JPS6359725B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for desalinating salt water using a photovoltaic electrodialysis device, and in particular, the output of a solar cell can be used to desalinate salt water with high efficiency even when the amount of solar radiation changes. The present invention relates to a method for desalinating salt water or seawater using a solar-powered electrodialysis device suitable for use in desalinating water.

[Background of the invention]

Electrodialysis is a method in which a potential difference is applied to an electrolyte solution, such as seawater, and an ion exchange membrane that is selective to ions of opposite sign is used to concentrate and dilute the electrolyte. This method has the characteristics of easy pretreatment of seawater (salt water), high strength of the ion exchange membrane, and low operability, so it is suitable for desalinating small and medium volumes of seawater. suitable for

In recent years, the performance of electrodialysis equipment for desalinating salt water (seawater) has significantly improved due to the establishment of high-temperature operation technology and the emergence of new technologies such as the development of new structures.

On the other hand, solar cells are seen as a promising new energy source and are being actively researched, with remarkable improvements in performance and reductions in manufacturing costs.

By the way, regions that are generally blessed with solar energy, such as the Middle East, are often poor in water resources, and are currently struggling to take measures to secure water sources. Therefore, interest in seawater desalination using solar energy is gathering in these regions. In particular, electrodialysis equipment that uses solar cells as a power source to desalinate seawater is considered to be a promising technology, reflecting recent technological innovations.

In such seawater desalination equipment that combines solar cells and electrodialysis equipment, DC power is used as separated energy for electrodialysis, so the output of the solar cells can be used directly, and solar heat can be used to generate electricity. Operating the dialysis machine at high temperatures has the advantage of reducing power consumption. Therefore, there is an advantage that a process that utilizes solar heat and sunlight simultaneously can be established.

By the way, the output of a solar cell varies greatly as the amount of solar radiation changes. Efficiently utilizing fluctuating power in this way is a challenge not only for electrodialyzers but also for various types of devices that serve as a load for solar cells. Fortunately, desalination plants can store fresh water, so they can be operated only during the day. In particular, the electrodialysis method is easy to start and stop, and the ion exchange membrane is resistant to microorganisms in seawater, so there is no need to take special precautions even when the system is stopped at night. Therefore, the problem with current seawater desalination methods using photovoltaic electrodialysis equipment is how to efficiently utilize the output of the solar cells, which fluctuates widely in response to changes in the amount of solar radiation.

Conventionally, the methods to solve this technical problem were as follows:
A method of smoothing power fluctuations using lead-acid batteries has been proposed (FY 1981 Sunshine Project Commissioned Research Results Report ``Research on Solar Energy Systems'').

However, this conventional method requires a large amount of expensive lead-acid batteries, is not easy to maintain and manage, and has the problems of a significant drop in power efficiency as the batteries charge and discharge. .

[Purpose of the invention]

An object of the present invention is to provide a method for desalinating salt water using a solar-powered electrodialysis device, in which the output of a solar cell, which varies greatly depending on the amount of solar radiation, can be used with high efficiency to operate the electrodialysis device. It is in.

[Summary of the invention]

The feature of the present invention is to operate the electrodialysis machine at the maximum output of the solar cells by adjusting the salt concentration of the water to be desalinated in accordance with the amount of solar radiation, taking into account the characteristics of the solar cells depending on the amount of solar radiation. The goal is to desalinate seawater.

That is, the present invention provides two circulation tanks, a freshwater tank and an intermediate concentration tank, for an electrodialysis device that desalinates salt water using a solar cell as a power source, and when the solar cell has a high output, the intermediate concentration tank and the electricity The desalinated water is desalinated to an intermediate concentration using a dialysis device, and the desalinated water is transferred from the intermediate concentration tank to the desalination tank, and when the output of the solar cell is low, the desalinated water is transferred to the desalination tank and the electrodialysis tank. This method is characterized by desalting to a freshwater concentration using a device, and in addition to this, raw water is added to the intermediate concentration tank to adjust the salt concentration.

Figure 1 is a power generation characteristic diagram showing the relationship between the output voltage and output current of a solar cell with respect to the amount of solar radiation.
The figure is an equivalent circuit diagram when an electrodialysis tank (electrical resistance R) is used as a load for a solar cell.
Electrodialyzers 2 with varying values are connected in series.

The characteristics of voltage E _i and current I _i at various amounts of solar radiation are shown by solid lines in FIG. The broken lines in the figure are the conditions under which the maximum power can be obtained, and the intersections of the solid lines and the broken lines are the maximum output conditions (voltage and current values, ie, optimum load resistance values) for each amount of solar radiation.

