Posts Tagged ‘Oak Ridge National Laboratory’
Friday, April 25, 2014 @ 11:04 AM gHale
Treating cadmium-telluride (CdTe) solar cell materials with cadmium-chloride improves their efficiency, but how that happens remained a mystery – until now.
After an atomic-scale examination of the thin-film solar cells, the light bulb has turned on and this decades-long debate about the materials’ photovoltaic efficiency increase after treatment appears solved.
A research team from led by the Department of Energy’s (DoE) Oak Ridge National Laboratory (ORNL), the University of Toledo and DoE’s National Renewable Energy Laboratory used electron microscopy and computational simulations to explore the physical origins of the unexplained treatment process.
Thin-film CdTe solar cells are a potential rival to silicon-based photovoltaic systems because of their theoretically low cost per power output and ease of fabrication. Their comparatively low historical efficiency in converting sunlight into energy, however, has limited the technology’s widespread use.
Research in the 1980s showed that treating CdTe thin films with cadmium-chloride significantly raises the cell’s efficiency, but scientists have been unable to determine the underlying causes. ORNL’s Chen Li, first author of a study on the subject, said the answer lay in investigating the material at an atomic level.
“We knew that chlorine was responsible for this magical effect, but we needed to find out where it went in the material’s structure,” Li said. “Only by understanding the structure can we understand what’s wrong in this solar cell — why the efficiency is not high enough, and how can we push it further.”
By comparing the solar cells before and after chlorine treatment, the researchers realized atom-scale grain boundaries ended up implicated in the enhanced performance. Grain boundaries are tiny defects that normally act as roadblocks to efficiency, because they inhibit carrier collection which greatly reduces the solar cell power.
Using state of the art electron microscopy techniques to study the thin films’ structure and chemical composition after treatment, the researchers found chlorine atoms replaced tellurium atoms within the grain boundaries. This atomic substitution creates local electric fields at the grain boundaries that boost the material’s photovoltaic performance instead of damaging it.
The research team’s finding, in addition to providing a long-awaited explanation, could help guide engineering of higher-efficiency CdTe solar cells. Controlling the grain boundary structure is a new direction that could help raise the cell efficiencies closer to the theoretical maximum of 32 percent light-to-energy conversion, Li said. Currently, the record CdTe cell efficiency is only 20.4 percent.
“We think that if all the grain boundaries in a thin film material could be aligned in same direction, it could improve cell efficiency even further,” Li said.
Friday, February 21, 2014 @ 09:02 AM gHale
A new suite of computer codes that closely model the behavior of neutrons in a reactor core, called neutronics, could give a more accurate way to analyze nuclear power reactors.
Technical staff at Westinghouse Electric Company, LLC, supported by the research team at the Consortium for Advanced Simulation of Light Water Reactors (CASL), used the Virtual Environment for Reactor Applications core simulator (VERA-CS) to analyze its AP1000 advanced pressurized water reactor (PWR). The testing focused on modeling the startup conditions of the AP1000 plant design.
“In our experience with VERA-CS, we have been impressed by its accuracy in reproducing past reactor startup measurements. These results give us confidence that VERA-CS can be used to anticipate the conditions that will occur during the AP1000 reactor startup operations,” said Bob Oelrich, manager of PWR Core Methods at Westinghouse. “This new modeling capability will allow designers to obtain higher-fidelity power distribution predictions in a reactor core and ultimately further improve reactor performance.”
The AP1000 reactor is an advanced reactor design with enhanced passive that builds on decades of Westinghouse’s experience with PWR design. The first eight units are currently under construction in China and the United States, and represent the first Generation III+ reactor to receive Design Certification from the U.S. Nuclear Regulatory Commission.
CASL is a U.S. Department of Energy (DoE) Innovation Hub established at Oak Ridge National Laboratory (ORNL), a part of DoE’s National Laboratory System. The consortium core partners are a strategic alliance of leaders in nuclear science and engineering from government, industry and academia.
“At CASL, we set out to improve reactor performance with predictive, science-based, simulation technology that harnesses world-class computational power,” said CASL Director Doug Kothe. “Our challenge is to advance research that will allow power uprates and increase fuel burn-up for U.S. nuclear plants. In order to do this, CASL is meeting the need for higher-fidelity, integrated tools.”
