WO2005031122A1

WO2005031122A1 - Method for improving the coefficient of efficiency in a closed steam plant process

Info

Publication number: WO2005031122A1
Application number: PCT/FI2004/050141
Authority: WO
Inventors: Matti Nurmia
Original assignee: Cuycha Innovation Oy
Priority date: 2003-10-01
Filing date: 2004-10-01
Publication date: 2005-04-07
Also published as: FI114560B; FI20035167A0

Abstract

A method for improving the coefficient of efficiency in a closed steam plant process, in which : the steam flow of the power plant goes through a mixing and superheating process, in which its molar mass is increased by mixing (D) an additive, preferably carbon dioxide, to it; and the mixture flow obtained is superheated at a high temperature (3) and expanded in an expansion process (4, 5) producing mechanical energy, in such a way that there can be one or more superheating and expansion processes; and the mixture flow leaving the expansion process (4, 5) in a superheated state is led to a heat exchanger (6), after which most of its water vapour is condensed in a nearly reversible condensation process (7), and - the condensate leaving the said condensation process (7) is pressurized, preheated in the heat exchanger (6), evaporated (1) and returned to the said mixing process (D), and - the additive leaving the condensation process (7) in a gaseous state is compressed (8, 10) and returned to the said mixing process (D).

Description

METHOD FOR IMPROVING THE COEFFICIENT OF EFFICIENCY IN A CLOSED STEAM PLANT PROCESS

The present invention relates to a method for improving the coefficient of efficiency in a closed steam power plant process, in which method: the process includes a high-pressure superheating process using pure water vapour and which water vapour is condensed at a pressure that is substantially lower than atmospheric pressure, and a mixture flow superheated to a high temperature is formed, using mixing and superheating processes, from the water-vapour flow and the flow of an additive that remains gaseous in the process, and - the said mixture flow is expanded in an expansion process producing mechanical energy, and most of the water vapour of the mixture flow exiting from the said expansion process is condensed in a condensation process, and - the condensate leaving the said condensation process and the additive are circulated in the said closed steam plant process.

The Rankine cycle used in steam power plants starts from a supercritical pressure at a temperature of about 580°. By condensing the steam at about 30°, the thermal energy can be efficiently exploited in the range between those temperatures. As the heat released in the combustion process is available at a considerably higher temperature, much of this energy will remain unconverted to mechanical work. Now that attempts are being made to reduce carbon dioxide emissions from thermal power plants, increasing attention is being paid to this drawback.

A favourite solution is a ""topping' cycle located above the temperature range of the Rankine cycle, i.e. a peak cycle, which releases its heat to the Rankine cycle. In the commonly used combi-power plants, this topping cycle is an open gas- turbine cycle. Such a process is disclosed in, for instance, publication US-6338241.

In publication DE-36054466, the open gas-turbine cycle is replaced with a closed process, in which an inert gas, such as nitrogen or carbon dioxide, or, however, preferably a noble gas such as xenon, is used. This gas is heated indirectly and expanded and its waste heat is transferred in a heat exchanger to a second cycle, which uses water or some other medium.

In an article, (K. einzierl, New Concepts for Coal-Fired Power Stations, VGB Conference, ^"Power Plant Engineering 2000', German Federal Ministry of Economy) , K. Weinzierl discloses a process with a closed topping cycle, in which potassium vapour is used at a temperature range of 820°C - 520°C.

In many solutions, a mixture of two media is used in the topping cycle. An example of this is the STIG (Steam Injection Gas Turbine) process, in which steam is injected to the intake flow of a gas turbine operating in an open cycle. This process is disclosed in, for instance, publication US-6446440.

A condenser can be connected to such an open-cycle process using a mixed medium, as disclosed in publication US-2832194. In it, the outlet flow of a high-pressure steam turbine is led to a combustion chamber, to which air and fuel are also fed. The mixture of steam and combustion gases that arises flows through a low-pressure turbine to a condenser, which is equipped with means for removing the uncondensed gases.