E _i under the maximum output condition with respect to the amount of solar radiation is approximately constant depending on the amount of solar radiation as shown in FIG. 1, and the output current value I _i varies greatly as the amount of solar radiation changes.

As mentioned above, in order to use solar cells efficiently, it is necessary to change the load resistance, that is, the electrical resistance R of the electrodialysis tank according to the amount of solar radiation, and to obtain the maximum efficiency voltage -
It is necessary to maintain current conditions.

FIGS. 3A, B, C, and D are diagrams schematically explaining conditions for obtaining maximum efficiency.

Figure A shows a general example of changes in the amount of solar radiation W over time. At times near sunrise and sunset when the amount of solar radiation W is small, the load resistance (electrical resistance R of the electrodialysis tank) is increased as shown in Figure B, while at times near noon when the amount of solar radiation W is large, the load resistance By reducing the resistance R,
Maximum output characteristics can be maintained.

Figure 6 C and D show the voltage E _i at maximum output characteristics.
and shows the change in current I _i . That is, in order to obtain the maximum output efficiency, it is necessary to realize the electrical resistance R _i of the electrodialysis tank that satisfies R _i =E _i /I _i at each time. The electrical resistance value R _i of this electrodialysis tank depends on the salt concentration C of the water to be desalinated which is circulated through the electrodialysis tank for desalination.

FIG. 4 is a line showing the relationship between the salt concentration C of the water to be treated, that is, the salt concentration C% of the liquid flowing in the electrodialysis tank, and the electrical resistance R of the electrodialysis tank. Here, the salt concentration C% of the water to be desalted is precisely the logarithmic average value of the salt concentrations at the inlet and outlet of the electrodialyzer.

This relationship diagram shows that the structure of the electrodialysis tank is such that the effective area of each ion exchange membrane is 0.23 m ² , the number of logarithms of ion exchange membranes is 200, and the spacing between the ion exchange membranes is 0.8.
An example is shown in which mm is used to desalinate an aqueous salt solution containing common salt as the main component at room temperature. The relationship between this salt concentration and the electrical resistance R of the electrodialysis device is uniquely determined by the structure of the electrodialysis tank. For example, in the process of desalinating seawater with a salt concentration of about 3.5% to freshwater with a salt concentration of 0.05%, the electrical resistance R of an electrodialysis tank is
changes greatly in a hyperbolic manner from 0.7Ω to 14Ω.

Therefore, as a method to operate the electrodialysis apparatus at maximum efficiency, the salt concentration of the water to be desalinated that flows through the electrodialysis tank is changed so that the load resistance R for the maximum output is achieved as shown in Figure 3B. One possible method is to adjust the electrical resistance of the electrodialyzer to obtain the maximum efficiency characteristics of the solar cell. In this method, when the output of the solar cell is low, the demineralized water with a low salt concentration (i.e., the area of high electrical resistance of the electrodialysis tank) is passed through the electrodialysis tank, while when the output of the solar cell is high, the water to be desalinated is passed through the electrodialysis tank. The operating conditions of the electrodialysis tank and the time fluctuations in the maximum output of the solar cell are made to match by flowing desalinated water (ie, the area where the electrical resistance R of the electrodialysis tank is low) through the electrodialysis tank.

By the way, as a typical operating method for conventional electrodialysis equipment, there is a known method in which batchwise, ie, desalinated water is circulated through an electrodialysis tank many times through a circulation tank to obtain fresh water with a predetermined salt concentration. ing. In this operating method, the salt concentration of the water to be desalinated flowing through the electrodialysis tank changes over time, but the pattern of the change in salt concentration over time is completely unrelated to fluctuations in the output of the solar cells. Therefore, in this batch operation direction, the electrodialysis tank is simply connected directly as a load to the solar cell, and the output of the solar cell that depends on the amount of solar radiation cannot be efficiently utilized.

In contrast, the present invention provides two circulation tanks, a freshwater tank and an intermediate concentration tank, in parallel to the electrodialysis tank,
By adjusting the salt concentration of the water to be desalinated and performing batch operation by separately using two circulation tanks when the output of the solar cells is low and when the output is high, it is possible to match the operating conditions of the electrodialysis tank with the time fluctuations of the output of the solar cells. It is designed to obtain maximum efficiency. In addition, when the solar cell output is high, the intermediate concentration tank is used to desalinate salt water to an intermediate concentration, and the intermediate concentration tank is also used to desalinate salt water to an intermediate concentration in response to fluctuations in the output of the solar cells. The salt concentration may be adjusted by supplying salt water.

