Research on aerial solar pond engineering technology

1. Introduction

1.1. Technical Field

This research involves solar pond engineering technology, specifically aerial solar pond technology applied for indoor lighting, heat collection, heat storage, thermal insulation, and temperature maintenance. This innovative approach breaks away from the traditional ground-based solar pond applications, creatively expanding the spatial application of solar ponds.

1.2. Background Technology

In natural environments, any pool that can collect and store solar energy and use it as a heat source is called a solar pond. Typically, it is composed of three layers: the Upper Convective Zone (UCZ) with freshwater, the Non-Convective Zone (NCZ) characterized by a salt concentration gradient, also known as the thermal insulating layer, and the Lower Convective Zone (LCZ) with uniform convection and saturated salt concentration. The heat collection mechanism of a solar pond relies on the salt gradient in the middle layer, which acts as a transparent thermal insulator. This layer permits short-wave solar radiation to penetrate and heat the lower convective zone while insulating the heat storage area from the air above, thereby reducing heat loss to the air.

Supported by this theory, our team conducted studies on the thermal stability of unsaturated solar ponds and the rate of salt diffusion. In experiments with two sets of solar ponds of equal salinity, both injected with freshwater layers of equal depth, it was observed that in outdoor ponds exposed to sunlight, the lower layer consistently maintained a higher temperature than the upper layer. Conversely, in indoor ponds where sunlight strikes the surface at an angle, the upper layer was warmer than the lower layer. This phenomenon demonstrates that solar ponds are capable of heat collection, storage, and insulation.

1.3. Experiment on Salt Diffusion in Indoor Solar Ponds

Time: Winter 2006 to Winter 2007

Location: Indoor Laboratories 1–3 at the Solar Pond Experimental Base in Shouguang City, Shandong Province.

Nine groups of solar ponds were set up indoors, each with a depth of 80 cm and a surface area of 0.4 m². To facilitate observation of the freshwater layer mixing, the freshwater was dyed black.

Temperature and salinity changes were monitored at fixed points and intervals within the vertical depth to investigate the effects of salt concentration and temperature on salt diffusion rates under natural environmental conditions.

1.4. Experimental Observations:

Group 1: Freshwater depth of 0.5 cm.

• In the 1‰ pond, horizontal surface diffusion was slow, but vertical diffusion was rapid. After 3 hours, diffusion had progressed to 3/5 of the vertical depth. Conductivity measurements taken on-site with a U.S. Thermal Company conductivity meter showed no gradient between the upper and lower layers.
• In the 5‰ pond, after freshwater addition, horizontal surface diffusion was rapid, while vertical sinking was not significant. After 5 minutes, the dyed freshwater layer had only diffused to a depth of 10 cm.
• The final diffusion completion time for this group was 19 days.

Group 2: Freshwater depth of 1.0 cm.

• In the 1‰ pond, horizontal surface diffusion was slow, but vertical diffusion was rapid. The freshwater sank 32 cm within 1 minute, and after 5 minutes, the dyed layer stabilized at a depth of 15 cm. After 12 hours, no gradient was detected between the upper and lower layers.
• In the 5‰ pond, after freshwater addition, horizontal surface diffusion was rapid, but vertical sinking was minimal. After 5 minutes, the dyed layer stabilized at a depth of 5 cm.
• The final diffusion completion time for this group was 37 days.

Group 3: Freshwater depth of 2 cm.

• In the 1‰ pond, the freshwater sank 32 cm within 1 minute, and the black dye at the bottom decomposed rapidly.
• The final diffusion completion time for this group was 78 days.

Group 4: Freshwater depth of 3 cm.

• In the 1‰ pond, after 48 hours, a gradient was observed only in the top 4 cm. After 8 days, the gradient layer remained unchanged. After 15 days, it decreased by 1 cm. After 21 days, diffusion was complete, with pond temperatures ranging from 4.0°C to 6.0°C during this period.
• The final diffusion completion times for ponds with salinities of 10‰, 15‰, and 20‰ were 136 days.
• For higher-salinity ponds, diffusion completion times were 129 days for 25‰ and 35‰, 122 days for 35‰ and 40‰, and 107 days for 50‰. This anomaly was attributed to sunlight entering through windows, creating a temperature gradient in the ponds and accelerating diffusion in ponds closer to the windows.

Groups 5–9: Similar phenomena were observed. Detailed parameters are recorded in the indoor pond salt diffusion progress table and the attached indoor pond salt diffusion records.

