Producer: Popular Science China
Producer: Chen Lin (Institute of Engineering Thermophysics, Chinese Academy of Sciences), Chen Jintao (Beihang University)
Producer: Computer Network Information Center, Chinese Academy of Sciences
Glass has appeared in our human society for thousands of years, but for a long time, glass is actually a luxury, almost the same level as gold and silver jewelry. For example, Jia Rong in "Dream of the Red Chamber" once asked Wang Xifeng to borrow a glass kang screen in order to "show off his wealth" to show that he was a high-society person. At the beginning of the twentieth century, because of the breakthrough of the production process, glass began to gradually enter the homes of ordinary people.
The most significant feature of glass is its strong light transmission, and people can enjoy both the sun and the scenery outside in the house.
Night inside the window (Image: veer gallery)
However, this property of glass also has drawbacks.
In the summer, the temperature of the room is increased by the sunlight that shines through the glass, and in the winter, the heat is lost through the glass, causing the temperature in the house to drop.
Current research shows that the energy consumption lost by door and window glass in China has accounted for about one-fifth of the total energy consumption of our entire society. That's a very surprising number, and one that we rarely notice.
So, how does glass waste energy?
The reason why glass wastes energy – thermal radiation
According to the basic principles of thermodynamics, we know that there are three main forms of heat transfer: thermal convection, heat conduction and thermal radiation.
Thermal convection refers to the phenomenon in which gases or liquids transfer heat due to flow. For example, having a window and closing it can block the convection of air indoors and outdoors, thereby reducing energy consumption.
Heat transfer refers to the transfer of heat from a place with a higher temperature to a place with a lower temperature. From the perspective of heat conduction, to get glass with good energy-saving effect, we can make the glass thicker, or make the glass into two layers, leave a hollow in the middle of the two layers, and even further, the hollow area is pumped into a vacuum, which is now common double-layer glass.
Thermal radiation refers to the phenomenon that an object radiates electromagnetic waves due to its temperature. From this point of view, the situation is slightly more complicated.
The purpose of our glass is to see the outside world. At the same time, the room will receive radiation from the sun through the glass to increase the internal temperature, but the indoor heater will radiate electromagnetic waves through the glass to cause the room temperature to decrease.
Then, the really good energy-saving glass is to not only let the sun radiate in, but also let the radiation that raises the temperature of the room run out easily, and control all the radiation. And a large amount of thermal radiation overflow is a key way for glass to waste energy.
Three forms of heat transfer (Image source: Reference 3)
For example, in winter, the source of indoor heat is mainly solar radiation and heater radiation.
Solar radiation will enter the room through the glass, and the indoor temperature is usually higher than the outdoor, coupled with radiators, heaters and other types of heating equipment will emit far-infrared radiation at room temperature, radiation hits the glass, so that the glass absorbs the heat of radiation and heats up. Subsequently, the glass will give this part of the energy to the ice and snow outside, resulting in a large amount of energy loss.
If we say, there is a way to let solar radiation enter more and heating equipment radiate less radiation out. Then, the consumption of this type of energy can be greatly reduced.
Different radiation passes through ordinary glass (Image source: Reference 1)
Today, scientists have developed a new material. After it is applied to the glass, the glass has the ability to regulate the sunlight and becomes a new type of energy-saving glass.
So, what is this new material? How does it make glass energy efficient? What are the advantages over traditional energy-saving glass? And listen to me slowly.
Metamaterials that regulate thermal radiation were successfully developed
The biggest difference between solar thermal radiation and indoor thermal radiation is that they are in different wavelength bands. Therefore, if the entry and exit conditions of the bands are set on the glass, we can artificially regulate their retention.
Based on this idea, researchers at the Institute of Engineering Thermophysics of the Chinese Academy of Sciences combined highly transparent polymer materials with nanocomponent units with specific structures to design a metamaterial (referring to a man-made material with special properties) that can regulate radiation.
Applying this artificial material to ordinary glass can turn ordinary glass into a new type of energy-saving glass.
The band where solar thermal radiation and indoor thermal radiation are located (Image source: Reference 1)
In this metamaterial, highly transparent polymer materials allow visible light to pass better, nanounits are responsible for screening radiation — when encountering short-wavelength solar radiation, it is activated and releases energy ; and for long-wavelength thermal radiation emitted by room-temperature objects, it forms a "big net" that reflects it back.
The combination of this metamaterial and glass can make the glass selectively pass through thermal radiation while maintaining good light transmittance, and then achieve the purpose of winter heat preservation and summer heat resistance.
Metamaterials absorb solar thermal radiation and hinder indoor thermal radiation spillage (Image source: Reference 1)
What is the energy-saving effect of the new glass? The researchers conducted an experiment: two glass house models, one made of ordinary glass and the other made of energy-efficient glass. The initial temperature in both models is 21 degrees Celsius, and the simulated daylight exposure device is turned on to allow them to receive direct sunlight at the same time.