In the process of publication US-4841721, 245-bar/371° water in a nearly critical state is fed to a pressure combustion process, from which the mixture of combustion gases and steam that is created is expanded in high and low-pressure turbines. In the outlet flow from the latter, the water vapour is condensed in a condenser operating at nearly normal pressure, in which the condensation heat is transferred to an auxiliary cycle using Freon-11.

In this process, the steam cycle that is converted to an open cycle acts as a topping cycle, the waste heat of which is transferred to an auxiliary cycle using another medium. As the vapour pressure of water at 24° is 30 mbar and the vapour density is only 0,02 kg/m³, the pure steam cycle cannot be extended substantially below 25°, even if cold cooling water were to be available. By using a medium with a vapour pressure higher than that of water in the auxiliary cycle, the auxiliary cycle can be extended below this limit, in which case the efficiency of the entire process will improve somewhat. This is advantageous when exploiting heat obtained at a low temperature, as in the ^ΛKalina process' of publication US-5440882 in which geothermal heat is exploited. In this, the medium used in the auxiliary cycle is two-component mixture, for example, of water and ammonia. Publication US-6347520 discloses an analogous process for exploiting the waste heat of a gas turbine.

In this connection, reference should also be made to the dual- cycle process of publication DE-19750589. In it, external heat is introduced to a first cycle, in which a medium is vaporized and condensed and the waste heat from it is transferred to a second cycle. The second cycle is a gas cycle, the waste heat from which is returned to the first cycle. The publication claims that the process disclosed can, in principle, convert thermal energy to mechanical work, without releasing waste heat to the environment.

Several of the solutions described here offer opportunities to achieve good efficiency, but their downside is the complexity and expense of dual- cycle processes and the increase in entropy occurring when transferring the large thermal flow from one cycle process to the other. In addition, many of the solutions use exotic, toxic, or highly reactive or expensive media. The only solutions of this group that are being used widely - combi power plants using an open gas-turbine cycle - require expensive additional measures when using solid fuels.

From the point of view of the present invention, the interesting solutions are those which do not use an auxiliary cycle, but in which the main cycle is instead modified with an additive. Publication US-4838027 discloses a closed process, the medium in which is a mixture of water and an additive that is more difficult to evaporate than water. Vapour consisting of both components passes through a first expansion process and is then led to a heat exchanger, in which at least part of the additive is condensed to form a first condensate. A second condensate is formed at a lower temperature and both condensates are heated in the heat exchanger. In this way, it is in principle possible to achieve a higher thermodynamic efficiency that when using pure water in the Rankine process, as the heat of evaporation of the additive can be brought to the process at a higher temperature than the heat of evaporation of water. However, the maximum temperature used in the embodiment examples is only 380° or 400°C, nor is the possibility of extending the process to higher temperatures than those used in known steam technology taken into account in the Claims or elsewhere in the publication.

A solution that is similar, but located at higher temperatures, is disclosed in patent publication US A 4196594. In it, the medium used in the power-plant process is a noble gas, such as helium or argon, to which a vapour is added, e.g., water vapour or zinc or cadmium vapour, the polytropic index (C_P/C_V) of which is lower than that of the actual medium. When the mixture expands in the power machinery, this vapour condenses and part of the heat of condensation transferring to the gas component of the mixture can be converted to mechanical work. An analo- gous process has been disclosed previously in publication US 4106294 while similar types of solution are disclosed in, for instance, publications US 4387576, US 5038567, and US 5754613.

An essential feature in these solutions is that one component of the medium undergoes an energy-releasing phase change during the expansion process. Such an expansion process is the inverse of the water-injection equipped compressor process. In the compressor process in question, water, which absorbs heat on evaporation, is added to the suction flow of the compressor, making the process nearly isothermal, i.e.. its effective polytropic index has a value close to 1. This reduces the compressor's power requirement at a given pressure ratio, compared to that of a normal, nearly isentropic compressor process. Correspondingly, in the expansion processes disclosed in the aforementioned publications, the additive added to the medium condenses, thus reducing the effective polytropic index of the medium. For a given pressure ratio, such a process will process less mechanical energy than a normal, nearly isentropic expansion process. Such an expansion process is not used in the present invention.

