The effect of Low-E surface at different positions on the performance of insulating glass

1. Introduction

With the gradual advancement of building energy conservation in various regions of the world, glass contractors have increasingly higher requirements for the energy-saving performance of doors and windows. In the future, the K value of doors and windows is required to be about 2.0-1.8W/m2.k. If you want to improve the energy-saving performance of doors and windows, The choice of glass for windows is a very important part.

Most of the insulating glass types currently in use are double-glass hollow, triple-glass hollow, double-glass Low-E hollow, and other products. With the improvement of energy-saving performance requirements for doors and windows, the configuration of insulating glass for doors and windows has also changed to double-glass Low-E hollow. (Off-line, double silver), three-glass Low-E hollow, three-glass two-piece Low-E or the use of warm edge, inflatable and other technical directions, the energy-saving performance of glass will be significantly improved.

People pay more and more attention to the selection of Low-E glass. Its product series, specifications, and varieties are becoming more and more refined. In response to different energy-saving performance requirements, more new products have emerged. To better understand and understand the performance of Low-E glass is the primary task of door and window designers.

2. The characteristics of Low-E glass　　 

Low-E glass (also known as low-emissivity coated glass) is the abbreviation of Low Emissivity Glass, which is a film product composed of multiple layers of metal or other compounds coated on the surface of the glass. The product has high transmittance to visible light, high reflectivity to infrared (especially mid-to-far infrared), and good heat insulation performance. It can play a role in controlling sunlight, saving energy, controlling heat, and improving the environment. The surface emissivity e of ordinary glass is about 0.84, and the surface emissivity of online Low-E glass is generally below 0.25. This low-radiation film with a thickness of 80-90nm has a high reflectivity to far-infrared heat radiation and can reflect more than 80% of far-infrared radiation, so Low-E glass has a good function of blocking and insulating radiation...

The sun's radiation rays can mostly be transmitted into the room through the hollow glass, bringing light and warmth to our lives. Indoor objects will re-radiate (long wave) due to their warm temperature, and transfer a certain amount of heat to the outdoors through the hollow glass. The hollow glass made of low-emissivity coated glass will transmit most of the visible light and solar energy in the sun's rays to the room, and effectively prevent the indoor re-radiation (long wave) from being transmitted to the outdoors through the glass, thus reducing the U value of doors and windows.

During the winter in the northern hemisphere, the design of low-radiation insulating glass in northern cities in various countries mainly considers shielding ultraviolet rays, high transmission of visible light (increasing the use of natural light), high transmission of near-infrared rays (obtaining solar heat), and resistance to long waves ( 2.5---50μm) low transmission (prevent indoor heat loss).

During the summer in the northern hemisphere, the design of low-radiation insulating glass in southern cities of various countries mainly considers shielding ultraviolet rays, high transmission of visible light (increasing the use of natural light), the low transmission of near-infrared rays (shielding solar heat) and long-wave (2.5- --50μm) low transmission (prevents heat radiation from outdoor objects (roads, buildings, etc.), and also prevents the loss of indoor air-conditioning).

^{Figure 1 the more difference design of low-radiation insulating glass 1}

3. The E value of Low-E glass and its effect on distinguishing between online Low-E glass and offline Low-E glass

Since Low-E glass is a film series product composed of one or several layers of a silver metal film or other compounds coated on the glass surface, its film layer has extremely low surface emissivity, which is suitable for far-infrared heat energy with a wavelength of 2.5µm-40µm Radiation goes back. The lower the emissivity e value of Low-E glass, the lower the U value of the glass, and the better its thermal insulation effect.

According to the different processing technology of Low-E glass, it is divided into online Low-E glass and offline Low-E glass

The general film system is composed of several to ten film layers, and the coating material is metallic silver, of which only the silver film layer plays a low-radiation effect, and the other film layers are all protective and over-film layers. Using the vacuum magnetron sputtering process, a single-layer, double-layer, or multi-layer silver functional film is plated on the glass surface, and a multi-layer dielectric film needs to be added on both sides of the silver.

To achieve higher energy-saving requirements in different regions, double-silver and triple-silver low-emissivity coated glass have appeared one after another. Its film structure is more complicated than ordinary Low-E film. Double silver (three silver) Low-E glass highlights the shielding of solar heat radiation by the glass. It filters sunlight to the maximum extent and solves the problem of high visible light transmission. The contradiction between the overrate and the low solar transmittance cannot be taken into account so that the effect of having higher visible light transmittance and a lower solar heat radiation transmittance at the same time is obtained.　　

