One of the common myths about solar power is that it doesn't do any good once the sun sets. The truth is that the sun's energy exists all day long. We just need to figure out new ways to capture it.

A team of physicists at the Harvard School of Engineering and Applied Sciences may have just come up with that new capture method. In a paper published last month in the Proceedings of the National Academy of Sciences, they propose creating a new type of energy harvester that would collect modest but valuable levels of infrared energy emitted by objects after the sun sets.

Principal investigator Federico Capasso, a professor of applied physics, called the idea "weird" and "counterintuitive" in a news release, but the new paper proves it will work.

The idea hinges on a process that is often called the Earth's radiation budget. Energy from the sun enters our atmosphere, where it is absorbed, reflected and ultimately emitted back up into space. Think of a blacktop driveway and how warm it remains after the sun sets in the summer – that's the part of the process that the Harvard researchers want to take advantage of with their new process.

The paper describes a new device that would absorb solar radiation during the day and then emit thermal radiation into the sky after the sun sets. Rather than wasting that energy, the proposed device — called an emissive energy harvester — would capture the energy as the warmed object cools off and then turn the energy into power. The researchers describe two types of devices that would achieve this pattern. They used real-world conditions in Oklahoma to mathematically prove their concept.

Lead author Steven J. Byrnes, a postdoctoral fellow, said the point of the paper was to prove if this whole emissive energy capture process would be worth the effort. "It's not obvious how much power you can generate this way, or whether it's worthwhile to pursue, until you sit down and do the calculation," he said.

The key, it turned out, was a forgotten electrical circuit first described back in 1968 by the man who invented some of the key technology in police radar guns. That circuit contained a valve-like diode that, when put to use in this new process, allowed the researchers to create an electrical current through the radiation process.

Byrnes said people have been studying these infrared diodes for at least 50 years, but recent advances in nanofabrication would make the application of the circuits possible.

The theoretical emissive energy harvester wouldn't solve all of the world's energy problems: "...the voltage will be relatively low," Byrnes explained – but it's another important step toward turning solar energy into a 24-hour power source.

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