Publication US-54444981 discloses a closed steam cycle, to which is added a gaseous catalyst' , in the examples given hydrogen or helium, the molecular weight of which is not higher than the mean molecular weight of the medium' . The properties of the water vapour differ from the properties of an ideal gas close to the critical point of water, while the steam containing such an additive differs from an ideal gas less than pure water vapour does. In principle, this permits a higher thermodynamic efficiency to be achieved, but such a ^""catalyst' further reduces the molar mass of the medium and, in practice, prevents the use of even normal superheating temperatures in the turbine process. The addition of hydrogen to the steam cycle creates a danger of explosions and causes material problems while, if noble gases are used, their loss will lead to high costs.

As the normal Rankine cycle allows heat to be removed from the cycle in nearly ideal conditions, the use of these methods can only significantly improve the thermodynamic efficiency of the process in cases in which they permit heat to be introduced to the process at temperatures that are higher than in the normal Rankine cycle. In theory, this is possible in the process of publication US A 4196594, if the medium used is a noble gas and the additive cadmium or zinc vapour. The process can be compared to K. Weinzierl' s cascade process described above, but it suffers from even greater technical difficulties, when the poisonous zinc or cadmium vapour condenses in the expansion process .

It was stated above that special measures are required to use solid fuels in gas turbines. An example of these is the gas- turbine process of US-6148602, the medium in which is a mixture of water vapour and carbon dioxide. In it, oxygen is separated from the air and part of it is used to produce a gas from a solid fuel. The gas is fed to a burner operating at excess pressure, into which oxygen and water are also fed, the mixture of carbon dioxide and water vapour that is created being expanded in a gas turbine.

Publication US-4498289 discloses a second gas-turbine process using oxygen combustion. In it, a liquid or gaseous fuel, oxygen, and carbon dioxide are fed at a pressure of about 3000 psi (204 bar) to a first burner, from which the combustion gases flow through a first turbine to a second burner. Oxygen and a fuel are added, and the combustion gases created flow through a heat exchanger to a condenser operating at the very high pressure of 294 psi (20 bar) . The C0₂ leaving the condenser is compressed to 3000 psi and returned to the first burner.

In the last two processes referred to, the medium used is a mixture of C0₂ and water vapour, but they suffer from limitations due to the nature of the gas-turbine cycle. The most important of these is that, even though the processes have a high peak temperature, their thermodynamic efficiency remains low, due to the low pressure ratios characteristic of the Brayton cycle. The pressure ratio is typically in the order of 10...24:1, so that, for example, heat produced in the range 400...1000° must be removed from the cycle at about 400°, which leads to a thermodynamic efficiency of only 31 %. As the thermal energy is introduced to the system in its internal combustion process, the use of solid fuel will require special arrangements, such as oxygen combustion, despite the costs of an oxygen plant and its high energy consumption.

The steam cycle offers a possibility to use a higher pressure ratio, but any substantial increase in the peak temperature of the cycle is prevented by the fact that the speed of sound in steam becomes too high. In efficient steam and gas turbines, the peripheral velocity of the blades approaches the speed of sound in the medium and, when the superheating temperature of the steam rises above about 600°, the necessary peripheral velocities can no longer be achieved in known turbine technology. In addition, the supercritical pressures used in the steam cycle place such great demands on strenght of the superheater that the superheating temperatures can not be substantially increased in the high-pressure section of the cycle using existing materials.

