Dual circuit geothermal thermal power plants. Encyclopedia of radio electronics and electrical engineering

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The double-circuit GeoTEP (Fig. 4.2) includes a steam generator 4, in which the thermal energy of the geothermal steam-water mixture is used to heat and evaporate the feed water of a traditional wet-steam steam turbine plant 6 with an electric generator 5. The geothermal water that has been used in the steam generator is pumped by a pump 3 into the return well 2. Dry cleaning Turbine plant feed water is carried out by conventional methods. The feed pump 8 returns the condensate from the condenser 7 to the steam generator.

In a double-circuit plant, there are no non-condensable gases in the steam circuit, so a deeper vacuum is provided in the condenser and the thermal efficiency of the plant increases compared to a single-circuit one. At the outlet of the steam generator, the remaining heat of geothermal water can, as in the case of a single-circuit Geothermal power plant, be used for heat supply needs.

Fig.4.2. Thermal scheme of a double-circuit GeoTPP

Gases, including hydrogen sulfide, are fed from the steam generator to the bubbling absorber and dissolved in the waste geothermal water, after which it is pumped into the disposal well. According to test data at the Ocean GeoTPP under construction (Kuril Islands), 93.97% of the initial hydrogen sulfide is dissolved in the bubbling absorber.

Temperature difference in the steam generator reduces the enthalpy of live steam of a double-circuit installation h₁ compared with a single-circuit, however, in general, the heat drop in the turbine increases due to a decrease in the enthalpy of the exhaust steam h₂. The thermodynamic calculation of the cycle is carried out as for a conventional steam turbine thermal power plant (see the section on solar steam turbine installations).

The flow rate of hot water from geothermal wells for an installation with a capacity of N, kW, is determined from the expression

, kg/s , (4.3)

where - temperature difference of geothermal water at the inlet and outlet of the steam generator, °C, - The efficiency of the steam generator. The total efficiency of modern double-circuit steam turbine GeoTEPs is 17.27%.

In deposits with a relatively low temperature of geothermal waters (100-200°C), double-circuit installations are used on low-boiling working fluids (freons, hydrocarbons). It is also economically justified to use such installations for utilizing the heat of separated water from single-loop GeoTPPs (instead of a heat exchanger in Fig. 4.1). In our country, for the first time in the world (in 1967), a power plant of this type based on freon R-12 with a capacity of 600 kW was built at the Paratunsky geothermal field (Kamchatka) under the scientific guidance of the Institute of Thermal Physics of the Siberian Branch of the USSR Academy of Sciences. The coolant temperature difference was 80...5^оC, cold water was supplied to the condenser from the river. Paratunka with an average annual temperature of 5^оS. Unfortunately, these works were not developed due to the former cheapness of fossil fuels.

At present, JSC "Kirovskiy Zavod" has developed a project and technical documentation for a double-circuit geothermal module with a capacity of 1,5 MW on freon R142v (backup coolant - isobutane). The power module will be fully manufactured at the factory and delivered by rail, construction and installation work and connection to the power grid will require minimal costs. It is expected that the factory cost for serial production of power modules will be reduced to about $800 per kilowatt of installed capacity.

Along with the GeoTPP running on a homogeneous low-boiling heat carrier, ENIN is developing a promising plant based on a mixed water-ammonia working fluid. The main advantage of such an installation is the possibility of its use in a wide temperature range of geothermal waters and steam-water mixture (from 90 to 220^оWITH). With a homogeneous working fluid, the deviation of the temperature at the outlet of the steam generator by 10...20^оC from the calculated one leads to a sharp decrease in the efficiency of the cycle - by 2.4 times. By changing the concentration of the components of the mixed heat carrier, it is possible to ensure acceptable performance of the installation at varying temperatures. The power of the ammonia-water turbine in this temperature range changes by less than 15%. In addition, such a turbine has the best weight and size parameters, and the water-ammonia mixture has better heat transfer characteristics, which makes it possible to reduce the metal consumption and cost of the steam generator and condenser compared to a power module based on a homogeneous heat carrier. Such power plants can be widely used for industrial waste heat recovery. They may have a strong demand in the international market for geothermal equipment.

The calculation of GeoTEU with low-boiling and mixed working fluids is carried out using tables of thermodynamic properties and h - s diagrams of the vapors of these liquids.

The possibility of using the thermal resources of the World Ocean, often mentioned in the literature, adjoins the problem of GeoTES. In tropical latitudes, the temperature of sea water on the surface is about 25^оC, at a depth of 500...1000 m - about 2...3^оC. Back in 1881, D'Arsonval expressed the idea of ​​using this temperature difference to produce electricity. The installation diagram for one of the projects for the implementation of this idea is shown in fig. 4.3.

Fig.4.3. Diagram of an ocean thermal power plant: 1 - pump for supplying warm surface water; 2 - low-boiling coolant steam generator; 3 - turbine; 4 - electric generator; 5 - capacitor; 6 - cold deep water supply pump; 7 - feed pump; 8 - ship platform

Pump 1 supplies warm surface water to steam generator 2, where the low-boiling coolant evaporates. Steam with a temperature of about 20°C is sent to the turbine 3, which drives the electric generator 4. The exhaust steam enters the condenser 5 and is condensed by cold deep water supplied by the circulation pump 6. The feed pump 7 returns the coolant to the steam generator.

When rising through the warm surface layers, deep water heats up to at least 7...8°C, respectively, the exhausted wet steam of the coolant will have a temperature of at least 12...13°C. As a result, the thermal efficiency of this cycle will be = 0,028, and for a real cycle - less than 2%. At the same time, the ocean CHP is characterized by high energy costs for its own needs, it will require very large consumption of warm and cold water, as well as a coolant, the energy consumption of pumps will exceed the energy generated by the unit. In the United States, attempts to implement such power plants near the Hawaiian Islands did not give a positive result.

Another ocean thermal power plant project - thermoelectric - involves using the Seebeck effect by placing thermoelectrode junctions in the surface and deep layers of the ocean. The ideal efficiency of such an installation, as for the Carnot cycle, is about 2%. Section 3.2 shows that the actual efficiency of thermal converters is an order of magnitude lower. Accordingly, for heat removal in the surface layers of ocean water and heat transfer in the deep layers, it would be necessary to construct heat exchange surfaces ("underwater sails") of a very large area. This is unrealistic for power plants of practically noticeable power. The low energy density is an obstacle to the use of ocean heat reserves.

Author: Labeish V.G.

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