Calculation of the thermal scheme of geoelectric power plants. Abstract: Geothermal energy. Geothermal power plants using hydrothermal steam
Currently, geothermal energy is used in 51 countries in electricity generation technologies. Over five years (from 2010 to 2015), the total capacity of geothermal power plants increased by 16% and amounted to 12,635 MW. A significant increase in the capacity of geothermal power plants is due to environmental safety, significant economic efficiency and high performance use of installed capacity.
Today, geothermal power plants (GEPs) are operated in 26 countries with an annual electricity generation of approximately 73,549 GW. The expected growth in the installed capacity of geothermal power plants by 2020 is about 21,443 MW (Fig. 1). The United States has significant indicators in the field of geothermal energy: the total installed capacity of geothermal power plants is 3450 MW with an annual electricity generation of 16.6 MW/h. The Philippines is in second place with a total geoelectric power capacity of 1870 MW, and Indonesia is in third place with 1340 MW. At the same time, the most significant increase in GeoPP capacity over the past five years has been noted in Turkey - from 91 to 397 MW, that is, by 336%. Next come Germany - by 280% (from 6.6 to 27 MW) and Kenya - by 194% (from 202 to 594 MW).
In modern geothermal energy, the most common are GeoPPs with a thermal design of a turbine installation, including additional expansion of geothermal steam, the total capacity of which is 5079 MW. GeoPP power units with a total capacity of 2863 MW operate on superheated geothermal steam. The total capacity of GeoPP power units with two stages of steam expansion is 2544 MW.
Geothermal binary power units with an organic Rankine cycle are becoming increasingly widespread, and today their total capacity exceeds 1800 MW. The average unit power of binary power units is 6.3 MW, power units with one separation pressure is 30.4 MW, with two separation pressures is 37.4 MW, and power units operating on superheated steam is 45.4 MW.
The main increase in the installed capacity of modern geothermal power plants in the world in recent years has been achieved largely due to the construction of new GeoPPs with binary cycle power units.
Technological schemes of modern GeoPPs can be classified according to the phase state of the geothermal coolant, the type of thermodynamic cycle and the turbines used (Fig. 2). Geothermal power plants operate on geothermal coolant in the form of superheated steam, steam-water mixture and hot water. The direct cycle of GeoPP is characterized by its use throughout the entire technological path as working environment geothermal coolant.
GeoPPs with a binary cycle are mainly used in fields with low temperature hot water(90-120 °C), which are characterized by the use of a low-boiling working fluid in the second circuit. Double-circuit GeoPPs involve the use of binary and combined binary cycles. In a combined cycle GeoPP, a steam turbine operates on geothermal steam, and the heat of waste or waste geothermal coolant in the form of a liquid phase is recovered in a secondary circuit binary power plant.
Condensing turbines of single-circuit GeoPPs operate on geothermal superheated steam, as well as on saturated steam separated from the steam-water mixture. Backpressure turbines are used in single-circuit geothermal power plants, which, along with generating electricity, provide heat to heating systems.
Currently in Russia, power units with back-pressure turbines are operated on the islands of Kunashir and Iturup (part of the Kuril ridge). The Omega-500, Tuman-2.0 and Tuman-2.5 power units were developed at the Kaluga Turbine Plant.
Back-pressure turbo units are much simpler in design than condensing units, so their price is significantly lower.
Quite often used technological schemes single-circuit GeoPPs with one, two and three separation pressures, the so-called SingleFlash, Double-Flash and Triple-Flash schemes, respectively. Thus, GeoPPs with two and three separation pressures involve the use of additional secondary steam obtained in the expander due to boiling of the separator. This makes it possible to increase the use of geothermal fluid heat compared to GeoPPs with one separation pressure.
Geothermal steam turbine units are produced by companies in Japan, the USA, Italy and Russia.
In table 1 presents the main manufacturers of modern steam turbine units and equipment for geothermal power plants. The design of geothermal turbines has a number of features that are due to the use of low-potential geothermal saturated steam as a working medium, which is characterized by corrosive aggressiveness and a tendency to form deposits.
