Thermal parameters of the hottest vacuum glass and

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Thermal parameters of vacuum glass and building energy conservation

China's building energy consumption accounts for more than 30% of the country's total energy consumption, and this proportion is still rising. The completed construction area in China is 2billion square meters every year, of which more than 95% are high-energy buildings. The energy consumption per unit building area in China is more than times that of developed countries, and windows are the opening of building envelope and the key part of building energy consumption. The energy consumption of windows accounts for about half of the whole building energy consumption. Improving the energy conservation of windows is an effective and fast measure to improve the level of building energy conservation

glass accounts for more than 80% of the whole window area, and the glass curtain wall with various advantages has become the main body of the wall, but the poor thermal insulation performance of many glass curtain walls that have been built has become a major weakness, affecting their promotion and use. Therefore, improving the thermal insulation performance of glass is the top priority

there are two ways of heat transfer through building glass: one is the heat transfer QT caused by the internal and external temperature difference, and the other is the heat QE introduced by solar radiation. The sum of the two is called relative heating, which is expressed in rhG:

rhg= QT +qe=k (t0 Ti) +se × SHGF

where k is the heat transfer coefficient, also known as K value, which means the heat transferred from indoor air to outdoor air through 1 ㎡ glass per unit time when the indoor and outdoor temperature difference is 1 degree. The legal unit of measurement in China is wm-2k-1. T0 is the outdoor temperature, Ti is the indoor temperature, and se is the shading coefficient, which means the ratio of the total solar radiation transmittance through the glass to the total solar radiation transmittance of 3mm thick ordinary flat glass. The higher the se value, the higher the proportion of solar radiation transmitted. SHGF is the heat gain factor of solar radiation, which means the solar radiation energy passing through 3mm thick ordinary glass in unit time at that time and place

if the heat transfer outside the room is positive, then in the above formula, when t0> Ti (such as summer), the leading term is positive, indicating that heat is transferred from outside the room into the room, and when t0

since solar radiation is from outside to inside, the second term in the above formula is always positive. Se parameters of glass should be selected according to factors such as region and orientation. For example, in hot areas with strong solar radiation (high SHGF), shading glass with low Se should be selected. In order to reduce the entry of solar energy and save energy consumption of air conditioning; In cold areas with weak solar radiation (low SHGF), high transmittance glass with high se should be selected to increase the entry of solar energy and save heating energy consumption

vacuum glass is a new type of energy-saving glass, and its structure is shown in Figure 1. Based on the principle of thermos bottle, it seals two pieces of glass around, with an interval of 0 2mm thin vacuum layer. Because there is no gas heat transfer, and there is a transparent low radiation film on the inner surface that acts as the silver film of the thermos, the thermal insulation performance of vacuum glass is far better than the "hollow" glass that has been widely used at present. The K value of standard vacuum glass and combined vacuum glass is lower than that of the wall materials widely used at present, but their thickness is only one tenth of the latter

standard vacuum glass with excellent heat insulation performance forms "safety vacuum glass" through the combination of "vacuum + hollow" or "vacuum + glue". Used as curtain wall glass, it solves the problem of thermal insulation of glass curtain wall. Vacuum glass with excellent window frames and sealing materials can form excellent energy-saving windows. In June, 2006, Beijing Shunda Mercer door and window production Co., Ltd. made 1400 pieces of l4+0.15v+n4+15a+n6 combined vacuum glass produced by xinliji company with the company's wooden window frame × The 1200mm wooden window has been tested by the national building materials industry building hardware plumbing product quality supervision, inspection and testing center, and the heat transfer coefficient (k value) is 0.8 wm-2k-1, reaching the 10th level of the national standard gb/t

for the control of solar radiant heat in relatively heated rhG, Low-E glass with different properties can be selected to make vacuum glass to obtain different shading coefficient se. Table 2 lists the solar radiation parameters of several float white glasses and Low-E glasses, and table 3 lists the solar radiation parameters of four vacuum glasses made of the glasses in Table 2. Different Low-E films have different shading coefficients. Those with high shading coefficients are called high permeability films, and some products have lower se than those listed in Table 1, which are called sunlight control films. The shading coefficient of vacuum glass made of different glasses is different, and when the same vacuum glass is installed, the se of Low-E film placed on the second or third surface from the outside to the inside is also different. The solar radiation heat gain coefficient SHGC, also known as the total solar radiation transmittance g, represents the ratio of the solar radiation energy entering the room through the glass to the total incident energy. The higher the value, the higher the proportion of solar energy entering the room. Therefore, different types of vacuum glass and installation methods can be selected to control the heat gain from solar radiation and achieve the purpose of energy saving. For example, in tropical areas with strong solar radiation, Low-E film with low shading coefficient should be selected and installed according to mode a to reduce solar heat gain and air conditioning energy consumption. Due to the different optical properties of Low-E films, even vacuum glasses with similar K values will have different radiation characteristics. The transmittance of visible light affects the daylighting of buildings, and too high reflectance of visible light will form "light pollution"; The speed control room system of the UV machine takes the software as the theme, which affects the UV irradiation in the room, and the position of the membrane will also affect the appearance color of the building. These factors should be comprehensively considered in the design

