GEOTHERMAL ENERGY

From the Earth’s interior to the surface, there comes a heat flow with the medium intensity of 0,05 W/m2. As different parts of the Earth’s crust possess unequal thermal conductivities, the intensity of heat flow varies considerably.
Hyperthermal regions, with higher heat flow intensities, are situated on tectonic areas near the margins of continental platforms. In such areas, at depths of several kilometres, there are rocks with temperatures of up to 300...500OC.

The energy of the rocks is called petrogeothermal energy, and the energy of the groundwaters warmed by these rocks, hydrogeothermal energy. Getting by chance into caves, between two impermeable layers, geothermal water can be transformed into steam. A characteristic of the majority of geothermal waters is the increased content of dissolved salts. Geothermal steam contains gases and vapors, which, sometimes, can be dangerous for the environment.

Figures 5.1 and 5.2 present images with water and steam, which spring from the Earth’s interior.

Figure 5.1


Geothermal energy can be of two types:

  1. high-temperature geothermal energy (characteristic of volcanic areas); the adjacent water tables reaching hundreds of degrees, with a partial evaporation, which is used in a central power station. The access to the water table is difficult. Sometimes, the depth of drilling can exceed 10,000 m;
  2. low-temperature geothermal energy; it is accessible in any part of the globe. The temperature of the Earth’s crust increases 3°C per 100 m in depth. The difference of the temperature could be used in district heating by recirculating the fluid in heat pumps, but it cannot be used in the production of electrical energy too.

Figure 5.2

Geothermal Power Plants


There are several uses of geothermal energy:

  1. direct use (figure 5.3);
  2. indirect use at a high potential (figure 5.4);
  3. indirect use at a reduced potential (figure 5.5).

Figure 5.3

Figure 5.4

Notations for figures 5.3, 5.4 and 5.5:


  • BW-borehole water;
  • WW-warm water;
  • NW-natural water;
  • PW- potable water ;
  • TN-thermal networks;
  • 1-aquifer;
  • 2-borehole for collecting geothermal energy ;
  • 3-heat exchanger with mixed flow of liquids;
  • 4-heat pump, HP;
  • 5-heat pump evaporator;
  • 6-heat pump condenser;
  • 7-scraped-surface heat exchanger;
  • 8-return borehole of geothermal water.

Figure 5.5

Geothermal waters are extracted by driving wells up to the aquifer. Water can rise under its own pressure from the closed aquifer systems. If pressure is not sufficiently high, pumps are installed on wells.
Slightly mineralized water can be used to produce domestic warm water or for other purposes. It may also be used as potable water. The degree of mineralization of potable water is adjusted from the heat exchanger, and, the temperature, from the heat pump.

Geothermal Power Stations

The steam or superheated water rises through the extraction well under its own pressure.
The turbines of the geothermal power stations work by salty steam, which can be obtained in three ways:

  1. directly from the well;
  2. by the expansion of the superheated water;
  3. by the evaporation of lower pressure water or a liquid with lower boiling temperature.

The following figures present several variants of geothermal power stations, as follows:

  1. with saturated steam, figure 5.6 ;
  2. with superheated water, with an expansion stage and a district heating plant, figure 5.7;
  3. with superheated water, with two expansion stages and a district heating plant, figure 5.8 ;
  4. with steam generator and closed operating circuit, figure 5.9.

 

Figure 5.6

 

Figure 5.7

 

Figure 5.8

Figure 5.9

Notations for figures 5.6, 5.7, 5.8 si 5.9:

  • 1-extraction well;
  • 2-butterfly valve;
  • 3-steam separator;
  • 4-turbine;
  • 5-electric separator;
  • 6-condenser;
  • 7-condensate pump;
  • 8-network water heater;
  • 10-return borehole;
  • 11-feed pump;
  • 12-steam generator.


  • BW-borehole water;
  • WW-warm water;
  • NW-natural water;
  • TN-thermal networks;

The advantages of geothermal energy:

  1. no products obtained by combustion;
  2. efficient heating systems.

The disadvantages of geothermal energy:

  1. the increase of regional seismicity by reducing the pressure in soil when extracting water or steam;
  2. air pollution with various gases resulting from the exploitation of steam;
  3. water pollution with different residual substances of the extracted water.