Geothermal energy. Geothermal water extraction technique. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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Geothermal energy is obtained from heat sources with high temperatures, it has some features. One of them is that the coolant temperature is significantly lower than the temperature during fuel combustion. Despite the fact that the total reserves of geothermal energy are large, its thermodynamic quality is low. These sources have much in common with industrial heat emissions and ocean thermal energy. The strategy for using geothermal energy is briefly discussed below.

Combination of opportunities and needs

Geothermal sources are always associated with attempts to generate electricity as the most valuable product, while the best way to utilize thermal energy is to use a combined mode (electricity generation and heating). Of course, electricity can be fed into the power system and transmitted through it to consumers along with electricity generated by other sources. At the same time, it is not superfluous to mention that the demand for heat at temperatures up to 100°C is usually even higher than for electricity. Thus, the use of geothermal energy in the form of heat is equally important. Electricity generation is likely to be of interest if the coolant has a temperature of more than 300°C, and will not be if the latter is below 150°C.

Heat is not easily transferred over distances of more than 30 km, so it must be used close to the extraction site. In cold climate zones, the heating of dwellings and industrial buildings creates a significant need for heat if the population density is more than 300 people per 1 km2 (more than 100 homesteads per 1 km2). Thus, a thermal plant with a capacity of 100 MW can serve a residential area of ​​approximately 20x20 km with a heat consumption of about 2 kW per homestead. A similar geothermal system has long been used in Iceland and, to a lesser extent, in New Zealand. Other major heat consumers are greenhouses (up to 60 MW/km in one unit for Northern Europe), fish farms, food drying plants and other technologies.

The scale of the use of geothermal energy is determined by several factors. The dominant cost is the capital cost of well construction, the cost of which increases exponentially with increasing depth. Since temperature increases with depth, and energy production increases with temperature, in most cases, the optimal well depth is limited to about 5 km. As a consequence, the scale of power plants is usually chosen to be more than 100 MW (electric or thermal - for high temperatures, only thermal - for low temperatures).

The total amount of heat recovered from a geothermal well can be increased by re-injection of waste and partially cooled water. This is a convenient way to get rid of waste water, which can be highly mineralized (contain up to 25 kg / m3 of salts) and are dangerous environmental pollutants. However, this leads to an increase in the cost of stations.

Heat Extraction Technique

The most successfully implemented projects have wells drilled directly into natural underground reservoirs of geothermal areas (Fig. 1). This method is used in the Geysers (California) and in Wairakei (New Zealand), where there is significant pressure in the wells. Similar methods are used to extract energy from aquifers in high-thermal areas where natural pressure is sufficient to dispense with pumping systems.

Recent developments are focused on extracting heat from dry rocks, as they can provide greater productivity than water sources. The leading group of specialists (Los Alamos Scientific Laboratory, USA) developed methods for crushing rocks by hydraulic fracturing using cold water injected under pressure into the well (Fig. 1). After preliminary rock crushing, water is injected through a supply well, filtered through rocks at a depth of about 5 km at a temperature of 250°C, warm water returns to the surface through a receiving well. Two such wells can provide energy for a gigawatt plant.

Fig.6.1. Layout of hydrothermal stations in the hyperthermal region (Geysers geothermal field, California): 1 - natural geyser; 2 - power station; 3 - deep well (5 km); 4 - steam-water source (~280°C); 5 - mantle; 6 - hot rocks

Electricity and thermal energy generation systems. The selection of heat exchangers and turbines for conventional geothermal sources is a complex task requiring specialized expertise. Several options for possible schemes of GeoTPP are shown in fig. 6.2.

If low-temperature sources are used to generate electricity, then other working fluids (for example, freon, toluene) have to be used instead of water to drive the turbines. New types of technology need to be more efficient. Particular difficulties may arise with heat exchangers due to the high concentration of various chemicals in well water. The capital cost of constructing a GeoTPP currently ranges from $1500 to $2500. per kilowatt of installed electric power, which is comparable to those for nuclear power plants and thermal power plants.

The main consumers of geothermal resources in the near and distant future will undoubtedly be heat supply and, to a much lesser extent, electricity generation. Priority of heat supply in the balance of use of geothermal electricity.

