Passive cooling is an age-old practice to keep people and buildings cool with little or no energy consumption. It can be as simple as leveraging shade from trees or other structures. Researchers increasingly are innovating to improve passive cooling and deploy environmentally friendly technologies as alternatives to air conditioning or other energy-intensive cooling methods.

A University of Wisconsin-Madison engineer is on a team led by the University at Buffalo that recently released results of research intended to “turbo charge” a passive cooling technique that uses sun-blocking nanomaterials to reflect heat away from buildings’ roofs. Thus far, scientists haven’t been able to achieve a huge gain with this type of technology’s cooling capabilities, so the materials are not used in real-life applications very often. But the new research made significant advances. 

The highlights

• The team came up with a unique design for a passive cooling system that essentially consists of two mirrors made from 10 thin layers of silver and silicon dioxide. They reflect certain light waves away from the structure and toward the sky. They also absorb sunlight and convert it to heat. 
• The innovation lowered the temperature in an outdoor environment by 22 degrees Fahrenheit. In a laboratory test that simulated nighttime environments, the system lowered the temperature by 25 degrees Fahrenheit.
• One of the system’s key innovations is the ability to provide cooling and heating capabilities from the same unit. Many passive cooling technologies simply reflect the light waves instead of harnessing them as a resource. This system harnessed enough solar energy to heat water to about 140 degrees Fahrenheit.
• The system is small — only about 27.5 inches squared — but has the potential to scale up enough to cover entire rooftops.

The applications

The scientists aim to get this technology in widespread use to reduce reliance on fossil fuels for heating and cooling. It could especially benefit communities that have limited or unreliable access to electricity. 

“Importantly, our system does not simply waste the solar input energy. Instead, the solar energy is absorbed by the solar spectral selective mirrors, and it can be used for solar water heating, which is widely used as an energy-efficient device in developing countries,” Qiaoqiang Gan, lead author and professor of electrical engineering at University at Buffalo, said in a news release. “It can retain both the solar heating and radiative cooling effects in a single system with no need of electricity. It’s really sort of a ‘magic’ system.”

The researchers will continue to perfect the technology and try to get it to the point where it captures enough solar energy to boil water. That could help with water purification in developing countries. One of the researchers launched a spin-off company, Sunny Clean Water, to commercialize this technology.