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Our planet is in many ways unique, especially when compared with its neighbors in the solar system. Most of the Earth’s surface, about 70%, is occupied by oceans, seas, rivers and lakes. However, only a small fraction of these water reserves are fresh. The lion's share is sea and ocean water, which can be drunk only if you want to get dehydrated, have problems with your kidneys and really love to re-read the instructions for an air freshener in the “room of thoughts”. In other words, salt water from its direct use will be more harm than good.

Today we will meet with you a study in which a group of scientists from the Massachusetts Institute of Technology (USA) developed a device that allows desalination of water using solar energy without any human involvement. What are the features of the invention, what is the principle of its operation and how effective is it? We learn about this from the report of the research group. Go.

Study basis


Despite the huge water reserves of the planet, given that only a fraction of them are suitable for consumption, almost a third of the world's population suffers from a shortage of drinking water. Using seawater as a source of fresh water is the most logical and relatively easy to implement way to solve this global problem.

As the scientists themselves say, modern desalination plants work quite efficiently, but they have a number of very banal shortcomings. One of which is their high cost and the need for a developed infrastructure that can support the work of such a complex mechanism.

Passive desalination systems that use solar energy are also quite effective. But the cost of the fresh water they produced is quite high, not to mention the low efficiency of converting solar energy to fresh water.

Recently, the emphasis of development has been put on the creation of more efficient passive systems. In the framework of such studies, it was possible to create systems based on the localization of solar heat. However, the conversion efficiency of solar radiation to steam is below 100% if the enthalpy of evaporation * is lost in the environment.
Evaporative enthalpy * - the amount of heat required to convert a liquid substance into a gaseous one.
Scientists believe that the key to increasing the efficiency of desalination systems can be the collection and reuse of vaporization enthalpy. This idea has already been implemented in large distillation plants. However, with small-sized distillation plants, certain technological difficulties arise.

Previous studies have already demonstrated compact solar heat localization systems that reused the enthalpy of vaporization to generate electricity, convert sea water to steam, or both. However, there is a theory that the effectiveness of such mechanisms can be much higher than it is now.

The authors of the work we are considering today state that the fundamental limitations of overall productivity and the corresponding design strategies for desalination plants have not been well studied. For this reason, we have no idea how exactly it is possible to create and improve precisely compact and inexpensive desalination systems in production.

To solve this puzzle, scientists conducted a series of analyzes to get a fundamental idea of ​​the heat and mass transfer inside the device. This knowledge can greatly contribute to the optimization of the desalination device being created, which will work more efficiently.

Scientists also created a prototype of a ten- stage TMSS ( thermally-localized multistage solar still) without the accumulation of salt, using inexpensive materials, and showed a record high efficiency of conversion of solar radiation into steam (385%) with a productivity of 5.78 l / m 2 per hour.

Research results



Image 1: TMSS prototype design diagram.

The first stage of the prototype, on which sunlight falls, consists of a layer of optically transparent silica (SiO 2 , silicon dioxide) airgel thermal insulation, a solar collector, a capillary wick * and a capacitor. All these layers are located along the direction of solar radiation ( 1a ).
Capillarity * - the effect of raising or lowering the liquid through narrow tubes, channels or porous bodies (capillary wick - from the wick in a kerosene lamp).
Each of the subsequent steps consists of a capillary wick and a condenser separated by air ( 1b ). The condenser of the last stage is located in the brine (highly concentrated brine) to maintain its temperature close to the environment, which provides a large vapor pressure gradient at each stage.

The solar collector, located between the silica layer and the first capillary wick, converts solar energy into heat. The airgel layer of silica suppresses heat loss from the solar collector due to conductivity, convection and radiation due to its ultra-low thermal conductivity and high opacity in the infrared range.

Thermal energy is transferred from the collector to a capillary wick attached to the rear side, where the brine rises capillary and evaporates due to elevated temperature. Steam passes through the air gap between the evaporator and the condenser, releasing thermal energy through condensation. Condensed pure water is collected at each stage, while the released thermal energy is transferred to start evaporation in the next stage, realizing the recirculation of the enthalpy.

The TMSS architecture provides high-performance desalination thanks to three key features that optimize heat and mass transfer.

