Changing solar energy not into electricity but hydrogen fuel and storing it for later can allow devices to run after the sun goes down, U.S. researchers say.

While solar energy has long been used as a clean alternative to fossil fuels such as coal and oil, it could only be harnessed during the day when the sun's rays were strongest, scientists at the University of North Carolina said.

If solar energy is going to have a shot at being a clean source for powering the planet, they said, a way to store it for nighttime use must be found.

The system developed at the university's Energy Frontier Research Center generates hydrogen fuel by using the sun's energy to split water into its component parts -- hydrogen and oxygen -- then storing the hydrogen while the oxygen is release into the air.

"But splitting water is extremely difficult to do," UNC chemistry Professor Tom Meyer, the research leader, said. "You need to take four electrons away from two water molecules, transfer them somewhere else, and make hydrogen, and, once you have done that, keep the hydrogen and oxygen separated. How to design molecules capable of doing that is a really big challenge that we've begun to overcome."

The system can turn the sun's energy into fuel while needing almost no external power to operate and releasing no greenhouse gases, the researchers said.

"So called 'solar fuels' like hydrogen offer a solution to how to store energy for nighttime use by taking a cue from natural photosynthesis," he said. "Our new findings may provide a last major piece of a puzzle for a new way to store the sun's energy -- it could be a tipping point for a solar energy future."

"When you talk about powering a planet with energy stored in batteries, it's just not practical," Meyer said. "It turns out that the most energy dense way to store energy is in the chemical bonds of molecules. And that's what we did -- we found an answer through chemistry."