The effect of porous media on the thermal storage performance of salinity-gradient solar pond

A solar pond is a system that uses a salt concentration gradient to store heat. The application of the system is conducive to solving the problems of long production cycles, low brine temperature, and salt algae in the solar pond lithium extraction process and the solar pond potassium enrichment process. Based on this, our research group laid porous media at the bottom of the solar pond and utilized the heat storage and adsorption capacity of porous media to improve the thermal storage performance of the solar pond. The experimental results show that foam ceramics have good thermal storage performance and the ability to reduce turbidity. Laying foam ceramics on the bottom of the solar pond can not only increase the thermal storage performance of LCZ but also help reduce the turbidity of the pool water. After adding porous media, the heat extraction position of the solar pool moves from the junction of LCZ and NCZ to the bottom of LCZ. In addition, adding transparent film on the upper surface of the solar pond is also conducive to improving the thermal storage performance of the solar pond and reducing the turbidity of the solar pond.


Introduction
At present, environmental issues and energy sustainable development issues have been widely concerned by people.China has taken the development of renewable energy (solar energy, geothermal energy, biomass energy, etc.) as an important part of its energy strategy [1][2] .The solar pond is a saltwater pond that uses a salt concentration gradient to store heat.The concentration of the working medium in the solar pond gradually increases from top to bottom.The working medium in the solar pool can be divided into three layers: Upper Convective Layer (UCZ), Non-Convective Layer (NCZ), and Lower Convective Layer (LCZ).NCZ has a large concentration gradient, which can inhibit the convective heat transfer in the upper and lower troposphere, thus storing the collected heat in the LCZ [3][4] .The solar pond has the advantages of no pollutant emission, low manufacturing, low costs, and no fossil fuel consumption.Therefore, it has been widely concerned by scholars at home and abroad [5] .
Haci et al. established two numerical models of solar ponds to study the thermal performance of aboveground and underground solar ponds; the simulation results showed that the maximum thermal efficiency of the aboveground solar pool is 25.93%, and the maximum thermal efficiency of the underground solar pool is 21.49% [6] .Xiao et al. built a solar greenhouse pond in Tibet and studied the influence of the greenhouse on the thermal storage performance of the solar pond according to the experimental results; the analysis results showed that the combination of greenhouse and solar pond could reduce the impact of low-temperature climate on the solar pond, shorten the freezing time of the solar pond, and enhance the thermal storage performance of the solar pond [7] .Li et al. added  nanoparticles to the LCZ of the solar pond.They studied the effect of nanoparticles on the temperature and thermal efficiency of the salinity-gradient solar pond through experiments.The experimental results showed that the average temperature of the LCZ of the solar pond increased by 1.7℃, and the thermal efficiency increased by 1.2~1.4times after adding nanoparticles [8] .Li et al. also studied the influence of salinity and turbidity on the temperature change rate and thermal storage performance of the solar pond by combining experiment and theory.The analysis results showed that the heating rate of the solar pond would increase with the increase of salinity, and the increase of salinity and turbidity would be conducive to improving the thermal storage performance of the troposphere [9] .Wang et al. used the brine from Qinghai Lake after extracting potassium to build a solar pond.The effects of different irrigation pool heights and different weather conditions on the performance of the solar pond were studied through experiments.The experimental results showed that the temperature of the solar pond increases gradually with the increase of the height of the irrigation pond; On rainy days, the temperature of the solar pond is relatively low [10] .
In recent years, the lithium extraction process and potassium enrichment process of solar ponds have been applied in western China [11][12] .The application of solar ponds can solve the problems of long production cycles, low brine temperature, and salt algae in these two processes.However, if the thermal storage performance of the solar pond can be further improved, it will be beneficial to the production efficiency of the lithium extraction process and the potassium enrichment process.Based on this, firstly, the research team analyzed the thermal storage performance and the ability to reduce the turbidity of porous media through indoor experiments.Then, a salt-gradient solar pond was built outdoors, and a variety of porous media were laid in LCZ.According to the experimental results, the influence of different porous media on the thermal storage performance and turbidity of the salt-gradient solar pond was analyzed.Finally, the effect of adding transparent plastic film on the solar pond's thermal storage performance and turbidity was studied through experiments.