1.5. Analysis of Experimental Results:

In Group 1, with a freshwater depth of 0.5 cm, the 20‰ pond (including ponds with salinities ranging from 5‰ to 50‰) completed diffusion in 19 days.

In Group 9, the 20‰ pond with a freshwater depth of 8 cm completed diffusion in 254 days.

These results can be interpreted as the time required for bottom water to overcome the salinity gradient and reach the surface within the 80 cm vertical depth, which was 19 days and 254 days, respectively.

The data indicate that solar ponds exhibit excellent thermal insulation properties. Their insulation effectiveness is determined by the thickness of the freshwater layer, pond depth, and the magnitude of the salinity gradient.

Inspired by these experiments, we propose a novel theory for the application of aerial solar ponds and subsequently invent the aerial solar ponds.

2. Structure of the Aerial Solar Pond

The technical objective of the aerial solar pond is to address the limitations of existing technologies by providing a solar pond window that combines the functions of daylighting, heat collection, heat storage, thermal insulation, temperature maintenance, and light control. Its integrated engineering design significantly enhances the heat collection, thermal insulation, soundproofing, and dust reduction capabilities of architectural spaces.

2.1. Technical Scheme

In January 2005, we completed the pilot test of this project using a four-glass three-cavity design. The internal structure comprises a sealed and transparent window body, with transparent partitions arranged vertically, dividing the inner cavity into at least three completely isolated medium-filled layers, namely, the outer, middle and inner layers. The middle layer is filled with air, argon gas, or vacuum-sealed, while the outer and inner layers are filled with a transparent thermal insulation liquid.

The front of the aerial solar pond window can have a rectangular or any other geometric shape. The outer and inner layers are filled with a transparent thermal insulation liquid which is antifreeze.

The thickness of the transparent thermal insulation liquid in the outer and inner layers ranges from 3 to 20 mm. The thickness of the vacuum, argon gas, or air layer ranges from 6 to 30 mm.

The outer and inner layers are equipped with inlets for the transparent thermal insulation liquid and outlets for gas release.

To prevent visible light from entering the room during summer, a reflective film curtain is installed on the inside of the inner layer.

2.2. Working Principle of the Aerial Solar Pond Window

Figure 4 shows a schematic cross-sectional view of the aerial solar pond.

1. Window body 2. Transparent cover plate 3. Transparent cover plate 4. Outer layer 5. Middle layer 6. Inner layer 7. Transparent thermal insulation liquid 8. Inlet 9. Outlet 10. Inlet 11. Outlet

The aerial solar pond window described in this invention features a sealed and transparent window body 1 with a rectangular front. Transparent partitions 2 and 3 are arranged vertically within the window body 1, dividing its inner cavity into three completely isolated medium-filled layers: outer layer 4, middle layer 5, and inner layer 6. The middle layer 5 is an air or argon gas layer with a thickness of 6 mm. The outer 4 and inner 6 layers are filled with a transparent thermal insulation liquid 7, with the outer layer having a thickness of 6 mm and the inner layer a thickness of 20 mm. The outer layer 4 is equipped with inlets 8 and outlets 9 for the transparent thermal insulation liquid 7, while the inner layer 6 has its own inlets 10 and outlets 11.

3. Optical Parameter Testing of Aerial Solar Ponds

On December 3, 2013, November 30, 2015, January 17, 2024, and March 26, 2024, we commissioned the National Glass Quality Inspection Center to conduct optical testing on different structures of the solar pond window components. The results are as follows:

3.1. Optical Parameters of the Components: Visible Light Transmittance and Direct Solar Light Transmittance

Testing Date: December 3, 2013

Sample Specifications and Quantity: 100×50 mm, 1 piece Sample Structure: Four-glass three-cavity Cross-link Dimensions: 4 mm (G) + 10 mm (medium) + 4 mm (G) + 6 mm (A) + 4 mm (G)

Test Parameters: See Table 1 for the Visible Light Transmittance Test Record and Table 2 for the Direct Solar Light Transmittance Test Record.

Test Results: Visible light transmittance is 79.62% (Table 1); direct solar light transmittance is 63.22% (Table 2).

