In a study published in PNAS, the research group led by Prof. PEI Gang and Prof. ZOU Chongwen from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences proposed a new method to continuously harvest and utilize energy from the sun and outer space all day long. Taking the sun as heat source at 6000K, and outer space as heat sink at 3K, they used spectrally self-adaptive intelligent coating technique to reduce the energy consumption for heating and cooling of buildings in an environmentally friendly way. The sun and the universe are the ultimate heat source and heat sink of the Earth's energy cycle. There are two main methods to capture the energy from the sun and the universe: photothermal (PT) for thermal collection and radiative cooling (RC) for clean cooling with no energy consumption. High temperature heat energy is directly obtained from the solar thermal through PT. RC can directly transmit residual heat energy through the atmospheric window through infrared radiation to the cold outer space, realizing direct utilization of outer space cryogenic temperature at a very long distance.
However, current PT and RC both rely on static spectral selective coatings. Infrared spectral conflict occurs during these processes. Given the single-target, single-function approach, researchers from USTC proposed a spectral adaptive regulation mechanism for continuous utilization of PT during daytime and RC during nighttime within a single device.
Researchers developed a spectral selective adaptive multilayer coating based on vanadium dioxide (VO2). This coating stays at metallic state under daytime solar irradiation, which makes it capable to work as a PT absorber. The solar absorptivity of the overall coating is 0.89, and the infrared emissivity is only 0.25, which shows a good performance in energy absorption. Meanwhile, without irradiation at night, this coating stays at the insulating state with unity emissivity within the atmospheric window, working as a selective RC emitter.
The spectrally self-adaptive absorber/emitter (SSA/E) applying above-mentioned coatings performed excellent adaptive switchable functions of PT mode during daytime and RC mode during nighttime. The results of the experiments showed that its surface temperature can be 170℃ higher than ambient temperature during daytime and 20℃ lower during nighttime.
SSA/E continuously operates with neither break nor energy waste, which greatly improves the overall efficiency of harvesting and utilizing cold and heat energy from the universe. This study provides a new approach to continuous energy capture, and a clean and cheap way for effective heating and cooling of buildings.
