Hydrogen from Solar Panels

While roofs across the world sport photovoltaic solar panels to convert sunlight into electricity, a Duke University engineer believes a novel hybrid system can wring even more useful energy out of the sun's rays.

nstead of systems based on standard solar panels, Duke engineer Nico Hotz proposes a hybrid option in which sunlight heats a combination of water and methanol in a maze of glass tubes on a rooftop. After two catalytic reactions, the system produces hydrogen much more efficiently than current technology without significant impurities. The resulting hydrogen can be stored and used on demand in fuel cells.

For his analysis, Hotz compared the hybrid system to three different technologies in terms of their exergetic performance. Exergy is a way of describing how much of a given quantity of energy can theoretically be converted to useful work.

"The hybrid system achieved exergetic efficiencies of 28.5 percent in the summer and 18.5 percent in the winter, compared to 5 to 15 percent for the conventional systems in the summer, and 2.5 to 5 percent in the winter," said Hotz, assistant professor of mechanical engineering and materials science at Duke's Pratt School of Engineering.

The paper describing the results of Hotz's analysis was named the top paper during the ASME Energy Sustainability Fuel Cell 2011 conference in Washington, D.C. Hotz recently joined the Duke faculty after completing post-graduate work at the University of California-Berkeley, where he analyzed a model of the new system. He is currently constructing one of the systems at Duke to test whether or not the theoretical efficiencies are born out experimentally.

Hotz's comparisons took place during the months of July and February in order to measure each system's performance during summer and winter months.

Like other solar-based systems, the hybrid system begins with the collection of sunlight. Then things get different. While the hybrid device might look like a traditional solar collector from the distance, it is actually a series of copper tubes coated with a thin layer of aluminum and aluminum oxide and partly filled with catalytic nanoparticles. A combination of water and methanol flows through the tubes, which are sealed in a vacuum.

"This set-up allows up to 95 percent of the sunlight to be absorbed with very little being lost as heat to the surroundings," Hotz said. "This is crucial because it permits us to achieve temperatures of well over 200 degrees Celsius within the tubes. By comparison, a standard solar collector can only heat water between 60 and 70 degrees Celsius."

Once the evaporated liquid achieves these higher temperatures, tiny amounts of a catalyst are added, which produces hydrogen. This combination of high temperature and added catalysts produces hydrogen very efficiently, Hotz said. The resulting hydrogen can then be immediately directed to a fuel cell to provide electricity to a building during the day, or compressed and stored in a tank to provide power later.

The three systems examined in the analysis were the standard photovoltaic cell which converts sunlight directly into electricity to then split water electrolytically into hydrogen and oxygen; a photocatalytic system producing hydrogen similar to Hotz's system, but simpler and not mature yet; and a system in which photovoltaic cells turn sunlight into electricity which is then stored in different types of batteries (with lithium ion being the most efficient).

"We performed a cost analysis and found that the hybrid solar-methanol is the least expensive solution, considering the total installation costs of $7,900 if designed to fulfill the requirements in summer, although this is still much more expensive than a conventional fossil fuel-fed generator," Hotz said.

Costs and efficiencies of systems can vary widely depending on location -- since the roof-mounted collectors that could provide all the building's needs in summer might not be enough for winter. A rooftop system large enough to supply all of a winter's electrical needs would produce more energy than needed in summer, so the owner could decide to shut down portions of the rooftop structure or, if possible, sell excess energy back to the grid.

"The installation costs per year including the fuel costs, and the price per amount of electricity produced, however showed that the (hybrid) solar scenarios can compete with the fossil fuel-based system to some degree," Hotz said. 'In summer, the first and third scenarios, as well as the hybrid system, are cheaper than a propane- or diesel-combusting generator."

This could be an important consideration, especially if a structure is to be located in a remote area where traditional forms of energy would be too difficult or expensive to obtain.

Hotz's research was supported by the Swiss National Science Fund. Joining him in the study were UC-Berkeley's Heng Pan and Costas Grigoropoulos, as well as Seung H. Ko of the Korea Advanced Institute of Science and Technology, Daejon.

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Rare Earth Metals May Trigger Wars

    Underneath these salt flats in Uyuni, Bolivia, lies one of the world's largest lithium reserves.

 

• Alternative energy is not the cure for energy security.
• A handful of countries, including China, dominate the markets for many rare earth metals.
• More domestic mining and new technologies for extracting the useful metals are needed. 

Breaking the fossil fuel addiction has a lot of nice benefits, but increasing energy security is not one of them, say researchers studying supply and demand of scarce metals used in making solar panels, wind generators and other alternative energy technologies.

