China's construction industry has made tremendous progress in the past two decades, but at the same time, building energy consumption has become an issue that cannot be ignored. Building energy conservation has become a key focus of China's energy conservation and emission reduction plan. The release of GB/T 51350-2019 "Technical Standards for Near Zero Energy Buildings" in 2019 shows that it is urgent to fully solve the problem of building energy consumption. At present, the heat transfer coefficient of traditional glass is relatively high, resulting in the energy consumption of building envelope structures accounting for 40% of the overall building energy consumption. However, energy-saving glass used in low-energy buildings is a breakthrough point in solving the problem. Aerogel glass has attracted much attention in recent years because of its excellent physical properties. According to the principle of energy-saving glass used in buildings, the main factors affecting the energy consumption of building glass are the heat transfer coefficient and shielding coefficient of the glass. At present, the mainstream energy-saving glass on the market can be divided into three categories: insulated energy-saving glass with insulation performance, hollow glass, and vacuum glass; Coating glass and dimming glass for energy-saving glass with shading performance; Heat absorbing glass with energy-saving properties (see Table 1). Aerogel material is a new material with three-dimensional space nano network structure. It has excellent physical properties such as low density (as low as 0.003g/cm), low thermal conductivity (0.013W/(m · K)), high porosity (as high as 98%), large specific surface area (800~1200m/g), low refractive index, high transmittance, good sound insulation effect and low dielectric constant (1.0~2.0). Various properties of aerogel make aerogel glass have good sound insulation and heat insulation effects, low heat transfer coefficient, good sunshade coefficient and high transparency.

1、 Aerogel glass

Aerogel was first successfully prepared by Kistler in 1931, while aerogel glass was first prepared by the European Nuclear Research Center in Geneva in 1977. Although aerogel glass has good light transmittance, low heat transfer coefficient and excellent sound insulation effect, due to its high cost, it was not further researched and developed at that time. It was the proposal of low-carbon and green buildings in recent years that attracted much attention. At present, aerogel glass can be divided into three categories, namely, monolithic aerogel glass, coated aerogel glass and particulate aerogel gel glass (see Table 2). Among them, particulate aerogel glass has the best application prospect due to its excellent physical properties, simple preparation process and high yield.

1.1 Whole aerogel glass

Among the three preparation processes of aerogel glass, the whole aerogel glass has the best solar thermal coefficient (SHGC), the lowest thermal conductivity and the highest transparency. According to the test, the solar thermal coefficient of the whole piece of aerogel glass with the thickness of 10mm is 0.9, while that of the granular aerogel glass with the same thickness is only 0.5; The heat transfer coefficient of 20mm thick monolithic aerogel glass is 0.5W/(m2 K) and the thermal conductivity is 0.017W/(m · K), while the heat transfer coefficient of granular aerogel glass with the same thickness is 0.7W/(m2 K) and the thermal conductivity is 0.025W/(m · K); The visible light transmittance of the whole piece of aerogel glass with the thickness of 20mm is 90%, while that of the granular aerogel glass with the same thickness is only 60%. However, due to the physical properties of gel itself, which leads to fragility and extremely low yield of whole aerogel, it is very difficult to prepare large volume whole aerogel, and the cost is very high. Therefore, it is currently only used in the laboratory stage and the demonstration project stage.

