Calculation of the thermal scheme of a geothermal power plant of binary type. Geothermal energy: technologies and equipment. Literature for self-study
Purpose of the lecture: show the possibilities and ways of using geothermal heat in power supply systems.
Heat in the form of hot springs and geysers can be used to generate electricity according to various schemes at geothermal power plants (GeoPPs). The most easily implemented scheme is a scheme using a pair of liquids with a low boiling point. Hot water from natural sources, heating such a liquid in the evaporator, turns it into steam, which is used in a turbine and serves as a drive for a current generator.
Figure 1 shows a cycle with one working fluid, for example, with water or freon ( but); cycle with two working fluids - water and freon ( b); direct steam cycle ( in) and a two-loop cycle ( G).
Technologies for the production of electrical energy largely depend on the thermal potential of thermal waters.
Picture. 1 - Examples of cycle organization for electricity generation:
I - geothermal source; II - turbine cycle; III - cooling water
High-potential deposits allow the use of practically traditional designs of thermal power plants with steam turbines.
Table 1 -Specifications geothermal power plants
Figure 2 shows the most simple circuit a small power plant (GeoES) using the heat of a hot underground source.
Water from a hot spring with a temperature of about 95 ° C is pumped by pump 2 to gas remover 3, where the gases dissolved in it are separated.
Next, the water enters the evaporator 4, in which it is converted into saturated steam and slightly overheated due to the heat of the steam (from the auxiliary boiler), which has previously been exhausted in the condenser ejector.
Slightly superheated steam does work in turbine 5, on the shaft of which there is a current generator. The exhaust steam condenses in condenser 6, which is cooled by water at normal temperature.
Figure 2-. Scheme of a small GeoPP:
1 - receiver hot water; 2 - hot water pump; 3 - gas remover;
4 - evaporator; 5 - steam turbine with a current generator; 6 - capacitor; 7 - circulation pump; 8 - cooling water receiver
Such simple installations were already in operation in Africa in the 1950s.
An obvious design option for a modern power plant is a geothermal power plant with a low-boiling working substance, shown in Figure 3. Hot water from the storage tank enters the evaporator 3, where it gives off its heat to some substance with a low boiling point. Such substances can be carbon dioxide, various freons, sulfur hexafluoride, butane, etc. Condenser 6 is a mixing type, which is cooled by cold liquid butane coming from a surface air cooler. Part of the butane from the condenser is supplied by the feed pump 9 to the heater 10, and then to the evaporator 3.
An important feature of this scheme is the possibility of operation in winter with low condensing temperatures. This temperature can be close to zero or even negative, because all the listed substances have very low freezing points. This allows you to significantly expand the temperature limits used in the cycle.
Picture 3. Scheme of a geothermal power plant with a low-boiling working substance:
1 - well, 2 - storage tank, 3 - evaporator, 4 - turbine, 5 - generator, 6 - condenser, 7 - circulation pump, 8 - surface air cooler, 9 - feed pump, 10 - working medium heater
Geothermal power station from immediate using natural steam.
The simplest and most affordable geothermal power plant is a backpressure steam turbine. Natural steam from the well is supplied directly to the turbine with subsequent release to the atmosphere or to a device that captures valuable chemicals. The backpressure turbine can be supplied with secondary steam or steam obtained from a separator. According to this scheme, the power plant operates without capacitors, and there is no need for a compressor to remove non-condensable gases from the capacitors. This installation is the simplest, capital and operating costs for it are minimal. It occupies a small area, requires almost no auxiliary equipment, and can easily be adapted as a portable geothermal power plant (Figure 4).
Figure 4 - Scheme of a geothermal power plant with the direct use of natural steam:
1 - well; 2 - turbine; 3 - generator;
4 - exit to the atmosphere or to a chemical plant
The considered scheme can become the most profitable for those areas where there are sufficient reserves of natural steam. Rational operation provides an opportunity effective work such an installation even with a variable well flow rate.
There are several such stations in Italy. One of them has a capacity of 4 thousand kW at a specific steam consumption of about 20 kg / s or 80 t / h; the other one with a capacity of 16 thousand kW, where four turbogenerators with a capacity of 4 thousand kW each are installed. The latter is supplied with steam from 7–8 wells.
Geothermal power plant with condensing turbine and direct use of natural steam (Figure 5) is the most modern scheme for generating electrical energy.
The steam from the well is fed into the turbine. Spent in the turbine, it enters the mixing condenser. A mixture of cooling water and condensate of the steam already exhausted in the turbine is discharged from the condenser into an underground tank, from where it is taken by circulation pumps and sent to the cooling tower for cooling. From the cooling tower, the cooling water again enters the condenser (Figure 5).
According to this scheme, with some changes, many geothermal power plants operate: Larderello-2 (Italy), Wairakei (New Zealand), etc.
Scope double-circuit power plants on low-boiling working substances (freon-R12, water-ammonia mixture,) is the use of heat from thermal waters with a temperature of 100 ... 200 ° C, as well as separated water in the deposits of steam hydrotherms.
Figure 5 - Scheme of a geothermal power plant with a condensing turbine and direct use of natural steam:
1 - well; 2 - turbine; 3 - generator; 4 - pump;
5 - capacitor; 6 - cooling tower; 7 - compressor; 8 - reset
Combined production of electrical and thermal energy
Combined production of electrical and thermal energy is possible at geothermal thermal power plants (GeoTPP).
The simplest diagram of a Geothermal power plant of a vacuum type for using the heat of hot water with a temperature of up to 100 ° C is shown in Figure 6.
The operation of such a power plant proceeds as follows. Hot water from well 1 enters storage tank 2. In the tank, it is freed from the gases dissolved in it and is sent to expander 3, in which a pressure of 0.3 atm is maintained. At this pressure and at a temperature of 69 ° C, a small part of the water turns into steam and is sent to the vacuum turbine 5, and the remaining water is pumped by the pump 4 to the heat supply system. The steam exhausted in the turbine is discharged into the mixing condenser 7. To remove air from the condenser, a vacuum pump 10 is installed. by gravity due to discharge.
The Verkhne-Mutnovskaya GeoTPP with a capacity of 12 MW (3x4 MW) is a pilot stage of the Mutnovskaya GeoTPP with a design capacity of 200 MW, created to supply the Petropavlovsk-Kamchatsky industrial region with electricity.
Figure 6 -. Scheme of a vacuum GeoTPP with one expander:
1 - well, 2 - storage tank, 3 - expander, 4 - hot water pump, 5 - vacuum turbine 750 kW, 6 - generator, 7 - mixing condenser,
8 - cooling water pump, 9 - fan cooling tower, 10 - vacuum pump
At the Pauzhetskaya Geothermal Power Plant (south of Kamchatka) with a capacity of 11 MW, steam turbines use only separated geothermal steam from a steam-water mixture obtained from geothermal wells. A large amount of geothermal water (about 80 of the total PVA consumption) with a temperature of 120 °C is discharged into the spawning Ozernaya River, which leads not only to loss of the thermal potential of the geothermal coolant, but also significantly worsens the ecological state of the river.
