Engineers develop non-electrical cooling system for tall urban buildings
A 'solar shelter' absorbs sunlight like a sponge and beams thermal radiation back to the sky.
By Peter Saunders
State University of New York (SUNY) at Buffalo engineers have designed a new system they claim can help cool buildings without consuming electricity, thus saving on utility bills. Specifically, it helps cool its surroundings by absorbing heat from the air inside a box, then transmitting that energy away.
To develop this radiative cooling system, the engineers first coated a sheet of aluminum with a clear polymer, polydimethylsiloxane. Then they installed the sheet inside a foam box and built a ‘solar shelter’ atop it, using solar-energy-absorbing materials for four outward-slanting walls and an inverted square cone within them (see photo).
The shelter served a dual purpose of absorbing incoming sunlight like a sponge and beaming thermal radiation emitted from the film into the sky. The combined shelter-and-box system measured about 18 x 10 x 10 in.
“The polymer stays cool as it dissipates heat through thermal radiation and can then cool down the environment,” explains co-author Lyu Zhou, a PhD candidate in electrical engineering at the university’s school of engineering and applied sciences. “This passive cooling won’t need a battery or other electricity source.”
Their research showed this approach could reduce the temperature of a small, enclosed space by about 6 C during the day and by about 11 C at night. To cool a building, many units would need to cover the roof.
“One of the innovations of our system is the ability to purposefully direct thermal emissions toward the sky,” says lead researcher Qiaoqiang Gan, PhD, associate professor of electrical engineering. “Normally, thermal emissions travel in all directions. We have found a way to beam them in a narrow direction, like how the design of a car headlight directs light in a certain direction. This enables the system to be effective in urban environments, where there are tall buildings on all sides. We use low-cost, commercially available materials and find they perform very well.”
“Radiative cooling is easy during the night because we don’t have solar input,” says assistant professor Haomin Song, PhD, “but daytime cooling is a challenge because the sun is shining. In this situation, you need to find strategies to prevent rooftops from heating up and emissive materials that don’t absorb solar energy. Our system addresses these challenges.”