[Abstract] This article conducts a large number of simulation calculations on the heat transfer coefficients and solar heat gain coefficients of various types of insulating glass, and analyzes the impact of various factors such as original film combination, partition type, use environment, etc. on the energy-saving indicators of insulating glass. and extent. On this basis, the insulating glass combination methods and usage conditions that should be correctly selected in construction and production design to achieve the best energy-saving effect are discussed.
[Keywords] Heat transfer coefficient of insulated glass, solar heat gain coefficient, building energy saving
1. Requirements for glass performance for building energy conservation
With the improvement of social and economic development, building energy consumption accounts for an increasing proportion of total social energy consumption. Currently, it is about 30% to 45% in Western developed countries. Although the economic development level and living standards of developing countries It is not high yet, but this proportion has reached 20%~25% and is gradually rising to 30%. In some large cities at low latitudes in the northern hemisphere, air conditioning has become a major component of peak electricity loads in summer. Regardless of Western developed countries or developing countries, building energy consumption is a major issue affecting the overall socio-economic development.
Among the four major enclosure components of doors, windows, walls, roofs, and floors that affect building energy consumption, doors and windows have the worst thermal insulation performance and are one of the main factors affecting indoor thermal environment quality and building energy conservation. As far as the current typical envelope components in some developing countries are concerned, the energy consumption of doors and windows accounts for about 40% to 50% of the total energy consumption of building envelope components. According to statistics, under heating or air conditioning conditions, the heat lost by single-glass windows in winter accounts for about 30% to 50% of the heating load. In summer, the cooling energy consumed by solar radiation heat entering the room through single-glass windows is about 30% to 50%. Accounting for 20%~30% of the air conditioning load. Therefore, enhancing the thermal insulation performance of doors and windows and reducing the energy consumption of doors and windows is an important step in improving the quality of indoor thermal environment and increasing the energy-saving level of buildings.
Insulated glass has outstanding thermal insulation performance and is an important material to improve the energy saving level of doors and windows. In recent years, it has been extremely widely used in buildings. However, with the continuous improvement of energy-saving standards, ordinary insulating glass can no longer fully meet the technical requirements of energy-saving design. For example, in the energy-saving design standards for areas with hot summers and cold winters, the heat transfer coefficient limit for exterior windows with large window-to-wall ratios reaches 2.5 W/m2K. In areas with hot summers and warm winters, this indicator reaches 2.0 W/m2K under some conditions. Therefore, on the one hand, we should vigorously promote insulating glass, a new product with excellent energy-saving properties, on the other hand, we should conduct an in-depth analysis and mastery of the various influencing factors of the energy-saving performance of insulating glass, and ensure that insulating glass is ensured from the aspects of the original glass sheet, spacer composition, and use environment. Glass can exert its best energy-saving performance.
2. Basic indicators of energy-saving characteristics of insulating glass
Among the many performance indicators of insulating glass for buildings, the main ones that can be used to judge its energy-saving characteristics are heat transfer coefficient K and solar heat gain coefficient SHGC. The heat transfer coefficient K of insulating glass refers to the heat transfer through 1 square meter of insulating glass per unit time when the air temperature difference on both sides of the glass is 1°C under stable heat transfer conditions, expressed in W/m2K. The lower the K value, the better the thermal insulation performance of the insulating glass, and the more significant the energy-saving effect during use. Solar heat gain coefficient SHGC refers to the ratio of the amount of solar radiation energy entering the room through the window glass to the amount of solar heat entering the room through an opening of the same size but without glass under the same conditions of solar radiation. When the SHGC value of the glass increases, it means that more direct solar heat can enter the room; when it decreases, it means that more direct solar heat is blocked outdoors. The impact of SHGC value on energy-saving effect is related to the different climate conditions of the building. In hot climate conditions, the impact of solar radiation heat on indoor temperature should be reduced. At this time, the glass needs to have a relatively low SHGC value; In cold climate conditions, solar radiation heat should be fully utilized to increase the indoor temperature. In this case, glass with a high SHGC value is required. Between the K value and the SHGC value, the former mainly measures the heat transfer process due to temperature difference, while the latter mainly measures the heat transfer caused by solar radiation. In the actual living environment, the two effects exist at the same time, so in In each building energy-saving design standard, windows achieve the specified energy-saving effect by limiting the combination conditions of K and SHGC.
