A team of engineers at the University of Colorado Boulder (CU Boulder) has developed a scalable manufactured metamaterial – an engineered material with extraordinary properties not found in nature – to act as a kind of air conditioning system for structures. It has the ability to cool objects even under direct sunlight with zero energy or water consumption.

When applied to a surface, the metamaterial film cools the object underneath by efficiently reflecting incoming solar energy back into space while simultaneously allowing the surface to shed its own heat in the form of infrared thermal radiation. This new material, which is described in a paper in Science, could provide an eco-friendly form of supplementary cooling for thermoelectric power plants, which currently require large amounts of water and electricity to maintain the operating temperatures of their machinery.

The material is a glass-polymer hybrid that measures just 50µm thick – slightly thicker than the aluminum foil found in a kitchen – and can be manufactured economically on rolls, making it a potentially viable large-scale technology for both residential and commercial applications.

"We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology," said Xiaobo Yin, co-director of the research and an assistant professor who holds dual appointments in CU Boulder's Department of Mechanical Engineering and the Materials Science and Engineering Program.

The material takes advantage of passive radiative cooling, the process by which objects naturally shed heat in the form of infrared radiation without consuming energy. Passive radiation provides some natural night time cooling and is used for residential cooling in some areas, but daytime cooling has historically been more of a challenge. Even a small amount of directly-absorbed solar energy is enough to negate passive radiation.

"Just 10–20 square meters of this material on the rooftop could nicely cool down a single-family house in summer."Gang Tan, University of Wyoming

The challenge for the CU Boulder researchers, then, was to create a material that could provide a one-two punch: reflect any incoming solar rays back into the atmosphere while still providing a means of escape for infrared radiation. To solve this, the researchers embedded visibly-scattering but infrared-radiant glass microspheres into a polymer film. They then added a thin silver coating underneath the film to achieve maximum spectral reflectance.

"Both the glass-polymer metamaterial formation and the silver coating are manufactured at scale on roll-to-roll processes," said Ronggui Yang, also a professor of mechanical engineering at CU Boulder.

During field tests in Boulder and Cave Creek, Arizona, the metamaterial successfully displayed an average radiative cooling power larger than 110W/m2 for a continuous period of 72 hours and larger than 90W/m2 in direct, noon-time sunlight. That cooling power is roughly equivalent to the electricity generated by solar cells over a similar area, but radiative cooling has the advantage that it occurs both day and night.

"Just 10–20m2 of this material on the rooftop could nicely cool down a single-family house in summer," said Gang Tan, an associate professor in the University of Wyoming's Department of Civil and Architectural Engineering and a co-author of the paper.

In addition to being useful for cooling buildings and power plants, the material could also help to improve the efficiency and lifetime of solar panels. In direct sunlight, panels can heat up to temperatures that hamper their ability to convert solar rays into electricity.

"Just by applying this material to the surface of a solar panel, we can cool the panel and recover an additional one to two percent of solar efficiency," said Yin. "That makes a big difference at scale."

The engineers have applied for a patent on the technology and are working with CU Boulder's Technology Transfer Office to explore potential commercial applications. They plan to create a 200m2 ‘cooling farm’ prototype in Boulder later this year.

"The key advantage of this technology is that it works 24/7 with no electricity or water usage," explained Yang "We're excited about the opportunity to explore potential uses in the power industry, aerospace, agriculture and more."

This story is adapted from material from the University of Colorado Boulder, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.