JPS6357086B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a salt water desalination method using an electrodialysis method using a solar cell as a power source.

[Background of the invention]

In the salt water desalination method using electrodialysis, a large number of cation exchange membranes that selectively permeate cations and anion exchange membranes that selectively permeate anions are alternately placed at intervals into an electrodialysis tank. Line them up,
When salt water is introduced between them and a DC voltage is applied from the outside, cations migrate toward the negative electrode and anions migrate toward the positive electrode. A chamber) and a chamber (desalination chamber) where cations and anions are reduced are generated alternately, so fresh water is obtained by taking out the liquid from this desalination chamber, and concentrated salt water taken out from the concentration chamber is discarded into the sea. This is what I did.

The electrodialysis method has characteristics such as simple pretreatment, high membrane durability, and low operating pressure, and is suitable for small to medium capacity desalination equipment. In recent years, new technologies such as high-temperature operation technology and new structural equipment using this method have been developed, and significant performance improvements have been observed. on the other hand,
Solar cells are seen as a promising new energy source and are being actively researched and developed, with remarkable progress being made in recent years in improving their performance and reducing manufacturing costs.

By the way, there are many regions, including the Middle East, that are generally blessed with solar energy but are not blessed with water resources. In these regions, there is a growing interest in desalination equipment that uses solar energy. Among these, electrodialysis freshwater equipment using solar cells as a power source is considered to be the most promising, reflecting recent technological innovations in both technologies.

An electrodialysis desalination system using solar cells as a power source has the following advantages.

(1) Electrodialysis equipment uses DC power as separation energy, so solar cell output can be used directly.

(2) Power consumption can be reduced by operating the electrodialysis machine at high temperatures using solar heat. In other words, it is possible to realize a process of simultaneously utilizing solar heat and sunlight.

However, the output of solar cells varies greatly depending on daily fluctuations in the amount of solar radiation. How to use this fluctuating power efficiently is a problem not limited to electrodialysis apparatuses but to devices in general that serve as a load for solar cells. Fortunately, desalination plants can store fresh water and can be operated only during the day. In particular, the electrodialysis method is easy to start and stop, and the membrane is resistant to microorganisms, so no special consideration is required even when the operation is stopped at night. Therefore, the technical issue in desalination methods and equipment using electrodialysis using solar power generation is how to efficiently use the battery output, which fluctuates widely depending on daily changes in solar radiation, for electrodialysis. be.

Conventionally, in order to solve this technical problem, a method has been proposed in which a lead-acid battery is used to smooth the power. (FY 1981 Sunshine Project Commissioned Research Results Report ``Research on Solar Energy Systems'').
However, this method requires a large amount of expensive lead-acid batteries, which are difficult to maintain and manage, and in addition, the efficiency decreases significantly as the batteries are charged and discharged.

[Purpose of the invention]

In view of the above-mentioned problems and the problems of the prior art, the present invention provides a method for desalinating salt water using an electrodialysis tank that utilizes solar cell output, which varies greatly depending on the intensity of solar radiation, with high efficiency.

[Summary of the invention]

A feature of the present invention is that an electrodialysis tank that desalinates salt water using solar cells as a power source is provided with a circulation tank that desalinates fresh water and two or more intermediate concentration circulation tanks. At the time of output, the raw water is desalted in stages using the electrodialysis tank and the intermediate concentration circulation tank,
Transferring intermediate concentration water at the final stage to a circulation tank for desalination to the fresh water level, and transferring desalination water from the circulation tank for desalination to the intermediate concentration level or fresh water depending on the output of the solar cell to the electrodialysis tank. and the circulating tank.

FIG. 2 shows the power generation characteristics of the solar cell, and FIG. 3 shows an equivalent circuit when an electrodialysis tank (electrical resistance R) is connected to the solar cell as a load. The voltage (Ei)-current (Ii) characteristics at various amounts of solar radiation are shown by solid lines in FIG. 2 when the resistance R of the load varies over a wide range. The broken line in FIG. 2 shows the operating conditions for obtaining the maximum power, and the intersection of the solid line and the broken line is the maximum output condition (voltage and current value, ie, load resistance value) under the solar radiation conditions of the solid line. As can be seen from FIG. 2, the voltage under the maximum output condition is approximately constant regardless of the amount of solar radiation, and the current value varies greatly depending on the amount of solar radiation.

From the above, in order to use solar cells efficiently,
By changing the load resistance (in this case, the electrical resistance of the electrodialyzer) according to the amount of solar radiation, the maximum efficiency voltage -
It can be seen that it is necessary to maintain current conditions.