Heat pumps
Heat pump- a device for transferring thermal energy from a source of low-grade thermal energy with a low temperature to a heat carrier consumer with a higher temperature. Thermodynamically, a heat pump is an inverted refrigeration machine. If in a refrigeration machine the main goal is to produce cold by taking heat from any volume by the evaporator, and the condenser discharges heat into the environment, then in a heat pump the situation is reversed (Figure 7). The condenser is a heat exchanger that generates heat for the consumer, and the evaporator is a heat exchanger that utilizes low-grade heat located in reservoirs, soils, wastewater, and the like. Depending on the principle of operation, heat pumps are divided into compression and absorption. Compression heat pumps are always driven by an electric motor, while absorption heat pumps can also use heat as an energy source. The compressor also needs a source of low-grade heat.
During operation, the compressor consumes electricity. The ratio of generated thermal energy and consumed electrical energy is called the transformation ratio (or heat conversion coefficient) and serves as an indicator of the efficiency of the heat pump. This value depends on the difference between the temperature levels in the evaporator and condenser: the greater the difference, the smaller this value.
By type of coolant in the input and output circuits, the pumps are divided into six types: "ground-water", "water-water", "air-water", "ground-air", "water-air", "air-air".
When using soil energy as a heat source, the pipeline in which the liquid circulates is buried in the ground 30-50 cm below the soil freezing level in the given region (Figure 8). To install a heat pump with a capacity of 10 kW, an earth circuit 350-450 m long is required, for laying which a plot of land of about 400 m² (20x20 m) is required.
Figure 7 - Scheme of heat pump operation
Figure 8 - Use of soil energy as a heat source
First of all, the advantages of heat pumps include cost-effectiveness: in order to transfer 1 kWh of thermal energy to the heating system, the HPP installation needs to spend 0.2-0.35 kWh of electricity .. All systems operate using closed circuits and practically do not require operating costs, except for the cost of electricity required to operate the equipment, which can be obtained from wind and solar power plants. The payback period of heat pumps is 4-9 years, with a service life of 15-20 years before major repairs.
The actual efficiency values of modern heat pumps are of the order of COP = 2.0 at a source temperature of −20 °C, and of the order of COP = 4.0 at a source temperature of +7 °C.
The structure of 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. Chemical 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 waters 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 supplied 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.
The temperature difference in the steam generator reduces the enthalpy of the live steam of a double-circuit installation h 1 compared to a single-circuit one, however, in general, the heat drop in the turbine increases due to a decrease in the enthalpy of the exhaust steam h 2 . 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 is the temperature difference of geothermal water at the inlet and outlet of the steam generator, °C, is 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-circuit 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 created, 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 ° C, cold water was supplied to the condenser from the river. Paratunka with an average annual temperature of 5 o C. Unfortunately, these works were not developed due to the former cheapness of organic fuel.
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 o C). With a homogeneous working fluid, the temperature deviation at the outlet of the steam generator by 10...20 o C 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 indicators, and the water-ammonia mixture differs the best performance heat exchange, which allows to reduce the metal consumption and the cost of the steam generator and condenser in comparison with the power module on a homogeneous coolant. 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.
Calculation of GeoTEU with low-boiling and mixed working fluids is carried out using tables of thermodynamic properties and h - s diagrams of 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 o C, at a depth of 500 ... 1000 m - about 2 ... 3 o C. Back in 1881, D "Arsonval expressed the idea of using this temperature difference to generate electricity. Scheme installations 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 is heated 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 the 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 costs of heat and cold water, as well as the heat carrier, the energy consumption of the 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.
Read and write useful
geothermal energy
Abstract.
Introduction.
The cost of electricity generated by geothermal power plants.
Bibliography.
Abstract.
This paper presents the history of the development of geothermal energy, both throughout the world and in our country, Russia. An analysis was made of the use of the deep heat of the Earth to convert it into electrical energy, as well as to provide cities and towns with heat and hot water in such regions of our country as Kamchatka, Sakhalin, and the North Caucasus. An economic justification for the development of geothermal deposits, the construction of power plants and their payback periods have been made. Comparing the energy of geothermal sources with other types of energy sources, we get the prospects for the development of geothermal energy, which should take an important place in the overall balance of energy use. In particular, for the restructuring and re-equipment of the power industry of the Kamchatka region and the Kuril Islands, partly Primorye and the North Caucasus, one should use their own geothermal resources.
Introduction.
The main directions for the development of generating capacities in the country's energy sector in the near future are the technical re-equipment and reconstruction of power plants, as well as the commissioning of new generating capacities. First of all, this is the construction of combined cycle plants with an efficiency of 5560%, which will increase the efficiency of existing thermal power plants by 2540%. The next step should be the construction of thermal power plants using new combustion technologies solid fuel and with supercritical steam parameters to achieve TPP efficiency equal to 46-48%. Further development nuclear power plants with new types of thermal and fast neutron reactors will also be received.
An important place in the formation of the Russian energy sector is occupied by the country's heat supply sector, which is the largest in terms of the volume of consumed energy resources, more than 45% of their total consumption. District heating systems (DH) produce more than 71%, and decentralized sources produce about 29% of all heat. More than 34% of all heat is supplied by power plants, approximately 50% by boilers. In accordance with the energy strategy of Russia until 2020. it is planned to increase heat consumption in the country by at least 1.3 times, and the share of decentralized heat supply will increase from 28.6% in 2000 to up to 33% in 2020
The increase in prices that took place in last years, for organic fuel (gas, fuel oil, diesel fuel) and for its transportation to remote regions of Russia and, accordingly, an objective increase in selling prices for electrical and thermal energy fundamentally change the attitude towards the use of renewable energy sources: geothermal, wind, solar.
Thus, the development of geothermal energy in certain regions of the country already makes it possible today to solve the problem of electricity and heat supply, in particular in Kamchatka, the Kuril Islands, as well as in the North Caucasus, in certain regions of Siberia and the European part of Russia.
Among the main directions of improvement and development of heat supply systems should be the expansion of the use of local non-traditional renewable energy sources and, first of all, geothermal heat of the earth. In the next 7-10 years, with the help of modern technologies of local heat supply, thanks to thermal heat, significant fossil fuel resources can be saved.
In the last decade, the use of non-traditional renewable energy sources (NRES) has experienced a real boom in the world. The scale of application of these sources has increased several times. This direction is developing most intensively in comparison with other areas of energy. There are several reasons for this phenomenon. First of all, it is obvious that the era of cheap traditional energy carriers has irrevocably ended. In this area, there is only one trend - the rise in prices for all their types. No less significant is the desire of many countries deprived of their fuel base for energy independence. Environmental considerations, including the emission of harmful gases, play a significant role. Active moral support for the use of renewable energy is provided by the population of developed countries.
For these reasons, the development of renewable energy in many states is a priority task of technical policy in the field of energy. In a number of countries, this policy is implemented through the adopted legislative and regulatory framework, which establishes the legal, economic and organizational foundations for the use of renewable energy. In particular, the economic foundations consist in various measures to support renewable energy at the stage of their development of the energy market (tax and credit benefits, direct subsidies, etc.)
In Russia practical use RES significantly lags behind the leading countries. There is no legislative and regulatory framework, as well as state economic support. All this makes it extremely difficult to practice in this area. The main reason for the inhibitory factors is the protracted economic trouble in the country and, as a result, difficulties with investments, low solvent demand, lack of funds for the necessary developments. Nevertheless, some work and practical measures for the use of renewable energy in our country are being carried out (geothermal energy). Steam-hydrothermal deposits in Russia are available only in Kamchatka and the Kuril Islands. Therefore, geothermal energy cannot take a significant place in the energy sector of the country as a whole in the future. However, it is able to radically and on the most economic basis solve the problem of energy supply to these regions, which use expensive imported fuel (fuel oil, coal, diesel fuel) and are on the verge of an energy crisis. The potential of steam-hydrothermal fields in Kamchatka is capable of providing from 1000 to 2000 MW of installed electric power from various sources, which significantly exceeds the needs of this region for the foreseeable future. Thus, there are real prospects for the development of geothermal energy here.