The experimental results show that after an hour has passed, the temperature of the ordinary glass room has increased to 27.3 degrees Celsius, and the temperature of the energy-efficient glass room has increased to 43 degrees Celsius.
Radiation-regulated glass house and ordinary glass room heating comparison test (image source: Reference 1)
The temperature of the two model rooms increased by 6.3 degrees Celsius and 22 degrees Celsius, respectively, compared to the initial temperature. That is to say, the energy-efficient glass house is 15.7 degrees Celsius higher than the ordinary glass house.
Radiation-regulated glass house and ordinary glass house test heating results (Image source: Reference 1)
This shows that energy-efficient glass has a better pass rate for solar thermal radiation, and at the same time has a better blocking effect on the spillover of indoor thermal radiation.
Experiments have shown that energy-efficient glass can help us make better use of solar energy and help us save energy better.
What are the advantages of energy-efficient glass?
Compared with the existing energy-saving glass, the advantages of the new energy-saving glass are mainly three points:
The first is efficiency. The use of new energy-saving glass can ensure indoor energy saving while trying to let more solar energy shine in, so it has a better effect on indoor temperature control.
The survey shows that the visible light transmittance of existing energy-saving glass is relatively low, generally 30% to 70%, but the new energy-saving glass can reach more than 85%, allowing more light to come in.
The second advantage is ease of production, transportation and use. The production of conventional energy-saving glass requires evaporation of various coatings in vacuum, which will produce higher production costs and inconvenient transportation. In addition, because the inside of the conventional energy-saving glass is vacuum, the replacement needs to be replaced as a whole.
For the new type of energy-saving glass, whether it is traditional glass or glass with certain energy-saving performance, as long as it has been formed, it does not need to be removed or other changes, only by coating this material, or pasting a film made of the material can be used.
The third advantage is flexibility. This metamaterial is a flexible material that can be applied to different scenarios.
For example, in the past, if you want to apply low-emissivity glass to tents, it is more difficult. Because low-emissivity glass needs to use double-layer, very thick glass to protect the functional layer inside, it lacks flexibility. Now, with this new technology, we can make a film of metamaterials, or directly make a coating and spray it on the tent.
Although energy-efficient glass performs well in the laboratory, its universality remains to be examined due to a wider range of environmental resistance tests, such as in cold areas and areas with strong salt spray.
Before a new technology research and development can be applied to life, it needs to improve rigorous scientific research experiments and inquiry analysis. But this does not prevent us from looking forward to the future application of this technology, because its own characteristics can bring us unlimited expectations.
What are the application prospects of energy-efficient glass?
The flexibility of energy-efficient glass gives scientists a wide range of application prospects.
For example, we need to grow out-of-season vegetables in agricultural greenhouses, but in some areas, because the overall latitude is particularly high, the sun is not enough, the temperature in the shed is low, and the planting effect is not ideal.
If this metamaterial is applied to the shed, it can raise and maintain the temperature in the shed, which is equivalent to the reinforced "plastic greenhouse 2.0".
Some scientists even have a big brain, thinking that if humans can grow vegetables on Mars in the future, they may be able to use this metamaterial to make better use of solar energy.
Vegetable greenhouses (Image source: Reference 4)
For example, this metamaterial can also be used to enhance the thermal insulation of the box. In the future, transparent boxes with a metamaterial coating and better thermal insulation may allow us to taste more delicious food and help the takeaway brother.
Energy-efficient glass also offers more technical imagination. At its core is actually selective radiation through light. If scientists can selectively pass through light, then can they also allow electromagnetic waves in all light ranges to pass selectively?
Sometimes our mobile phone signal is not particularly good indoors, because the electromagnetic waves from the base station outside the house cannot be transmitted inside the house. If we can update and upgrade the current technology, can we selectively get the electromagnetic waves we want and exclude the interference of other clutters? Even, can we isolate the electromagnetic waves that are radiated by the human body and harmful electromagnetic waves into the room?
Perhaps in the near future, scientists will be able to successfully solve problems and lead us to a glimpse of the truth.
bibliography:
1. "Perspective new technology" 20210417 The glass _CCTV program that regulates sunlight Official website-CCTV-10_CCTV(cctv.com)
2. Beijing cityscape picture material outside the glass window _ID: VCG211129496296-VCG.COM
3. Definition and difference between heat conduction, heat convection, and thermal radiation_Baidu knows (baidu.com)
4. How to solve the problem of planting light in the construction of modern vegetable greenhouses (11467.com)
5. It's so hard to deliver takeaways, why are so many people rushing to do it? _Revenue (sohu.com)