Next, transfer the intermediate concentration water desalinated using the intermediate concentration tank to the fresh water tank, and use the fresh water tank to reduce the intermediate concentration water to fresh water concentration when the solar cell output is low (for example, after sunrise the next day). Fresh water can be obtained by desalination.

FIG. 5 is an explanatory diagram showing an example of a flowchart of an electrodialysis apparatus used for carrying out the present invention.

Reference numeral 2 denotes an electrodialysis tank that serves as a load for the solar cell.Flow paths 3 and 4 for circulating the water to be desalinated are provided in the demineralization chamber of the electrodialysis tank 2. A water tank 6 is arranged in parallel. Further, valves 7 to 10 are provided in the upper and lower pipes of the intermediate concentration tank 5 and the fresh water tank 6, respectively. Further, a supply pipe 12 for supplying raw water via a valve 11 is provided above the intermediate concentration tank 5 . On the other hand, a pump 13 is provided in the middle of the common flow path 3 of the intermediate concentration tank 5 and the fresh water tank 6, and the water to be desalinated in the intermediate concentration tank 5 or the fresh water tank 6 is transferred to the electrodialysis tank by opening and closing valves 7 to 10. It is configured so that it can be circulated to 2.

Further, the flow path 4 on the outlet side of the electrodialysis tank 2 is connected to an intermediate concentration tank and a fresh water tank 6, and its extended end is connected to a fresh water tank 15 via a valve 14. In addition, in the electrodialysis tank 2, the parts related to concentrated water and polar liquid are omitted.

Next, a salt water desalination method according to the present invention will be explained using the apparatus configured as described above.

FIGS. 6A, B, and C are diagrams illustrating an example of an operating method to which the present invention is applied along the passage of time from sunrise to sunset. Note that in the figure, a is the sunrise time, b is the time when the freshwater tank is switched to the intermediate concentration tank, c is the midday hour (12 o'clock), and d is the sunset time.

In the figure, first of all, when the battery output is low and time has not passed since sunrise (point a in Figure 6), the intermediate concentration that has been desalted from raw water to an intermediate concentration in the intermediate concentration tank the previous day, as shown in Figure 5, is Of the concentrated water, an amount equivalent to the amount of fresh water produced per day is transferred to the fresh water tank 6, valves 7 and 8 are closed, valves 9 and 10 are opened, and the pump 13 is activated to transfer the intermediate concentration water to the electrodialysis tank. and desalinate to obtain fresh water with a predetermined concentration.

When the output of the solar cells is low until a predetermined time (a-b) has elapsed from sunrise, intermediate concentration water with low salt concentration (that is, the electrical resistance of the electrodialysis tank is high)
As shown in FIG. 3, high power usage efficiency can be obtained.

The fresh water obtained by the treatment is discharged into a fresh water tank 15 by closing the valve 10 and opening the valve 14 and operating the pump 13.

Next, valves 9 and 14 are closed, valves 7 and 8 are opened, and the intermediate concentration water in the intermediate concentration tank 5 is pumped to the pump 13.
It is circulated to an electrodialysis tank for desalination.

This period of operation is suitable for treating water with high battery output and high salt concentration.

Further, since the battery output tends to increase until the sun reaches its midpoint, the valve 11 is opened to supply raw water to the intermediate concentration tank 5 to adjust the salt concentration to be high.

That is, between b and c, raw water is supplied in an amount exceeding the maximum desalination capacity in the electrodialysis tank 2, and the salt concentration is increased over time. It is preferable that this change in salt concentration over time corresponds to an increase in the output of the battery.
Therefore, the amount of raw water supplied is optimally adjusted by changing the opening degree of the valve 11 in conjunction with the battery output.

After the mid-south hour (between C and D in Figure 6),
As the battery output decreases, the salt concentration decreases, and the battery's maximum output condition is automatically established. If the operation is continued until sunset in this manner, intermediate concentration water can be obtained for desalination treatment in the fresh water tank 6 the next day. In addition, in this operation as well, raw water can be supplied as needed and optimal conditions can be strictly controlled. A certain amount of intermediate concentration water obtained by the above operation is sent to the freshwater tank 6 by the pump 13 with the valve 10 opened and the valve 8 closed, and is subjected to desalination treatment on the next day.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 The method of the present invention was carried out using a DC power supply that simulated the output characteristics of a solar cell, and a 3.5% saline solution was desalted to a salt concentration of 500 ppm. As an electrodialysis tank, the current carrying area per ion exchange membrane is 0.23, which is the same as the electrical resistance characteristic of the electrodialysis tank shown in Figure 4.
m ² and the number of logarithms of the ion exchange membrane was 200. At the start of operation, 4.2 m ³ of 0.35% saline solution was introduced into the freshwater tank, and 1.4 m ³ of the same concentration saline solution was supplied to the intermediate concentration tank. The maximum output of the DC power supply was 10 kW, the period was a sine wave of 16 hours, and the electrodialysis device was operated for the first 8 hours.