3.2. Optical Parameters of Components: Infrared Transmittance Testing Date: November 30, 2015

Sample Specifications and Quantity: 300×300×30 mm, 1 piece Sample Structure: Four-glass three-cavity

Cross-link Dimensions: 4 mm (G) + 4 mm (medium) + 4 mm (G) + 6 mm (A) + 4 mm (G) + 4 mm (medium) + 4 mm (G)

Testing Requirements: According to ISO 9050:2003, values were taken every 50 nm from 800 nm to 2500 nm, and averaged

Testing Results: infrared transmittance was 16.6%

3.3. Optical Parameters of Components: Direct Solar Infrared Transmittance

Testing Date: January 10, 2024

Sample Specifications and Quantity: 350×200 mm, 2 pieces Sample Structure: Three-glass two-cavity Sample 017 Cross-link Dimensions: 5 mm (G) + 6 mm (Liquid) + 5 mm (G) + 20 mm (AR) + 5 mm (G)

Testing Requirements: Insulated and tested as a single piece Testing Standard: GB/T 2680-2021.05.13

Testing Item: Direct solar infrared transmittance Testing Results: 46.5%

The detailed solar infrared transmission curve for aerial solar pond sample 017（Figure 5） shows visible light transmittance greater than 80%, with mid to far infrared transmittance near zero. This optical performance is highly promising and significantly ahead of others.

The detailed solar infrared transmission curve for aerial solar pond sample 017（Figure 5） shows visible light transmittance greater than 80%, with mid to far infrared transmittance near zero. This optical performance is highly promising and significantly ahead of others.

Sample 016:

Testing Date: January 10, 2024

Sample Specifications and Quantity: 350×200 mm, 2 pieces Sample Structure: Three-glass two-cavity

Sample 016 Cross-link Dimensions: 5 mm (G) + 6 mm (AR) + 5 mm (G) + 20 mm (Liquid) + 5 mm (G) Testing Requirements: Insulated and tested as a single piece

Testing Item: Direct solar infrared transmittance Testing Standard: GB/T 2680-2021.05.13 Testing Results: 35.4%

The curve for data 016 is shown in Figure 6, and it shows visible light transmittance is over 80%, with near-infrared blocking ratio greater than data 017. Mid and far infrared blocking ratio is nearly 100%. By contrasting the curves of data 016 and 017, the key factor affecting visible light transmittance and infrared blocking is the thickness of the thermal insulation medium, as shown in Figure 7.

3.4. Direct Solar Infrared Transmittance under Different Mediums

Testing Date: March 26, 2024；Sample Specifications and Quantity: 350×200 mm, 2 pieces Sample Structure: Three-glass, two-cavity

Sample 0276 Cross-link Dimensions: 5 mm (G) + 6 mm (Liquid) + 5 mm (G) + 20 mm (AR) + 5 mm (G) Testing Item: Direct solar infrared transmittance

Testing Results: 48.0%；Sample 0277 Cross-link Dimensions: 5 mm (G) + 6 mm (AR) + 5 mm (G) + 20 mm (Liquid) + 5 mm (G) Testing Item: Direct solar infrared transmittance

Testing Results: 37.0%

The curve for data 277 shows slight variations due to changes in the medium, but the overall trend remains consistent, as illustrated in figure 10, which compares data 276（Figure 8） and 277（Figure 9).

3.5. Optical Response of Aerial Solar Pond

Given the superior optical parameters of the solar pond glass panels, in the summer of 2023, we retrofitted the windows of a workshop in the University Technology Park in Weifang, Shandong, with solar pond engineering glass panels. The glass panels were produced by Shandong Meixin Glass Technology Co., Ltd., the window structures by Hebei Xunyi Doors and Windows Co., Ltd., and the medium filling was completed on-site.

The workshop covers an area of 155.3 m², as shown in Figure 11. A total of 43 solar pond engineering glass panels, covering 20 m², were installed as shown in solar pond engineering glass table 3. Specific areas include:

South balcony lighting area: 6.92×1.56 m² (Figure 12) Southeast bedroom lighting area: 1.75×1.56 m² (Figure 13) East bathroom lighting area: 0.92×1.56 m² (Figure 14) Northeast bedroom lighting area: 1.75×1.56 m² (Figure 15) Northeast kitchen lighting area: 3.62×1.56 m² (Figure 16) Northeast bathroom lighting area: 1.06×1.56 m² (Figure 17)

During construction, special attention was given to the thermal insulation properties for the summer months. We utilized a three- glass, two-cavity cross-linked structure with the following configuration: 5 mm (G) + 6 mm (medium) + 5 mm (G) + 20 mm (AR) + 5 mm (G).

On August 1, 2023, the workshop was officially operational, showcasing the following key optical and functional characteristics:

3.5.1. 3.5.1.