Firstly, the vaporization enthalpy is recirculated through a multi-stage configuration in which the latent heat generated in the previous stage is used in the next stage to activate the evaporation.

Secondly, unlike traditional approaches to the localization of solar heat, the performance of which depends on heat-insulating absorbent materials that provide absorption of solar energy and water evaporation on one interface, the TMSS architecture shares these functions: the absorption of solar energy occurs on the front side, while while interfacial heating and the resulting fumes are on the other side of the stage.

This design allows the use of inexpensive materials in the development, since there is no need for a solar collector with moisture-absorbing properties or in special capillary wicks with a certain degree of solar absorption.

Thirdly, vertically arranged installation steps with adjustable tilt angles can significantly reduce spurious heat losses due to the small contact area between the thin-film evaporator and bulk brine ( 1a ). In addition, this architecture allows the installation to work at different positions of the Sun, caused by geography or seasonal changes.

Researchers note that in order to achieve the best performance, many design parameters should be optimized, including the width of the device ( a), the thickness of the air gap ( b ) between each stage of the device and the total number of stages ( n ). For this prototype, a step height of 10 cm was chosen, since it is approximately equal to the length of the capillary wick. The choice of b and n was determined taking into account the heat and mass transfer at each stage.

For example, decreasing b can decrease the vapor transfer resistance, but increase the conductivity loss through the gap. This conductivity loss at this stage can be reused by the next stage to accelerate evaporation, however, it reduces the formation of steam at the previous high-temperature stage, which reduces the overall efficiency of converting solar radiation into steam.

If you increase the number of steps (n ), in theory, the efficiency will increase, but this “bonus” will become less when the number of steps is critical and the efficiency begins to fall due to the inevitable heat loss from the side walls of the steps.

To determine what the values ​​of a , b and n should be , scientists created a theoretical model.

The model showed that for such a device (10 cm high), the air gap ( b ) should be 2.5 mm, which corresponds to the peak efficiency (650%) of the conversion of solar radiation into steam.

Knowing that a = 10 cm and b= 2.5 mm, you can set the optimal value for the number of steps. It was found that the efficiency of the installation will increase very slightly if the number of steps ( n ) exceeds 20 pieces (efficiency of about 600%).

Scientists decided to use an air gap of not 2.5, but 5 mm. Thus, you can be sure that the gap is larger than the typical droplet size on the condenser, then the condensate will not touch the evaporator and can be collected.

The number of steps was 10 to demonstrate that even such a small device can work efficiently.

Given the selected parameters ( a = 10 cm, b = 5 mm and n = 10), scientists suggested that the efficiency will be about 417%.

Do not forget about the side walls of the steps, since heat loss due to an increase in their number can reduce the efficiency of the device. Therefore, insulator layers 1.27 cm thick were added to the sides.

Also, the theoretical model shows a decrease in efficiency when the air gap increases to 100 mm (from 417% to 300%) and to 1.5 cm (from 417% to less than 250%).


Image No. 2

The simulation result was the TMSS prototype, shown in Figure 2a. This ten-stage device consists of eleven nylon frames (Nylon PA12), which were made using 3D printing. A commercially available 10x10 cm solar collector (B-SX / TL / ZZ-1.88) was installed on the rear of the first frame. There was also a 10x10 cm glass plate with a thickness of 1 mm with an antireflection coating on the front to protect the collector ( 2a ). A monolithic silica airgel (9.5x9.5 cm and 5 mm thick) was placed between the solar collector and the glass plate and served as transparent thermal insulation. The remaining 10 frames were identical to each other. In each of them, a capacitor of 10x10 cm aluminum plate and 0.5 mm thick was placed ( 2b) The capacitor was coated with a 1 μm Teflon layer, which allowed the droplets to drain and not linger on the capacitor. The contact and advance angles on the hydrophobic coating were 108.2 ° and 103.2 °, respectively ( 2c and 2d ). Hysteresis with a small contact angle (～ 5 °) made it easy to remove condensed droplets of millimeter scale under the action of gravity. To effectively collect desalinated water, a slit with an inclination angle of ～ 5.7 ° was made in the lower part of the frame, which was connected to the outlet.

