In a changing climate, energy-efficient methods of cooling buildings and vehicles are needed. Researchers at Linköping University have now shown that passive radiative cooling electrical tuning can be used to control the temperature of a material at ambient temperature and atmospheric pressure.
“To cool buildings, for example, traditional air conditioning systems are mainly used today, which require large amounts of energy and use refrigerants that are harmful to the environment. With the help of passive radiative cooling, the cold of space could be used to supplement normal air conditioning and reduce energy consumption,” says Magnus Jonsson, professor and group leader for organic photonics and nano-optics at Linköping University.
Passive radiant cooling takes advantage of the fact that thermal energy can leave an object in the form of infrared radiation. All objects emit heat as infrared light – trees, buildings, water and even people.
Different materials emit different amounts of infrared heat. This depends on the material’s ability to absorb infrared radiation – the better it absorbs infrared heat, the better the material dissipates the heat. For example, ordinary white writing paper is good at absorbing infrared heat and consequently emitting it. Metals, on the other hand, are rather bad at it, as most of the heat is reflected.
Due to the atmosphere’s ability to transmit light in the infrared wavelength range, the cold in space, where the temperature is around -270 degrees Celsius, can be used to remove heat from objects on Earth. The temperature difference can result in a net transport to the outside. An object can therefore be kept at a lower temperature than the ambient temperature with the help of passive radiative cooling.
This effect was used far back in history, for example to make ice cream in warmer climates. However, in recent years, materials science research has increasingly focused on this phenomenon and developed new materials with a high ability to emit infrared heat without being heated by the sun’s rays.
Researchers at Linköping University have now shown that a device’s temperature can be regulated by electrically adjusting the extent to which it emits heat through passive radiant cooling. The concept uses a conductive polymer to electrochemically tune the device’s emissivity.
The results were published in the journal Cell Reports Physical Science.
“You can compare it to a thermostat. Currently we can regulate the temperature by 0.25 degrees Celsius. That might not sound like much, but the point is that we have shown that it is possible to perform this tuning at room temperature and under normal pressure,” says Debashree Banerjee, senior research engineer at Linköping University and lead author of the study.
The researchers believe that, having shown that it is possible, there is potential to advance both materials and devices. In the long term, systems are conceivable that can be laid on a roof like a solar cell and thus control the infrared heat radiation of the house and cool it if necessary. The process requires extremely low energy consumption and causes minimal environmental pollution. Further areas of application can also be clothing and wallpaper, which can be tuned to heat flows and improve thermal comfort indoors with reduced energy consumption.
In another study published in Advanced Science, the same research group developed a thermoelectric device that operates on the same principle of radiant cooling, complemented by solar heating. It is based on creating a temperature differential between two cellulosic materials, one of which contains carbon black to also absorb the heat of the sun. The materials are connected to a material that converts the temperature difference into an electrical potential. If the device is exposed to the sky, an electrical voltage of 60 mV is induced even in moderate sunlight, but the concept also works at night as the two wood materials are designed for different heat dissipation abilities.
“We use not only the sun but also space as a source of energy,” says Mingna Liao, a PhD student in the group and lead author of the article in Advanced Science.
In order to be able to carry out controlled measurements for both studies, the researchers built a sky simulator. In this way, the measurements were not affected by changes in the environment, as they are outdoors. The sky simulator consists of a tube with aluminum-coated sides that reflect the radiation. A container placed on the ground contains a material that absorbs thermal radiation and is cooled with liquid nitrogen to simulate the cold of space.
The research was funded by the Knut and Alice Wallenberg Foundation, the Wallenberg Wood Science Center, the Swedish Defense Research Agency (FOI), the Swedish Research Council and the Strategic Research Area for Advanced Functional Materials (AFM) at Linköping University.