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A passive cooling system

Researchers at MIT developed a passive cooling system that relies on evaporation and radioation without the use of electricity.

The system combines previous standalone cooling designs that each provide limited amounts of cooling power, in order to produce significantly more cooling overall. It 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 vapour and infrared radiation to pass through. The evaporation of water (rising up from the layer below) provides some of the cooling power. In addition, infrared radiation radiates some of the heat straight up through the air and into space, thanks to Earth’s extreme transparent atmosphere. The top layer, being a good insulator, is also highly solar-reflecting, limiting the amount of solar heating of the device, even under strong direct sunlight.

Below the aerogel is a layer of hydrogel. This is another sponge-like material, but one which pore spaces are filled with water rather than air. 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.

Lastly, there is a mirror-like layer that reflects any incoming sunlight that reaches it,  sending it back up through the device rather than letting it heat up the materials and thus reducing their thermal load.

The system was tested, using a small version, just 4 inches across, on the rooftop of a building at MIT, proving its effectiveness even during suboptimal weather conditions, and achieving 9.3 C of cooling (18.7 F).

The system could be used to preserve food crops and supplement conventional air conditioners in buildings, with no need for power and only a small need for water.

The main challenge right now is the production of the aerogel, which is key to the system’s overall efficiency, but at present is expensive to produce. The team is trying to make the process less expensive as well as to find alternative materials that can provide the same insulating function at a lower cost.

Photo: Courtesy of Zhengmao Lu

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