The history of the development of geothermal energy.
Along with huge resources of fossil fuels, Russia has significant reserves of the earth's heat, which can be multiplied by geothermal sources located at a depth of 300 to 2500 m, mainly in the fault zones of the earth's crust.
The territory of Russia is well explored, and today the main resources of the earth's heat are known, which have significant industrial potential, including energy. Moreover, almost everywhere there are reserves of heat with a temperature of 30 to 200°C.
Back in 1983 in VSEGINGEO an atlas of the resources of thermal waters of the USSR was compiled. In our country, 47 geothermal deposits have been explored with reserves of thermal waters, which allow you to get more than 240 10³ m³ / day. Today in Russia specialists from almost 50 scientific organizations deal with the problems of using the heat of the earth.
More than 3,000 wells have been drilled to use geothermal resources. The cost of geothermal research and drilling already carried out in this area, in modern prices, is more than 4 billion rubles. dollars. So in Kamchatka, 365 wells have already been drilled in geothermal fields with a depth of 225 to 2266 m and used up (still in Soviet time) about 300 million. dollars (in current prices).
The operation of the first geothermal power plant was started in Italy in 1904. The first geothermal power plant in Kamchatka, and the first in the USSR, the Pauzhetskaya Geothermal Power Plant was put into operation in 1967. and had a power of 5 mW, subsequently increased to 11 mW. New impulse The development of geothermal energy in Kamchatka was given in the 90s with the emergence of organizations and firms (JSC Geoterm, JSC Intergeotherm, JSC Nauka), which, in cooperation with industry (primarily with the Kaluga Turbine Plant), developed new progressive schemes, technologies and types of equipment for converting geothermal energy into electrical energy and secured a loan from the European Bank for Reconstruction and Development. As a result, in 1999 Verkhne-Mutnovskaya GeoTPP (three modules of 4 MW each) was put into operation in Kamchatka. The first block of 25mW is introduced. the first stage of the Mutnovskaya GeoTPP with a total capacity of 50 MW.
The second phase with a capacity of 100 MW can be commissioned in 2004
Thus, the immediate and quite real prospects for geothermal energy in Kamchatka have been determined, which is a positive undoubted example of the use of renewable energy in Russia, despite the serious economic difficulties in the country. The potential of steam-hydrothermal fields in Kamchatka is capable of providing 1000 MW of installed electric power, which significantly exceeds the needs of this region for the foreseeable future.
According to the Institute of Volcanology, Far Eastern Branch of the Russian Academy of Sciences, the already identified geothermal resources make it possible to fully provide Kamchatka with electricity and heat for more than 100 years. Along with the high-temperature Mutnovskoye field with a capacity of 300 MW(e) in the south of Kamchatka, significant reserves of geothermal resources are known at the Koshelevskoye, Bolshe Bannoy, and in the north at the Kireunskoye deposits. Heat reserves of geothermal waters in Kamchatka are estimated at 5000 MW (t).
Chukotka also has significant reserves of geothermal heat (on the border with the Kamchatka region), some of them have already been discovered and can be actively used for nearby cities and towns.
The Kuril Islands are also rich in the reserves of the earth's heat, they are quite enough to supply heat and electricity to this territory for 100,200 years. Reserves of a two-phase geothermal coolant have been discovered on Iturup Island, the capacity of which (30 MW(e)) is sufficient to meet the energy needs of the entire island in the next 100 years. Here, wells have already been drilled at the Ocean geothermal field and a GeoPP is being built. There are reserves of geothermal heat on the southern island of Kunashir, which are already being used to generate electricity and heat supply to the city of Yuzhno Kurilsk. The bowels of the northern island of Paramushir are less studied, but it is known that this island also has significant reserves of geothermal water with a temperature of 70 to 95 ° C, and a GeoTS with a capacity of 20 MW (t) is also being built here.
The deposits of thermal waters with a temperature of 100-200°C are much more widespread. At this temperature, it is advisable to use low-boiling working fluids in the steam turbine cycle. The use of double-circuit Geothermal power plants on thermal water is possible in a number of regions of Russia, primarily in the North Caucasus. Geothermal deposits are well studied here with a reservoir temperature of 70 to 180 ° C, which are located at a depth of 300 to 5000 m. Geothermal water has been used here for a long time for heat supply and hot water supply. In Dagestan, more than 6 million m of geothermal water is produced annually. About 500 thousand people in the North Caucasus use geothermal water supply.
Primorye, the Baikal region, and the West Siberian region also have reserves of geothermal heat suitable for large-scale use in industry and agriculture.
Conversion of geothermal energy into electrical and thermal energy.
One of the promising areas for using the heat of highly mineralized underground thermal waters is converting it into electrical energy. For this purpose, a technological scheme was developed for the construction of a Geothermal power plant, consisting of a geothermal circulation system(GCC) and steam turbine plant (STP), the scheme of which is shown in Fig.1. Distinctive feature Such a technological scheme from the well-known is that in it the role of an evaporator and a superheater is performed by a downhole vertical counterflow heat exchanger located in the upper part of the injection well, where produced high-temperature thermal water is supplied through the surface pipeline, which, after transferring heat to the secondary coolant, is pumped back into the formation. The secondary coolant from the condenser of the steam turbine plant enters the heating zone by gravity through a pipe lowered inside the heat exchanger to the bottom.
The Rankine cycle is at the heart of the work of vocational schools; t,s is a diagram of this cycle and the nature of the change in the temperatures of heat carriers in the evaporator heat exchanger.
Most important point during the construction of GeoTPP is the choice of the working fluid in the secondary circuit. The working fluid selected for a geothermal installation must have favorable chemical, physical and operational properties under given operating conditions, i.e. be stable, non-flammable, explosion-proof, non-toxic, inert to construction materials and cheap. It is desirable to choose a working fluid with a lower coefficient of dynamic viscosity (less hydraulic losses) and with a higher coefficient of thermal conductivity (improved heat transfer).
It is practically impossible to fulfill all these requirements at the same time, therefore, it is always necessary to optimize the choice of one or another working fluid.
The low initial parameters of the working bodies of geothermal power plants lead to the search for low-boiling working bodies with a negative curvature of the right boundary curve in the t, s diagram, since the use of water and steam leads in this case to a deterioration in thermodynamic parameters and to a sharp increase in the dimensions of steam turbine plants, which is significant increases their value.
It is proposed to use a mixture of isobutane + isopentane in the supercritical state as a supercritical agent in the secondary circuit of binary energy cycles. The use of supercritical mixtures is convenient because the critical properties, i.e. the critical temperature tc(x), the critical pressure pc(x) and the critical density qc(x) depend on the composition of the mixture x. This will allow, by selecting the composition of the mixture, to select a supercritical agent with the most favorable critical parameters for the corresponding temperature of the thermal water of a particular geothermal field.
