New passive cooling device could trim users’ power bill

Researchers say ambient temperature drops by 9.3 degrees Celsius

By Sudeep Sonawane
September 23, 2022

Scientists have developed a method of passive cooling using principles of evaporation and radiation. This device uses water and needs no electricity. It will supplement cooling appliances that use power, researchers say.
Passive cooling works by blocking heat with insulation materials, using heat absorbing panels and regulating air movement to keep a room cool without using cooling appliances that use electricity.
Experts from Massachusetts Institute of Technology [MIT] made this device that cools ambient temperature by 19 degrees Fahrenheit (9.3 degrees Celsius), according to a report published by Science Daily this Tuesday.
A drop in temperature around 9.3 degrees Celsius safely preserves food 40 per cent longer under extreme humid conditions. It could triple safe storage time in dry conditions, device makers claim. The device they made combines radiative cooling, evaporative cooling, and thermal insulation in a slim package.
The journal Cell Reports Physical Science first published the paper written by MIT postdoctoral associate Zhengmao Lu, Arny Leroy, professors Jeffrey Grossman and Evelyn Wang, and two other researchers.

Their system combines previous stand-alone cooling designs that each provide limited amounts of cooling power to produce significantly more cooling overall. This cooling is enough to reduce stored food losses in countries already suffering from limited supplies.
More research would trim the cost of one key part of the device, the researchers say. Eventually, a commercial version could fulfil the cooling needs of consumers in developing countries where a lack or poor supply of electricity and water limits the use of conventional cooling appliances.
“This technology combines some good features of previous technologies such as evaporative cooling and radiative cooling,” says Lu. By using this combination, he says, “We show that you can achieve significant food life extension, even in areas where you have high humidity which limits the capabilities of conventional evaporative or radiative cooling systems.”
The new system could reduce the work load of conventional air-cooling appliances installed in rooms. It achieves this by sending cool water to the condenser, hottest part of the AC. “Lowering the condenser temperature effectively increases the air-conditioner efficiency. This potentially saves energy,” says Lu.
“By combining those features in a synergistic way, we are now able to achieve high cooling performance, even in high-humidity areas where previous technology generally cannot perform well.”
The system consists of three layers of material, which together provide cooling as water and heat pass through the device.
The top layer is an aerogel, a material consisting mostly of air enclosed in the cavities of a sponge-like structure made of polyethylene. The material is highly insulating, but freely allows both water vapor and infrared radiation to pass through. The evaporation of water (rising from the layer below) provides some of the cooling power. The infrared radiation, takes advantage of the extreme transparency of Earth’s atmosphere at those wavelengths. It radiates part of the heat up through the air and into space. This is unlike air conditioners that push hot air into the surrounding environment.
Below the aerogel is a layer of hydrogel ­– another sponge-like material, but one whose pore spaces have water rather than air. It is like material currently used commercially in products such as cooling pads or wound dressings. This provides the water source for evaporative cooling, as water vapor forms at its surface and the vapor passes up right through the aerogel layer and out to the environment.
A mirror-like layer below this reflects incoming sunlight back up through the device rather than letting it heat the materials and lessens their thermal load. The top layer of aerogel, being a good insulator, is also highly solar-reflecting, limiting the amount of solar heating of the device, even under strong direct sunlight.
The device looks like a solar panel. It cools instead of putting out electricity. For example, it could be the roof of a food storage container. Or, it could send chilled water through pipes to cool parts of an existing air conditioning system and improve its efficiency.
The only maintenance needed for this system is adding water for evaporation. Water consumption is low. Therefore, refilling water once every four days is enough in hot and dry regions. Remarkably, only once a month in wet regions.

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