Experimental device and content
For the indoor experiment, four beakers with a volume of 2000 mL were used.One of which did not add any porous media, and the other three beakers were respectively paved with foam ceramics (SiC), ceramic pellets (Al2O3), and coal slag, with porosity of 0.5, 0.52, and 0.55, respectively.In the experiment, first, the porous media with the same initial temperature and the same volume (both are 200 mL) was measured and placed at the bottom of the beaker.Next, the thermocouple was buried in the porous medium and connected to the data recording device.Finally, the working medium was slowly injected into several beakers.The thermal storage performance and the ability to reduce the turbidity of porous media were analyzed according to the measured working medium temperature and turbidity.
In order to carry out outdoor experiments, our research team built two salinity-gradient solar ponds with the same geometric parameters.The solar pond is made of red bricks, with an inclination of 60°.In order to improve the thermal insulation performance of the solar pond, a layer of compressed benzene plate is arranged on the inner side of the wall.To prevent leakage of the working medium from the solar pond, a corrosion-resistant black plastic is laid on the upper side of the compressed benzene plate, which is also conducive to enhancing the solar pond's ability to absorb solar radiation energy.To realize the contrast experiment, we laid foam ceramics with a thickness of 3 cm at the bottom of one of the solar ponds.The geometric dimension of the upper-end face of the solar pond is 1.2 m × 1.2 m, bottom geometric dimension is 0.5 m × 0.5 m.Its structure is shown in Figure 1.The solar pond was filled in early October, and different concentrations of NaCl solution were used for each layer.When the filling was completed, the salinity of LCZ (thermal reservoir) was 12.5%, and the thickness was 20 cm.The salinity of NCZ gradually decreased from 12.5% in the bottom layer to 1% in the top layer, with a thickness of 25 cm.UCZ is made of tap water, with a thickness of 10 cm.
The experimental site is located in Yingkou.The annual sunshine hours of Yingkou are 2206.9~2904.3h, and the annual total solar radiation is 5064.3MJ/m 2 , which belongs to the area with abundant solar energy resources (Class III area).The daily average maximum temperature in Yingkou in October is 17℃, and the daily average minimum temperature is 8℃.During the experiment, we used the SRS300 portable solar radiation meter to measure the change of outdoor solar radiation intensity all day.We used the K-type thermocouple and TA612C thermocouple thermometer to measure the temperature of the working medium in the solar pond.We used the WGZ-1B digital turbidity meter and handheld salinity meter to measure the turbidity and salinity of the working medium in the solar pond.

Control equation
The solar pond is a complex system with dual diffusion of hot salt.When analyzing the thermal storage performance of solar ponds, it is necessary to consider both the influence of salinity gradient on salt diffusion and temperature gradient on salt diffusion.Control equations for thermal diffusion and salt diffusion [13] are: where T, ρ, Cp, and S are, respectively, the temperature, density, constant pressure specific heat capacity, and salinity of the working medium in the solar pond; t is the time; V is the velocity field of the working fluid in the solar pool; kT is the thermal diffusion coefficient; ks is the thermal diffusion coefficient; ̇ is the heat generation rate per unit volume of the solar pool.

Solar radiation penetration model
The amount of solar radiation absorbed by the working fluid in the solar pool Qr [14] : is the amount of solar radiation received on the surface of the solar pool; γ is the reflectance ratio of solar radiation energy on the surface of the solar pool; α is the absorption ratio of solar radiation energy by impurities on the liquid surface of the solar pool; τ (c, z)is the radiation penetration ratio of the working fluid at different flooding depths z; c is the turbidity of the working fluid.

