As the world gets warmer, the use of energy-hungry air conditioning systems is expected to increase exponentially, putting pressure on existing power grids and bypassing many locations with little or no reliable electrical power. Now, an innovative system developed at the Massachusetts Institute of Technology provides a way to use passive cooling to conserve food crops and to supplement traditional air conditioners in buildings, with no energy and only a small need for water.
The system, which combines radiative cooling, evaporative cooling and thermal insulation in a thin package that can resemble existing solar panels, can provide up to 19 degrees Fahrenheit (9.3 degrees Celsius) of cooling from ambient temperature, enough to allow food to be safely stored for an extended period of time. 40 percent longer under extremely humid conditions. The safe storage time can be tripled under drying conditions.
The results were reported today in the journal Cell Reports Physical Sciences, in a research paper by MIT postdoc Zhengmao Lu, Arny Leroy PhD ’21, Professors Jeffrey Grossman and Evelyn Wang, and two others. While more research is needed in order to bring down the cost of a key component of the system, researchers say such a system could eventually play an important role in meeting cooling needs in many parts of the world where electricity shortages or water use limit the use of water. conventional cooling systems.
The system cleverly combines previous independent cooling designs that each provide limited amounts of cooling energy, in order to produce more cooling overall – enough to help reduce food loss from spoilage in parts of the world already with limited food supplies. In recognition of this potential, the research team received partial support from the Abdul Latif Jameel Laboratory for Water and Food Systems of the Massachusetts Institute of Technology.
“This technology combines some good features of previous techniques such as evaporative cooling and radiative cooling,” Lu says. Using this combination, he says, “we show that you can achieve significant nutritional lifespan, even in areas where you have high humidity,” limiting the potential of traditional evaporative or radiant cooling systems.
In places where there are existing air conditioning systems in buildings, the new system can be used to significantly reduce the load on these systems by sending cold water to the hottest part of the system, which is the condenser. “By lowering the condenser temperature, you can effectively increase the efficiency of the air conditioner, and that way you can save energy,” says Lu.
He also said that other groups have also been seeking to use passive cooling technologies, but that “by combining these features in a synergistic way, we can now achieve high cooling performance, even in high-humidity areas where previous technology generally could not perform well.”
The system consists of three layers of materials, which together provide cooling as water and heat pass through the device. In practical terms, the device could resemble traditional solar panels, but instead of turning off the electricity, it would provide cooling directly, for example by serving as a roof for a food storage container. Or it can be used to send chilled water through pipes to cool parts of an existing air conditioning system and improve its efficiency. The only maintenance required is adding water to evaporate, but the consumption is so low that this only needs to be done once every four days in the hottest and driest areas, and only once a month in the wetter areas.
The top layer is a gelatinous material consisting mostly of air surrounded by cavities of a sponge-like structure made of polyethylene. The material is highly insulating but freely allows both water vapor and infrared rays to pass through. Water evaporation (rising from the bottom layer) provides some of the cooling energy, while infrared radiation, which takes advantage of the extreme transparency of Earth’s atmosphere at those wavelengths, radiates some heat directly through the air and into space—unlike air conditioners, which blow Hot air in the immediate surroundings.
Underneath the airgel is a layer of hydrogel – another sponge-like substance, but one whose pores are filled with water instead of air. It is similar to the material currently used commercially for products such as cooling pads or wound dressings. This provides a water source for evaporative cooling, as water vapor forms on its surface and the vapor passes directly upward through the aerosol layer and exits to the environment.
Below that, a mirror-like layer reflects off any incoming sunlight that has reached it, sending it back through the device rather than allowing it to heat up the materials and thus reduce their heat load. And the top layer of the aerogel, being a good insulator, is very reflective of the sun, which limits the amount of solar heating of the device, even under strong direct sunlight.
“The novelty here is really just the combination of radiative cooling, evaporative cooling and thermal insulation together in one structure,” Lu explains. The system, using a small version, just 4 inches wide, has been tested on the roof of an MIT building, proving to be effective even during suboptimal weather conditions, Lu says, and achieves 9.3 degrees Celsius (18.7 Fahrenheit) cooling.
“The challenge previously was that evaporites often didn’t handle solar energy absorption well,” Lu says. “With these other materials, usually when they’re under the sun, they get heated up, so they can’t get that high cooling energy at the ambient temperature.”
The properties of the airgel material are key to the overall efficiency of the system, but it is currently very expensive to produce these materials, requiring special equipment for critical point drying (CPD) to slowly remove solvents from the microporous structure without damaging it. The main characteristic that must be controlled to provide the desired properties is the size of the pores in the antenna, which is made by mixing polyethylene with solvents, allowing it to settle like a bowl of O-gel, and then obtaining solvents from it. The research team is currently exploring ways to make this drying process less expensive, such as using freeze drying, or finding alternative materials that can provide the same insulation function at a lower cost, such as air-gap-separated membranes.
While other materials used in the system are readily available and relatively inexpensive, Lu says, “Aerogel is the only material that is a product from the lab that requires further development in terms of mass production.” It’s impossible to predict how long this development might take before this system becomes viable for widespread use, he says.
“This work represents a new and interesting approach to systems integration for passive cooling technologies,” says Xiulin Ruan, a professor of mechanical engineering at Purdue University, who was not associated with this research. Rowan adds, “By combining evaporative cooling, radiative cooling and insulation, it has better cooling performance and can be effective in a wider range of climates than evaporative cooling or radiative cooling alone. The work could have great practical applications, such as in food preservation, if The system could have been made at a reasonable cost.”
The research team included Linan Zhang of the MIT Department of Mechanical Engineering and Jatin Patel of the Department of Materials Science and Engineering.