KR20160051022A - Efficient open cycle ocean thermal energy conversion (OTEC) using vacuum membrane distillation (VMD) for selective power generation and seawater desalination - Google Patents

Abstract

Provided is efficient open cycle ocean thermal energy conversion (OTEC) using vacuum membrane distillation (VMD) for selective power generation and seawater desalination, wherein the OTEC withdraws seawater on a surface layer to flow into the VMD placed in a vacuum chamber by enabling steam generated from a VMD device to flow into a turbine to generate power by operating the turbine, and a heat exchange device is provided so as to desalinate condensed steam by heat exchanging the steam used in a turbine power generation device with deep water at a lower temperature.

Description

[0001] The present invention relates to an open-type seawater temperature difference generation and desalination apparatus using a vacuum film evaporation method,

The present invention relates to an open seawater temperature difference power generation and desalination apparatus that does not use carbon dioxide as a working fluid as well as a freon refrigerant and ammonia. More specifically, the vacuum film distillation method is applied to the seawater temperature difference generation process to dramatically improve the evaporation rate of seawater, thereby selectively increasing power and fresh water production per unit time.

The evaporated water vapor can be used only for one purpose of power generation or seawater desalination, and the operation can be controlled so that some of it is used for power generation and the remainder is used for desalination, thereby minimizing the volume of the plant required for conventional seawater temperature difference generation The present invention also relates to a seawater temperature difference power generation and desalination apparatus, a power generation method, and a desalination method, which not only increase power and fresh water production dramatically but also optimize relatively.

Deep sea water refers to seawater resources that are deep within 200m or less of the depth of the sea, and Korea has abundant deep ocean water resources in the East Sea. Fig. 1 is a schematic view showing ocean surface water and deep water. Marine deep seawater has fewer pathogenic microorganisms compared to surface seawater and is clean, suitable as drinking water, rich in inorganic nutrients, and degradable soluble organic matter that causes microbial contamination, and is also suitable as aquaculture water. It maintains a constant temperature throughout the year, facilitating the management of biological and water quality. It is able to create a good aquaculture environment and has a balanced balance of useful substances including various minerals and essential trace elements. It is getting attention.

In general, the ocean temperature difference generation is a power generation system that produces electricity by using ocean surface water with high temperature and deep ocean water with low water temperature as vapor heat source and condensation heat source, respectively. The temperature difference between ocean surface water and deep sea cold water Is used as thermal energy to produce electricity. The ocean temperature difference power generation is a method of generating electricity by gasifying ammonia and low boiling point liquid such as ammonia and freon, and turning the turbine to a gas having a high vapor pressure. The gas after turning the turbine is cooled by deep cold water Liquid < / RTI > FIG. 2 shows the principle of temperature difference generation using known ocean surface water and deep water.

In order to develop the economical ocean temperature difference, it is necessary to continuously obtain a large amount of ocean surface waters and deep ocean water. The deep ocean water used as cooling water is almost unlimited in the east coast of Korea and the water temperature is 2 ° C or less. And can be utilized as cooling water for power generation.

In particular, since surface water is heated by solar heat and is subject to seasonal and geographical influences, it is difficult to secure surface water at high temperatures throughout the year in the case of Korea. However, since summer water temperature exceeds 25 ℃, seasonal temperature difference generation is possible. It is very likely to be used in Pacific Island countries.

In the conventional seawater temperature difference power generation, the refrigerant used to drive the turbine is operated in a closed mode that produces electricity by heat-exchanging through compression, condensation, expansion and evaporation processes. In the case of the closed-phase temperature difference power generation, the refrigerant gasified by the ammonia o low-boiling point liquid is used, and the power generation efficiency is determined according to the characteristics of the refrigerant.

Chlorofluorocarbon (hereinafter referred to as CFC) and hydrofluorofluorocarbon (hereinafter referred to as HCFC), which are derived from methane or ethane, have been mainly used as refrigerants for refrigerator, air conditioner, heat pump and the like. The collapse of the stratospheric ozone layer by CFCs and HCFCs has emerged as an important global environmental issue, and the production and consumption of CFCs and HCFCs, which disrupt stratospheric ozone, are regulated by the Montreal Protocol,

Accordingly, the present invention is an open-type seawater temperature difference power generation apparatus that does not use carbon dioxide as a working fluid as well as a freon refrigerant and ammonia. By applying the vacuum film distillation method to the seawater temperature difference generation process, the evaporation rate of seawater is remarkably improved, And fresh water production, as well as new power generation and desalination methods to easily control the ratio of power and fresh water production.

