Analysis of performance optimization strategies for doors and windows②

Approach 2. Adjust the cavity thickness of the hollow glass

We can understand from formula (2) that the influencing factors of the heat transfer coefficient of insulating glass are the physical properties of the filled gas, the gas concentration, the thickness of the gas layer, and the temperature difference between the two glass cavities. Of course, they are also related to the glass. The surface emissivity of the glass, the thickness of the glass and other factors are related; therefore, for the established two pieces of glass, adjusting the distance between the two pieces of glass, that is, setting different spacer widths, can obtain the best heat transfer coefficient of the glass, and The optimal heat transfer coefficient is obviously different according to different environmental conditions. As shown in Table 2, we see that based on the EN673 standard calculation, the air layer U value of 16A is the lowest, and for this configuration, the U value of 16A is 0.14~0.24 lower than the U value of 12A, that is, the performance is improved by 10%-15 %; When using spacers with a width greater than 16A, because the gas layer is too wide, more convective heat transfer is brought about, which further increases the U value of the glass. Calculated based on the JGJ151 standard, the U value of the interval layer of 12A is the lowest, and for this configuration, the U value of 12A is 0.04~0.06 lower than the U value of 16A, that is, the performance is improved by 2%-4%; therefore, we usually say that 12A The width of the hollow spacers up to 16A can obtain an excellent heat transfer coefficient of insulating glass. This is why we often see that the specifications of spacers for insulating glass tend to be concentrated in 12A to 16A. Figure 3 is a linear comparison chart of the heat transfer coefficient of insulating glass with different gases and different thicknesses.

The test conditions of EN 673 standard and JGJ151 standard are quite different. The boundary conditions of the two standards are shown in Table 3; the reason why the calculation results are different under different standard conditions is related to the test environment conditions of the standard system. Therefore, we say that for a certain type of filling gas, which width of the spacer bar can make the insulating glass obtain the best heat transfer coefficient, it should be determined according to the specific gas type and the standard system (test environmental conditions) of the product. To design the optimal hollow glass structure, it is necessary to refer to the actual climatic conditions of the place where the product is used.

^{Figure 3 The heat transfer coefficient of insulating glass}

Approach 3. Use Low-E coated glass

The thermal conductivity ht of the insulating glass system can be calculated by the following formula

^{The thermal conductivity ht of the insulating glass system}

Where hs is the thermal conductivity of the insulating glass gas gap layer, and its value depends on the gas thermal conductivity hg of the gas gap layer and the radiant heat conductivity hr of the two pieces of glass in the hollow glass gas gap layer; where hr can be calculated according to the following formula

^{The two pieces of glass in the hollow glass gas gap layer hr}

Where ε1 and ε2 are the corrected emissivity of the inner surface of the inner and outer sheets of the insulating glass; it can be seen that the heat transfer coefficient of the insulating glass is closely related to the surface emissivity of the inner and outer substrates that make up the insulating glass unit; therefore, to obtain better We usually choose low-emissivity coated glass as the insulating glass substrate to obtain better thermal insulation performance;

Low-E energy-saving glass is used as a key material for the external maintenance structure of the building. The main reason is: the functionality of the building maintenance structure includes __- heat preservation, shading, and beauty, and Low-E glass can balance these major elements The contradiction between the two; it can not only achieve the insulation of the building, avoid a large amount of heat loss through the glass, but also achieve good shading characteristics, allowing more visible light in the solar spectrum to enter the room, and less solar radiation Heat enters through the glass; at the same time, it has a wider choice of space, a variety of performance parameter matching, and a variety of colors to meet the design requirements of different regions and different architectural styles. Low-E film can realize the effective use of solar radiation through the spectral selectivity of the silver layer of the functional layer; the heat in the solar radiation directly penetrates the glass and the glass absorbs the radiation again, the lower the heat insulation performance of the glass The better, the better the shading performance; the smaller the proportion of the object’s thermal radiation absorbed by the glass and the lower the emissivity, the better the glass’s thermal insulation performance.
High-performance Low-E insulating glass can obtain an extremely low heat transfer coefficient. 

Table 4 is a comparison of parameters obtained by various combinations of insulating glass (Low-E grades are selected from Saint-Gobain's products).

Approach 4. Application of special glass technology

With the continuous development of glass technology, energy-saving glass technology is changing with each passing day. There is already a lot of energy-saving glass with special deep processing technology, and good application effects have been achieved, such as vacuum glass, hot mirror glass, inner suspension glass, intelligent dimming Low-emission energy-saving glass, aerogel insulating glass, etc.

a) Vacuum glass application:

Traditional vacuum glass is a special glass in which two sheets of flat glass are separated by tiny support, and the periphery of the glass is sealed by fiber welding. The gas in the middle is evacuated through the exhaust hole, and then the exhaust hole is closed to maintain the vacuum layer. The heat preservation performance of vacuum glass is excellent, mainly because the existence of the vacuum layer greatly reduces the heat convection and conduction loss; at the same time, the vacuum glass can also be combined with Low-E glass to form a vacuum composite insulating glass to obtain excellent performance parameters. The insulation performance of a piece of 6mm vacuum glass is equivalent to that of a 370mm thick solid clay brick wall. Compared with single-layer glass, it can save 700MJ of energy per square meter per year, which is equivalent to 192 kilowatt-hours of electricity and 1,000 tons of standard coal. Energy-saving efficiency is extremely high. The ideal vacuum glass also has good sound insulation performance. The vacuum glass with a thickness of 6mm can reduce indoor noise below 45dB (the noise reduction value is around 33-35dB). Vacuum glass technology has developed rapidly in recent years, and its performance has been continuously improved. Gradually improve the fixing of supporting points, low heat transfer, air extraction hole sealing technology, edge sealing technology, and other technical issues, as well as the problem of substrate tempering, and has also made great breakthroughs, which can greatly reduce the safety hazards of glass breakage after installation.

The vacuum glass itself, has a very low heat transfer coefficient. But for the vacuum glazing system, we need to take the thermal bridge problem at the edge of the glass seriously. If the vacuum composite insulating glass truly takes advantage of its comprehensive performance, it is recommended to use warm-edge spacers at the edge of the glass to solve the problem of thermal bridges at the edge, to achieve better thermal insulation performance of the entire window. If the design of the single piece of vacuum glass and the embedded node of the profile can be solved, and the treatment of the edge without thermal bridge can be realized, this will be a solution with a lot of room for development in the future.

b) Application of aerogel insulating glass

The hollow glass cavity is not filled with air or other inert gases, nor is it evacuated into a vacuum state, but filled with transparent solid insulation material. In this way, hollow glass with better insulation performance can still be obtained. Silicon aerogel materials have very low thermal conductivity. The silicon particles contain microporous materials, and the wavelengths of visible light are much smaller. The thermal conductivity test data of aerogel insulating glass is shown in Figure 4.

^{Figure 4 The thermal conductivity test data of aerogel insulating glass}