As a secondary coolant, a low-boiling hydrocarbon isobutane is used, the thermodynamic parameters of which correspond to the required conditions. Critical parameters of isobutane: tc = 134.69°C; pk = 3.629 MPa; qk = 225.5 kg/m³. In addition, the choice of isobutane as a secondary coolant is due to its relatively low cost and environmental friendliness (unlike freons). Isobutane as a working fluid has found wide distribution abroad, and it is also proposed to use it in a supercritical state in binary geothermal energy cycles.
The energy characteristics of the installation are calculated for a wide range of temperatures of produced water and various modes of its operation. In all cases, it was assumed that the condensation temperature of isobutane tcon =30°C.
The question arises about the choice of the smallest temperature differenceêtfig.2. On the one hand, a decrease in êt leads to an increase in the surface of the evaporator heat exchanger, which may not be economically justified. On the other hand, an increase in êt at a given temperature of thermal water ts leads to the need to lower the evaporation temperature ts (and, consequently, the pressure), which will adversely affect the cycle efficiency. In most practical cases, it is recommended to take êt = 10÷25ºС.
The results obtained show that there are optimal parameters for the operation of a steam power plant, which depend on the temperature of the water entering the primary circuit of the heat exchanger steam generator. With an increase in the evaporation temperature of isobutane tz, the power N generated by the turbine increases by 1 kg/s of the secondary coolant consumption. At the same time, as tg increases, the amount of evaporated isobutane decreases per 1 kg/s of thermal water consumption.
As the temperature of thermal water increases, so does optimum temperature evaporation.
Figure 3 shows the graphs of the dependence of the power N generated by the turbine on the evaporation temperature ts of the secondary coolant at various temperatures of thermal water.
For high-temperature water (tt = 180ºС), supercritical cycles are considered, when the initial vapor pressure pн= 3.8; 4.0; 4.2; and 5.0 MPa. Of these, the most effective in terms of obtaining maximum power is the supercritical cycle, close to the so-called "triangular" cycle with an initial pressure pn = 5.0 MPa. During this cycle, due to the minimum temperature difference between the heat carrier and the working fluid, the temperature potential of thermal water is used to the fullest extent. Comparison of this cycle with the subcritical one (pn=3.4MPa) shows that the power generated by the turbine during the supercritical cycle increases by 11%, the flow density of the substance entering the turbine is 1.7 times higher than in the cycle with pn=3 ,4 MPa, which will lead to an improvement in the transport properties of the coolant and a reduction in the size of the equipment (supply pipelines and turbine) of the steam turbine plant. In addition, in the cycle with pH = 5.0 MPa, the temperature of the waste thermal water t, injected back into the reservoir, is 42ºС, while in the subcritical cycle with pH = 3.4 MPa, the temperature tн = 55ºС.
At the same time, an increase in the initial pressure to 5.0 MPa in the supercritical cycle affects the cost of the equipment, in particular, the cost of the turbine. Although the dimensions of the flow part of the turbine decrease with increasing pressure, the number of turbine stages simultaneously increases, a more developed end seal is required, and, most importantly, the thickness of the casing walls increases.
To create a supercritical cycle in the technological scheme of GeoTPP, it is necessary to install a pump on the pipeline connecting the condenser with the heat exchanger.
However, factors such as the increase in power, the reduction in the size of the supply pipelines and the turbine, and the more complete actuation of the thermal potential of thermal water, speak in favor of the supercritical cycle.
In the future, it is necessary to look for coolants with a lower critical temperature, which will make it possible to create supercritical cycles using thermal waters with a lower temperature, since the thermal potential of the vast majority of explored deposits in Russia does not exceed 100÷120ºС. In this regard, the most promising is R13B1(trifluorobromomethane) with the following critical parameters: tc = 66.9ºС; pk = 3.946 MPa; qk= 770kg/m³.
The results of evaluation calculations show that the use of thermal water with a temperature of tk = 120ºС in the primary circuit of the GeoTPP and the creation of a supercritical cycle with an initial pressure of pn = 5.0 MPa in the secondary circuit on freon R13B1 also makes it possible to increase the turbine power up to 14% compared to the subcritical cycle with initial pressure pn = 3.5 MPa.
For the successful operation of the GeoTPP, it is necessary to solve the problems associated with the occurrence of corrosion and salt deposits, which, as a rule, are aggravated with an increase in the mineralization of thermal water. The most intense salt deposits are formed due to the degassing of thermal water and the disruption of the carbon dioxide balance as a result of this.
In the proposed technological scheme, the primary coolant circulates in a closed circuit: reservoir - production well - surface pipeline - pump - injection well - reservoir, where conditions for water degassing are minimized. At the same time, it is necessary to adhere to such thermobaric conditions in the surface part of the primary circuit, which prevent degassing and precipitation of carbonate deposits (depending on temperature and salinity, the pressure must be maintained at 1.5 MPa and above).
A decrease in the temperature of thermal water also leads to precipitation of non-carbonate salts, which was confirmed by studies conducted at the Kayasulinsky geothermal site. Part of the precipitated salts will be deposited on the inner surface of the injection well, and the bulk will be carried to the bottomhole zone. The deposition of salts at the bottom of the injection well will contribute to a decrease in injectivity and a gradual decrease in the circular flow rate, up to a complete stop of the GCS.
To prevent corrosion and scaling in the GCS circuit, an effective HEDPK (hydroxyethylidene diphosphonic acid) reagent can be used, which has a long-term anti-corrosion and anti-scale effect of surface passivation. Restoration of the passivation layer of OEDFK is carried out by periodically pulsed injection of a reagent solution into thermal water at the mouth of a production well.
To dissolve the salt sludge that will accumulate in the bottomhole zone, and therefore to restore the injectivity of the injection well, a very effective reagent is NMA (concentrate of low molecular weight acids), which can also be introduced periodically into the circulating thermal water in the area before the injection pump.
Therefore, from the above, it can be suggested that one of the promising directions for the development of the thermal energy of the earth's interior is its conversion into electrical energy by building two-circuit GeoTPPs on low-boiling working agents. The efficiency of such a transformation depends on many factors, in particular, on the choice of the working fluid and the parameters of the thermodynamic cycle of the secondary circuit of the GeoTPP.
The results of the computational analysis of cycles using various heat carriers in the secondary circuit show that the most optimal are supercritical cycles, which allow increasing the turbine power and cycle efficiency, improving the transport properties of the coolant and more fully adjusting the temperature of the initial thermal water circulating in the primary circuit of the GeoTPP.
It has also been established that for high-temperature thermal water (180ºС and above), the most promising is the creation of supercritical cycles in the secondary circuit of the GeoTPP using isobutane, while for waters with a lower temperature (100÷120ºС and above), when creating the same cycles, the most suitable heat carrier is freon R13B1.
Depending on the temperature of the extracted thermal water, there is an optimal temperature for the evaporation of the secondary heat carrier, corresponding to the maximum power generated by the turbine.
In the future, it is necessary to study supercritical mixtures, the use of which as a working agent for geothermal energy cycles is the most convenient, since by selecting the mixture composition, one can easily change their critical properties depending on external conditions.
Another direction in the use of geothermal energy is geothermal heat supply, which has long been used in Kamchatka and the North Caucasus for heating greenhouses, heating and hot water supply in the housing and communal sector. An analysis of world and domestic experience indicates the prospects of geothermal heat supply. Currently, geothermal heat supply systems with a total capacity of 17,175 MW are operating in the world, and more than 200,000 geothermal installations are operated in the United States alone. According to the plans of the European Union, the capacity of geothermal heating systems, including heat pumps, should increase from 1300 MW in 1995 to 5000 MW in 2010.
