A crystal film that is many microns thinner than a human hair could passively warm crops and keep them frost-free through the winter—without using a single kilowatt of electricity.
This may sound too good to be true, but it is in fact the latest discovery from a team of researchers who have taken the ambient warmth in the atmosphere, and harnessed it to increase ground temperatures by almost 5°C.
Agriculture—as well as infrastructure like telephone lines and electric cables—suffers under low temperatures, with crops frequently decimated by the cold. Some estimates show that between 5 and 15% of global crop production is lost to the ravages of frost each year.
It’s a huge sustainability challenge that the farming industry has been trying to solve for some time, with a mix of protective plastic sheeting, heaters, insulating blankets and films during the bitter months. But these either chew up lots of energy, or are famously inefficient and still result in a net heat loss.
Enter a team of researchers from China, who were intent on finding a technological fix for this problem. As raw materials, they picked two main ingredients: a silicon-like metalloid called germanium, and zinc-sulfide, a naturally-occurring salt. The crystals that make up these two ingredients have ‘nanophotonic’ properties, meaning that at nanoscopic scales they are able to interact with and respond to wavelengths of light, a trait the researchers realized that they could exploit to trap heat.
Before they started building the device, they first developed an equation to guide them on what precise combination of these materials would lock in the most heat. This suggested that about five layers of germanium and zinc sulfide on a bed of silicon was the optimal arrangement.
The whole thing is almost invisibly thin: the germanium and zinc sulfide layers only measured 4.12 microns thick, which is thinner than a sheet of paper. But with this combination, the researchers found they could exploit incoming and outgoing wavelengths of light in a very interesting way.
On the one hand, the film was able to reflect wavelengths in the 8 to 14 micrometer range: usually these carry heat that radiates out into the atmosphere and dissipates into the cold, but in this case, the film instead blocks that outward radiation, bouncing those wavelengths back down and trapping them in the air beneath the film. This curbs heat loss. On the other hand, the film is able to absorb wavelengths in the 5 to 8, and 14 to 16 micrometer range from the surrounding atmosphere—and this actually results in a further passive warming of the air below the film.
This was already an exciting achievement, but until the researchers conducted tests outside the laboratory, they couldn’t be sure of its effectiveness for crops and other applications. So, they took their invention out into the field at night during the wintertime, laid their ultrathin film over the stop of a U-shaped structure, and used a thermometer-like device to measure the temperature below. Then they compared this to the performance of two other film-like covers on the market.
This field experiment showed temperatures that were a whopping 4.4°C warmer than those allowed by one type of alternative film, and 2.1°C warmer compared to the other. These cozier conditions were maintained under different types of weather, the researchers found, and while there was some heat loss under the nanophotonic film, it was up to fourfold lower than its competitors.
The researchers think that if the film is fine-tuned to further exploit wavelengths, and if it’s tested in extreme environments like deserts, even greater heat savings of up to 60 watts per square meter could be achieved.
For crops, this invention holds the potential of a less frigid future that keeps the nip of frost at bay—and gets more greens directly onto our plates instead of leaving them to waste away on the field. The film “heralds a promising approach to energy conservation in diverse scenarios,” the researchers believe, “thereby paving the way for a new paradigm in the pursuit of carbon neutrality.”
Li et. al. “Night-time radiative warming using the atmosphere.” Light Science & Applications, 2023.