To measure whether a building is energy efficient, three main factors are considered, namely heat loss (heat exchange), heat convection, and heat conduction and radiation. Heat conduction is the heat transfer by the molecular motion of the materials used in doors and windows, which is transferred through one side of the material to the other, resulting in heat loss; radiation is mainly directly transmitted in the form of rays, leading to heat loss; convection is through doors and windows The gap causes the circulating flow of hot and cold air, which causes heat exchange through gas convection, resulting in heat loss. The heat loss of doors and windows is mainly through the following methods: loss of heat conduction through the profile, loss of radiant heat through glass, and loss of air convection through the gap between the door and window. Therefore, the following key technical measures can be taken to improve the energy-saving effect of building doors and windows.
1. Select the appropriate orientation and window-to-wall area ratio to reduce solar radiation heat transfer. According to the change law of the solar height angle and azimuth angle, the south window of the building can reduce solar radiation heat in summer and increase solar radiation heat in winter. Is the most favorable building orientation. For example, the place where we are located (Suzhou) is dominated by southeast wind in summer. The southeast-facing building is conducive to natural ventilation, and the solar radiation is not very strong, so in Suzhou, the southeast direction is the recommended direction.
In order to avoid the large window opening area from affecting the energy consumption of the building, in the relevant standards for building energy efficiency, the building window is required, that is, the window-to-wall area ratio. Its meaning is the ratio of the total area of the transparent parts of the windows and balcony doors on the exterior wall surface of the entire building to the total area of the exterior wall surface of the whole building. The area of the window here emphasizes the area of the transparent part, that is, the area of the window with the lighting function. If the back of the window is provided with a wall or a block, it should not be included in the window area.
Generally, the heat transfer thermal resistance of a window is much smaller than the heat transfer thermal resistance of a wall. Therefore, the cold and heat consumption of a building increases with the increase of the window-to-wall area ratio. On the contrary, the smaller the window-to-wall area ratio design, the more heat The smaller the loss, the better the energy saving effect. Generally speaking, the solar radiation intensity and sunshine rate are different in different directions, and the solar radiant heat obtained by the windows is also different. The solar radiation intensity and solar irradiance are high in the south and north directions, and the solar radiant heat obtained by the windows is more. GB50176-1993 "Code for Thermal Design of Civil Buildings" stipulates that when the wall of a residential building is designed for minimum heat transfer resistance, the area ratio of windows and walls in each direction is: not more than 0.20 in the north direction and not more than 0.25 in the east and west directions (single-layer windows). Or 0.30 (double-layer window); the south direction is not greater than 0.35.
2. Choosing a suitable window type to reduce heat Convection door window type is the first factor that affects energy saving performance. The commonly used window types are sliding windows and casement windows, which have poor energy-saving effects, while casement windows have excellent energy-saving effects. The sliding window slides back and forth on the sliding rail of the window frame. There is a large space in the upper part and a gap between the pulleys in the lower part. The convection exchange between the upper and lower sashes forms a large heat loss. All kinds of heat insulation profiles can not achieve energy saving effect as window frames.
Casement windows usually have rubber sealing strips between the sash and the window frame. After the sash is closed, the sealing rubber strips are pressed tightly, there is almost no gap, and it is difficult to form convection. The heat loss is mainly from the glass, window sash and window frame profiles. Heat conduction, radiation heat dissipation, air leakage at the contact position of the sash and the window frame, and air leakage between the window frame and the wall.
In general, the thermal insulation performance of casement windows is about 20% higher than that of sliding windows, because the heat consumption of cold wind penetration of the window accounts for a certain proportion of the heat consumption of the entire window (about 10% to 30%), and the airtightness of casement windows is higher. Sliding windows are 1-2 levels, so from the perspective of energy saving, casement windows should be used as much as possible.
3. Reasonable selection of insulating glass to control the radiant heat in the window. The glass area accounts for 65% to 75% of the window area. It is mainly caused by the loss of energy through heat radiation. Therefore, we must choose a reasonable glass for building doors and windows. Glass is selected to control the radiant heat transfer through doors and windows, thereby ensuring the energy saving effect of the overall building.