3.2 Online Low-E Glass　　

On-line low-emissivity coated glass has high hardness and is not easy to scratch, so it is called hard coating. Its production is carried out at a certain temperature, which is called chemical vapor deposition. The coating material is tin oxide. The film thickness is 20 times that of offline, but it is also quite thin.
The spectrum of online Low-E glass shows the characteristics of tin oxide conductive film, while the spectrum of offline Low-E glass shows the characteristics of silver and tin oxide composite film. Both have a good transmission for visible light, while the latter for near-infrared light. It has much higher reflection than the former, and the latter has less absorption and higher reflection for far-infrared radiation than the former. Therefore, compared with the online Low-E glass, the offline Low-E glass has a lower shading coefficient and a lower heat transfer coefficient.

In the application, the glass varieties should be selected according to the design requirements for the performance of doors and windows. At the same time, the U value of Low-E glass, the shading coefficient SC, and the E value of Low-E glass should be considered. The glass database should be selected in the glass database. The glass U value requirements are also in line with the glass varieties that meet the design requirements for the shading coefficient SC. The E value of different Low-E glass has great fluctuations in the glass U value and shading coefficient. A correct understanding of the e value of Low-E glass will have great reference value for the selection of glass varieties.

3.3 The function of Low-E coating surface in different positions of insulating glass

^{Figure 2 The analysis of the energy-saving effect of Low-E insulating glass}

As shown in the figure, the outdoor side surface of the two pieces of insulating glass is the first surface, the indoor side inner surface is the fourth surface, and the second and third surfaces are respectively located in the inner middle of the insulating glass.

The Low-E coating is located on the 2nd or 3rd side, which has little effect on the U value of the two insulating glass but has different shading coefficients (see No. 6-7 in Table 1).

When Low-E glass is on the third side (5mm+12Ar+6mm):

U=1.728, SC=0.819, g=0.712;

When Low-E glass is on the second side (5mm+12Ar+6mm):

U=1.729, SC=0.593, g=0.618　　 

When the low-emissivity coating in the northern hemisphere is located on the third side, the northern hemisphere will get a lot of solar energy in winter; and the northern hemisphere will increase the cost of cooling in summer. Take the northern hemisphere city of New York as an example. New York is in a cold (B) climate zone, and the climate characteristics are cold in winter and hot in summer. Therefore, determining the location of the Low-E glass surface should consider both the solar heat obtained in the northern hemisphere in winter and the air conditioning in summer in the northern hemisphere. The energy consumed for cooling, the shading coefficient (SC) should not be too large. Considering comprehensively, it is recommended that the window glass in the New York area should have a low-emissivity coating on the second side, which prevents solar heat from entering the room and has good heat insulation. 

4. The effect of the configuration of the two-cavity three-glass hollow and different Low-E glass surfaces on the glass performance　　 

We used a well-known thermal engineering software to calculate the energy-saving effect of the two-cavity three-glass hollow Low-E glass coating surface at different positions. Table 1 is summarized.

^{Table 1 The change table of the glass U value shading coefficient SC when the coating surface of Low-E insulating glass is in different positions.}

4.1 Glass surface determination

The glass surface of double-glazed insulating glass, the outdoor side is the first surface, and the second and third surfaces from the outdoor to the indoor area, in turn, the fourth surface is the indoor side, and the outdoor side is still the first surface in the case of triple-glass, and so on. 2, 3, 4, 5, and the 6th surface is the indoor side.

4.2 Three-glass hollow monolithic Low-E glass coating surface is placed in different positions, the parameters are shown in Table 2　　 

   　　    　   ^{Table 2 hollow glass configuration U SC g e glass data code film surface position}　    　　

Table 2 Three-glass hollow monolithic Low-E glass coating surface when the glass U value shading coefficient SC comparison table is in different positionsIt can be seen from the above table that the U value of the glass does not change much at different positions of the membrane surface. U is between 1.471 and 1.452. The effect of heat preservation and energy saving is the same for different positions, but the shading coefficient is from 0.481 to 0.715. Variety. From the perspective of the U value, the four options (the second side, the third side, the fourth side, and the fifth side) all have the same value. From the perspective of the shading coefficient, the second side is the smallest and the fifth side is the largest. The greater the shading coefficient, the more solar energy is obtained, and vice versa. If you choose the shading coefficient (0.715) on the 5th side, you can get more heat in the northern hemisphere in winter, but you will pay more for air conditioning in summer in the northern hemisphere. We have to determine the corresponding film surface position according to the design requirements. In the cold (B) area, we suggest that it should be placed on the second and fourth sides. The northern hemisphere can get enough heat in winter, and the northern hemisphere will pay the relatively little air-conditioning cost in summer.
4.3 The placement of the coating surface of the three-glass hollow two-piece Low-E glass is different, and the parameters are shown in Table 3.

