Discussion on energy-saving auxiliary design methods for double-glazed glass curtain walls?

Compared with traditional glass curtain walls, double-glazed curtain walls can improve the thermal performance of buildings to a certain extent. However, whether its advantages can be exerted depends on whether the design of the building and curtain wall is reasonable. At present, the world At present, the research progress of double-glazed curtain wall energy-saving design is still mainly based on basic research, which is still far from direct guidance of engineering design. Based on the review of the current status of theoretical and experimental research at home and abroad, this article is based on the current domestic design of this type of building. cooperation methods and processes, and proposes to adopt corresponding auxiliary design methods according to different tasks in the three stages of scheme design, preliminary design and construction drawing design, emphasizing that the summary of past design experience should be strengthened to guide architects to correctly grasp the scheme design stage The combination of double-layer glass curtain wall and architectural design achieves effective energy saving

1 Background

The popularity of glass curtain walls in modern buildings, especially high-rise office buildings, continues to increase. It is almost the unanimous choice for office building facades in major cities around the world. One of the main reasons is that people's aesthetic tendencies are still influenced by the modernist style. Glass curtain walls not only have a simple, transparent and modern appearance, but can even symbolize the strength and image of a company. Therefore, they are favored by many owners and architects. However, people have also paid a heavy price for this unique appearance. Since the thermal insulation and thermal insulation properties of traditional glass curtain walls are far inferior to those of traditional walls, and there is a lack of reasonable measures to reduce solar overheating in summer, the cost of this type of building has been greatly increased. Air conditioning and heating energy consumption. Recently, our country has introduced energy-saving standards for public building design, which has put forward more stringent thermal requirements for glass curtain walls.

Compared with traditional glass curtain walls, the unique interlayer design of Double Skin Falade (hereinafter referred to as DsF) not only provides more possibilities for improving the thermal insulation performance of the curtain wall, but more importantly The most important thing is to provide a shelter for the sunshade components, so that they can effectively shade the sun without damaging the appearance of the building. In addition, it can also reduce building energy consumption by enhancing ventilation and cooling, so it is highly praised by architects. However, using DSF will not only increase the initial investment and sacrifice considerable building area, but also its maintenance costs will be higher than those of ordinary curtain walls. Moreover, it cannot be simply assumed that DSF must have better thermal performance such as thermal insulation, heat insulation, ventilation, etc. At present, "most types of DSF cannot reduce heating and cooling loads at the same time. Different types can only be combined or changed according to specific circumstances." Only by setting up the system can it be substantially improved compared to the traditional insulated glass plus external shading solution. For example, the most prominent issue is how to reasonably design the size of the mezzanine, because its too small space will directly affect the DSF in the summer and transition seasons. If the ventilation and cooling effect is too large, the space utilization efficiency will be reduced. If you rely entirely on mechanical ventilation, although the mezzanine width can be reduced, there is also the problem of how to optimize the ventilation and cooling effect and fan energy consumption. In addition, the mezzanine is naturally ventilated. It is also greatly affected by outdoor wind speed and direction. Currently, there is no satisfactory comprehensive method to guide the selection of design conditions, predict operating effects, and guide the design of node structures. There are many less successful practices in countries around the world. Example. It can be seen that the decision-making process of DSF selection and design is affected by aesthetics, thermal comfort and energy saving (Figure 1), and is not a simple matter of "just use it"

2 Related foreign research. Current situation

DSF’s energy-saving research has been carried out abroad, especially in Europe, for more than 20 years, mainly focusing on the research on heat transfer processes, research on integration with building operation modes, and full life cycle analysis. .Among them, the first two aspects have achieved more research results. As the most powerful basis for guiding engineering decisions, full life cycle analysis is the ultimate goal of the entire research, and the research combined with the building operation model is to combine the microscopic heat transfer research results. The link with the macroscopic large-scale model that combines the curtain wall system, building and air-conditioning system is crucial to realize the simulation of the building's energy consumption throughout the year.

The research on the heat transfer process mainly includes both theory and experiment. research on aspects.

The theoretical research is divided into nodal control volume method and computational fluid dynamics simulation method. The purpose of experimental research is to explore the empirical formula of heat transfer coefficient or convective heat transfer coefficient. These research results can assist in establishing numerical models. For example, Grabe's research found that the natural flow resistance coefficient in the interlayer cannot adopt the resistance coefficient under mechanical flow given in the manual. The glass surface convection heat transfer coefficient and the heat transfer coefficient of the entire window both increase with The temperature difference between the inner and outer glass changes linearly, but the former changes significantly, and the latter change can be ignored in mild climate areas.

