Solar desalination technology

Water resources are one of the material foundations for the survival and development of human society. With the increase of the world's population and changes in human life style, the shortage of freshwater resources has increasingly attracted people’s attention and attention. In general, China is a poor country with a population that accounts for 20% of the world's population. Water resources account for only 6%, and per capita possession is only 1/4 of the world's per capita possession. The lack of water resources has seriously hampered the economic development of our country and destroyed the ecological environment. As an open source incremental technology, seawater desalination has become an important way to solve water resources problems. However, our country’s technology in this respect is still far from compared with foreign countries.

The gap with foreign countries

1. Technology

The basic research on the core components, materials, and hydropower co-production of low-temperature and multi-effect technologies needs to be in-depth, equipment verification and environmental conditions cannot meet the requirements of technological development, and there is a lack of engineering practices for the design, manufacture, installation, commissioning, and operation and maintenance of large-scale seawater desalination devices. There is an urgent need to form complete sets of technology and training teams through scale demonstration. Key components and materials such as reverse osmosis membrane modules, high-pressure pumps, energy recovery, and water treatment chemicals are still mainly imported, lacking large-scale reverse osmosis desalination complete sets of engineering technologies and practices, and urgently require the formation of high-pressure pumps, energy recovery, membrane modules, etc. The independent technology and mass production of key equipment will form complete sets of technologies through scale demonstrations to deal with foreign companies’ domestic competition. The concept of nuclear seawater desalination has been proposed for many years, and there is still a lack of engineering practice; the interface between nuclear reactors and seawater desalination is still at the research and design stage, and the process needs to be opened to form a complete set of technology and equipment systems.

2. Industrial scale.

China's seawater desalination projects are mostly on the 1,000-ton scale, while foreign countries have reached the level of 100,000 tons. China's desalination water production accounts for only 0.3% of the world's total production, which is a significant gap with foreign countries.

3. Implementation mechanism.

There is no special agency for overall coordination and no industry alliance has been formed. Desalination of seawater must be planned in a coordinated manner in the areas of government guidance, industry coordination, industrial policy, and technological innovation, and the interests of all parties must be fully coordinated in order to create synergy and promote industrial development.

4. Demonstration and input.

The state's lack of capital investment in scale demonstration projects has resulted in insufficient demonstration of scale, which has constrained the development of technology and transformation of results in this area.

Solar water desalination technology development status

With regard to the current seawater desalination technology, the cost issue has been the biggest obstacle to desalination technology. Take 10,000 tons/day desalination plant for reverse osmosis, the general chemical consumption is 0.3~0.5 yuan/ton water; the power consumption is about 2.2~ 2.5 yuan/ton water; membrane replacement cost 0.3~0.5 yuan/ton water; employee wage and welfare about 0.2 yuan/ton water; fixed assets depreciation fee 0.9~1.2 yuan/ton water; equipment maintenance and maintenance cost 0.2~0.4 yuan/ton Water; management fee is less than 0.1 yuan/ton water. In this case, without considering loan interest, the water cost for the general membrane desalination project is 4.2-5.4 yuan/ton water, of which the power cost and depreciation are the two largest parts of the cost of water production.

To save energy and some special needs, people have developed solar desalination technology. Solar desalination methods can be divided into two major categories: direct and indirect methods.

Direct method

The direct method is a method of applying solar collectors to turn sunlight into heat energy to directly heat seawater and distill it into fresh water. It is divided into shallow disk type (also known as ceiling type, horizontal type), tilt type and multi-effect type three distillation devices.

(1) Shallow disk type: It consists of a shallow tray (20~40mm) and transparent cover. The sun shines through the transparent cover onto the black matter in the pan, causing it to generate heat, warming the sea water in the pan, producing steam, and the steam hitting the transparent cover is cooled by the outside air, condenses into water droplets, and flows into the freshwater collecting tank. In the case of concentrated seawater discharge prior to deposition of scale, the water production rate is relatively low, and the daily production water is usually about 2 to 4 kg/(m2·d). Japan's screen factory in Pingfeng Island real-faced trial of this method found that: the adoption of insulation measures can increase water production 20~30%, shallow shallow sea water production when the water is high, but too easy to generate scale, through filtration, precipitation, pickling and scratching Sterilization and other methods to prevent or remove scale in the shallow pan. This law has been promoted in Kuwait, Greece, India and Japan.

