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  • Simon Simard
  • Sustainable Energy

    A powerful new battery could give us electric planes that don’t pollute

    A manufacturing trick with magnetic fields produces a battery that may discharge fast enough to get an aircraft off the ground.

    • by James Temple
    • October 30, 2018
    • CMU mechanical engineering professor Venkat Viswanathan and MIT materials science professor Yet-Ming Chiang (right) are developing a new battery specifically designed for an advanced hybrid plane.

    Brightly colored molecular models line two walls of Yet-Ming Chiang’s office at MIT. Chiang, a materials science professor and serial battery entrepreneur, has spent much of his career studying how slightly different arrangements of those sticks and spheres add up to radically different outcomes in energy storage.

    But he and his colleague, Venkat Viswanathan, are taking a different approach to reach their next goal, altering not the composition of the batteries but the alignment of the compounds within them. By applying magnetic forces to straighten the tortuous path that lithium ions navigate through the electrodes, the scientists believe, they could significantly boost the rate at which the device discharges electricity.

    That shot of power could open up a use that has long eluded batteries: meeting the huge demands of a passenger aircraft at liftoff. If it works as hoped, it would enable regional commuter flights that don’t burn fuel or produce direct climate emissions.

    Viswanathan, an assistant professor of mechanical engineering at Carnegie Mellon, initiated and is leading the research project. He and Chiang are now collaborating with 24M, the lithium-ion battery manufacturer Chiang cofounded in 2010, and Zunum Aero, an aircraft startup based in Bothell, Washington, to develop and test prototype batteries specifically designed for the needs of an advanced hybrid plane.

    Venkat Viswanathan, an assistant professor of mechanical engineering at Carnegie Mellon.
    Courtesy of Carnegie Mellon University

    High stakes

    Eliminating greenhouse-gas emissions from airplanes is one of the hardest challenges in the climate puzzle. Air travel accounts for around 2% of global carbon dioxide emissions and is one of the fastest-growing sources of greenhouse-gas pollution. 

    But there are no clean alternatives today for more than a tiny sliver of air travel, because the batteries powering electric cars are still too expensive, heavy, and otherwise poorly suited for aviation.

    More than a dozen companies, including Uber, Airbus, and Boeing, are already exploring the potential to electrify small aircraft, creating the equivalent of flying taxis that can cover around 100 miles (161 kilometers) on a charge. The hope is that these one- or two-passenger vehicles—in most cases envisioned as autonomous vertical takeoff and landing aircraft—could shorten commutes, ease congestion, and reduce vehicle emissions. But these would largely replace car rides for the rich, not displace air travel.

    Viswanathan and Chiang are aiming higher. The initial plan is to develop a battery that could power a 12-person plane with 400 miles (644 kilometers) of range—enough to make trips from, say, San Francisco to Los Angeles, or New York to Washington. In a second phase, they hope to enable an electric plane capable of carrying 50 people the same distance.

    Such planes would still be equipped with a combustion engine and carry fuel. But the fuel would largely be on board to achieve the US Federal Aviation Administration’s “reserve requirement” for safety, which instructs aircraft to carry enough to land at an airport 200 miles (322 kilometers) from the intended destination. In a normal flight, the planes shouldn’t have to tap into that fuel.

    Simon Simard

    Airborne

    The appeal of the project to a startup like Zunum is obvious: the better that batteries get at meeting the needs of aircraft, the bigger the market that hybrid or electric planes can potentially address.

    Last year, the company announced plans to deliver a line of “hybrid to electric” aircraft with room for 12 passengers in 2022.

    At launch, the company intends to offer a hybrid plane with a gas turbine and two battery packs capable of flying around 700 miles (1,127 kilometers), as well as an all-electric version with three battery packs and a range of less than 200 miles. (Unlike the planes Viswanathan and Chiang have in mind, the hybrid model would draw heavily on the on-board fuel.) But crucially, the plane itself is expected to feature an open architecture that allows owners to switch out these modules over time, enabling them to upgrade to better batteries developed in the future or shift from hybrid to all-electric operation.

    Zunum has secured capital from Boeing, JetBlue, and the State of Washington’s Clean Energy Fund. JetSuite, a Dallas-based charter flight company, has agreed to purchase up to 100 of the planes. Other startups, including Eviation Aircraft and Wright Electric, are also working to develop small electric planes for commuter-length flights.

    This clip shows what happens when magnetic forces are applied to magnetic microrods mixed in with electrode materials.
    Courtesy of MIT researcher Jonathan Sander

    Planes are rarely used for regional travel, representing less than 1% of trips under 500 miles, according to the US Bureau of Transportation Statistics. Airlines have shied away from shorter flights largely because most of the fuel is burned during takeoff, meaning longer routes are far more economical. And given the high costs and hassles of flying, consumers largely opt for cars, trains, or buses instead for this travel range.

