In the photo on the left, a disk of the new insulating material blocks and reflects visible light, hiding the MIT logo beneath it. But seen in infrared light, at right, the material is transparent and the logo is visible. Source: MITAccording to MIT professor and head of the mechanical engineering department Evelyn Wang, much research has been done on how to minimize heat loss, but not on the difficult problem of minimizing heat gain. Wang and her team, along with researchers at the Pontifical Catholic University of Chile, have come up with a solution: a device that uses a process known as radiative cooling.

Unique insulation prevents incoming sunlight from heating up the device while simultaneously radiating infrared light (heat) away from the device. This efficient process requires no moving parts or electricity to cool the device well below the ambient air temperature. The key is aerogel, a type of insulation made of polyethylene foam. Aerogel is lightweight and both blocks and reflects the visible rays of sunlight but allows heat-carrying infrared rays to escape through it. Previous attempts were unsuccessful because although the materials used for insulation could keep the heat out, they also blocked the infrared rays, preventing the radiative cooling process from taking place.

Aerogels are mainly made of air and provide thermal insulation. Their structure is made up of tiny foam-like formations. The polyethylene aerogel developed by the researchers blocks more than 90% of incoming sunlight and allows 80% of the heat rays to pass outward. As a result, it can dramatically cool down the material, such as metal or ceramic plate, placed below the insulating layer, and that plate could, in turn, cool whatever is in contact with it, such as a container or liquid passing through coils, in order to provide cooling for air, water or produce.

In field tests, the performance of the radiative cooling device was measured under full sunlight, both with the insulating material in place (left) and without it (right). Source: MITSuch a system could be used, for example, to keep vegetables and fruit from spoiling, potentially doubling the time the produce could remain fresh, in remote places where reliable power for refrigeration is not available, explained MIT graduate student Arny Leroy. It could also be a low-cost add-on, providing additional cooling to supplement a conventional system. “Whatever system you have,” Leroy said, “put the aerogel on it, and you’ll get much better performance.”

The system was field-tested in the Atacama Desert town of San Pedro and at MIT in Cambridge, Massachusetts. It is described in a paper in the journal Science Advances.