The high transparency (﹥ 95%) of glass and silica airgel, as well as the high absorption capacity (～ 93%) of the solar collector, were measured using a UV-Vis-NIR spectrophotometer ( 2e ).

The most interesting thing is that ordinary paper towels 10 cm wide and 15 cm long, which were attached to the back of each capacitor ( 2f ), were used as a capillary wick . The cellulose fibers of these towels create numerous micropores with diameters ranging from 10 to 100 microns ( 2g and 2h ), which create capillary pressure and provide quick transport of water.

The total cost of materials used to create the entire installation was about 1.54 dollars. At the same time, 70% of the cost falls on nylon frames. They are partially hollow, but if you use completely hollow frames, then the cost will decrease.


Image No. 3

Initial assessment of the characteristics of the tested installation (scheme 3a) was carried out in laboratory conditions. The artificial sun generated a flow of 1000 watts per m 2 .

For a detailed assessment of thermal characteristics, 12 thermocouples were simultaneously used, which measured the temperature response in real time: 10 pairs controlled the temperature of the evaporator / condenser of each stage ( T 1 - T 10 ); 1 pair recorded the temperature of the last stage capacitor ( T b ) and 1 more pair recorded the ambient temperature ( T atm ) ( 2a and 3a ). The data collected on the loss of temperature and mass were processed by a computer. The temperature dynamics of 10 steps for 3 hours is shown in3b .

Due to the high thermal resistance of the airgel and the isolation of the side walls of the steps, the temperature of the first step literally reached 15 ° C in 15 minutes ( T 1 ), and then it reached a stable state of 72 ° C. The remaining steps also gradually reached such a stable state after 100 minutes from the start of irradiation.

Although the last-stage capacitor was inserted into the water tank, its temperature was still slightly higher than the ambient temperature ( T b～ 25 ° C) in a stable state due to thermal resistance through a thin aluminum sheet.

The mass change rate for a 10-stage device gradually increased and was maintained at a constant level of ～ 0.89 g / min after the establishment of a thermal stable state.

A similar dynamics of the behavior of the steps was described at the modeling stage ( 3c ), which considers the temperature-dependent vapor concentration and diffusion at each stage of the setup.

Condensed water began to flow from the outlet of the first stage about 8 minutes after turning on the artificial sun. Following it happened with the subsequent steps.


Demonstration of the start of the installation of the TMSS.

When the TMSS entered steady state after 100 minutes, there was a continuous flow of water from all ten holes.


Demonstration of the installation in stable mode.

The total weight loss was about 150 g, and about 113 g of water was collected after 3 hours of operation. Lost water was mostly represented by droplets that remained on the condenser, and steam leakage during operation of the installation. If we subtract the contribution of evaporation in unlit conditions, it turns out that
the steam production rate of the ten-stage TMSS in the stationary mode was 5.78 l / m 2 per hour.

Further, to better understand the mechanism of heat and mass transfer inside the TMSS, an analysis of the temperature and steam flow of each stage in a stationary state ( 3d ) was carried out . The temperature of each stage was averaged over the last hour of measurement (i.e., from 120 minutes to 180 minutes of the test).

The temperature measurement showed a linear decline between the steps due to the same thermal resistance of each of them. To assess the contribution of each stage, the concentration of saturated steam was calculated based on the temperature of the evaporator and the vapor stream.

The steam flow showed an exponential decrease with each subsequent stage (3d) due to heat loss on the side wall and the non-linear relationship between temperature and vapor concentration. In total, the first three steps made the largest contribution - about 45% of the total steam flow. This observation in practice shows why adding a large number of steps will be simply inefficient and irrational.

To clearly demonstrate the importance of recirculation of the enthalpy of vaporization, a comparative analysis of the performance of a ten-stage device with a single-stage was performed. The efficiency of a single-stage system was only 81% ( 3 ), as predicted by the theoretical model (about 83%). The corresponding water output was 1.21 l / m 2 per hour, which is about five times less than the capacity of a ten-stage installation ( 3s ).

The insulation of the side walls and its importance have also been tested. In the absence of insulation, the efficiency dropped to 286%, while in the presence of isolation it should reach 326% ( 3 ).