Korean Patent Laid-Open Publication No. 10-2011-0101754 basically uses ocean surface water used as condensation cooling water and ocean surface water used as vaporizing hot water or power plant discharge water in the ocean temperature difference power generation cycle and compared with the existing ocean temperature difference power generation cycle There is disclosed a configuration related to a multi-stage cycle type offshore thermal power generation system using deep sea water and ocean surface water or power plant drain water, which can increase power generation efficiency by utilizing a small amount of deep sea water. Japanese Patent Laid-Open Publication No. 10-2011-0101754 relates to a seawater temperature difference power generation apparatus comprising an evaporator for vaporizing a refrigerant in a liquid state by heat exchange with a hot water discharged from a condenser of an external power plant, A condenser installed in the sea for heat exchange with the seawater in the sea to liquefy the gaseous refrigerant, and a condenser for sending the liquefied refrigerant to the evaporator so as to circulate the refrigerant in the refrigerant circulation tube And a refrigerant circulation pump for circulating the refrigerant. Korean Patent Laid-Open Publication No. 10-2013-914 discloses a method for reusing energy contained in a waste water discharged from a condenser of various power plants using a turbine including a nuclear power plant, There is disclosed a configuration for a high efficiency temperature difference power generation system in which power is produced by constituting a closed circuit which is reused through a temperature difference power generation system using a temperature difference of seawater and which supplies low temperature water used in a temperature difference power generation system to cooling water of a power plant condenser. Korean Patent Publication No. 10-1356122 discloses a turbine 1 in which a high-temperature, high-pressure working fluid flows from a compressor; A turbine 2 through which the working fluid from the turbine 1 passes through the inside of the evaporator and into which a working fluid with increased temperature and pressure flows; A regenerator for exchanging heat between the high-temperature refrigerant flowing out of the turbine 2 and the low-temperature refrigerant flowing out of the pump; A deep water which is supplied to the heat source of the condenser to condense the high temperature and high pressure refrigerant which has flowed out from the turbine 2 and has passed through the inside of the regenerator and a condenser which is supplied to the heat source of the evaporator to evaporate the refrigerant flowing out of the pump, There is disclosed a configuration relating to an ocean surface temperature multi-stage turbine power generation cycle for ocean surface water and deep water heat source including surface water. However, in the above-mentioned prior art, the surface seawater as a technical feature of the present invention is taken in and introduced into a vacuum film evaporator in a vacuum chamber (which is much smaller than the vacuum chamber of an existing open type sea water temperature power generator) There has not been disclosed a configuration for generating a temperature difference and desalination which evaporates and becomes steam, and this water vapor travels to rotate the turbine and to desalinate. In addition, a method of selectively adjusting the power generation amount and the seawater desalination amount and adjusting the production ratio of the power and the fresh water as needed may not be disclosed.

Unlike a conventional method in which a refrigerant used for driving a turbine in a seawater temperature difference power generation is operated in a closed mode in which electric power is produced by heat exchange while undergoing compression, condensation, expansion and evaporation processes, As well as a selective generation / desalination method employing a vacuum membrane distillation method as an open seawater temperature difference power generation system that does not use carbon dioxide as a working fluid and a desalination apparatus.

In the ocean temperature difference power generation cycle using ocean surface water and deep sea water of the present invention as a heat source, the surface sea water is taken and introduced into the vacuum film evaporator to evaporate on the surface of the membrane to convert it into gas phase water vapor, Vacuum film evaporator; A turbine device for introducing a part of steam generated from the vacuum film evaporator into the turbine to rotate the turbine; A heat exchanger and a desalination device for introducing the remaining portion of the steam generated from the vacuum film evaporator into the second condenser and performing heat exchange with the low temperature deep water to condense the steam; A heat exchanger and a desalination device for desalinating condensed water vapor by exchanging water vapor used in the turbine with low temperature deep water; And a divider for adjusting the flow rate of gas and liquid per unit time after the evaporator, after the cryogenic pump, and after the first condenser according to the optimal power generation to desalination ratio.

By using ocean surface water used as cooling water for condensation and ocean surface water used as vaporizing hot water in marine temperature difference power generation system, it is possible to construct power and desalination combined system using infinite ocean thermal energy, And the power generation and desalination production can be appropriately adjusted in real time as needed to optimize the production efficiency by smoothly responding to the seasonal and local environment.

In addition, when the VMD evaporator is applied instead of the flash evaporator, the volume of the VMD evaporator to produce the same amount as the production amount of the flash evaporator is remarkably reduced, and the installation and maintenance cost is reduced. When the evaporator of the same size is used, When used, it is possible to produce power and desalination water several tens times higher than flash evaporation. The total production amount of electric power and fresh water is proportional to the size of the vacuum chamber and the effective surface area of the film of the vacuum film evaporator installed therein, so that it is possible to optimize the size of the vacuum chamber according to the desired production amount. Therefore, the initial investment cost of the evaporator and the mounting structure (ship, barge, etc.) is reduced, and the power generation amount is increased.

Fig. 1 is a schematic view showing ocean surface water and deep water.
FIG. 2 shows the principle of temperature difference generation using known ocean surface water and deep water.
3 is a schematic diagram of a marine temperature difference generation system using the vacuum film evaporator of the present invention.
Fig. 4 shows a schematic diagram of an open type temperature difference generation for power generation.
FIG. 5 shows the process of development of concentrated-water and pressure-retarded osmosis (PRO) in fresh water.

The concept and operation sequence for open OTEC is as follows. In the evaporator, a part of the surface seawater flowing from the outside is evaporated by the difference of water vapor pressure. At this time, the pressure inside the evaporator should be lower than the water vapor pressure corresponding to the temperature of the surface sea water, and in general, the pressure of 1-3% of the atmospheric pressure is maintained.

Since the amount of water evaporated is less than 1% of the amount of surface seawater input, most of the surface seawater is returned to the ocean. The reason why a large amount of surface sea water should be injected is that the temperature difference between the surface sea water and the deep water which is the driving force of evaporation is small (about 20 degrees Celsius), so that the temperature polarization phenomenon caused by absorbing latent heat on the surface of water or water drops The temperature change of the fuel cell) must be minimized. The evaporated water vapor causes the wind to rotate the turbine to generate electricity.

The water vapor used for power generation is sent to the condenser. Deep seawater having a low temperature flows into the condenser, and the heat of the steam used in the rotation of the turbine is taken away from the heat exchanger and condensed. Since the imported deep water is only for heat exchange, it is used for the production of condensate in the heat exchanger, and then the whole amount is discharged to the ocean. Direct condensers are used to directly contact the surface of the incoming deep water and the steam passing through the turbine, and a surface condenser is used for desalination purposes. . At this time, since condensed water vapor in the surface condenser is fresh water, the condensation effect is the same as the desalination process using the phase change of seawater.

3 is a schematic diagram of a marine temperature difference generation system using the vacuum film evaporator of the present invention.