Surface convective heat transfer loss
The heat loss at the liquid level of the solar pool Qs includes radiation heat loss, convective heat loss, and evaporation heat loss [14] .
Qs=0.97σ (ε a 4 - w 4 )+1.57u(Tw-Ta) + (psw-Hr• psa) [a 2 u 2 +b 2 (Tw-Ta) 2/3 ] 1/2 where σ is the Boltzmann constant; ε is the blackness of the ambient atmosphere; Ta and Tw are the ambient temperature and the liquid level temperature of the solar pool; u is the wind speed at the liquid level; psw is the saturated vapor pressure of water in the solar pool; Hr is the relative humidity at the liquid level; pSA is the partial pressure of water vapor in the air at the liquid level; a is the forced convective evaporation coefficient at the liquid level; b is the natural convective evaporation coefficient at the liquid level.

Side wall and bottom heat loss
The heat transfer process on the side walls and bottom surfaces of the solar pool can be regarded as a one-dimensional non-stationary heat transfer process.For the heat loss of the sidewall, it is divided into several layers for accurate calculation.Heat flux on the sidewalls of each layer of the solar pool Φw, I am as follows: Φw, i=Aw, i (Ti-Ta)/Rw, i where Aw, I and Ti are the sidewall area and working fluid temperature of the i-th layer of the solar pool; Rw, i is the area thermal resistance at the sidewall of the i-th layer of the solar pool.
The heat flux at the bottom of the solar pool Φb is: Φb=Ab (Tb-Ts)/Rb where Ab is the bottom area of the solar pool; Tb is the temperature of the working fluid at the bottom of the solar pool; Ts is the soil temperature; Rb is the area of thermal resistance at the bottom of the solar pool.

Analysis of thermal storage performance and the ability to reduce the turbidity of porous media
Figure 2 shows the comparison of the thermal storage performance of different porous media.It can be seen from Figure 2 that after slowly injecting 1800 mL of hot water with a temperature of 80℃ into each beaker, according to the measurement results, the temperature of boiler slag is the highest, the temperature of ceramic balls is moderate, and the temperature of foam ceramics is the lowest.According to the principle of thermodynamics, the larger the overall heat capacity of the porous media material is, the smaller the temperature rise is after absorbing the same heat, resulting in a lower material temperature under the same initial temperature.Therefore, the overall heat capacity of porous media from high to low is foam ceramics, ceramic balls, and boiler slag.
It can also be seen from Figure 2 that after the completion of hot water injection, the water temperature under different conditions will gradually decrease, but the reduction range is different.Among them, when the porous medium is not added, the temperature reduction is the largest, and when the porous medium is added, the temperature reduction is small.This shows that porous media materials have a certain thermal storage capacity.The smaller the temperature reduction is, the stronger the thermal storage capacity of porous media will be.It can be seen that the thermal storage performance of the three porous media from strong to weak are foam ceramics, ceramic balls, and boiler slag.
In this paper, the turbidity removal rate is introduced to measure the turbidity change of the working medium.The calculation equation of the turbidity removal rate is shown in the following.
Figure 3 shows the comparison of the ability to reduce the turbidity of different porous media.

Figure 3. Comparison of the ability to reduce the turbidity of different porous media
It can be seen from Figure 3 that under the condition that the volume and initial turbidity of the working medium are roughly the same (the volume is 1800 mL, and the turbidity is 35 ntu), according to the measurement results, the reduction process of the working medium turbidity mainly occurs in the first 24 hours.During the experiment, the turbidity decreased significantly in the first 5 hours, and the trend of turbidity reduction gradually stabilized after 24 hours.We found that foam ceramics had the best ability to reduce turbidity.The turbidity removal rate was about 62.5% at 5 hours and gradually reached more than 85% after 24 hours.The ability to reduce the turbidity of boiler slag in the early stage is roughly the same as foam ceramics.The turbidity removal rate can reach 60% at 5 hours.After 5 hours, the ability to reduce turbidity is gradually weaker than that of foam ceramics.The turbidity removal rate in the stable stage is close to 80%.The ability to reduce the turbidity of ceramic pellets is poor.At 5 hours, the turbidity removal rate is about 53.4%.After 24 hours, the turbidity removal rate gradually tends to 72%, which is only about 3% higher than that without porous media.To sum up, in order to achieve a good turbidity reduction effect, foam ceramics should be selected as the turbidity reduction material for the working medium in the solar pond.