In the USSR, geothermal waters were used in the Krasnodar and Stavropol Territories, Kabardino-Balkaria, North Ossetia, Checheno-Ingushetia, Dagestan, Kamchatka Oblast, Crimea, Georgia, Azerbaijan and Kazakhstan. In 1988, 60.8 million m³ of geothermal water was produced, now in Russia it is produced up to 30 million. m³ per year, which is equivalent to 150÷170 thousand tons of reference fuel. At the same time, the technical potential of geothermal energy, according to the Ministry of Energy of the Russian Federation, is 2950 million tons of standard fuel.
Over the past 10 years, the system of exploration, development and exploitation of geothermal resources has collapsed in our country. In the USSR, research work on this problem was carried out by the institutes of the Academy of Sciences, the ministries of geology and the gas industry. Exploration, appraisal and approval of deposit reserves were carried out by institutes and regional subdivisions of the Ministry of Geology. Drilling of productive wells, field development, development of technologies for re-injection, treatment of geothermal waters, operation of geothermal heat supply systems were carried out by subdivisions of the Ministry of Gas Industry. It included five regional operational departments, the Soyuzgeotherm scientific and production association (Makhachkala), which developed a scheme for the prospective use of geothermal waters of the USSR. The design of systems and equipment for geothermal heat supply was carried out by the Central Research and Design and Experimental Institute of Engineering Equipment.
At present, comprehensive research work in the field of geothermy has ceased: from geological and hydrogeological studies to the problems of purification of geothermal waters. Exploratory drilling is not carried out, the development of previously explored deposits is not carried out, the equipment of existing geothermal heat supply systems is not being modernized. The role of state administration in the development of geothermy is negligible. Geothermal specialists are scattered, their experience is not in demand. Analysis of the current situation and development prospects in new economic conditions Russia, let's do it on the example of the Krasnodar Territory.
For this region, of all renewable energy sources, the most promising is the use of geothermal waters. Figure 4 shows the priorities for the use of renewable energy for heat supply to objects in the Krasnodar Territory.
IN Krasnodar Territory up to 10 million m³/year of geothermal water with a temperature of 70÷100ºС is extracted annually, which replaces 40÷50 thousand tons of organic fuel (in terms of reference fuel). There are 10 fields in operation with 37 wells, 6 fields with 23 wells are under development. Total number of geothermal wells77. 32 hectares are heated by geothermal waters. greenhouses, 11 thousand apartments in eight settlements, 2 thousand people are provided with hot water. Explored operational reserves of geothermal waters of the region are estimated at 77.7 thousand cubic meters. m³ / day, or during operation during the heating season - 11.7 million. m³ per season, predicted reserves, respectively, 165 thousand. m³/day and 24.7 mln. m³ per season.
One of the most developed Mostovskoye geothermal field, 240 km from Krasnodar in the foothills of the Caucasus, where 14 wells were drilled with a depth of 1650÷1850m with flow rates of 1500÷3300 m³ / day, a temperature at the mouth of 67÷78º C, a total salinity of 0.9÷1, 9g/l. By chemical composition geothermal water almost meets the standards for drinking water. The main consumer of geothermal water from this field is a greenhouse complex with a greenhouse area of up to 30 hectares, which previously operated 8 wells. Currently, 40% of the greenhouse area is heated here.
For heat supply of residential and administrative buildings of the village. Bridge in the 80s, a geothermal central heating point (CHP) was built with an estimated thermal power of 5 MW, the diagram of which is shown in Fig. 5. Geothermal water in the central heating center comes from two wells with a flow rate of 45÷70 m³/h each and a temperature of 70÷74ºС into two storage tanks with a capacity of 300m³. To utilize the heat of waste geothermal water, two steam-compressor heat pumps with an estimated thermal power of 500 kW were installed. The geothermal water used in heating systems with a temperature of 30÷35ºС before the heat pump unit (HPU) is divided into two streams, one of which is cooled to 10ºС and drained into the reservoir, and the second is heated up to 50ºС and returned to the storage tanks. The heat pump units were manufactured by the Moscow Kompressor plant on the basis of refrigeration machines A-220-2-0.
The regulation of the thermal power of geothermal heating in the absence of peak reheating is carried out in two ways: by passing the coolant and cyclically. With the latter method, the systems are periodically filled with geothermal coolant with simultaneous draining of the cooled one. With a daily heating period Z, the heating time Zn is determined by the formula
Zn = 48j/(1 + j), where is the heat output coefficient; design air temperature in the room, °С; and actual and calculated outdoor air temperature, °С.
The capacity of storage tanks of geothermal systems is determined from the condition of ensuring the normalized amplitude of air temperature fluctuations in heated residential premises (± 3 ° C) according to the formula.
where kF is the heat output of the heating system per 1°C of the temperature difference, W/°C; Z \u003d Zn + Zpp period of operation of geothermal heating; Zp pause duration, h; Qp and Qp is the calculated and seasonally average heat output of the heating system of the building, W; c volumetric heat capacity of geothermal water, J/(m³ ºС); n number of geothermal heating starts per day; k1 is the heat loss coefficient in the geothermal heat supply system; A1 amplitude of temperature fluctuations in a heated building, ºС; Rnom total indicator of heat absorption of heated premises; Vc and Vts capacity of heating systems and heating networks, m³.
During the operation of heat pumps, the ratio of geothermal water flow rates through the evaporator Gi and the condenser Gk is determined by the formula:
Where tk, to, t is the temperature of geothermal water after the condenser, the heating system of the building and HPI evaporators, ºС.
It should be noted the low reliability of the used designs of heat pumps, since their operating conditions differed significantly from the operating conditions of refrigeration machines. The ratio of discharge and suction pressures of compressors when operating in the mode of heat pumps is 1.5÷2 times higher than the same ratio in refrigeration machines. Failures of the connecting rod and piston group, oil facilities, and automation led to the premature failure of these machines.
As a result of the lack of control of the hydrological regime, the operation of the Mostovskoye geothermal field after 10 years, the pressure at the wellhead decreased by 2 times. In order to restore the reservoir pressure of the field in 1985. three injection wells were drilled, a pumping station was built, but their work did not give a positive result due to the low injectivity of the reservoirs.
For the most promising use of geothermal resources in the city of Ust-Labinsk with a population of 50 thousand people, located 60 km from Krasnodar, a system of geothermal heat supply with an estimated thermal power of 65 MW has been developed. Of the three water-pumping horizons, Eocene-Paleocene deposits were selected with a depth of 2200÷2600m, formation temperature 97÷100ºС, salinity 17÷24g/l.
As a result of the analysis of existing and prospective heat loads in accordance with the scheme for the development of the city's heat supply, the optimal, calculated, thermal power of the geothermal heat supply system was determined. A technical and economic comparison of four options (three of them without peak boilers with a different number of wells and one with reheating in the boiler) showed that the scheme with the peak boiler (Fig. 6) has the minimum payback period.
The geothermal heat supply system provides for the construction of the western and central thermal water intakes with seven injection wells. Operating mode of thermal water intakes with re-injection of cooled coolant. Double-circuit heat supply system with peak reheating in the boiler room and dependent connection existing systems building heating. Capital investment in the construction of this geothermal system amounted to 5.14 million. rub. (in prices of 1984), payback period 4.5 years, estimated savings of substituted fuel 18.4 thousand tons of reference fuel per year.
