Volume 7, Issue 2
http://www.coe.berkeley.edu/labnotes/0307
Commuter-friendly version


Green Aluminum
by Paul Spinrad

In the Hall-Héroult process, aluminum is extracted from alumina in large cells charged with megawatt amounts of electrical current. Large aluminum plants run hundreds of pots.

"Twenty years ago, someone in China might drink beer from a barrel out of a mug," explains materials science and engineering professor James W. Evans, "Now they're going to drink out of an aluminum can."

World demand for aluminum is expected to continue rising, but aluminum production consumes enormous amounts of energy and generates significant greenhouse gas emissions. By outfitting an aluminum smelting plant with wireless sensors, Evans and his team, in collaboration with mechanical engineering professor Paul Wright, are exploring ways to reduce emissions while increasing efficiency. A project of CITRIS (Center for Information Technology Research in the Interest of Society), the effort demonstrates how information technology can reduce energy usage, hazardous emissions and overall environmental impact.

Aluminum production relies on an energy-intensive process called electrowinning, which extracts pure aluminum from alumina derived from mined bauxite. Electrowinning takes place in 3- by 10-meter electrolytic cells, which may number in the hundreds in a large plant. Each cell receives one megawatt or more of continuous DC, like charging a huge battery. Chemical reactions inside the cell free the elemental aluminum, which collects in a molten pool at the bottom of a 960°C salt electrolyte bath, where it is siphoned out.

Inside each cell, a solid salt crust naturally forms above the hot bath. Whenever this crust is breached, the cell emits greenhouse gases, including hydrogen fluorides (HFs) and perfluorocarbons (PFCs). The crust is broken intentionally and re-sealed with alumina during routine maintenance, but the greater environmental threat occurs when a crust cracks spontaneously and goes undetected. "Fairly frequently a cell will have a hole in its crust that the operator doesn't know about," Evans explains, "This allows atmospheric air to contact the molten salt electrolyte, which causes excess emissions until the problem is discovered."

Historically, the industry has let this pollution happen rather than expending the effort to inspect each pot. Monitoring never progressed past visual inspection because the pots' enormous electrical currents always made extra wiring a safety hazard.

"These pots cost $2 million to $3 million dollars each, and they were never instrumented, except for the one measurement of voltage," Evans notes. "My son's a pilot, and I asked him what kind of plane you could buy for that much. He said you could get a slightly used Learjet. You'd find all sorts of gauges in the cockpit of one of those, but these cells are still from the era of the Sopwith Camel."

Berkeley researchers Evans and Wright installed wireless temperature gauges on aluminum processing pots at an experimental plant. These "motes" transmitted readings to a central laptop via low-power FM, allowing the operator to see the pots' real-time status. Breaks in the crust showed up as sudden temperature jumps inside the pots' gas exit ducts; after the breaks were patched, temperatures returned to normal. Using this simple system, plant operators can fix breaks when they happen rather than let HFs and PFCs leak undetected for hours or days.

To keep things low maintenance, some motes ran off energy scavenged by thermoelectric generators (TEGs) rather than batteries that would need replacing. The TEGs generated current from the temperature differential between hot duct gases and the outside air. A finned metal heat sink radiated heat away on the cool side to boost power.

Such sensors now let researchers study and refine electrowinning in ways that have been impossible throughout the more than 100-year history of the process. "We have a real opportunity here," says Evans, "Using wireless sensors, we can finally instrument these things properly and figure out how to save energy. Current aluminum cells are only 40% to 45% efficient."

Wireless monitoring could improve other industries, Evans says, whether or not they have electric safety issues. The promise of streamlining large operations by bringing in a laptop and slapping on some magnetically attached sensors is irresistible. Evans, Wright and their team have experimented with wireless monitoring for copper production and are considering motes for wine barrels, industrial looms and paper mills.

"CITRIS has launched many projects for civil applications, like security, transportation and HVAC," explains Wright, "But industrial applications can also be improved. In fact, solving the power and environmental problems from industrial processing may be even more crucial for a green future in society. These issues grow hotter by the day."

Typical cargo ships carry 2,000 containers, each of which can hold 20 metric tons, and large ports unload multiple ships per day. Containers might carry almost anything, and their manifest documents, if they can be believed, are often vaguely labeled, for example, as "clothing."

Containerized shipping moves far more material across our borders than any other mode of transport. Each year, major ports in the United States handle thousands of cargo ships and millions of containers, and each container can carry up to 20 thousand kilograms of cargo. If just one or two kilos of highly enriched uranium were to make its way through this system, someone could use it to build a suitcase nuclear weapon. How do you detect the attempted smuggling of small amounts of fissionable material buried deep inside containers carrying lead bricks or any other legitimate cargo?

"We're looking for fissionable material rather than other radioactive material, which makes detection harder," explains nuclear engineering professor Eric B. Norman. "Highly radioactive Cobalt-60 or Cesium-137 can make a 'dirty bomb' that will scare people, but fissile materials like Uranium-235 and Plutonium-239 are both tougher to spot and far more potentially dangerous."