Effect of porous media on thermal storage performance and turbidity of solar pond
Figure 4 shows the distribution of working medium temperature in the solar pond on the 2nd, 8 th , and 17th day after filling.It can be seen from Figure 4 that with the continuous test process, the working medium temperature in the solar pond will gradually increase whether or not porous media is added at the bottom of the solar pond.In contrast, after laying porous media (foam ceramics) at the bottom of the solar pond, the temperature of the working medium in the solar pond will be higher, especially the temperature of LCZ.Under the two conditions, the maximum temperature difference of LCZ is 2.1℃, the maximum temperature difference of NCZ is 0.7℃, and the temperature difference of UCZ is smaller on the 17th day after the completion of filling, which indicates that the thermal storage effect of porous media is mainly reflected in LCZ.
It can also be seen from Figure 4 that when no porous media is added to the bottom of the solar pond, the maximum temperature appears at the junction between LCZ and NCZ.When porous media is added to the bottom of the solar pond, the highest temperature appears at the bottom of the solar pond.This is because although the salinity of LCZ is basically the same, leading to the existence of thermal convection in this area, the heat storage of porous media has a great impact on the medium working temperature.Finally, the heat is concentrated at the bottom of the pool.Therefore, for the solar pond with porous media, the best heat extraction location is at the bottom of the solar pond.
Figure 5 shows the distribution of working medium turbidity in the solar pond on the day of completion of perfusion and on the 10th and 20th days.It can be seen from Figure 5 that the turbidity of the working medium in the solar pond gradually increases with the continuous testing process.On the whole, the turbidity at the bottom of the solar pond is relatively high.From the bottom of the pond to the liquid level, the turbidity gradually decreases.The turbidity of the working medium in LCZ has the largest change in the vertical direction.In contrast, the turbidity of the working medium in NCZ and UCZ has a small change, which indicates that the dust falling into the solar pond is mainly concentrated at the bottom of the solar pond after sedimentation.After adding porous media, the turbidity at the bottom of the pool increases a little.However, with the increase in height, the turbidity of the solar pond decreases sharply, and the average turbidity in the pond is lower than that without porous media.This is because the adsorption of porous media is conducive to the precipitation of dust in the solar pond, resulting in most of the dust settling at the bottom of the pond.In conclusion, although the adsorption of porous media increases the turbidity at the bottom of the solar pond, it effectively reduces the overall turbidity of the solar pond, thus improving the transparency of the working medium in the pond.