Figures 7A, B, and C are diagrams showing electrodialysis characteristics with respect to cycle time, where Figure A is the relationship between time, the amount of power generated by the solar cell (kW), and the power consumed for electrodialysis, and Figure B is the relationship between time, the amount of power generated by the solar cell (kW), and the power consumed for electrodialysis. Changes in the salt concentration of the salt water to be treated in the freshwater tank and the intermediate concentration tank, and Figure C is a diagram showing the power utilization efficiency of the electrodialysis tank.

As is clear from the figure, the salt concentration of the water in the fresh water tank reached 500 ppm 1 hour and 55 minutes after the start of operation, and in the subsequent operation of the intermediate concentration tank, a 3.5% saline solution was output from the DC power supply for up to 4 hours (from the start of operation). Added accordingly. After 8 hours (from the start of operation), the salt concentration of the water in the intermediate concentration tank is 0.35, the same as at the start of operation.
%. The power usage efficiency during operation is C
As shown in the figure, it remained high except for a drop just before the end of freshwater tank operation, and an average of 91% was obtained.

Example 2 Using the same DC power output as in Example 1, an electrodialysis apparatus was operated in a conventional batch manner to desalinate a saline solution. In this batch operation, a desalination water tank and a concentration water tank are provided, and an aqueous salt solution is circulated from each tank to the desalination chamber and concentration chamber of the electrodialysis tank, respectively, and when the salt concentration of the water in the desalination tank is reduced to the freshwater concentration, , we adopted a common method of extracting the fresh water and circulating new water to be treated. Introduce 0.8 ^m3 of 3.5% saline solution as raw water into the desalination water tank, and the freshwater concentration is 500 ppm.
Batch operation (6 times) was carried out until .

The results are shown in FIGS. 7A and 7B.

During each batch operation, as shown in Figure 8B, the power usage efficiency is always approximately 100% once, but the time when the power usage efficiency is low is overwhelmingly long.
The average power usage efficiency during hourly operation was 66%. In the conventional batch method, the method of the present invention (Example 1)
It was found that the power usage efficiency was significantly lower than that of the conventional method.

〔Effect of the invention〕

According to the present invention, since the fluctuating output of the solar cell can be used efficiently, there is an excellent effect that the desalination efficiency can be increased.

[Brief explanation of the drawing]

Figure 1 is a voltage-current characteristic diagram showing the power generation characteristics of solar cells, Figure 2 is an equivalent circuit diagram of an electrodialysis device using solar cells, and Figure 3 is a schematic diagram showing the operating conditions of solar cells at their maximum output. , FIG. 4 is a diagram showing the relationship between the salt concentration of the water to be treated circulating through the electrodialysis apparatus and the electrical resistance of the electrodialysis tank, FIG. 5 is a flowchart of the electrodialysis apparatus according to the method of the present invention, and FIG. Figure A,
B and C are diagrams showing operation details according to the present invention, No. 7
Figures A, B, and C are diagrams showing the results of operation according to the embodiment of the present invention, and Fig. 8 is a diagram showing the results of operation according to the conventional batch method. DESCRIPTION OF SYMBOLS 1...Solar cell, 2...Electrodialysis tank, 5...Intermediate concentration tank, 6...Fresh water tank, 7-10...Valve, 13...Pump.

Claims (1)

[Scope of Claims] 1. Two circulation tanks, a fresh water tank and an intermediate concentration tank, are provided for an electrodialysis apparatus that desalinates salt water using a solar cell as a power source, and when the solar cell has a high output, the intermediate concentration tank and the The raw water is desalinated to an intermediate concentration using an electrodialysis device, and the desalinated water is transferred from the intermediate concentration tank to the desalination tank, and when the output of the solar cell is low, the desalinated water is connected to the desalination tank and the electricity. A saltwater desalination method characterized by desalting to a freshwater concentration using a dialysis device. 2. Two circulation tanks, a fresh water tank and an intermediate concentration tank, are provided for an electrodialysis device that desalinates salt water using a solar cell as a power source, and the intermediate concentration tank and the electrodialysis device are used when the solar cell has a high output. The raw water is desalinated to an intermediate concentration, and the raw water is added to the intermediate concentration tank to adjust the salt concentration, and the desalted water is transferred from the intermediate concentration tank to the desalination tank. A method for desalinating salt water, characterized in that desalinated water is desalinated to a freshwater concentration using the desalination tank and the electrodialysis device.