Modern advanced technologies for increasing the efficiency of geothermal turbines include:
- intra-channel moisture separation in the turbine flow section, including peripheral moisture separation, moisture removal through slots in hollow nozzle blades and a separator stage;
- systems for periodic washing of the flow part and end seals on an operating turbine;
- application of technology for controlling the physical and chemical properties of geothermal coolant using surfactant additives;
- reduction of losses in turbine cascades by optimizing the geometry of nozzles and working blades, including the use of highly efficient saber-shaped blades.
Thus, in the design of the geothermal steam turbine of JSC KTZ with a capacity of 25 MW for the Mutnovskaya GeoPP, special devices for moisture separation are used, which make it possible to remove up to 80% of the liquid phase in the form of large drops and liquid films from the flow part. Starting from the fourth turbine stage, a developed system of peripheral moisture separation is used in the flow section. In the seventh and eighth stages of both turbine flows, in-channel moisture separation in the nozzle grids is used. Enough effective method moisture removal is the use of a special turbine separator stage, which makes it possible to increase the turbine efficiency by almost 2%.
The salt content of steam entering the flow path of GeoPP turbines depends on the mineralization of the initial geothermal fluid and the efficiency of phase separation in separation devices. The efficiency of separation devices largely determines the degree of entrainment of the turbine flow path with scale deposits, and also affects the intensity of droplet impact erosion of turbine blades and corrosion cracking of the metal elements of the turbine flow passage.
In the technological schemes of modern geothermal power plants, vertical and horizontal separators are used. Vertical separators are used mainly at GeoPPs built with the participation of New Zealand specialists in New Zealand, the Philippines and other countries. Horizontal separators are used in geothermal power units in Russia, the USA, Japan and Iceland. Moreover, up to 70% of GeoPPs in the world operate with vertical separators. Vertical separators are capable of providing an average dryness of steam at the outlet of up to 99.9%. Moreover, their efficiency significantly depends on operating parameters: flow and pressure of wet steam, moisture content of the steam-water mixture (SWM), liquid level in the separator, etc.
In Russia, horizontal separators have been developed and are being used at GeoPP power units, which are characterized by high efficiency and small-sized characteristics. The degree of steam dryness at the separator outlet reaches 99.99%. These developments were based on the research and technology of enterprises producing equipment for nuclear power plants, shipbuilding and other industries. Such separators are installed and successfully operate in the modular power units of the Verkhne-Mutnovskaya GeoPP and at the first stage of the Mutnovskaya GeoPP (Fig. 3).
The advantage of binary plants, which consists primarily in the ability to produce electricity based on a low-temperature heat source, has largely determined the main directions of their application. It is especially advisable to use binary settings for:
- energy supply (also autonomous) to regions with low-temperature geothermal resources;
- increasing the capacity of existing GeoPPs operating on high-temperature geothermal coolant, without drilling additional wells;
- increasing the efficiency of using geothermal sources through the use of binary units in technological schemes of newly designed combined geothermal power plants.
Thermophysical, thermodynamic and other properties of organic low-boiling substances have a significant impact on the type and efficiency of the thermal cycle, technological parameters, design and characteristics of equipment, operating modes, reliability and environmental friendliness of binary plants.
In practice, about 15 different low-boiling organic substances and mixtures are used as the working fluid of binary plants. In fact, at present, geothermal binary power units mainly operate on hydrocarbons - about 82.7% of the total installed capacity of binary power units in the world, fluorocarbons - 6.7%, chlorofluorocarbons - 2.0%, water-ammonia mixture - 0.5 %, no data on the working fluid for 8.2%.
Combined binary cycle geothermal power plants are distinguished by the fact that the geothermal fluid from the primary circuit not only provides heat to the secondary circuit, but is also directly used to convert the heat into mechanical work in the steam turbine.
The steam phase of the geothermal two-phase coolant is used directly to generate electrical energy by expansion in a steam turbine with back pressure, and the condensation heat of the geothermal steam (as well as the separator) is sent to the second low temperature circuit, in which an organic working fluid is used to generate electricity. The use of such a combined GeoPP scheme is especially advisable in cases where the source geothermal fluid contains a large amount of non-condensable gases, since the energy costs for removing them from the condenser can be significant.
