by Julia Rosen Wednesday, September 4, 2013
Most of the existing solutions for renewable energy storage represent riffs on our current energy infrastructure. They are either inherent to existing fuel sources (concentrated solar power), or linked directly to the grid (flywheels, pumped storage and compressed air energy storage). But what if the future looks radically different from today, as history has often shown it can? What if the future of energy is based on hydrogen produced by renewable energy sources?
“If you look at the history of fuel that humans have used, you follow that chain from wood to coal and oil to natural gas and on down the line,” says Ted Brekken, a professor of electrical engineering at Oregon State University who specializes in renewable energy systems. What happens as you do this, he says, is that you get less carbon. “Hydrogen is the energy carrier in all of those [fuels].” Extending this logic, some think that hydrogen fuel is just the natural end of that progression, he says. And hydrogen also provides an elegant means of storing energy — once it exists as a gas or liquid, it can be transported across the country, used to generate electricity and power vehicles, or sit in a tank for years until needed.
Because hydrogen burns clean, both its production and consumption would be carbon-free if it could be generated by renewable energy sources. For instance, the voltage generated by a traditional solar panel can be used to drive the electrolysis of water, which produces hydrogen fuel and oxygen as a byproduct. “We’re converting the energy from sunlight into energy held by the chemical bonds within the hydrogen,” says Dan Esposito, a postdoctoral fellow working on solar fuels at the National Institute of Standards and Technology in Gaithersburg, Md. The idea isn’t new — it’s called photosynthesis.
Esposito and his colleagues have advanced a promising technique to produce “solar hydrogen,” as it’s commonly called. As they reported recently in Nature Materials, they collect solar energy on a silicon-based solar cell covered by a nanometer-thick insulating layer of silicon dioxide. This insulator is crucial to their design — it prevents the extremely efficient light-harvesting layer from corroding over time, overcoming a long-standing trade-off in solar fuel production between stability and efficiency. Electrons generated by solar absorption then tunnel through the insulating layer into a cell where water is separated into oxygen and hydrogen through electrolysis.
Other researchers are pursuing the same goal — for example, scientists at Lawrence Berkeley National Laboratory have invented an “artificial forest” for water splitting — but Esposito says solar fuel research has a ways to go before it becomes commercially viable.
Nonetheless, the possibilities of solar fuel, no matter how far off, are exciting, Esposito says. “One idea that has been discussed is to have these huge solar fuel-generating plants out in the ocean,” he says, where solar energy and water — the only inputs — abound. “As you split water to produce hydrogen, you could put that hydrogen right into a tanker and bring it back to port.”
While solar hydrogen has some major advantages, like burning clean and meeting a wide variety of energy needs, there are downsides as well. For example, while hydrogen fuel is denser than fossil fuels by mass, it is much less dense by volume. “If you take two trucks going down the road,” Brekken says, “and one is trucking gasoline, and the other is trucking compressed gas hydrogen, there’s going to be way more energy in the gasoline than the hydrogen just because you are not able to pack enough of the hydrogen gas in the tanker and gasoline is so ridiculously energy-dense.” This is the kind of stubborn economic challenge that, like all renewable energy sources, hydrogen fuel must overcome, he says.
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