By Neal Singer
The idea of building a solar collector out of pieces the size of glitter at first seems peculiar. How would the tiny pieces be joined together? How would electricity be harvested from each piece? The costs seem prohibitive to wire the back of each tiny cell so that electrons — converted from incoming photons — can be properly channeled.
Project lead Greg Nielson holds a solar cell test prototype with a microscale lens array fastened above it that together will help create a concentrated photovoltaic unit . (Photo by Randy Montoya)
Yet Sandia benchtop aggregations of exactly such tiny photovoltaic (PV) collectors have resulted in inexpensive and efficient electricity-generating cells that have aroused commercial interest.
The cells are fabricated of crystalline silicon, using microelectronic and microelectromechanical systems (MEMS) techniques.
Project lead investigator Greg Nielson (1749-2) says the research team has identified more than 20 benefits of scale for its microphotovoltaic cells. These include new applications, improved performance, potential for reduced costs, and higher efficiencies.
For large-scale power generation, says project participant Murat Okandan (1749-2), “One of the biggest scale benefits is a significant reduction in manufacturing and installation costs compared with current PV techniques.”
Part of the potential cost reduction comes about because microcells require relatively little material to form well-controlled and highly efficient devices.
From 14 to 20 micrometers thick (a human hair is approximately 70 micrometers thick), they are 10 times thinner than conventional 6-inch-by-6-inch, brick-sized cells, yet perform at about the same efficiency.
“So they use 100 times less silicon to generate the same amount of electricity,” says Murat. “Since they are much smaller and have less mechanical deformations for a given environment than the conventional cells, they may also be more reliable over the long term.”
Another manufacturing convenience is that the cells, because they are only hundreds of micrometers in diameter, can be fabricated from commercial wafers of any size, including today’s 300-millimeter (12-inch) diameter wafers and future 450-millimeter (18-inch) wafers. Further, if one cell proves defective in manufacture, the rest still can be harvested, while if a brick-sized unit goes bad, the entire wafer may be unusable. Also, brick-sized units fabricated larger than the conventional 6-inch-by-6-inch cross section to take advantage of larger wafer size would require thicker power lines to harvest the increased power, creating more cost and possibly shading the wafer. That problem does not exist with the small-cell approach and its individualized wiring.
Other unique features are available because the cells are so small. “The shade tolerance of our units to overhead obstructions is better than conventional PV panels,” says Greg, “because portions of our units not in shade will keep sending out electricity where a partially shaded conventional panel may turn off entirely.”
Because flexible substrates can be easily fabricated, high-efficiency PV for ubiquitous solar power becomes more feasible, says Murat.
“Eventually units could be mass-produced and wrapped around unusual shapes for building-integrated solar, tents, and maybe even clothing,” he says. This would make it possible for hunters, hikers, or military personnel in the field to recharge batteries for phones, cameras, and other electronic devices as they walk or rest.
Storage at the chip level
Even better, such microengineered panels could have circuits imprinted that would help perform other functions customarily left to large-scale construction with the attendant need for field construction design and permits.
Says Sandia field engineer Vipin Gupta (6338), “Photovoltaic modules made from these micro-sized cells for the rooftops of homes and warehouse could have intelligent controls, inverters, and even storage built in at the chip level. Such an integrated module could greatly simplify the cumbersome design, bid, permit, and grid integration process that our solar technical assistance teams see in the field all the time.”
A commercial move to microscale PV cells would be a dramatic change from conventional silicon PV modules composed of arrays of 6-inch-by-6-inch wafers. However, by bringing in techniques normally used in MEMS, electronics, and the light-emitting diode (LED) industries, the change to small cells should be relatively straightforward, Vipin says.
Each cell is formed on silicon wafers, etched, and then released inexpensively in hexagonal shapes, with electrical contacts prefabricated on each piece, by borrowing techniques from integrated circuits and MEMS.
Offering a run for their money to conventional large wafers of crystalline silicon, electricity presently can be harvested from the Sandia-created cells with 14.9 percent efficiency. Off-the-shelf commercial modules range from 13 to 20 percent efficient.