according to ASHRAE standard, the outdoor temperature 3 safety net in summer is 1.7 ℃ and the indoor temperature is 23.9 ℃ in buildings or high-altitude performances. When solar radiation flux Φ E is 783wm-2, 3mm glass transmittance τ S=0.87, solar radiation heat gain factor SHGF= τ s Φ e. Then the relative heating of this vacuum glass installed in a way:

rhg=k △ t+se × SHGF=K? T+ Se τ s Φ E

=6.71+313.7=+320.4 (wm-2)

a positive sign indicates that the thermal power is transmitted from outside the room to inside, which is "heat gain"

nearly 98% of this relative heat increase will be caused by solar radiation (the second term in the calculation)

according to ASHRAE standard, the outdoor temperature at night in winter is -17.8 ℃, and the indoor temperature is 21.1 ℃. At this time, the relative heating:

rhg= K △ t=0.86 × (-17..1) = -33 (wm-2)

the negative sign indicates that the thermal power is transmitted from indoor to outdoor, which is "heat loss"

using this Low-E vacuum glass has obvious advantages in winter, especially in cold areas. In sunny areas or facades with long sunshine exposure time, the heat gain is still large in summer, and shading measures or Low-E vacuum glass with lower shading coefficient should be adopted

however, it should be pointed out that the results calculated according to the standard provisions listed in Table 4 make the performance of various glasses comparable, but in fact, the heat exchange capacity of building glass is not only related to the parameters of the glass itself (such as K value and shading coefficient se, etc.), but also depends on various environmental factors (such as the geography, climate, sunshine conditions, etc. of the location of the building) and the characteristics of the building itself (such as orientation, height, etc.). Environmental factors are ever-changing, such as solar radiation flux Φ E cannot be a constant value, nor can it always be as high as 783wm-2 specified in ASHRAE standard. Therefore, building energy consumption in the construction industry has a set of professional standards and measurement methods, which can be more close to the actual estimation of building energy consumption

vacuum glass products have been successfully applied to more than ten completed projects such as Beijing Tianheng building, of which Tianheng building is the most popular. The building is the world's leading full vacuum glass office building and the world's leading building that uses vacuum glass as a large-area curtain wall. The total construction area is 57238 square meters, with 22 floors above the ground. The West and North facades adopt semi hidden frame vacuum glass curtain walls of 7000 square meters, the East and south facades adopt vacuum glass aluminum alloy heat breaking windows of 2500 square meters, and the vacuum glass adopts "hollow + vacuum + hollow" safety composite glass. The K value is 1.0 wm-2k-1, which has been tested by the spaceman construction engineering quality supervision and inspection center, which has been imagined and created by many Chinese material scientists, It exceeds the national standard gb/t thermal insulation window's highest level 10 standard. The shielding coefficient is 0.56. According to experts' calculations, compared with the installation of other kinds of glass, the building can save electricity expenses by 620000 yuan at least and 4.23 million yuan at most every year

to sum up, we can draw the following conclusions:

1. The thermal parameters related to vacuum glass and energy conservation have reached a very good level, and are emerging in building energy conservation

2. The thermal parameters related to vacuum glass and energy saving especially depend on the parameters of coating. With the development of coating technology, the K value is lower, and the vacuum glass with different shading coefficients will constantly update the existing parameters to achieve better energy-saving effect

3. Vacuum glass does not exclude deep processing technologies such as insulating glass and laminated glass. The combination of various technologies will produce combined safety vacuum glass with better heat insulation and sound insulation performance, which can meet various requirements of building doors, windows and curtain walls

4. At present, vacuum glass is still the newborn in the glass deep processing industry. With the support of government policies and guidelines, we should speed up the progress of industrialization, make a large number of products with better quality and lower price available, and promote the development of building energy conservation in China

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