1. Production and development of concentrated water and desalination using ocean temperature difference

As a design example of the temperature difference generation using the vacuum film evaporator of the present invention, the evaporation flow rate of the evaporated steam was determined first (power generation of 200 kW, 3.5 kg / s) and the whole system was designed.

The surface seawater (20-35 degrees) is taken and introduced into the vacuum evaporator in the vacuum chamber. At this time, a part of the water evaporates on the surface of the membrane, is converted into gas phase water vapor, passes through the membrane, and is collected in the vacuum chamber. The temperature of this input water is about 25-30 degrees, and the concentration is 35,000 ppm, which is the concentration of normal surface sea water (see 1 in FIG. 3).

The surface water passing through the evaporation membrane distillation apparatus is slightly lower in temperature and slightly higher in concentration, in proportion to the quantity of the permeated water that has passed through the membrane beforehand. However, since the membrane permeation rate is relatively small (about 0.6%) compared to the ocean surface layer, the change in temperature and concentration is not significant. Most non-evaporated surface water that has passed through the vacuum film evaporator is again released to the ocean (see 2 in FIG. 3).

The pressure of the vacuum chamber should be lower than the water vapor pressure corresponding to the temperature of the input surface water. Therefore, 0.01-0.03 atm is appropriate (refer to 3 in FIG. 3). The flow rate of the water vapor entering the turbine is equal to the evaporation flow rate of the water vapor produced in the membrane evaporator, and the speed of the water vapor and the temperature after passing the turbine are determined according to the diameter and expansion coefficient of the turbine channel (see FIG.

The low temperature deep seawater (5-8 캜) is introduced, passed through a heat exchanger, and then discharged to the ocean, and the temperature of the discharged water is about 7-10 캜 (see 5 and 6 in Fig. 3). Since this low temperature deep water is used only for heat exchange without mixing with other water, the composition ratio of each ion is different from that of surface sea water, but the total concentration is kept at 35,000 ppm like ordinary surface sea water. The hot water vapor that had been used to drive the turbine contacts the surface of this cold heat exchanger and condenses into water (see 7 in FIG. 3). Here, if direct contact is used, no condenser is needed, and the surface of the incoming low-temperature deep water and the water vapor passing through the turbine are in direct contact with each other and condensation occurs. When the condensates are all collected, the temperature is about 9-12 degrees, which is slightly higher than the deep low temperature water. Since this condensate is fresh water produced by removing salts through phase transformation from the evaporation of seawater, its concentration is close to 0.0 ppm (see 7 in FIG. 3).

As shown in FIG. 3, the amount of water introduced into the vacuum evaporation apparatus is evaporated on the surface of the membrane, converted into vapor phase water vapor, passed through the membrane, collected in a vacuum chamber, evaporated by water vapor, The steam generated from the turbine is condensed by heat exchange with the low temperature deep water and becomes liquid fresh water.

1.1 Classification by VMD Configuration and Operation

The membrane evaporation method used in step 1 of FIG. 3 can be performed by four methods: direct contact membrane distillation, vacuum membrane distillation, air-gap membrane distillation, and sweep-gas membrane distillation . In this case, the present invention uses a vacuum film evaporation (VMD) method in which the basic mechanism is similar to the OTEC evaporator, which can replace the evaporator of open OTEC.

1.1.1 Application Principle

The vacuum evaporation method is a vacuum evaporation method in which the upper part and the lower part of the lower part separated by the membrane are vacuumed by using a pump and the upper part is evaporated at the upper surface of the membrane when hot water is introduced from the right side and discharged to the left, Pass through the membrane with the difference between the water vapor pressure and the vacuum pressure as the driving force. The collected water vapor is desalinated using an external condenser. This vacuum film evaporation method is a great advantage that the resistance to heat transfer and mass transfer through the membrane is very small as compared with the direct film evaporation method.

According to the cell example of the present invention, when the surface layer seawater having a temperature of 26 占 폚 is introduced into the evaporation apparatus of a vacuum film, for example, The temperature of the effluent water deprived of latent heat will be slightly lower than the influent, and the temperature of the water vapor moving from the vacuum chamber to the turbine will be 21-24 ° C with a difference of about 5 ° C.

Therefore, it is necessary to specially design the material and the module of the membrane because the difference in the water vapor pressure across the membrane is small. The membrane should be basically hydrophobic, and the membrane thickness should be thinner (less than 0.1 mm) and larger than the pore size of the conventional commercialized products by about 0.2-0.3um or more. This is because the difference in water vapor pressure due to the temperature difference between the surface seawater and the deep water is relatively small so that the evaporation efficiency should be increased by increasing the porosity of the membrane and even if wetting occurs, the water passing through the membrane without evaporation greatly changes the effective surface area of the membrane Because it does not significantly affect evaporation efficiency or the concentration of freshwater produced.

1.1.2 Basic Mechanism

At the higher temperature surface, 26 degrees of surface water seeps and evaporates and moves to the opposite side through the membrane. The reason for evaporation is to keep the pressure on the opposite side of the membrane that is holding the vacuum lower than the water vapor pressure of surface seawater I will. The mechanism by which water vapor passes through the pores of the membrane is explained by Knudsen diffusion and viscous flow. Knudsen diffusion is the transfer of material by collision between water vapor particles and the inner wall of the pore, rather than collision between particles, when the pore of the membrane is smaller than the average free path of the water vapor. Viscous flow is caused by the difference in vapor pressure across the membrane It is a mechanism of convective mass transfer by steam gas flow.

1.1.3 Module

The module of the film evaporation method of the present invention includes (1) plate and frame, (2) tubular capillary or hollow fiber, and (3) spiral wound.

Plates are made in a periodic stack of membranes and spacers. This is the most common form of module because it is very easy to install and operate. However, it is not preferred for practical industrial use because it has a drawback that the membrane area ratio per unit volume is smaller than other modules.

