Common plastics could provide seasonal passive heating and cooling for buildings

by Khushi Srivastava

Coatings made of common materials like plastics can provide more energy savings and thermal comfort than standard building envelopes by limiting radiant heat transfers between buildings and their surroundings to particular wavelengths.

Princeton and UCLA researchers have created a passive system that cools and warms buildings in the summer and winter.

In an article published on June 27 in the journal Cell Reports Physical Science, they report that by limiting radiant heat flows between buildings and their surroundings to specific wavelengths, coatings made from common materials can achieve energy savings and thermal comfort that exceed what traditional building envelopes can.

“With the increase in global temperatures, maintaining habitable buildings has become a global challenge,” said Jyotirmoy Mandal, an assistant professor of Civil and Environmental Engineering at Princeton University. “We can manage heat in buildings by modifying the optical properties of their envelopes to utilize how radiation behaves in our surroundings. Buildings exchange the majority of their heat with the environment as radiation.

Radiant heat, transported by electromagnetic waves, is everywhere; we feel it when sunshine warms our skin or an electric coil heats a room. Controlling building temperature by radiant heat is a widespread method. Most buildings employ window coverings to hide sunlight, and many have white roofs and walls to reflect it.

“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,” said Aaswath Raman, an associate professor of Materials Science and Engineering at UCLA. “In recent years, there has been a lot of interest in cool roof coatings that reflect sunlight.” However, cooling walls and windows presents a far more nuanced and intricate challenge.”

Roofs typically provide an unobstructed view of the sky. This enables cool roof coatings to reflect sunlight and release long-wave heat upwards and eventually into space. Walls and windows, on the other hand, typically overlook the ground and nearby buildings. During hot weather, they are warmed by heat emitted from heated streets, pavements, and neighboring buildings. This means that, while walls and windows transmit heat to the sky, they are also heated by the earth. During cold weather, the terrestrial environment cools, allowing heat to escape from walls and windows.

The researchers discovered a solution to this problem by examining how heat flows between buildings and the ground level, as well as between structures and the sky. Radiant heat travels from buildings to the sky through a tiny region of the infrared spectrum known as the atmospheric transmission window, which the researchers refer to as the narrowband. At ground level, radiant heat travels across the full infrared spectrum, which the researchers refer to as broadband.

“By coating walls and windows with materials that only radiate or absorb heat in the atmospheric window, we may limit broadband heat gain from the ground in the summer and loss in the winter while retaining the cooling influence of the sky. We believe that this concept is unprecedented and goes beyond what typical roof and wall enclosures can do.” Mandal made a note.

The findings have a substantial impact for two reasons. First, the researchers demonstrate in the article that many common and low-cost construction materials emit heat in the narrowband while blocking broadband heat. Polyvinyl fluoride, which is already used as a siding material, and other common plastics might be adapted for this use.

“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.” Raman noticed. “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 confidence is that the potential energy impacts on a building size are significant. The researchers discovered that seasonal energy savings from their technique are comparable to the benefits of painting dark roofs white. This could be valuable if air conditioning costs and heat-related deaths continue to rise globally. Mandal and Raman intend to pursue this discovery further.

“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.” Mandal notes. “In fact, the advantages of the systems and materials we demonstrate are greatest for structures in the global south. So, it could be a more equitable option in resource-poor communities, especially as cooling demands and heat-related deaths rise.”

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