There is a long list of elements, mostly rare metals, that are currently mined only in a handful of countries. Without them a lot of new technologies would be stopped in their tracks. What's needed are new sources, which means more mining and better technologies for extracting the useful metals from ores.

"We are almost completely dependent on imports," said geologist James Burnell of the Colorado Geological Survey. "Trade wars are developing with the rare earth elements."

Burnell is slated to present a paper about the resource demands of alternative energy technologies on Nov. 2 at the annual meeting of the Geological Society of America in Denver.

Elements such as gallium, indium, selenium, tellurium and high-purity silicon are needed to make photovoltaic panels. For high capacity batteries like those used in hybrid and electric cars, manufacturers need zinc, vanadium, lithium and rare earth elements. Fuel cells require platinum group minerals.

One of the world's biggest suppliers and consumers of scarce metals game is none other than China, said Burnell. They are already beginning to throw their weight around. A possible sign of what's ahead for many important elements may be China's recent announcement about the element indium, which is used to make flat panel displays. China supplies the world with indium, Burnell said.

"They put the world on notice that they will stop exporting indium in the future," said Burnell.

Another strategic element that China could soon stop exporting is neodymium, which is used to make high-strength magnets for gearless wind generators. China is planning on building 330 giga-watts of wind generator capacity within its own borders. That will require more neodymium than they currently export, Burnell said.

Other big players are Chile and Argentina, which supply the Western world with lithium, cobalt and manganese.

"The bottom line is that we really have to look for more," said Burnell. There is a disconnect in the public mind about alternative energy tech and the mining required to get the elements needed for those technologies, he added.

"There are 30 pounds of rare earth metals in a Prius," said Burnell. Those have to be mined somewhere and if they are not mined domestically, there are energy security issues.

Among the countries that are particularly concerned about by China's announced slowdown in exporting some rare earth elements is Japan, said Yasushi Watanabe of the Institute for Geo-Resources and Environment in Tsukuba, Japan. Watanabe is also scheduled to present a paper on the matter at the same meeting.

Among the things Watanabe is looking at are the sorts of rocks that need to be explored to find new sources in other countries. There will also be a great need to find new ways to extract the valuable metals from different ores in which they are found, he said.

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Early Life Crippled By Natural Nukes

Ancient nuclear reactors buried in lake and shallow ocean sediments may have cooked early microbes, according to a new study. And radiation from the deposits could have delayed the onset of our modern-day, oxygen-rich atmosphere, and even had a hand in shaping the genetics of primordial life.

Natural nuclear reactors dating to 2 billion years ago have been found in Gabon, Africa. Though long since exhausted, scientists know from the unusually low quantity of the Uranium-235 isotope in the rock that they once went critical, and hosted a sustained fission reaction that went on for as long as two hundred thousand years.

A billion years earlier, such deposits could have been common, say Laurence Coogan and Jay Cullen of the University of Victoria. The first oxygen-producing bacteria colonized lakes and shallow seas, and likely created oxygen 'oases' in an otherwise nitrogen-dominated world.

"Oxygen oases would have been hot spots for uranium concentration," Cullen said, because oxygen dissolved in water would draw uranium out of rocks and sediments. "Back then, there was so much more 235U that a softball-sized chunk of uranium would be enough for it to go critical."

If the researchers are right, wherever there were oxygen-producing bacteria, there were also natural nuclear reactors. Radiation could have damaged the bugs' DNA, either directly from the reactors or as leftover atoms of radioactive strontium (Sr) and iodine (I) made their way into the food chain.

 Igneous rocks on Iceland. Ancient nuclear reactors buried in lake and shallow ocean sediments may have cooked early microbes, according to a new study. (image right)

In short, organisms that produced oxygen 3 billion years ago were shooting themselves in the foot by spawning toxic nuclear reactors. That may explain why it wasn't until around 2.3 billion years ago that oxygen finally started building up in the atmosphere. By then, Cullen said, most of the readily available nuclear fuel was used up.

However, it's also possible the reactors had a positive effect on early life.

"Modern cyanobacteria are quite good at dealing with ionizing radiation," Cullen said. "The question you have to     ask is, 'Why?' Well, maybe there was some selective pressure back then that forced them to develop that resistance."

The researchers' work was published in the latest issue of the journal GSA Today.

Radiation is harmful because it causes uncontrolled mutations in organisms' DNA. But mutation is also the engine of evolution. Cullen said it's possible that natural nuclear reactors may have molded the genetic makeup of early life forms. 

"There is no doubt that sources of radiation from geology, the sun, or cosmic rays will definitely cause mutation, and they were almost certainly all higher back then," Paul Falkowski of Rutgers University said.

One way to test that model might be to test ancient rocks for concentrations of lead (Pb) that would indicate whether or not natural nuclear reactors were common in antiquity.

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