The capillary tubular or hollow fiber module eliminates the disadvantage of the small membrane area ratio of the flat membrane. If the flat membrane is represented by its geometric structure in transverse thickness, the hollow fiber is expressed as inner diameter, outer diameter, and length. While the density of the membrane is very high, the pressure required to introduce the influent into the small caliber can be the key. Tubular modules are often used because they are larger in diameter (about 1 cm) than hollow fibers and can support themselves. The density of the membrane is smaller than that of the hollow fiber, but it is also advantageous that the membrane is larger than the flat membrane.

Spiral wound modules are a common form of reverse osmosis. Flexible flat membranes are modules that are rolled together with a spacer, which requires a higher pressure to create a fluid flow between membrane and membrane than other modules. Although the degree of integration of the membrane is high, membrane fouling is apt to occur in the structure. Commercial spiral wound is usually made longer than 1 meter. In the case of evaporation method, it is not preferable to make the length of about 30-50 cm or more because of heat loss according to the length of film. Therefore, spiral wound is less suitable for film evaporation than other modules. Especially when applied to the VMD-OTEC, an apparatus of the present invention, the space in which water vapor molecules can move through diffusion is very small compared to other modules.

1.1.4 Material of membrane

Membrane materials used in the membrane evaporation method are polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polypropylene (PP). Each membrane is characterized by its physical properties: pore size, liquid entry pressure (LEP), porosity, tortuosity, and thermal conductivity. LEP is the maximum pressure of water passing through the membrane surface. If the water pressure at the higher inlet water side is locally higher than this LEP, evaporation does not occur at the surface of the membrane and membrane wetting through water directly occurs do. The disadvantage of this wetting is that the VMD-OTEC of the present invention is much smaller than the DCMD as described above. However, it is desirable to lower the probability of occurrence of wetting as much as possible.

As the material of the film used in the present invention, a PVDF film is suitable, and the heat transfer coefficient is in the range of 0.17-0.21 W / Km, porosity = 0.7-0.8, and LEP = 100-200 kPa. The porosity of the membrane varies depending on the manufacturer, but it is usually at least 0.2 μm and at most 1.0 μm. Even with the same porosity, mass transfer of water vapor is easy when the pore size is large. On the other hand, if the pore is too large, the LEP is reduced, and the above-mentioned wetting can easily occur even at low water pressure. Even in the case of VMD, if the film pore is too large to cause wetting seriously, the production efficiency may be seriously degraded.

PTFE membranes have higher thermal conductivity (0.25-0.29 W / Km) and higher porosity (porosity = 0.8-0.9) than PVFD. Because of the high porosity, LEP is slightly higher than PVFD to 100-300 kPa, the membrane diameter is within the general range (0.2-1.0 μm), and PTFE often uses PP as its support.

The PP itself can be used as a membrane of the evaporation method. The thermal conductivity is 0.11-0.2 W / Km and the porosity is about 0.70. In the case of PP, the film evaporation efficiency is similar to PVDF.

When applying the VMD to an open OTEC, wetting is not particularly a problem, as mentioned above. This is because evaporation continues to occur on the surface of the water, since the water will eventually remain in the vacuum chamber even if it passes directly in the form of a liquid. If the water that passes through the wetting process is all evaporated, this vapor will also help turn the turbine. If the entire volume does not evaporate and the remaining volume accumulates continuously, it must be temporarily shut down to drain this water, which is to ensure that the dead water does not secondarily block the outer wall surface of the membrane.

One of the characteristics of the open-type temperature difference power generation is that the temperature of the surface seawater and deep seawater and the corresponding vacuum pressure difference are given as fixed values so that the material of the membrane should be optimized to the open temperature difference development. In the hollow fiber, the inner diameter and outer diameter of the membrane, the aperture of the pores, the porosity, and the tortuosity of the membrane are appropriately adjusted so long as the membrane does not undergo geometric deformation in the steady state, , The resistance to evaporation can be minimized.

≪ Example 1 >

The evaporator (flash evaporator), which is an integral part of the open-type temperature difference generator, aims to spray the surface seawater by sprout method and evaporate. Depending on the target power generation, the volume of the evaporator is determined, with a floor area of 18 m ² and a height of 6.9 m for 200 kW and a diameter of 9.2 m and a height of about 20 m for 1.8 MW. Thus, the volumes are 165.6 m ³ and 1506 m ^3, respectively. The steam evaporation of the 200 kW open-air temperature difference power generation is designed to be 3.5 kg / s, and for 1.8 MW, 33.4 kg / s is appropriate.

When the evaporator of the open type temperature difference power generator of the present invention is replaced with a vacuum film evaporator, the decrease in volume for the same vapor evaporation amount can be calculated as follows. For example, a comparison is made based on a 1.8 MW open-type temperature difference generator.

The volume required for a typical membrane filtration device is the surface area of the membrane divided by the degree of integration of the membrane. In the case of a hollow fiber membrane, the degree of integration is inversely proportional to the outer diameter of the membrane, and is about 1000 m ³ / m ² when the outer diameter is 1 mm. The area of the required membrane is obtained by dividing the steam evaporation amount (kg / s) of the open-type temperature difference power generation by the flux of the vacuum film evaporator.

The factors determining this flux are the pressure difference across the membrane, temperature difference, film thickness, pore size, porosity, and tortuosity. The difference in pressure is determined by the temperature of surface sea water flowing into the evaporator and the temperature of water vapor flowing out of the evaporator. Thus, when the temperature of surface sea water and deep water is constant, this difference in water vapor pressure, .

