In some parts of the world, when it gets hot, we want to turn on the air conditioner or make the AC cooler. The thing is, other than making us pay more for the electricity bill, ACs can contribute to greenhouse gases.
Many researchers have tried to make new innovations that can help tackle this problem. Recently, they’ve come up with a plant-based film with cool features: it gets cooler when exposed to sunlight and it comes in different textures and bright, rainbowlike colors.
Given enough time and development, this film could keep buildings, cars, and other structures cool in the future, without the need for external power.
The project’s principal investigator Silvia Vignolini explained, “To make materials that remain cooler than the air around them during the day, you need something that reflects a lot of solar light and doesn’t absorb it, which would transform energy from the light into heat.
“There are only a few materials that have this property, and adding color pigments would typically undo their cooling effects.”
There is a phenomenon called passive daytime radiative cooling (PDRC), which is the ability of a surface to emit its own heat into space without getting absorbed by the air or atmosphere. Therefore, without any electrical power, such surface can become several degrees colder than the air around it.
When used on buildings or other structures, materials with this feature can help limit the use of air conditioning and other cooling methods that take a lot of power.
Films and colors
According to one of the researchers, some paints and films in development nowadays can achieve PDRC, but most of them are white or have mirrored finish.
So, if a building owner wants to have a building with blue-colored PDRC paint, that wouldn’t be quite possible (for now), as colored pigments absorb specific wavelengths of sunlight and only reflect the colors we see—that can cause undesirable warming effects in the process.
Now, there is a way to show colors without the use of pigments. Take soap bubbles—they show different colors on their surfaces. These colors are the result of how light interacts with different thicknesses of the bubble’s film: a phenomenon called structural color.
Vignolini’s research partly covers identifying the causes behind different types of structural colors in nature. In one case, her group found that cellulose nanocrystals (CNCs) from cellulose found in plants are viable materials to make colorful films without adding any pigment.
After hearing a talk from the first researchers who have created previous cooling film material, Vignolini learned that cellulose is also one of the few naturally occurring materials that can promote PDRC.
“I thought wow, this is really amazing, and I never really thought cellulose could do this,” Vignolini said.
Making a film from the cellulose
In the research, Vignolini and team layered colorful CNC materials with a white-colored material made from ethyl cellulose. It produced a colorful bi-layered PDRC film, and they made films with different colors: vibrant blue, green, and red.
These colors were an average of nearly 40F cooler than the surrounding air when placed under sunlight. Moreover, a square meter of the film could generate over 120 Watts of cooling power, proving its ability to outcompete residential air conditioners.
Although the film looks and sounds good, it still has its challenges.
Per one of the researchers, the most challenging aspect of this search was finding a way to make the two layers stick together on their own. The CNC films were brittle, and the ethyl cellulose layer had to be plasma-treated to get good adhesion.
Nonetheless, films developed in this research were robust and could be prepared several meters at a time in a standard manufacturing line.
The future of these films
These films aren’t a novel innovation—as mentioned before, researchers have created them before. And since then, more researchers focus on improving the films’ aesthetic appearance.
After modifying findings from previous research, the scientists have tried to make cellulose-based cooling films that are glittery and colorful.
They’ve also adjusted the ethyl cellulose film to have different textures. Think of it like the differing looks and qualities of wood finishes used in architecture and interior design.
With these “tinkering” people in the future would have more options when incorporating PDRC in their homes, businesses, cars, and other structures.
In the near future, the researchers plan to find ways to make these films more functional. According to one researcher, CNC materials can be used as sensors to detect environmental pollutants or weather changes.
That additional feature could provide more benefits if combined with the cooling power of their CNC-ethyl cellulose films.
For instance, a cobalt colored PDRC on a building façade in an urban area compact with cars might someday keep the building cool. The detectors then could alert officials to higher levels of smog-causing molecules in the air.
It’s still unclear whether we can have this innovation at a large scale shortly, but perhaps some other development may be applicable sooner than the others.
Developing better air conditioners for the planet
Scientists have developed a form of air conditioning that’s more eco-friendly, with the potential to reduce environmental harm caused by our current ACs.
To refresh our memories, existing ACs contribute a lot to global warming because the current refrigerants are a lot more potent than carbon dioxide. When they leak accidentally, they can also cause significant damage.
And with this new development, the scientists say that the device works by making a refrigerant change between being a gas and a liquid.
When liquid becomes a gas, it expands and absorbs heat, which cools the inside of a fridge. Then, a compressor turns the gas back into a liquid, releasing heat. In our current ACs, this heat gets released outside buildings.
To make them more environmentally friendly, scientists tested solid refrigerants. These solid ones, according to the researchers, won’t leak into the environment from air conditioning units.
One class of solid refrigerants, barocaloric materials, work similarly to traditional gas-liquid cooling systems. The solids use pressure changes to go through heat cycles. The difference is that the pressure drives a solid-to-solid change.
So, the material will remain solid. The changes are within the molecular structure.
How the solid refrigerant works
These solid materials contain long, flexible molecular chains that are typically floppy and disordered. Under certain pressure, the chains can become more ordered and rigid—this change releases heat.
Think of this whole process like melting wax, but the wax doesn’t liquify. So when the pressure is released, the material reabsorbs the heat, completing the cycle.
There’s a little caveat: solid-to-solid systems need vast amounts of pressure to work effectively, and producing such pressures needs expensive, specialized equipment—rendering it impractical.
The researchers had actually found barocaloric materials that can act as refrigerants at much lower pressures. They can also work in a cooling system they’ve built from scratch.
But now, they’ve made a device that cools spaces in a practical way, which has three main parts. One is a metal tube packed with solid refrigerant and an inert liquid, which can be water or oil.
Another is a hydraulic piston or cylinder that applies pressure to the liquid. And finally, the liquid helps transfer that pressure to the refrigerant and helps carry heat through the system.
The materials also work as functional refrigerants, turning pressure changes into cool rooms and fridges.
Unfortunately, for now, the system is still in the testing stage. So, we can’t get it immediately.
Principal investigator Dr. Jarad Mason, “Our system still doesn’t use pressures as low as those of commercial refrigeration systems, but we are getting closer.”
Researcher Dr. Adam Slavney added, “We’re really hoping to use this machine as a testbed to help us find even better materials. With an optimal material, solid-state refrigerants could become a viable replacement for current air conditioning and other cooling technologies.”
Compared to the film, the barocaloric materials seem like a solution that’s closer to realization, but let’s see.
Sources
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