About the application of low-E coated glass in architecture
Abstract: Coated glass is a glass deep processing product made of a film of metal, metal oxide, etc., plated on one or two surfaces of glass by physical or chemical methods. Different film colors and different reflectance of light make the buildings decorated with coated glass crystal brilliant. low-E coated glass can control the incidence of sunlight and reduce the energy consumption of air conditioning, while low-radiation coated glass can limit the indoor heat radiation loss, which has significant energy-saving effect in cold areas.
Key words: coating, low-E, shading coefficient, relative heat gain
Coated glass (Reflective glass) is also known as reflective glass. Coated glass is the coating of one or more layers of metal, alloy or metal compound film on the surface of the glass to change the optical properties of the glass to meet certain specific requirements. Coated glass according to the different characteristics of the product, can be divided into the following categories: heat reflection glass, Low radiation glass (low-E), conductive film glass, etc.
In architecture, the most used is heat-reflecting glass.
low-E glass is generally plated on the glass surface of one or more layers of metal such as chromium, titanium or stainless steel or its compounds composed of thin film, so that the product is rich in color, for visible light has an appropriate transmission, infrared has a high reflectivity, high absorption of ultraviolet, in addition, but also has good transmission, therefore, also known as sunlight control glass.
1: The preparation principle of low-E glass
Two current Low-E glass production methods
(1) Online high temperature pyrolytic deposition method: Online high temperature pyrolytic deposition method "Low-E" glass in the United States has a number of company products. Such as PPG's Surgate200, Ford's Sunglas H.R. "P". These products are made during the float glass cooling process. Liquid metal or metal powder is sprayed directly onto the surface of the hot glass, and as the glass cools, the metal film layer becomes part of the glass. As a result, the film is hard and durable. The "Low-E" glass produced by this method has many advantages: it can be hot-bent, tempered, does not have to be used in a hollow state, and can be stored for a long time. Its disadvantage is that the thermal performance is relatively poor. Unless the film is very thick, its "u" value is only half that of the sputtering "Low-E" coated glass. If you want to improve its thermal properties by increasing the thickness of the film, its transparency is very poor.
(2) Off-line vacuum sputtering method: off-line production of Low-E glass, is the current international general use of vacuum magnetron sputtering coating technology. Unlike pyrolytic deposition, the sputtering process is off-line. According to the different transmission position of the glass, there are horizontal and vertical points. The production of "Low-E" glass by sputtering process requires a layer of pure silver film as a functional film. The sterling silver film is between two layers of metal oxide film. The metal oxide film provides protection to the sterling silver film, and acts as an intermediate layer between the film layers to increase the color purity and light transmission. In the vertical production process, the glass is vertically placed on the shelf, sent into the vacuum environment of 10-1 Pa order, and the appropriate amount of process gas (inert gas Ar or reaction gas O2, N2) is injected, and the vacuum degree is kept stable. The target Ag, Si, etc. are embedded into the cathode, and a magnetic field is placed in the horizontal direction perpendicular to the cathode to form a magnetic-controlled target. With the magnetron target as the cathode and DC or AC power supply, the process gas is ionized to form plasma under the action of high voltage. Under the combined action of electric and magnetic fields, electrons carry out high-speed spiral motion and collide with gas molecules to produce more positive ions and electrons. Under the action of electric field, the positive ions reach a certain energy and impact the cathode target, and the sputtered target is deposited on the glass substrate to form a film. In order to form a uniform film layer, the cathode target moves back and forth near the glass surface. In order to obtain a multilayer film, multiple cathodes must be used, each of which moves back and forth on the glass surface to form a certain film thickness. The horizontal method is largely similar to the vertical method. The main difference is the placement of the glass, which is transmitted by horizontally arranged wheels through the cathode, and the degree of vacuum changes after the glass is passed through a series of pinpointed valves. When the glass reaches the main sputtering chamber, the coating pressure is reached, the metal cathode target is fixed, and the glass moves. As the glass passes through the cathode, a film is formed.
At present, the target products of domestic and most of the imported magnetron sputtering coating production lines are sun-controlled film glass plated with elemental film and metal film. This kind of product process is relatively simple, and the requirements for equipment are low. Therefore, these production lines cannot meet the requirements of plating LOW-E glass.
The production of "Low-E" glass by sputtering method has the following characteristics: due to a variety of metal target options, and a variety of metal target combinations, the production of "Low-E" glass by sputtering method can be in a variety of configurations. In terms of color and purity, sputtering is also better than thermal spray plating, and because it is off-line, it is also more flexible in terms of new product development. The main advantage is that the "u" value of the "Low-E" insulating glass produced by sputtering is better than the "u" value of the pyrolysis product, but its disadvantage is that the silver oxide film is very fragile, so it can not be used like ordinary glass. It must be made into hollow glass, and before it is made into hollow products, it is not suitable for long-distance transportation.
2: The application principle of low-E glass in architecture
Shading Coefficient: abbreviated SC, referred to in GB/T2680 as shading coefficient (abbreviated Se). It is an important limiting indicator for glass in building energy saving design standards, which refers to the ratio of the amount of solar radiation energy through the window glass and the amount of transparent glass through the same area of 3mm. SC is calculated by dividing the total solar transmittance of the sample glass by the total solar transmittance of the standard 3mm white glass (0.889 in the theoretical value of GB/T2680, 0.87 in the international standard), SC=SHGC÷0.87(or 0.889). The smaller the shading factor, the better the performance of blocking sunlight heat radiation to the room. However, only in hot climate areas and large window and wall ratio, low shading coefficient of glass is conducive to energy saving, in cold areas and small window and wall ratio, high shading coefficient of glass is more conducive to the use of solar heat to reduce heating energy consumption and achieve energy saving.
