Reasonable selection and comprehensive performance comparison of energy-saving glass for building glass curtain wall

[China Glass Network] Introduction

The proportion of heat consumption in the building components is higher, and the heat loss and cold wind penetration loss account for about 60% of the total energy consumption of the heating. Therefore, the architectural glass system has a great impact on building energy conservation, and its funds in building energy conservation. The expenditure is also large.

The price of plastic steel windows that truly meet the national energy-saving standards is relatively high. For example, the cost per square meter of plastic single glass windows, plastic double glass windows and plastic steel hollow glass windows is 250, 350, 450 yuan respectively. The low-radiation insulating glass windows of plastic steel are more expensive. Therefore, the amount of energy-saving glass in China only accounts for about 30% of architectural glass, and it is mainly composed of single-layer heat-absorbing glass and heat-reflecting glass that control the transmission of summer sunlight. The amount of insulating glass is still relatively small, compared with western developed countries. There is a big gap. With the development of China's social economy, the application of energy-saving glass will become increasingly popular.

1. Energy-saving principle of low-emissivity glass

The temperature of the sun is about 5800K, and the main energy of the radiation is concentrated in the wavelength range of 0.2~2μm. The energy of visible light and near-infrared light segment accounts for a large proportion. It is necessary to reduce the solar radiation energy of these two sections into the room. Energy saving in summer buildings.

Conventional sunshade energy-saving glass mainly refers to heat-absorbing glass, heat-reflecting glass, low-emissivity glass, and its solar radiation transmission performance comparison.

The heat absorbing glass mainly converts the solar radiant energy into heat energy (the temperature rise of the glass itself) by absorption, and then scatters into the indoor and outdoor in the form of convection and radiation, thereby reducing the heat of the daylight through the glass to realize the energy saving of the building in summer. Divided into two categories:

1.1, ontology coloring

A special coloring agent is added to the batch of the colorless transparent flat glass, and is produced by a float process or a flat pull process.

1.2, surface coating

An endothermic and colored oxide film is sprayed on the surface of the glass. The heat-reflecting glass is a coated glass whose surface is coated with a film of metal, non-metal and its oxide, and the reflection of sunlight energy through these layers prevents the summer sunlight from entering the room. The heat-reflecting glass also has a good mirror image effect, which can beautify the street view and the building facade, but improper selection will cause light pollution.

Low-emissivity glass is also a coated glass with the same shading properties as heat-reflecting glass. Low-emissivity glass not only has remarkable shading performance, but also has excellent heat preservation performance because its heat transfer coefficient K value is smaller than other types of energy-saving glass.

The mechanism of temperature difference heat transfer and energy saving of low-emissivity glass is more complicated. The heat loss of a glass system consists of two parts: the heat of the solar radiation and the "convection heat transfer" of the glass itself. The heat exchange process between the actual glass system and the indoor and outdoor air is a complex heat exchange in a large space. The "convection heat transfer" of the inner and outer surfaces of the glass includes convection and radiation. The lower the radiance of the glass surface, the greater the radiant heat transfer resistance and the smaller the heat transfer coefficient of the glass.

Ordinary white glass has a high transmittance in the visible and near-infrared sections of solar radiation, that is, most of the energy in solar radiation can pass through. In winter, the indoor heating temperature is about 18-20 °C, according to Wien's displacement law:

λmT=2.8976×10-3

That is, the large monochromatic radiation wavelength of the infrared radiation of the indoor wall is about 10 μm. At this time, the ordinary white glass has a small infrared radiation penetration rate for the normal temperature object with λ>3 μm, and the indoor infrared radiation does not penetrate the glass to the outside, passive Solar houses and solar collectors use this property of ordinary white glass.

It can be seen that ordinary white glass itself has good thermal insulation performance, but the surface radiance of ordinary white glass is larger than that of low-radiation glass, and some infrared radiation is still absorbed by the glass body and then “convective heat transfer” to the outside, resulting in indoor heat. loss.

The low-radiation glass with low surface radiation force can not only prevent the indoor infrared radiation from escaping, but also has a stronger reflection ability than ordinary white glass, which is equivalent to increasing the "convective heat transfer" thermal resistance of the glass system itself. The low-emission film must be placed on the side of the convective heat transfer resistance to effectively increase the heat transfer resistance and improve the insulation performance).

For example, the surface emissivity of 3mm thick ordinary white glass is ε=0.84, the heat transfer coefficient of the outer surface is 23.3W/(m2·°C), the heat transfer coefficient of the inner surface is 8.7W/(m2·°C), and the total heat transfer coefficient. K=6.2W/(m2·°C); the inner surface film emissivity of ε=0.088 for 3mm thick low-emissivity glass, the internal surface heat transfer coefficient can be reduced to 4.14W/(m2·°C), and the external surface heat transfer coefficient is not The total heat transfer coefficient is reduced to K = 3.47 W / (m 2 · ° C), and the heat transfer amount of the single-layer low-emissivity glass ε = 0.088 is only 0.56 times that of the single-layer ordinary glass (ε = 0.84).

