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neuraminidase. In our simulations, we will be able to use neuraminidase inhibitors as both treatment and prophylaxis. (A vaccine against H5N1 has been developed and recently began clinical trials but because the vaccine is not yet proven or available, we will focus our simulations on seeing whether the antiviral drugs together with traditional public health measures might stop an epidemic.)

Preliminary results announced in late February are reported at www.sciam. com. In April, we will complete similar flu pandemic simulations in the EpiSims Portland model.

Our hope is that the ability to realistically model populations and disease outbreaks can help health officials make difficult decisions based on the best possible answers to "what if" questions.

The creation of models such as TR ANSIMS that simulate human movements through urban environments was the computational breakthrough that made EpiSims possible, and epidemiology is only one potential application for this kind of individual-based modeling. We are also in the process of creating and linking simulations of other sociotechni-cal systems, including environmental and atmospheric pollution, telecommunications, transportation, commodity markets, water supplies and power grids, to provide virtual laboratories for exploring solutions to a wide variety of real-world problems. ®

Scalable, Efficient Epidemiological Simulation. Stephen Eubank in Proceedings of the 2002

ACM Symposium on Applied Computing, pages 139-145; 2002.

Six Degrees: The Science of a Connected Age. Duncan J. Watts. W. W. Norton, 2004.

Containing Pandemic Influenza with Antiviral Agents. Ira M. Longini, Jr., et al. in American

Journal of Epidemiology, Vol. 159, No. 7, pages 623-633; April 1, 2004.

Modelling Disease Outbreaks in Realistic Urban Social Networks. Stephen Eubank et al. in

A sample EpiSims animation and additional data from the Portland smallpox simulations can be viewed at

The actual response chosen made little difference compared with the time element.




ie automated speed traps that ward the approaches to Nabern, Germany, seem to be the only things that can wipe the smile off Rosario Berretta's face. "Please slow down here," he murmurs darkly as our vehicle nears the outskirts of the picturesque Swabian village. Berretta leads a team that is preparing a fleet of 60 of Daim-lerChrysler's latest hydrogen fuel-cell car, the F-Cell, for testing worldwide. The aim is to allow automakers to evaluate the pollution-free, energy-efficient vehicles in diverse driving conditions. The curly-headed engineer is eager for visitors to try out the F-Cell's quick pickup off the line, one of the benefits of having an electric motor under the hood. But such maneuvers have to wait until the sharp eyes of the camera traps get small in the rearview mirror.

Despite its high-tech propulsion system, the F-Cell looks, performs and handles like a Toyota Corolla, a Ford Focus or any other conventional small car. Thus, the F-Cell seems less like a next-generation prototype and more like a real-world car. The sole clue of anything out of the ordinary is the unfamiliar whir of a compressor—a noise that Berretta vows company engineers will soon muffle.

DaimlerChrysler is not alone in its quest for the ultimate clean vehicle. After a decade of focused research and development, the auto industry worldwide has passed a milestone with the arrival of the first test fleets of seemingly roadworthy fuel-cell cars. Twenty of Honda's latest FCX and 30 of Ford's Focus FCV fuel-cell-powered compacts will soon be on the highways. General Motors plans to provide 13 fuel-cell-powered vehicles to the New York City metropolitan area for evaluation next year. Already 30 Daim-lerChrysler-built fuel-cell buses are plying the streets of 10 European cities, and three more will shortly each be servicing Beijing and Perth.

Meanwhile nearly every other car company—particularly, Toyota but also Nissan, Renault, Volkswagen, Mitsubishi and Hyun-

Although fleets of fuel-cell prototypes are hitting the streets, basic technical and market obstacles must be hurdled before the clean, hydrogen-powered cars reach dealer showrooms

By Steven Ashley dai, among others—is operating at least a few prototype vehicles as well, one indication of the substantial funds carmakers are investing to perfect the technology. Today between 600 and 800 fuel-cell vehicles are reportedly under trial across the globe. And suppliers have emerged to develop and provide the components needed to build the prototypes. If all goes well, these developments will mark a midway milestone on the road to the initial commercialization of the fuel-cell car by the early part of the next decade.

