Over the last few years, we've been keeping tabs on a cooling system developed by a team led by Professor Shanhui Fan at Stanford University that draws thermal heat from a building and beams it into space through a process called radiative cooling – all without requiring any external power source. Now the researchers have paired this technology with a solar panel, designing a system with the same rooftop footprint that could generate electricity and passively cool a building at the same time.

Air conditioners are major energy hogs, and with global temperatures on the rise the problem is only set to get worse. When we first looked at the radiative cooling system being developed by Fan and his associates in 2013, it was suggested the passive rooftop panels could be used in place of solar panels used to generate electricity to run AC systems, or to cool buildings in off-grid areas. But now the team has found an even better solution, in a dual-purpose device that means passive cooling doesn't have to mean sacrificing electricity generation for rooftop real estate.

"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 Fan.

The hybrid device takes the form of a double-layered solar panel, with the upper sun-facing layer consisting of the same semiconductor materials used on existing rooftop solar panels, while the bottom layer is made of materials that convert a building's thermal heat into a particular wavelength of infrared light that is able to pass directly through the atmosphere, allowing it to be beamed into space.

To test the potential of the concept, the team built a prototype device with a diameter roughly that of a pie plate and mounted it to the roof of a building at Stanford. Because the prototype didn't include a layer of metal foil as is normally found in solar cells, which would have prevented the infrared light from escaping, it wasn't possible to test whether the device produced electricity. However, the top layer, which was absorbing sunlight, reached 24° C (43° F) above the ambient temperature, while the radiative cooling layer shielded below fell to 29° C (52° F) below the ambient temperature.

"This shows that heat radiated up from the bottom, through the top layer and into space," says Zhen Chen, who is now a professor at the Southeast University of China but who led the experiments as a postdoctoral scholar in Fan's lab.

Having demonstrated the cooling capabilities of the system, the team is now working to design solar cells that don't require metal liners so a single device can both cool a building and generate electricity.

"We think we can build a practical device that does both things," Fan says.

The team's research appears in the journal Joule.