of the lava flow. It has been used in Hawaii after the 1973 eruption, but it is shut down in present due to cooling of the rock, after a use of 10 years.

At the 10^th World Energy Conference, Harvestable Geothermal Resources had been evaluated at 2.1 • 10²¹ kJ, meaning that this resource, could cover the population’s needs for house heating and domestic water for a period of 8.33 • 10⁷ years.

The Geothermal Energy contained in continental rocks having a temperature of over 150 ˚C, situated no deeper than 3km below surface is about 4kJ from which 8 • 10¹⁹ kJ having a temperature of over 250 ˚C and could be used to produce electricity.

In order to transform the Earth’s heat into electricity a big role plays the Geothermal Power Plant.

Another use of Earth’s temperature is for storing the thermal energy. This can be used in order to heat up or cool down closed spaces or even houses.

Geothermal Power plant

The Geothermal Energy is so unique due to its many forms in comparison with other Renewable Energy sources. It can be used to produce domestic water, to heat up homes, to produce electricity, or even to keep buildings at constant temperature.

The best of Geothermal Energy is that being renewable (as long as we choose carefully the processes to transform the hot water into electricity and then take care of putting it back) this type of energy alone, could theoretically, sustain all people energy necessities, a distinctive characteristic that other type of renewable energy doesn’t have.

Advantages

High thermal performance;

Only one pipe buried in small depth;

Usual digging methods.

Disadvantages

High liquid volume on high length which needs raised antifreeze measures;

High price for pipes;

High installation costs;

Limited cooling capacity due to pressure loss.

Bibliography

• John W. Lund, Characteristics, Development and Utilization of Geothermal Resources, Geo-Heat Center, USA
• White, D.E., Buffler, L.J.P., and Truesdell, A.H Vapor-Dominated Hydrothermal Systems Compared with Hot-Water Systems, Economic Geology, 2001.
• www.episodes.org/backissues/33igc/21.pdf
• www.britannica.com/EBchecked/topic/229724/geology/14203/Geothermal-energy
• http://wapi.isu.edu/geo_pgt/Mod04_Earth/mod4.htm

Iconography

• Figure 1 / Figure 8 http://geophysics.ou.edu/geomechanics/notes/heatflow/global_heat_flow.htm
• Figure 3 http://hays.outcrop.org/images/keller3e/14_26.jpg
• Figure 6 http://rglsun1.geol.vt.edu/gif/RadiogenicModel.gif
• Figure 7 www.seas.upenn.edu/~electric/images/education/hotdryrock.jpg
• Figure 9 /Figure 10 http://greenzoneenergy.com/images/closed_loop_system_pondlake.gif

English