Electric Vehicles: Fuel CellsC Hochgraf,General Motors Fuel Cell Activities, Honeoye Falls, NY, USA Published by Elsevier B.V.. Hydrogen fuel cells are one of the most promising al-terna
Trang 1Electric Vehicles: Fuel Cells
C Hochgraf,General Motors Fuel Cell Activities, Honeoye Falls, NY, USA
Published by Elsevier B.V.
Introduction – Why Fuel Cells?
Hydrogen fuel cells are one of the most promising
al-ternatives to internal combustion engine hybrids and
pure battery electric power for propelling passenger
vehicles
Compared to internal combustion engine hybrid
ve-hicles burning hydrocarbon fuels, fuel cell veve-hicles offer
three primary advantages First, the fuel cell system
produces no tank-to-wheel carbon dioxide emissions and
no other harmful emissions such as oxides of nitrogen,
carbon monoxide, or particulates Second, the fuel cell
system offers the potential for approximately 30% higher
well-to-wheel energy efficiency Third, the hydrogen fuel
consumed by the fuel cell can be produced from a variety
of renewable sources including carbon-free methods such
as electrolysis of water
Compared to pure battery-run electric vehicles, the
fuel cell vehicle offers three primary advantages First,
the fuel cell vehicle has more than twice the driving
range of a vehicle using existing batteries Second, it
offers a much shorter refueling time, enabling brief
re-fueling stops on long trips Third, at cold temperatures,
the fuel cell system can warm up much faster than a
battery and therefore produce full power in a shorter
period of time
A fuel cell vehicle can be refilled with compressed
hydrogen at a rate of 2.0 kg hydrogen per minute To
recharge a battery electric vehicle at an equivalent rate
would require the battery and charger to handle 2.5 MW
of power Such a charger would be 400 times larger than
that typically used for battery electric vehicles
At –30 1C, many high-energy lithium battery
chem-istries cannot provide high power, that is, they cannot
support discharge C-rates of 10 or more To get full
power capability, the battery would need to be warmed
up However, the time and energy required to accomplish
this for a battery are significantly longer than for a
well-designed fuel cell system Improved, but not full, power
capability can be obtained at 30 1C by using
lower-energy-density chemistries such as those using
nano-particle lithium titanium oxide
The primary disadvantages of fuel cell systems,
compared to gasoline hybrids, are the high present-day
cost, shorter than required fuel cell stack life, poor
en-ergy density of fuel storage, and lack of a widespread
hydrogen fueling infrastructure Fuel cell system cost,
while higher than a gasoline hybrid, is projected to be
lower than that of an equivalent full-range electric
vehicle with advanced batteries Cost is being lowered and durability increased through engineering develop-ment efforts The developdevelop-ment of higher-energy-density hydrogen storage is an area of active research
Several studies have shown that the cost in the near term of producing, distributing, and dispensing hydrogen for use in fuel cell electric vehicles is in the range of US$2–3 per gallon of gasoline equivalent (on a cost-per-kilometer basis, not including taxes) Hydrogen is pro-duced in large quantities for industrial uses including oil refining and fertilizer production The economics of hydrogen production by steam methane reforming are well understood The US Department of Energy is tar-geting long-term costs of US$1.0–1.5 per gallon of gas-oline equivalent The primary challenges are the initial cost to deploy the fueling infrastructure and delays in getting approval to site dispensing stations due to the absence of uniform building codes and standards for hydrogen Support of government policy is often cited as being essential to overcoming these challenges
Requirements of an Automobile Fuel Cell Powertrain
The requirements for an automobile propulsion system have evolved over many decades Consumers’ expect-ations for on-road vehicle propulsion are guided by ex-perience, which is almost entirely with internal combustion engines
Consumers expect vehicles to start instantly, accel-erate quickly, drive for long periods without refueling, refuel in a few minutes, be cost effective, last for decades, and be safe to operate and service These high-level re-quirements are quantified inTable 1
Achieving the desired driving range is particularly challenging for the fuel cell vehicle because the energy storage density of compressed hydrogen gas is less than
1 kW h L1 at 70 MPa compared to gasoline’s energy storage density of greater than 8 kW h L1 As a result, the requirement for fuel cell efficiency is significantly higher than that of a gasoline engine
Packaging the propulsion system to fit into a standard vehicle’s dimensions and shape drives compact and lightweight design solutions for the fuel cell system
Powertrain Configuration
A fuel cell powertrain consists of a fuel cell stack with balance of plant components for air supply, fuel control,
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