Hydrogen fuel cells are still too expensive to be used widely in vehicles, so researchers at GE are taking a different tack: they're slashing the size of the fuel cell to a bare minimum while relying on two distinct kinds of advanced battery technologies to deliver the necessary horsepower under a wide range of driving conditions.
The technology is essentially an advanced version of today's hybrid-vehicle technologies. While GE is developing it to make a cheaper fuel-cell bus, the resulting technology could be applicable to diesel or gasoline hybrids too--and could make it into cars someday. GE's effort, which will draw on advances in other hybrid projects at the company, is scheduled to produce a prototype in three years.
An existing generation of demonstration fuel-cell buses is now three to four times more expensive than ordinary buses, which, along with the necessary hydrogen fueling stations, makes them too expensive to be practical. But in terms of adopting hydrogen as a fuel, buses do hold clear advantages over cars, says Bill Van Amburg, senior vice president of Weststart-Calstart, a not-for-profit organization currently developing fuel-cell buses. A city bus can been filled at a central location (requiring less infrastructure) and has far more room on board to store hydrogen.
To address fuel-cell cost, which comes largely from the use of expensive catalysts such as platinum, researchers at GE's labs in Niskayuna, NY, are drastically reducing the size of the fuel cells, which are "by far the most expensive component of the bus"--significantly more expensive than batteries, says Vlatko Vlatkovic, a leader in electronics and energy-conversion research at GE.
For the horsepower needed for acceleration or high speeds, the bus will instead rely on advanced battery technology. In fact, GE will use two kinds of batteries to do distinct jobs: one for the big bursts of acceleration power essential to getting the bus moving from a dead stop, and a second for storing lots of electricity to supplement the fuel cell during high-speed or uphill driving. As with hybrid cars, the power for these batteries would come from energy recaptured during braking and from excess charge from the fuel cell.
For the first job, GE researchers are evaluating new high-power, yet safe, lithium-ion batteries from A123 Systems, whose batteries are now used in a line of professional power tools (see "Safer Lithium-Ion Batteries"). A123 researchers are redesigning their batteries for the much larger packs needed in buses. Vlatkovic says that the company is also considering ultra-capacitors, another type of energy-storage device that can take in and deliver charge very quickly, although it can't store as much energy as a battery.
For the second job--delivering longer-lasting, though less-intense, power--GE is considering a family of exotic high-temperature batteries that use melted sodium metal. GE has developed advanced sodium-metal chloride batteries for a hybrid locomotive project. The battery boasts high-energy storage capacity, but it hasn't been used as much as lithium-ion batteries have been, in part because the high temperatures rule out its use in laptops and cell phones. But Vlatkovic says the sodium batteries could be less expensive than lithium-ion batteries, and therefore more attractive for bulk energy storage. The batteries, which operate at about 300 degrees Celsius, could be insulated to keep the temperatures high enough inside the battery while also keeping them safe.
Central to GE's development efforts is creating the control systems required for switching seamlessly between different ways of storing and delivering power. This, as with the company's work with batteries, will draw on earlier work with hybrids.
Vlatkovic says the effort could be a huge boon to all kinds of hybrid vehicles. "The prime source of energy can be, in principle, anything," he says. Instead of a small fuel cell, GE could use a small advanced diesel engine running on renewable biofuels or equipped to get the most from new ultra-low sulfur diesel (see "How Diesel Technology Could Cut Oil Imports").
Such a diesel hybrid may prove to be about as efficient and clean as a fuel-cell-powered vehicle, says Van Amburg, when the costs of making hydrogen are considered. But, he adds, "there's still room for debate." GE's effort is part of a $49 million program funded by the Federal Transit Administration to help make fuel-cell transit buses practical. Hydrogen-fuel-cell-powered vehicles could reduce pollution in cities, since they emit only water, and they have an advantage over battery-only electric vehicles in that refueling times are typically faster. But other options might make more sense from an energy-efficiency or environmental perspective. While fuel cells only emit water vapor, the cheapest ways of making hydrogen fuel use fossil fuels, emitting greenhouse gases in the process. And the manufacture and transport of hydrogen consumes energy, too.
Source : Technology Review
