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1. Introduction   

With the continuous development of the economy and science and technology, people's lives are becoming more and more colorful. With the rise of high-rise buildings and the rapid increase in the number of cars, glass has been widely used in construction, automobiles, and other fields due to its advantages of flatness and transparency, rich colors, beautiful appearance, and easy cleaning, and has become an irreplaceable building and automobile material. As people's requirements for living and working environments are getting higher and higher, glass has gradually evolved from a pure lighting material to a variety of functions such as energy-saving, privacy protection, noise reduction, and environment optimization.   

From the perspective of technology research and development, building energy conservation is one of the key points of energy conservation and emission reduction in various countries. At present, building energy consumption in various countries accounts for about one-third of the total energy consumption of society. According to estimates, among building energy consumption in various countries, heat transfer energy consumption through doors and windows accounted for about 28% of building energy consumption, and energy consumption through air penetration through doors and windows accounted for about 27% of building energy consumption. The total energy consumption of doors and windows accounted for building energy consumption. 55%. The high proportion of high energy consumption puts huge pressure on building energy conservation. With the development of the economy and society, this proportion continues to rise. Therefore, glass doors and windows have become a weak link in building energy conservation and have energy conservation potential. There are many types of architectural glass. Among them, Low-E coated glass (Low-E) has attracted people's attention at the beginning of its invention because of its remarkable energy-saving effect. Low-E glass not only has high visible light transmittance but also has a strong feature of blocking infrared rays. It can play the dual functions of natural lighting and heat insulation and energy saving. It is currently recognized as one of the ideal window glass materials. In winter, Low-E glass can effectively reduce the loss of indoor heat, and it can also block the direct transmission and secondary radiation of outdoor sunlight in summer, thus playing the role of energy-saving and consumption reduction [1]. The use of Low-E glass in the construction field can bring greater energy saving and emission reduction effects. The cost of using Low-E glass is far lower than that of ordinary glass in the long run. 

The online Low-E Glass 1

The online Low-E Glass 1

Glass coating technology is divided into two categories: online method and offline method. Among them, the coating process is set on the glass production line, and the high-heat surface of the glass is immediately chemically coated after the glass is produced. This combination of production and deep processing is called online coating. This process makes full use of the fresh and clean surface and thermal advantages of float glass, eliminating the need for pre-treatment procedures such as transportation, cleaning and heating, and vacuuming procedures for offline coated glass. Its specific coating atmosphere, temperature, and production scale endow chemical Vapor-deposited coated glass has many excellent properties. The transparent conductive film deposited on the glass surface is an online tin oxide-based film. It has high durability that can last the same life as buildings, high hardness, strong bonding with the glass substrate, and low cost. 

Advantage.   

In this article, by measuring the photothermal properties of two original glass and two online coated glasses, comparing the related performance data and curves of these four glasses, it more intuitively demonstrates the excellent energy-saving effect of online Low-E glass products. In addition, it analyzed how to combine online Low-E with offline Low-E and vacuum glass to obtain energy-saving glass with better performance.   

2. Testing equipment and samples   

2.1 The testing equipment is as follows:

NumberEquipment nameInstrument model
1Architectural glass visible and near infrared spectrophotometerGlasSpec2500
2UV-visible spectrophotometerTU1900
3Non-contact surface resistance testerStraometer G
4Emissivity meterAE1
5Portable energy-saving glass on-site comprehensive test systemGlasSmart1000

The name and model of Low-E glass testing Instrument

Figure 1 Test instrument GlasSpec2500

Figure 1 Test instrument GlasSpec2500

Figure 2 Test instrument GlasSmart1000 

Figure 2 Test instrument GlasSmart1000  

2.2 Test sample  

The test samples are shown in Table 2:

NumberSample nameSample thickness
1Original white glass5mm
2Ford Blue Original Glass5mm
3Online solar control film glass5mm
4Online Low-E coated glass5mm

3. Analysis of inspection and calculation results

3.1 Single glass test spectrum data   

The transmission and reflection properties of the four samples in the wavelength range of 190-2500nm were measured with a spectrophotometer. The transmission and reflection curves of the four samples in the range of 190-2500nm are shown in Figure 3 to Figure 6. Comparing these four pictures, it can be seen that the online Low-E glass has good transparency to visible light and high reflectivity to infrared light. Therefore, in winter, Low-E glass can effectively reduce the infrared electromagnetic field of indoor objects. Radiation can reduce the loss of indoor heat. In summer, it can also block the direct transmission of outdoor sunlight and secondary radiation, which can reduce the operating costs of heating in winter and air conditioning in summer, thereby saving energy and reducing consumption.

Figure 3 Transmission and reflection curves of ordinary white glass (5mm) in the range of 190-2500nm

Figure 3 Transmission and reflection curves of ordinary white glass (5mm) in the range of 190-2500nm

Figure 4 Transmission and reflection curves of Ford blue glass (5mm) in the range of 190-2500nm

Figure 4 Transmission and reflection curves of Ford blue glass (5mm) in the range of 190-2500nm

Figure 5 Transmission and reflection curves of online solar control film glass (5mm) in the range of 190-2500nm

Figure 5 Transmission and reflection curves of online solar control film glass (5mm) in the range of 190-2500nm

Transmission and reflection curves of online Low-E film glass (5mm) in the range of 190-2500nm

Figure 6 Transmission and reflection curves of online Low-E film glass (5mm) in the range of 190-2500nm

The portable energy-saving glass on-site comprehensive test system GlasSmart1000 was used to measure the four samples respectively, and the thermal values obtained are shown in Table 3.