Glass energy efficiency is evaluated by the heat transfer coefficient. The smaller the heat transfer coefficient, the better the thermal insulation performance and the higher the energy saving. The larger the heat transfer coefficient, the worse the thermal insulation performance and the lower the energy saving. Various types of glass have different heat transfer properties, as shown in Table 1: Table 1 The heat transfer coefficient of common glass (W / ㎡.K) Table 1 The heat transfer coefficient of common glass (W / ㎡.K)
It can be seen from Table 1 that single glass has the highest heat transfer coefficient and is not suitable for energy-saving doors and windows. Insulating glass has better energy saving performance than single glass, and Low-E insulating glass has the best energy saving performance.
Insulating glass refers to a product in which two or more pieces of flat glass are separated by a spacer frame and sealed with a sealant, so that a dry gas space is formed between the glass layers. Because a certain thickness is formed between two pieces of glass and the flow of air or other gas layers is restricted to reduce the convection and conductive heat transfer of the glass, it has better heat insulation capabilities.
Low-e glass (also known as Low-E glass) has good spectral selectivity. Based on a large amount of visible light, it can block a considerable part of infrared rays from entering the room, especially far infrared rays are almost completely reflected back by them without passing through the glass. It not only maintains high light transmittance, but also reduces indoor heat load to a large extent. It can block the transmission of heat through glass and has a good energy saving effect.
The hollow product made of LOW-E glass can improve the heat resistance by one to two times than ordinary insulating glass, and has excellent solar control and thermal control capabilities. In the hot summer, the outdoor temperature is higher than the indoor temperature. The LOW-E insulating glass can prevent outdoor heat from radiating to the indoor. In the cold winter, because the indoor temperature is higher than the outdoor temperature, the LOW-E insulating glass can prevent the indoor temperature from radiating to the outside. Prevents diffusion loss of temperature. LOW-E insulating glass has good heat insulation, making it the most ideal choice for energy-saving windows and doors.
4. The selection of suitable window frame materials reduces the heat transfer window profiles to approximately 15% to 30% of the opening area of the outer window, which is another weak link of energy loss in the outer window of the building. The total heat loss occupies a certain proportion, so the selection of window profiles is also crucial. The thermal conductivity of the window frame material determines the different energy consumption of the doors and windows. The larger the thermal conductivity, the stronger the heat transfer capacity and the greater the energy consumption. The window frame materials commonly used in China are aluminum alloy, PVC plastic, steel, heat-insulating aluminum alloy, etc. Table 2 lists the thermal conductivity of the four window frame materials.
As can be seen from the table, the thermal conductivity of PVC plastic material is the lowest, which is good for heat insulation, but its strength, durability, and fire resistance are not as good as those of aluminum alloy materials. From the perspective of environmental protection, plastic profiles will emit toxic fumes when sawing. The toxic gas "dioxin" released during a fire is not an environmentally friendly product and is not recommended for use. The thermal conductivity of steel is very large, and the heat conduction is fast, which is not conducive to energy saving. Aluminum alloy profiles have a good ability to reflect ultraviolet, visible light, and infrared rays. The surface reflection ability is related to the surface state and color. The reflection ability to heat radiation can reach 90%, which is very beneficial for blocking solar radiation heat. However, the high thermal conductivity of aluminum alloy is not conducive to energy saving.
In order to avoid the shortcomings of aluminum alloys, heat-resistant aluminum alloy profiles have been developed. The principle of heat-resistant aluminum alloy profiles is to penetrate a heat insulation strip in the middle of the aluminum and break the aluminum profile to form a broken bridge, which effectively prevents heat conduction. It retains the advantages of aluminum profiles, and also greatly reduces the heat transfer coefficient of aluminum profiles. It has a better energy saving effect and is the best choice for energy saving of doors and windows.
