A new way to provide cooling without power
Device developed at MIT could provide refrigeration for off-grid locations.

MIT researchers have devised a new way of providing cooling on a hot sunny day, using inexpensive materials and requiring no fossil fuel-generated power.

The passive system, which is primarily being hailed as a way to preserve food and medications in hot off-grid locations, is essentially a high-tech version of a parasol.

The system allows emission of heat at mid-infrared range of light that can pass straight out through the atmosphere and radiate into the cold of outer space, punching right through the gases that act like a greenhouse. To prevent heating in the direct sunlight, a small strip of metal suspended above the device blocks the sun's direct rays.

The new system is described this week in the journal Nature Communications. In theory, the system they designed could provide cooling of as much as 20º Celsius (36º Fahrenheit) below the ambient temperature in a location like Boston, the researchers say. So far, in their initial proof-of-concept testing, they have achieved a cooling of 6º C (about 11º F). For applications that require even more cooling, the remainder could be achieved through conventional refrigeration systems or thermoelectric cooling.

Other groups have attempted to design passive cooling systems that radiate heat in the form of mid-infrared wavelengths of light, but these systems have been based on complex engineered photonic devices that can be expensive to make and not readily available for widespread use, the researchers say. The devices are complex because they are designed to reflect all wavelengths of sunlight almost perfectly, and only to emit radiation in the mid-infrared range, for the most part. That combination of selective reflectivity and emissivity requires a multilayer material where the thicknesses of the layers are controlled to nanometer precision.