Geothermal energy is a long-term source of energy worldwide. With the geothermal energy stored in the top three kilometers of the Earth's crust, theoretically, the current energy demand could be covered for more than 100,000 years. However, only a small part of this energy is technically usable and the effects on the earth's crust during extensive heat dissipation are not yet clear.
When geothermal energy is used, a distinction is made between direct use, that is, the use of heat in itself, and indirect use, the use for conversion into electricity in a geothermal power plant. With the help of restrictions, you can also get combined heat and electricity to optimize efficiency. In sparsely populated areas or on sites of power plants far from settlements with heat demand, it is difficult to carry out cogeneration processes.
Direct use of geothermal energy
The first applications can be found in the baths of the Roman Empire, the Middle Kingdom of the Chinese and the Ottomans for the construction of spas.
In Chaudes-Aigues, in the center of France, is the district's first historic geothermal heating network dating from the 14th century.
Today there are many uses for geothermal energy in industry, crafts and in residential buildings.
Heating and cooling with geothermal heat
For most applications, only relatively low temperatures are needed. From deep geothermal energy often the required temperatures can be directly available. If this is not enough, the temperature can be raised with heat pumps, as is often the case with geothermal energy near the surface.
In relation to heat pumps, geothermal energy is generally used to heat and cool buildings, as well as for the preparation of domestic hot water.
Another possibility is natural cooling, where the water with the flat surface temperature, that is, the average annual temperature of the site, is used directly to cool the building (without the use of a heat pump). This natural cooling has the potential to replace millions of electric air conditioners worldwide.
Another direct application is to avoid the freezing of bridges, roads or airports. Again, no heat pump is needed. This includes the laying of frost-free water pipes.
For the use of heat in hot springs of geothermal energy, deep water is used at temperatures between 40 and 150 ° C. Thermal water is usually brought to the surface from a depth of 1,000 to 4,500 meters through a well, and it transfers most of its thermal energy through a heat exchanger to a second circuit of the "secondary" heating network. Then it is cooled and pressed through a second hole with a pump on the ground, in the layer from which it was extracted.
Generation of electrical energy through geothermal energy
For the generation of electricity, geothermal energy was used for the first time in Larderello in Tuscany. In 1913, Count Piero Ginori Conti built a power plant there, in which steam turbines generated 220 kW of electric power. Today there are around 750 MW of installed electrical power. Under Tuscany, the magma is relatively close to the surface. This hot magma increases the temperature of the soil to such an extent that a use of geothermal energy is economically possible.
In the generation of hydrothermal energy, water temperatures of at least 80ºC are needed. Hydrothermal deposits of hot and dry steam with temperatures above 150 ° C can be used directly to drive a steam turbine.
For a long time, thermal water was used exclusively for the supply of heat in the construction area. The newly developed Organic Rankine Cycle (ORC) plants allow the use of temperatures above 80ºC for the generation of electrical energy. These work with an organic medium that evaporates at relatively low temperatures. This organic vapor drives the generator of energy through a turbine. The fluids used â € <â €
For systems in a smaller power range (<200 kW), it is also possible to design motor units such as Stirling motors.
The generation of electricity from deep geothermal energy is basic and controllable, in existing plants often more than 8,000 hours of operation are achieved per year.
Generation of electricity through high-notch deposits
Generation of electricity from geothermal energy is traditionally carried out in countries that have high enthalpy deposits, where temperatures of several hundred degrees Celsius are found at relatively low depths (<2000 m).
Depending on the pressure and temperature, the deposits may be dominated by water or steam. In modern production techniques, refrigerated liquids are re-injected, so that practically no negative environmental effects occur, such as the smell of sulfur compounds.
Generation of electricity through deposits Niederenthalpiel
In the Niederenthalpiel deposits, the maximum energy efficiency possible due to the low temperature extends between the supply and the return system lower than in the Hochenthalpiel deposits.
The optimal choice of working fluid (for example, the Kalina process with ammonia) attempts to make more efficient use of the distance between the flow and return temperatures. However, it must be taken into account that the safety requirements for the handling of ammonia may be different than when using various organic work equipment.
Own electricity consumption, in particular for the supply of circulating pumps in the thermal water cycle, in these plants can amount up to 25% of the amount of power produced.