Falling nighttime temperatures are a fundamental fact, which can damage crops, equipment and infrastructure. A team of scientists has created a new type of film that selectively absorbs and reflects different wavelengths of infrared light to effectively keep objects warm, helping to protect crops and infrastructure from damage caused by overnight cold temperatures.
The Earth's atmosphere is transparent to certain infrared wavelengths (i.e., thermal energy), which means they can pass right through the atmosphere and into the extremely cold temperatures of space. Scientists are exploiting this phenomenon to develop radiative cooling systems for buildings, but it's also why surface temperatures drop dramatically overnight.
These temperature fluctuations and subsequent impacts such as frost and dew can damage crops, power lines, and other equipment and infrastructure exposed to the elements. Active heating often requires the use of heaters, which not only consumes energy but also has a huge impact on the environment.
But in a new study, scientists at the Chinese Academy of Sciences have created a radiant heating system that works passively. The research team created a nanophotonic film that selectively reflects and absorbs different wavelengths of light to maximize the warmth provided to the object it covers. The key is to reflect wavelengths in the 8 to 14 micron range where the atmosphere is transparent, while absorbing wavelengths in the 5 to 8 and 14 to 16 ranges (radiation bands).
The film is made from five ultra-thin layers of alternating germanium and zinc sulfide. These materials have a reflectance of 0.91 in the transparent window and an absorptivity of 0.7 in the radiation band. In outdoor testing, the film successfully kept the covered surface 2.1°C (3.8°F) warmer than a surface that reflected all wavelengths and 4.4°C (7.9°F) warmer than a surface that absorbed all wavelengths.
It sounds like a problem that a blanket or other basic covering could solve, but the team says this technology is far more effective at protecting vulnerable surfaces and objects. Additionally, if this technology is installed on a building, it can keep the building at a comfortable temperature overnight while also saving a fortune on energy bills.
The research was published in the journal Light: Science and Applications.