期刊文章分类查询,尽在期刊图书馆
于萱
成都信息工程大学银杏酒店管理学院 611743
Abstract：Passive building is the use of passive building energy-saving technologies which have the characteristic of low-power, low-cost, low-technology. This paper will focus on the difference of vacuum glazing and insulating glazing in passive building. The aim of this report is to find out the suitable material of glazing in passive design. A brief introduction about the development of glazing technology will be given first. Next, comparison and contrast between these two kinds of glass will contain the areas of thermal insulation, sound insulation, application areas, etc. After these comparisons, the improvement direction of insulating glazing will be given. Finally, there will be a summary of each glazing, followed by a suggested choice of glazing which is more appropriate to be used in the future.
Key words：Passive design; vacuum glazing; insulating glazing; energy performance
1. Introduction
As there is a significant portion of energy is consumed by buildings, especially in developed countries, the awareness of building’s energy efficiency keeps increasing all over the world (Sadineni, Madala and Boehm, 2011). In order to reduce the energy consumption and protect the global environment in the area of architecture, the idea of passive building starts to play a more and more important role in the architectural design stage. Passive building means the building uses passive energy efficiency strategies to optimize building’s envelope, so that it is able to extremely improve building’s performance in terms of thermal insulation and air tightness, and then decrease the demand for cooling and heating system by using the efficient indoor ventilation system (Henze, 2005). According to make the most of natural ventilation, day lighting, solar radiation, or some other passive energy efficiency strategies, the building can create a high level of interior thermal and lighting environment, and reduce the use of active strategies, such as air conditioning system, electrical lighting, etc. (Henze, 2005). At present, there are many passive strategies for raising energy efficiency of building envelope through the improvements in terms of external walls, roofs, doors and windows, floors, or some other envelope components (Sadineni, Madala and Boehm, 2011), and this essay will focus on window’s passive design.
The main material of a window is glass, so in some extents it is the glass which determines the energy efficiency performance of a window. The document will make the comparison and contrast between two typical types of passive window structures, one is vacuum glazing, and the other one is insulating glazing, and then find out the strengths and weaknesses of each glazing. With the analysis of their advantages and disadvantages, it is expected to be aware of the more appropriate passive window structure in different contexts. The aim of the essay is to help improve the building’s energy efficiency through the passive design of the window. After the introduction part, there will be an illustration of the structure of two window designs. Then the compare and contrast between vacuum glazing and insulating glazing in the area of thermal insulation, sound insulation, prevent dewing the performance, application areas, and sealing performance. Based on the results of these comparisons and contrasts, a result will be given to determine which type of window structure has a more valuable prospect. For the glazing which has a relatively poor performance, some suggestions will be provided in the following section to improve its performance.
2. Structure
Insulating glazing consists of two or more glass panes, and the gap between two glass panes is insulated by sealant from the outside (Guo and Sang, 2006). In the beginning, the gap was filled with dry air, and at present it often uses other gases whose U-value is lower than air (Hart et al., 2012). The thickness of the gap should not be shorter than 6mm, otherwise the window could not play a role of thermal insulation efficiently (Hart et al., 2012). In order to achieve the standardized production of the framework of insulating glazing, the thickness has been unified to several specific sizes, which encompasses 6mm, 8mm, 12mm, 15mm, and 18mm (Hart et al., 2012). The glass pane used to be plate glass, but now it is added a layer of Low-E coating for the purpose of improving its thermal insulating capability (Hart et al., 2012). In the light of different use places and purposes, the raw material and window’s structure will be changed a little. Firstly, the glass plane can be white glass, coloured glass, coated glass, or tempered glass and laminated glass which have been processed for two times (Guo and Sang, 2006). Secondly, as mentioned before, the gap can be filled with different gases by different requirements (Guo and Sang, 2006). For example, argon can improve the performance of thermal insulation, while sulfur hexafluoride is able to increase the capability of sound insulation (Guo and Sang, 2006). Thirdly, how to seal the in-between gap can be different. It mainly has two sealing methods, single sealing and double sealing (Hart et al., 2012). Single sealing means to use the gap is only sealed once, but double sealing will have another layer of sealant to avoid the outside moisture, which will make the insulating glazing lose efficiency, comes into the dry gap(Hart et al., 2012). Therefore, double sealing insulating glazing usually has a longer life span than single sealing one.