The present invention is intended to eliminate the defects of the state of the art referred to above. The characteristic features of the invention are stated in the accompanying Claims. According to the invention, a power-plant process using a closed Rankine cycle equipped with a parallel cycle is used, in which the speed of sound in the medium is reduced by mixing an additive, preferably carbon dioxide, to the steam. The advantageousness of carbon dioxide and other gases with two or more atoms is due to the fact that their rotational and vibrational degrees of freedom are activated at high temperatures, leading to an increase in the specific heat of the gases in question. By using such a mixture, the mean pressure stage of the cycle can be substantially increased from the values used in a normal Rankine cycle. As a result, the temperature of the exit flow of the low-pressure turbine will rise correspondingly while a reduction of the thermodynamic efficiency of the process is prevented by transferring this heat to the feed water of the process. By selecting the mixture ratio of the steam and additive in such a way that thermal capacity flow of the exitflow is nearly the same as that of the condensed water flow arising from it, this preheating is thermodynamically more advantageous than preheating of the turbine feed water using bleed steam from the turbines, as used in the prior art. This heat transfer can also be limited to only part of the said outflow, or part of the said condensed water flow, so that the mixture ratio can be used as the degree of freedom, when optimizing the efficiency of the entire power plant. This is advantageous in applications, in which thermal energy is produced in addition to electrical energy.

The process of the present invention further differs from the normal Rankine process in that the vapour component of the medium is condensed nearly reversibly in a condensing column, in which the gaseous additive maintains a constant pressure and in which an efficient column structure of sufficient height is used to prevent the mixing of the flows of condensing water and the medium at different heights.

In embodiment 1, the molar ratio of the mixture flow is 1:1 and condensation takes place at a pressure of 0,2 bar, in such a way that the vapour pressure drops form 0,1 bar to 0,017 bar while the temperature correspondingly drops from 48° to 15°. Though in this case the mean condensing temperature is about 33°, the condensated water is obtained at a temperature of nearly 48 ° .

As depicted in embodiment 2, this ^Λ sliding' condensation process can be used to produce thermal energy, for example, for district-heating requirements, thermodynamically more economically than when using bleed or back-pressure steam condensing at the constant temperature of the Rankine process.

If very cold cooling water is available, the efficiency of the process can be increased by several percentage units by reducing the condensation pressure, for example, to 0,07 bar.

At a molar ratio of 1:1, condensation then begins at 27° and half of the water vapour will have condensed by 15,5°. It is thus possible to achieve lower condensation temperatures than in the normal Rankine cycle, while simultaneously avoiding the problems that would be caused by a low steam density at low temperatures .

Thanks to the higher density of the medium and its higher superheating temperature, a gas turbine can be advantageously used in the medium-pressure stage. Fe/Cr and Ni/Cr-alloys, for instance, which have an upper operational temperature of 1100° - 1200°C in an oxidizing environment, can be considered as materials for superheaters operating at a high temperature.

In the following, embodiments of the present invention are described with reference to the accompanying drawings:

Figure la shows schematically the power-plant process of Embodiment 1,

Figure lb shows one double column construction used in heat exchangers and condensers, Figure lc shows details of a second double column construction, Figure 2a shows a T, s-diagram of the steam component of the process of Embodiment 1, Figure 2b shows a T, s-diagram of the carbon dioxide component of the process of Embodiment 1, Figure 3 shows the CHP power-plant process of an Embodiment 2 , Figure 4 shows the power-plant process of Embodiment 3, in which there are two mixture superheating stages,

Figure 5 shows the marine-engine process of Embodiment 4.

Embodiment 1

Figure la shows the process schematically. At point D, a C0₂ flow is mixed with the exit flow of a high-pressure turbine 2 and the mixture flow is reheated in a superheater 3. From there the mixture flow is led into a medium-pressure turbine 4, a low-pressure turbine 5, and, through a heat exchanger 6, into a condenser 7. The C0₂ leaving the condenser is led to a low- pressure compressor 8 and then to a condenser 9, in which most of the water vapour is condensed, and then through a compressor 10 back to the mixing point D. The condensation water leaving the condensation column 7 and the condenser 9 is pressurized and preheated in the heat exchanger 6, from which the feed water is led to the vaporization and superheating piping 1 and from there to a high-pressure turbine 2.