At present, it can be calculated. Because the actual measurement of K value is limited by cost and it is difficult to collect a large amount of various types of data, the analysis process of this article will use the Window5.2 software developed by the Lawrence Berkeley Laboratory in the United States for simulation calculations. The software can calculate related parameters such as K value and SHGC value of various types of glass, and its calculation results can approximately replace the actual measured values. In order to ensure the consistency of the calculation results, unless otherwise specified, this article uses the NFRC series standard environmental condition setting data in the calculation and analysis.
3. Analysis of influencing factors of energy saving indicators
3.1 Glass thickness:
The heat transfer coefficient of insulating glass is directly related to the product of the thermal resistance of the glass (the thermal resistance of the glass is 1mK/W) and the thickness of the glass. When the thickness of the glass is increased, the blocking ability of the glass to heat transfer will inevitably increase, thereby reducing the heat transfer coefficient of the entire insulating glass system. Calculation for ordinary insulating glass with a 12 mm air spacer layer shows that when both pieces of glass are 3mm white glass, K=2.745W/m2K, and when both pieces of glass are 10mm white glass, K=2.64 W/m2K, a decrease of 3.8%. left and right, and the change in K value and the change in glass thickness are basically linear. It can also be seen from the calculation results that increasing the thickness of the glass does not have a great effect on reducing the K value of insulating glass. The combination of 8+12+8 only reduces the K value by 0.03 W/m2K compared with the commonly used 6+12+6 combination. The impact on building energy consumption is minimal. The changes of the hollow system composed of heat-absorbing glass or coated glass are similar to those of white glass, so in the following analysis of other factors, the commonly used 6mm glass will be the main one. When the thickness of the glass increases, the energy of sunlight penetrating the glass into the room will decrease accordingly, resulting in a decrease in the solar heat gain coefficient of the insulating glass. As shown in Figure 2, when the hollow is composed of two pieces of white glass, the thickness of the single glass is increased from 3mm to 10mm, and the SHGC value is reduced by 16%; when the hollow is composed of green glass (selecting typical parameters) + white glass, the SHGC value is reduced About 37%. The influence of different manufacturers and different colors of heat-absorbing glass will be different, but within the same type, the thickness of the glass will have a greater impact on the SHGC value, and it will also have a greater impact on the visible light transmittance. Therefore, when selecting insulating glass composed of heat-absorbing glass in a building, the impact of glass thickness on the intensity of solar energy obtained indoors should be considered based on the design parameters of the building's energy consumption and on the premise of meeting the structural requirements. When coated glass forms a hollow structure, the thickness will have varying degrees of impact depending on the type of substrate, but the main factor will be the type of coating.
The K value of insulating glass is obtained through actual laboratory measurement, and the SHGC value is calculated from spectral data.
3.2 Type of glass:
The types of glass that make up hollow glass include white glass, heat-absorbing glass, solar control coating, Low-E glass, etc., as well as deep-processed products produced from these glasses. There will be slight changes in the optical and thermal properties of the glass after it is heat-bent and tempered, but it will not cause significant changes to the hollow system, so only the original glass sheets that have not been further processed are analyzed here. Different types of glass have very different energy-saving characteristics when used as a single piece. When combined into a hollow glass, the combination of various forms will also show different changing characteristics.Heat-absorbing glass reduces the transmittance of solar heat and increases the absorption rate through body coloring. Since the air flow speed on the outdoor glass surface is greater than indoors, it can take away more heat from the glass itself, thereby reducing solar radiation. The extent to which heat enters the room. Different color types and different depths of heat-absorbing glass will greatly change the SHGC value and visible light transmittance of the glass. However, the emissivity of heat-absorbing glass in various color series is the same as that of ordinary white glass, which is about 0.84. Therefore, under the same thickness, the heat transfer coefficient K value is the same when forming insulating glass. Several representative 6mm-thick heat-absorbing glass from different manufacturers were selected. The hollow combination method is heat-absorbing glass + 12mm air + 6mm white glass. Table 1 lists various energy-saving characteristic parameters. Calculation results show that heat-absorbing glass can only control the heat transfer of solar radiation and cannot change the heat transfer caused by temperature differences.