The thickness of the film is generally not more than 0.1 mm, the pore diameter is in the range of 1-3 microns, the porosity is generally in the range of 0.7-0.8, and the tortuosity is inversely proportional to the porosity. In this case, the final flux is determined by the hydrophobicity of the film, which can be estimated as the contact angle, which is about 0.1 g / m ² s. The required membrane area is calculated as 334,000 m ² , and the required volume is 334 m ³ divided by the degree of integration, which is about 5 times smaller than the 1.8 MW evaporator.

Therefore, the former tower-type evaporator can be installed in a two-dimensional plane within 1-2 meters in height. In the case of the conventional open-type temperature difference power generation, the temperature of the water vapor discharged from the evaporator is determined by the vacuum pressure of the evaporator.

That is, when the evaporator vacuum pressure is set to the saturated water vapor pressure of water, it corresponds to the temperature. The water droplets are formed inside the evaporator, and the evaporation of the water occurs in the process of rising and falling again. For this reason, a relatively large volume is required and the vacuum pressure must be maintained at 0.03 atm or less.

However, in the case of membrane evaporation, the integrated form of the membrane can be designed and applied in a variety of ways, and the evaporation of water can occur two-dimensionally on the membrane surface, effectively reducing the vacuum pressure inside the integrated module. If the pressure inside the membrane evaporation module is set to the water vapor pressure corresponding to the deep sea water temperature of 8 degrees Celsius, the flux of the membrane is increased by about 5 times, and the volume of the evaporator of the conventional open type temperature difference power generation is about 20-30 times smaller This is necessary for the membrane evaporator.

That is, the vapor evaporator can be installed by reducing the evaporator volume of 1500 m ³ to a maximum of 50 m ³ (= 5 m × 5 m × 2 m), and the same amount of generated electricity can be expected.

≪ Example 2 >

Fig. 4 shows a schematic diagram of an open type temperature difference generation for power generation. Using a pump, the surface seawater is taken and introduced into a flash evaporator. As an example, in the case of an open type temperature difference generator for 200 kW, surface sea water at 26 degrees Celsius is taken at a flow rate per unit time of 620 kg / s (1 in FIG. 4).

In the evaporator, 0.5-0.6 percent of the incoming surface water is evaporated by steam, and most of the inflow surface water is slightly lower in temperature and then released to the ocean. The temperature of the evaporator effluent water is lower than that of the influent surface seawater because it consumes latent heat per unit mass when the surface seawater evaporates. When the vacuum pressure inside the evaporator is determined, the pressure serves as the water vapor pressure of the surface sea water to be evaporated, thus determining the temperature inside the evaporator, that is, the temperature of the evaporated water vapor (2 in FIG.

The flow rate per unit time of the evaporated steam can be divided into the turbine and the second condenser. In this case, when k = 1, k = 1, only the temperature difference desalination is carried out without generating temperature difference. When k = 0, the total amount of vaporized steam participates in the temperature difference generation, It means desalination. The fraction of evaporated water vapor by k contributes to the temperature difference desalination, and the fraction of k-1 contributes to the temperature difference evolution.

At this time, non-condensable gas is sent to the turbine and the second condenser at the same fraction. The amount of this non-condensable gas generated per unit time is very small compared to the amount of surface seawater, deep seawater, and water vapor, and may be calculated as a percentage of the amount of water vapor evaporation, not included in the mass transfer equation. Condensing gas introduced into the second condenser flows out and is sent to the compressor, and the rest is sent to the compressor through the turbine and the first condenser and discharged to the atmosphere (3 in FIG. 4). The water vapor sent directly from the divider to the second condenser is condensed as it passes through the heat exchanger inside the condenser and immediately desalinated (4 in FIG. 4).

The water vapor sent from the divider to the turbine turns the turbine using the thermal energy and mechanical energy it had, and electricity is generated. The temperature of the steam leaving the turbine is about 10 degrees Celsius lower than the temperature of the incoming steam because the steam converts some of the heat energy it had to turn the turbine into mechanical energy and the conversion efficiency is less than 100% , And the quantitative decrease in steam temperature is determined by the expansion rate of the turbine (5 in Fig. 4).

The water vapor used for the rotation of the turbine is sent to the first condenser for desalination (6 in FIG. 4). The water vapor evaporated in the evaporator is divided into fractions of k and k-1, respectively, in the reservoir, passes through the first condenser and the turbine, and then the whole amount is collected again in the reservoir (7 in FIG. The low-temperature deep seawater of the first and second condensers is used by being taken in the low temperature pump (8 in FIG. 4).

The collected low-temperature oceanic deep seawater is sent to a first condenser for desalination and a second condenser for desalination, after generating electricity at k'-1 and k 'fractions. The k 'value is appropriately adjusted according to the temperature, pressure, and the flow rate per unit time of each of the water vapor flowing into and out of the turbine, in general, where the partitioning rate k after the evaporator and the partitioning rate k' after the low temperature deep- It is possible to optimize the final desalination / power production design ratio of the overall system (FIG. 4, 9).

The temperature difference between the steam passing through the turbine and the steam directly to the second condenser without passing through is about 10 degrees Celsius or more. Therefore, the amount of low-temperature deep water necessary for condensation becomes larger in the first condenser than in the second condenser. In other words, the temperature of the deep seawater used in the first condenser is between the original low temperature at the time of entry into the cryogenic pump and the temperature of the steam passing through the turbine, which is about 10-12 degrees Celsius. This temperature is much lower than the temperature of the water vapor evaporated in the evaporator entering the second condenser and is slightly higher than the inlet temperature of the deep ocean water. The energy use of the temperature difference power generation is due to a large part of the pump for the intake of ocean surface waters and ocean deep water, so that a portion (fraction k '') of the low temperature deep ocean water used in the first condenser is sent directly to the second condenser Can be used for heat exchange for condensation in the second condenser by mixing with the low temperature deep water inflow of fraction k '(FIG. 4, 10).

