Cheaper way to passively heat and cool homes on horizon

Researchers have found a new process to cool buildings in the summer and keep them warmer during the winter, according to a new study in Cell Reports Physical Science.

Their new method manipulates the movement of radiant heat from buildings’ vertical walls and windows. It works by using common building materials including plastics, polymer resins, and ceramics, that preferentially absorb and emit a specific range of infrared radiation (radiant heat).

The findings offer a sustainable way to reduce reliance on active forms of cooling and heating, which could have important impacts for low-income communities with limited to no access to them.

“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,” says Aaswath Raman, associate professor of materials science and engineering at the University of California – Los Angeles in the US, who led the research.

“In recent years there has been massive interest in cool roof coatings that reflect sunlight. But cooling walls and windows is a much more subtle and complex challenge.”

Infrared (IR) is a type of electromagnetic radiation with wavelengths longer than visible light.

It is invisible to the human eye, but we can detect IR light with our skin as the heat radiated from warm objects. The Earth and other terrestrial objects, like buildings, absorb sunlight and re-emit it as IR-radiation. This is why the ground feels hotter on a sunny day.

Like the rainbow of frequencies that make up visible light, IR is made up of a range of frequencies – from shorter “near-infrared” to the longer “far-infrared” waves.

IR radiation is lost from buildings to the sky in a narrow portion of the IR-spectrum known as the “atmospheric transmission window”. But buildings also exchange a broad range of IR with the Earth through their vertical surfaces, which causes them to lose heat in the winter and retain it in the summer.

This makes cooling buildings with less skyward-facing surfaces a challenge.

The new research demonstrates that common materials capable of preferentially absorbing and emitting radiant heat within the atmospheric window (while reflecting other IR wavelengths) could stay cooler in the summer and warmer during the winter than conventional building materials – such as white paints and composites.

“We were particularly excited when we found that materials like polypropylene, which we sourced from household plastics, can selectively radiate or absorb heat in the atmospheric window very effectively,” Raman says.

“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.”