Don't be afraid of radiation! It turns out that it has these functions...

Grain Full, the eighth of the 24 solar terms, symbolizes frequent rainfall. "In the middle of April, Grain Full means that things are small and full." It means that the rain is abundant in the south and the wheat grains in the north are just beginning to become full.

Image source: Photo Network

"All things grow with the sun." After Grain Full, the days are longer and the solar radiation is stronger.

When walking on the road, have you ever observed the glass of roadside buildings? Many of them are not completely transparent glass, but coated glass with various colors. This is not only for aesthetics, but also closely related to radiation.

Photo of heat-reflective glass (Note: from the Internet)

01

What is the relationship between heat and radiation?

Heat-reflective glass is a glass that is coated with a heat-reflective coating on its surface through vacuum coating and other technologies. When applied to building surfaces over a large area, it can block most of the sun's heat from the building through heat reflection and radiation, thereby effectively reducing heat absorption and lowering the temperature inside the building in summer.

The temperature rises because the object continues to absorb heat. Heat can be transferred through conduction, convection and radiation. Among them, "radiation" refers to the emission of electromagnetic waves by an object into the surrounding space, which is thermal radiation. The radiation capacity is related to temperature. The higher the temperature, the stronger the radiation capacity. Therefore, a hotter object will radiate more heat into the surrounding space. The material and color of the surface of an object will affect its reflection and absorption of thermal radiation. Radiative cooling uses this reflection and absorption property to achieve cooling.

Working principle diagram of heat-reflective glass (Note: from the Internet)

Schematic diagram of three ways of heat transfer (Note: from the Internet)

02

Can radiation be used for cooling?

1. The scientific principles of radiant heat transfer

Radiative heat transfer is based on the physical principles of absorbing and emitting radiation. When an object absorbs radiation, it absorbs energy from the surrounding space, causing the temperature to rise. Conversely, when an object emits radiation, it releases energy to the surrounding space, causing the temperature to drop. Radiative heat transfer is the transmission of energy in the form of electromagnetic waves. The process does not require a medium, which means that heat can be transferred in a vacuum, so it is very common in space.

2. Technical principles of radiative cooling

"Radiative cooling" refers to a method in which a hot object radiates heat to an external low-temperature space through the infrared transparent window of the atmosphere using blackbody radiation, thereby achieving cooling.

Schematic diagram of radiation-related principles (Note: from the Internet)

As long as the temperature of an object is higher than 0K, it will generate thermal radiation. Due to the different surface conditions, molecular structure and temperature of the radiating object, the wavelength of the radiation is also different. Most of the thermal energy of objects on the surface is transferred through radiation heat exchange, and the heat of the objects is discharged into the outer space with a temperature close to absolute zero through the "atmospheric window" in the form of 13μm electromagnetic waves, achieving the purpose of self-cooling.

Schematic diagram of radiation system (Note: from the Internet)

03

Radiative heat transfer is used in every aspect of our lives

1. Aerospace

Many of the spacecraft equipment used by astronauts in space are in a vacuum state and cannot undergo heat conduction and convection. Their cooling and heat dissipation mostly use radiation heat transfer technology.

Schematic diagram of spacecraft (Note: from the Internet)

2. Agriculture

Greenhouses also use radiation to increase temperature. During the day, sunlight will penetrate the greenhouse film and glass into the greenhouse, absorbing the heat of solar radiation to increase the temperature of the greenhouse, but the long-wave radiation heat reflected from the ground cannot be dissipated through the greenhouse, thereby reducing heat loss to achieve warming.

Greenhouse schematic diagram (Note: from the Internet)

Radiative heat transfer is also widely used in life. For example, microwave ovens and infrared stoves that we often use all achieve heating effects through radiative heat transfer. In addition, radiative heat transfer is also needed in industrial production for material heating, melting, evaporation and other processes.

Microwave oven working diagram

Schematic diagram of infrared stove (Note: from the Internet)

04

What does the future hold for radiant heat transfer?

Due to its unique advantages, the related technologies of radiation heat transfer have a very wide range of application scenarios and have played an increasingly important role in refrigeration technology, energy-saving buildings, energy conversion, material preparation and other fields. However, the related technologies of radiation heat transfer also face many challenges in their future development.

1. Radiation needs to be further improved in efficiency and reasonable energy storage

Although the total amount of radiation is huge, the energy flux density is low and unstable. For example, the solar radiation energy is very large, but it is restricted by many factors such as day and night, season, and weather. At noon on a sunny day in summer, the solar radiation is the strongest, but it is only one-fifth of that on a cloudy day. To obtain higher power, a large collection device is often required. Therefore, it is necessary to further improve the energy conversion efficiency, develop more efficient radiation energy conversion equipment, and design effective energy storage methods to cope with energy instability.

2. The actual production process needs to improve stability and durability

The stability and durability of radiation materials play a vital role in practical applications. Radiation emission and absorption are greatly affected by the color and cleanliness of the surface of the object. In practical applications, the equipment is prone to water and dust accumulation. At the same time, the equipment will rust and deteriorate after long-term use, which will seriously affect the energy absorption efficiency. Therefore, in actual production applications, it is also necessary to consider efficient, reasonable and low-cost equipment maintenance and care.

3. Reduce the production cost of equipment

With the development of technology, some radiation equipment materials can be mass-produced, but the efficiency is low, the cost is high, and it cannot be compared with conventional fossil energy for the time being, and the equipment life is short. Therefore, in actual use, radiation conversion equipment should comprehensively consider local electricity prices and weather conditions, make full use of radiation energy in various bands, and analyze the equipment investment payback period to reasonably select the equipment type and scale of construction.

Radiation application diagram of each band (Note: from the Internet)

References:

[EB/OL] https://pixabay.com/zh/photos/architecture-reflection-modern-1758454/

[EB/OL] https://www.sohu.com/a/375505731_376393

[EB/OL] https://www.ithome.com/0/669/888.htm

[EB/OL] http://www.zhileng.com/tech/xs/2022/1022/70669.html

[EB/OL] http://home.ustc.edu.cn/~wulj/

[EB/OL] https://www.51miz.com/tupian/1554647.html?keyword_id=158744

[EB/OL] https://www.sohu.com/a/408777934_120777133?_trans_=010001_grzy

[EB/OL] https://m.isolves.com/sh/zs/2020-12-07/34225.html

[EB/OL] https://www.jiaheu.com/topic/813143.html

[EB/OL] https://www.renrendoc.com/paper/193949656.html

Source: Beijing Society of Refrigeration

Contributor: Xu Qian, Associate Professor, School of Energy and Environmental Engineering, University of Science and Technology Beijing

(Recipient of the Beijing Association for Science and Technology's 2021-2023 Youth Talent Support Project)

Zheng Qiang, a master's student at the School of Energy and Environmental Engineering, University of Science and Technology Beijing

Editor: Guru