Image No. 4

The graph above shows a comparison of the effectiveness of the tested TMSS installation (marked with an asterisk) and previously developed equivalents. As we can see, the indicators of the developed installation literally break all records.

The next important indicator that the researchers checked was the degree of desalination of the TMSS prototype using water with a 3.5% NaCl content as an example. After desalination, the mineralization of water (0.0005 wt.%) Was reduced by four orders of magnitude ( 5a ).


Image No. 5

Moreover, the international standard for drinking water, established by the World Health Organization, is 0.02 wt.%.

Another important aspect is the accumulation of salts, which can interfere with the continuous operation of the installation. To test the resistance of the prototype to this problem, a test was carried out in which the installation was irradiated with light at 1500 W / m 2 for 1.5 hours. The total laboratory solar radiation was 5.25 kWh per m 2 , which exceeds the average annual daily solar radiation in the United States. These 3.5 hours simulated the day, after which the radiation was turned off to simulate the night. Such conditions lead to the rapid accumulation of salts and a reduction in diffusion time. On 5b shows the dynamics of accumulation and salt rejection for 5.18 hour test. In general, the evaporator showed a high ability to remove salt throughout the test.

Salt accumulation was observed only in the two upper corners, which had the greatest diffusion resistance, since they were at the farthest distance from the brine (the dynamics of salt accumulation in these corners is shown by a white dashed line at 5b ).

The first two hours of salt did not accumulate, since NaCl has a high diffusion solubility in water. But after 2 hours, the salt began to crystallize, and after 3.5 hours, about 45% of the area in 4x4 cm corners were covered with salt. However, after 15 hours of normal operation, the accumulated salt completely diffuses.

All the above results of tests and observations were obtained in laboratory conditions. Naturally, environmental conditions cannot be controlled, as in a laboratory. Therefore, similar tests were carried out, but already in the open air (the test was carried out in July 2019).


Image No. 6

The prototype was located on the roof of the institute's campus ( 6a and 6b ). To assess changes in temperature, 12 thermocouples were also used, and a pyranometer was used to assess changes in incident solar flux. The camera recorded all the changes, including the amount of water collected in a special cylinder with a volume of 100 ml.

The open-air experiment began at 11:10 local time and ended at 16:00. The temperature of each stage rose rapidly during the first hour, when the temperature of the solar collector exceeded the ambient temperature by more than 30 ° C ( 6s ).


Demonstration of the installation during the test in the open.

Water began to flow out of the first stage after 20 minutes. The solar flux varied significantly from 200 to 800 W m 2 due to scattered clouds ( 6d ), which led to fluctuations in the temperature of the solar collector ( 6c ). Due to cloud cover, the quite expected fluctuation in the temperature of the solar collector from 50 to 65 ° C was observed.

In images 6e, it is clearly visible how much water was collected during the experiment: 72 ml in 4.5 hours, i.e. 2.6 l kW -1 per hour.

It is logical that such a small device will not be able, given the variability of the weather, to satisfy the daily water rate for humans (about 3.2 l). To do this, you need to assemble an array of prototypes with an area of ​​1 m 2 (10 per 10 pieces), which can collect about 10-20 liters of water per day, depending on weather conditions and the season.

For a more detailed familiarization with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue


In this work, scientists described the characteristics of their prototype desalination plant using sunlight. While most modern installations require either large financial investments or certain conditions (both natural and infrastructural), the prototype created is very cheap and very effective. The total cost of materials amounted to only 1.54 dollars, and the water yield of 5.7 l m2 per hour.

Scientists call the foundation of their creation an understanding of the principles of thermal and mass transfer inside the device being developed. After all, if you know what and how is happening with the various participants in the process, you can adjust their behavior.

The problem of fresh water is becoming more and more every year, although many do not notice it, because they live in conditions of fairly trouble-free access to water resources. However, there is a problem and cannot be ignored. This study shows how effective simple and cheap devices that implement the fundamental principles of the natural sciences can be. All ingenious is simple. This phrase sounds often enough, although sometimes it is used for other purposes, but in the case of the prototype we examined today, it fits perfectly.

Friday off-top:

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Thank you for your attention, stay curious and have a great weekend everyone, guys! :)

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