Relative heat increase: refers to the sum of heat obtained and lost through the glass, considering the influence of temperature difference heat transfer and solar radiation on the room. Relative heat increase =(outdoor temperature - indoor temperature)X heat transfer coefficient K+ solar irradiation intensity X shading coefficient SCX0.87. When it is greater than 0, it means that the indoor heat is getting more and more; When the value is less than 0, it means that more and more heat is being lost from the room. When the outdoor temperature is high in hot weather, the first term of the formula is positive, and the heat transfer to the room is smaller. At this time, the smaller the K value and SC value, the smaller the relative heat increase of the glass, which is conducive to reducing the refrigeration energy consumption. When the outdoor temperature is low in cold weather, the first term of the formula is negative, heat transfer to the outdoor, and the second term of solar radiation heat transfer to the indoor, then the larger the SC, the more favorable the heat entering the solar radiation is to make up for the heat lost to the outdoor. Therefore, in cold climates, the higher the glass SC value, the more energy consumption can be reduced.
Heat transfer coefficient: referred to as K value or u value (for glass, the two are only different for short). It is an important limit value of building energy saving design standards for glass, which refers to the heat transfer through 1 square meter glass per unit time when the temperature difference between the two sides of the glass is 1 degree under stable heat transfer conditions, expressed in W/(m2k) or W/(m2 ℃). The U value in foreign countries is expressed in British units as Btu/hr/ft2/F, and the U value in British units is multiplied by a conversion factor of 5.678 to obtain the U value in metric units. The lower the heat transfer coefficient, the better the thermal insulation performance of the glass. The heat transfer coefficient of single ordinary glass is about 5.8W /(m2K), and that of single Yaohua Low-E is about 3.6W/(m2K). Ordinary 6+12+6 insulating glass is about 2.9W /(m2K), and the Low-E hollow heat transfer coefficient of the same configuration is below 1.9W/(m2k).
Solar radiation directly transmits heat through heat transfer and convection conduction. The total thermal power Q transmitted through each square meter of glass can be expressed as follows:
Q=630Sc+U (inside T - outside T)
In the formula, 630 is the solar energy intensity through 3mm transparent glass, (T inside -T outside) is the air temperature on both sides of the glass, which is related to the environment, Sc and U are the inherent parameters of the glass itself, and their meanings are as follows: Sc - the shading coefficient of the glass, the value range is 0 ~ 1, which reflects the shading effect of the glass on the direct radiation of the sun. U - The heat transfer coefficient of glass, which reflects the glass's ability to conduct heat.
3: low-E glass has the following characteristics:
(1) Excellent thermal performance: The heat loss of the exterior door and window glass is the main part of the building energy consumption, accounting for more than 50% of the building energy consumption. Relevant research data show that the heat transfer of the inner surface of the glass is mainly radiation, accounting for 58%, which means that to change the performance of the glass to reduce the loss of heat energy, the most effective way is to inhibit the radiation of the inner surface. The radiant rate of ordinary float glass is as high as 0.84, when plated with a layer of silver based low radiation film, its radiant rate can be reduced to below 0.15. Therefore, the manufacture of building doors and Windows with Low-E glass can greatly reduce the transmission of indoor heat energy to the outside caused by radiation, and achieve the ideal energy saving effect. Another significant benefit of reduced indoor heat loss is environmental protection. In cold season, the emission of harmful gases such as CO2 and SO2 caused by building heating is an important source of pollution. If Low-E glass is used, due to the reduction of heat loss, the fuel consumed for heating can be greatly reduced, thus reducing the emission of harmful gases.
The heat through the glass is bidirectional, the heat can be transferred from the indoor to the outdoor, and vice versa, and is carried out at the same time, only the problem of transferring heat. In winter, the indoor temperature is higher than the outdoor, requiring insulation. In summer, the indoor temperature is lower than that of the room, and the glass is required to be insulated, that is, the outdoor heat is transferred to the room as little as possible. LOW-E glass can achieve the requirements of winter and summer, both heat preservation and heat insulation, and play an environmentally friendly and low-carbon effect. (2) Good optical performance: the visible light transmittance of Low-E glass varies from 0% to 95% in theory (6mm white glass is difficult to do), and the visible light transmittance represents the indoor lighting. Outdoor reflectivity from 10%-30% or so, outdoor reflectivity is visible light reflectivity, representing reflective intensity or dazzling degree, so far, China requires the visible light reflectivity of the curtain wall is not more than 30%.
The above characteristics of Low-E glass make it increasingly widely used in developed countries. China is a country with relatively low energy consumption per capita, and building energy consumption has accounted for about 27.5% of the country's total energy consumption. Therefore, vigorously developing the production technology of Low-E glass and promoting its application fields will certainly bring significant social and economic benefits.
In the United States and Europe, low-radiation (Low-E) coated glass has received great attention because of its superior properties. In particular, the German Wschvo regulations have made the rapid development of Low-E glass.
European manufacturers began laboratory research on "Low-E" in the late 1960s. In 1978, interqane of the United States successfully applied "Low-E" glass to buildings.
The superiority of "Low-E" is beyond question. Since 1990, the use of "Low-E" in the United States has been increasing at a rate of 5% per year. In the future, whether "Low-E" will become the dominant position of window glass is not known, but owners and door and window companies attach great importance to energy-saving doors and Windows.
References:
1: Deep Processing of Flat Glass, Wuhan University of Technology. :
2: Glass Surface and Surface Treatment, Wang Chengyu, Tao Ying, China Building Materials Industry Press.
3: The energy-saving effect of Low-E glass in Buildings, Tong Fei and Zhang Zhiga
