Using the sun's heat and light together could be an efficient way to produce hydrogen, which in turn could be used to power cars, homes and businesses.

Combining The Sun's Heat And Light Could Boost Water-Splitting Efficiency

George Douglas | NREL

Using the sun


George Douglas, NREL

Photo of large mirros used in the production of hydrogen.

Using large mirrors such as these to magnify the sun's heat and light could potentially increase the efficiency of producing hydrogen as much as 60 percent over using electricity or heat alone.

The sun warms us and gives us light. We're accustomed to combining the benefits of Sol's gifts when we bask in the sunshine, enjoying the view or a good book. Researchers now say that using the sun's heat and light together could be an efficient way to produce hydrogen, which in turn could be used to power cars, homes and businesses.

Energy from the sun can split water into hydrogen and oxygen, and the hydrogen then can be used in a fuel cell to produce electricity. This water splitting can be done either through electrolysis by using solar cells to convert sunlight into electricity to produce hydrogen or by concentrating heat from the sun to high enough temperatures to break the hydrogen and oxygen bond.

But what happens if you combine the two, heat and electricity, and apply them together to produce hydrogen from water? A demonstration by Australian company Solar Systems suggests that using both could increase the efficiency of producing hydrogen by as much as 60 percent over using electricity or heat alone. That's a prospect that intrigues researchers at NREL and elsewhere. With such a boost in efficiency, they say, hydrogen from sunshine and water could compete economically with other ways of producing hydrogen from renewable energy resources.

It works this way. The infrared (heat) portion of the sunshine is concentrated into a light pipe that pushes water vapor temperatures up to 1,300 degrees Celsius (more than 2,300 degrees Fahrenheit), weakening the bonds joining the water's hydrogen and oxygen. The membrane of a high-temperature electrolyzer is zapped with electricity to split the water vapor as it contacts the membrane. Since the strength of the bonds holding the hydrogen and oxygen together as water have been diminished by the sunshine's heat, it is easier for electricity from the sunshine's light to split the H2O.

NREL's Robert McConnell, Exploratory Research Project Leader in the National Center for Photovoltaics, is excited about the prospect.

"The approach (Solar Systems) has developed is very elegant," he said.

McConnell recently reported on the economic potential of this hybrid method of harvesting hydrogen at the U.S. Hydrogen Conference in California. His "back of the envelope" analysis shows that the concept has the potential of meeting the U.S. Department of Energy's long-term goal of producing hydrogen at about $2 per kilogram, roughly the equivalent of a gallon of gasoline. A hydrogen fuel cell is considerably more efficient than a gasoline engine, and would get double the fuel economy. And no emissions result from hydrogen produced from sunshine and water.

The next step, McConnell said, is to repeat the Australian company's demonstration on a small scale, to validate its findings. Once that's done, a variety of disciplines, including mechanical, chemical and electrical engineers and solar cell experts, will need to be brought together to further develop the technology.


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