Of the total low-temperature deep-seated water intake, the fraction of the low-temperature deep-seated water used in the second condenser is k '+ k "(1-k'). At this time, the fraction k 'has the temperature of the deep seawater taken and the fraction k' '(1-k') has the temperature of the deep water passing through the first condenser (11 of FIG.

(1-k ') that have passed through the first condenser and have not been sent to the second condenser, and (1-k') (1-k ' The fractions are collected together and released into the ocean. Since the sum of these two fractions equals one, the deep seawater taken by the low temperature pump is used for optimized heat exchange in the first and second condensers and then discharged to the surface ocean (12 in FIG. 4).

Finally, the amount of water discharged per unit time is equal to the total amount of surface seawater and deep water inflow minus the amount of water vapor evaporated in the evaporator. As noted in step 7, this amount of evaporated water vapor corresponds to the amount of desalination collected in the reservoir (13 in FIG. 4). The condensing gases collected in the first and second condensers are all sent to the compressor and discharged into the air (14 in FIG. 4).

1.2 Effect of replacing evaporator with vacuum evaporator

The technical feature of the present invention is to replace the evaporator requiring a large volume, which is the main device of the open type temperature difference generator, with a vacuum film evaporator. (1) the volume of the evaporator can be drastically reduced, (2) the steam evaporation rate can be dramatically increased under the same operating conditions, (3) the two gains are selectively optimized, You can maximize / optimize incidence. (4) By selectively controlling the generation of electricity and fresh water, it is possible to optimize the entire production according to the acceptance by coping with regional seasonal changes smoothly.

1.2.1 Troubleshooting Wetting

The fundamental problem of membrane evaporation is wetting. That is, when the raw water at a high temperature passes over the surface of the membrane, due to its high water pressure, the water must not evaporate and pass directly through the pores of the membrane. The original structure of open temperature difference power generation has no problems with this wetting. In the original design, the nozzles are designed to be properly designed and sprayed with water droplets from the nozzles, and these droplets rise in the vacuum chamber and evaporate on the surface during the falling time.

When the vacuum film evaporation method is used, evaporation of water vapor occurs on the surface of the membrane, not on the surface of the water droplet like open type. Thus, the surface area of the membrane can be increased to easily control / increase the evaporation amount of water. At this time, the degree of integration of the film depending on the type of the module is a very important design factor. The evaporated water vapor passes through the pores of the membrane before it is directed to the turbine, and passing through the pores in the form of a fluid, rather than passing it in the form of vapor in the form of vapor, is called wetting.

Because the VMD-OTEC of the present invention is wetting compared to the original DCMD, even if the wetting occurs locally, water passing through the membrane will escape to the drain or remain inside the module and will participate in normal evaporation. There is little side effect. Depending on the operating conditions, the structure of the module, and the size of the pores, small droplets or bubbles may also form on the surface of the vacuum side membrane. In any case, the surface seawater present in the liquid takes part in the evaporation at the membrane surface or, if not, passes through the pores of the membrane immediately and is released back into the ocean. Therefore, even if wetting occurs, it is possible to reduce the wetting degree by reducing the water pressure on the inlet side by controlling the inlet water pressure and the inlet water pressure of the ocean surface water without immediately stopping the operation.

1.2.2. Flux troubleshooting

Using the most common parameters, the flux of the VMD is calculated to be about 0.83 g / m ² s. This calculation uses the most basic VMD theory and the values used are porosity = 0.75, tortuosity = 2.0, pressure difference = 0.80 kPa (water vapor pressure difference between 26 and 22 degrees), membrane pore = 0.2 um. That is, 1 m ² membrane per second produces 0.83 gram water.

Described that 33.4 kg / s of water vapor must be evaporated to produce 1.8 MW of power above. The area of the membrane to produce this water vapor is about 40,000 m ² . Approximately 400 m ² / m ³ of the density of the flat membrane can be used to integrate a 400 square meter flat membrane within one cubic meter.

In this case, the required volume is 100 m ³ , more than 4000 m ³ It is about 40 times smaller. Hollow fiber module can increase the integration level by about 5-10 more than flat membrane. Therefore, when hollow fiber VMD is implemented instead of flat membrane, it is possible to realize same flux with volume about 400 times smaller than the volume required for conventional open temperature difference power generation. (The VMD is not limited in height, as is conventional evaporators.)

1.2.3 Design

The present invention aims at reducing the cost of open-type temperature difference power generation, increasing efficiency, and selectively producing electricity and fresh water as needed.

For example, if the 1.8 MW evaporator is 9.2 meters in diameter, simulate an evaporator of the same diameter with a height of 1 meter and replace it with a hollow fiber module. At this time, since the volume is 27 m ³ , the area of the collectable hollow fiber membrane is maximum 27 m ³ x 4000 m ² / m ³ = 108,000 m ² .

The amount of water vapor generated is 108,000 m ² x 0.83 g / m ² s = 90 kg / s, which is about three times the original open design. That is, even though the height of the evaporator is reduced by 15 times, the estimated power generation amount can increase from 1.8 MW to 5.4 MW. Accumulating hollow fiber membranes as large as the volume of the original open evaporator would increase the steam generation rate (at 33.4 kg / s) to 13,360 kg / s and the power generation rate (at 1.8 MW) to 720 MW .

In this way, by using the deep sea water used as condensation cooling water and the surface water used as vaporizing hot water in the offshore temperature difference power generation system, the power generation system is constituted by infinite offshore energy, thereby increasing the efficiency of the closed offshore temperature difference power generation system .