A widely used commercial tool called a pick-and-place machine — the current standard for the mass assembly of electronics — can place up to 130,000 pieces of glitter per hour at electrical contact points preestablished on the substrate; the placement takes place at cooler temperatures. The cost is approximately one-tenth of a cent per piece with the number of cells per module determined by the level of optical concentration and the size of the die, likely to be in the 10,000 to 50,000 cell per square meter range. An alternate technology, still at the lab-bench stage, involves self-assembly of the parts at even lower costs.
Solar concentrators — low-cost, prefabricated, optically efficient microlens arrays — can be placed directly over each glitter-sized cell to increase the number of photons arriving to be converted via the photovoltaic effect into electrons. The small cell size means that cheaper and more efficient short focal length microlens arrays can be fabricated for this purpose.
High-voltage output is possible directly from the modules because of the large number of cells in the array. This reduces costs associated with wiring, due to reduced resistive losses at higher voltages.
The project combines expertise from Sandia’s Microsystems Center, Photovoltaics and Grid Integration Department; the Materials, Devices, and Energy Technologies Group; and the National Renewable Energy Lab’s Concentrating Photovoltaics Group.
Involved in the process, in addition to Greg, Murat, and Vipin, are Jose Luis Cruz-Campa (1749-1), Paul Resnick (1749-1), Tammy Pluym (1746), Peggy Clews (1746), Carlos Sanchez (1746), Bill Sweatt (1512), Tony Lentine (1727), Anton Filatov (1749-1), Mike Sinclair (1816), Mark Overberg (1742), Jeff Nelson (6338), Jennifer Granata (6335), Craig Carmignani (6335), Rick Kemp (1815), Connie Stewart (1815), Jonathan Wierer (1123), George Wang (1126), Jerry Simmons (1120), Jason Strauch (1717), Judith Lavin (6338), and Mark Wanlass (NREL).
The work is supported by DOE’s Solar Energy Technology Program and Sandia’s LDRD program, and has been presented at four technical conferences this year.
The ability of light to produce electrons, and thus electricity, has been known for more than a hundred years.
Are there other applications beyond terrestrial solar power? “Absolutely!” says Greg. “We envision this technology impacting many areas of Sandia’s business including satellites and remote sensing, in addition to supporting warfighters.” -- Neal Singer
Engineers working on Sandia’s newest supercomputer have received some welcome recognition for their hard work: Red Sky made the November 2009 Top500 list as the 10th fastest computer in the world.
Sandia engineers achieved Red Sky’s top-10 performance by temporarily aggregating the Labs’ newest institutional machine with a second system being constructed using the same architecture and components.
SUPERFAST — KATHRYN CHAVEZ (9323) checks the status of systems that make up Sandia’s Red Sky supercomputer. Red Sky has made the Top500.org list as the 10th fastest supercomputer on the planet. Sandia engineers achieved Red Sky’s top-10 performance by temporarily aggregating Sandia’s newest institutional machine with a second system being constructed using the same architecture and components. Story on page 4. (Photo by Randy Montoya)
That second system, sponsored by the DOE’s Office of Energy Efficiency and Renewable Energy, will sit adjacent to Red Sky and be operated by Sandia to support work done at the National Renewable Energy Laboratory.
Red Sky achieved a peak performance of more than 500 teraflops (or 500 trillion mathematical operations per second), and an impressive 433.5 teraflops against the Linpack benchmark commonly used for ranking supercomputing speed.
In addition to raw horsepower, the Labs’ newest supercomputer has been designed to maximize its energy efficiency.
“Red Sky should really be called Green Sky,” says John Zepper (9320), senior manager of Computing Systems & Technology Integration. “This machine is the most energy-efficient high-performance system we have deployed to date.”
The system uses a newly designed power distributing system that significantly reduces power leakage and a unique cooling system that is more than 95 percent efficient in cooling the system’s multitude of computer racks.