The results of thermodynamic calculations show that, under all equal initial conditions, the use of a binary power unit in combined cycle geothermal power plants can increase the capacity of a Single-Flash GeoPP by 15%, and a DoubleFlash GeoPP by 5%. Currently, binary plants are produced at factories in the USA, Germany, Italy, Sweden, Russia and other countries. Information on some technical characteristics of binary installations produced by various manufacturers is presented in table. 2.
In Fig. Figure 4 presents data on the cost of installed power of 1 kW for the construction of various GeoPPs with turbine units using geothermal steam and low-boiling organic working fluid, indicating the dependence of the cost of GeoPP on the cycle used and the temperature of the geothermal geofluid.
The most promising Russian geothermal energy projects are the expansion of the Mutnovskaya GeoPP (50 MW) and the Verkhne-Mutnovskaya GeoPP (12 MW) with combined (binary cycle) power units with a capacity of 10 and 6.5 MW, respectively, due to the recovery of heat from their waste coolant without drilling additional wells, as well as the construction of the second stage of the Mutnovskaya GeoPP with a capacity of 50 MW.
Conclusions
1.
In the global geothermal energy industry, technological schemes with GeoPPs of direct, binary and combined cycles are used - depending on the phase state and temperature of the geothermal coolant.
2.
The main increase in the total installed capacity of GeoPPs in the world in recent years is due to the development of binary geothermal energy technologies.
3.
The specific cost of the installed capacity of geothermal power units significantly depends on the temperature of the geothermal coolant and sharply decreases with its increase.
GEOTHERMAL ENERGY
Skotarev Ivan Nikolaevich
2nd year student, department physicists SSAU, Stavropol
Khashchenko Andrey Alexandrovich
scientific supervisor, can. physics and mathematics sciences, Associate Professor, St. State Agrarian University, Stavropol
Nowadays humanity doesn’t think much about what it will leave to future generations. People mindlessly pump and dig up minerals. Every year the population of the planet is growing, and therefore the need for more more energy carriers such as gas, oil and coal. This cannot continue for long. Therefore, now, in addition to the development of the nuclear industry, the use of alternative energy sources is becoming relevant. One of promising directions in this area is geothermal energy.
Most of the surface of our planet has significant reserves of geothermal energy due to significant geological activity: active volcanic activity in the initial periods of the development of our planet and also to this day, radioactive decay, tectonic shifts and the presence of magma areas in earth's crust. In some places on our planet, especially a lot of geothermal energy accumulates. These are, for example, various valleys of geysers, volcanoes, underground accumulations of magma, which in turn heat the upper rocks.
Speaking in simple language Geothermal energy is the energy of the Earth's interior. For example, volcanic eruptions clearly indicate the enormous temperature inside the planet. This temperature gradually decreases from the hot inner core to the Earth's surface ( figure 1).
Figure 1. Temperature in different layers of the earth
Geothermal energy has always attracted people due to its potential. useful application. After all, man in the process of his development came up with many useful technologies and looked for profit and profit in everything. This is what happened with coal, oil, gas, peat, etc.
For example, in some geographic areas, the use of geothermal sources can significantly increase energy production, since geothermal power plants (GEP) are one of the cheapest alternative energy sources because the upper three-kilometer layer of the Earth contains over 1020 J of heat suitable for generating electricity. Nature itself gives a person a unique source of energy; it is only necessary to use it.
There are currently 5 types of geothermal energy sources:
1. Geothermal dry steam deposits.
2. Sources of wet steam. (a mixture of hot water and steam).
3. Geothermal water deposits (contain hot water or steam and water).
4. Dry hot rocks heated by magma.
5. Magma (molten rocks heated to 1300 °C).
Magma transfers its heat to rocks, and their temperature rises with increasing depth. According to available data, the temperature of rocks increases on average by 1 °C for every 33 m of depth (geothermal step). There is a wide variety of geothermal energy temperature conditions around the world that will determine technical means for its use.