1.1.1 Preparation of whole aerogel

The preparation process of SiO2 aerogel mainly includes three steps: sol gel, aging and drying (see Figure 1). Two single silicon sources, TEOS and TMOS, are mainly used to prepare transparent aerogels. However, due to the rigid silicon oxygen bond (Si-O-Si) linkage, the mechanical properties of the framework of such SiO2 aerogels are poor, and it is relatively difficult to prepare the whole piece of aerogels, and the damage rate after preparation is very high. Therefore, how to improve mechanical properties without affecting transparency has become a focus of attention. In terms of silicon source selection, some experts selected MTES and MTMS, which are commonly used in the preparation of flexible aerogels, as silicon sources, to prepare transparent flexible aerogels by adding surfactants and adjusting the preparation process. In 2007, Kanamori et al. used MTES as a single silicon source, methanol as a solvent, oxalic acid and ammonia as acid base catalysts, and cetyltrimethylammonium bromide as a surface modifier (CTAB) to prepare a whole transparent flexible aerogel with a visible light transmittance of 60%, 75%, and 80% (see Figure 2). At the same time, it has a certain compression resilience, providing a new research idea for the development of the whole transparent aerogel. In 2016, Shuimizu and others prepared corresponding aerogels using ETMS and VTNS for the first time, both of which showed good light transmittance and mechanical properties (see Figure 3). Because of its low thermal conductivity, high transparency and good mechanical properties, this kind of transparent flexible aerogel has a good prospect in the development of building envelope. At present, the whole aerogel glass demonstration projects abroad are mainly based on this kind of aerogel. In addition to improving the transparency of flexible aerogels, many experts have conducted a lot of research on improving the mechanical strength of transparent aerogels with TEOS and TMOS as silicon sources. For example, using TEOS, TMOS and MTES, MTMS composite, hybrid precursor method is used to prepare aerogels, but the flexibility will be improved while the transparency will be reduced. This research is in progress.

1.1.2 Simulation of monolithic aerogel glass for buildings in different climate regions

Schultz et al. prepared aerogel blocks with 15mm and 20mm core materials, and sandwiched them between two 4mm glasses. The heat transfer coefficients of the two sample centers are 0.66W/(m2 K) and 0.5W/(m2 K) respectively, and the total solar transmittance is 76% and 75%. Cb A et al. found that the thermal conductivity of the whole gel glass for building is 0.017W/(m · K) and 0.007W/(m · K) under normal pressure and vacuum conditions respectively, and the visible light transmittance is as high as 90%, and the haze is 0.01, which is the same as that of float glass, and can reduce the noise by 3 dB more than that of traditional glass. Wang Huan et al. took the library of Guangzhou University as a sample (see Figure 4), and used the building performance simulation analysis software IES<VE>to simulate the cooling load of building air conditioners for five types of glass, including aerogel glass, in hot summer and warm winter areas. The results show that the radiant heat generated by the solar radiation of glass in hot summer and warm winter areas is the main consumption reason of the cooling load. From this, it can be seen that the shading performance of glass is the main factor affecting the cooling load of air conditioners. Finally, they concluded that aerogel glass can significantly reduce the cooling load of air conditioners in hot summer and warm winter areas.

Wang et al. prepared 15mm whole aerogel glass for energy consumption test in Harbin, Beijing and Shanghai, which can save 20%, 11% and 9% of energy consumption in one year respectively, proving that aerogel glass has great energy saving advantages, and the energy saving effect is more obvious in cold regions. When using three-layer glass to further reduce the U-value of glass, the highest energy savings in severe cold areas can reach 73%. There are results indicating that the heat transfer coefficient and shielding coefficient play a decisive role in the energy consumption of glass.

1.1.3 Application examples of whole aerogel glass

Berardi et al. analyzed the thermal test, lighting test and other disciplines of the whole aerogel glass, and applied the integrated aerogel glass to the renovation project of the teaching building completed in the 1960s.

During the renovation process, use 300mm × 400m × The 12mm small aerogel block is compounded with 4mm thick ordinary float glass, and the glasses in different rooms of the teaching building are replaced in proportion by 40%, 60%, 80% and 100% respectively. Compared with the original sample, the U value of the glass increases with the increase of the replacement proportion of the aerogel area. The U value of the blank sample is 1.5W/(m2 K), the U value of the 40% replacement area is 1.2W/(m2 K), and the U value of the 60% replacement area is 0.9W/(m2 K) The U-value for 80% replacement area is 0.7W/(m2 K), and the U-value for 100% replacement area is 0.6W/(m2 K). The heating and cooling tests proved that when the replacement amount of aerogel reached 60%, the energy consumption of refrigeration at the same temperature could be kept unchanged, while the heating energy consumption decreased with the increase of replacement rate. In terms of sunshine, the UDI can be improved by using 40% replacement amount (the UDI using a single transparent glass is 36.2, and the 40% replacement area can reach 36.8), but the continued increase of the replacement area of aerogel will have a certain impact on sunshine availability. Through testing and simulation, it is concluded that up to 80% energy saving effect can be achieved through the replacement of aerogel glass.

