Researchers have been exploring how to turbocharge a passive cooling technique — known as radiative or sky cooling — with sun-blocking nanomaterials that emit heat away from building rooftops. While progress has been made, this eco-friendly technology isn’t commonplace because researchers have struggled to maximize the materials’ cooling capabilities.
A radiative cooling system was developed that consists of what are essentially two mirrors made of ten extremely thin layers of silver and silicon dioxide that are placed in a V-shape. The mirrors absorb incoming sunlight, turning solar power from visible and near-infrared waves into heat. The mirrors also reflect mid-infrared waves from an emitter (a vertical box in between the two mirrors), which then bounces the heat they carry into the sky. Since the thermal emission from both surfaces of the central thermal emitter is reflected to the sky, the local cooling power density on the emitter is doubled, resulting in a high temperature reduction.
In tests, the system lowered the temperature inside a test system in an outdoor environment under direct sunlight by more than 12 °C (22 °F). In a test box in a laboratory, which was meant to simulate the night, it lowered the temperature by more than 14 °C (25 °F). It simultaneously captured enough solar power to heat water to about 60 °C (140 °F).
While the system tested was only 70 square centimeters (27.5 inches), it could eventually be scaled up to cover rooftops, with the goal of reducing society’s reliance on fossil fuels for cooling and heating. It also could aid communities with limited access to electricity.
Most radiative cooling systems scatter the solar energy, which limits the system’s cooling capabilities. Even with a perfect spectral selection, the upper limit for the cooling power with an ambient temperature of 25 °C is about 160 watts per square meter. In contrast, the solar energy of about 1000 Watts per square meter on top of those systems was simply wasted.