Faced with ever tighter governmental regulatory limits on exhaust emissions, forecasts of impending oil shortages and a potential global warming catastrophe caused by greenhouse gases, the motor vehicle industry and national governments have invested tens of billions of dollars during the past 10 years to bring to reality a clean, efficient propulsion technology that is intended to replace the venerable internal-combustion (IC) engine [see "Vehicle of Change," by Lawrence D. Burns, J. Byron McCormick and Christopher E. Borroni-Bird; Scientific American, October 2002]. Critics, however, still question the industry's actual interest in producing a truly green car and whether this R&D effort is really enough to yield success anytime soon. Suspicions linger that work on fuel-cell vehicles is a smokescreen intended to shield business as usual long into the future. Car company executives reply that they foresee no better option to the hydrogen fuel-cell vehicle in the long run, because all alternatives, such as hybrid vehicles (which combine IC engines with electrochemical batteries), still burn petrochemical fuels and produce carbon dioxide and pollutants.

Overview/Green Machines

Stumbling Blocks a two-hour drive, say, the 140 or so miles from Nabern to Frankfurt am Main on the German autobahn, would be enough to reveal the most telling distinction between the F-Cell and your typical IC engine car. In something less than 90 minutes, you would be stuck on the roadside out of fuel and with little prayer of finding a fill-up. Neither the F-Cell nor any of its hydrogen-powered kindred carries enough fuel to get anywhere near the 300-mile minimum driving range that car owners expect. And because hydrogen service stations are still few and far between, refueling would be problematic at best. So despite bright hopes and the upbeat pronouncements by automakers, considerable technical and market challenges remain that could delay introduction of the fuel-cell family car for years, if not decades.

Before early adopters can trade in their Toyota Priuses and Honda Accord Hybrids for something even greener, car manufacturers and their suppliers must somehow figure out how to do several things: boost onboard hydrogen storage capacity substantially, cut the price tags of fuel-cell drive trains to a hundredth of the current costs, increase the power plants' operating lifetimes fivefold, and enhance their energy output for SUVs and other heavy vehicles. Finally, to operate these vehicles, a hydrogen fueling infrastructure will be required to replace the international network of gas stations.

Even some of the automakers remain unconvinced that all this will happen soon: "High-volume production could be 25 years off," says Bill Reinert, national manager for Toyota's advanced technology group. "I'm less than hopeful about reducing costs sufficiently, and I'm quite pessimistic about solving hydrogen storage issues and packaging these large systems in a marketable vehicle." One telling sign that fuel-cell vehicles are still works in progress: nearly all car company representatives call for more government investment in basic research and hydrogen distribution systems to help overcome these roadblocks.

Stack Issues a fuel-cell car, bus or truck is essentially an electric vehicle powered by a device that operates like a refuelable battery. Unlike a battery, though, a fuel cell does not store energy; it uses an electrochemical process to generate electricity and will run as long as hydrogen fuel and oxygen are fed to it [see box on page 66].

At the core of the automotive fuel cell is a thin, fluorocar-bon-based polymer—a proton-exchange membrane (PEM)— that serves as both the electrolyte (for charge transport) and a physical barrier to prevent mixing of the hydrogen fuel and the oxygen. Electricity for powering a fuel-cell car is produced when electrons are stripped from hydrogen atoms at catalysis sites on the membrane surface. The charge carriers—hydrogen ions or protons—then migrate through the membrane and combine with oxygen and an electron to form water, the only exhaust produced. Individual cells are assembled into what are called stacks.

Engineers chose PEM fuel cells because they convert up to

■ The motor vehicle industry recently passed a milestone when it fielded test fleets of reasonably practical fuel-cell cars some 10 years after the first prototypes hit the road. During that period, carmakers and governments spent several tens of billions of dollars on research and development, but much more will be needed before initial commercialization can take place.

■ Despite stricter pollution limits, potential oil shortages and the threat of global warming, volume production of fuel-cell vehicles is not expected before midway through the next decade and, perhaps, much later.

■ Significant improvements in onboard hydrogen storage capacity, fuel-cell durability and power as well as substantially lower costs will be required before fuel-cell cars can approach marketability. A hydrogen production and distribution system must also be built.

Tens of billions of dollars have been spent on fuel-cell vehicles during the past 10 years.

Hydrogen Fuel-Cell Prototype

Air pump

Fuel-ce system radiator

Air pump

Fuel-ce system radiator

Electric motor

Hydrogen tanks

Electric motor

Hydrogen tanks

Power-train radiator

Honda's 2005 FCX model is typical of current hydrogen fuel-cell technology. The four-seat compact, which has a top speed of 93 miles per hour, offers a driving range greater than 200 miles. Equivalent fuel economy is 62 miles per gallon in city driving and 51 mpg on the highway. The FCX's fuel-cell stack, which was designed by Honda for low-cost

Fuel-cell stack — Fuel-cell system box Power-control unit manufacturing, features a hydrocarbon polymer membrane that offers improved durability. An ultracapacitor—a device that stores energy in the fields between electrically charged plates—provides extra power during passing maneuvers or hill climbing. Reclaimed energy from a regenerative braking system is stored by the ultracapacitor.

