All objects emit heat as infrared light including trees, buildings, water and even humans. Different types of materials emit different amounts of infrared heat. This variation depends on the ability of the material to absorb infrared radiation; the better it is at absorbing infrared heat, the better the material is at emitting the heat. For example, ordinary white writing paper is good at absorbing infrared heat and, consequently, at emitting it. By contrast, metals are bad at both.
Due to the atmosphere’s ability to transmit light in the infrared wavelength range, coldness in outer space, where the temperature is about -270 degrees Celsius, can be used to remove heat from objects on earth. As a result of the temperature difference, there can be a net transport out. An object can therefore get a lower temperature than the ambient temperature with the help of passive radiative cooling.
Researchers at Sweden’s Linköping University have now shown that the temperature of a device can be regulated by electrically tuning the extent to which it emits heat through passive radiative cooling. The concept, described in in the journal Cell Reports Physical Science, uses a conducting polymer to electrochemically tune the emissivity of the device. Now that this approach has been proven, there is potential to further develop both materials and devices.
In the long term, the researchers envision passive radiative cooling systems that can be laid on a roof, much like solar cells, thus controlling the infrared thermal radiation from the house and cooling it when needed. This technology requires extremely little energy consumption and causes minimal pollution. Other areas of application may include tunable clothing and wallpaper designed to improve thermal comfort indoors, while lowering energy consumption.