The transfer of radiant heat, which we experience when hot surfaces raise temperatures in our bodies and surroundings, poses challenges for cooling buildings that have fewer surfaces facing the sky. A research team has unveiled a novel passive cooling technology that involves applying specialized materials to walls and windows, enhancing their ability to control heat exchange with the environment at ground level. This innovation has the potential to diminish the need for air conditioning, offering a more eco-friendly, cost-effective, and scalable solution, particularly for low-income communities with limited access to heating and cooling systems.
As global temperatures climb, the demand for sustainable cooling methods is becoming more urgent. Researchers from UCLA and their partners have developed an economical and adaptable technique to cool buildings during summer while providing warmth in winter.
Under the guidance of Aaswath Raman, an associate professor at the UCLA Samueli School of Engineering, the research team has published findings in Cell Reports Physical Science regarding a new way to regulate radiant heat transfer through widely used building materials for improved thermal management.
Radiant heat, which we notice when warm surfaces heat our living spaces, travels through electromagnetic waves across a wide spectrum at ground level, between buildings and their surroundings such as streets and nearby structures. In contrast, heat exchanges between buildings and the sky occur within a much narrower infrared spectrum, referred to as the atmospheric transmission window. This difference in radiant heat movement has historically complicated efforts to cool buildings that do not have ample sky-facing surfaces. Such structures are challenging to cool during hot weather as they absorb heat from the ground and adjacent surfaces, and they struggle to retain heat in colder months as outdoor temperatures drop.
“In historical cities like Santorini, Greece, or Jodhpur, India, building designs aimed at reflecting sunlight have been in practice for centuries,” remarked Raman, who directs the Raman Lab at UCLA Samueli. “While there’s been a surge of interest in cool roof surfaces that reflect sunlight, addressing the cooling needs of walls and windows is a more intricate task.”
Taking inspiration from successful applications of ultra-white reflective paint on roofs, which reflects sunlight and allows heat to escape into the sky, the researchers sought to create a similar passive cooling method for walls and windows. They managed to show that materials that preferentially absorb and emit radiant heat within the atmospheric window maintain cooler temperatures than standard building materials in summer and remain warmer in winter.
“It was particularly thrilling to discover that common materials like polypropylene, derived from everyday plastics, can effectively radiate or absorb heat in the atmospheric window,” said Raman. “These materials, though seemingly ordinary, offer scalability that could lead to their widespread use in regulating building temperatures soon.”
Moreover, the team’s strategy incorporates easily sourced, economical materials, providing the added advantage of energy savings by lessening dependency on air conditioning and heating systems that can be expensive and contribute to greenhouse gas emissions.
“Our proposed mechanism is entirely passive, making it a sustainable solution for achieving seasonal temperature regulation while tapping into energy savings,” stated Jyotirmoy Mandal, the lead author of the study and a former postdoctoral researcher in Raman’s lab, who is now an assistant professor at Princeton University specializing in civil and environmental engineering.
The research team believes this new method can be easily scaled up, which would have a significant impact on low-income communities that lack adequate heating and cooling systems, particularly those facing increased health risks due to extreme weather events worldwide.
Raman and his team are investigating ways to illustrate this passive cooling effect on a larger scale, aiming to gauge real-world energy savings, especially for communities in Southern California that are susceptible to heat.
The study received funding from various sources, including the Schmidt Science fellowship, the Rhodes Trust, the Alfred P. Sloan Foundation, and the National Science Foundation. Co-authors of the paper include John Brewer, a recent Ph.D. graduate from Raman’s lab, Jyothis Anand from Oak Ridge National Lab, Arvind Ramachandran from Arizona State University, and independent researcher Sagar Mandal.
