Researchers from UCLA and Princeton University have created a passive system that allows buildings to be heated in the winter and cooled in the summer. According to the journal Cell Reports Physical Science, coatings made of common materials can achieve thermal comfort and energy savings beyond what can be achieved by traditional building envelopes by limiting radiant heat flows between buildings and their surroundings to particular wavelengths.
Researchers propose using common materials to control radiant heat moving into and out of buildings. Image Credit: Princeton University
With the increase in global temperatures, maintaining habitable buildings has become a global challenge. Buildings exchange a majority of heat with their environment as radiation, and by tailoring the optical properties of their envelopes to exploit how radiation behaves in our environment, we can control heat in buildings in new and impactful ways.
Jyotirmoy Mandal, Researcher and Assistant Professor, Department of Civil and Environmental Engineering, Princeton University
People experience radiant heat when sunlight hits their skin or when a room is heated by an electric coil. Radiant heat is transported by electromagnetic waves and is present everywhere. Controlling radiant heat to regulate building temperature is common practice. To reflect sunlight, most building owners paint their walls and roofs white and use window shades to block sunlight.
If we look at historical cities like Santorini in Greece or Jodhpur in India, we find that cooling buildings by making roofs and walls reflect sunlight has been practiced for centuries. In recent years there has been massive interest on cool roof coatings that reflect sunlight. But cooling walls and windows is a much more subtle and complex challenge.
Aaswath Raman, Researcher and Associate Professor, Department of Materials Science and Engineering, University of California, Los Angeles
Roofs typically have an unobstructed view of the sky, which allows cool roof coatings to reflect sunlight and emit long-wave heat upwards, eventually sending it into space. In contrast, walls and windows primarily face the ground and nearby buildings. During hot weather, they absorb heat radiating from streets, pavements, and adjacent structures. As a result, even though walls and windows release heat towards the sky, they simultaneously absorb more heat from the ground. In colder weather, the terrestrial environment cools down, drawing heat away from walls and windows.
The researchers discovered a solution by understanding the different ways heat transfers between buildings and the ground compared to buildings and the sky. Heat radiates from buildings to the sky within a specific portion of the infrared spectrum called the atmospheric transmission window, which the researchers referred to as narrowband. At ground level, however, heat radiates across the entire infrared spectrum, which the researchers termed broadband.
By coating walls and windows with materials that only radiate or absorb heat in the atmospheric window, we can reduce broadband heat gain from the ground in the summer, and loss in the winter, while maintaining the cooling effect of the sky. We believe that this idea is unprecedented, and beyond what traditional roof and wall envelopes can achieve.
Jyotirmoy Mandal, Researcher and Assistant Professor, Department of Civil and Environmental Engineering, Princeton University
The findings are significant for two key reasons. Firstly, the researchers demonstrate in their article that many common and affordable building materials can radiate heat in the narrowband while blocking broadband heat. Materials like polyvinyl fluoride, which is already used for siding, could be repurposed for this application. Additionally, even more common plastics could be utilized for this purpose.
Raman said, “We were really excited when we found that materials like polypropylene, which we sourced from household plastics, selectively radiate or absorb heat in the atmospheric window. These materials border on the mundane, but the same scalability that makes them common also means that we could see them thermoregulating buildings in the near future.”
The second reason for optimism is the substantial potential energy impacts at the building scale. The researchers observed that seasonal energy savings achieved through their mechanism are comparable to the benefits of painting dark roofs white. This innovation could be particularly valuable as air conditioning costs and heat-related casualties continue to rise globally. Mandal and Raman plan to further their research in this area.
Mandal said, “The mechanism we proposed is completely passive, which makes it a sustainable way to cool and heat buildings with the seasons and yield untapped energy savings. In fact, the benefits of the mechanisms and materials we show are highest for buildings in the global south. So, it could be a more equitable solution in resource-poor communities, even more as they see increasing cooling demands and heat-related mortalities.”
The project received support from the Schmidt Science Fellowship, the Alfred P. Sloan Foundation, and the National Science Foundation.
Journal Reference:
Mandal, J., et al. (2024) Radiative cooling and thermoregulation in the earth’s glow. Cell Reports Physical Science. doi.org/10.1016/j.xcrp.2024.102065