That is, as schematically shown in Fig. 4, the load resistance R (electrodialysis tank electrical resistance) is increased at times near sunrise and sunset when the amount of solar radiation W is small, and the load resistance R is increased near noon when the amount of solar radiation is large. By reducing , it is possible to maintain the maximum output characteristic for extracting the maximum output from the solar cell.

By the way, the electrical resistance of the electrodialysis tank changes depending on the salt concentration of the water to be treated, that is, the salt water flowing through the electrodialysis tank.

Figure 5 shows the treated material obtained by desalinating an aqueous salt solution containing salt as a main component in an electrodialysis tank with an effective area of 0.23 m ² per membrane, 200 pairs of membranes, and a membrane spacing of 0.8 mm at room temperature. The relationship between the water salt concentration (more precisely, the logarithmic average value of the salt concentration at the inlet and outlet of the electrodialysis tank) and the electrical resistance of the electrodialysis tank is shown. For example, seawater (salt concentration approximately 3.5
%) to fresh water (salt concentration 0.05%), the electrical resistance of the electrodialysis tank changes greatly from 0.7Ω to 14Ω.

Therefore, if the salt concentration of the water to be treated flowing through the electrodialysis tank is changed according to the amount of solar radiation so that it becomes higher when the solar cell output is high, the conditions for obtaining the maximum output from the solar cells can be satisfied. Can be done. In other words, when the output of the solar cell is low, it can process water with low salt concentration, and when the output is high, it can process water with high salt concentration.
What is necessary is to match the operating conditions of the electrodialysis tank with the fluctuations in the output of the solar cell.

By the way, the basic and most frequently used operating method for electrodialysis desalination equipment is the batch operation method, in which the water to be treated is circulated many times through a circulation tank and into an electrodialysis tank to obtain fresh water with a predetermined salt concentration. ing. In this operation method, the salt concentration of the water to be treated flowing through the electrodialysis tank changes over time, but the aspect of this change is completely unrelated to the fluctuations in the output of the solar cells. High power utilization efficiency cannot be obtained simply by directly connecting the device and the solar cell.

In the electrodialysis saltwater desalination apparatus of the present invention,
When performing batch operation of an electrodialysis machine, multiple circulation tanks are installed for the water to be desalinated, the concentration range for each circulation tank is determined, and the circulation tanks are used according to the output of the solar cells. Therefore, when the output is high, a high concentration liquid is partially desalted, and when the output is low, a low concentration liquid is desalted, and the electrodialysis apparatus is operated with high power utilization efficiency. Furthermore, according to a preferred embodiment of the present invention, the partially desalinated liquid at high output is stored;
If low output continues for a long time due to cloudy weather, etc., by desalting this, the output during cloudy weather can be effectively utilized.

[Embodiments of the invention]

An example of the present invention will be explained with reference to FIG. In this example, two intermediate concentration circulation tanks and one circulation tank for desalination to fresh water are used.
It goes without saying that more than one group may be used.

In FIG. 1, the apparatus includes an electrodialysis tank 1, three types of circulation tanks: a circulation tank 2A for partially desalinating raw water to an intermediate concentration, and a circulation tank 2B for desalting from the intermediate concentration to a lower intermediate concentration. , a circulation tank 2C (hereinafter, the concentration corresponding to circulation tank 2B is referred to as high intermediate concentration, and the concentration corresponding to circulation tank 2C is referred to as low intermediate concentration), which further desalinates fresh water (salt concentration 500 ppm in the case of drinking water), high intermediate concentration. It is composed of a concentrated liquid storage tank 3B, a low intermediate concentration liquid storage tank 3C, and the like.

When the battery output is low, not long after sunrise, the low intermediate concentration liquid stored in the storage tank 3C from the previous day's operation is circulated to the electrodialysis tank 1 using the circulation tank 2C to obtain fresh water. Next, when the battery output increases sufficiently, the high intermediate concentration liquid stored in the storage tank 3B from the previous day's operation is desalted to a low intermediate concentration using the circulation tank 2B and stored in the storage tank 3C. When the battery output increases further and reaches its peak, the raw water is desalted to a high intermediate concentration using circulation tank 2A, and storage tank 3B
Store in.

The relationship between the battery output and the corresponding type of circulation tank (salt concentration) in the above operating method is schematically shown in FIG. As is clear from the figure, when the battery output is low, a low salt concentration solution flows to the electrodialysis tank (therefore, the electrical resistance of the electrodialysis tank is high), and when the battery output is high, a high salt concentration solution flows to the electrodialysis tank. (Thus, the electrical resistance of the electrodialysis tank is low), this allows operation close to the maximum output characteristics as shown in Figure 4, making it possible to use the solar cell output with high efficiency. . In addition, on cloudy days, the low output state of the battery continues for a long time, so we increased the capacity of storage tanks 3B and 3C, and gradually reduced the amount of intermediate concentration liquid stored in these tanks during periods of continuous sunny weather. By increasing the amount and consuming it on cloudy days, it is possible to maintain highly efficient operating conditions all day long even on cloudy days.