    Zunum chief executive Ashish Kumar, previously an executive at Microsoft and Google, believes hybrid planes could change these habits—in large part by cutting the cost of fuel and, in turn, fares. “In most parts of the world you could double your domestic air miles as people get off the highway and into faster aircraft,” he says.

    Simon Simard

    As batteries improve, hybrid and electric planes can cut into a much bigger share of air transportation as well. By 2035, Kumar expects, hybrid planes will able to reach distances as great as 1,500 miles (2,414 kilometers), at which point air travel represents 82% of trips, according to the BTS.

    A greedy battery

    During a meeting in Chiang’s office in early September, Viswanathan underscored the challenges of electrifying aviation by pulling up a chart displaying the discharge profile of a battery pack across a flight path. It’s an alpine wall in the first minutes of the flight. Then it drops dramatically to a long, flat plateau as the plane reaches cruising altitude. 

    In other words, a battery must be able to deliver a massive amount of power at takeoff, and pack enough energy density to cruise for at least hundreds of miles. But to work within the confines of aircraft physics and economics, it also needs to be as long-lasting and light as possible, and capable of rapid charging—or at least, as Zunum plans, able to be easily swapped for a fully charged battery between flights. 

    Viswanathan notes that a standard Tesla-style battery pack may check the first two boxes. But takeoff would be like driving a Model S in “ludicrous mode” for four minutes instead of a few seconds, generating a massive amount of heat. 

    “You’d fry the battery,” he says.

    That would radically shorten the lifetime of very expensive battery packs.

    This clip shows what happens when magnetic forces are applied to magnetic droplets mixed in with electrode materials.
    Courtesy of MIT researcher Jonathan Sander

    Getting lithium-ion batteries to discharge at a rate fast enough for aircraft requires making it easier for ions and electrons to flow through the battery, particularly the electrodes. One option is to make the electrode materials more porous or thinner, but either of those changes would come at a steep cost to energy density.

    So instead, the researchers are exploring ways to straighten the twisting paths through tightly packed carbon, cobalt compounds, and other materials in the electrodes.

    As in many a magical illusion, the trick relies on magnets.

    In a 2016 paper in Nature Energy, Chiang, MIT researcher Jonathan Sander, and colleagues showed that mixing magnetic nanoparticles into the electrode materials, and applying a light magnetic field, helped to create aligned pathways through the electrodes.

    Subsequent tests found the discharge capacity of these electrodes, or the rate at which electrons can travel out of the battery, was more than double that of conventional lithium-ion batteries—without sacrificing energy density.

    “It’s opening up a whole new direction in what we can get out of batteries for electric aviation,” Chiang said.

    The researchers are now working with 24M in Cambridge, Massachusetts, where Chiang also serves as chief scientist, to develop and test prototype batteries using this magnetic approach. If all goes well, Zunum will then work with the researchers to evaluate the prototypes in what are known as “copper bird” tests, in which all the plane’s electricity systems are evaluated on the ground. Eventually, they could be tested in actual flights as well. 

    Simon Simard

    Just the start

    Until the batteries are actually created and evaluated, it remains to be seen how well this approach will really work. And even in the best-case scenario, the field is still probably decades away from electrifying more than a fraction of total air miles.

    Richard Anderson, an aerospace engineer and director of Embry-Riddle Aeronautical University’s Eagle Flight Research Center, points out that batteries are at least 20 times heavier than fuel for a given amount of energy output. He is skeptical that companies pursuing hybrid commuter flights, like Zunum, can find enough ways to offset that added weight in the next few years. He also thinks the field is overestimating how quickly hybrid planes will be able to reach longer distances—while underestimating the regulatory challenges they’ll face.

    The MIT and Carnegie researchers themselves are quick to say that other big battery improvements will still be required to extend the range of electric planes, which may necessitate a shift to entirely different chemistries. On top of that, planes will probably need to be fundamentally redesigned to reduce energy demands, potentially by redistributing motors or changing the shape of the body to reduce drag, Viswanathan says.

    But he and Chiang are working to develop a technical capability that would be required regardless of any other advances. Even if other battery engineers find ways to make electric planes to fly a thousand miles, they’ll still need enough power to get off the ground.

    This article has been updated to clarify the roles of the researchers.

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    Venkat Viswanathan, an assistant professor of mechanical engineering at Carnegie Mellon.
    Courtesy of Carnegie Mellon University
    Simon Simard
    Simon Simard
    Simon Simard
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