Geothermal energy can be used in two main ways - to generate electricity and to heat various objects. Geothermal heat can be converted into electricity if the coolant temperature reaches more than 150 °C. It is precisely the use of the internal regions of the Earth for heating that is the most profitable and effective and also very affordable. Direct geothermal heat, depending on the temperature, can be used for heating buildings, greenhouses, swimming pools, drying agricultural and fish products, evaporating solutions, growing fish, mushrooms, etc.
All geothermal installations existing today are divided into three types:
1. stations, the basis for which are dry steam deposits - this is a direct scheme.
Dry steam power plants appeared earlier than anyone else. In order to obtain the required energy, steam is passed through a turbine or generator ( figure 2).
Figure 2. Geothermal power plant of direct circuit
2. stations with a separator using hot water deposits under pressure. Sometimes a pump is used for this, which provides required volume incoming energy carrier - indirect scheme.
This is the most common type of geothermal plant in the world. Here the waters are pumped under high pressure to generator sets. The hydrothermal solution is pumped into the evaporator to reduce the pressure, resulting in the evaporation of part of the solution. Next, steam is formed, which makes the turbine work. The remaining liquid may also be beneficial. Usually it is passed through another evaporator to obtain additional power ( figure 3).
Figure 3. Indirect geothermal power plant
They are characterized by the absence of interaction between the generator or turbine and steam or water. The principle of their operation is based on reasonable use underground water moderate temperature.
Typically the temperature should be below two hundred degrees. The binary cycle itself consists of using two types of water - hot and moderate. Both streams are passed through a heat exchanger. The hotter liquid evaporates the colder one, and the vapors formed as a result of this process drive the turbines.
Figure 4. Schematic of a geothermal power plant with a binary cycle.
As for our country, geothermal energy ranks first in terms of potential possibilities for its use due to the unique landscape and natural conditions. The discovered reserves of geothermal waters with temperatures from 40 to 200 °C and a depth of up to 3500 m on its territory can provide approximately 14 million m3 of hot water per day. Large reserves of underground thermal waters are found in Dagestan, North Ossetia, Checheno-Ingushetia, Kabardino-Balkaria, Transcaucasia, Stavropol and Krasnodar territories, Kazakhstan, Kamchatka and a number of other regions of Russia. For example, in Dagestan, thermal waters have been used for heat supply for a long time.
The first geothermal power plant was built in 1966 at the Pauzhetsky field on the Kamchatka Peninsula to supply electricity to surrounding villages and fish processing plants, thereby promoting local development. Local geothermal system can provide energy to power plants with a capacity of up to 250-350 MW. But this potential is only used by a quarter.
The territory of the Kuril Islands has a unique and at the same time complex landscape. Power supply to the cities located there comes with great difficulties: the need to deliver means of subsistence to the islands by sea or air, which is quite expensive and takes a lot of time. Geothermal resources of the islands at the moment allow you to receive 230 MW of electricity, which can meet all the region’s needs for energy, heat, and hot water supply.
On the island of Iturup, resources of a two-phase geothermal coolant have been found, the power of which is sufficient to meet the energy needs of the entire island. On the southern island of Kunashir there is a 2.6 MW GeoPP, which is used to generate electricity and heat supply to the city of Yuzhno-Kurilsk. It is planned to build several more GeoPPs with a total capacity of 12-17 MW.
The most promising regions for the use of geothermal sources in Russia are the south of Russia and Far East. The Caucasus, Stavropol, and Krasnodar Territories have enormous potential for geothermal energy.
The use of geothermal waters in the central part of Russia requires high costs due to the deep occurrence of thermal waters.
IN Kaliningrad region There are plans to implement a pilot project for geothermal heat and power supply to the city of Svetly based on a binary GeoPP with a capacity of 4 MW.
Geothermal energy in Russia is focused both on the construction of large facilities and on the use of geothermal energy for individual homes, schools, hospitals, private shops and other facilities using geothermal circulation systems.
In the Stavropol Territory, at the Kayasulinskoye field, the construction of an expensive experimental Stavropol Geothermal Power Plant with a capacity of 3 MW was started and suspended.
