GE’s Two-Battery Strategy for Fuel-Cell Buses
Design effort could make fuel cells practical while pushing the state of the art in hybrid propulsion for all kinds of vehicles.
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.
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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.
