Glass can be seen everywhere in our daily life, from the mirrors used in the morning when we get up to wash, to the glass curtain walls of buildings, all of which are made of glass. Glass is an important building material, one of which is Low-E glass.
With the rapid development of science and technology, glass has gradually entered the high-tech industry from traditional industries, and its application has extended from buildings to solar panels and LCD TV panels. In recent years, it has been widely used in touch products such as tablet computers and mobile phones, and even in the automotive industry. The design of green buildings emphasizes the improvement of energy efficiency and environmental tolerance to achieve the purpose of environmental symbiosis and co-prosperity. Therefore, the use of building materials in green buildings must be adjusted according to the different climates in different places. One new building material being used is Low-E, Energy Efficient Glass.
What is Low-E glass?
Low-E glass (Low Emissivity) is a film product composed of multiple layers of metal or other compounds coated on the surface of the glass. The coating layer has the characteristics of high transmission of visible light and high reflection of middle and far-infrared rays. In simple terms, this glass transmits light, but when thermal energy (infrared) from the sun or HVAC hits the Low-E glass, it is reflected into the original space instead of being passed through the glass.
How does Low-E Glass Works?
Low-E glass usually looks tinted but is not the same as tinted glass. Colored glass is made by adding alloy materials to the glass itself, while the surface of Low-E glass has a microscopic thin layer (usually multiple layers) composed of various metal particles. These thin layers of metal turn the Low-E glass into a filter, allowing it to filter light and in turn have excellent thermal insulation properties.
Different types of energy have different wavelengths. By adding thin layers of various metals to the glass surface, you can choose which type of energy to filter. If you want you can to filter out heat (long wavelengths, i.e., infrared) while allowing light (shorter wavelengths) to enter.
Manufacturing Process and Classification of Low-E Glass:
At present, there are two mature process technologies for commercial manufacturing of Low-E glass, namely, vacuum magnetron sputtering process; (offline Low-E,) a physical method, and chemical vapor deposition process; (online Low-E,) a chemical process.
What are the Low-E glass Manufacturing Processes?
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Vacuum magnetron sputtering process:
Vacuum magnetron sputtering technology refers to a technology that uses the magnetic field of a cathode surface to form electron traps so that electrons drift close to the cathode surface. The cathode consists of the material to be deposited onto the glass, and is the target of an electric field, where electrons collide at high pressure. Transparent glass is placed at a certain distance from the cathode, inside a vacuum chamber filled with inert gas. The inert gas is usually Argon. High voltage is applied between the cathode and the anode, and a glow discharge is generated to generate plasma. Under the action of the electric field, the electrons collide with argon atoms when they fly to the glass substrate, and the collision ionizes new electrons.
As the number of collisions increases, under the action of the electromagnetic field, new electrons gradually move away from the target surface, fly to the glass substrate, and deposit on the glass substrate. The ionized argon ions are accelerated to the target, and bombard the target surface with high energy, resulting in sputtering, in which the target atoms or molecules are also deposited on the glass substrate to form a thin film, which is the phenomenon of physical vapor deposition. The coated glass must meet certain aesthetics while pursuing energy saving. Therefore, the target needs to move back and forth on the surface of the glass substrate, so that the coated film can be uniform and have a certain thickness. This method is called offline vacuum magnetron. Sputtering is a novel new technology for material synthesis and processing.
Because the vacuum magnetron sputtering process and equipment are used to coat the Low-E film on the glass plate, there is a molecular bond between the film layer and the surface of the glass plate. The stability between the film layer and the glass surface is low, and the film layer itself is relatively soft and easy to be scratched or worn. The film is easily oxidized and denatured when exposed to the air for a long time, so the offline Low-E glass film is also called soft film. Due to the characteristics of its soft film, offline Low-E glass cannot be used bare or exposed to the environment. Low-E glass is usually made into insulating glass, and the Low-E film is in the air cavity of the insulating glass which protects the Low-E film from scratches and abrasions. A dry inert gas (Ar) is be filled between the two layers of glass to protect the Low-E film layer from being oxidized. This also reduce the U value of the heat transfer coefficient of the insulating glass and increases its heat insulation.
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Chemical vapor deposition process:
Chemical vapor deposition is a chemical process which uses one or several gas-phase compounds or elemental substances to perform chemical reactions on the surface of the substrate to form thin films. A chemical reaction occurs on a solid and produces a solid substance. There is a firm chemical bond between the online Low-E film and the surface of the glass plate. The film itself is very hard, not easily scratched or worn, and there is no oxidation or denaturation problem when the film is exposed to the air for a long time. Therefore, the online Low-E glass film is also commonly known as the hard film. Hard film online Low-E glass can be used bare or used alone.
What are the Classifications of Low-E Glass?
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By functional layers:
- Single silver Low-E coated glass usually contains only one functional layer silver layer, plus other metal and compound layers. The total number of film layers may reach 5 layers.
- Double silver Low-E coated glass has two functional silver layers, plus other metal and compound layers. The total number of film layers may reach 9 layers. However, the technical process control of double silver Low-E glass is much more difficult than that of single silver.
- Three silver Low-E coated glass has three functional silver layers, plus other metal and compound layers. The total number of film layers may reach more than 13 layers.