During the first generation of nuclear energy, performance and safety margins ended up at conservative levels as industry and researchers gained experience with the operation and maintenance of what was then a new and complex technology. Over the past 50 years, nuclear scientists and engineers gained a deeper understanding of the reactor processes, further characterizing nuclear reactor fuel and structure materials.
By making use of newly available computing resources, CASL’s research aims for a step increase in the improvements in reactor operations that have occurred over the last several decades.
“CASL has been using modern high-performance computing platforms such as ORNL’s Titan, working in concert with the INL Fission computer system, for modeling and simulation at significantly increased levels of detail,” said CASL Chief Computational Scientist John Turner. “However, we also recognized the need to deliver a product that is suitable for industry-sized computing platforms.”
With that understanding, CASL designed the Test Stand project to try out tools such as VERA-CS in industrial applications. CASL partner Westinghouse ended up selected as the host for the first trial run of the new VERA nuclear reactor core simulator (VERA-CS). Westinghouse chose a real-world application for VERA-CS: The reactor physics-analysis of the AP1000 PWR, which features a core design with several advanced features. Using VERA-CS to study the AP1000 provides information to further improve the characterization of advanced cores compared to traditional modeling approaches.
Westinghouse’s test run on VERA-CS focused on modeling one aspect of reactor physics called “neutronics,” which describes the behavior of neutrons in a reactor core. While neutronics is only one of VERA’s capabilities, the results provided by VERA-CS for the AP1000 PWR enhance Westinghouse’s confidence in their startup predictions and expand the validation of VERA by incorporating the latest trends in PWR core design and operational features.
The CASL team now is working on extending the suite of simulation capabilities to the entire range of operating conditions for commercial reactors, including full-power operation with fuel depletion and fuel cycle reload.
Wednesday, November 20, 2013 @ 04:11 PM gHale
By identifying fundamental forces that change plant structures during pretreatment processes used in the production of bioenergy, there may be more effective ways to convert woody plant matter into biofuels.
Pretreatment subjects plant material to extremely high temperature and pressure to break apart the protective gel of lignin and hemicellulose that surrounds sugary cellulose fibers.
“While pretreatments are used to make biomass more convertible, no pretreatment is perfect or complete,” said Department of Energy’s Oak Ridge National Laboratory’s (ORNL) Brian Davison, who was a coauthor of a research study and paper on the subject.
“Whereas the pretreatment can improve biomass digestion, it can also make a portion of the biomass more difficult to convert,” he said. “Our research provides insight into the mechanisms behind this ‘two steps forward, one step back’ process.” Also, pretreatment is the most expensive stage of biofuel production.
The team’s integration of experimental techniques including neutron scattering and X-ray analysis with supercomputer simulations revealed unexpected findings about what happens to water molecules trapped between cellulose fibers.
“As the biomass heats up, the bundle of fibers actually dehydrates — the water that’s in between the fibers gets pushed out,” said ORNL’s Paul Langan. “This is very counterintuitive because you are boiling something in water but simultaneously dehydrating it. It’s a really simple result, but it’s something no one expected.”
This process of dehydration causes the cellulose fibers to move closer together and become more crystalline, which makes them harder to break down.
In a second part of the study, the researchers analyzed the two polymers called lignin and hemicellulose that bond to form a tangled mesh around the cellulose bundles. According to the team’s experimental observations and simulations, the two polymers separate into different phases when heated during pretreatment.
“Lignin is hydrophobic so it repels water, and hemicellulose is hydrophilic, meaning it likes water,” Langan said. “Whenever you have a mixture of two polymers in water, one of which is hydrophilic and one hydrophobic, and you heat it up, they separate out into different phases.”
Understanding the role of these underlying physical factors — dehydration and phase separation — could enable scientists to engineer improved plants and pretreatment processes and ultimately bring down the costs of biofuel production.
“Our insight is that we have to find a balance which avoids cellulose dehydration but allows phase separation,” Langan said. “We know now what we have to achieve — we don’t yet know how that could be done, but we’ve provided clear and specific information to help us get there.”
Wednesday, October 30, 2013 @ 07:10 PM gHale
There could soon be new types of nuclear fuel pellets that would be safer in the event of a nuclear disaster.
