As described in the previous article --The Analysis of the Energy-saving Characteristics of Insulating Glass①, about the Analysis of Influencing Factors of Energy-saving Indicators, We could know that there have many influencing factors of energy-saving indicators and we will continue to introduce the remaining 6 factors to come to a conclusion.
Due to the unique low-emissivity characteristics of the Low-E glass film surface, when the insulating glass is composed, the difference in the placement of the film surface will cause the insulating glass to have different optical properties. Taking the Low-E glass as an example, according to the calculation of the 6+12+6 combination with white glass, when the coating surface is placed in 4 different positions (outdoor is 1# position, indoor is 4# position), Table 3 shows the changes in the energy-saving characteristics of insulating glass. According to the results, the K value of the hollow glass is the smallest when the film surface position is 2# or 3#. That is, the thermal insulation performance is the best. The solar heat gain coefficient at position 3# is more significant than that at position 2#. This difference is a crucial factor to pay attention to when using Low-E glass under different climatic conditions. In cold climates, people hope to get more solar radiant heat while keeping indoor heat preservation. At this time, the coating surface should be located at position 3#; in hot climates, people hope that the less solar radiant heat was entering the room, the better when the coating surface should be located at 2# position.
Figure 3 The influence of Low-E glass film surface position on energy saving
If the design needs building energy-saving or color decoration, when the heat-absorbing glass and Low-E glass are used to form a hollow in a hot area, it can be seen from Table 3 that the heat transfer coefficients when the membrane surface is at the 2# or 3# position are all Is the smallest, but the solar heat gain coefficient at position 3# is much smaller than that at position 2#, and the Low-E film layer should be at position 3#.
The thermal conductivity of insulating glass is about one and a half lower than that of monolithic glass, mainly due to the function of the gas spacer layer. In addition to air, there are inert gases such as argon and krypton. Because the thermal conductivity of the gas is very low (air 0.024 W/m²K; argon 0.016 W/m²K), the thermal resistance of the insulating glass is greatly improved. 6+12+6 white glass hollow combination, when filled with air, the K value is about 2.7 W/m² K, when filled with 90% argon, the K value is about 2.55 W/m² K, and when filled with 100% argon, about 2.53 W/m² K, when filled with 100% krypton, the K value is about 2.47 W/m² K. Compared with the two inert gases, argon is rich in air, easy to extract, and low in cost, so it is widely used. No matter what kind of gas is filled, the SHGC value and visible light transmittance of the hollow glass remain unchanged under the same thickness.
The thickness of the commonly used insulating glass spacer layer is 6 mm, 9 mm, 12 mm, and so on. The thickness of the gas spacer is directly related to the size of the heat transfer resistance. In the case of the same glass material and sealing structure, the larger the gas spacer layer, the larger the heat transfer resistance. But when the thickness of the gas layer reaches a certain level, the growth rate of heat transfer resistance is very low. Because when the thickness of the gas layer increases to a certain level, the gas will produce an inevitable convection process under the action of the temperature difference between the glasses, thereby reducing the effect of the gas layer thickening. As shown in Figure 4, when the gas layer is increased from 1 mm to 9 mm, the K value of white glass hollow is reduced by 37% when filled with air, the K value of Low-E hollow glass is reduced by 53% when filled with air, and 59% when filled with argon. When increasing from 9 mm to 13 mm, the descending speed starts to slow down. After 13 mm, the K value rebounded slightly. Therefore, for the hollow glass assembly with a thickness of 6 mm, an increase in the thickness of the gas spacer layer exceeding 13 mm will not produce significant energy-saving effects.
Figure 4 The emissivity of Low-E glass
We can also see from Figure 4 that when the gas spacer layer increases, the K value of Low-E insulating glass decreases faster than that of ordinary insulating glass. This feature makes it necessary to use a particular combination of two gas layers with different thicknesses when composing the three-glass insulating glass. The thickness of the spacer layer at the Low-E glass part should not be less than the thickness of the spacer layer at the white glass part. For example, in the case of 6 mm glass hollow combination, the K value of white glass+6 mm+white glass+12 mm+Low-E is 1.48 W/m² K; the K value of white glass+9 mm+white glass+9 mm+Low-E is 1.54 W/m² K; The K value of white glass+12 mm+white glass+6 mm+Low-E is 1.7 0W/m² K.