It can be seen from the data in Table 3 that the K values of ordinary white glass, colored glass (Ford blue glass) and online silver-white film are all-around 5.36, while the SC values of colored glass and online silver-white film glass samples are relatively low, ranging from 0.75 to 0.77. In between, the K value for online Low-E glass is around 3.56, and the SC value is around 0.82. Compared with ordinary white glass, solar control coated glass can reduce the SC value, while the K value is almost unchanged; low-emissivity coated glass can reasonably control the SC while greatly reducing the K value. It can be considered that the energy-saving effect of solar control film glass mainly depends on the shading coefficient SC. The energy-saving of low-emissivity coated glass mainly comes from the combined effect of K value and shading coefficient SC, of which the K value is more prominent. In addition, Low-E glass has a strong feature of blocking infrared rays, while ensuring high visible light transmittance (visible light transmittance up to 80%), and can play the dual effects of heat insulation, energy-saving, and natural lighting.  

3.2 insulating glass calculation data  

The combined ordinary white, online silver-white film, and online Low-E glass into the hollow glass (the glass thickness is 5mm), and the air gap layer is 12mm. The calculated light and heat parameters of the glass components are shown in Table 4.

It can be seen from Table 4 that after Low-E glass and ordinary white synthetic hollow glass, the K value drops to about 1.9, and the SC value is about 0.72. In addition, the visible light transmittance of Low-E hollow glass is still relatively high, at 72.5 %, close to natural light, good lighting, plus a lower K value, a higher SC value.  

3.3 online Low-E and offline Low-E are used together to calculate the results  

Online Low-E is used in conjunction with offline Low-E (as shown in Figure 7), the K value will be lower, and the performance parameters are shown in Table 5.

Figure 7 Schematic diagram of the use of offline Low-E glass and online Low-E glass 

Glass structureK value SC value
 6 Single silver offline Low-E Glass + 9 Ar + 6 Ordinary white glass1.650.57
 6 Single silver offline Low-E Glass + 9 Ar + 5 Offline Low-E glass1.410.53
6 Single silver offline Low-E Glass + 12 Ar + 6 Ordinary white glass1.540.57
 6 Single silver offline Low-E Glass + 12 Ar + 5 Offline Low-E glass1.310.53

Table 5 Online Low-E and offline Low-E calculation results

It can be seen from Table 5 that the online Low-E and offline Low-E are used together, and the online film layer is placed on the indoor side air surface, which can significantly reduce the K value of the insulating glass by about 0.23-0.24, and at the same time, the SC value can also be reduced...  

The reason why the K value of the whole glass can be reduced after using the online Low-E is that it is closely related to the calculation formula of the K value (the U value mentioned in the formula).   

The calculation of the heat transfer coefficient specified in Appendix A of "Technical Regulations for the Application of Building Glass" JGJ113-2015 is as follows:

The heat transfer coefficient of single glass and laminated glass should be calculated according to the following formula:

1/U= 1/hɛ +1/ht + 1/hi

In the calculation formula:

U - Glass heat transfer coefficient

hɛ - Outdoor surface heat transfer coefficient

ht - Glass thermal conductivity

hi - Indoor surface heat transfer coefficient

The indoor surface heat transfer coefficient hi is calculated as follows:

The indoor surface heat transfer coefficient should be calculated as follows:

h=3.6+4.4ɛ/0.837

In the calculation formula:

h - Indoor surface heat transfer coefficient [ W/( m² * K)]

ɛ - Corrected emissivity of glass interior surface

If the surface of the glass interior is not coated with low-emissivity film, the value can be set as 8 [ W/( m² * K)]

The use of online Low-E glass changes the surface correction rate ɛ of the glass interior, thus changing the heat transfer coefficient of the interior surface and lowering the K value of the glass.  

3.4 Online Low-E Glass is used in conjunction with vacuum glass  

Vacuum glass is a kind of glass with good thermal insulation performance. It is favored by ultra-low energy consumption buildings and landmark buildings. At present, the production capacity is relatively low and the cost is relatively high. The complicated process of vacuum glass and the lower pass rate than hollow glass are some of the reasons for its high cost. Vacuum glass and Low-E glass are inseparable. At least one piece of the original glass must be Low-E glass to reduce the radiation heat transfer and thus the overall K value. In the production process of vacuum glass, due to the long processing cycle and complicated process route, the low-E film has high requirements for physical properties such as wear resistance. The stability of the film is also an important factor affecting the pass rate of vacuum glass.  

Online Low-E Glass is known for its "hard film". The film layer is exposed to the air for a long time without oxidation or denaturation. Therefore, it is more suitable for use with vacuum glass, and the pass rate may be improved. The calculation results of performance parameters such as the K value of the online Low-E vacuum glass are shown in Table 6.

Glass structureK value SC value
6 Ordinary white glass + V + 6 Ordinary white glass2.150.87
5 Online Low-E Glass + V + 6 Ordinary white glass0.900.77
6 Ordinary white glass + 12 Air + 5 Online Low-E Glass + V + 6 Ordinary white glass0.750.68

It can be seen from Table 6 that the online Low-E is used as the original sheet of vacuum glass, and the K value of vacuum glass can be reduced to below 0.9. At the same time, the qualification rate of vacuum glass may be improved and the cost may be reduced. It is a development of vacuum glass technology.

4. Conclusion  

Online Low-E Glass, as a unique piece of coated glass, has the advantages of high durability with the same life span as buildings, high hardness, firm combination with the glass substrate, and low cost. It is hoped that Online Low-E can give full play to its advantages and be used in buildings rationally, making outstanding contributions to the building energy conservation.

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