New materials could end up encasing uranium-bearing fuel as an alternative to zirconium alloys, which have seen use as the outer layer of nuclear fuel pellets for the last 50 years, said a team of scientists from the University of Tennessee (UT) and Oak Ridge National Laboratory (ORNL), who will present their work at the AVS 60th International Symposium and Exhibition in Long Beach, CA.
Using sophisticated computer analyses, the UT and ORNL team identified the positive impact of several possible materials that exhibit resistance to high-temperature oxidation and failure, on reactor core evolution, thereby buying more time to cope in the event of a nuclear accident.
“At this stage there are several very intriguing options that are being explored,” said Steven J. Zinkle, the Governor’s Chair in the Department of Nuclear Engineering at the University of Tennessee and Oak Ridge National Laboratory. There is evidence that some of the new materials would reduce the oxidation by at least two orders of magnitude.
“That would be a game-changer,” he said. The materials examined include advanced steels, coated molybdenum and nuclear-grade silicon carbide composites (SiC fibers embedded in a SiC matrix).
The next step, he added, involves building actual fuel pins from these laboratory-tested materials and exposing them to irradiation inside a fission reactor. Once they perform as desired, the new fuel concepts would likely end up tested in a limited capacity in commercial reactors to then enable larger deployment possibilities.
Though it would take years before any new fuel concepts end up used commercially, given the rigorous and conservative qualification steps required, Zinkle said, these new materials may eventually replace the existing zirconium alloy cladding if they prove to be safer.
The typical core of a nuclear power plant uses the heat generated by fission of uranium and plutonium in fuel rods to heat and pressurize water. Steam then generates to drive steam turbines for electricity production. Water continuously circulates as a coolant to harness the thermal energy from the fuel and to keep the core from overheating.
The cooling pumps are a critical part of the reactor design because even when a nuclear reactor shuts down, the power it generates from radioactive decay of fission products remains at 1 percent of its peak for hours after shutdown. Given that nuclear power plants generate a staggering sum of energy under their nominal operating conditions (~4 GW of thermal energy), even one percent power levels after shutdown prove substantial. That’s why it’s essential to have cool water circulating continuously even after the shutdown occurs. Otherwise you risk overheating and ultimately melting the core – like leaving a pot boiling on the burner.
That’s basically what happened at Fukushima. On March 11, 2011, engineers at the plant managed to initially safely shut down the plant following a massive earthquake, but then a large tsunami knocked out the backup generators running the water pumps an hour later. What followed were explosions associated with hydrogen generated from the reduction of steam during high temperature oxidation of core materials, and releases of radioactive fission products. The accident displaced the local population and will take years and require a significant cost to clean-up.
Fukushima has had a profound impact on the safety culture of the industry, said Zinkle. Despite the fact that not a single U.S. nuclear power plant was unsafe or shut down in the wake of the accident, the Nuclear Regulatory Commission has issued new requirements to enhance their safety, including increasing the requirements for backup power generation on site.
Monday, August 27, 2012 @ 05:08 PM gHale
Knowing the position of missing oxygen atoms could be the key to cheaper solid oxide fuel cells with longer lifetimes.
That means new microscopy research could enable scientists to map these vacancies at an atomic scale.
Although fuel cells hold promise as an efficient energy conversion technology, they have yet to reach mainstream markets because of their high price tag and limited lifespans.
Overcoming these barriers requires a fundamental understanding of fuel cells, which produce electricity through a chemical reaction between oxygen and a fuel. As conducting oxygen ions move through the fuel cell, they travel through vacancies where oxygen atoms used to be. The distribution, arrangement and geometry of such oxygen vacancies in fuel cell materials should affect the efficiency of the overall device, researchers said.
“A big part of making a better fuel cell is to understand what the oxygen vacancies do inside the material: How fast they move, how they order, how they interact with interfaces and defects,” said Department of Energy’s Oak Ridge National Laboratory’s (ORNL) Albina Borisevich. “The question is how to study them. It’s one thing to see an atom of one type on the background of atoms of a different type. But in this case, you want to see if there are a few atoms missing. Seeing a void is much more difficult.”