Vacuum glazing’s two glass panes are sealed up and make the in-between gap becomes vacuum, and the thickness of the gap is about 0.1-0.2mm (Igesolutions.com, 2015). At least one of the glass panes has a Low-E coating layer, which is the same as insulating glazing, and the thermal insulation performance of this glazing works under a similar principal to vacuum bottle (Liu, Di and Tang, 2011). Vacuum glazing can be regarded as a plate vacuum bottle, and the pressure of the gap is lower than 10-1Pa, which means the thermal transmission within the gap can be almost ignored (Liu, Di and Tang, 2011). However, there are two differences between vacuum glazing and vacuum bottle. The first distinction is that the glazing should be able to let light come through, so it cannot use non-transparent coating as vacuum bottle. Thus vacuum glass uses another kind of transparent Low-E coating (Eames, 2008). The other difference is the shape. Vacuum glass cannot have a cylindrical due to the specific usage, so it needs some supports to make sure the gap always has the same thickness (Eames, 2008). However, because support may cause the thermal transmission between two glass panes, the diameter of the support must be controlled between 0.3-0.5mm to avoid redundant thermal transmission and for the purpose of aesthetics issues, as well (Eames, 2008).
3. Comparison and Contrast of Energy Efficiency Performance
3.1 Thermal Insulation
Due to the different structures, heat transfer mechanism of the vacuum glazing and insulating glazing is also different. Heat transfer can be performed through three ways：conduction, convection and radiation. Middle vacuum layer of vacuum glazing reduces heat transfer through conduction and convection into low level, therefore, the heat transfer in these two ways can be neglected. In vacuum glazing, heat transfer contains: Radiation heat transfer, Supports heat transfer and Residual gas heat transfer. Moreover, the residual gas heat transfer can be neglected in qualified products. Due to the air layer of insulating glazing, gas radiation heat transfer is existent. In insulating glazing, heat transfer contains: Radiation heat transfer, gas radiation heat transfer (including conduction and convection).
Therefore, in order to reduce heat transfer caused by the temperature difference, vacuum glazing and insulating glazing should reduce the radiation heat transfer. Effective method is to use low-e glass, under the condition of the demand of other optical performance, radiation rate of low-e glass is the lower the better. The vacuum glazing is not only to ensure that must be vacuum but also minimize the heat transfer from supports and minimize residual gas heat transfer to negligible levels. However, insulating glazing should minimize the gas heat transfer. In order to reduce gas heat transfer and give attention to other factors such as sound insulation, the air layer of the insulating glazing is 9mm to 24mm, commonly in the majority with 12mm. To reduce the gas heat transfer, inert gas such as argon and krypton can be used to instead of air, but even so, gas heat transfer is still dominant. Therefore, vacuum glazing has better heat preservation and heat insulation performance than insulating glazing.
Figure 1: Sections of Insulating Glazing and Vacuum Glazing
3.2 Sound Insulation
Urban noise is mainly divided into daily noise, traffic noise, equipment noise and construction noise (Zhong and Wu, 2015). Traffic and equipment noise mainly concentrated in the low frequency noise (200 Hz ~ 300 Hz) and the middle frequency (500 Hz ~ 1000 Hz). Daily noise is in every frequency and construction noise is intermittent. Glass can effectively reduce the noise. The transmission of sound needs a medium. Insulating glazing sound transmission through the air, and vacuum glazing has vacuum layer, sound cannot travel through gas, mainly spread through supports. Due to the characteristic of the vacuum glazing structure, it can effectively overcome the weakness of insulating glazing that easy to resonate in low and middle frequency, develop its good sound insulation effect. In most frequency band, especially low frequency band, the sound insulation performance of vacuum glazing is better than that of insulating glazing.
3.3 Prevent Dewing Performance
Dewing is refers to when the surface temperature is lower than the air dew point temperature, the phenomenon of condensation water will appear near the surface. (Tang and Xu, 2014). When the outdoor temperature is below – 3ºC, low-e insulating glazing is condensation on the inner surface, while when outdoor temperature is below - 56 ºC, condensation will appear in the large area of vacuum glass’s surface. Vacuum glazing sealed all round and internal is the vacuum state, therefore, the possibility of its internal condensation does not exist. In addition, in the case of outdoor temperature is low, the interior side surface temperature of vacuum glazing is higher than the insulating glazing. In general, the indoor side vacuum glazing surface temperature is higher than the dew point, therefore, when the outdoor temperature is very low, condensation will not occur on vacuum glazing.
3.4 Application Areas
In the cold region solar radiation is beneficial to maintain the indoor temperature in winter. Therefore, low refractive index, shading coefficient is small and the glazing that cannot effectively use the solar energy heating is not suitable for use. Transparent insulating glazing heat preservation performance is good, can be used. However, insulating glazing has burst problems in plateau low pressure area. Due to the internal of vacuum glazing is the vacuum state, there is no burst problem for vacuum glazing in plateau low pressure area. Therefore, vacuum glazing can be applied to the original and high altitudes areas. Theoretically, in the plateau area the differential pressure and the stress on vacuum glazing are small, therefore, the security is high.
