New Reactor Paves the Way for Efficiently Producing Fuel from Sunlight

Solar energy has long been touted as the solution to our energy woes, but while it is plentiful and free, it can't be bottled up and transported from sunny locations to the drearier -- but more energy-hungry -- parts of the world. The process developed by Haile -- a professor of materials science and chemical engineering at the California Institute of Technology (Caltech) -- and her colleagues could make that possible.

The researchers designed and built a two-foot-tall prototype reactor that has a quartz window and a cavity that absorbs concentrated sunlight. The concentrator works"like the magnifying glass you used as a kid" to focus the sun's rays, says Haile.

At the heart of the reactor is a cylindrical lining of ceria. Ceria -- a metal oxide that is commonly embedded in the walls of self-cleaning ovens, where it catalyzes reactions that decompose food and other stuck-on gunk -- propels the solar-driven reactions. The reactor takes advantage of ceria's ability to"exhale" oxygen from its crystalline framework at very high temperatures and then"inhale" oxygen back in at lower temperatures.

"What is special about the material is that it doesn't release all of the oxygen. That helps to leave the framework of the material intact as oxygen leaves," Haile explains."When we cool it back down, the material's thermodynamically preferred state is to pull oxygen back into the structure."

Specifically, the inhaled oxygen is stripped off of carbon dioxide (CO₂) and/or water (H₂O) gas molecules that are pumped into the reactor, producing carbon monoxide (CO) and/or hydrogen gas (H₂). H₂can be used to fuel hydrogen fuel cells; CO, combined with H₂, can be used to create synthetic gas, or"syngas," which is the precursor to liquid hydrocarbon fuels. Adding other catalysts to the gas mixture, meanwhile, produces methane. And once the ceria is oxygenated to full capacity, it can be heated back up again, and the cycle can begin anew.

For all of this to work, the temperatures in the reactor have to be very high -- nearly 3,000 degrees Fahrenheit. At Caltech, Haile and her students achieved such temperatures using electrical furnaces. But for a real-world test, she says,"we needed to use photons, so we went to Switzerland." At the Paul Scherrer Institute's High-Flux Solar Simulator, the researchers and their collaborators -- led by Aldo Steinfeld of the institute's Solar Technology Laboratory -- installed the reactor on a large solar simulator capable of delivering the heat of 1,500 suns.

In experiments conducted last spring, Haile and her colleagues achieved the best rates for CO₂dissociation ever achieved,"by orders of magnitude," she says. The efficiency of the reactor was uncommonly high for CO₂splitting, in part, she says,"because we're using the whole solar spectrum, and not just particular wavelengths." And unlike in electrolysis, the rate is not limited by the low solubility of CO₂in water. Furthermore, Haile says, the high operating temperatures of the reactor mean that fast catalysis is possible, without the need for expensive and rare metal catalysts (cerium, in fact, is the most common of the rare earth metals -- about as abundant as copper).

In the short term, Haile and her colleagues plan to tinker with the ceria formulation so that the reaction temperature can be lowered, and to re-engineer the reactor, to improve its efficiency. Currently, the system harnesses less than 1% of the solar energy it receives, with most of the energy lost as heat through the reactor's walls or by re-radiation through the quartz window."When we designed the reactor, we didn't do much to control these losses," says Haile. Thermodynamic modeling by lead author and former Caltech graduate student William Chueh suggests that efficiencies of 15% or higher are possible.

Ultimately, Haile says, the process could be adopted in large-scale energy plants, allowing solar-derived power to be reliably available during the day and night. The CO₂emitted by vehicles could be collected and converted to fuel,"but that is difficult," she says. A more realistic scenario might be to take the CO₂emissions from coal-powered electric plants and convert them to transportation fuels."You'd effectively be using the carbon twice," Haile explains. Alternatively, she says, the reactor could be used in a"zero CO₂emissions" cycle: H₂O and CO₂would be converted to methane, would fuel electricity-producing power plants that generate more CO₂and H₂O, to keep the process going.

The work was funded by the National Science Foundation, the State of Minnesota Initiative for Renewable Energy and the Environment, and the Swiss National Science Foundation.

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ESA’s Mercury Mapper Feels the Heat

The Mercury Magnetospheric Orbiter (MMO) has survived a simulated voyage to the innermost planet. The octagonal spacecraft, which is Japan's contribution to BepiColombo, and its ESA sunshield withstood temperatures higher than 350°C.

This is a taste of things to come for the spacecraft. BepiColombo will encounter fully ten times the radiation power received by a satellite in orbit around Earth and, to simulate this, the Large Space Simulator (LSS) at ESA's ESTEC centre in the Netherlands had to be specially adapted.

