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Discuss the development and problems of energy-saving doors and windows from a technical perspective

1. Development of energy-saving doors and windows

The window is an important element in the composition of the building and is usually a dominant feature of the building's appearance. In addition, windows provide lighting, vision, and fresh air for building users. At the same time, windows also play a pivotal role in affecting energy use, power demand, and the environment. A large number of studies have shown that solar heat gain and heat loss through the windows account for a large proportion of the heating and air conditioning load of the building. Reasonable window design and selection can reduce electric lighting load, power peak demand, and air-conditioning load, thereby avoiding expensive electricity expenditure. As a developed country or developing country, with large building energy consumption, building energy conservation is imperative. Windows is an important part of building energy conservation. Therefore, vigorous research and development of energy-saving windows have certain practical significance.

2. Performance evaluation index of energy-saving windows

At present, there are often the following four indicators for evaluating the energy-saving performance of windows.

2.1 Heat transfer coefficient (U)

When there is a temperature difference between indoor and outdoor, the window will lose or gain heat through a combination of conduction, convection, and long-wave radiation through the window frame and glass. The U value represents the overall heat transfer capacity and heat preservation capacity.

2.2 Solar Heat Gain Coefficient (SHGC)

Regardless of the outdoor temperature, windows can obtain heat through direct or indirect solar radiation. The solar heat gain capacity of a window can be characterized by the solar heat gain coefficient or shading coefficient of the window.

2.3 Visible light transmittance (VT)

Visible light transmittance is an optical property of glass that describes the amount of visible light passing through the glass. Increasing the visible light transmittance can provide sufficient natural lighting, thereby reducing electrical lighting and related air-conditioning loads, thereby affecting building energy consumption.

2.4 Air penetration (AL)

The air infiltration caused by the gap between the window sash and the window frame will also bring about the loss and accumulation of heat. This effect is usually quantified by the air permeability through a unit area window under certain pressure conditions.

3. Factors affecting energy saving of windows 

3.1 glass

As the main part of the window, glass plays a decisive role in the overall energy-saving effect of the window. The material of the glass itself, the structure of the glass used in the window, and the combination of glass and frame materials are all key factors that affect the energy-saving effect of windows. Simply increasing the thickness of the glass cannot effectively improve the heat insulation performance of the glass. Therefore, in the current market, single-layer glass windows have disappeared, and been replaced by double-layer glass that uses an air cavity in the middle to reduce heat transfer. However, double glazing did not significantly improve the visible light transmittance and solar heat gain coefficient. Therefore, the key to the design of energy-saving windows is to use glass with better energy-saving performance to improve the energy-saving performance of windows.

3.2 Frame material 

The material of the window frame determines the physical properties of the window such as the thickness, structural strength, weight, and durability of the window frame, and also affects the thermal performance of the window. At present, the most widely used frame materials in the market are aluminum alloy window frames and plastic-steel windows with plastics as frame materials and profiles. Plastic-steel windows have good thermal insulation performance due to the multi-cavity structure. The heat transfer coefficient is only 1/357 of that of steel and 1/1250 of aluminum. At the same time, they are weather-resistant, corrosion-resistant, air-tight, and water-tight. It has good performance in terms of performance and sound insulation.

3.3 Sealing parts

The window is opened and closed by hardware accessories. It is the most prone to wear and continuous activity. The effectiveness of its function not only directly leads to safety problems, but also affects the insulation performance, water tightness, and airtightness of building windows. For casement windows, multi-lock point hardware can greatly improve the sealing performance of the window than single-lock point hardware. At the same time, the quality of the opening sealing material not only affects the heat preservation and energy-saving effect of the building but also relates to the waterproof performance of the wall. At present, the gaps between the four sides of the steel-plastic window frame and the wall are usually filled with polyurethane foam. This type of material not only has a filling effect but also has good sealing, heat preservation, and heat insulation properties. In addition, the most commonly used sealing materials include silica gel and EPDM strips. The sealant strip is used to seal between the glass and the fan and frame and plays an important role in watertight, airtight, and energy-saving in plastic steel windows.

Three factors affecting energy-saving of windows 1

 Three factors affecting energy-saving of windows 1

4. Measures to improve the energy-saving performance of windows

4.1 The choice of glass

To design high-performance energy-saving windows, the most important thing is to select and match the most reasonable window production plan among various glass processes, so as to meet the requirements of window energy-saving. Among them, double-layer and multilayer insulating glass and colored glass are commonly used options. Multilayer glass can improve the heat preservation performance of glass, while tinted glass can reduce solar radiation heat and glare.

4.1.1 Laminated glass

The same is transparent colorless glass, the U value of single-layer glass is 1.09, the U value of double-layer insulating glass is less than 0.5, and the U-value of three-layer double insulating glass is only 0.31. It can be seen that the multi-layer glass can make up for the heat loss of the building. However, experiments have also shown that although the heat transfer loss of multilayer glass is significantly reduced, the reduction in visible light transmittance and solar heat gain coefficient is not satisfactory.

The laminated glass 1

The laminated glass 1

4.1.2  Tinted glass and photochromic glass

Coloring will reduce the solar heat gain coefficient of single-layer glass, but the visible light transmittance drops faster, and the color reduces the solar heat gain coefficient to a limited extent. If you need to greatly reduce the solar heat gain coefficient, you can use a reflective coating to increase the reflectivity of the material surface. However, because these coatings have a strong influence on the outside world, they will reflect sunlight as strongly as a mirror, which will have an impact on nearby buildings. Moreover, tinted glass cannot adjust its own performance with changes in the surrounding environment. In many cases, it can neither meet the energy-saving requirements nor meet the environmental requirements of the owners. In comparison, the cover glass is an ideal window material. This material can transform from a transparent state under dark indoor conditions to a dark state under bright outdoor conditions, allowing enough light to pass through to provide illumination while blocking excessive sunlight that can cause glare and air conditioning. Moreover, its production is not high, and it is an ideal material for making energy-saving building windows.