2. Linking with the development of pressure-retarded osmosis (PRO) using byproducts, concentrated water and fresh water

PRO is a method of generating electricity using concentration differences. While OTEC utilizes liquid phase and gas phase transitions, PRO is characterized by the use of differences in concentration in the liquid phase. A part of the surface water flowing into the open type temperature difference power generation device participates in evaporation and becomes a fresh water, and a certain part of the fresh water passes through the PRO membrane and contributes to the PRO generation. Therefore, from the viewpoint of the amount of generated electricity, it is natural that the amount of electricity generated is smaller than the total temperature difference generated.

2.1 PRO Mechanism

The mechanism of PRO is as follows. A PRO draw solution with a concentration similar to surface seawater is introduced into the membrane module with an osmotic pressure of about 30 bar. Fresh water flows into the opposite side of the membrane as a feed of PRO. The osmotic pressure of the feed is almost zero or very small compared to the induction number. Due to the difference in osmotic pressure across the membrane, fresh water flows from the feed side through the membrane to the draw solution. At this time, the volume of the moved water is expressed in dV. Due to the passage of fresh water, the concentration of the draw solution decreases and becomes brackish water. The pressure on the draw solution is increased in proportion to the increased volume, and this pressure is used to turn the secondary turbine to generate electricity.

2.2 Pressure-retarded osmosis evolution of the present invention

The core role of the seawater temperature difference generator in the present invention is to normally produce concentrated water and fresh water at the same flow rate and introduce it into the pressure delay osmosis device shown in FIG. Pressure-delayed osmosis is a reverse osmosis of seawater desalination using forward osmosis with the use of osmotic pressure differences between two solutions of different concentrations. When the pressure of the high concentration solution is maintained at a pressure lower than the osmotic pressure, there is a water-flux to the high concentration solution through the membrane by the osmotic pressure, and thereby the energy is produced by rotating the turbine using the water pressure of the induction channel.

FIG. 5 shows the process of development of concentrated-water and pressure-retarded osmosis (PRO) in fresh water. The condensed water (1 in FIG. 5) evaporated in the vacuum chamber and the produced fresh water (2 in FIG. 5) in which steam discharged from the turbine is heat-exchanged with low temperature deep water to become liquid water flows into the heat exchanger. The production fresh water passes through the heat exchanger and its flow rate (for example, 1 kg / s) and concentration (0.0 ppm) are maintained, but the temperature is increased by 16-20 degrees (3 in FIG. The condensate flows through the heat exchanger while its flow rate and concentration remain constant, but the temperature is equal to the temperature of the fresh water produced after the heat exchanger (16-20 ° C) (4 of FIG. 5).

The temperature-regulated produced fresh water and the concentrated water are respectively the feed (5 in FIG. 5) and the draw solution (6 in FIG. 5) of the pressure delay osmotic device. Inside the membrane module, a part of water on the incoming side flows through the membrane and flows toward the induction solution (7 in FIG. 5). Some of the water that has not passed through the membrane can be reused as part of the acquisition. However, when water osmosis occurs through the membrane, the concentration of fresh water through the pressure delay osmotic device will increase to several hundreds of ppm tens of degrees since a portion of the high concentration salt on the induction side can be transferred to the incoming side. Therefore, although it is impossible to use for food and beverage, its concentration can be reused as a part of the intake because it is significantly lower than the induction solution. Alternatively, if necessary, reverse osmosis can be used to produce fresh water with a concentration close to 0 ppm.

Low-concentration fresh water that is not to be reused can be released or, in some cases, used for appropriate purposes. At this time, the temperature is about 16-20 ° C as in the case of the water supply, but it can be mixed with the surface water to be introduced into the vacuum chamber if the temperature can be increased to the surface water level by using a low-cost heat source or a part of the waste heat source. Since the concentration of this vaporized water is lowered, the evaporation rate of water at the same vacuum pressure increases and thus the overall efficiency can be increased.

A portion of the concentrated water that has passed through the heat exchanger becomes the inductive solution and is transferred to the pressure delay osmotic unit. The rest goes to the pressure exchanger. The respective water pressures are kept equal (Pex1) (8 in Fig. 5).

The yield through the membrane is calculated as the product of membrane area and permeation flux. The permeate flux is proportional to the osmotic pressure difference between the inlet and the inductive solution, and this proportional constant is the water permeability coefficient (A). The permeability coefficient is an intrinsic value of the produced film, and the higher the flux, the higher the flux under the same operating conditions.

When the concentration of the inductive solution is 70,000 ppm and the concentration of the inlet water is 0.0 ppm, the difference in concentration is about twice the difference in the concentration of general seawater and fresh water, so the osmotic pressure across the pressure delay osmotic device is about 60 bar (860 psi) do. This pressure is nearly equal to the water pressure applied to the high concentration solution by the pump in the reverse osmosis unit. The power produced per unit area is proportional to the permeability constant A and the square of the difference in osmotic pressure.

The flow rate of water to the pressure exchanger is the sum of the flow rate of the inductive solution and the permeate flow rate through the membrane at the inlet (9 in FIG. 5). At this time, the water pressure increases in proportion to the permeate flow rate (Pex 2) (10 in FIG. 5). Since the input water is fresh water, when it is mixed with the induction solution, the concentration of the induction solution is decreased. Therefore, the osmotic pressure difference in the steady state is slightly less than the actual calculation, which is called concentration polarization. This concentration polarization can be minimized by speeding up the flow rate across the membrane.

The difference between the water pressure (Pex1) of the original induction solution and the water pressure (Pex2) of the induction solution passing through the pressure delay osmosis device is converted into mechanical energy to rotate the turbine and generate electric energy. Of the inductive solutions that have passed through the pressure exchanger (with reduced concentration), some or all of the original inductive solution that has not passed through the pressure delay osmotic device can be reused (11 of FIG. 5) Mixed with the effluent water, and the concentration can be lowered to a level close to the surface sea water (12 in FIG. 5).

