The Analysis of the Energy-saving Characteristics of Insulating Glass①

1. Building energy-saving requirements for glass performance

With the improvement of the social and economic development, the proportion of building energy consumption in the total energy consumption of society is increasing. It is about 30% to 45% in western developed countries, and most developing countries have reached 20% to 25%, maybe some developing countries are gradually rising to 30%. In some large cities, summer air conditioning has become a significant component of peak power loads. Regardless of whether Europe and the United States are mainly developed countries or developing countries dominated by India and Turkey, building energy consumption is a significant issue that affects the overall social and economic development. Among the four major enclosure components that affect making energy consumption, doors and windows, walls, roofs, and ground, doors, and windows have the worst thermal insulation performance, which is one of the main factors affecting indoor thermal environment quality and building energy conservation. As far as typical enclosure components are concerned, the energy consumption of doors and windows accounts for about 40% to 50% of the total energy consumption of building enclosure components. According to statistics, under heating or air-conditioning conditions, the heat loss of a single-glass window in winter accounts for about 30% to 50% of the heating load, and the amount of cold consumed by solar radiation heat entering the room through a single-glass window in summer is about 30%~50%. It accounts 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 are essential links to improve the quality of the indoor thermal environment and increase the level of building energy conservation.

Insulating glass has outstanding thermal insulation properties and is an important material to improve the energy-saving level of doors and windows. It has been extremely widely used in buildings in recent years. However, with the continuous improvement of energy-saving standards, ordinary insulating glass can no longer fully meet the technical requirements of the energy-saving design. For example, in the energy-saving design standards for areas with hot summers and cold winters, the heat transfer coefficient of external windows for large window-to-wall ratios is limited to 2.5 W/m² K. For regions with hot summers and warm winters, this index reaches 2.0 W/m² K under certain conditions. Therefore, we should vigorously promote insulating glass, a new product with excellent energy-saving characteristics, on the one hand. On the other hand, we must deeply analyze and master the various factors affecting the energy-saving performance of insulating glass and ensure the hollow glass from the original glass, partition composition, and use environment. Glass can play its best energy-saving performance.

^{The energy-saving windows and doors 1}

2. The fundamental indicators of the 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 the heat transfer coefficient K and the 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 between the two sides of the glass is 1 °C under stable heat transfer conditions, expressed in W/m²K. The lower the K value, the better the insulation performance of the insulating glass, and the more significant the energy-saving effect during use. The solar heat gain coefficient SHGC refers to the ratio of solar radiation energy entering the room through the window glass and the 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, more direct sun heat can enter the room, and when it decreases, more direct sun heat is blocked outside. The influence of SHGC value on the energy-saving effect is related to the different climatic conditions in which the building is located. In hot climate conditions, the influence of solar radiant heat on indoor temperature should be reduced. At this time, glass is required to have a relatively low SHGC value; In cold climates, the solar radiation heat should be fully utilized to increase the indoor temperature. At this time, glass with a high SHGC value is required. Between the K value and the SHGC value, the former mainly measures the heat transfer process caused by the temperature difference. The latter mainly measures the heat transfer generated by solar radiation. In the actual living environment, both effects exist at the same time. In each building energy-saving design standard, the combination conditions of K and SHGC are limited to make the windows achieve the required energy-saving effect.

^{The solar films tester 1}

It is currently calculated about them. Because the actual measurement of the K value is limited by cost, it is challenging to collect various types of large amounts of data, so the analysis process in this article will use the Window5.2 software developed by the Lawrence Berkeley Laboratory in the United States for simulation calculation. The software can calculate the K value and SHGC value, and other related parameters of various types of glass, and the computed results can approximate the actual measured values. To ensure the consistency of the calculation results, except for special instructions, this article uses the NFRC series standard environmental condition setting data in the calculation and analysis.

3. The Analysis of Influencing Factors of Energy-saving Indicators

3.1 The thickness of the glass

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 glass thickness is increased, the resistance of the piece of glass to heat transfer will inevitably be raised, thereby reducing the heat transfer coefficient of the entire insulating glass system. Calculating for ordinary insulating glass with 12mm air gap layer, when the two pieces of glass are 3mm white glass, K=2.745W/m² K, when both are 10mm white glass, K=2.64 W/m² K, a decrease of 3.8% The relationship between the change of K value and the transformation of glass thickness is linear. It can also be seen from the calculation results that increasing the glass thickness does not have a significant effect on reducing the K value of insulating glass. The 8+12+8 combination method only reduces the K value by 0.03 W/m² K compared to the commonly used 6+12+6 combination. The impact of building energy consumption is minimal. The open system composed of heat-absorbing glass or coated glass has similar changes to white glass, so the commonly used 6mm glass will be the main factor in analyzing other factors below.