Research combined with building operation modes includes research on natural passage improvement under specific building layout conditions and research on combined air conditioning systems. There are two main calculation methods: using commercial energy consumption simulation software and secondary development based on network method calculation principles. The research results of Dutch scholar Paassen show that simple nighttime mechanical ventilation can reduce the installed capacity and energy consumption by 40%. If controllable windows and weather prediction systems are considered, the energy saving potential reaches 70%. The Gratia simulation in Belgium found that only when the curtain wall spacing is less than There will be no obvious pressure loss until 40cm. In most curtain walls, the main pressure loss occurs at the air outlet; for the south curtain wall, the size of the natural ventilation opening has a great impact on the temperature in the interlayer, but the impact is very small in the front. Used at night Drafts are very feasible, but you need to be very careful during the day to prevent hot air from entering the room. The use of draft chimneys can ensure that even if there is no wind pressure, the top floor building can achieve ventilation effects, but the size of the opening needs to be adjusted to balance the ventilation volume on each floor. In his doctoral thesis, Li Baofeng did a lot of experimental research on the thermal performance of DSF under China's cold winter and hot summer climate conditions, and proposed that the external circulation type is more suitable for this climate, and the interlayer width should be 400mm, and summer ventilation should be based on wind pressure. Instead of heat pressing, the ideal materials for sunshade louvers and interior and exterior curtain walls are perforated aluminum alloy and high-transparency glass respectively.

At the same time, the actual measurement results of some buildings provide designers with valuable experience. Pasquav's test results at the Siemens Building in Germany show that if short-term high temperatures are allowed, the entire building can be naturally ventilated throughout the year. If there is no suspended ceiling, the effect of night ventilation can be greatly improved. The building uses a ground source heat pump system for cooling, which can meet year-round requirements. The author also points out that DSF is not energy-saving everywhere and in any building. Some boundary conditions of simulation assumptions need to be carefully considered. For example, the temperature around the curtain wall is often several degrees higher than the meteorological observation temperature; for DSF with each partition, the curtain wall The amount of air exchange depends more on the wind speed and direction rather than the chimney effect. In the design, the double-layer curtain wall space should be separated from the vents to prevent hot air from flowing back in summer, or a small area of ??DSF should be designed instead of the entire surface, or like Germany Dehi, the center can completely open the exterior curtain wall in the hot season, but this design has a higher cost and reduces the thermal insulation performance in winter.

There are few studies on DSF from the perspective of full life cycle analysis, especially full-year building energy consumption analysis. The main research indicators include full life cycle economics and full life cycle greenhouse gas emissions. A more representative example is Turkish scholar Cetiner’s analysis of Istanbul’s mild climate. The results show that a double-skin curtain wall is the most energy-saving solution than a single-skin curtain wall by 23%, while the cheapest single-skin curtain wall is 25% cheaper than the cheapest double-skin curtain wall. In the double curtain wall system, the most energy-saving solution can reduce energy consumption by 33.9%, and the most expensive solution can save 7.7% of funds during the entire life cycle.

Based on the above research status, some conclusions from the existing research results can be used to guide design, but they are still unable to meet actual needs. Due to the complex heat transfer process of DSF, the different scales during the day and throughout the year, and the difficulty in obtaining accurate outdoor boundary conditions, there is currently no accurate and reliable method for simulating the annual energy consumption of DSF buildings. At the same time, the existing calculation methods are complex to operate and have high requirements on engineers. They are not suitable for guiding the work in the design stage in engineering applications, but are suitable for use in the detailed design stage.

3 Integration with the architectural design process

At present, research on DSF energy conservation is mainly concentrated in European and American universities, scientific research institutes and a few powerful engineering design companies. The scientific researchers engaged in this area all have a profound background in thermal engineering knowledge, and the research ideas basically start from the analysis of the DSF heat transfer process, with the goal of exploring reasonable mathematical models and computer simulation methods, and some achievements have been made. Results, such as energy consumption simulation software, CFD (ComututlonalRuldlFluidDynamics computational fluid dynamics) simulation software, and several other recognized methods that can assist energy consumption research. However, these methods are generally complex and not only impossible for architects to master, but even engineers with a background in thermal engineering need special training to master them. In other words, DSF's energy-saving research is still a cutting-edge field, and there is still a long way to go before it can be applied to actual projects, especially to guide architects in design.

However, engineering practice will not stop progress because scientific research is not fully developed. In fact, in recent years, some luxury office buildings and apartments in domestic megacities such as Beijing and Shanghai have successively adopted DSF. However, there are generally varying degrees of gaps between the actual operating results of these projects and those advertised. The use of DSF in some real estate projects is not entirely due to energy-saving effects and economic considerations, but for the purpose of real estate speculation. After all, DSF has a fashionable appearance and advanced concepts, and it is expected that a large number of such buildings will emerge in China in the future. Engineering practice urgently needs the guidance of experience and theoretical research, but so far, there are no satisfactory answers to many basic questions about the energy-saving effect of DSF, such as: ① Is DSF suitable for the climates of Beijing, Shanghai, and Shenzhen? Is the energy-saving effect good? Obvious? ② Is DSF suitable for facades with different orientations at the same time? Is its structure the same? ③ What type of building is DSF most suitable for? Is it worth using in residential buildings?... These questions seem simple and basic, but in fact they are very sharp, because for any plan For projects using DSF, these questions must be answered at the beginning of the work. Although a large number of foreign practices have accumulated some experience, the actual operating performance of DSF varies greatly depending on the climatic conditions of different cities and even the conditions surrounding a single building.

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