The cost of electricity generated by geothermal power plants.
The costs of research and development (drilling) of geothermal fields account for up to 50% of the total cost of a GeoTPP, and therefore the cost of electricity generated at a GeoPP is quite significant. Thus, the cost of the entire pilot industrial (OP) Verkhne-Mutnovskaya GeoPP [capacity 12 (3 × 4) MW] amounted to about 300 million rubles. However, the absence of transportation costs for fuel, the renewability of geothermal energy and the environmental friendliness of electricity and heat production allow geothermal energy to successfully compete in the energy market and, in some cases, produce cheaper electricity and heat than traditional IES and CHP. For remote areas (Kamchatka, Kuril Islands), GeoPPs have an unconditional advantage over thermal power plants and diesel stations operating on imported fuel.
If we consider Kamchatka as an example, where more than 80% of electricity is produced at CHPP-1 and CHPP-2, operating on imported fuel oil, then the use of geothermal energy is more profitable. Even today, when the process of construction and development of new GeoPPs at the Mutnovsky geothermal field is still underway, the cost of electricity at the Verkhne-Mutnovskaya GeoPP is more than two times lower than at the CHPP in Petropavlovsk Kamchatsky. The cost of 1 kWh(e) at the old Pauzhetskaya GeoPP is 2¸3 times lower than at CHPP-1 and CHPP-2.
The cost of 1 kWh of electricity in Kamchatka in July 1988 was between 10 and 25 cents, and the average electricity tariff was set at 14 cents. In June 2001 in the same region, the electricity tariff for 1 kWh ranged from 7 to 15 cents. At the beginning of 2002 the average tariff in OAO Kamchatskenergo was 3.6 rubles. (12 cents). It is clear that the economy of Kamchatka cannot successfully develop without reducing the cost of electricity consumed, and this can only be achieved through the use of geothermal resources.
Now, when restructuring the energy sector, it is very important to proceed from real prices for fuel and equipment, as well as energy prices for different consumers. Otherwise, you can come to erroneous conclusions and forecasts. Thus, in the strategy for the development of the economy of the Kamchatka region, developed in 2001 at Dalsetproekt, without sufficient justification, the price of 1000 m³ of gas was set at $50, although it is clear that the real cost of gas will not be lower than $100, and the duration of the development of gas fields will be 5 ÷10 years. At the same time, according to the proposed strategy, gas reserves are calculated for a life of no more than 12 years. Therefore, the prospects for the development of the energy sector in the Kamchatka region should be associated primarily with the construction of a series of geothermal power plants at the Mutnovsky field [up to 300 MW (e)], the re-equipment of the Pauzhetskaya GeoPP, whose capacity should be increased to 20 MW, and the construction of new GeoPPs. The latter will ensure the energy independence of Kamchatka for many years (at least 100 years) and will reduce the cost of electricity sold.
According to the assessment of the World Energy Council, of all renewable energy sources, the GeoPP has the lowest price per 1 kWh (see table).
|
power |
use power |
Price installed |
in the last |
||||
| 10200 | 55÷95(84) | 2÷10 | 1÷8 | 800÷3000 | 70,2 | 22 | |
| Wind | 12500 | 20÷30(25) | 5÷13 | 3÷10 | 1100÷ 1700 | 27,1 | 30 |
| 50 | 8÷20 | 25÷125 | 5÷25 | 5000÷10000 | 2,1 | 30 | |
| tides | 34 | 20÷30 | 8÷15 | 8÷15 | 1700÷ 2500 | 0,6 |
From the experience of operating large GeoPPs in the Philippines, New Zealand, Mexico and the USA, it follows that the cost of 1 kWh of electricity often does not exceed 1 cent, while it should be borne in mind that the power utilization factor at GeoPPs reaches 0.95.
Geothermal heat supply is most beneficial with the direct use of geothermal hot water, as well as with the introduction of heat pumps, which can effectively use the heat of the earth with a temperature of 10÷30ºС, i.е. low-grade geothermal heat. In the current economic conditions of Russia, the development of geothermal heat supply is extremely difficult. Fixed assets must be invested in drilling wells. In the Krasnodar Territory, with the cost of drilling 1 m of a well 8 thousand rubles, its depth is 1800 m, the costs amount to 14.4 million rubles. With an estimated well flow rate of 70 m³ / h, a triggered temperature difference of 30º C, round-the-clock operation for 150 days. per year, the utilization rate of the estimated flow during the heating season is 0.5, the amount of heat supplied is 4385 MWh, or in value terms 1.3 million rubles. at a tariff of 300 rubles/(MWh). At this rate, well drilling will pay off in 11 years. At the same time, in the future, the need to develop this area in the energy sector is beyond doubt.
Conclusions.
1. Almost throughout Russia there are unique reserves of geothermal heat with coolant temperatures (water, two-phase flow and steam) from 30 to 200º C.
2. In recent years, based on major fundamental research, geothermal technologies have been created in Russia that can quickly ensure the efficient use of earth heat at GeoPPs and GeoTS to generate electricity and heat.
3. Geothermal energy should take an important place in the overall balance of energy use. In particular, for the restructuring and re-equipment of the power industry of the Kamchatka region and the Kuril Islands and partly of Primorye, Siberia and the North Caucasus, one should use their own geothermal resources.
4. Large-scale introduction of new heat supply schemes with heat pumps using low-grade heat sources will reduce fossil fuel consumption by 20÷25%.
5. To attract investments and loans to the energy sector, it is necessary to carry out effective projects and guarantee timely return borrowed money, which is possible only with full and timely payment for electricity and heat supplied to consumers.
Bibliography.
1. Conversion of geothermal energy into electrical energy using a supercritical cycle in the secondary circuit. Abdulagatov I.M., Alkhasov A.B. "Heat power engineering.-1988 No. 4-p. 53-56".
2. Salamov A.A. "Geothermal power plants in the energy sector of the world" Thermal power engineering 2000 No. 1-p. 79-80"
3. Heat of the Earth: From the report "Prospects for the development of geothermal technologies" Ecology and Life-2001-No. 6-str 49-52.
4. Tarnizhevsky B.V. "State and prospects for the use of renewable energy sources in Russia" Industrial Energy-2002-No. 1-p. 52-56.
5. Kuznetsov V.A. "Mutnovskaya geothermal power plant" Power stations-2002-№1-p. 31-35.
6. Butuzov V.A. "Geothermal heat supply systems in the Krasnodar Territory" Energy Manager-2002-No. 1-p.14-16.
7. Butuzov V.A. "Analysis of geothermal heat supply systems in Russia" Industrial Energy-2002-No. 6-pp. 53-57.
8. Dobrokhotov V.I. "The use of geothermal resources in the energy sector of Russia" Thermal power engineering-2003-№1-p.2-11.
9. Alkhasov A.B. "Improving the efficiency of geothermal heat use" Thermal Power Engineering-2003-No. 3-p.52-54.
Geothermal energy is energy derived from the natural heat of the earth. This heat can be achieved with the help of wells. The geothermal gradient in the well increases by 1 0C every 36 meters. This heat is delivered to the surface in the form of steam or hot water. Such heat can be used both directly for heating houses and buildings, and for the production of electricity. Thermal regions exist in many parts of the world.