In the condensation column 7, the condensation heat must be transferred to the cooling water at the same time as the internal mass and heat transfer of the process between the liquid and gas phase is held close to thermodynamic equilibrium. In this example, the construction (Fig. lb) disclosed in the non-public Finnish patent application No. 20025057 is used. The cooling water is brought to the column 7 from the connection 21 and it leaves the column from the connection 22. The water flows downwards in pipes 10, while the structures 11 guide the water flow to helical paths. The mixture of water vapour and C0₂ is brought to the column from the connection 23 and it flows upwards in the space 12 between the pipes 10. The C0₂ leaves from the connection 24 and the condensation water from the connection 25. Regularly arranged structures 13 with a flat or other shape are located in the space 12 to guide the flow of gas and liquid. The structures in question form a regular, three-dimensional labyrinth, in which the gas and vapour phase at a low pressure flow upwards while the condensation water arising from it flows downwards. The heat-exchanging surface of the column can be increased by using hollow structures 14, the internal space of which is connected to the internal space of the said pipes 10

(Figure lc) .

In the labyrinth described above, it is possible to provide a sufficient cross-sectional area for these flows, while simultaneously bringing the phases flowing in the opposite direction into a good heat-exchanging contact with each other and with the cooling water flowing in the pipes 10. This permits a nearly reversible condensation of the water vapour at a mean temperature that is close to the condenser temperatures used in the prior art . Similar constructions are used in the heat exchanger 6, in which the mixture of water vapour and C0₂ leaving the low-pressure turbine 5 flows in the space 12 between the pipes 10 and the feed water to be preheated flows inside the pipes 10.

The mixing of the water-vapour and carbon-dioxide flows is an irreversible process, in which the entropy of each kilomol of each component increases by the value S_MIX = - R In x, in which x is the 'pressure ratio' of the mixture, i.e. the molar proportion of the component in question in the mixture. In this example, the molar proportion of each component is 0,5, so that S_Mix ⁼ 0,693 R = 5,76 kJ/K for each kilomol of the mixture. As the components of the mixing process behave nearly like ideal gases, the mixing is nearly isothermic and takes place in this example at 300° .

The T, s diagram of the steam component of this embodiment is show in Figure 2a. Part A - B represents the preheating of 240- bar feed water in the heat exchanger 6 to a temperature of 290° while part B - C represents its evaporation and superheating to 575°. Part C - D represents its expansion in the high-pressure turbine 2, from which it leaves at a pressure of 30 bar and a temperature of 300°. At point D, it is mixed with a C0₂ flow, which is also at 30 bar and 300°. The mixing is shown as an isothermal process D - E, in which the steam pressure of 30 bar drops to its 15-bar partial pressure in the mixture.

The superheating of the steam component in the superheater 3 to a temperature of 850° C is depicted by part E - F and its expansion in the medium and low-pressure turbines by part F - G. The steam component exhausts at 0,1 bar at 300° and is cooled in the heat exchanger 6 to 48° (part G - H) . It transfers in a nearly saturated state to the condensation column 7 and is condensed in the column in the temperature range 45° - 15° (part H - A) , in such a way that half of its condenses when the temperature drops to 33°, at which the vapour pressure of the water is 0,05 bar.

The T, s diagram of the C0₂ component of the process is shown in Figure 2b, in which the part A' - B' represents the compression, in the low-pressure compressor 8, to a pressure of 1,5 bar, of the C0₂ leaving the condensation column 7 at a pressure of 0,2 bar and 15°. Enough water (not shown) is injected into the compressor to saturate the exhausting C0₂ with water vapour at 55°. Part B' - C represents the condensation of this water vapour in the condenser 9 while part C - D' represents the compression of the C0₂ to 30 bar in the compressor 10. The mixing of the C0₂ leaving the compressor at 300° with the steam component at point D is shown as part D' - E' and its superheating to 850° C as part E' - F' . Its expansion in the medium and low-pressure turbines is shown by part F' - G' and its cooling in the heat exchanged 6 to 48° by part G' - H' . The temperature of the C0₂ drops in the condensation column 7 to 15°, while its partial pressure increases to 0,2 bar (part H' - A' ) .