55 percent of the fuel energy put into them into work output; the efficiency figure for an IC engine is approximately 30 percent. Other benefits include relatively low-temperature operation (80 degrees Celsius); reasonably safe, quiet performance; easy operation; and low maintenance requirements.

The prospect of a commercial fuel-cell car by 2015 will depend on improvements in membrane technology, which makes up as much as 35 percent of the cost of a fuel-cell stack. Researchers list several needed enhancements such as low fuel crossover from one side of a membrane to the other, augmented chemical and mechanical stability of the membrane for greater durability, control over undesired by-reactions, and higher tolerance to contamination by fuel impurities or from unwanted reaction by-products such as carbon monoxide. Most of all, what is required is an across-the-board reduction in costs.

News of a "breakthrough" in membrane technology created a considerable stir in fuel-cell research circles last fall. PolyFuel, a small company in Mountain View, Calif., announced that it had created a hydrocarbon polymer membrane that it says offers superior performance and lower costs than current perfluorinated membranes. "It looks like a piece of sandwich wrap," James Balcom says, chuckling. The Poly-Fuel chief executive boasts a variety of reasons why his cellophanelike film performs better than the more common per fluorinated membranes, notably DuPont's Nafion material. The hydrocarbon membrane can run at higher temperatures than current membranes—up to 95 degrees C, which allows the use of smaller radiators to dissipate heat. It lasts 50 percent longer than fluorocarbon versions, he claims, while generating 10 to 15 percent more power and operating at lower (less troublesome) humidity levels. And whereas fluorocarbon membranes cost about $300 per square meter, the PolyFuel materials potentially cost half the price [see box on next page]. Although many other researchers remain skeptical about hydrocarbon membranes, Honda's newest FCX fuel-cell cars incorporate them.

Catalyst Conundrum the other key to the operation of a PEM membrane is the thin layer of platinum-based catalyst that coats both of its sides and that represents 40 percent of the stack cost. The catalyst prepares hydrogen (from the fuel) and oxygen (from the air) to take part in an oxidation reaction by assisting both molecules to split, ionize, and release or accept protons and electrons. On the hydrogen side of the membrane, a hydrogen molecule (containing two hydrogen atoms) must attach to two adjacent catalyst sites, thereby freeing positive hydrogen ions (protons) to travel across the membrane. The complex reaction on the oxygen side occurs when a hydrogen ion and an electron mate with oxygen to produce water. This latter sequence must be finely controlled because it can yield destructive by-products such as hydrogen peroxide, which degrades fuel-cell components.

Because of the high cost of the precious metal ingredients, researchers are searching for ways to lower the platinum content. Their efforts include not only finding methods to raise the activity of the catalyst so less can be used for the same power output but also determining how to form a stable catalyst structure that does not degrade over time and avoiding side reactions that contaminate the membrane. One recent success in boosting catalytic activity was achieved by 3M Corporation researchers, who created nanotextured membrane surfaces covered with "forests of tiny columns" that significantly increased the catalysis area. Other work has concentrated on materials ranging from nonprecious metal catalysts such as cobalt and chromium to catalysts consisting of fine dispersions of particles embedded in porous composite structures.

Onboard Storage one of the biggest worries among proponents of fuel-cell vehicles is how engineers will manage to stuff enough hydrogen onboard to provide the driving range that consumers demand. Five to seven kilograms will take a car up to 400 miles, but current fuel-cell prototypes hold from 2.5 to 3.5 kilograms. "Nobody really knows how to store twice that amount in a reasonable volume," says Dennis Campbell, chief executive of Ballard Power Systems in Vancouver, the dominant fuel-cell-stack maker.

Typically hydrogen is stored in pressure tanks as a highly compressed gas at ambient temperature. Many engineering teams are working on doubling the pressure capacity of today's 5,000-psi (pounds per square inch) composite pressure tanks. But twice the pressure does not increase the storage twofold. Liquid-hydrogen systems, which store the fuel at temperatures below -253 degrees C, have been tested successfully

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