In 1999, the Verkhne-Mutnovskaya GeoPP was put into operation ( figure 5).
Figure 5. Verkhne-Mutnovskaya GeoPP
It has a capacity of 12 MW (3x4 MW) and is a pilot stage of the Mutnovskaya GeoPP with a design capacity of 200 MW, created to supply power to the industrial region of Petropavlovsk-Kamchatsk.
But despite the great advantages in this direction, there are also disadvantages:
1. The main one is the need to pump waste water back into the underground aquifer. Thermal waters contain large amounts of salts of various toxic metals (boron, lead, zinc, cadmium, arsenic) and chemical compounds(ammonia, phenols), which makes it impossible to discharge these waters into natural water systems located on the surface.
2. Sometimes an operating geothermal power plant may stop working as a result of natural changes in the earth's crust.
3. Find suitable place to build a geothermal power plant and obtaining permission from local authorities and consent from residents for its construction can be problematic.
4. The construction of a GeoPP may negatively affect land stability in the surrounding region.
Most of these shortcomings are minor and completely solvable.
In today's world, people do not think about the consequences of their decisions. After all, what will they do if they run out of oil, gas and coal? People are used to living in comfort. They won’t be able to heat their houses with wood for a long time, because a large population will need a huge amount of wood, which will naturally lead to large-scale deforestation and leave the world without oxygen. Therefore, in order to prevent this from happening, it is necessary to use the resources available to us sparingly, but with maximum efficiency. Just one way to solve this problem is the development of geothermal energy. Of course, it has its pros and cons, but its development will greatly facilitate the continued existence of humanity and will play a big role in its further development.
Now this direction is not very popular, because the world is dominated by oil and gas industry and large companies are in no hurry to invest in the development of a much-needed industry. Therefore, for the further progress of geothermal energy, investments and government support are necessary, without which it is simply impossible to implement anything on a national scale. The introduction of geothermal energy into the country's energy balance will allow:
1. increase energy security, on the other hand, reduce the harmful impact on the environment compared to traditional sources.
2. develop the economy, because the released funds can be invested in other industries, social development states, etc.
In the last decade, the use of non-traditional renewable energy sources has experienced a real boom in the world. The scale of use of these sources has increased several times. It is capable of radically and on the most economic basis solving the problem of energy supply to these areas, which use expensive imported fuel and are on the verge of an energy crisis, improve the social situation of the population of these areas, etc. This is exactly what we see in Western European countries (Germany, France, Great Britain), Northern Europe (Norway, Sweden, Finland, Iceland, Denmark). This is explained by the fact that they have high economic development and are very dependent on fossil resources, and therefore the heads of these states, together with business, are trying to minimize this dependence. In particular, the development of geothermal energy in the Northern European countries is favored by the presence of a large number of geysers and volcanoes. It’s not for nothing that Iceland is called the country of volcanoes and geysers.
Now humanity is beginning to understand the importance of this industry and is trying to develop it as much as possible. The use of a wide range of diverse technologies makes it possible to reduce energy consumption by 40-60% and at the same time ensure real economic development. And the remaining needs for electricity and heat can be met through more efficient production, through restoration, through combining the production of thermal and electrical energy, as well as through the use of renewable resources, which makes it possible to abandon certain types of power plants and reduce emissions carbon dioxide by about 80%.
References:
1.Baeva A.G., Moskvicheva V.N. Geothermal energy: problems, resources, use: ed. M.: SO AN USSR, Institute of Thermophysics, 1979. - 350 p.
2.Berman E., Mavritsky B.F. Geothermal energy: ed. M.: Mir, 1978 - 416 pp.
3.Geothermal energy. [ Electronic resource] - Access mode - URL: http://ustoj.com/Energy_5.htm(access date 08/29/2013).
4. Geothermal energy in Russia. [Electronic resource] - Access mode - URL: http://www.gisee.ru/articles/geothermic-energy/24511/(date of access: 09/07/2013).