Fig.6.2. Examples of the organization of the cycle for the production of electricity. Cycle with one working fluid, for example, with water or freon (a); cycle with two working fluids - water and freon (b); direct steam cycle (s) and double-circuit cycle (d); I - geothermal source; II - turbine cycle; III - cooling water

Geothermal technology for extracting thermal energy from the subsoil is a set of methods, means and processes for extracting, processing and delivering a heat carrier with a given quality and market level of economic efficiency of its use. The use of low-temperature geothermal energy of shallow depths can be considered as some technical and economic phenomenon or a real revolution in the heat supply system. In less than 10 years, a multi-variant technology has been developed in the United States and hundreds of thousands of operating heat supply systems have been built. At least 50-80 thousand new systems are put into operation every year. This technology is being successfully implemented in other countries of the world: Sweden, Switzerland, Canada, Austria, Germany, Russia. In 2002, there were about 450 such systems operating in the world with a total capacity of 2.9 GW (t), with an average of -10 kW (t).

Surface (shallow) geothermal systems are used to heat and cool various types of residential buildings (from individual to multi-apartment), gas stations, supermarkets, churches, educational institutions, etc.

The essence of the technologies under consideration, represented by near-surface systems (mining and energy installations) with heat exchange in wells and channels, is to create an underground heat exchanger, with a closed or open circuit, located at a shallow depth (50 - 300 m) and connected to a heat pump installed inside heated room (Fig. 6.3). At the same time, in the territory of Central Russia, rock temperatures in the range from 7 to 15°C can be used.

These systems extract not only geothermal energy stored in rocks or water, but also solar energy. The specific share of this or that energy used by the installation depends on the depth of the heat exchanger, the climatic and hydrogeological conditions of the area.

In Russia, there is a positive experience in the construction and operation of such geothermal installations. In particular, in the Yaroslavl region, a heat supply system for a large rural school has been built and is operating for the second year, three more units of this type are being designed and built.

a)

b)
Fig.6.3. Surface (shallow) geothermal system with heat exchange: a - horizontal channels; b - in wells

An assessment of geothermal technologies used in world practice shows that they can be used to provide a wide range of consumers of thermal energy: from an urban microdistrict to an individual house. On the basis of geothermal circulation systems (GCC), consisting of a doublet of deep (up to 1,5 - 2,5 km) wells, using heat pumps and peak reheating, high-temperature heating modes (90 ° C and above) are obtained with a heat output of up to several tens of MW. The technology of ground source heat pumps in wells 50 - 150 m corresponds to medium temperature and low temperature conditions, for commercial (shops, offices, etc.) and municipal (schools, hospitals, etc.) applications and housing and communal services, with a power of up to 0,1-0,4, XNUMX MW.

On fig. 6.4 shows schemes of heat supply with geothermal water.

Fig.6.4. Heat supply technologies with extraction of geothermal energy (click to enlarge): a) on the basis of GCC; b) on the basis of a deep heat exchanger; c) ground source heat pumps

The main criterion for evaluating the energy-saving, economic and environmental effects of geothermal installations with an electric heat pump is the coefficient of use of primary energy carriers (PIEC), which is determined by the product of efficiency. electricity generation (CPIe = 0,30 - 0,35) by the average, over the lifetime of the installation, the heat pump conversion factor (CHPTC). The range of SFTC that can be achieved using geothermal sources, from soil to reservoir brines, at temperatures from 5 - 7°C to 35 - 40°C, from 3 to 7 units and above. Thus, depending on the type of source, KIPI levels from 1,1 to 2,5 units can be obtained, which is 1,2 to 7,0 times higher than for traditional boilers (Fig. 6.5).

The efficiency of a geothermal plant with an electric HP is as much higher as compared to a traditional boiler plant, the greater the ratio of their KPIs. Hence, the savings in energy consumption and the reduction of harmful emissions: 20 - 70%.

Rising prices for imported fuel and transportation costs today predetermined the accelerated development of geothermal energy in Kamchatka, the Kuril Islands and in the northern regions of Russia.

On fig. 6.5 shows the coefficients for the use of primary energy carriers in traditional and geothermal boilers.

Fig.6.5. Primary Energy Resource Utilization Rate (PIEC) for conventional (t) and geothermal (d) boilers

Russia has many years of experience in researching geothermal fields, carrying out drilling operations on them and operating the GeoPP. For more than 30 years, the Pauzhetskaya GeoPP (south of Kamchatka) has been providing the cheapest electricity to the village of Ozernaya, where the main production of red caviar is concentrated. Back in 1967, Russia was the first country in the world to create a GeoPP with a binary cycle using low-grade heat (hot water - 95°C) at the Paratunsky geothermal field in Kamchatka.

Author: Magomedov A.M.

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