How an MIT researcher's zero-electricity cooling system could reduce food shortages

Billions of people will soon be buying their first air conditioner (AC) in countries with fast-growing economies and already-dangerous levels of heat and humidity, such as India and Brazil. Whether they'll be taking up environmentally-friendlier units will be a question of whether affordable solutions exist compared to the pollution-heavy models that have long dominated the market since the 1900s.

And that's not all. Of the estimated 2.8 billion people living in the hottest parts of the world, many of whom are already exposed to life-threatening temperatures, only around eight percent currently have access to air conditioners (ACs). This number is a lofty 90 percent in the US and Japan, revealing climate inequality among some of Earth's most vulnerable members.

A breakthrough in cooling systems' technology that requires zero electricity and little maintenance

Ensuring an eco-friendly and accessible cooling system solution appears to be the primary motivation behind a recent study by researchers at MIT. Interesting Engineering (IE) interviews Dr. Zhengmao Lu, a postdoctoral associate in the Department of Materials Science and Engineering at MIT and the study's lead author.

"We developed a passive cooling technology that does not rely on electricity. It provides large energy savings with minimal water consumption even in humid places where previous passive cooling technologies typically do not work well," Dr. Lu tells IE.

In a statement, the team described the device as resembling a regular solar panel, which would provide cooling rather than electricity. Picture the top of a food storage container which, rather than producing power, provides cooling for the food.

The only maintenance needed is adding water for evaporation. However, because of the low water consumption, the team explained that this would only need to be done once a month in wetter locations and once every four days in the hottest, driest regions.

A unique combination of radiative cooling, evaporative cooling, and thermal insulation technologies

The system combines radiative, evaporative, and thermal insulation technologies in a slim package that resembles existing solar panels. According to a recent press release, the system can provide up to about 19 degrees Fahrenheit (9.3 degrees Celsius) cooling from the ambient temperature.

Under these temperatures and in very humid conditions, it could be possible to safely store food for about 40 percent longer than without cooling. In dryer conditions, the safe storage time could triple.

Although each of the three cooling methods employed in the recent MIT research has shown success as standalone technologies, the team's combination of the different methods in one device makes their work stand out. The breakthrough being this combo produced the new passive, zero-energy technology.

In a statement, Lu explains, "the novelty here is really just bringing together the radiative cooling feature, the evaporative cooling feature, and also the thermal insulation feature all together in one architecture."

The MIT design relied on the engineering of material in three layers

Three layers of material make up the system, which collectively offers cooling when heat and water move through it.

A top layer of 'airy' aerogel allows the passage of infrared light and water vapour

Critical to the functioning of the cooling system, an aerogel- a substance primarily formed of air contained in voids of a spongelike structure made of polyethylene makes up the top layer. Despite being extremely insulating, the material freely permits the passage of infrared light and water vapor.

As opposed to radiative air conditioning systems, which usually cool using circulating water running in pipes in thermal contact with the surface and which release hot air due to the power needed to operate them, the aerogel also allows the passage of infrared radiation, which is essential for the upward release of heat into the air.

A layer of 'water-filled' hydrogel makes up the second layer

A layer of hydrogel, another sponge-like substance whose pore-holes are filled with water instead of air, sits below the aerogel. This layer resembles the materials already utilized in manufacturing goods like cooling pads and wound dressings. Water vapor condenses at its surface and rises straight through the aerogel layer, which serves as the water supply for evaporative cooling.

A third 'mirror-like' layer reflects any incoming sunlight

Below the hydrogel is a layer that resembles a mirror. This structure reflects any sunlight which enters it- sending it back up through the device rather than allowing it to heat the materials below. In this way, the third layer helps radiate thermal energy away from the cooling device.

Additionally, even under intense direct sunshine, the device's solar heating is kept to a minimum thanks to the top layer of aerogel, which is both a good insulator and a strong solar reflector.

Real-life applications can extend to off-grid locations and remedy food shortage issues

The real-life applications of the zero-energy (passive) cooling system include shelf-life extension for perishable food and produce- especially in areas not connected to the grid or with unreliable electricity supplies. In regions where food resources are already scarce, the device could cut food waste due to spoilage.

In part because of this potential, the Abdul Latif Jameel Water and Food Systems Lab at MIT have provided some support to the research team.

Lu further highlights that the novel device can also increase the efficiency of existing air conditioning systems for buildings. The passive cooling system can significantly reduce the load on conventional systems by ferrying cool water to the hottest part of the system- the condenser.

"By lowering the condenser temperature, you can effectively increase the air conditioner efficiency, so that way you can potentially save energy," reveals Lu.

The system could play a significant role in meeting the cooling needs of many parts of the world- but first, costs need to be reduced

"While our cooling design is not restricted to one set of materials, in our initial demonstration, one of the materials we used called polyethylene aerogel is not readily scalable," admits Dr. Zhengmao Lu to IE.

Lu reveals that one essential step during the synthesis of this material is the critical point drying (CPD) process - which is currently still quite expensive and can be energy intensive. The following steps for the research could therefore include replacing the polyethylene aerogel with cheaper alternatives with similar properties.

Alternatively, Lu and colleagues may look into replacing the traditional CPD step entirely with more affordable drying methods, such as evaporative drying techniques, while maintaining the materials' integrity.

Lu explains that the other materials used in the system are readily available and relatively inexpensive. Still, the professor also acknowledges that it's impossible to anticipate how long it might take for this system to be developed to the point where it is practical for widespread use.

Once a solution has been found, MIT researchers are confident that such a system could play a significant role in meeting the cooling needs of many parts of the world where cost or a lack of electricity or water limits the use of conventional cooling systems.

Why natural wind and air fans do not work in hotter environments

"Maybe not quite specific to this study, but for cooling in general, it is useful to distinguish between delivering cooling at higher-than ambient temperatures and delivering cooling at sub-ambient temperatures," reveals Dr. Zhengmao Lu to IE.

Lu explains that in the former case, using natural wind or air fans, one can rely on heat conduction, or convection, to the ambient air.

However, in the latter case, Zu highlights that natural wind and air fans will not work when the environment is hotter. In fact, they will cause more environmental heating- so thermal insulation is beneficial in this case.

"We need to rely on something else such as evaporation, thermal radiation, or active air conditioning to reach below-ambient temperatures," explains Zu.

"Not only did our structure stay colder than the pure evaporation limit, but it also did so with much less water consumption"

Whilst the study's results were not entirely surprising to the researchers, and they admit to IE that they were 'quite excited' when they saw the cooling structure consistently staying below the temperature one can achieve with pure evaporative cooling.

Zu tells IE, "not only did our structure stay colder than the pure evaporation limit, but it also did so with much less water consumption."