A device was developed that can sit outside under blazing sunlight on a clear day without using any power to cool things down by more than 23 °F (13 °C). The device, which has no moving parts, works by a process called radiative cooling. It blocks incoming sunlight to keep from heating it up, and at the same time, efficiently radiates infrared light — which is essentially heat — that passes straight out into the sky and into space, cooling the device significantly below the ambient air temperature.
The key to the function of the inexpensive system is insulation made of a polyethylene foam called an aerogel. This lightweight material blocks and reflects the visible rays of sunlight so that they don’t penetrate through it. But it’s highly transparent to the infrared rays that carry heat, allowing them to pass freely outward. Such a system could be used, for example, as a way 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.
Radiative cooling is the main process that most hot objects use to cool down. They emit mid-range infrared radiation, which carries the heat energy from the object straight off into space because air is highly transparent to infrared light. The new device is based on a concept that also uses radiative cooling but employs a physical barrier (a narrow strip of metal) to shade the device from direct sunlight to prevent it from heating up. That device worked but it provided less than half the amount of cooling power that the new system achieves because of its highly efficient insulating layer.
The biggest input of heat preventing the earlier device from achieving deeper cooling was from the heat of the surrounding air. The problem is that almost all insulating materials are also very good at blocking infrared light and so would interfere with the radiative cooling effect. The solution came through the development of a new kind of aerogel. Aerogels are lightweight materials that consist mostly of air and provide very good thermal insulation, with a structure made up of microscopic foamlike formations of some material. The new insight was to make an aerogel out of polyethylene, the material used in many plastic bags. The result is a soft, squishy, white material that’s so lightweight that a given volume weighs just 1/50 as much as water.
The key to its success is that while it blocks more than 90 percent of incoming sunlight, thus protecting the surface below from heating, it is very transparent to infrared light, allowing about 80 percent of the heat rays to pass freely outward. The result is that it can dramatically cool down a plate, made of a material such as metal or ceramic, placed below the insulating layer, which is referred to as an emitter. That plate could then cool a container connected to it or cool liquid passing through coils in contact with it to provide cooling for produce or air or water.
The system could be used to provide an initial cooling stage for electric refrigeration, thus minimizing the load on those systems to allow them to operate more efficiently with less power.