Using tape and aluminum foil, UCLA engineers built a passive radiative cooler that lowers outdoor temperatures without electricity or refrigerants. The team’s cooler design has solar reflectance, long-wavelength infrared (LWIR) emittance and optical selectivity comparable to leading radiative coolers, the engineers said.
The readily available materials with which the cooler is made, combined with its ease of manufacture and performance, suggest that this design could be widely used for low cost passive thermal cooling.
Engineers’ interest in scotch tape and aluminum-based radiative cooling designs stemmed from their belief that using these materials would allow them to design a cooler that would be inexpensive and demonstrate repeatable optical performance.
Two layers of tape over aluminum foil ultimately make up the design. Scotch tape’s adhesive and plastic film 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 the Earth’s atmosphere and therefore easily lost to space.
Engineers designed a carbon-free, energy-free cooler that uses common household items. According to the engineers, the design could serve as a standard reference for future chiller designs that require a generic radiative cooling component for 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 moderately selective infrared (IR) emittance, that is, a selective proportion of heat was lost to LWIR radiation. The inherent absence of highly emissive chemical bonds in non-LWIR thermal wavelengths made both the tape adhesive and plastic selectively LWIR emissive.
When aluminum foil was used as a rear reflector, it produced a modest weighted solar reflectance. The researchers also tested another design using sputter-coated silver, which absorbs less solar radiation than aluminum foil.
With silver as the rear reflector, the design produced high solar reflectance which made it capable of daytime radiative cooling as well as nighttime cooling.
At night, the aluminum scotch tape design achieved a temperature drop of 7°C below ambient. The spray-coated silver design allowed for a 2-3°C temperature drop during the day.
The researchers then tested their first design with an additional layer of a polyethylene convection screen to reduce heat absorption. With this setup, they achieved a temperature drop of up to 11°C overnight. “The steep temperature drop we achieved can be further utilized to generate electricity through thermoelectric processes,” said researcher Jyotirmoy Mandal.
Wide availability and consistent quality of scotch tape and foil support the reproducibility of the chiller design. It could serve as a standard reference, both for chiller designs that require a generic radiative cooling component for their operation, and as a control for radiative cooling experiments, the team members said. It could also provide a basis for defining and classifying the selectivity of thermal emitters.
The design of the thermal cooler could make radiative cooling available for a range of applications and in low resource environments. To encourage adoption of their cooling design as a standard, the researchers are making 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).