Researchers at Princeton and UCLA have developed a passive mechanism to cool buildings in the summer and warm them in the winter.
In an article published June 27 in the journal Cell Reports Physical Science, they report that by restricting radiant heat flows between buildings and their environment to specific wavelengths, coatings engineered from common materials can achieve energy savings and thermal comfort that goes beyond what traditional building envelopes can achieve.
Radiant heat, carried by electromagnetic waves, is ubiquitous – we feel it when sunlight warms our skin, or when an electric coil heats up a room. Regulating building temperature by controlling radiant heat is a common practice. Most buildings use window shades to block sunlight, and many paint roofs and walls white to reflect the sun.
Roofs usually have an open view of the sky. This allows cool roof coatings to reflect sunlight and radiate long-wave heat skywards and eventually to space. Walls and windows, on the other hand, mostly have the ground and neighboring buildings in view. During hot weather, they are warmed by heat radiating from hot streets, pavements and nearby buildings in view. This means that even as walls and windows radiate heat to the sky, they are heated even more by the earth. During cold weather, the terrestrial environment becomes colder, draining heat from walls and windows.
The researchers realized the way around this problem lies in the different way heat moves between buildings and the area at ground level and the way it moves between buildings and the sky. Radiant heat moves from buildings to the sky in a narrow portion of the infrared spectrum known as the atmospheric transmission window, so the researchers call this narrowband. At ground level, radiant heat moves across the entire infrared spectrum, and the researchers call that broadband.
The findings’ impact is significant for two important reasons. First, the researchers show in the article that many common and low-cost building materials radiate heat in the narrowband and block broadband heat. Material such as polyvinyl fluoride, already used as siding material, could be adapted for the purpose, as could even more common plastics.
The second reason for optimism is that the potential energy impacts at the building scale are substantial. The researchers noted that seasonal energy savings with their mechanism are comparable to the benefits of painting dark roofs white. This could be useful as air conditioning cost and heat related casualties continue to soar worldwide. Mandal and Raman plan to continue this research further.