3.5 Sealing Performance
Insulating glazing is generally adopted double seal structure. The internal seal using butyl rubber, outer seal using polysulfide rubber. According to Tang and Xu(2014), insulating glazing failure caused by water vapor permeability accounted for 63% of total failure number. Sealant is high molecular polymer organic matter. With time goes by, it appears aging and the water vapor will seep into the cavity. At that time when the temperature decrease the insulating glazing inner surface will appear the phenomenon of condensation or frosted, affect the heat preservation and heat insulation effects. If the low-E glass is used, it will damage the low-E film. Vacuum glass is mostly adopted glass solder sealing. This way is not easy to age and has some advantages including low gas permeability, good sealing and will not produce a lot of stress on the glass edge.
3.6 Summary
Vacuum glazing and insulating glazing not only have different structures but also have different heat transfer mechanisms. Vacuum glass heat transfer by radiation heat transfer, supports heat transfer and residual gas heat transfer, while insulating glazing is made up of gas heat transfer and radiation heat transfer. Both of two types of glazing can reduce the radiation heat transfer by choosing low-e glass, vacuum glazing should minimize supports heat transfer and residual gas heat transfer and insulating glass should minimize the gas heat transfer. Compared with insulating glazing, vacuum glazing is thinner and lighter. Secondly, the sound insulation effect of vacuum glazing in traffic noise, especially low frequency and middle frequency band, is better. Thirdly, prevent dewing performance of vacuum glazing is better and vacuum glazing can be safely used in the plateau area. Finally, the sealing way of insulating glazing is easy to age, while vacuum glass sealing way has better durability. Through the above comparison that: Compared with insulating glazing, vacuum glazing performance advantages are more obvious. In addition, vacuum glazing also has other advantages: Firstly, the wind load resisting strength of vacuum glazing is better. Due to the vacuum glazing structure is the solid combination of two pieces of glass, it has strong rigidity and the wind load resisting strength is twice than the same thickness of insulating glazing. Secondly, because of the manufacturing process and equipment are advanced, the service life of the vacuum glazing is far longer than sealed insulating glazing with organic materials.
Vacuum glazing has huge development potential. Construction should consider its use value, economic benefits, social benefits, and create a beautiful living environment. Using vacuum glazing meets the requirements of energy conservation and environmental protection, while the initial investment increase, later use cost reduction. Because the cost of construction is temporary and the cost of operation is for a long time. Vacuum glazing has more economic benefits. According to Tang (2009), using vacuum glazing the initial investment will increase 0.5%-1% of total initial investment, while the operation process will save 5%-10% energy consumption, benefit rate significantly improves. Tang (2009) provides examples, build a five square meters of vacuum glazing building, electricity can save $2.5 million (CNY) a year, equivalent to save 1000 tons of coal, and greatly reduce the pollution of the environment. With a building area of 3600m2 ordinary residential building in north China as an example, using vacuum glazing rather than ordinary insulating glazing can save at least 80% of energy consumption, 50000 (CNY) in operation and the annual total pollutant emissions decrease 84t.
External window is a major part of the structure, not only should meet the basic function such as lighting, ventilation, heat dissipation, ornamental, shall also have the good heat preservation, heat insulation, sound insulation performance, etc. Energy consumption of building external window is big, according to the research, the heat consumption of building external window accounts for 34.4% of the total heat consumption of the building result from external window area is large. Building external window is the weakest link of building envelope in heat insulation and heat preservation, while it is the main building component to reduce energy consumption. Development of energy-saving building, comprehensive promotion of energy-saving technology, constantly resources and improving comprehensive utilization of resources can realize sustainable development. It is not only an important economic problem, is also the important strategy. With the development of science and technology, new material, new craft and new technology will appear constantly. Vacuum glazing will constantly improve the quality of itself, production technology and equipment will also be constantly updated, production will continue to expand, the cost will decrease and combination of vacuum glazing varieties will continue to increase. The structure of vacuum glazing is simple and solid. Sound insulation, heat insulation and environmental protection effects are more than three times stronger than the insulating glazing. In sum, promoting vacuum glazing technology and production has positive significance on relieving the energy crisis. To emphasize is that if the vacuum glazing assembles insulating glazing into a variety of "super glazing". It has more superior physical properties and will appear in a variety of applications.
4. Improvements of Insulating Glazing
As mentioned in the last part, vacuum glazing is superior to insulating glazing in most cases, because of the cutting-edge technologies that used in it. However, there are still some strategies that can help insulating glazing improve its performance in terms of passive design.