Table 1 Effects of different types of heat-absorbing glass on hollow energy-saving characteristics
Sunlight control coated glass is a layer of metal or metal compound film coated on the surface of the glass. The film not only makes the glass appear rich in color, but also its main function is to reduce the SHGC value of the glass and limit the direct entry of solar heat radiation. indoor. Different types of coatings will cause great changes in the SHGC value and visible light transmittance of the glass, but they have no obvious reflection effect on far-infrared thermal radiation. Therefore, when the sunlight control coated glass is used alone or in a hollow space, the K value is different from that of white glass is similar.
Low-E glass is a coated glass with a high reflectance for far-infrared rays in the wavelength range of 4.5 to 25 microns. In the environment around us, the heat transfer caused by temperature differences is mainly concentrated in the far-infrared band. White glass,The reflectivity of heat-absorbing glass and solar control coated glass for far-infrared thermal radiation is very small, and the absorption rate is very high. The absorbed heat will increase the temperature of the glass itself, which will cause the heat to be transferred to the lower temperature side again. On the contrary, Low-E glass can reflect more than 80% of the far-infrared heat radiation transmitted from the side with higher temperature, thereby avoiding secondary heat transfer due to the increase in its own temperature, so Low-E glass has great Low heat transfer coefficient.
3.3 The emissivity of Low-E glass:
The heat transfer coefficient of Low-E glass is directly related to the emissivity of its film surface. The lower the emissivity, the higher the reflectivity of far-infrared rays, and the lower the heat transfer coefficient of the glass. For example, when the film surface emissivity of a 6mm single-piece Low-E glass is 0.2, the heat transfer coefficient is 3.80 W/m2K; when the emissivity is 0.1, the heat transfer coefficient is 3.45 W/m2K. Changes in the K value of a single piece of glass will inevitably cause changes in the K value of insulating glass, so the heat transfer coefficient of Low-E insulating glass will change with the change in the emissivity of the low-emissivity film layer. The data shown in Figure 3 shows the change of the hollow K value by the film surface emissivity when the combination of white glass and Low-E glass is 6+12+6. It can be seen that when the emissivity decreases from 0.2 to 0.1, the K value only decreases by 0.17 W/m2K. This shows that compared with the change of monolithic Low-E, the change of K value in Low-E hollow is not very significantly affected by the radiation rate.
3.4 Low-E glass coating surface location:
Due to the unique low-emissivity characteristics of the Low-E glass film surface, when forming insulating glass, different placement positions of the coating surface will produce different optical properties of the insulating glass. Taking Yaohua Low-E as an example, according to the calculation of 6+12+6 combination with white glass, when the coated surface is placed in 4 different positions (position 1# outdoors and position 4# indoors), The changes in energy-saving characteristics of insulating glass are shown in Table 3. According to the results, the K value of the insulating glass when the membrane surface is at 2# or 3# is the smallest, that is, the thermal insulation performance is the best. The solar heat gain coefficient at position 3# is greater than that at position 2#. This difference is a key factor to pay attention to when using Low-E glass under different climate conditions. In cold climate conditions, people hope to obtain more solar radiation heat while insulating indoors. At this time, the coating surface should be located at the 3# position; in hot climate conditions, people hope that the less solar radiation heat entering the room, the better. The coating surface should be at position 2#.
Table 3 Impact of Low-E glass film surface position on energy saving
For the purpose of building energy saving or color decoration design, heat-absorbing glass and Low-E glass are used to form a hollow structure in hot areas, it can be seen from Table 3 that the heat transfer coefficient of the membrane surface is at the 2# or 3# position. is the smallest, but the solar heat gain coefficient at position 3# is much smaller than that at position 2#. At this time, the Low-E film layer should be located at position 3#.