5. Dvorov I.M. Deep heat of the Earth: ed. M.: Nauka, 1972. - 208 p.
6.Energy. Material from Wikipedia - the free encyclopedia. [Electronic resource] - Access mode - URL: http://ru.wikipedia.org/wiki/Geothermal_energy(date of access: 09/07/2013).
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 spent in the steam generator is pumped by pump 3 into the return well 2. Dry cleaning turbine plant feed water is being conventional methods. Feed pump 8 returns condensate from condenser 7 to the steam generator.
In a double-circuit installation, there are no non-condensable gases in the steam circuit, therefore a deeper vacuum is ensured in the condenser and the thermal efficiency of the installation increases compared to a single-circuit one. At the exit from 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 diagram double-circuit geothermal power plant
Gases, including hydrogen sulfide, are supplied from the steam generator to the bubble absorber and dissolved in the waste geothermal water, after which it is pumped into the disposal well. According to tests at the Ocean Geothermal Power Plant 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 live steam in a double-circuit installation h 1 compared to a single-circuit one, however, in general, the heat difference in the turbine increases due to a decrease in the enthalpy of 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 plants).
The consumption of hot water from geothermal wells for an installation with a capacity 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 overall efficiency of modern double-circuit steam turbine geothermal power plants is 17.27%.
In fields with relatively low temperatures of geothermal waters (100-200°C), double-circuit plants using low-boiling working fluids (freons, hydrocarbons) are used. It is also economically justifiable to use such installations for recycling the heat of separated water from single-circuit geothermal power plants (instead of the district heating heat exchanger in Fig. 4.1). In our country, for the first time in the world (in 1967), a power plant of this type was created using R-12 refrigerant with a capacity of 600 kW, built at the Paratunsky geothermal field (Kamchatka) under the scientific leadership of the Institute of Thermophysics of the Siberian Branch of the USSR Academy of Sciences. The coolant temperature difference was 80...5 o 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.
Currently, JSC "Kirovsky Plant" has developed the design and technical documentation of a double-circuit geothermal module with a capacity of 1.5 MW using freon R142v (reserve coolant - isobutane). The energy module will be completely 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 mass production of power modules will be reduced to approximately $800 per kilowatt of installed capacity.
Along with GeoTES using a homogeneous low-boiling coolant, ENIN is developing a promising installation using a mixed water-ammonia working fluid. The main advantage of such an installation is the possibility of its use in a wide range of temperatures of geothermal waters and steam-water mixtures (from 90 to 220 o C). With a homogeneous working fluid, a deviation of the temperature at the outlet of the steam generator by 10...20 o C from the calculated one leads to a sharp reduction in efficiency cycle - 2.4 times. By changing the concentration of the components of the mixed coolant, it is possible to ensure acceptable installation performance at changing temperatures. The power of the ammonia water turbine in this temperature range varies by less than 15%. In addition, such a turbine has better 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 using a homogeneous coolant. Such power plants can be widely used for waste heat recovery in industry. They may have a strong demand in the international geothermal equipment market.
Calculation of GeoTEI with low-boiling and mixed working fluids is carried out using tables of thermodynamic properties and h - s diagrams of vapors of these liquids.
Related to the problem of geothermal power plants is the possibility of using the thermal resources of the World Ocean, which is often mentioned in the literature. 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 produce electricity. Scheme installations for one of the projects for implementing this idea are shown in Fig. 4.3.
Fig.4.3. Scheme 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 into steam generator 2, where the low-boiling coolant evaporates. Steam with a temperature of about 20° C is sent to turbine 3, which drives electric generator 4. The exhaust steam enters condenser 5 and is condensed by cold deep water supplied by circulation pump 6. Feed pump 7 returns the coolant to the steam generator.
When rising through warm surface layers, deep water heats up to at least 7...8° C, respectively, the exhausted wet coolant steam 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, ocean thermal power plants are characterized by high energy costs for their own needs; very large costs of warm and cold water, as well as the coolant, 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 produce 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. Low energy density is an obstacle to the use of ocean heat reserves.
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3.4 CALCULATION OF GEOTHERMAL POWER PLANT
Let us calculate the thermal circuit of a binary type geothermal power plant, according to.