In this way, after the process of Ocean Thermal Energy Conversion (OC-OTEC) process, it is possible to produce about 10% of OC-OTEC by generating byproduct concentrate and pressure-retarded osmosis (PRO) There is an effect that can be. In addition, when the VMD evaporator is applied instead of the flash evaporator, the evaporator volume is reduced to about 10% and the cost is reduced. In the same volume, steam can be produced at least 10 times. Also, since the initial investment cost of the evaporator and the mounting structure (ship, barge, etc.) is reduced, the power generation amount is increased.

Claims (13)

A vacuum film evaporator for collecting the surface sea water and introducing it into the vacuum chamber;
A turbine generator for introducing water vapor generated from the vacuum film evaporator into the turbine and rotating the turbine to generate electricity;
And a heat exchanger for desalinating condensed water vapor by exchanging water vapor used in the turbine power generator with deep low-temperature water, characterized in that a selective open-type seawater temperature difference power generation and desalination apparatus
A vacuum film evaporator for collecting the surface sea water and introducing it into the vacuum chamber;
A turbine generator for introducing water vapor generated from the vacuum film evaporator into the turbine and rotating the turbine to generate electricity;
A first heat exchanger for desalinating condensed water vapor by exchanging water vapor used in the turbine generator with deep low temperature deep water;
A second heat exchange device for exchanging heat between the remaining condensed water evaporated in the vacuum chamber and the produced fresh water produced by heat exchange with the low temperature deep water;
A pressure delayed osmosis unit adapted to use the concentrated water and the produced fresh water that have passed through the second heat exchange unit as the inlet and the induction solution;
And a pressure exchanger using part of the concentrated water that has passed through the second heat exchanger. The selective open-type seawater temperature difference power generation and desalination apparatus using the vacuum film evaporation method
The method according to claim 1 or 2, wherein the pressure of the vacuum chamber is controlled to 1-3% of the atmospheric pressure, and the flow rate of steam is 0.5 to 6% of the flow rate. Power generation and desalination equipment
The method according to claim 1 or 2, wherein the seawater supplied to the vacuum chamber is surface-layer seawater at a temperature of 20-35 ° C and the seawater supplied to the heat exchanger is deep-seated water at 5-8 ° C. Optional open seawater temperature difference power generation and desalination equipment
The vacuum film evaporator as set forth in claim 1 or 2, wherein the upper and lower ends of the vacuum film evaporator are separated by a membrane and the inside of the lower end portion is evacuated to allow the seawater to flow into the membrane at the upper end thereof, And the generated water vapor is passed through the membrane by the propulsion force of the difference between the water vapor pressure and the vacuum pressure. The selective open sea water temperature difference generation and desalination apparatus using the vacuum film evaporation method
The desalination apparatus according to claim 2, wherein the first heat exchanger is a water-cooled type and the second heat exchanger is an air-cooling type.
The vacuum evaporation method according to claim 1 or 2, wherein the vacuum film formed in the vacuum film evaporator is any one selected from a plate and frame, a tubular capillary, a hollow fiber and a spiral wound. Optional open seawater temperature difference power generation and desalination equipment
The vacuum film evaporation method according to claim 1 or 2, wherein the material of the vacuum film formed in the vacuum film evaporator is any one selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polypropylene Selective open seawater temperature difference power generation and desalination system using
Collecting the surface seawater and flowing it into a vacuum film evaporator in a vacuum chamber;
The vacuum evaporation system separates the upper and lower parts of the membrane into a membrane and the inside of the lower part is in a vacuum state so that seawater flows into the membrane at the upper part and is discharged to evaporate at the upper surface of the membrane. Passing the generated steam through the membrane as a propulsion force, and generating steam by rotating the turbine;
And a step of desalting the condensed water vapor by exchanging the water vapor used in the turbine generator with the low temperature deep water.
Collecting the surface seawater and flowing it into a vacuum film evaporator in a vacuum chamber;
Introducing water vapor generated from the vacuum film evaporator into the turbine and rotating the turbine to generate electricity;
Desalting the condensed water vapor by heat-exchanging the water vapor used in the turbine generator with deep low-temperature water;
Sending the produced fresh water produced by heat exchange with the condensate remaining in the vacuum chamber and the deep seawater at low temperature to the second heat exchanger;
And a pressure-delayed osmosis unit adapted to utilize the concentrated water and the produced fresh water that have passed through the second heat exchange unit as the inlet and the induction solution, and the step of generating the selective open-type seawater temperature difference generation and desalination method using the vacuum film evaporation method
a) withdrawing surface sea water and introducing it into an evaporator;
b) evaporating 0.5-0.6 percent of the surface seawater incoming from the evaporator with water vapor and discharging the remaining surface water to the sea;
c) dividing the vaporized vapor into turbine and second condenser;
d) condensing the water vapor sent to the second condenser through the heat exchanger inside the condenser to desalinate the water vapor, and discharging the non-condensable gas to the compressor to the atmosphere;
e) the water vapor introduced into the turbine is used for rotation of the turbine, desalinated through a first condenser, and discharged to the atmosphere through a non-condensing gas to a compressor, Sea water temperature difference generation and desalination method
The method as claimed in claim 11, wherein step c) is performed by passing the water vapor evaporated in the evaporator through the second condenser and the turbine, and then collecting the water vapor again in the water storage tank. And desalination methods
The method of claim 11 or 12, wherein the low-temperature deep seawater of the first and second condensers is used to take and use deep seawater at low temperatures, and a portion of the deep-sea deep seawater used in the first condenser is introduced into the low- And is used for heat exchange for condensation in the second condenser by mixing with the inflow amount. The selective open-type water temperature difference generation and desalination method using the vacuum film evaporation method

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