3.5 Type of spacer gas:
The thermal conductivity of insulating glass is about 1.5 times lower than that of single-piece glass, which is mainly due to the role of the gas spacer layer. In addition to air, the gas filled inside the insulating glass also includes inert gases such as argon and krypton. Since the thermal conductivity of gas is very low (air 0.024W/mK; argon 0.016W/mK), the thermal resistance performance of insulating glass is greatly improved. For a 6+12+6 white glass hollow combination, the K value is approximately 2.7 W/m2K when filled with air, approximately 2.55 W/m2K when filled with 90% argon, and approximately 2.53 W/m2K when filled with 100% argon, the K value is approximately 2.47 W/m2K when filled with 100% krypton gas. Compared with the two inert gases, argon is rich in content in the air, is easier to extract, and has low cost of use, so it is widely used. No matter what kind of gas is filled, the SHGC value and visible light transmittance of insulating glass remain basically unchanged under the same thickness.
3.6 Thickness of gas spacer layer:
Commonly used insulating glass spacer layer thicknesses are 6mm, 9mm, 12mm, etc. The thickness of the gas spacer layer is directly related to the size of the heat transfer resistance. When the glass material and sealing structure are the same, the larger the gas spacer layer, the greater the heat transfer resistance. However, when the thickness of the gas layer reaches a certain level, the growth rate of heat transfer resistance becomes very small. Because when the thickness of the gas layer increases to a certain extent, the gas will produce a certain 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 increases from 1mm to 9mm, the K value drops by 37% when the white glass insulating glass is filled with air, by 53% when the Low-E insulating glass is filled with air, and by 59% when filled with argon gas. When increasing from 9mm to 13mm, the descending speed begins to slow down. After 13mm, the K value rebounded slightly. Therefore, for a 6mm thick glass insulated combination, increasing the thickness of the gas spacer layer beyond 13mm will not produce significant energy saving effects. 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 characteristic makes it necessary to use a special combination of two gas layers with different thicknesses when forming triple-glass insulating glass. The thickness of the spacer layer in the Low-E part should not be less than the thickness of the spacer layer in the white glass part. For example, when 6mm glass is insulating combined, the K value of white glass + 6mm + white glass + 12mm + Low-E is 1.48 W/m2K; the K value of white glass + 9mm + white glass + 9mm + Low-E is 1.54W/m2K; The K value of white glass+12mm+white glass+6mm+Low-E is 1.70W/m2K.
3.7 Types of spacers:
The performance of the sealing material at the edge of the insulating glass has a certain impact on the K value of the insulating glass. Usually, most intervals use the aluminum strip method. Although it is light in weight and simple to process, its thermal conductivity is large, resulting in a reduction in the thermal resistance of the edge of the insulating glass. When the outdoor temperature is particularly cold, frost may form on the edges of indoor glass. The warm edge sealing system represented by Swiggle strips has better thermal insulation performance and greatly reduces the stress on the edge of the insulating glass.The heat transfer coefficient effectively reduces the frosting phenomenon on the edges. At the same time, it can reduce the central K value of white glass hollows by more than 5% and the central K value of Low-E insulating glasses by more than 9%.
Table 4 Thermal conductivity of various edge sealing materials
3.8 Installation angle of insulating glass:
Under normal circumstances, insulating glass is placed vertically, but the application range of insulating glass is becoming more and more extensive. If it is used in greenhouses or sloped roofs, 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 ultimately lead to changes in the heat transfer coefficient of the insulating glass. Taking the commonly used 6+12+6 white glass air-filled combination as an example, Figure 5 shows the changes in the K value of insulating glass at different angles (Note: Affected by different calculation formulas used in different angle ranges, the data in the figure are for analysis only. Reference), the commonly used K value in the vertical position (90°) is 2.70W/m2K, and the K value in the horizontal position (0°) is 3.26 W/m2K, an increase of 21%. Therefore, when the insulating glass is placed horizontally, the impact of the increased K value on the energy-saving effect of the building must be considered. However, it should be noted that the K value change trend in Figure 5 refers to the environmental conditions where the indoor temperature is greater than the outdoor temperature, and the change is not obvious under the opposite conditions.