Our geothermal power plant consists of two turbines:
The first operates on saturated water vapor obtained in an expander. Electrical power - ;
The second one operates on saturated vapor of refrigerant R11, which evaporates due to the heat of water removed from the expander.
Water from geothermal wells with pressure pgw and temperature tgw enters the expander. The expander produces dry saturated steam with a pressure of pp. This steam is sent to a steam turbine. The remaining water from the expander goes to the evaporator, where it is cooled and ends back into the well. Temperature difference in evaporation plant= 20°C. The working fluids expand in turbines and enter condensers, where they are cooled with water from the river at temperature thw. Heating of water in the condenser = 10°C, and subheating to saturation temperature = 5°C.
Relative internal efficiencies of turbines. Electromechanical efficiency of turbogenerators = 0.95.
The initial data is given in Table 3.1.
Table 3.1. Initial data for calculating GeoPP
Schematic diagram of a binary type GeoPP (Fig. 3.2).
Rice. 3.2. Schematic diagram of GeoPP.
According to the diagram in Fig. 3.2 and the initial data we carry out calculations.
Calculation of the circuit of a steam turbine operating on dry saturated water steam
Steam temperature at the turbine condenser inlet:
where is the temperature of the cooling water at the condenser inlet; - heating water in the condenser; - temperature difference in the condenser.
The steam pressure in the turbine condenser is determined from tables of properties of water and water steam:
Available heat drop per turbine:
where is the enthalpy of dry saturated steam at the turbine inlet; - enthalpy at the end of the theoretical process of steam expansion in the turbine.
Steam consumption from the expander to the steam turbine:
where is the relative internal efficiency of the steam turbine; - electromechanical efficiency of turbogenerators.
Geothermal water expander calculation
Equation heat balance expander
where is the flow rate of geothermal water from the well; - enthalpy of geothermal water from a well; - water flow from the expander to the evaporator; - enthalpy of geothermal water at the exit from the expander. It is determined from tables of properties of water and water vapor as the enthalpy of boiling water.
Expander Material Balance Equation
By solving these two equations together, it is necessary to determine and.
The temperature of geothermal water at the outlet of the expander is determined from the tables of the properties of water and water vapor as the saturation temperature at the pressure in the expander:
Determination of parameters at characteristic points of the thermal circuit of a turbine operating in freon
Freon vapor temperature at the turbine inlet:
Freon vapor temperature at the turbine outlet:
The enthalpy of freon vapor at the turbine inlet is determined from the p-h diagram for freon on the saturation line at:
240 kJ/kg.
The enthalpy of the freon vapor at the outlet of the turbine is determined from the p-h diagram for the freon at the intersection of the lines and the temperature line:
220 kJ/kg.
The enthalpy of the boiling freon at the outlet of the condenser is determined from the p-h diagram for the freon on the curve for the boiling liquid by temperature:
215 kJ/kg.
Evaporator calculation
Geothermal water temperature at the evaporator outlet:
Evaporator heat balance equation:
where is the heat capacity of water. Take =4.2 kJ/kg.
From this equation it is necessary to determine.
Calculation of the power of a turbine operating on freon
where is the relative internal efficiency of the freon turbine; - electromechanical efficiency of turbogenerators.
Determining pump power for pumping geothermal water into a well
where is the pump efficiency, assumed to be 0.8; - average specific volume of geothermal water.
Electric power of GeoPP
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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 produce electricity using various schemes at geothermal power plants (GEP). The most easily feasible scheme is the one using steam of liquids having a low boiling point. Hot water from natural sources, heating such a liquid in an 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 water or freon ( A); cycle with two working fluids – water and freon ( b); direct steam cycle ( V) and double-circuit cycle ( G).
Technologies for the production of electrical energy largely depend on the thermal potential of thermal waters.
Drawing. 1 - Examples of organizing a cycle for electricity production:
I – geothermal source; II – turbine cycle; III – cooling water
High-potential deposits allow the use of almost 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 (GeoPP) using the heat of a hot underground source.
Water from a hot spring with a temperature of about 95 °C is supplied 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 was previously 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 is condensed in condenser 6, cooled with water at normal temperature.