“The Red Sky project has leveraged Sandia internal intellectual property and expertise in partnership with Intel and Sun Microsystems to deliver a leading-edge high-performance computing system,” says Rob Leland, director of Computing and Network Services Center 9300.
Red Sky uses a number of innovative technologies, such as interconnect switches designed jointly by Sandia and Sun. These switches were used to build the first implementation of a 3-D torus interconnect topology using InfiniBand networking. The system is also believed to be the first InfiniBand-based system that uses optical interconnect cables exclusively.
For computing, the system uses Sun’s latest high-density dual node computer blades, which come with Intel’s new Nehalem processors.
Red Sky began supporting a limited set of Sandia users this past June and is expected to be in full production this January. -- Stephanie Holinka
Environmentalists and car enthusiasts have been anxiously awaiting next year’s full-scale debut of electric cars that can run up to 40 miles on a single charge. But before those vehicles end up in your driveway, the lithium-ion batteries that power them will have been through some serious abuse — including being crushed, pounded with nails, and heated to boiling hot temperatures — to test the limits of what they can safely handle and provide critical scientific data for developing the next generation of batteries.
PETE ROTH examines a component undergoing tests at Sandia’s Battery Abuse Testing Laboratory. (Photo by Randy Montoya)
Sandia’s Battery Abuse Testing Laboratory (BATLab) has been at the forefront of this effort, doing everything imaginable to hybrid and plug-in electric hybrid batteries in the relatively safe confines of a lab to make sure that once they hit the road, they will provide safe and reliable transportation. And now, the BATLab is getting ready to offer more in-depth quantitative analysis as it prepares to beat up even more batteries.
During a Nov. 18 visit to be briefed on Sandia’s capabilities and programs, Deputy Secretary of Energy Daniel Poneman announced that Sandia’s BATLab will receive $4.2 million in stimulus funds to modify and enhance its existing facility. The funding is part of a $104.7 million American Recovery and Reinvestment Act package awarded to seven DOE national laboratories, to provide important technological insights to further develop the nation’s clean energy efforts.
“It’s so terribly important that we keep the nation’s work moving in this direction,” Poneman said. “We as a nation have relied on the national laboratories since the time of World War II, well over half a century, to keep this nation strong, to keep us at the cutting-edge of science, of innovation in the service of the nation, and in the service of the American people. The scientists and engineers who have been working all these years at Sandia are owed a great debt of gratitude by the American people for the tremendous progress they have made in keeping our nation safe.”
For years, the nation has relied on Sandia’s BATLab to test everything from regular small cells about the size of a laptop computer battery up to modules and packs weighing several hundred pounds for the DOE-funded FreedomCAR hybrid vehicle initiative. And while the BATLab team has been recognized for its ability to perform scientific analysis and a full range of measurements, members face a number of limitations.
“The equipment and facilities that we currently have allow us to do only one test at a time, so our throughput has been somewhat limited,” says Pete Roth (2546), lead researcher for Sandia’s FreedomCAR program. “The new equipment and upgrades that we will be able to implement will enhance the amount and range of testing and diagnostics that we can do, and we expect to at least be able to double our throughput.”
Those upgrades include fire suppression, improved lighting, and advanced electrical systems, as well as new software and analytical equipment to help diagnose battery responses and provide data for manufacturers.
Such improved efficiency will allow Sandia to continue to build on its past success, and offer increasingly valuable contributions to the nation’s FreedomCAR effort.
“Pete and his team are already internationally recognized for this work, and this funding will help us to sustain that leadership position into the future as the auto manufacturers start to implement these lithium battery modules and packs into their vehicles,” says Tom
Wunsch (2546), manager of Advanced Power Sources R&D Dept. 2546.
“This funding is an answer to a lot of our hopes and aspirations for where this program could go. Even though we’ve had programmatic support, we’ve been getting by on a shoestring in terms of facilities and equipment support,” Pete says. “I think this is going to take us into a whole new regime of conducting the
science and testing we’ve always dreamed of doing. It’s going to be very beneficial.”. -- Stephanie Hobby