Embodiment 2

Figure 3 shows a CHP power plant, in which there is only a medium-pressure turbine and in which the condenser operates at an adjustable pressure, for example, in the range 0,5 - 2 bar. The operation of the power plant can be optimized to the desired electrical and thermal output by altering the condensation pressure and the molar ratio of the mixture of C0₂ and water vapour. As the partial pressure of the water vapour changes during the condensation process, the condensation heat is released in this and other embodiments of the present invention over a substantial temperature range, so that it can be exploited, for example, in district-heating production at a higher thermodynamic efficiency than the condensation heat of back-pressure steam condensing at a constant pressure.

The speed of rotation of the turbine 4 is optimized according to the process variables and the electrical power produced by the generator 31 is transmitted to the power grid using a frequency converter 32. As the C0₂ is obtained from the condenser at a relatively high pressure, its compression requires less power and water injection is not required in the compressor 10.

In this, as in other embodiments of the present invention, it is possible to reduce the flue-gas flow arising at a given output by using oxygen or oxygen-enriched air in the combustion process, preferably using the method disclosed in patent publication WO 03/038359. This reduces the amount of equipment and costs required to utilise the thermal content of the flue gases and remove the emissions from them. The C0₂ can also be removed more economically from the flue gases, preferably by using the process disclosed in patent publication WO 03/035221.

Embodiment 3

In the example shown in Figure 4, the mixture of water vapour and additive goes through two superheating and expansion processes. The steam leaving the high-pressure turbine 2 at 25 bar/375° is mixed with a C0₂ flow at point D and the mixture is superheated to 750°. The mixture leaves the medium-pressure turbine 4 at 2,5 bar/375°, is superheated to 750°, and is expanded in the low-pressure turbine 5 to 0,25 bar/375°. The mixture leaving the low-pressure turbine 5 flows through the heat exchanger 6, heating the feed water, which transfers from the heat exchanger at 365° to the evaporator/superheater 1. The mixture leaves the heat exchanger at 50° and transfers to the condensation column 7, from which the condensation water flow returns at 48° to the heat exchanger 6.

The C0₂ leaves the condenser at 0,25 bar/15° and is compressed in the low-pressure compressor 8 to 1,2 bar. Enough water is injected to the compressor to saturate the flow leaving it with water vapour at 50° (not shown) . The water vapour is condensed in the condenser 9 and the C0₂ is compressed in the compressor 10 to 25 bar/365⁰ and mixed at point D with the exit flow of the high-pressure turbine.

Embodiment 4 : marine engines

In the example (Figure 5) applied for marine propulsion, the output of the machinery is regulated by altering the flow travelling through the main turbine 4 and the speed of rotation of the turbine. A generator 31 feeds a propeller motor 33 through a frequency converter 32 while an auxiliary turbine 34 drives the compressors 8 and 9. Due to the electrical power transmission, an astern turbine is not required. In this, as in the other examples, the mixture ratio of steam and additive can be adjusted to optimize the efficiency of the machinery for each output level. A high-pressure steam turbine (not shown) described in example 1 can also be added to this example.

These embodiments are intended to illustrate some variations of the implementation of the present invention and are not intended to depict entire power plants. For example, they do not depict any known process in which the heat of the flue gases are used to preheat the combustion air. The energy required for preheating can be reduced and recovery of the carbon dioxide created can be facilitated by using oxygen or oxygen-enriched air in the combustion in the manner disclosed in embodiment 2.

The Rankine process equipped with the parallel cycle of the present invention offers the following advantages:

- improvement of the Rankine process so that the ability of the power plant to use all kinds of fuel is retained,

- the possibility to retrofit the process to existing steam power plants,

- improvement of the efficiency of the Rankine cycle, using a parallel cycle, without the increase in entropy caused by a separate topping cycle,

- as the additive it is advantageous to use carbon dioxide, which has good thermodynamic and chemical properties and which is cheap and nearly non-toxic,

- the possibility to use lower condensing temperatures than in a normal Rankine cycle in such a way that the density of the medium remains sufficient in the low-pressure stage, - thermodynamically advantageous exploitation of the heat of the flow leaving the turbine process, for heating the feed water and for other purposes,

- The use of the mixture ratio of the steam and additive as a degree of freedom in the optimization of the process,

- the pressurization of the additive in the condensation column reduces the compression power required and the size of the compressor.