Figure 2-. Scheme of a small GeoPP:
1 – hot water receiver; 2 – hot water pump; 3 – gas remover;
4 – evaporator; 5 – steam turbine with current generator; 6 – capacitor; 7 – circulation pump; 8 – cooling water receiver
Such simple installations operated in Africa already in the 50s.
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.
Important feature This circuit is able to work in winter with low condensation temperatures. This temperature can be close to zero or even negative, since all of the listed substances have very low temperatures freezing. This allows you to significantly expand the temperature limits used in the cycle.
Drawing 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 substance heater
Geothermal power plant With direct using natural steam.
The simplest and most affordable geothermal power plant is a steam turbine plant with back pressure. Natural steam from the well is supplied directly to the turbine and then released into the atmosphere or into a device that captures valuable chemicals. The backpressure turbine can be supplied with secondary steam or steam obtained from the 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 are minimal. It occupies a small area and requires almost no auxiliary equipment and can be easily adapted as a portable geothermal power plant (Figure 4).
Figure 4 - Scheme of a geothermal power plant with direct use of natural steam:
1 – well; 2 – turbine; 3 – generator;
4 – exit to the atmosphere or to a chemical plant
The considered scheme may be the most profitable for those areas where there are sufficient reserves of natural steam. Rational operation provides the opportunity efficient work such an installation even with variable well flow rates.
There are several such stations operating in Italy. One of them is with a power of 4 thousand kW at specific consumption steam about 20 kg/s or 80 t/h; the other has 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.
Steam from the well is supplied to the turbine. Spent in the turbine, it enters the mixing condenser. A mixture of cooling water and condensate of 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 flows into the condenser (Figure 5).
Many geothermal power plants operate according to this scheme with some modifications: Larderello-2 (Italy), Wairakei (New Zealand), etc.
Application area double-circuit power plants using 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 at hydrothermal steam deposits.
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 (GeoTES).
The simplest geothermal power plant scheme vacuum type for using the heat of hot water with temperatures 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 accumulator tank 2. In the tank, it is freed from gases dissolved in it and 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 vacuum turbine 5, and the remaining water is pumped by pump 4 into the heat supply system. The steam exhausted in the turbine is discharged into mixing condenser 7. To remove air from the condenser, a vacuum pump 10. A mixture of cooling water and exhaust steam condensate is taken from the condenser by pump 8 and sent for cooling to the ventilation cooling tower 9. The water cooled in the cooling tower is supplied to the condenser by gravity due to vacuum.
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 power to the Petropavlovsk-Kamchatsky industrial region.
Figure 6 -. Diagram of a vacuum geothermal power plant 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, only separated geothermal steam from the steam-water mixture obtained from geothermal wells is used in steam turbines. A large amount of geothermal water (about 80 total consumption of PVA) with a temperature of 120 °C is discharged into the spawning river Ozernaya, which leads not only to the loss of the thermal potential of the geothermal coolant, but also significantly worsens the ecological condition 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 coolant consumer with a higher high temperature,. Thermodynamically, a heat pump is an inverted refrigeration machine. If in refrigeration machine the main goal is to produce cold by extracting heat from any volume by an evaporator, and the condenser discharges heat into the environment, then in a heat pump the picture is the opposite (Figure 7). The condenser is a heat exchanger that produces 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 coefficient (or heat conversion coefficient) and serves as an indicator of efficiency heat pump. This value depends on the difference in 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, 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 this region(Figure 8). To install a heat pump with a capacity of 10 kW, an earthen circuit 350-450 m long is required, for the installation of which a plot of land with an area of about 400 m² (20x20 m) will be required.
Figure 7 – Heat pump operation diagram
Figure 8 - Using soil energy as a heat source
The advantages of heat pumps include, first of all, efficiency: to transfer 1 kWh of thermal energy to the heating system, the heat pump installation needs to spend 0.2-0.35 kWh of electricity. All systems operate using closed loops and require virtually no operating costs, other than the cost of electricity required to operate the equipment, which can be obtained from wind and solar power plants. The payback period for 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.