Norman's research group, a collaboration between UC Berkeley and Lawrence Livermore National Laboratory, uses a process called "active interrogation" to sniff out hidden fissile materials. Radioactive materials have traditionally been detected passively, as with Geiger counters, but active interrogation bombards the target with neutrons and then analyzes what comes out as a result. Previous methods measured the number of neutrons produced, but analyzing the resulting gamma rays provides a more detailed picture: A burst of gamma rays with very high energy levels (2.6 MeV and higher) indicates that the material is either fissile or heavy in sulfur or calcium. If this gamma radiation falls off with time, following a characteristic profile, that confirms that the material is fissile.

In practical terms, each cargo container must be tested in one minute to prevent backing up port operations. So Norman's group has prototyped a "nuclear car wash" at Livermore Lab that scans containers as they roll through on tracks. At the entrance, an underground particle accelerator shoots an upward stream of deuterium ions through a deuterium gas target. This creates helium and generates a beam of neutrons that shoots into the container. The level of neutron irradiation produced is comfortably below anything that could trigger a dangerous chain reaction in a fissionable target and, while it can briefly turn some cargo slightly radioactive, the effect soon dissipates.

"We haven't seen any showstoppers yet, and we aren't giving anyone a harmful dose of radiation," says Norman.

Flanking the container's path in the nuclear car wash are detector arrays with plastic scintillators that glow in response to gamma radiation. Photomultiplier tubes amplify this glow, and light detectors capture it for measurement.

Norman's group is testing the system on samples of highly enriched uranium (HEU) buried in big stacks of plywood, whose high hydrogen content characterizes many types of cargo and also absorbs neutrons, making detection more difficult. Conducting such experiments requires enormous administrative expertise in addition to scientific knowledge, since HEU samples are controlled by a complex and highly redundant system of security procedures and safeguards. Only one person on the team is authorized to physically handle the material, and all such handling must be logged. When the researchers want to move the sample into a different stack of plywood, they need to call the handler. All testing takes place at Livermore Lab, which is one of very few sites authorized to possess kilogram amounts of HEU.

Within the next year, Norman expects the Department of Homeland Security, which currently funds the research, to hold a Grand Challenge–style competition to decide which cargo scanning techniques should be fielded in U.S. ports. In the long run, multiple techniques will be used together to make up for each one’s weaknesses. If there's any doubt, a container will be shunted aside for detailed, open-lid inspection.

"We have to assume that the bad guys are as smart as we are," Norman explains, "and maybe they can fool the different techniques individually. But we can make them slip up trying to evade all of them at once. Only one of them has to work."

Women of Behrampada, a community on the outskirts of Mumbai. Students from Engineers for a Sustainable World—Berkeley educated Behram residents in water and sanitation issues, as part of the Haat MeN Sehat ("Health In Hand") project, an international effort which began in Behrampada.

Jayashri Srikantiah (B.S.’91 EECS) loves to ask what’s fair. Is it fair to imprison a Muslim man without due process? Is it fair to deport an undocumented Mexican woman who has testified against her husband for abusing her? As director of Stanford Law School’s Immigrants’ Rights Clinic, Srikantiah confronts these questions every day, helping law students protect the rights of noncitizens.

“I think immigrants’ rights are a major civil rights issue of our time,” she says. “I really connect with the immigrants’ rights movement, and that’s where I get the passion for my work. It informs everything I do, from working with students to bringing cases to court, to my own scholarship. It’s what makes me care so much about this field.”

Srikantiah and her family immigrated to San Jose from Bombay when she was a young girl. (She’s now a naturalized American citizen.) Though her family didn’t face legal challenges, she knows what it’s like to be an immigrant wanting to be treated like everyone else. But she never imagined herself as a lawyer.

“I enjoyed math and the technical side of things and had a great time as an EECS student at Berkeley,” Srikantiah says. “I also really enjoyed writing, debate and being involved in the South Asian community on campus. I minored in South Asian Studies.”

After graduating from Berkeley, Srikantiah worked at Intel as an electrical engineer. But she missed writing, she says, and decided to make it part of her career. Law school seemed a natural next step, so after two years at Intel, she enrolled in New York University’s School of Law.

Though no longer in a technical field, Srikantiah says her engineering training prepared her for the rigors of law school, from analyzing a subject to taking tests, to simply feeling confident that she could complete a law degree. It was after law school that she got her first exposure to immigration law. “When I was clerking on the Ninth Circuit, I saw a lot of immigration cases that were incredibly compelling, and I related to them on a personal level because I’m an immigrant myself.”

After graduating from law school in 1996, she joined a private law firm, and in 1998 took a staff attorney job at the American Civil Liberties Union (ACLU), working in its Immigrants’ Rights Project. Eventually, she became the associate legal director of the ACLU of Northern California. In 2004, she accepted the director’s position at Stanford Law School and launched the clinic the next year. Today, she supervises a dozen law students each semester who represent individual clients and work on broader advocacy projects. One of her clinic’s biggest successes so far is helping to secure a Supreme Court victory in Lopez v. Gonzales, a case dealing with the immigration consequences of a drug possession offense.

In fact, Srikantiah’s students often represent immigrants with less than clean records, people with old drug convictions who have served time and who find themselves in an immigration muddle. Srikantiah knows that these cases face major challenges, both legally and socially, but she views each case as an opportunity to learn compassion for every person, she says, even if the client has made mistakes. The ultimate goal, she believes, is to find the just solution.

“We get so much demand for our work that it can be overwhelming, but I love being in this world,” she says. “I don’t see myself anywhere else.”