Solar heat could help generate both electricity and hydrogen fuel at the same time in a system that scientists in Switzerland and the United States call "hydricity." Such a system could supply electricity round-the-clock with an overall efficiency better than many photovoltaic cells, researchers add.
There are two ways solar energy is used to generate electricity. Photovoltaic cells directly convert sunlight to electricity, while solar thermal power plants—also known as concentrating solar power systems—focus sunlight with mirrors, heating water and producing high-pressure steam that drives turbines.
Photovoltaic cells only absorb a portion of the solar spectrum, but they can generate electricity from both direct and diffuse sunlight. Solar thermal power plants can use more wavelengths of the solar spectrum, but they can only operate in direct sunlight, limiting them to sun-rich areas. Moreover, the highest conversion efficiencies reported yet for solar thermal power plants are significantly less than those for photovoltaic cells.
Scientists now suggest that coupling solar thermal power plants with hydrogen fuel production facilities could result in "hydricity" systems competitive with photovoltaic designs.
Today’s solar thermal power plants operate at temperatures of up to roughly 625 degrees C. However, the researchers noted that solar thermal power plants are more efficient at higher temperatures. What’s more, when they reach temperatures above 725 degrees C they can split water into it’s constituents, hydrogen and oxygen.
An integrated "hydricity" system would produce both steam for generating electricity and hydrogen for storing energy. And each makes the other more efficient. Set to produce hydrogen alone, its production efficiency approaches 50 percent, the researchers claim. This is because the high-pressure steam the system generates can easily be used to pressurize hydrogen. The substantial amount of power needed to compress hydrogen fuel for later transport and use is often neglected when it comes to calculating hydrogen production efficiency.
Furthermore, this new solar thermal energy design can generate electricity with standalone efficiencies approaching up to an unprecedented 46 percent, researchers say. This is because the high-temperature steam leaving high-pressure turbines can run a succession of lower-pressure turbines, helping make the most of the solar thermal energy the system collects.
Moreover, the hydrogen fuel the system generates can be burned to generate electricity after nightfall, for round-the-clock power. The researchers say the efficiency of this hydrogen-to-electricity system could reach up to 70 percent, comparable to the highest reported hydrogen fuel cell efficiencies.
Altogether, the researchers say the sun-to-electricity efficiency of hydricity, averaged over a 24-hour cycle, might approach roughly 35 percent, nearly the efficiency attained using the best multijunction photovoltaic cells combined with batteries. In addition, they note that the hydrogen fuel the system produces could find use in transportation, chemical production, and other industries. Finally, unlike batteries, stored hydrogen neither discharges over time nor degrades with repeated use.
The scientists at Purdue University in West Lafayette, Ind., and the Federal Polytechnic School of Lausanne in Switzerland detailed their findings online 14 December in the journal Proceedings of the National Academy of Sciences.
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.