The visible light transmittance is excellent. The windows equipped with solar pond panels are so transparent that they appear as if no glass is present, making it nearly indistinguishable to the naked eye.

3.5.2. 3.5.2.

The outer cavity of the solar pond is filled with 6 mm of light-transmitting, thermal insulation liquid. Although this liquid absorbs infrared radiation and heats up, the heat is effectively dissipated through outdoor air convection, resulting in a comfortable indoor environment that remains cool in summer and warm in winter.

3.5.3. 3.5.3.

At 14:00 on February 7, 2025, the outdoor ambient temperature was -4.8°C, while the balcony temperature (without auxiliary heat sources) was 25°C, making it feel as warm as spring to the human body. See Figure 18.

4. Engineering Technology Evaluation

4.1. Optical Evaluation

The maximum visible light transmittance of the solar pond glass panel is 80.05%, as shown in Table 1; the peak direct solar transmittance reaches 79.304%. Within the spectral range of 1400 to 1800 nm, the transmittance varies between 0 and 0.063. The high transmittance in the visible spectrum coupled with the ultra-low transmittance in the infrared spectrum embodies the inventive essence of the aerial solar pond glass panel.

4.2. Sealing Performance Evaluation

A key aspect of ensuring the safe operation of aerial solar pond is the sealing performance between glass segments. While initial trials of aerial solar ponds were completed in 2003, the issue of glass sealing was not fully resolved until a collaboration with Shandong Meixin Glass Technology Co., Ltd. in 2023 successfully addressed this challenge.

The advantages of this sealing process include:

• Flexible and warm edge
• Composite flexible material, free of metals and materials with high thermal conductivity, addresses heat dissipation at the edges of insulated glass, enhancing overall energy efficiency, and reduces the thermal conductivity of the spacer to 0.25 W/m·K.
• Extreme airtightness
• This technology achieves very low water vapor permeability and gas transmission rates, forming an effective and long-lasting barrier against moisture ingress and gas leakage.

4.3. Mechanical Performance Evaluation

Unique bonding.

An unique bonding method and capability prevent seal failure in insulated glass due to butyl rubber overflow, tearing, or weak connections.

Flammable material

The flexible material, which can contract and expand easily, diminishes the image distortion caused by insufficient flatness of tempered glass.

4.4. Weather Resistance Evaluation

Since August 1, 2023, the aerial solar pond has undergone preliminary testing for one full cycle, enduring temperatures as high as 60°C and as low as -20°C. The three-glass two-cavity structure has shown no signs of gas leakage or medium seepage.

4.5. Processing Requirements

An aerial solar pond has exceptionally high transparency and excellent infrared blocking capabilities. Its digital manufacturing process must occur in a dust-free and sterile sealed workshop.

4.6. Comparability with Similar Products and Economic Evaluation

Currently, Low-E glass and rare earth-coated glass offer similar functions. However, these products require substantial resources and energy for manufacturing. In contrast, the key material for solar pond engineering glass is water, an inexhaustible and inexpensive natural resource. With 88% high transparency and 84% effective thermal insulation, it represents a unique strategic resource for global promotion.

5. Discussion

5.1. Optical Distortion

Due to the filling medium, image magnification and distortion reduces the optical performance of the solar pond. Future product innovation should focus on mitigating these effects through multi-point glass connections and improving glass processing techniques.

5.2. Digital and Sterile Filling Process

Ensuring long-term cleanliness and high transparency of the glass requires a digitized and sterile filling process for adhesives and mediums. Current production lines urgently need improvements and enhancements.

6. Conclusion

The aerial solar pond engineering technology is an original green technology that builds upon traditional land-based solar pond thermal utilization systems. It breaks through conventional theoretical frameworks and innovatively establishes the aerial solar pond engineering technology. In natural environments, this technology utilizes sunlight to integrate lighting, heat collection, storage, insulation, dust removal, sound insulation, and thermal isolation into a single system. It stands at the forefront of promoting sustainable development in the construction industry for human society, significantly expanding the new frontiers of solar pond thermal utilization. This technology represents a groundbreaking advancement in the field of future energy systems with the potential to impact the world.

The high transparency and thermal insulation efficiency of solar pond glass suggest extensive application prospects. The glass can be widely used in residential and commercial construction, greenhouse engineering, curtain walls, automobiles, yachts, trains, and low-altitude aircraft. With rapid technological advancements and strategic collaboration among scientists worldwide, aerial solar pond technology is poised to become a significant sustainable energy solution in a future green, low-carbon society.