3.9 Changes in outdoor wind speed:
When testing or calculating the heat transfer coefficient of a piece of insulating glass in accordance with domestic and foreign standards, the convection heat transfer on the indoor surface is generally set to a natural convection state, and the outdoor surface is set to a forced convection state with a wind speed of about 3~5m/s. However, when it is actually installed on a high-rise building, the wind speed on the outer surface of the glass will increase as the height increases, which will enhance the heat transfer capacity of the outer surface of the glass and the heat transfer coefficient of the insulating glass will increase slightly. Comparing the data in Figure 6, when the wind speed increases 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, and the K value of the Low-E insulated increases by 0.1 W/ m2K. For high-rise building structures with small window-to-wall ratios, the above changes in the K value will not have a big impact on the energy-saving effect. However, for high-rise buildings with pure curtain walls, in order to even if the top-floor room can maintain a good thermal environment, the impact of increased wind speed at high altitudes on the energy-saving effect should be considered.
3.10 Changes in adopting different standards:
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 NFRC100 and NFRC200, and the international ISO standard is ISO15099. The European prEN ISO 10077 and prEN 13363 standards mainly adopt the relevant regulations of ISO. Chinese glass heat transfer coefficient test standard is GB8484, and an equivalent standard is added to JGJ113-2003. According to the heat transfer coefficient calculation conditions of ISO10292, the optical thermal performance of glass can be tested or calculated according to GB/T2680. These standards are not exactly the same in terms of the environmental conditions set for testing or simulation calculations, mainly in terms of indoor and outdoor temperature differences, convection heat transfer coefficients (or wind speed), solar radiation intensity, etc. This will have a certain impact on the final test or simulation calculation results, but the comparison of simulation calculations using different standards shows that different standards have little impact on the SHGC value and a slight impact on the heat transfer coefficient K value. Taking 6+12+6 air-filled Low-E insulating glass as an example, based on different standard environmental settings, the K value results calculated using Window5.2 are shown in Table 5.
Table 5 Impact of different standard parameter settings on K value
| Room temperature | Indoor convection（W/ m2K） | Outdoor temperature | Sun radiation（W/ m2） | Wind speed（m/s） | Outdoor convection（W/ m2K） | Changes in K value of Low-E insulating glass | |
| NFRC100-2001 Winter | 21℃ | -18℃ | 0 | 5.5 | 26.0 | 1.923 | |
| ASHRAE Winter | 21.1℃ | -17.8℃ | 0 | 6.7 | 25.4 | 1.943 | |
| ISO15099 Winter | 20℃ | 3.6 | 0℃ | 300 | - | 20.0 | 1.958 |
| ISO15099 Russia | 21℃ | 3.6 | -26.6℃ | 300 | - | 20.0 | 1.998 |
| GB8484 | 18℃ | -20℃ | 0 | 3.0 | Standard test |
4. The conclusion
The wide application of insulating glass has greatly promoted the development of building energy saving. At the same time, the gradual improvement of building energy saving standards will also promote the continuous improvement of insulating glass to achieve better energy saving characteristics. Through the above detailed data analysis of the original piece combination, spacer type, and usage environment of insulating glass, it can be concluded that the important factors affecting the energy-saving characteristics of insulating glass are the type of original glass piece and the thickness and type of the spacer layer. Among them, Low-E glass has made a huge leap in the energy-saving effect of insulating glass with its excellent optical and thermal properties. The average annual consumption of Low-E glass around the world has reached 120 million m2. Some European countries are legislating to encourage the use of Low-E glass. Industry associations in Japan and the United States have taken certain measures to encourage the increase in the popularity of Low-E glass. With the gradual deepening of the concept of sustainable development and awareness of building energy conservation, high-performance insulating glass products will continue to develop and have broader market prospects.
