On a hot roof in Cambridge, Massachusetts, a small experimental device sat in the sun without a fan, compressor or electrical connection. Its job was simply to get colder than the air around it. Water moved through one part of the device, heat escaped through another, and layers of insulating material helped keep the cooling from being undone by sunlight.That rather quiet experiment points to a problem that is becoming harder to ignore. Cooling is increasingly important as temperatures rise, yet conventional air conditioning depends on electricity and reliable infrastructure. In places where either is scarce, keeping food, water or indoor spaces cool can be difficult. An MIT team has been testing a different approach, combining several passive cooling methods in a single compact structure.
MIT researchers combined three cooling effects without electricity
The idea is not to replace air conditioning with a single miracle material. Instead, the system brings together three familiar physical effects that have usually been used separately: evaporation, heat loss by infrared radiation and thermal insulation.Individually, each has limitations. Evaporative cooling becomes less effective when the surrounding air is already humid. Radiative cooling can struggle when sunlight warms the surface it is supposed to cool. Insulation does not create cooling by itself; it simply slows the movement of heat.The MIT design places these functions together so that the weaknesses of one component are partly offset by the others. As reported by MIT News, in tests, the small device produced a temperature reduction of about 9.3 degrees Celsius, or 18.7 degrees Fahrenheit, relative to the surrounding air.That is a substantial difference for something that does not consume electricity. The researchers described the system in a 2022 paper published in Cell Reports Physical Science. The work involved Zhengmao Lu, Arny Leroy, Jeffrey Grossman, Evelyn Wang, Lenan Zhang and Jatin Patil.
The three-layer device reflects sunlight, evaporates water and sheds heat
The device is designed around three main layers. The arrangement matters because sunlight, water and heat all have to move in particular directions.At the top is an aerogel made from polyethylene. Although it looks solid, much of the material is actually empty space. Its porous structure traps air, giving it strong insulating properties, while still allowing water vapour and infrared radiation to pass through.That combination is useful here. Water from the layer underneath can evaporate upwards, taking heat with it. At the same time, thermal radiation can pass through the upper material and escape towards the sky.
PC: MIT
The distinction from a conventional air conditioner is important. An air conditioner moves unwanted heat from inside a building to the outside air. This passive system can send some of its heat upward through infrared radiation, taking advantage of wavelengths at which the atmosphere is relatively transparent.Beneath the aerogel is a hydrogel. Its pores hold water, providing the supply needed for evaporation. As water reaches the surface and changes into vapour, it carries heat away from the structure.The bottom layer has a different task. It behaves like a mirror, reflecting sunlight back upwards instead of allowing that energy to be absorbed and turned into heat. The aerogel itself also helps with solar protection. Its insulating and light-reflecting properties reduce the amount of heating caused by strong sunlight.It is a fairly simple sequence: reflect as much sunlight as possible, prevent heat from moving in, evaporate a small amount of water and allow infrared energy to escape.
Why humidity has been a problem
Evaporative cooling works best when the surrounding air can accept more water vapour. In dry conditions, that is relatively easy. In humid weather, the air is already carrying a large amount of moisture, so evaporation slows down.That has made many evaporative cooling systems less useful in humid regions, even though those areas can have significant cooling needs.Radiative cooling has its own complications. A surface may be able to radiate heat towards the sky, but if it absorbs too much sunlight during the day, the solar heating can cancel much of that benefit.The MIT design attempts to deal with both problems at once. Evaporation provides one route for removing heat, while radiative cooling provides another. The insulating layer helps stop heat from returning, and the reflective layer limits solar gain.In the rooftop experiment, the researchers deliberately tested the concept under weather conditions that were not ideal for passive cooling. The prototype was only about four inches across, but it still produced the measured cooling effect.The result suggests that the combination, rather than any individual component, is what gives the system its wider operating range.The electricity-free system could give harvested crops a longer life
The most direct application may have little to do with buildings.
In regions without dependable electricity, harvested crops can spoil before they reach consumers. Refrigeration can extend their useful life, but conventional refrigeration requires equipment, energy and maintenance that may not be readily available.The passive device could potentially be incorporated into the roof of a food-storage container. Instead of generating electricity like a solar panel, the panel-like structure would provide cooling directly.According to the MIT team’s estimates, the system could lower temperatures enough to extend the safe storage period for food by roughly 40% under very humid conditions. In drier environments, the potential storage period could be about three times longer.Those figures are tied to the particular conditions tested and modelled, rather than representing a universal increase in food shelf life. Still, the difference illustrates why even a modest temperature reduction can matter where refrigeration is limited.Water use is also relatively small. The system needs water for evaporation, but not a continuous flow. Under the hottest and driest conditions described by the researchers, the water supply might need replenishing roughly every four days. In wetter conditions, it could last for about a month.That would make the system quite different from many water-intensive evaporative cooling arrangements.
Passive cooling could cut the energy needed by conventional AC systems
The device does not necessarily have to operate on its own. In a building that already has air conditioning, passive cooling could be used to reduce the temperature of water entering parts of the cooling system. The researchers have suggested directing the chilled water towards the condenser.This is a less obvious application, but potentially an important one. Air conditioners become more efficient when the temperature at which their condenser operates is reduced. If a passive system can provide some of that cooling without drawing electricity, the mechanical system has less work to do. In effect, the passive device would not have to cool an entire building. It could take on a smaller part of the job and reduce the load placed on the powered equipment.
The cooling concept works, but its key material is costly to make
There is, however, a practical obstacle between a rooftop prototype and widespread use. The aerogel gives the design much of its usefulness, but producing it is currently expensive. The polyethylene has to be formed into a delicate porous structure, and solvents must be removed without collapsing or damaging that structure. The MIT process uses critical point drying, which requires specialised equipment. The size of the pores also needs to be controlled because the internal structure determines how well the material performs.The researchers have been looking at less expensive ways of producing the aerogel, including freeze-drying. They have also considered whether other materials could perform the same insulating role.One possibility is a design based on membranes separated by an air gap. Such an arrangement would not necessarily have the same properties as the aerogel, but could offer a cheaper route to the insulation required.The other components are considerably less troublesome. Water-based hydrogel materials are already used commercially, and reflective surfaces are not difficult to produce.
The small cooling system could have much wider uses
The appeal of the design lies partly in its flexibility. A panel installed on a roof could be used to cool a storage space. The same basic architecture could potentially supply chilled water to another cooling system.It also does not need the grid to operate. There is no compressor, electrical fan or powered refrigeration cycle. The basic energy flows come from sunlight being reflected away, water evaporating and heat radiating towards the sky. That makes the technology particularly relevant to locations where electricity is unreliable or expensive. It could also be useful in places where extending conventional cooling infrastructure is difficult.The system is not entirely maintenance-free, since its water supply has to be replenished. And its present manufacturing cost remains a serious limitation.Xiulin Ruan, a mechanical engineering professor at Purdue University who was not involved in the research, described the combination of evaporative cooling, radiative cooling and insulation as a promising integration of existing passive approaches. He also pointed to cost as an important condition for practical applications.