According to various estimates, the temperature in the center of the Earth is at least 6,650 0C. The cooling rate of the Earth is approximately equal to 300-350 0C per billion years. The earth contains 42 x 1012 W of heat, of which 2% is contained in the crust and 98% in the mantle and core. Modern technologies do not allow reaching heat that is too deep, but 840,000,000,000 W (2%) of available geothermal energy can provide the needs of mankind for a long time. The regions around the edges of the continental plates are best place for the construction of geothermal stations, because the bark in such areas is much thinner.
Geothermal power plants and geothermal resources
The deeper the well, the higher the temperature, but in some places the geothermal temperature rises faster. Such places are usually located in areas of high seismic activity, where tectonic plates collide or break. That is why the most promising geothermal resources are located in zones of volcanic activity. The higher the geothermal gradient, the cheaper it is to extract heat, by reducing drilling and pumping costs. In the most favorable cases, the gradient may be so high that the surface water is heated to the desired temperature. Geysers and hot springs are examples of such cases.
Below the earth's crust is a layer of hot and molten rock called magma. Heat arises there, primarily due to the decay of natural radioactive elements such as uranium and potassium. The energy potential of heat at a depth of 10,000 meters is 50,000 times more energy than all the world's oil and gas reserves.
Zones of the highest underground temperatures are located in regions with active and young volcanoes. Such "hot spots" are found at tectonic plate boundaries or where the crust is so thin that heat from the magma can pass through. Many hotspots are located in the Pacific rim, which is also called the “Ring of Fire” due to a large number volcanoes.
Geothermal power plants - ways to use geothermal energy
There are two main uses for geothermal energy: direct heat and electricity generation. Direct use of heat is the simplest and therefore the most common method. The practice of direct use of heat is widespread in high latitudes at the boundaries of tectonic plates, for example in Iceland and Japan. The water supply in such cases is mounted directly in deep wells. The resulting hot water is used to heat roads, dry clothes, and heat greenhouses and residential buildings. The method of generating electricity from geothermal energy is very similar to the direct use method. The only difference is the need for a higher temperature (more than 150 0C).
In California, Nevada and some other places, geothermal energy is used in large power plants. So, in California, about 5% of electricity is generated by geothermal energy, in El Salvador, geothermal energy produces about 1/3 of electricity. In Idaho and Iceland, geothermal heat is used in a variety of applications, including home heating. Thousands of homes use geothermal heat pumps to provide clean and affordable heat.
Geothermal power plants - sources of geothermal energy.
dry heated rock– In order to use the energy in geothermal power plants contained in dry rock, water at high pressure pumped into the breed. Thus, fractures existing in the rock are expanded, and an underground reservoir of steam or hot water is created.
Magma A molten mass that forms under the Earth's crust. The temperature of the magma reaches 1200 0С. Although small volumes of magma are found at accessible depths, practical methods for generating energy from magma are under development.
Hot, pressurized groundwater containing dissolved methane. Electricity generation uses both heat and gas.
Geothermal power plants - operating principles
Currently, there are three schemes for the production of electricity using hydrothermal resources: direct using dry steam, indirect using water steam and mixed production scheme (binary cycle). The type of conversion depends on the state of the medium (steam or water) and its temperature. Dry steam power plants were the first to be mastered. To generate electricity for them, steam coming from the well is passed directly through the turbine / generator. Power plants with indirect type of electricity generation are by far the most common. They use hot underground water (up to 182°C) which is pumped at high pressure into generator sets on the surface. Mixed geothermal power plants differ from the previous two types of geothermal power plants in that the steam and water never come into direct contact with the turbine/generator.
Geothermal power plants operating on dry steam
Steam power plants operate primarily on hydrothermal steam. The steam goes directly to a turbine that feeds a generator that produces electricity. The use of steam eliminates the need to burn fossil fuels (there is also no need to transport and store the fuel). These are the oldest geothermal power plants. The first such power plant was built in Larderello (Italy) in 1904, and it is still in operation. Steam technology is used at the Geysers power plant in Northern California, the largest geothermal power plant in the world.
Geothermal power plants on steam hydrotherms
These plants use superheated hydrotherms (temperatures above 182°C) to produce electricity. The hydrothermal solution is forced into the evaporator to reduce pressure, because of this, part of the solution evaporates very quickly. The resulting steam drives a turbine. If liquid remains in the tank, it can be evaporated in the next evaporator for even more power.
Geothermal power plants with a binary cycle of electricity production.
Most geothermal areas contain moderate temperature water (below 200°C). Binary cycle power plants use this water to generate energy. Hot geothermal water and a second, additional liquid with a lower boiling point than water are passed through a heat exchanger. The heat from the geothermal water evaporates a second liquid, the vapors of which drive turbines. Since it is a closed system, there are practically no emissions into the atmosphere. Temperate waters are the most abundant geothermal resource, so most geothermal power plants of the future will operate on this principle.
The future of geothermal electricity.
Steam tanks and hot water are only a small part of geothermal resources. Earth's magma and dry rock will provide cheap, clean, virtually inexhaustible energy once the appropriate technologies are developed to utilize them. Until then, the most common producers of geothermal electricity will be binary cycle power plants.
For geothermal electricity to become a key element of the US energy infrastructure, methods must be developed to reduce the cost of its production. The US Department of Energy is working with representatives of the geothermal industry to reduce the cost of a kilowatt-hour to $0.03-0.05. New geothermal power plants with a capacity of 15,000 MW are predicted to appear in the next decade.
Practice #6
Target: get acquainted with the principle of operation of GeoTPP and ocean thermal energy conversion technologies (OTEC), as well as with the methodology for their calculation.
Lesson duration- 2 hours
Working process:
1. On the basis of the theoretical part of the work, get acquainted with the principle of operation of the GeoTPP and the technologies for converting the thermal energy of the ocean (PTEC.
2. In accordance with the individual task, solve practical problems.
1. THEORETICAL PART
Use of ocean thermal energy
Ocean thermal energy conversion technology (OTEC) generates electricity from the temperature difference between warm and cold ocean water. Cold water is pumped through a pipe from a depth of more than 1000 meters (from a place where the sun's rays never reach). The system also uses warm water from an area close to the surface of the ocean. The sun-heated water passes through a heat exchanger with low-boiling chemicals such as ammonia, which creates a chemical vapor that drives the turbines of power generators. The vapor is then condensed back into liquid form using chilled water from the deep ocean. Tropical regions are considered to be the best place to place PTEC systems. This is due to the greater temperature difference between water in shallow water and at depth.
Unlike wind and solar farms, ocean thermal power plants can produce clean electricity around the clock, 365 days a year. The only by-product of such power units is cold water, which can be used for cooling and air conditioning in administrative and residential buildings near the power generating facility.
Use of geothermal energy
Geothermal energy is energy derived from the natural heat of the earth. This heat can be achieved with the help of wells. The geothermal gradient in the well increases by 1°C every 36 meters. This heat is delivered to the surface in the form of steam or hot water. Such heat can be used both directly for heating houses and buildings, and for the production of electricity.
According to various estimates, the temperature at the center of the Earth is at least 6650 °C. The rate of cooling of the Earth is approximately equal to 300-350 ° C per billion years. The earth emits 42·10 12 W of heat, of which 2% is absorbed in the crust and 98% in the mantle and core. Modern technology does not allow reaching heat that is released too deeply, but even 840000000000 W (2%) of available geothermal energy can provide the needs of mankind for a long time. Areas around the edges of the continental plates are the best place to build geothermal plants because the crust in such areas is much thinner.
