Researchers at Stanford University are developing a rooftop device designed to generate solar energy and cool off buildings at the same time.
According to Stanford, the sun-facing layer of the device is nothing new: It’s made of the same semiconductor materials that have long adorned rooftops to convert visible light into electricity. The novelty lies in the device’s bottom layer, which is based on materials that can beam heat away from the roof and into space through a process known as radiative cooling.
“We’ve built the first device that one day could make energy and save energy, in the same place and at the same time, by controlling two very different properties of light,” says Stanford electrical engineer Shanhui Fan, senior author of an article appearing Nov. 8 in the journal Joule.
In radiative cooling, objects – including our own bodies – shed heat by radiating infrared light. That’s the invisible light night-vision goggles detect, explains Stanford. Normally, this form of cooling doesn’t work well for something like a building because Earth’s atmosphere acts like a thick blanket and traps the majority of the heat near the building rather than allowing it to escape.
Fan’s cooling technology takes advantage of the fact that this thick atmospheric blanket essentially has holes that allow a particular wavelength of infrared light to pass directly into space. In previous work, Fan had developed materials that can convert heat radiating off a building into the particular infrared wavelength that can pass directly through the atmosphere. These materials release heat into space and could save energy that would have been needed to air-condition a building’s interior. That same material is what Fan placed under the standard solar layer in his new device, says Stanford.
Zhen Chen, who led the experiments as a postdoctoral scholar in Fan’s lab, says the researchers built a prototype about the diameter of a pie plate and mounted their device on the rooftop of a Stanford building. Then, they compared the temperature of the ambient air on the rooftop with the temperatures of the top and bottom layers of the device. The top layer was hotter than the rooftop air – which made sense because it was absorbing sunlight. But, as the researchers hoped, the bottom layer of the device was significantly cooler than the air on the rooftop.
“This shows that heat radiated up from the bottom, through the top layer and into space,” explains Chen, who is now a professor at the Southeast University of China.
What they weren’t able to test is whether the device also produced electricity. The upper layer in this experiment lacked the metal foil normally found in solar cells that would have blocked the infrared light from escaping. The team is now designing solar cells that work without metal liners to couple with the radiative cooling layer.
“We think we can build a practical device that does both things,” notes Fan.
The research was supported by the Stanford University Global Climate and Energy Project, the National Science Foundation, and the National Natural Science Foundation of China. Postdoctoral scholars Wei Li of Stanford and Linxiao Zhu of the University of Michigan, Ann Arbor, co-authored the paper with Fan.