(2) Tilt type: Two kinds of tilt-type sub-distributor tray type and inclined material absorbent wool type. The inclined shallow disk type is to change the water tray from horizontal to inclined, so that the seawater flows along the slope, so as to increase the collection efficiency and promote evaporation. The working principle is the same as that of the shallow disk type, but the water layer is shallower. With this method, water production can be increased by 30 to 60%, but technical difficulties are more. It adjusts the flow of seawater according to the amount of evaporation, and it is easy to precipitate scale due to the shallow water layer. There are no reports of using this type of device at present. The sloping hair type is designed to store seawater by absorbing water on the surface of a tilted water tray to make the seawater layer shallower so as to reduce the heat capacity and increase the amount of water produced. There are still some problems to be solved in this method, so there is no practical example yet.

(3) Multi-effect type: The heat when the shallow disk type and the inclined type both condense the steam into water droplets is wasted. In order to increase the efficiency of the use of thermal energy, this part of the heat is used to repeatedly heat seawater to increase the amount of water produced, and a multi-effect distiller has been developed. The working principle is that the collector board is heated by sunlight, and the water absorption hair is on the opposite side of the collector board, and the seawater is immersed. When the seawater evaporates due to heat, the water vapor meets the following cold separator to condense into water droplets, and the heat generated when condensing Heat the seawater on the back of the baffle to allow it to evaporate. This repeated evaporation - concentration - evaporation, and finally the heat through the bottom of the radiator plate discharge. Reflectors can also be installed to increase heat collection to increase seawater production. Using this method, the daily water production is 7.5 to 10 times that of the shallow disk type. The key to this method is the water-absorbing material, the adhesive and the seawater corrosion of the collector plate.

Indirect method

The indirect method uses a heat collector to convert light energy into heat energy, or solar cells to convert light energy into electric energy, and use it as a method for making fresh water from the energy of a seawater desalination device. This method is divided into the following categories:

(1) Distillation: Under normal pressure, the boiling point of seawater is about 100.5°C. Evaporation of seawater requires about 2261 kJ/kg (540 kcal/kg) of heat, and the same amount of heat can be released when the steam condenses into water. According to this principle, people try to reuse the heat of water condensation to save energy. According to different utilization methods, the distillation method is divided into multi-stage flash evaporation method, multi-effect distillation method and vapor compression distillation method.

Multi-stage flash evaporation method: This method is applicable to larger-scale devices, and has a smaller adaptation range to load changes and higher costs.

Multi-effect distillation method: This method has a relatively high thermal efficiency, and it has a relatively large range of adaptability to changing heat sources. Therefore, the development speed is relatively fast, but it is currently not as widely used as multi-stage flash evaporation.

Steam compression method: When using this method, the power source of the compressor is provided externally at the start of the machine, and after the operation, it is driven by the latent heat of seawater vapor. This method is suitable for small desalination devices. The power of the compressor can also be powered by solar or sunlight. However, the cost of this method is relatively high and remains to be further explored.

(2) Reverse Osmosis: This method uses fresh water pressure to separate fresh water, and supplies the power required to drive the high-pressure pump through solar power.

(3) Electrodialysis: When the temperature of seawater rises, the viscosity of seawater decreases. Therefore, in this system, solar cells can be used to supply the energy required for electrodialysis, and solar heat can be used to increase the temperature of seawater, thereby reducing the consumption. Electricity. This method has not yet been promoted and applied.

The solar desalination plant began with a relatively simple direct method platter type distiller and developed various indirect methods that can improve energy utilization. The advantages of the direct method are: (1) Simple equipment structure, low construction cost, easy maintenance, (2) low cost of water production, and (3) application in small-scale scattered areas. However, this method has low water production capacity and can only solve drinking water. Among them, the inclined distillation apparatus needs to be further studied, and the multi-effect distillation apparatus can save energy, have high water production capacity, and develop relatively quickly. It is expected that there will be great development in the future. The indirect method is more complicated and the cost is higher, but its ability to create water is strong and it is suitable for medium-scale freshwater production. At present, some technical problems have been gradually solved, and the cost issue will be a major obstacle to popularization and application.

The indirect method is very important in energy gathering technology. The innovation in this area will influence the development and application of this law. The combination of a collector-type heat collector and a multi-stage flash device is very effective. If the cost of solar power can be rapidly reduced, it will be possible to promote it. At present, the best solution is a seawater desalination unit that combines a non-light collecting type and a multi-effect type distiller, and is studying a combination of a vacuum heat insulation type heat collector and a multi-effect type distiller. The South China Sea islands are located in the tropics, have long sunshine, and have sufficient solar energy to use. The development of solar desalination plants has extremely important significance for these areas and is worthy of study. As these islands are far from the mainland, they require simple equipment, easy operation, management and maintenance. From the current domestic and international research situation, the direct distillation method is most advantageous. At present, shallow disk distiller has been widely applied, tilted distiller is being studied, and multiple effects have not been reported. Although the law is relatively difficult, there are many problems to be solved, but there are promising developments that are worthy of research and exploration.

