Using “Scotch tape” and aluminum foil, engineers at UCLA built a passive radiative cooler that lowers outdoor temperatures without electricity or refrigerants. The team’s cooler design exhibits solar reflectance, long wavelength infrared (LWIR) emittance, and optical selectivity that is comparable to leading radiative coolers, the engineers said. The readily available materials with which the cooler is made, combined with its ease of fabrication and performance, suggest that this design could be widely used for low-cost passive thermal cooling. The engineers’ interest in “Scotch tape”-and-aluminum-based radiative cooling designs stemmed from their belief that the use of these materials would enable them to design a cooler that would be low-cost and would demonstrate reproducible optical performance. Two layers of tape on alumuninum foil ultimately comprise the design. The adhesive and the plastic film in “Scotch tape” have chemical bonds that absorb and radiate heat in the LWIR wavelength. The LWIR wavelength is suitable for passive cooling because it is poorly absorbed by Earth’s atmosphere and therefore easily lost to space. Engineers have designed a zero-carbon, zero-energy cooler that uses common household items. According to the engineers, the design could serve as a standard reference for forthcoming cooler designs that require a generic radiative cooling component for their operation, as well as a control for radiative cooling experiments. Courtesy of Jyotirmoy Mandal and Aaswath P. Raman, UCLA. When the researchers made optical measurements of the cooler, they found that the design had a moderately selective infrared (IR) emittance — that is, a selective proportion of heat was lost through LWIR radiation. The intrinsic lack of highly emissive chemical bonds in non-LWIR thermal wavelengths made both the adhesive and the plastic in the tape selectively LWIR-emissive. When aluminum foil was used as a back reflector, it yielded a modest weighted solar reflectance. The researchers also tested an alternate design using sputter-coated silver, which absorbs less solar radiation than aluminum foil. With silver as a back reflector, the design yielded a high solar reflectance that made it capable of daytime radiative cooling as well as nighttime cooling. At night, the aluminumized “Scotch tape” design achieved a 7 °C temperature drop below the ambient temperature. The sputter-coated silver design achieved a 2 to 3 °C temperature drop during daytime. The researchers then tested their first design with an added layer of a polyethylene convection shield to reduce heat absorption. With this setup, they achieved a temperature drop of up to 11 °C during the nighttime. “The large temperature drop we achieved can be further used to generate electricity through thermoelectric processes,” researcher Jyotirmoy Mandal said. Wide availability and consistent quality of “Scotch tape” and aluminum foil support the reproducibility of the cooler design. It could serve as a standard reference, both for cooler designs that require a generic radiative cooling component for their operation, and as a control for radiative cooling experiments, team members said. It could also provide a basis for defining and classifying thermal emitter selectivity. The thermal cooler design could make radiative cooling available for a range of applications and in low-resource settings. To encourage the adoption of their cooling design as a standard, the researchers are making the data from their experiments on the optical performance of the design publicly available online. The research was published in the Journal of Photonics for Energy (www.doi.org/10.1117/1.JPE.12.012112).