^{Figure 1 The relationship between the K value of insulating glass and the thickness of the glass}

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 hollow glass, as shown in Figure 2, when the insulating glass 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 (select typical parameters) + white glass, it decreases Around 37%. The degree of influence of different manufacturers and different colors of heat-absorbing glass will be other. Still, in the same type, the glass thickness will have a more significant impact on the SHGC value, and it will also have a considerable effect on the visible light transmittance. Therefore, when selecting insulating glass composed of heat-absorbing glass in buildings, the influence of the thickness of the glass on the solar energy intensity 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 the coated glass is hollow, the thickness will have varying degrees of influence depending on the type of substrate, but the main factor will be the type of film layer.

^{Figure 2 The relationship between SHGC value and glass thickness}

3.2 The different type of glass

The glass types that make up the hollow include white glass, heat-absorbing glass, solar control coating, Low-E glass, etc., as well as deep-processed products produced from these glasses. The optical and thermal characteristics of the glass after hot bending and tempering will slightly change. Still, it will not cause noticeable changes to the hollow system, so only the original glass that has not been further processed is analyzed. Different types of glass have very other energy-saving characteristics when used in a single piece. When the composite is hollow, the combination of various forms will also show different changing features.The heat-absorbing glass reduces the solar heat transmittance and increases the absorption rate through the body's coloring. Since the airflow speed on the outdoor glass surface is greater than that of the indoor, it can take away more heat from the glass itself, thereby reducing solar radiation—the degree of heat entering the room. Different color types and different shades of heat-absorbing glass will significantly change the glass's SHGC value and visible light transmittance. However, the emissivity of the heat-absorbing glass of various color series is the same as that of ordinary white glass, which is about 0.84. Therefore, in the same thickness, the heat transfer coefficient K value is the same when forming the hollow glass. Select several representative heat-absorbing glasses of 6mm thickness from different manufacturers. The open combination method is heat-absorbing glass+12mm air+6mm white glass. Table 1 lists various energy-saving characteristic parameters. The calculation results show that the heat-absorbing glass can only control the heat transfer of solar radiation but cannot change the heat transfer caused by the temperature difference. Table 1 The influence of different types of heat-absorbing glass on hollow energy-saving characteristics

^{Table 1 The influence of different types of heat-absorbing glass on hollow energy-saving characteristics}

Solar control coated glass is coated with a layer of metal or metal compound film on the surface of the glass. The film layer makes the glass rich in colors, but its primary function is to reduce the solar heat gain coefficient SHGC value of the mirror and limit the direct solar heat radiation. Indoor. Different coatings will significantly change the SHGC value and visible light transmittance of the glass but have no pronounced reflection effect on the far-infrared heat radiation. Therefore, when the solar control coated glass is used in a single insulating glass, the K value and white Glass are similar. Low-E glass is a coated glass that has a high reflectance of far-infrared rays in the wavelength range of 4.5~25 microns. In the environment around us, the heat transfer caused by the temperature difference is mainly concentrated in the far-infrared band, white glass,Heat-absorbing glass and solar control coated glass has low reflectivity and high absorptivity for far-infrared heat radiation. 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 high temperature, thus avoiding the secondary heat transfer caused by the increase of its temperature, so Low-E glass has a very 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 smaller 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 6mm monolithic Low-E glass is 0.2, the heat transfer coefficient is 3.80 W/m² K; when the emissivity is 0.1, the heat transfer coefficient is 3.45 W/m² K. The change of the K value of a single piece of glass will inevitably cause the change of the K value of the hollow glass, so the heat transfer coefficient of Low-E insulating glass will change with the change of the emissivity of the low emissivity film. The data shown in Figure 3 shows the transformation of the insulating glass K value by the emissivity of the film surface when the white glass and Low-E glass are combined with 6+12+6. It can be seen that when the emissivity is reduced from 0.2 to 0.1, the K value only decreases by 0.17 W/m² K. This shows that compared with the change of monolithic Low-E glass, the shift in K value of Low-E insulating glass is not very significantly affected by the emissivity.

^{Figure 3 The emissivity of Low-E glass}

Due to article space limitations, we will continue to analyze the influencing factors of building glass energy-saving indicators in the last article The Analysis of the Energy-saving Characteristics of Insulating Glass②...

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