1.2 Coated aerogel glass

Coated aerogel glass is also called aerogel coated glass. Due to its imperfect process, short service life and insignificant energy-saving effect, there are few studies at home and abroad. Kim et al. soaked the float glass in the SiO2 aerogel precursor solution and dried it under normal pressure to prepare the coated aerogel glass. The data speculated that when the coating thickness was 100 μ At m, the thermal conductivity of the coating itself is 0.016W/(m · K), and the transmittance exceeds 90%. Yang Lixiu et al. prepared aerogel solution by acid alkali two-step method, and then realized aerogel glass coating by dip pulling method and rotary coating method. Zhao E dissolved the aerogel with methanol, and then uniformly coated the gel on the glass after the aerogel was evenly dispersed by the rotating coating method. The coated aerogel glass was successfully prepared, which reduced the thermal conductivity of the glass from 1.332W/(m · K) to 1.302W/(m · K), but at the same time, the transmittance decreased by 12%. It was found that this sample had a certain self-cleaning ability. In addition, Zhao E et al. [30] also carried out SiO2 aerogel coating on the surface of FTO glass, and successfully prepared FTOSiO2 aerogel coating composite glass with low emissivity and low thermal conductivity, with emissivity of 0.53 and thermal conductivity of 0.247W/(m · K). Li Jin et al. used the two-step method to prepare 80nm VO2 film by polymer precipitation method and 1 μ SiO2-VO2 composite aerogel coated glass was prepared by compounding m SiO2 aerogel film on the glass surface. The thermal conductivity is 0.019W/(m · k), and it has thermochromic property. The transmittance in the visible light region reaches 45%, and the transmittance control amplitude is 46%.

1.3 Granular aerogel glass

Granular aerogel glass, because its filler aerogel particles diffuse solar radiation, allows solar radiation to enter, but does not allow solar radiation to pass through, thus presenting a transparent but opaque state. This makes the adaptive building application of particle aerogel glass to scenes that do not require visual interaction, such as airport shed roof, railway station curtain wall, basketball stadium transparent curtain wall, etc. The sample of particulate aerogel glass.

Lv Yajun et al. prepared eight kinds of granular aerogel glasses with different structures. They found that the size and thickness of the gel filled particles had a certain impact on the light transmission and heat insulation properties of the aerogel. From the test data, when the size of the aerogel particles increased from 0.41mm to 0.93mm, the transparency increased significantly under the same filling thickness, and there was no obvious change when the size of the aerogel particles continued to increase; When the particle size increases from 0.41mm to 2.7mm, the heat transfer coefficient of aerogel glass increases by 15% under the same filling thickness. With the same particle size, the greater the filling thickness, the worse the transparency, and the higher the heat transfer coefficient. Compared with insulating glass of the same size, the heat transfer coefficient of granular aerogel glass can be reduced by 51.43% at most. Under the irradiation of infrared heating lamp, the heat insulation effect is improved by 5.1~10.2 ℃.

1.3.1 Preparation of transparent particle aerogel

Compared with the preparation of a whole transparent aerogel, the preparation cost and technical requirements of transparent particle aerogels are lower. With the traditional preparation method of SiO2 aerogels, TEOS and TMOS are better choices for silicon sources. Due to the need to produce finished products in granular form, the requirement for mechanical strength is reduced, and the main focus is on improving transparency. According to relevant tests, when TEOS and TMOS are used as a single silicon source to prepare SiO2 aerogels, the lower the water consumption is, the better the transparency is. After preparation, they can be crushed and sieved for use.