There are several ways to get energy at GeoTPP:
· Direct scheme: steam is sent through pipes to turbines connected to electric generators;
· Indirect circuit: similar to the direct circuit, but before entering the pipes, the steam is cleaned of gases that cause the destruction of the pipes;
· Mixed circuit: similar to the direct circuit, but after condensation, gases that have not dissolved in it are removed from the water.
2. PRACTICAL PART
Task 1. Determine initial temperature t2 and amount of geothermal energy E o (J) aquifer thickness h km at depth z km, if the characteristics of the reservoir rock are given: density p gr \u003d 2700 kg / m 3; porosity but = 5%; specific heat C gr =840 J/(kg K). temperature gradient (dT/dz) in °C / km, select according to the table of task options.
Average surface temperature t o take equal to 10 °C. Specific heat capacity of water From to = 4200 J/(kg K); density of water ρ \u003d 1 10 3 kg / m 3. Calculate with respect to surface area F \u003d 1 km 2. The minimum allowable formation temperature is taken equal to t1=40 ° C.
Determine also the time constant of extraction of thermal energy o (years) when water is injected into the reservoir and its consumption V \u003d 0.1 m 3 / (s km 2). What will be the thermal power extracted initially (dE/dz) τ =0 and after 10 years (dE/dz) τ =10?
Problem 1 is devoted to the thermal potential of geothermal energy concentrated in natural aquifers at a depth z (km) from the earth's surface. Usually the thickness of the aquifer h (km) is less than its depth. The layer has a porous structure - rocks have pores filled with water (porosity is estimated by the coefficient α). The average density of hard rocks of the earth's crust p gr =2700 kg/m 3 and the coefficient of thermal conductivity λ gr =2 W/(m·K). The change in ground temperature towards the earth's surface is characterized by a temperature gradient (dT/dz), measured in °C/km or K/km.
The most common on the globe are areas with a normal temperature gradient (less than 40 ° C / km) with a density of heat fluxes outgoing towards the surface of ≈ 0.06 W / m 2. The economic feasibility of extracting heat from the bowels of the Earth is unlikely here.
In semi-thermal areas, the temperature gradient is 40-80 °C/km. Here it is advisable to use the heat of the bowels for heating, in greenhouses, in balneology.
In hyperthermal areas (near the boundaries of the platforms of the earth's crust) the gradient is more than 80 °C/km. It is expedient to build a GeoTPP here.
With a known temperature gradient, it is possible to determine the temperature of the aquifer before the start of its operation:
T g \u003d T o + (dT / dz) z,
where T o is the temperature on the Earth's surface, K (° C).
In calculation practice, the characteristics of geothermal energy are usually referred to 1 km 2 of the surface F.
The heat capacity of the reservoir C pl (J / K) can be determined by the equation
C pl \u003d [α ρ in C in + (1- α) ρ gr C gr ] h F,
where p in and C in are, respectively, the density and isobaric specific heat
p gr and C gr - density and specific heat capacity of the soil (formation rocks); usually p gr \u003d 820-850 J / (kg K).
If you set the minimum allowable temperature at which you can use the thermal energy of the reservoir T 1 (K), then you can estimate its thermal potential by the start of operation (J):
E 0 \u003d C pl (T 2 -T 1)
The time constant of the reservoir τ 0 (possible time of its use, years) in the case of removal of thermal energy by pumping water into it with a volumetric flow rate V (m 3 / s) can be determined by the equation:
τ 0 \u003d C pl / (V ρ in C in)
It is believed that the thermal potential of the reservoir during its development changes according to the exponential law:
E=E 0 e -(τ / τ o)
where τ is the number of years since the start of operation;
e is the base of natural logarithms.
Thermal power of a geothermal reservoir at time τ (years from the start of development) in W (MW):
Task 2 It is believed that the actual efficiency η oceanic thermal power plant, using the temperature difference of surface and deep waters (T 1 -T 2) = ∆T and operating according to the Rankine cycle, is half the thermal efficiency of the plant operating according to the Carnot cycle, η t k . Estimate the possible value of the actual efficiency of the OTES, the working fluid of which is ammonia, if the water temperature on the ocean surface t , °С, and the water temperature at the depth of the ocean t2 , °С. What is the consumption of warm water V , m/h will be required for OTES with a capacity of N MW?
Task 2 is devoted to the prospects of using the temperature difference between the surface and deep ocean waters to generate electricity at the OTES operating according to the well-known Rankine cycle. As a working fluid, the use of low-boiling substances (ammonia, freon) is supposed. Due to small temperature differences (∆T=15÷26 o C), the thermal efficiency of a plant operating according to the Carnot cycle is only 5-9%. The real efficiency of a plant operating on the Rankine cycle will be half that. As a result, in order to obtain a share of relatively small capacities at OTES, large consumptions of "warm" and "cold" water are required and, consequently, huge diameters of inlet and outlet pipelines.
Q 0 =p V C p ∆T,
where p is the density of sea water, kg / m 3;
C p - mass heat capacity of sea water, J / (kg K);
V - volumetric water flow, m 3 / s;
∆T \u003d T 1 -T 2 - temperature difference between surface and deep waters
(cycle temperature difference) in °C or K.
In an ideal theoretical Carnot cycle, the mechanical power N 0 (W) can be defined as
N 0 \u003d η t k Q o,
or taking into account (1) and the expression for the thermal efficiency of the Carnot cycle η t k:
N 0 \u003d p C p V (∆T) 2 /T 1.
Task 3 Double-circuit steam-water geothermal power plant with electric power N receives heat from water from geothermal wells with a temperature t gs . Dry saturated steam at the outlet of the steam generator has a temperature 20 0 C lower than t gs . The steam expands in the turbine and enters the condenser, where it is cooled by water from environment with temperature t xv . The cooling water is heated in the condenser by 12 0 C. The condensate has a temperature 20 0 C higher than t xv . Geothermal water leaves the steam generating plant at a temperature 15 0 C higher than the condensate. Relative turbine internal coefficient η oi , electrical efficiency of the turbogenerator η e =0.96. Determine the thermal efficiency of the Rankine cycle, steam flow and specific heat flow, water flow from geothermal wells and from the environment.
In a single-circuit steam turbine GeoTEP, the enthalpy of dry saturated steam after separation is determined by the temperature of geothermal water t gw. From tables of thermodynamic properties of water and water vapor or h-s diagrams. In the case of a double-circuit GeoTEU, the temperature difference in the steam generator Δt is taken into account. Otherwise, the calculation is carried out as for a solar steam turbine TPP.
Steam consumption is determined from the ratio
kg/s,
where η t is the thermal efficiency of the cycle,
η оі - Relative internal efficiency of the turbine,
η e is the electric efficiency of the turbogenerator,
N is the power of the GeoTEU, kW,
The flow rate of hot water from geothermal wells is determined from the formula
, kg/s,
consumption of cold water from the environment for steam condensation
, kg/s,
where c = 4.19 kJ/kg∙K is the heat capacity of water,
η pg is the efficiency of the steam generator,
Δt pg – temperature difference of geothermal water in the steam generator, 0 С,
Δt xv - temperature drop of cold water in the condenser, 0 C.
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.
| Quantities and their units | Task options | |||||||||
| N, MW | ||||||||||
| t min., 0 С | ||||||||||
| t min., 0 С | ||||||||||
| η oi , % |