Engineers talk about the power of the Sun in units called the solar constant. This is how much energy is received every second through a square metre of space at the distance of Earth's orbit.

"Previously, the LSS was capable of simulating a solar constant or two. Now it has been upgraded to produce ten solar constants," says Jan van Casteren, ESA BepiColombo project manager.

The improvements have been achieved in two ways: the lamps from the simulators are being used at their maximum power and the mirrors that focus the beam have been adjusted.'

Instead of producing a parallel beam of light 6 m across, they now concentrate the light into a cone just 2.7 m in diameter when it reaches the spacecraft. This creates a beam so fierce that a new shroud with a larger cooling capacity had to be installed to 'catch' the light that missed the spacecraft and prevent the chamber walls from heating up.

BepiColombo consists of separate modules. The MMO will investigate the magnetic environment of Mercury. It is kept cool during its six-year cruise to Mercury by the sunshield. These are the two modules that have now completed their thermal tests.

"The sunshield test was successful. Its function to protect the MMO spacecraft during the cruise phase was demonstrated," says Jan.

Once at Mercury, most of the Sun's fearsome heat will be prevented from entering BepiColombo by special thermal blankets. They consist of multiple layers including a white ceramic outer layer and several metallic layers to reflect as much heat as possible back into space.

"The tests allowed us to measure the thermal blanket's performance. The results allow us to prepare some adjustments for the tests of the Mercury Planetary Orbiter next year," says Jan.

In addition to enduring temperatures of 350°C, ESA's Mercury Planetary Orbiter (MPO) will go where no spacecraft has gone before: down into a low elliptical orbit around Mercury, of between just 400 km and 1500 km above the planet's scorching surface.

At that proximity, Mercury is worse than a hot plate on a cooker, releasing floods of infrared radiation into space. So, the MPO will have to deal with this as well as the solar heat. The MPO begins its tests in the LSS in the summer.

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Is the Hornet Our Key to Renewable Energy? Physicist Discovers That Hornet's Outer Shell Can Harvest Solar Power

"The interesting thing here is that a living biological creature does a thing like that," says physicist Prof. David Bergman of Tel Aviv University's School of Physics and Astronomy, who was part of the team that made discovery."The hornet may have discovered things we do not yet know." In partnership with the late Prof. Jacob Ishay of the university's Sackler Faculty of Medicine, Prof. Bergman and his doctoral candidate Marian Plotkin engaged in a truly interdisciplinary research project to explain the biological processes that turn a hornet's abdomen into solar cells.

The research team made the discovery several years ago, and recently tried to mimic it. The results show that the hornet's body shell, or exoskeleton, is able to harvest solar energy. They were recently published in the German journalNaturwissenschaften.

Discovering a new system for renewable energy?

Previously, entomologists noted that Oriental wasps, unlike other wasps and bees, are active in the afternoon rather than the morning when the sun is just rising. They also noticed that the hornet digs more intensely as the sun's intensity increases.

Taking this information to the lab, the Tel Aviv University team studied weather conditions like temperature, humidity and solar radiation to determine if and how these factors also affected the hornet's behavior, but found that UVB radiation alone dictated the change.

In the course of their research, the Tel Aviv University team also found that the yellow and brown stripes on the hornet abdomen enable a photo-voltaic effect: the brown and yellow stripes on the hornet abdomen can absorb solar radiation, and the yellow pigment transforms that into electric power.

The team determined that the brown shell of the hornet was made from grooves that split light into diverging beams. The yellow stripe on the abdomen is made from pinhole depressions, and contains a pigment called xanthopterin. Together, the light diverging grooves, pinhole depressions and xanthopterin change light into electrical energy. The shell traps the light and the pigment does the conversion.

A biological heat pump

The researchers also found a number of energy processes unique to the insect. Like air conditioners and refrigerators, the hornet has a well-developed heat pump system in its body which keeps it cooler than the outside temperature while it forages in the sun. This is something that's not easy to do, says Prof. Bergman.

To see if the solar collecting prowess of the hornet could be duplicated, the team imitated the structure of the hornet's body but had poor results in achieving the same high efficiency rates of energy collection. In the future, they plan to refine the model to see if this"bio-mimicry" can give clues to novel renewable energy solutions.

The research team also discovered that hornets use finely honed acoustic signals to guide them so they can build their combs with extraordinary precision in total darkness. Bees can at least see what they are doing, explains Prof. Bergman, but hornets cannot -- it's totally dark inside a hornet nest.

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