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Application of Online Low-E Glass in Near-Zero Energy Building

Low-E coated glass is made by coating a thin film with low-emissivity properties on the glass surface by physical or chemical methods. It is referred to as low-e glass, so it is also called Low-E glass. At present, there are two mature process technologies for commercial manufacturing of Low-E glass, namely, the vacuum magnetron sputtering process using physical methods and the chemical vapor deposition process using chemical methods.

Application of Online Low-E Glass in Near-Zero Energy Building

The most important characteristic of Low-E glass is energy saving, which is expressed in terms of emissivity. The lower the emissivity, the better its energy-saving performance. Currently used in the passive house market, the emissivity of the offline single-silver Low-E is between 0.07 and 0.15, and the emissivity of the online Low-E is between 0.16 and 0.20.

The contribution of Low-E glass to energy saving is reflected in two aspects. One is that Low-E film can reduce the heat transfer caused by the temperature difference between the two sides of the glass (ie temperature difference heat transfer). On the other hand, Low-E film can effectively block the sun. Radiation, thereby limiting the radiant heat energy (ie, radiant heat transfer) that the sun shines through the glass. In practical applications, Low-E glass is generally made into hollow glass, vacuum glass and other structures.

Heat transfer coefficient of hollow glass

The heat transfer between indoor and outdoor through the glass is divided into three steps:

  • The heat exchanged between outdoor air and the glass surface, referred to as “outdoor heat exchange”.
  • The heat transferred through the glass body referred to as “glass heat transfer”;
    Indoor The heat exchanged between the air and the glass surface is referred to as “indoor heat exchange”.
  • In the past, only the glass body, that is, the “heat transfer of glass” has been tried, such as making hollow or vacuum glass and coating the surface of the internal glass, etc., these technical means can only reduce the “heat transfer of glass”. When the thickness of the gas-filled layer of single-cavity insulating glass exceeds a certain value (usually 12~15mm), the heat transfer coefficient U value increases due to the increase in the heat transfer of the gas directional circulation. If you insert a piece of glass or other plates to separate the gas layer, you can limit the gas circulation heat transfer. After stacking, the total thickness of the gas filled layer can be very large, which can increase the total gas thermal resistance and reduce the circulation heat transfer. This is the advantage of multi-cavity hollow .

Passive houses generally use triple insulated glass and generally use two Low-E or three Low-E configurations. Offline Low-E is commonly used because of its lower emissivity.

Ways to reduce the heat transfer coefficient of insulating glass

To reduce the heat exchange between indoor and outdoor air and the glass surface, that is, to reduce “outdoor heat exchange” and “indoor heat exchange”, it can also be achieved by plating Low-E film to reduce surface emissivity. Because the outdoor environment has very high requirements on the abrasion resistance and acid and alkali resistance of the film, the current technology is difficult to realize, so it is not considered to make a fuss on outdoor heat exchange, but it can be reduced by plating Low-E film on the indoor surface. Indoor heat exchange. Compared with the original contribution of “glass heat transfer” to reduce the heat transfer coefficient U value, “glass heat transfer” and “indoor heat transfer” contribute to reducing U value. The advantage is that it neither changes the glass structure nor increases the glass. Weight, but the U value can be further reduced.

One of the advantages of on-line Low-E: The film made on the surface of high-temperature glass is firmly bonded to the glass and is scratch-resistant, so it is also called “hard coating” and can be used as a single piece. On the contrary, one of the shortcomings of offline Low-E using metallic silver as the functional layer: the film layer has poor hardness and is not resistant to corrosion, so it is also called “soft coating” and cannot be used as a single piece. Therefore, regardless of the theory or the existing technical conditions, it is feasible to use online Low-E film to reduce the U value on the indoor surface.

Thermal comfort is a subjective evaluation of satisfaction with the surrounding thermal environment (ISO 7730). There are aspects to analyze whether a thermal environment is comfortable:

  •  Physical aspect: According to the thermal balance relationship between the heat generated by human activities and the heat loss of the body wearing clothes under the action of the external environment, analyze the impact of the environment on the comfort of the human body and satisfy the comfort of the human body conditions of.
  • Physiological aspect: Study the human body’s physiological response to heat and cold stress, such as skin temperature, skin humidity, perspiration rate, blood pressure, body temperature, etc., and use the physiological response to distinguish the comfort level of the environment.
  • Psychological aspect: Analyze people’s subjective feelings in a hot environment, and use psychological methods to distinguish between cold and hot and comfortable levels of the environment. This article only studies the difference in thermal comfort caused by physical heat transfer between human skin and glass.
  • Building energy efficiency design emphasizes the reduction of building energy consumption, that is, the energy consumption of equipment required for the building to maintain the visual level of the indoor physical environment. This also leads to passive design, including natural ventilation, natural light utilization and other strategies. Its potential target is zero energy consumption of the building.
  • Correspondingly, the design aimed at comfort focuses on the experience of the building users. The most typical design type is the proof design based on the healthy design goal. The potential goal set is the comfort and satisfaction of the building users. Enlarging the design goals of the two can be found: one is to minimize energy consumption, and the other is to maximize comfort. If the two are placed in the relationship of the building skin, it is easy to find that the characteristics of the skin material play a very important balancing role in the contradictory relationship between energy saving and comfort.

The evaluation system of movable houses includes two sets of indicators, namely “actual energy consumption” and “indoor comfort? The coefficient is less than 0.85W/(m2·K), which is based on the fact that the average temperature of the inner surface is higher than 17°C under the design conditions.” This is called the “thermal comfort criterion.” This criterion clearly states that the heat transfer coefficient of the outer window is determined by It is determined by the average temperature of the inner surface. Studies have shown that when the indoor air temperature is 20°C and the temperature of the inner surface of the enclosure is higher than 17°C, the human body will not feel the obvious cold radiation from the enclosure. That is to say. , The heat transfer coefficient index of the outer window must ensure that the average temperature of the inner surface of the outer window is not lower than 17 ℃ under the environment in which it is located, in order to meet the thermal comfort requirements of passive ultra-low energy buildings.

Concluding remarks

The online Low-E film has the conditions to be exposed to the air. Through the comparison of the U value of the three-glass and two-cavity Low-E hollow glass, it is found that the hollow glass with lower heat transfer coefficient can be obtained by arranging it on the indoor surface of the hollow glass. plan. Analyzing the temperature difference between the indoor surface of the three-glass two-cavity Low-E insulating glass and the room temperature, and the radiation heat transfer per unit area, it is found that the temperature difference between the indoor surface of the outer window and the indoor environment is used to measure thermal comfort. If it is inaccurate, placing the online Low-E film on the indoor surface of the hollow glass has better thermal comfort.

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