The reason why the power consumption fluctuates minutely over time in FIG. 6 is because the salt concentration of the liquid flowing through the electrodialysis tank periodically fluctuates during batch operation.
The period naturally varies depending on the relative size of the circulation tank capacity and the electrodialysis tank capacity.

Specific examples and comparative examples will be described below.

Example 1 Using the method and device explained in Fig. 1 using a DC power supply that simulates the output characteristics of a solar cell.
A 3.5% saline solution was desalted to a salt concentration of 500 ppm. The electrodialysis tank used was an electrodialysis tank with a current carrying area of 0.23 m ² per membrane and a membrane number of 200, the electrical resistance characteristics of which were shown in FIG. 5 above. The capacity of all three circulation tanks was 1.2 m ³ , and the amount of liquid charged was 1.0 m ³ . The capacity of the two intermediate concentration liquid storage tanks was 5 m ³ . The output of the DC power supply was a sine wave with a period of 16 hours with a maximum output of 10 kW, and the electrodialysis device was operated for the first 8 hours.

Results obtained by operating three types of circulation tanks A, B, and C corresponding to each concentration from raw water (3.5wt%) to 1.5%, from 1.5% to 5000ppm, and from 5000ppm to freshwater (500ppm). is shown in Figure 7. As can be seen in the figure, the relationship between the power generation amount (output of the DC power supply) shown by the broken line and the power consumption amount in electrodialysis shown by the solid line shows a fairly good agreement, and the 8-hour average power usage efficiency (consumption x 100 / power generation) amount) was 83%, and the amount of water produced was 6 m ³ /8 h.

Comparative Example 1 An electrodialysis apparatus was operated batchwise with the same DC power output as in Example 1. In this batch operation, one demineralized water circulation tank and one concentrated water circulation tank are installed, and the aqueous salt solution is circulated from each tank to the electrodialysis tank desalination chamber and concentration chamber, and the liquid in the desalted water circulation tank is When the salt concentration drops to the freshwater concentration, we used the standard method of extracting the freshwater and filling it with new water to be treated. Figure 8 shows the results obtained by operating under the conditions of a (food) salt concentration of raw water (water to be treated) of 3.5%, an amount of raw water charged into the desalinated water circulation tank of 0.8 m ³ , and a freshwater concentration of 500 ppm. According to the figure, there is a considerable difference between the power generation amount (DC power output) shown by the broken line and the power consumption in electrodialysis, with an 8-hour average power usage efficiency of 66% and a water production amount of
It was 4.8m ³ /8h.

〔Effect of the invention〕

According to the present invention, using relatively inexpensive equipment,
The solar cell output, which varies greatly depending on the amount of solar radiation, can be used directly with high efficiency to operate the electrodialysis device. Also, store the desalted solution to an intermediate concentration.
By desalinating this on cloudy days, it is possible to utilize solar cell output with high efficiency even on cloudy days.

[Brief explanation of the drawing]

Figure 1 is the flow of the electrodialysis desalination equipment according to the present invention, Figure 2 is a voltage-current diagram showing the power generation characteristics of the solar cell, Figure 3 is the equivalent circuit diagram of the solar cell, and Figure 4 a. , b, c, and d are schematic diagrams showing the conditions for using the solar cell at its maximum output, Figure 5 is a diagram showing the relationship between the salt concentration of the liquid flowing through the electrodialysis tank and the electrical resistance of the electrodialysis tank, and Figure 6 is a diagram showing the conditions for using the solar cell at maximum output. FIG. 7 is a schematic diagram showing the operating style of the embodiment, FIG. 7 is a characteristic diagram showing the operating results according to the embodiment of the present invention, and FIG. 8 is a characteristic diagram showing the operating results according to the conventional batch method. 1...Electrodialysis tank, 2A, 2B, 2C...Demineralized water circulation tank, 3B, 3C...Intermediate concentration liquid storage tank, 4...
…pump.

Claims (1)

[Claims] 1. For an electrodialysis tank that desalinates salt water using a solar cell as a power source, a circulation tank that desalinates up to one fresh water and two or more intermediate concentration circulation tanks are provided, and the electrodialysis cell is The method includes sequentially desalinating raw water in stages using a tank and an intermediate concentration circulation tank, and transferring the intermediate concentration water at the final stage to a circulation tank that desalinates the fresh water, and increasing the intermediate concentration according to the output of the solar cell. A method for desalinating salt water, characterized in that desalinated water or desalinated water in a circulation tank for desalination to fresh water is desalinated by the electrodialysis tank and the circulation tank.