The functional layer of the Low-E film is the silver layer. Silver is one of the substances with the lowest emissivity in nature. Coating a nano-level silver layer on the surface of the glass can reduce the emissivity of the glass from 0.84 to 0.02- 0.12. This layer of coating has the characteristics of high transmission of visible light and high reflection of mid-and far-infrared rays. It filters sunlight into a cold light source, which not only meets the requirements of natural lighting but also ensures a comfortable room temperature.
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By transmittance:
- High transmittance type: Light transmittance is more than 70%, which allows a large amount of visible light to enter the room. The low-E glass used for winter warmth in northern China is generally high-transparency Low-E glass, which can not only keep the interior warm but also meet the needs of natural lighting.
- Medium transmission type: The light transmittance is 50%-70%, which has a strong blocking effect on visible light.
- Low transmittance type: the transmittance is below 50%, which has a strong blocking effect on visible light. Low-transmittance Low-E glass is suitable for places with strong sunlight. While meeting the normal lighting needs, the light will not be too dazzling.
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By shading coefficient (Sc):
The shading coefficient refers to the coefficient of the ability of glass to block sunlight. The higher the value of the shading coefficient Sc, the more solar radiation enters the room through the glass, which is suitable for the northern regions with long winters. The lower the value of shading coefficient Sc, the better the blocking effect of direct solar radiation, which can reduce the direct solar radiation entering the room, which is suitable for southern regions with long summers.
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By heat transfer coefficient (U):
The U value of the heat transfer coefficient indicates how much heat is transferred between one square meter of the inner and outer surfaces of the glass within a unit time and temperature difference. Under the same indoor and outdoor temperature difference, a low U value means that the glass transmits less heat energy due the temperature difference, and the glass has good thermal insulation performance. This is suitable for areas with high indoor thermal insulation requirements in winter.
Application of Low-E Glass in Building Energy Saving:
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High latitude and cold regions
In North America and Northeast China, these regions have long winters, extremely low temperatures in winter, short daylight hours, and weak light intensity. High-transmittance Low-E glass can maximize the use of natural light due to its better light transmittance. At the same time, a coating surface is placed on the inner layer of the Low-E glass to achieve a better thermal insulation effect.
Application principle: The indoor temperature is higher than the outdoor, the far-infrared heat radiation mainly comes from the indoors, and the Low-E glass can reflect it indoors to keep the indoor heat from leaking. For part of the solar radiation from the outdoors, Low-E glass can still allow it to enter the room. This part of the energy is absorbed by the indoor objects and then converted into far-infrared heat radiation and left indoors.
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Areas with hot summer and warm winter
In Southeast Asia, most of the regions are hot in summer and warm in winter. The temperature rises rapidly in summer, and the demand for heating in winter is lower than that in northern regions. Single-silver or double-silver Low-E glass can be used in these areas, so it is difficult for outside heat to enter the room, reducing the energy consumption of air conditioning cooling.
Application principle: The outdoor temperature is higher than the indoor temperature, and the far-infrared heat radiation mainly comes from the outdoors. Low-E glass can reflect the mid-and far-infrared rays in the sunlight, and only allow the visible light to enter, so that the sunlight becomes a cold light source and prevents the heat from entering the room. This not only achieves the purpose of energy-saving but also meets the needs of indoor lighting.
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Areas with hot summer and cold winter
In areas like Nevada, USA and Central China, the climatic conditions of these regions are such that the four seasons are clearly distinguished. The temperature difference between winter and summer is large, and the climatic conditions are relatively complex and more changeable. In these areas, low-transmittance and shading-type Low-E glass can be used. This can reduce the light transmittance of the glass in summer, prevent outdoor heat radiation from entering the room, and make the heating of the inside of the building less obvious. In winter, although the transmittance of Low-E glass is relatively low, it can meet the normal lighting needs, and at the same time, it can prevent indoor heating and other heat from being transmitted outdoors.
In these types of climates, it is necessary to consider the heat dissipation in summer, and at the same time pay attention to keeping warm in winter. With proper Low-E glass, a balance can be found between summer and winter, to maximize the energy-saving effect and achieve a win-win situation for lighting needs.
Contribution of Low-E Glass to Energy Saving:
- Low-E film can reduce the heat transfer caused by the temperature difference on both sides of the glass (i.e., temperature difference heat transfer).
- Low-E film can effectively block solar radiation, thereby limiting the radiant heat of the sun shining through the glass.
Features of Low-E Glass:
- The elegant and beautiful blue-gray appearance meets the requirements of aesthetic design.
- Good low radiation and solar control performance, with low reflected light, low shading coefficient, and low U value.
- The low-radiation performance helps to block outdoor high temperatures in summer, reducing air-conditioning electricity bills.
- Low-E properties help buildings maintain the indoor temperature in winter, reduce heating loads, and save energy and reduce carbon.
- The highly durable in-line coating ensures that the product is easy to handle and process over long distances.
- It can be cut, glued, strengthened, bent, and glazed as easily as ordinary glass.
- There is no need to remove the edge when installing, which reduces labor costs and improves efficiency.
- It can be installed and used in a single piece, and the film surface will not be oxidized and discolored.
- The color is stable before and after the online film layer processing, and the color of each glass is the same.
- Available in a variety of sizes and thicknesses.