The tinted glass 1

The tinted glass 1

The photochromic glass 1

The photochromic glass 1

4.1.3 Filling gas in the interlayer

Filling gas in the middle of the interlayer can improve the energy-saving performance of laminated glass windows. In order to achieve the best effect, it is necessary to strictly control the thickness and the gas-filled into the insulating glass. Krypton and argon are two commonly used inert gases, which are non-toxic, colorless, odorless, and chemically stable. The heat preservation performance of glass filled with krypton gas is better than that of argon gas, but the cost of krypton gas is higher. However, there is always a problem of gas leakage, so this process is very dependent on the sealing quality of the glass. When the technology meets the requirements and can provide good quality assurance, it is a good choice to fill the window with inert gas to improve the energy-saving performance of the window.

4.1.4 Coated glass

In most commercial buildings, Low-E coatings are usually used to reduce solar heat gain. Although this reduces the beneficial solar heat gain that can be used to supplement heating, it is obviously more important to control solar heat gain. The solar control range of Low-E glass can be greatly changed while maintaining a low U value.

The coating glass 1

The coating glass 1

4.2 Frame material

4.2.1 Composite frame material

The so-called composite frame material is mainly composed of two or more single materials. Tests have proved that the performance of the frame material composited by metal and non-metal can be fully optimized. For example, the plastic steel frame material can be made into a complex section by using the plasticity of the plastic, so as to provide the best section for installing the sealing strip and the inlaid clip, which can greatly improve the air-tightness and water-tightness of the window. In the window frame design, the metal frame material can face the outdoors, and the plastic frame can face the interior. This aspect can meet the requirements of the appearance of the building, and at the same time, it can also prevent the indoor side from exposing the metal surface, which is beneficial to prevent condensation and the discomfort of coldness when touched. Plastic frame materials are arranged indoors to avoid direct sunlight, reduce aging and extend life.

The composite frame material of insualting glass 1

 The composite frame material of insualting glass 1

4.2.2 Window frame-filling material

After the emergence of new materials, corresponding structural cooperation is needed to make them exert reasonable performance. The same material uses different structures, and there is a big difference in performance. The window frame adopts multi-cavity heat insulation strips, and the sealing adopts multi-channel sealing and glass and accessories matching so that the heat transfer coefficient of the window frame is greatly reduced. In order to increase the thermal resistance of the frame, the width of the heat-insulating strip or the thickness of the heat-insulating core connecting the inner and outer aluminum frames should be increased. In order to make the frame section compact, according to the principle of heat flow length, under the same width of the heat insulation strip, it can be designed into a "bow" shape to increase the length of the heat flow, increase the thermal resistance, and further reduce the heat transfer coefficient. Properly widening the heat insulation strip can effectively reduce the K value, but if it is too large, the structure will not be compact, and measures to increase the thermal resistance must be sought. According to the principle of heat conduction, the heat conduction surface can be designed as a superimposed multi-cavity to increase thermal resistance. The sealing of the gap when the other window is closed is a weak link. In addition to solving the problem of convection heat loss, the sealing structure design of the window should also consider solving the problem of heat conduction loss. Therefore, the design of the sealing strip needs to consider the multi-head sealed cavity in addition to the sealing performance to form a multi-cavity space to improve the heat transfer coefficient. Matching with other parts.

4.3 Installation of windows

The installation of energy-saving windows should be fixed by welding, expansion bolts, or nails according to different material conditions. No matter what kind of fixing method is used, the installation must be firm. Increase the overlap of the opening gap of the window and reduce the width of the opening gap. According to the materials used, the cross-sectional shape, and the location of the device, various sealing strips are used for sealing to improve the air-tightness level of the outer window. The installation of seam strips and sealing strips on the windows should be straight, and the combination with the windows should be firm and tight. When using plastic windows, various hardware, fasteners, sealing strips, spacers, spacers, sealing materials, and heat preservation materials that are in close contact with the plastic profile should be compatible with PVC in performance. In addition, plastic windows are sensitive to temperature, so they should be stored in a warehouse (shed) below 50°C after entering the site, and away from heat sources. Pay attention to the installation of plastic windows and glass, and avoid construction at low temperatures.

The installation of energy-saving windows 1

 The installation of energy-saving windows 1

5. Conclusion

(1) Improvement of glass. High-performance windows can be made of insulating glass (consisting of one layer of Low-E + one layer of ordinary float glass or two or more layers of Low-E float glass) or photochromic glass, surrounded by high-strength and high-airtight composite adhesives. Bonding and sealing two or more pieces of glass with sealing strips and glass strips. The middle is filled with dry gas, and the frame is filled with desiccant to ensure the dryness of the air between the glass sheets. In addition, in buildings in cold regions, windows should have the characteristics of low U value and high solar heat gain coefficient, while in hot areas, low U value and low solar heat gain coefficient. Therefore, various glass needs to be used in different areas. Different types should be assembled.

(2) Improvement of window frames. Develop and adopt new composite frame materials, the combination of metal materials and non-metal materials. And add a widened heat insulation strip in the section of the window frame.

(3) Improvement of sealing parts. In addition to the sealing performance, the design of the sealing strip needs to consider the multi-head sealed cavity to form a multi-cavity space to obtain the heat transfer coefficient and other parts. High-performance and durable hardware accessories can improve the energy-saving performance of windows.

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