The performance of the sealing material at the edge of the insulating glass has a particular influence on the K value of the insulating glass. Under normal circumstances, most of the spacers use aluminum strip spacers. Although they are light in weight and simple to process, their thermal conductivity is large, which reduces the thermal resistance of the insulating glass at the edge. When the outdoor temperature is freezing, frost will occur on the edge of the glass indoors. The warm edge sealing system represented by the Swiggle rubber strip has better heat insulation performance, dramatically reduces the heat transfer coefficient of the edge of the hollow glass, effectively reduces the edge frosting, and can also reduce the white glass empty glass. The central K value is reduced by more than 5%, and the significant K value of Low-E insulating glass is reduced by more than 9%.
|Edge material||Double seal aluminum strip||Hot melt butyl/U type||Aluminum strip Swiggle||Impermeable steel Swiggle|
|Thermal conductivity W/mk||10.8||4.43||3.06||1.36|
Table 4 The thermal conductivity of various edge sealing materials
In general, insulating glass is used vertically, but the application range of insulating glass is becoming more and more extensive. If it is applied to a greenhouse or a sloped roof, its angle will change. When the angle changes, the convection state of the internal gas will also change, which will inevitably affect the heat transfer effect of the gas and eventually cause the heat transfer coefficient of the hollow glass to change. Taking the commonly used 6+12+6 white glass air-filling combination as an example, Figure 5 shows the change of the K value of insulating glass at different angles
Figure 5 The change of square angle of insulating glass
(note: affected by different calculation formulas for different angle ranges, the data in the figure is for analysis), the commonly used vertical placement (90°) state K value is 2.70W/m² K, and the horizontal placement (0°) K value is 3.26 W/m² K, an increase of 21%. Therefore, when insulating glass is used horizontally, the impact of the rise of K value on the energy-saving effect of the building must be considered. However, it should be noted that the changing trend of the K value in Figure 5 refers to an environmental condition where the indoor temperature is greater than the outdoor temperature, and the change is not evident under the opposite conditions.
When testing or calculating the heat transfer coefficient of a piece of insulating glass according to the standards of various countries and regions in the world, the convective heat transfer on the indoor surface is generally set to a natural convection state, and the outdoor surface is in a forced convection state with a wind speed of about 3~5m/s. However, when installed on a high-rise building, the wind speed on the outer surface of the glass will increase with the increase in height, which will strengthen the heat transfer capacity of the outer surface of the glass, and the heat transfer coefficient of the insulating glass will slightly increase.
Figure 6 The influence of outdoor wind speed on energy-saving characteristics
Comparing the data in Figure 6, when the wind speed is increased from 5m/s used in the test standard to 15m/s, the K value of the white glass hollow increases by 0.16 W/m2K the K value of the Low-E insulating glass increases by 0.1 W/ m2K. For high-rise building structures with a small window-to-wall ratio, the change mentioned above of K value will not significantly impact the energy-saving effect. Still, for high-rise buildings with pure curtain walls, to maintain an excellent thermal environment in the top room, it is necessary to consider the impact of higher altitude wind speed on energy-saving effects.
The test or simulation calculation conditions for the heat transfer coefficient and SHGC value of insulating glass are slightly different in the standards of various countries. The United States adopts NFRC 100 and NFRC 200. The international ISO standard is ISO 15099, the European pr EN ISO 10077 and pr EN 13363 standards mainly adopt the relevant ISO regulations, and the Chinese glass heat transfer coefficient test standard is GB8484. The equivalent The heat transfer coefficient calculation conditions of ISO 10292 can test or calculate the optical, thermal performance of glass according to GB/T 2680. These standards are not entirely the same in the environmental conditions of testing or simulation calculation, mainly in terms of indoor and outdoor temperature difference, convective heat transfer coefficient (or wind speed), solar radiation intensity, etc. This will have a particular impact on the final test or simulation calculation results. Still, the comparison of simulation calculations using different standards shows that various measures have little effect on the SHGC value and slightly impact the heat transfer coefficient K value. Taking 6+12+6 air-filled Low-E insulating glass as an example, according to different standard environmental settings, the K value calculated using Window 5.2 is shown in Table 5.
|Indoor temperature||Indoor convection|
|Outdoor temperature||Sun radiation|
|Outdoor convection（W/m² K）||Changes in K value|
Table 5 The influence of different standard parameter settings on K value
4. The summary
The wide application of insulating glass has extensively promoted building energy efficiency, and the gradual improvement of building energy efficiency standards will also encourage the continuous realization of better energy-saving characteristics of insulating glass. Through the above-detailed data analysis of the original sheet combination, interval type, and use environment of the insulating glass, it can be concluded that the important factors affecting the energy-saving characteristics of the insulating glass are the type of the original glass sheet and the thickness and type of the spacer layer. Among them, Low-E glass has made a considerable leap in the energy-saving effect of insulating glass with its excellent optical, thermal properties. The average annual consumption of Low-E glass in the world has reached 120 million m². Some European countries are enacting legislation to encourage the use of Low-E glass. Industry associations in Japan and the United States have taken specific measures to promote the increase in the popularity of Low-E glass. In developing countries, especially China, Low-E insulating glass in the construction industry is also undergoing rapid development. Equipment provided by Jinan Lijiang Automation Equipment Co,.Ltd, online Low-E series products produced by Yaohua Glass, and those created by CSG Glass and Yaopi Glass offline Low-E products have achieved sound energy-saving effects in practical applications. With the gradual deepening of sustainable development and the awareness of building energy conservation, high-performance insulating glass products will surely get continuous development and have a broader market prospect.