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Wednesday, October 5, 2022

Passive cooling systems can benefit off-grid locations. MIT News

As the world warms, the use of power-hungry air conditioning systems is projected to increase significantly, straining existing power grids and bypassing many locations with little or no reliable power. Now, an innovative system developed at MIT offers a way to preserve food crops and supplement conventional air conditioning in buildings using passive cooling, requiring no electricity and very little water. It offers.

Combining radiative cooling, evaporative cooling, and insulation in a slim package similar to existing solar panels, the system can cool from ambient temperatures down to 19 degrees Fahrenheit (9.3 degrees Celsius) and provides food-safety. is enough. Approximately 40% longer storage in very humid conditions. Safe storage time can be tripled under dry conditions.

The findings are reported today in the journal cell report physical science, MIT Postdoc Zhengmao Lu, Arny Leroy PhD ’21, Professors Jeffrey Grossman and Evelyn Wang, and two other papers. Further research is needed to bring down the cost of one critical component of the system, but researchers believe that ultimately such a system could meet the cooling needs of many regions of the world that are either underpowered or underpowered. said it could play an important role in meeting Water limits the use of traditional cooling systems.

This system cleverly combines previous stand-alone cooling designs that each provide a limited amount of cooling power to produce significant cooling overall. This is enough to reduce food loss through spoilage in parts of the world already suffering from limited food supplies. Partially supported by Systems Lab.

“This technology combines some of the best features of previous technologies, such as evaporative and radiative cooling,” says Lu. Using this combination, he says, “we have shown that even in areas with high humidity, we can significantly extend the life of food.” This limits the capabilities of conventional evaporative or radiative cooling systems.

Where there are existing air conditioning systems in the building, the new system can significantly reduce the load on these systems by directing chilled water to the hottest part of the system, the condenser. “By lowering the condenser temperature, we can effectively increase the efficiency of the air conditioner, potentially saving energy,” he said.

Other groups have also pursued passive cooling technologies, but said, “The synergistic combination of these features allows for high cooling performance, even in areas of high humidity where previous technologies have typically failed to perform well. Now,” he says.

The system consists of three layers of material that provide cooling as water and heat pass through the device. In effect, the device resembles a traditional solar panel, but instead of producing electricity, it provides direct cooling, such as acting as the roof of a food storage container. Alternatively, it can be used to pipe cold water to cool parts of an existing air conditioning system and improve its efficiency. The only maintenance required is the addition of water for evaporation, but consumption is so low that this is only once every four days in the hottest and driest areas and once a month in the wettest areas. increase.

The top layer is an airgel, which consists of air trapped in cavities in a sponge-like structure mostly made of polyethylene. This material is highly insulating and allows both water vapor and infrared radiation to pass freely. Evaporation of water (rising from the layers below) provides some of the cooling power, while infrared radiation takes advantage of the extreme transparency of the Earth’s atmosphere at those wavelengths to channel some of the heat straight through the air. Radiate upwards and emit into space. — Unlike air conditioners that blow hot air into the surrounding environment.

Underneath the airgel is a layer of hydrogel. This is another sponge-like material, but its pores are filled with water instead of air. This is similar to materials currently used commercially for products such as cooling pads and wound dressings. This provides a water source for evaporative cooling. This is because water vapor forms on its surface and the vapor escapes through the airgel layer to the environment.

The underlying mirror-like layer reflects incident sunlight that reaches it and sends it back through the device rather than heating the material, reducing the heat load. The top layer is also highly reflective of sunlight, limiting the amount of solar heat the device receives even in strong direct sunlight.

“The novelty here is the combination of radiative cooling, evaporative cooling, and thermal insulation all in one architecture,” explains Lu. The system was tested on the roof of an MIT building using a smaller version, just four inches in diameter, and proved effective in suboptimal weather conditions, achieving cooling of 9.3 C (18.7 F), he said. says Lu.

“The challenge so far has been that evaporative materials often don’t handle solar absorption well,” says Lu. “These other materials typically heat up in the sun, so you can’t get high cooling power at ambient temperatures.”

The properties of the airgel material are key to the efficiency of the overall system, but currently this material is expensive to manufacture. This is because special equipment for critical point drying (CPD) is required to slowly remove the solvent without damaging the delicate porous structure. An important property that needs to be controlled to provide the desired properties is the pore size of the airgel. This involves mixing a polyethylene material with a solvent, allowing it to harden like a bowl of jelly, and then adding the solvent. The research team is now exploring ways to make this drying process less expensive, such as using freeze-drying, or finding alternative materials that can provide the same insulation at a lower cost, such as membranes separated by air gaps. I’m here.

Other materials used in the system are readily available and relatively inexpensive, Lu said. And it’s impossible to predict how long the development of the system will take before it’s practical and widely used, he says.

The research team included Renan Chan from the Massachusetts Institute of Technology’s Department of Mechanical Engineering and Jatin Patil from the Department of Materials Science and Engineering.

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