ORNL scientists used scanning transmission electron microscopy to determine the distribution of oxygen vacancies in a fuel cell cathode material below the level of a single unit cell. The team verified its findings with theoretical calculations and neutron experiments at the lab’s Spallation Neutron Source.
“Even though the vacancy doesn’t generate any signal in the electron micrograph, it’s still a big disturbance in the structure,” Borisevich said. “You can see that the lattice expands where vacancies are present. So we tracked the lattice expansion around vacancies and compared it with theoretical models, and we were able to develop a calibration for this type of material.”
By providing a means to study vacancies at an atomic scale, the ORNL technique will help inform the development of improved fuel cell technologies in a systematic and deliberate fashion, in contrast to trial and error approaches.
Beyond its relevance to applications in fuel cells and information storage and logic devices, ORNL coauthor Sergei Kalinin said the team’s research is also building a bridge between two scientific communities that traditionally have had little in common.
“From my perspective, it is physics marrying electrochemistry,” Kalinin said. “The idea is that vacancies are important for energy, and vacancies are important for physics. The materials that physicists like to study are exactly the same as the materials used for fuel cells, and unless we understand how vacancies behave at interfaces, ferroic domain walls, and in thin films, we will not be able to fully appreciate the physics of these systems.”
Tuesday, May 15, 2012 @ 03:05 PM gHale
There is now a new way to accurately measure gas bubbles in pipelines.
In the end, it all comes down to safety as the ability to measure gas bubbles in pipelines is vital to the manufacturing, power and petrochemical industries.
In the case of harvesting petrochemicals from the seabed, warning of bubbles present in the harvested crude is crucial because otherwise when these bubbles come up from the seabed (where pressure is very high) to the surface where the rig is, the reduction in pressure causes these bubbles to expand and could cause a “blow out.” A blow out is the sudden release of oil and/or gas from a well and issues with the blow out preventer were key in Deepwater Horizon oil spill in the Gulf of Mexico in 2010.
Currently, the most popular technique for estimating the gas bubble size distribution (BSD) is to send sound waves through the bubble liquid and compare the measured attenuation of the sound wave (loss in amplitude as it propagates) with that predicted by theory.
The key problem is the theory assumes the bubbles exist in an infinite body of liquid. If in fact the bubbles are in a pipe, then the assumptions of the theory do not match the conditions of the experiment. That could lead to errors in the estimation of the bubble population.
There is now a new method, which takes into account that bubbles exist in a pipe, said Professor Tim Leighton from the Institute of Sound and Vibration Research at the University of Southampton, who is leading a project team.
Leighton and his team started the work as part of an ongoing program to devise ways of accurately estimating the BSD for the mercury-filled steel pipelines of the target test facility (TTF) of the $1.4 billion Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL), one of the most powerful pulsed neutron sources in the world.
The research explores how measured phase speeds and attenuations in bubbly liquid in a pipe might invert to estimate the BSD (independently measured using an optical technique). This new technique, appropriate for pipelines such as TTF, gives good BSD estimations if the frequency range is sufficiently broad.
“The SNS facility was built with the expectation that every so often it would need to be shut down and the now highly radioactive container of the mercury replaced by a new one, because its steel embrittles from radiation damage,” Leighton said. “However, because the proton beam impacts the mercury and generates shock waves, which cause cavitation bubbles to collapse in the mercury and erode the steel, the replacement may need to be more often than originally planned at full operating power. Indeed, achieving full design power is in jeopardy.
“With downtime associated with unplanned container replacement worth around $12 million, engineers at the facility are considering introducing helium bubbles, of the correct size and number, into the mercury to help absorb the shock waves before they hit the wall, so that the cavitation bubbles do not erode the steel. Oak Ridge National Laboratory (ORNL) and the Science and Facilities Research Council (Rutherford Appleton Laboratory, RAL) commissioned us as part of their program to devise instruments to check that their bubble generators can deliver the correct number and size of bubbles to the location where they will protect the pipelines from erosion.
“This paper reports on the method we devised half-way through the research contract. It works, but just after we designed it the 2008 global financial crash occurred, and funds were no longer available to build the device into the mercury pipelines of ORNL. A more affordable solution had to be found, which is what we are now working on. The original design has been put on hold for when the world is in a healthier financial state. This has been a fantastic opportunity to work with nuclear scientists and engineers from ORNL and RAL.”