According to the information I am looking for, I personally prefer a new type of hot distillation seawater desalination - adsorption solar desalination system

The basic working principle of adsorption solar desalination system

Some solid substances such as zeolite activated carbon, lanthanum chloride, calcium chloride, etc., have strong adsorption characteristics to water vapor. By using this feature, these substances can be made into the adsorption and desorption beds of water vapor, and then the solar energy collector device can be used to drive the adsorption and desorption processes, and a solar adsorption desalination device can be formed. FIG. 1 is a schematic view of a solar adsorption type seawater desalination system.

In this device, the adsorbent bed containing adsorbent material can directly absorb sunlight, and the solar collector is both a generator (corresponding to the desorption process) and an adsorber (corresponding to the adsorption steam process). The adsorption process and the desorption process alternate. At night, the solar water heated by solar energy is used to heat the evaporator during the day. The seawater in the evaporator is heated to evaporate, and the steam enters the solar collector adsorber bed. The latent heat released from the adsorption bed adsorbing steam is taken away by the cooling system at the back of the bed. The brine formed in the evaporator exits the device. Finally, the adsorbent bed adsorbs steam sufficiently to reach saturation. During the day, the temperature in the adsorbent bed is gradually increased by the heat collector of the solar collector. When the partial pressure of water vapour on the surface of the adsorbent is greater than the saturation partial pressure of water vapor in the condenser, the valve 3 is opened, and the vapour enters the condenser and is condensed into product fresh water. The latent heat released during the condensation of water vapor is carried away by the cooling water and completes a cyclic process. In order to make the system work continuously, it is also possible to make a plurality of adsorption beds, staggering their adsorption and desorption times.

Characteristics and Key Technologies of Adsorption Solar Desalination System

Compared with the traditional seawater desalination method, the adsorption solar desalination technology has the following advantages:

1. The efficiency of producing fresh water is high, and the coefficient of energy performance is large. The coefficient of performance is an indicator describing the relative size of the energy input and output of a seawater desalination system. The coefficient of performance of a seawater desalination device is defined as the ratio of the heat required to produce fresh water to the heat supplied by the system. . The large coefficient of performance indicates that the supply of energy is converted into fresh water with high efficiency.

2. Many adsorbent materials are ordinary chemical materials. Cheap and easy to purchase, especially zeolites and activated carbon, are very cheap. Using this material as an adsorbent can greatly reduce the operating costs of the system. In addition, zeolites and activated carbon are non-toxic and odorless substances. Can fully meet the requirements of environmental protection.

3. The use of solar energy as a heat source not only saves electricity, but also can be used in remote areas and islands where electricity is scarce.

The system can be driven either by solar thermal energy or by the residual heat of other processes.

4. In addition to the low-power pump, the entire unit has no other moving parts. The system runs smoothly and quietly.

5. Seawater is concentrated in only one container and does not come into contact with the hottest parts of the system, reducing corrosion and scaling. The corrosion of seawater is less, and the requirements for other parts of the material are not high, so that the cost of corrosion protection can be reduced. Although the adsorptive solar desalination system has many advantages, there are still no commercial examples to date. The following key technologies for desalination of solar desalination have yet to be resolved:

1. Desorption temperatures required for general adsorbent materials are high, all between 80 degrees Celsius and 150 degrees Celsius, while solar collectors operate at high temperatures with low efficiency, resulting in low overall system efficiency. However, with the increasing maturity and perfection of solar energy application technology, the current use of heat pipe vacuum tube solar collectors to obtain 80 to 150 degrees Celsius medium temperature has reached a higher efficiency. Therefore, using the adsorption principle, the technical route of using solar energy for seawater desalination has regained the attention of people.

2. The thermal conductivity of the general adsorbent material is low, resulting in limited heat and mass transfer in the adsorption-desorption device. Therefore, the design of an efficient adsorption-desorption device is the key to the system. The process of heat and mass transfer is strengthened and the operating temperature of the total system is reduced. It is an improvement direction for designing an efficient adsorption-desorption device.

3. The energy density of solar energy is very low, and the thermal energy converted from solar energy is limited! The main direction of improving the system performance coefficient is to fully utilize thermal energy, reuse the latent heat of condensation of water vapor, and realize multi-effect operation of the system.

4. For multiple adsorption-desorption system, especially the combination of adsorption process and multi-effect distillation process, the coordinated operation of various components in the system is also the key to improve the system efficiency. Therefore, the development of a suitable automatic control system It is a prerequisite for improving the overall system efficiency.

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