Effect of transparent plastic film on thermal storage performance and turbidity of solar pond
In order to strengthen the thermal storage performance of the solar pond, scholars have taken a variety of measures.Our research team covered the upper end of the solar pond with plastic film (at this time, no porous media was added to the solar pond) and analyzed the impact of this measure on the thermal storage performance and turbidity of the solar pond through the experimental results.Figure 6 shows the distribution of working medium temperature in the solar pond before and after covering it with transparent plastic film.It can be seen from Figure 6 that the upper end of the solar pond was not covered with plastic film 24 days after the experiment.It was found that the temperature of the solar pond gradually increased after the completion of the water injection.However, the temperature distribution of the solar pond is basically the same on the 18th and 24th days, which indicates that the solar pond and the external environment have basically reached a heat balance.On the 25th day, the upper-end face of the solar pond was covered with plastic film and then operated for 4 days.The measurement shows that the temperature of the working medium in the solar pond is further increased because the plastic film can effectively reduce the convective heat transfer between the solar pond and the external environment, thus playing a certain thermal insulation effect.
It can also be seen from Figure 6 that when the upper end of the solar pond is not covered with plastic film, the temperature of LCZ rises slightly with the decrease of the submergence depth, which is relatively uniform on the whole.The temperature of NCZ and UCZ decreases gradually with the decrease of submergence depth, and the lowest temperature is located at the upper end of UCZ.When the upper end of the solar pond is covered with plastic film, the variation range of LCZ temperature increases significantly, and the highest temperature appears near the interface between LCZ and NCZ.This is because the plastic film on the upper end of the solar pool reduces the heat dissipation between the solar pond and the external environment.There is obvious natural convection in the LCZ.There is no obvious natural convection in the NCZ.Therefore, under the condition that the salinity of the LCZ is constant, the high-temperature brine diffuses from the bottom of the LCZ to the bottom of the NCZ.Then, it is difficult to diffuse upward, resulting in a high temperature in the upper area of the LCZ.If you want to extract heat from the solar pond with plastic film on the upper end, the best heat extraction location is at the upper part of the LCZ.
In addition, it can be seen from Figure 6 that when the upper end of the solar pond is covered with plastic film, the temperature of UCZ increases with the decrease of the submergence depth, and the lowest temperature of the whole solar pond appears at the bottom of UCZ.This is because after covering the upper end of the solar pool with plastic film, a small greenhouse will be formed between the film and the liquid level.The temperature of this greenhouse is higher than the temperature of UCZ.The heat exchange between the two makes the upper temperature of UCZ higher than its lower temperature Figure 7 shows the distribution of working medium turbidity in the solar pond before and after covering it with transparent plastic film.It can be seen from Figure 7 that when the upper end of the solar pond is not covered with plastic film, the average turbidity of the solar pond rises from 1.92 ntu to 4.12 ntu from the 10th day after the solar pond is filled.This change process has a certain upward trend.From the 11th day to the 20th day, the turbidity of the solar pond increased significantly due to sandstorms and dust.On the 20th day, the average turbidity of the solar pond increased to 12.16 ntu.After the 21st day, the upper end of the solar pond was covered with plastic film.Although the turbidity at the bottom of the solar pond was still increasing, the overall turbidity was gradually decreasing.On the 25th day, the average turbidity of the solar pond decreased to 10.75 ntu.At this time, the turbidity of the solar pond gradually decreased from the bottom of the pond to the liquid level, indicating that after the plastic film was covered, no new dust fell on the free surface.The original dust gradually settled to the bottom of the pond, making the overall turbidity of the solar pond gradually decrease.Therefore, it is necessary to cover the solar pond with a transparent cover for long-term operation.

Conclusions
The improvement of the thermal storage performance of the solar pond is beneficial to improve the production efficiency of the potassium extraction process and potassium enrichment process.Based on this, this paper analyzes the thermal storage performance and the ability to reduce the turbidity of porous media through indoor experiments.It studies the influence of porous media and the addition of a transparent cover on the thermal storage performance and turbidity of the solar pond through outdoor experiments.The results show that by comparing the thermal storage capacity and the ability to reduce the turbidity of the three porous media, it is found that the thermal storage capacity and the ability to reduce the turbidity of foam ceramics are the best when the porosity is close to each other.Laying foam ceramics on the bottom of the solar pool can not only increase the thermal storage performance of LCZ but also reduce the turbidity of the pool water.
After covering the upper surface of the solar pond with transparent film, the heat loss of the solar pond is reduced.The overall temperature of the solar pond is increased, and the turbidity of the solar pond is reduced by covering.

Figure 1 .
Figure 1.Schematic diagram of the solar pond structure

Figure 2 .
Figure 2. Comparison of thermal storage performance of different porous media

Figure 4 .
Figure 4. Effect of porous media on the heat storage capacity of solar pond

Figure 5 .
Figure 5.Effect of porous media on the turbidity of solar pond

Figure 6 .
Figure 6.Distribution of working medium temperature in the solar pond before and after covering plastic film

Figure 7 .
Figure 7. Distribution of working medium turbidity in the solar pond before and after covering with transparent plastic film