Therefore, low-emissivity glass with high transmittance in the visible light region, low transmittance in the near-infrared region, and small heat transfer coefficient is suitable for use in cold regions of the north.

It can be seen from the comparison of the table that the low-radiation glass has a lower heat transfer coefficient, and the low-radiation film also has the shading property at the same time, which causes the solar heat to be reduced in summer, and the comprehensive energy-saving performance is excellent throughout the year. Compared with other energy-saving glass, its influence on indoor lighting is also relatively small.

2. Time-dependent heat transfer of low-radiation insulating glass windows

Since the glass is very thin and the heat capacity is small, the calculation of the dynamic air conditioning load with a time interval of 1 h is regarded as no late heat transfer, and the hourly heat transfer analysis of the architectural glass system can be performed according to the steady state method. The results of time-dependent heat transfer calculation for low-emissivity glass in winter (south) and summer (west) in a region.

The outdoor air temperature (°C) from 0:00 to 23:00 in winter is: -5.55, -7.19, -8.29, -9.34, -10.27, -1.15, -11.65, -12.04, -11.8, -10.52, - 8.57 , -6.33, -4.21, -2.56, - 1.65, -1.57, -1.72, -2.03, -2.44, -2.94, - 3.48, -4.02, -4.53, - 4.98.

The illuminating time (W/m2) of the southward sun from 8:00 to 16:00 is: 47, 311, 506, 618, 654, 614, 498, 298, 36.

The outdoor air temperature (°C) from 0:00 to 23:00 in summer is: 26.77, 25.4, 25.4, 24.32, 24.23, 23.25, 23.07, 25.19, 27.6, 29.65, 32.4, 34.65, 35.9, 36.65, 37.1, 37.55, 36.2, 35.05, 34, 32.115, 31.5, 29.57, 28.43, 27.5.

The illuminating illuminance (W/m2) of the westward sun from 6:00 to 16:00 is: 2,83,109,135,151,158,162,362,542,661,692,630,445. The indoor temperature in winter and summer is 18 and 26 °C, respectively.

It can be seen that in the winter without sunshine (mainly at night), the low-radiation hollow glass window has less heat consumption and better thermal insulation performance, but in the winter sunshine, the net transmission of the ordinary hollow glass window to the room in the south of the building The heat (calculated into the solar radiant heat) is higher, and the low-radiation glazing causes heat transfer to the room due to its shading.

The amount is reduced to a lower level. Therefore, in the winter sunshine time (mainly during the day), the energy saving effect of the ordinary insulating glass in the south of the building is better.

The low-radiation glazing's shading performance and low heat transfer coefficient make it a low net heat transfer to the room during summer sunshine hours. However, since the indoor and outdoor air temperature difference amplitude of the air-conditioned room in summer is significantly reduced compared with the winter, the energy-saving advantage of the low-radiation hollow glass window with small heat transfer coefficient is not significant in the summer non-sunlighting time.

In the southern region, which is dominated by air conditioning in summer, the energy-saving effect of low-radiation insulating glass windows is mainly due to its shading performance, which is the same as other sun-shading energy-saving glass. Therefore, the energy-saving effect and other energy-saving effects of low-radiation insulating glass windows in the south are adopted. The gap between the glasses is small. Low-emissivity glass is more suitable for the outer windows of the east, west and north of the cold regions of the north. If the low-radiation glazing is applied to the south, it will affect the solar energy utilization of the buildings in winter.

3. Thermal comfort of low-emissivity glass

Under the temperature difference heat transfer (excluding solar radiation), the hourly temperature of the low-emissivity glass system, the indoor and outdoor air temperature calculation conditions are the same as in Figure 2.

From this, the difference between winter and summer insulation performance of low-emissivity glass and ordinary glass can be compared. Since the low-emission film layer (heat exchange heat resistance) of the single-layer low-emissivity glass is disposed on the inner surface of the glass, the temperature of the single-layer low-emissivity glass is closer to the outdoor environment temperature, and the summer is higher than ordinary white glass, and the winter is higher than ordinary. The white glass is low, the thermal comfort is reduced, and the possibility of icing on the inner surface of the winter window increases.

The double glass system is the opposite. The inner glass temperature of the low-radiation insulating glass is closer to the indoor ambient temperature in winter and summer than the ordinary insulating glass, which indicates the thermal comfort and thermal insulation of the low-radiation insulating glass during non-sunlight hours. The performance is better than ordinary insulating glass.

4, the conclusion

Glass curtain wall is widely used in public building envelopes. The glass curtain wall has transmissive and reflective properties, which can create a bright indoor light environment, blending internal and external space, and have strong expressiveness in architectural form, becoming a modern urban building. a sign. As the main maintenance component of the building, the glass curtain wall is not only a key part affecting energy consumption, but also an important factor affecting the comfort of the room.