4.1 Coloured Insulating Glazing
Coloured glass is able to absorb a part of solar radiation, and since outside air veto avoids the glass breaking. Since outside air circulation speed is larger than inside one, coloured glass can contribute to dissipate heat on the external glass surface (Guo and Sang, 2006). Thus, the window is able to reduce the total amount of solar radiation that comes into the room through insulating glazing. Compared with transparent insulating glass, coloured insulating glass has a higher performance in preventing outdoor heat comes in, but its ability of keeping indoors warm is almost the same as transparent insulating glass (Guo and Sang, 2006). In addition, coloured glass will break if it is heated unevenly, so when the glass has to be used in a place with large temperature difference between day and night, it is necessary to take actions to avoid the glass from breaking (Guo and Sang, 2006).
4.2 Position of Low-E Coating
Insulating glazing with two glass panes has four surfaces from outdoor to indoor. According to the research, it is indicated that shielding coefficient will have a great change if the Low-E coating is put on the different surface. If the coating is put on the inside surface of the external glass pane, its shielding coefficient will be much smaller than whose coating is put on the outside surface of the internal glass pane (Hart et al., 2012). Therefore, for relatively warm areas, Low-E coating is suggested to be on the inside surface of the external glass pane, which helps to dissipate heat; while for relatively cold areas, Low- E coating should be on the outside surface of the internal glass pane to achieve heat preservation.
4.3 Gas in the Gap
Because some kinds of gas have a relatively low U-value, such as argon, krypton, CO2, SF6, if the gap is filled with these gases, it is reported that the thermal resistance performance of the window can be raised up 5% approximately (Hart et al., 2012). Since CO2 has acidic property which will corrode the sealant and Low-E coating in a long-term view, it is not recommended to be used for insulating glazing (Hart et al., 2012). Then SF6 will have harmful impacts on the ozone layer, so it cannot be used, as well (Hart et al., 2012). Argon and krypton do not have any negative influences, but krypton which has the best performance is difficult to acquire and costs a lot, so the most efficient method is to mixing use the argon and krypton to help improve window’s thermal performance (Hart et al., 2012).
4.4 Thickness of Gap
Gap’s thickness has a positive correlation to the U-value of insulating glazing, but when the thickness reaches to a specific value, the growth rate of U-value can be quite small, because the gas will have convection to reduce its U-value (Guo and Sang, 2006). Based on the research, the most reasonable thickness of the gap should be 12.7mm, and it is recommended choosing 12mm due to the limitation of industrial standard (Guo and Sang, 2006).
4.5 Material for Sealing the Gap
It is common to use aluminium as the material of sealant, which has advantages in low mass, easy to be made, but aluminium has a high heat conductivity that will reduce the U-value of edges of insulating glass (Guo and Sang, 2006). In this case, when the outside temperature drops to a specific low level, the interior side of insulating glazing will have condensation (Guo and Sang, 2006). The solution here is using the Swiggle instead of aluminium, because the Swiggle has a high performance in thermal insulation, and is able to raise the energy efficiency performance of the building by avoiding condensation (Guo and Sang, 2006).
5. Conclusion
Summing up, insulating glazing and vacuum glazing are two sustainable window structures in passive building design. These two types of glazing all consists of two glass panes, but insulating glazing’s gap between two glass panes is filled with specific gases, while vacuum glazing’s gap is vacuum. With regard to the comparisons and contrasts in their energy efficiency performance, vacuum glazing seems to be superior to insulating glass in most of aspects, but insulating glazing’s upfront cost is lower than vacuum glazing, which is convenient for those who do not have enough capital in the beginning. Moreover, insulating glazing’s performance can be improved by using coloured glass appropriately, adding Low-E coating in the suitable position in light with the site’s climate, filling the in-between gap with argon and krypton, keeping the thickness of the gap to 12mm, using the Swiggle as the material of sealant, or some other strategies which are not listed in the essay.
In conclusion, compared with insulating glazing, vacuum glazing has the more hopeful prospect relatively, due to its high performance in sustainable area. However, vacuum glazing focuses on high-end market more at present, and it is not quite popular among residential market which plays a significant role in property market (Eames, 2008). Therefore, it is necessary to promote vacuum glazing to be used in more buildings, so that window’s passive design can make more contributions to sustainable development.
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论文作者:于萱
论文发表刊物:《防护工程》2018年第30期
论文发表时间:2019/1/15
标签：期刊论文; 成都论文; 尽在论文; 银杏论文; 管理学院论文; 文章分类论文; 信息工程论文; 《防护工程》2018年第30期论文;