As is apparent from the above description and the embodiments presented, the implementation variations of the present invention are extremely diverse and are thus not limited to the examples described above.

Claims

1. A method for improving the coefficient of efficiency in a closed steam plant process, in which method: - the process includes a high-pressure superheating process using pure water vapour and which water vapour is condensed at a pressure that is substantially lower than atmospheric pressure, and a mixture flow superheated to a high temperature is formed, using mixing and superheating processes (D, 3) , from the water-vapour flow and the flow of an additive that remains gaseous in the process, and the said mixture flow is expanded in an expansion process (4, 5) producing mechanical energy, and - most of the water vapour of the mixture flow exhausting from the said expansion process is condensed in a condensation process (7), and the condensate leaving the said condensation process and the additive are circulated in the said closed steam plant process, characterized in that carbon dioxide, or some other inert, two or more-atom gas, is used as the said additive, which raises the molar mass of the said mixture flow to a value suit- able for gas turbines of the prior art, and the gaseous mixed flow exiting in a superheated state from the said expansion process (4,5) flows, at least to a substantial extent, through a heat exchanger (6), in which it heats the condensate leaving the said condensation process, and the steam component of the mixture flow leaving the said heat exchanger (6) is condensed in nearly reversible conditions in a condensation column (7), in which the gaseous additive is pressurized to nearly the pressure of the mixture flow leaving the said expansion process.

2. A method according to Claim 1, characterized in that

- the mixture ratio of the said water-vapour flow and the additive is selected in such a way that the thermal capacity flow of the mixture flow leaving the said expansion process (4, 5) is suitable for heating the condensation water flow returning from the said condensation process (7) .

3. A method according to Claim 1 or 2, characterized in that

- a steam flow exiting from a steam turbine (2) , operating at a high pressure and forming part of the said steam power-plant process, is used as the water-vapour flow depicted in Claim 1.

4. A method according to any of Claims 1 - 3, characterized in that - the said expansion process includes one or more intermediate superheating processes.

5. A method according to any of Claims 1 - 4, characterized in that - the water-vapour component of the mixture flow depicted in Claim 1 is condensed in a resevoir (7), in which these are located in the space (12) between pipes (10) structures (13) with a flat or other shape in a regular geometrical arrangement, in order to promote the transfer of heat between the condensation process taking place in the said intermediate space and the cooling water flowing in the said pipes, and

- which structures (13) overlap with each other in such a way that regular, three-dimensional, labyrinth paths are formed, along which the said mixture flow flows upwards, while remain- ing close to a thermodynamic equilibrium with the condensate flowing downwards along the said paths, and

- the said reservoir (7), together with the structures (10, 13) in it, forms an effective column structure of sufficient height, which prevents mixing between the flows of condensation water and the medium at different heights and the exchange of heat between them.

6. A method according to any of Claims 1 - 5, characterized in that

- the said structures (14) with a plate or other shape are hollow and their internal space is connected to the internal space of the said pipes (10) , in order to make the thermal transfer taking place from the condensation process to the cooling water more efficient.

7. A method according to any of Claims 1 - 6, characterized in that

- the said plates or other structures (13, 14) are given such a shape and/or angle of tilt and/or their mutual distances are arranged in such a way that they guide the phases of the condensation process to helical paths.

8. A method according to any of Claims 1 - 7, characterized in that

- structures (11) with a helical or other shape are placed in the pipes (10) referred to in Claim 1, in which the cooling water of the condensation process flows, in order to improve the heat transfer of the condensation process.

9. A method according to any of Claims 1 - 8, characterized in that - structures similar to the structures (10, 11, 13, 14) referred to in any of Claims 5 - 8 are used in the heat exchanger (6) referred to in Claim 1, in such a way that the intake water of the turbine process flows in the space (12) and the feed water flows in the opposite direction in the pipes (10).