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Each year Technology Review honors 35 innovators under 35 who are tackling important problems in transformative ways. Candidates come from around the globe, some from regional TR35 competitions run by TR’s international editions.

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    David Bradwell

    Cheap, reliable batteries to store renewable energy.

    In the fall of 2007, David Bradwell, an MIT grad student, created a new kind of battery–one that might eventually be used to store massive amounts of solar and wind energy for use at night or when the wind isn’t blowing. Unlike existing batteries, it has active components that are liquid, which enables it to handle high currents without fracturing (the battery is kept at 700 degrees Celcius with the help of insulation). Last year Bradwell’s research attracted a total of about $11 million from the U.S. Department of Energy’s new Advanced Research Projects Agency-Energy (ARPA-E) and the French oil company Total.

    Bradwell’s battery is based on an electrolyte that can dissolve a compound consisting of two metals, such as magnesium and antimony. Applying a current in one direction splits the compound, and the two metals are deposited onto opposite electrodes. When no electricity is delivered, a voltage difference between the electrodes drives a current in the other direction. That generates electricity and causes the metals to recombine in the electrolyte.

    The system could eventually cost less than $100 per kilowatt-hour for a new installation–about the same as pumping water up a hill to be released later to spin a turbine (the cheapest conventional approach for large-scale energy storage), says Arun Majumdar, the director of ARPA-E. The battery, however, would have the advantage of working in places without hills or large amounts of water, where many renewable power resources are located.

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    Hany Eitouni

    Making safer batteries with solid polymers.

    Hany Eitouni has built batteries that are safer, longer-lasting, and able to store more energy in a smaller space than the conventional lithium-ion cells commonly used today. His technology, Eitouni says, could be used in next-generation electric cars and even in the electric grid, which would be a new application for lithium-ion batteries.

    While working at the Lawrence Berkeley National Lab, Eitouni figured out how to replace the most dangerous component of lithium-ion batteries: a flammable liquid electrolyte that conducts electricity between the positive and negative electrodes. The more energy packed into a battery, the higher the danger that the liquid electrolyte will catch fire. Previous researchers had tried to sidestep this problem by using gel polymers for the electrolyte, but even these contained flammable solvents.

    The solution was a solid material that is made of two linked polymer chains. One polymer is almost as conductive as a traditional liquid electrolyte but a lot less flammable; the other, which is also less flammable, provides mechanical stability so that the electrolyte doesn’t turn into goo. And the battery lasts longer than traditional lithium-ion or previous lithium-polymer cells because the polymer doesn’t react with the charged electrodes.

    To commercialize the technology, Eitouni cofounded Seeo in Berkeley, CA, in 2007. He says that the startup’s battery keeps 90 percent of its storage capacity after 2,000 charges (traditional rechargeable batteries lose nearly a third of their capacity after about 500 charges). It also stores 50 percent more energy per kilogram than commercial lithium-ion batteries. Seeo is building a pilot factory that will make large battery packs to smooth out spikes in supply and demand on the electric grid. It’s expected to be completed in 2011.

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    Peter Meinhold

    Engineering a better bug for biofuels.

    As a biofuel, ethanol is relatively easy to make, but it has a lower energy density than gasoline and can’t be transported through existing pipelines designed for petroleum fuels. Isobutanol, however, can be sent through these pipelines, and its energy density is close to that of gasoline. It can also be turned into jet fuel, and it can be used as a raw material for the manufacture of plastics and many other chemicals normally derived from petroleum.

    Both ethanol and isobutanol are made from sugars produced by breaking down biomass. But it’s not easy to produce isobutanol with the help of microbes like the ones that ferment those sugars into ethanol. So Peter Meinhold rewired the yeast genome, replacing genes that controlled ethanol fermentation with genes for a enzymatic pathway that would produce isobutanol. He cofounded Gevo in 2005 to commercialize the technology and produce isobutanol that would be cost-competitive with petroleum-based fuels.

    Gevo has also enhanced the isobutanol-producing capacity of its yeast by developing a system that continously removes isobutanol as it is produced. (Otherwise, high concentrations of isobutanol would inhibit the growth of the yeast.) The company is also developing new versions of the yeast that can feed on sugars produced from grasses and wood chips. In 2009, Gevo announced the startup of a million-gallon-per-year demonstration facility retrofitted into an ethanol plant. The company has set a goal of going to market by 2012.

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    Jochen Mundinger

    Reducing the carbon footprint of travel.

    Frustrated by trying to coördinate different modes of transport to get from Switzerland to a conference in Poland four years ago, Jochen Mundinger had an idea for a search engine that would find the fastest, cheapest, or most ecological way to get from A to B. Mathematically, what’s involved is a network optimization problem under a particular set of constraints–a perfect fit for Mundinger, who was trained as a network analyst and works at the Swiss Federal Institute of Technology in Lausanne. In 2007, he turned the idea into a company, RouteRank.

    Type in, say, “Basel” and “Munich” in ­RouteRank and the system will find not only flights but also trains, public transport, and driving routes–and let you combine them. A click of your mouse lets you sort the results according to what you consider most important, whether that’s price, travel time, or environmental considerations. RouteRank calculates the carbon dioxide emissions associated with each itinerary and lets you offset them by connecting you to Myclimate, a Swiss-based nonprofit foundation. Today RouteRank provides customized commercial service to Nokia, the conservation group WWF, and, most recently, the Swiss government. One surprising discovery, says Mundinger, is that the fastest way of getting somewhere isn’t always the least green. Next steps include expanding beyond Europe.

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    Lyndon Rive

    Leasing solar power.

    What will it take to get you to install solar panels on your roof? Lyndon Rive, solar’s master salesman, wants to know. Thanks to an innovative leasing program, among other sales enticements, SolarCity has become the largest residential solar installer in the United States. The company, which is based in Foster City, CA, has installed more than 8,000 solar systems since 2006. It tripled in size this year, and Rive, its CEO and cofounder, expects it to double next year.

    To reduce the high up-front costs for customers, Rive will lease homeowners the panels at a rate based on the size of their system. As the panels produce power, surplus electricity is sold to the local utility, and Rive says that those sales, combined with the savings from using less power from the grid, will typically reduce the homeowner’s electric bill by more than enough to offset the lease payments. He has hired a team to create software that can manage hundreds of thousands of solar projects in 1,000 jurisdictions, each with its own particular requirements. By saving “pennies here and pennies there,” he says, and increasing the volume of installations, Rive is driving down the costs of solar power. His hope is that solar will be able to survive without government subsidies in six to eight years.

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    Chris Rivest

    Printing cheaper solar cells.

    The lowest-cost solar panels on the market are made using thin-film solar cells that cost about 80 cents per watt of electricity they produce; costs for other types of cells can be as high as $2 per watt. Those prices are too high if solar power is to displace coal and natural gas. But Chris Rivest has a plan to reduce the price of solar cells to well under 50 cents per watt.

    Rivest cofounded SunPrint in 2008 to build cheaper solar cells using a process called acoustic printing, originally developed by Xerox for ink-jet printers. Focusing a sound wave onto a pool of ink causes droplets to spatter onto a nearby surface. Rivest and his cofounders designed and built an acoustic printer to deposit layers of ink containing cadmium telluride, one of the most cost-effective solar-cell materials available, on glass, plastic, or metal. Because acoustic printing provides finer control than other printing methods, the technique uses 50 percent less cadmium telluride and eliminates further processing steps that require expensive tools. Rivest expects commercial production of solar panels to begin within a year or so.