There are two main ways to produce heat-insulating profiles. One is to use heat-insulating strips and aluminum profiles to form "insulation bridges" through mechanical tooth opening, strip-piercing, and rolling processes. Another type is to inject a heat-insulating material into the heat-insulation cavity of an aluminum alloy profile, and then solidify to remove the broken bridge metal to form a "heat-insulating bridge", which is called a "cast-type" heat-insulating profile. The inner and outer sides of the heat-insulating profile can be profiles with different cross-sections or different color profiles with different surface treatment methods.
However, due to the influence of the region and climate, avoid large differences in linear expansion coefficients of thermal insulation materials and aluminum profiles, resulting in large stresses and gaps between the two during thermal expansion and contraction; at the same time, the combination of thermal insulation materials and aluminum profiles As a whole, the doors and windows are also subject to the same stress as aluminum. Therefore, it is required that the thermal insulation material must have tensile strength, flexural strength, expansion coefficient and elastic modulus close to those of aluminum alloy profiles, otherwise the thermal insulation bridge will be broken and destroyed.
5. To improve the airtightness of doors and windows and reduce the airtightness of air convection hot windows refers to the ability to prevent air penetration when the doors and windows are closed. The level of airtightness of doors and windows has a great impact on the loss of heat. The change in outdoor wind will adversely affect the room temperature. The higher the level of airtightness, the less the heat loss and the smaller the impact on room temperature. Therefore, improving the airtightness of doors and windows and reducing the penetration of cold wind is also a way to improve the energy efficiency of windows.
The air-tightness of windows of residential buildings and public buildings is stipulated in GB50176-1993 "Code for Thermal Design of Civil Buildings": in areas where the average outdoor wind speed is greater than or equal to 3.0m / s in winter, for buildings with 1 to 6 floors, it should not be low Level III of the current national standard "Classification and Testing Methods for Air Permeability of Building Exterior Windows"; for 7 to 30-story buildings, it should not be lower than level II of the above standard; the average outdoor wind speed in winter is less than 3.0 In the area of m / s, for buildings with 1 to 6 floors, it should not be lower than the level VI specified in the above standard; for buildings with 7 to 30 floors, it should not be lower than the level III specified in the above standard.
Air penetration is mainly between window frames and sashes, between fan frames and glass, and between window frames and walls. The ideal sealing strip can minimize the air penetration between the window frame and the sash, and between the window frame and the glass. The sealing strip is divided into rubber strip, plastic strip or rubber-plastic combination according to the material; the strip is strip-shaped, brush-shaped, and sheet-shaped according to the shape; and the fixing methods are pasting, squeezing or nailing. The sealing rubber strip must have sufficient tensile strength, good elasticity, good temperature resistance and aging resistance, and the cross-sectional structure size must match the profile of doors and windows.
Because the bad quality of the rubber strip has poor aging resistance, after long-term exposure to the sun, the rubber strip hardens after aging, loses its elasticity, and is easy to fall off, which not only has poor sealing performance, but also causes safety risks due to loose glass. The quality of the opening sealing material directly affects the sealing between the window frame and the wall, which not only affects the thermal insulation and energy saving effect of the house, but also relates to the waterproof performance of the wall. Therefore, the opening sealing material must also be selected correctly.
The gaps between the four sides of the door and window frame and the wall are usually filled with polyurethane foam. Such materials not only have a filling effect, but also have good sealing and heat insulation properties. In addition, more sealing materials are also used. There are silicone, EPDM strips. When the sealing performance of the window fails to meet the requirements of energy-saving standards, appropriate sealing measures should be taken, such as setting a sealing strip or sealant made of rubber, felt, etc. in the gap to improve the airtightness of the window.
6. Concluding remarks With the successive introduction of China's building energy-saving standards, energy-saving doors and windows have become more and more popular in the market. In recent years, China has done a lot of work in the research and development of energy-saving doors and windows and the introduction of technology. Generally speaking, the energy saving of doors and windows has been greatly improved, but compared with advanced countries, there is still a large gap. We should learn from advanced foreign technologies and combine China's national conditions carry out practical research to improve the grade of doors and windows to meet the needs of the development of building energy efficiency.