ENGINEERING TECHNOLOGY

Scientists achieve 24 consecutive hours of energy from the sun and deep space


Energy and environmental protection have always been hot topics. How to obtain and use renewable energy more efficiently is the key to the sustainable development of human society and the tireless pursuit of scientists.

Recently, Pei Gang, professor of the School of Engineering Sciences of the University of Science and Technology of China, and Zou Chongwen, researcher of the National Synchrotron Radiation Laboratory, jointly proposed a new energy utilization method, developed a spectral adaptive intelligent coating, solved the “spectral conflict” in the process of photothermal conversion and radiation refrigeration, and realized the continuous energy capture from the solar heat source and the space cold source for 24 hours.

The relevant research results have been published in the Proceedings of the National Academy of Sciences of the United States.

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Outdoor experimental apparatus diagram and data map (including experiment and simulation) Courtesy of the University of Science and Technology of China

Start by confronting “spectral conflicts” head-on

Cold and heat are one of the most important terminal forms of energy, with about 51% of the world’s energy consumed annually in the form of cooling or heat. However, the current supply of cold and heat mainly relies on traditional fossil energy, which will undoubtedly further exacerbate environmental problems.

Relying on renewable energy sources for cooling and heating is therefore of great significance for global energy conservation and reduction of greenhouse gas emissions.

“Compared with the Earth’s environment, the sun with a temperature of about 6,000 K and space at 3 K are the ultimate heat and cold source for the Earth.” Zhao Bin, co-first author of the paper, introduced it to China Science News.

Photothermal conversion obtains high-temperature thermal energy through the direct use of solar radiation; sky radiation refrigeration can directly emit surface energy through the atmospheric window in the form of infrared radiation to low-temperature space to obtain low-temperature cooling.

In fact, the two technologies have the same principle and similar device, if the two physical processes are integrated in the same device, not only can achieve the dual functional characteristics of night cooling and daytime heat collection, but also greatly improve the time utilization rate and energy income of the device.

“But there is an inherent ‘conflict’ between photothermal conversion and sky radiation cooling for the spectral selectivity requirements of coatings, with the former requiring low emissivity throughout the mid-infrared band and the latter requiring high emissivity in the atmospheric window band.” Zhao Bin said.

At present, the commonly used photothermal conversion and sky radiation refrigeration techniques are to collect heat and cold through different spectral selective coatings, but most methods are static and single-function, and can only use different fixed coatings to provide heating during the day and cooling at night.

The very few reported comprehensive utilization of CSP-sky radiation refrigeration are also based on static non-selective coatings, although it can achieve dual-function coupling, but the heat collection and cooling performance is much lower than that of a single photothermal conversion and sky radiation refrigeration technology.

How to solve the “conflict” and achieve the superposition and coupling of the functions of the two devices without affecting their respective performance is the work that Pei Gang’s team has been doing.

Vanadium dioxide that “transforms”

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

The team targeted vanadium dioxide films. Zou Chongwen introduced, “Vanadium dioxide is a typical strongly correlated transition metal oxide, which has special metal-insulator phase transition characteristics, and the phase transition temperature is about 68 degrees Celsius. ”

When the temperature is below 68 degrees Celsius, vanadium dioxide is a non-conductive insulator that can transmit both visible light and infrared light; when the temperature exceeds 68 degrees Celsius, vanadium dioxide will instantly “transform” into a low-resistance conductor and can block infrared rays.

Zou Chongwen said that the use of vanadium dioxide, a dynamic infrared spectral characteristic in the process of temperature-induced phase change, combined with the coating design of multi-layer films, is expected to achieve adaptive spectral smart coatings and solve the “spectral conflicts” in the process of photothermal conversion and sky radiation refrigeration.

The phase transition characteristics of vanadium dioxide films are closely related to their quality, so high-quality vanadium dioxide film preparation is the key to smart coatings.

“Vanadium atoms have multiple chemical valence states, and what we need is +4-valent vanadium dioxide with a perfect stoichiometric ratio, and the proportion of atoms in the growth process is first controlled during the preparation process.” Zou Chongwen said, “A little more or less atomic proportions have a great impact on the phase transition characteristics of thin films. ”

In addition, +4 valent vanadium dioxide has various phase structures, of which only a certain single oblique phase structure has this phase transition characteristic. Zou Chongwen admits that the preparation of this pure phase structure of vanadium dioxide film is still a challenge.

After unremitting efforts, Zou Chongwen’s team successfully developed a spectral adaptive intelligent coating based on vanadium dioxide phase change materials by means of molecular beam epitaxy and magnetron sputtering.

It can be applied to building energy saving, deep space exploration and other fields

The study found that this smart coating is in a metallic state under the daytime solar irradiation, the overall solar absorption rate of the coating is 0.89, and the infrared emissivity is only 0.25, which is manifested as the characteristics of photothermal absorption; under the condition of no irradiation at night, it is in an insulated state, the coating has a high emissivity in the atmospheric window band, and the low emissivity in the remaining mid-infrared bands is manifested as radiation refrigeration characteristics.

To explore the performance of the spectral adaptive smart coating in real-world weather conditions, the team conducted outdoor experiments on a clear autumn day in Urumqi.

The measured results show that the surface temperature of this coating can be 170 °C higher than the ambient temperature during the day, and 20 °C lower than the ambient temperature at night, with the adaptive function of light and heat conversion during the day and radiation refrigeration at night, and at the same time, it can achieve 24-hour all-weather operation, which greatly improves the comprehensive efficiency of cold and heat energy capture.

Reviewers commented on the work, “This study proposes a very novel way to capture renewable energy from the sun and space that will generate new research interest.” ”

“Related technologies can be applied to building energy conservation, automotive temperature control, photovoltaic cooling, deep space exploration and other fields.” Pei Gang said that in the next step, research will be carried out in the direction of large-scale preparation of materials, efficient collection and transmission of cold and heat, and reverse cooling and heat regulation. (Source: China Science Daily Wang Min)

Related paper information:https://doi.org/10.1073/pnas.2120557119



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