Researchers at the Massachusetts Insititute of Technology have further advanced the technology used to achieve passive cooling — a method that does not require electricity at all. In their recent attempts, the post-doctoral researcher Zhengmao Lu and his colleagues achieved passive cooling up to 19 degrees Fahrenheit (9.3 degrees Celsius), a university press release said.
The system combines two standalone passive cooling technologies that have been used previously and then added thermal insulation to provide significantly more cooling, which hasn't been achieved before. Not only does the system free you up from having to dig a hole underground to make a fridge, but the only maintenance it would require is also the addition of water. The frequency of this would also depend on the humidity of the area.
The researchers demonstrated their technology on the rooftop of a building at MIT with boxes that were four inches (10 cm) across and could easily be mistaken for a solar panel.
The devices they made consist of three layers of material that perform the dual role of cooling water as well as letting heat pass through. The top layer is made of aerogel, a sponge-like structure made from polyethylene with air enclosed in cavities. Although insulating in nature, the material allows water vapor and infrared radiation to pass through it.
Below the aerogel is a layer of hydrogel which is another sponge-like material, but its cavities are filled with water. Finally, a mirror-like layer reflects all incoming light back up to the other components of the device so that components heat up and not the contents of the storage box.
When the water in the hydrogel is heated, it turns into water vapor and rises upward (evaporative cooling), taking along some of the heat. The vapor can also pass through the aerogel, which also allows infrared radiation (radiative cooling) to carry some heat from the device straight up through the air and into space.
The cooling thus achieved could be used to store food for 40 percent longer under humid conditions and thrice longer under dryer conditions, the press release claimed.
The technology can also be used to lower the load that air conditioning compressors go through by cooling them. This would increase the air conditioner efficiency and lead to energy savings too. However, there is a major roadblock before this technology can be scaled up commercially.
Previous attempts at using passive cooling have gained partial success since the evaporative materials used in the process would heat up under the Sun and be unable to provide sufficient cooling. The aerogel used in these experiments was developed by the MIT team and involves an expensive manufacturing process.
Solvents used in the manufacture of the aerogel need to be removed slowly without damaging the aerogel structure. This is achieved with specialized equipment that facilitates critical point drying (CPD), which increases the cost.
The researchers are now working to determine if inexpensive methods such as freeze drying or the use of alternative materials could avoid the need for CPD, thereby reducing costs. As of now, the team doesn't know when exactly this would be possible.
Findings of the research conducted can be found in the journal Cell Reports Physical Science.
Passive cooling relying on evaporation and radiation, while offering great energy-saving opportunities, faces challenges with low ambient cooling powers, environmental heating, high water usage, and climate condition constraints. To overcome these shortcomings, here, we present insulated cooling with evaporation and radiation (ICER), which utilizes a solar-reflecting layer; an infrared-emitting evaporative layer; and an infrared-transparent, solar-reflecting, and vapor-permeable insulation layer. One major advantage of ICER is that it synergistically combines thermal insulation, evaporative cooling, and radiative cooling. Consequently, it consistently achieves below-wet-bulb temperatures with much less water consumption than pure evaporation while reaching 9.3°C below the ambient temperature under direct sunlight. With unfavorable climate conditions, ICER delivers 96 W/m2 daytime cooling power at the ambient temperature and shows a 300% enhancement over the state-of-the-art radiative cooler. During the summer months, without electricity, ICER can extend food shelf life by 40% in humid climates and 200% in dry climates with low water-refilling frequencies.