^{Table 3 Comparison table of U-value shading coefficient SC of three-glass hollow two-piece Low-E glass coating surface at different positions.} 

It can be seen from the above calculation table that the use of two pieces of Low-E glass is mainly to improve the energy-saving performance of the glass configuration and obtain a better energy-saving effect. Analysis of the 3\5 and 2\4 groups from the different film surface positions The location is better (that is, the 3rd, 5th, and the 2nd and 4th sides), the U value of the glass is the same as 1.183. The use of two-piece Low-E glass has a significant improvement in energy-saving performance, but the shading coefficient is quite different ( SC 0.603--0.401) is very different. Which combination to choose depends on the design requirements. We recommend using the 2\4 combination, that is, the membrane surface is placed on the second and fourth sides, which will achieve greater energy-saving effects. For details, please refer to the data provided in Table 1.

5. The effect of Low-E insulating glass on energy-saving performance after inflating　　 

It can be seen from Table 4 that for the same type of glass (the E value of Low-E glass is 0.30 or 0.24 or 0.16 or 0.09), the changes after comparison are quite different after inflation, such as:

^{Table 4 Glass U value change table after inflating with different glass configurations}

The difference in U value of the first and second groups of ordinary insulating glass is 2.66-2.502=0.158　　

The third and fourth groups have an e value of 0.30, and the difference in U value is 2.079-1.837=0.242　　

The 5th and 6th group e value is 0.24, the U value difference is 1.983-1.728=0.255　　

The 7th and 8th group e value is 0.16, the U value difference is 1.854-1.576=0.278　　

The 9th and 10th group e value is 0.09, and the U value difference is 1.740-1.445=0.295　　 

As the E value of Low-E insulating glass continues to decrease, the contrast difference of the glass U value after inflation will increase from 0.242 to 0.295, an increase of 21.9%. As the E value of the glass decreases, its energy-saving effect becomes more obvious. For example, the difference between the first and second groups of ordinary insulating glass (0.158) after inflation is compared with the difference (0.295) of the 9th and 10th groups with the best e-value of the glass (0.295-0.158=0.137), an increase of 86.7%. Therefore, the lower the e-value of the glass, the better the energy-saving performance, and the greater the decrease in the U-value of the glass after inflation.

If Low-E glass is added to the hollow glass and filled with inert gas, the U value of the glass can be greatly reduced. The 17th group of Table 4 is the three-glass hollow with a U value of 1.776. The 21st group is the three-glass hollow with Low-E glass with a U value of 1.263, an increase of 40.6%. The 22nd group is the three-glass hollow Low-E filling gas. Its U value is 1.043 compared with the ratio of 17 groups, and the increase rate reaches 70.2%. Therefore, if you want to greatly reduce the U value of the glass, you must use Low-E glass and fill it with gas to achieve better energy-saving effects.

From the curve in Figure 2, it is more intuitive to see the change in the U value of the glass after inflation. For specific values, please refer to Table 4 "Glass U Value Change Table after Different Glass Configurations Are Inflated".

^{Figure 3 U-value curve of insulating glass after inflation}

The curve change in the above figure clearly shows that as the e-value of the glass decreases, the U-value of the glass after inflation decreases, whiw-E glass to make hollow glass, the following issues should be considered:　　

6. Suggestion

When choosing Low-E glass to make hollow glass, the following issues should be considered:

6.1. The change of Low-E glass e value affects the overall performance of Low-E insulating glass.

6.2. The relationship between the placement position of the glass SC and the Low-E surface when the three-glass hollow is used.

6.3 When using a three-glass hollow, the relationship between the glass U value and the placement position of the Low-E surface.

6.4 Determine the reasonable position of the insulating glass and the Low-E coating surface according to the design requirements.

6.5 adopts Low-E glass, and it is necessary to consider filling with inert gas to obtain a better energy-saving effect.

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