1.3.2 Simulation of particulate aerogel glass for buildings in different climate regions

Chen Youming et al., using optical model and dynamic heat transfer model, proposed to conduct simulation test on the adaptability of particulate aerogel glass with solar energy utility as an indicator through the research on the double-layer insulating glass model, three-layer insulating glass model, high permeability Low-E insulating glass model, double silver Low-E insulating glass model and sunshade Low-E insulating glass model. The conclusion shows that aerogel glass has year-round climate applicability in severe cold areas (Harbin), cold areas (Beijing), mild areas (Kunming) and hot summer and cold winter areas (Changsha). Compared with the above five types of glass, granular gel glass is prominent in energy saving and consumption reduction. The order of its adaptability is severe cold>cold>hot summer and cold winter>mild areas. This conclusion also conforms to the rule of the weatherability of the whole piece of aerogel glass. Zhang Yi et al. studied the indoor thermal environment of granular aerogel gel glass residential buildings in summer, and concluded that the energy consumption of aerogel glass as the building envelope structure was 26.06% lower than that of other building envelope structures. In terms of lighting, although granular aerogel glass could not achieve visual interaction, and its visible light transmittance was lower than that of hollow glass, its indoor light was soft after scattering, and there was no glare, It can fully meet the relevant lighting design requirements.

At present, there has been some progress in the application of aerogel glass at home and abroad, but the relatively mature application technologies are concentrated in Europe and the United States. The domestic production is still in the laboratory stage or pilot test stage, and the production has not been expanded. Garnier and others are in Italy facing a 4m building × 4m × The 3m building was rebuilt. After the glass was replaced with the insulating glass made of two 4mm low emissivity coated glass, different translucent aerogel particles were filled with 16mm thick particles to study the sound insulation, heat insulation and light transmission properties of the particle aerogel glass. Through testing, the U value of the prepared aerogel particle glass was 1.4W/(m · K). In winter, compared to unfilled insulating glass, the energy-saving effect reaches more than 50%, the visible light transmittance is reduced by 10%, and indoor noise is reduced by more than 3dB. There are already a large number of construction test points abroad (see Figure 8-9). Granular aerogel glass has different requirements in different architectural applications. From the outside of the building, it can ensure excellent decoration effect while greatly reducing building energy consumption. From the inside of the building, it can avoid glare. Although the visual interaction effect is poor, the visible light transmittance can fully meet the application standards, and can be used in sports halls, churches, airports and other buildings with light transmission and thermal insulation needs. The Detroit Academy of Arts in the United States has applied aerogel composite glass for energy-saving transformation, in addition to CABOT aerogel glass system in Nobel Hall of the State University of New York, Stony Brook, etc.

2、 Conclusion and Outlook

At present, the industrialization of aerogel glass in China is relatively low, and the cost is relatively high, which is more in the laboratory stage or demonstration project stage. The manufacturers of particulate aerogel glass in the world are concentrated in European and American countries. In 2015, small-scale mass production first appeared in Changsha, China. In addition to excellent energy-saving effect, low-energy gel glass for building also has many excellent properties, such as excellent heat insulation, sound insulation and noise reduction, wind pressure resistance, incombustibility and flame retardancy, anti condensation, no danger of cold and hot self explosion, and long service life, but it has not yet been further developed. In order to realize the popularization of aerogel glass for low energy consumption buildings, the following problems need to be solved:

1) Compared with traditional thermal insulation materials, the cost of raw materials, preparation process and production equipment of SiO2 aerogel is higher, which has brought greater resistance to the development and popularization of aerogel glass. What needs to be explored are: how to use low-cost silicon sources to prepare aerogels with good transparency and flexibility, how to change the drying method to reduce the production process cost, how to reduce the production cycle of aerogels, and how to reduce the cost of labor and machinery.

2) The relevant domestic standards are not perfect, leading to difficulties in popularizing aerogel glass. The quality of aerogel glass products is uneven, and the situation of shoddy products may appear in the future, which is extremely unfavorable for the development of a new type of building materials. Relevant departments and enterprises need to work together to draft relevant material testing standards and engineering application technical specifications.

3) At present, granular gel glass is still the main glass at home and abroad, but the performance of the whole piece of aerogel glass is far better than that of granular gel glass. It is necessary to develop a new composite process or cross-linking method, so that the SiO2 aerogel has a certain flexibility and integrity while having transparency, reducing its brittleness and damage probability. In the context of dual carbon and energy conservation and emission reduction, aerogel glass can greatly reduce the energy consumption of building envelope, so it is an extremely important part of zero energy consumption buildings. Although the current domestic aerogel glass is still dominated by laboratories or demonstration projects, the aerogel glass will serve the society in the future, with broad application prospects and development space.