Scientific Invention: Intelligent Coating Realizes "Daytime Heat Collection And Night Cooling"

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Professor Zou and Professor Pei from Chongwen College of science and technology of China jointly proposed a new research method of synchrotron radiation technology in China. They have developed a spectrum adaptive intelligent coating, which solves the "spectral conflict" in the process of solar thermal conversion and radiation cooling, and can capture energy from solar heat sources and space cold sources 24 hours a day.

The related research results were published in the proceedings of the National Academy of Sciences.

Starting from facing "spectrum conflict"

Cold and heat are one of the most important terminal forms of energy. About 51% of the global energy is consumed in the form of cold or heat each year. However, at present, the supply of these two kinds of energy mainly depends on the traditional fossil energy, which will undoubtedly further aggravate the environmental problems.

Therefore, relying on renewable energy to achieve refrigeration and heating is of great significance for global energy saving and reducing greenhouse gas emissions.

"Compared with the earth's environment, the sun with a temperature of about 6000K and space at 3K are the ultimate heat and cold sources for the earth." Zhao Bin, the co first author of the paper and a postdoctoral fellow of the school of Engineering Science and technology of China University of science and technology, introduced it to China Science Daily.

Through the direct utilization of solar radiation, the solar thermal conversion can obtain high temperature heat; In this way, the cold radiation from the sky can be obtained by cooling the surface of the sky directly.

In fact, the two technologies have the same principle and similar devices. If the two physical processes are integrated into the same device, it can not only present the dual function characteristics of night cooling and daytime heat collection, but also greatly improve the time utilization rate and energy income of the device.

"However, there is an inherent" conflict "between photothermal conversion and sky radiation cooling for the spectral selectivity of the coating. The former requires a low emissivity in the whole mid infrared band, while the latter requires a high emissivity in the atmospheric window band." Zhao Bin said.

At present, the commonly used solar thermal conversion and sky radiation cooling technologies collect heat and cold through different spectral selective coatings, but most of the methods are static and single function, and can only use different fixed coatings to provide heating in the daytime and cooling at night.

A few of the reported comprehensive utilization of solar thermal conversion sky radiation refrigeration is also based on static non selective coating. Although it can achieve dual function coupling, its heat collection and refrigeration performance is much lower than that of single solar thermal conversion and sky radiation refrigeration technology.

How to solve the "conflict" and realize the function superposition coupling of the two devices without affecting their performance is the work of Pei Gang team.

Vanadium dioxide that can be transformed

In this study, Pei Gang's team innovatively proposed a spectrum adaptive regulation mechanism, that is, the spectral selection performance of the coating can be "dynamically adjusted" according to the energy capture mode.

The team is targeting vanadium dioxide thin films. Zou Chongwen said, "vanadium dioxide is a typical strong correlation transition metal oxide. It has a special metal insulator phase transition characteristic, and the phase transition temperature is about 68 ℃

When the temperature is lower than 68 ℃, vanadium dioxide is a non-conductive insulator, which can transmit visible light and infrared light at the same time; When the temperature of resistance is over 68 ℃, the resistance of vanadium can be changed to higher than 68 ℃.

Zou Chongwen said that using the dynamic infrared spectrum characteristics of vanadium dioxide in the process of temperature-induced phase change, combined with the coating design of multilayer films, it is expected to realize adaptive spectral intelligent coating, and solve the "spectral conflict" in the process of solar thermal conversion and sky radiation cooling.

The reporter learned that the phase transition characteristics of vanadium dioxide thin film are closely related to its quality, so the preparation of high-quality vanadium dioxide thin film is the key to intelligent coating.

"Vanadium atoms have multi valence states, and what we need is + 4 valence vanadium dioxide with perfect stoichiometric ratio. In the preparation process, the atomic proportion in the growth process should be controlled first." "A little more or less atoms have a great influence on the phase transition characteristics of the films," Zou said

In addition, vanadium dioxide with + 4 valence also has various phase structures, among which only a certain monoclinic phase structure has this phase transition characteristic. Zou admitted that it is still a challenge to prepare vanadium dioxide thin films with pure phase structure.

Through unremitting efforts, Zou Chongwen team has successfully developed a spectrum adaptive intelligent coating based on vanadium dioxide phase change material by means of molecular beam epitaxy and magnetron sputtering.

It can be applied in many fields

The results show that the smart coating is in metal state under solar irradiation in the daytime. The solar absorptivity is 0.89, and the infrared emissivity is only 0.25; In the other bands, the coatings have low emissivity at night.

In order to explore the performance of spectrum adaptive intelligent coating under real weather conditions, the team conducted outdoor experiments in Urumqi on a sunny autumn day.

The experimental results show that the surface temperature of the coating can be 170 ℃ higher than the ambient temperature in the daytime and 20 ℃ lower than the ambient temperature at night. It has the adaptive functions of light heat conversion in the daytime and radiation cooling at night. At the same time, it can realize 24-hour operation, which greatly improves the comprehensive efficiency of cold and thermal energy capture.

"This study proposes a very novel way to capture renewable energy from the sun and space, which will arouse new research interest." The reviewer commented.

"Relevant technologies can be applied to building energy conservation, vehicle temperature control, photovoltaic cooling, deep space exploration and other fields." Pei Gang said that next, they will carry out research on large-scale material preparation, efficient collection and transmission of cold and heat, and reverse cooling and heating regulation