Recent heat waves have hit regions like Northern Europe and the Pacific Northwest that they’ve traditionally gone through without much air conditioning. As people in those areas adjust to the new reality, we are likely to see a change in electricity use, with increased demand typically in areas further south. The stress that these changes put on the grid could add to the challenge of rapidly moving away from fossil fuels.
Materials that passively heat or cool the environment can reduce energy demand by addressing some of these needs without using energy. Some of these materials Reflect the incoming sunlight to me Keep it from overheating space, while others actively radiate heat away In space, which is great if you’re only concerned about the heat. But many of these areas experience seasons and have times when eliminating stray heat increases energy use as well.
Now, a team of researchers at Nankai University has discovered a way to get it all done: cool air heating and cooling just by heating things up — all without the need for any energy input.
The basics of passive materials are pretty simple. To heat, you need a material that absorbs light and releases energy in the form of heat. Cooling can be as simple as reflecting that light away. In a more complex form, it is also possible to incorporate materials that radiate energy far into infrared wavelengths that are not absorbed by the atmosphere, allowing photons to escape into space.
Usually, you are faced with one choice or the other – materials cannot easily switch from absorbing sunlight to reflecting it. The best thing you can generally do is to turn a capacity on or off so that (for example) the material will stop reflecting sunlight under some conditions. But even some of these methods require energy to switch between countries.
For the new material, the research team was inspired by the folding and unfolding of the leaves of mimosa plants, which change shape based on environmental conditions. The idea was to use something like this to switch between heating and cooling states based on the temperature in the environment.
For this idea to work, they used a polymer that changed its shape in response to temperature. The polymer consists of three distinct subunits that can adopt different conformations when placed under stress. When polymer sheets are stretched at high temperatures (90°C), they expand and contract at temperatures typical of the indoor environment. This temperature sensitive paper was then combined with a transparent paper that did not respond to temperatures. The resulting bilayer sheet will experience asymmetric stresses due to temperature change, causing it to coil when cooled and flatten again when heated.
On its own, the temperature-sensitive plate wouldn’t be particularly useful, so the researchers had to combine it with two other materials. One was a third layer of the temperature-sensitive paper with two main properties: it reflects visible wavelengths and emits photons in the infrared, allowing it to radiate heat. The second is a dark substrate that absorbs visible light.
The latter device included a layer of dark substrate that, when exposed to sunlight, would absorb it and turn it into heat. Furthermore, there is a three-layered sheet, which changes shape based on temperature, and reflects sunlight as it is emitted in the infrared.
At lower temperatures, the temperature-sensitive paper rolls, exposing the dark, sunlight-absorbing substrate, heating things up. However, once it heats up, the paper will open up and cover it. Now, instead of an absorbent surface, the surface becomes reflective, which prevents it from warming the area. However, any heat in the area covered by this system can radiate far, because the reflective surface emits infrared radiation.
The researchers, creatively, call these two states the heating and cooling modes. About 73 percent of incoming sunlight is absorbed when it is in the heating mode. In contrast, switching to cooling mode means that only 35 percent of incoming sunlight is absorbed, and mid-infrared emissions increase by 67 percent.
While the reflective plate is thin and appears fragile, the researchers tested one for more than 500 rolling/non-screwing cycles, and it survived without any apparent problems. The only problem the team saw was that the reflective layer was not in strong contact with the non-reflective layer when it was untied, which limited the amount of heat that could transfer between the two. Since the reflective layer is responsible for radiating this heat away, this limited the overall efficiency of the system.
Another obvious limitation is that this material needs a fair amount of space to work because the reflective surface rolls into a tube. So that will need to be managed before it can be incorporated into something like a building material. However, as a first pass in one material adapted to heating and cooling, the concept looks great, and it is possible that some implementation details will be sorted out in future iterations.