Gas-powered electronics
Your next computer may run on gas — hydrogen gas, that is,
as fuel-cell power comes to portable electronics.
Robin Tichy
Product Engineer
Todd Sweetland
Technology Manager
Micro Power Electronics Inc.
Hillsboro, Oreg.
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Portable electronic
devices used for extended periods of time, like this
handheld bar-code scanner, make ideal candidates for
hybrid-fuel-cell/battery power sources. A fuelcell/battery
hybrid power source can power this scanner all day on a
single charge of hydrogen fuel.
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The energy density of PEM
fuel cells compares favorably with current state-oftheart
battery power systems. The energy conversion stack makes up
most of the mass in a fuel cell while energy capacity is
controlled by the lighter stored-fuel quantity. Thus, fuel
cells produce better energy densities at higher storage
capacities.
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It's happening in consumer, industrial, and military gear.
Notebook computers, MP3 players, and cell phones feel its
effects daily. It is the need for greater energy reserves that
let electronics with new features operate longer. In other
words, electronic equipment of all types needs higher-capacity
batteries; or, at least a way to recharge without plugging into
an ac outlet.
No one can deny battery systems have improved. Energy
density, the amount of stored electrical energy per unit weight,
has steadily risen over the last 40 years. The possibilities for
further improvement are good, but battery technology is maturing
and the pace of improvements slowing.
The device that uses a chemical reaction to generate
electricity is called a cell. Batteries consist of two or more
electric cells connected in a manner to boost the voltage or
current available from a single cell. Cells that generate power
through one-way chemical reactions are called primary cells.
Chemicals inside primary cells get consumed until the battery is
dead and must be replaced. Typical of primary-cell batteries
today is the alkaline battery, which replaced the old
carbon-zinc system.
Secondary-cell, or rechargeable, batteries replace-their
electrical energy by forcing an electric current through the
battery in the opposite direction from normal current flow. This
drives the working ions back to their original electrodes
restoring the charge. One of the earliest secondary batteries
still used today is the lead-acid battery. Other secondary-cell
technologies include nickel-metal-hydride (NiMH) and lithium-ion
(Li-ion.)
Multiplying power demand by the length of time the device
must operate gives the energy needs of a system. For example, if
a portable electronic device draws 2 W for 10 hr, the battery
must provide 20 W-hr of energy. The fact is most batteries
supply more total energy when current demands are low and less
energy when demands are high.
The active material mass of a specific type of battery is
directly proportional to the amount of energy it must deliver.
Primary-cell batteries provide the best energy-to-weight ratio.
The most advanced primary cells deliver about 800 W-hr/kg. Their
disadvantage is, of course, that once drained of energy they
must be replaced. Secondary-cell batteries typically have a much
lower energy-to-weight ratio. One of the best is Li-ion; it
reaches energy densities of 200 W-hr/kg. Though not able to meet
the overall capacity of primary cells, secondary-cell batteries
do offer the advantage of energy replacement through recharging.
So any given secondary-cell battery supplies many times its
energy capacity over its lifetime. Of course, the nature of
secondary-cell batteries requires that an alternate energy
source restores the consumed battery energy.
Other costs and restrictions also limit the use of
secondary-cell batteries. For example, the typical operating
temperature for a Li-ion battery is from 0 to 40°C. Operation
outside that range can shorten battery life and reduce the
number of recharge cycles the battery accepts before it requires
replacement. It can also result in dangerous conditions
culminating in destruction of the battery.
While batteries still represent the best way to power most
portable applications, there comes a time when batteries alone
cannot meet system demands. Then it's time to consider
alternative power sources: fuel cells and hybrid systems.
Fuel cells use an external fuel that it converts to
electrical power by an electrochemical converter. In fuel cells,
hydrogen undergoes a chemical reaction with oxygen to form
water. A by-product of that reaction is the generation of
electrical power. The fuel is either hydrogen gas or a simple
hydrocarbon such as methanol. The fuel cell continues to operate
as long as fuel holds out. Just like putting more gas in the
tank of your car when it nears empty, just add more hydrogen
fuel to keep the fuel cell working.
Fuel-cell research centers around two basic technologies:
proton-exchange-membrane fuel cells (PEMFC) and direct-methanol
fuel cells (DMFC.) Of the two, PEMFC is the more mature
technology; but DMFC promises to be easier to apply to portable
systems.
Most of a fuel cell's mass is in its stack of electrolyte,
electrodes, and current collectors where electricity is
produced. The fuel component accounts for little of the overall
weight. Because the amount of fuel determines energy capacity,
larger fuel capacity results in negligible weight gains while
greatly boosting energy-to-weight ratios.
A diverse group of companies today have prototypes ready for
small-scale manufacturing and are currently setting up
partnerships to bring products to market. Surprisingly, many
companies funding this research intend to use fuel-cell
technology in their products rather than manufacture and market
the fuel cell itself. Toshiba, NEC, Sony, Samsung, Sanyo,
Panasonic, and Hitachi are all actively developing
prototype fuel cells to power notebook computers, either
directly or in hybrid systems paired with Li-ion batteries.
Hybrid sources most often combine a high-energy/low-power
component with a low-energy/ highpower component. Hybrid power
systems help extend run time or enhance power capability while
minimizing system weight or volume. Examples of hybrid power
sources are battery/battery (such as Li-ion and Zn/air),
battery/capacitor, fuel cell/ capacitor, and fuel cell/battery.
Fuel-cell/battery hybrids eliminate the ac power cord for a
host. The fuel cell recharges the battery and thus makes
operating time effectively infinite. This hybrid model provides
relatively simple integration for fuel-cell technology.
The tasks facing early adopters of fuel cells and hybrid
systems are formidable. Many find additional restrictions
imposed on their designs such as a need for continuous airflow,
a restrictive temperature operating range, and the need to
control exhaust gas emissions and temperatures. And they face
new regulations in operating their devices. The FAA has already
expressed concerns about permitting hydrogen-filled devices in
the sealed environment of an airplane cabin. Today's regulations
ban such devices from commercial airliners.
Still, the idea of virtually unlimited portable ontime
entices many companies to pursue the goal. High costs and
regulatory uncertainties make the best candidates for fuel cells
those possessing highend market value. Likewise, look for
adoption in areas where costs and benefits gained by
incorporating the new power technology outweigh current
limitations or potential liabilities.
Handheld scanners and portable computers are typical of the
products that will benefit from fuel-cell technology. These
devices often feature full-color LCD touchscreens, multiple
wireless systems such as Wi-Fi and Bluetooth, optical sensors
for 2D and 3D bar codes, and RFID interrogation. The plethora of
features create high energy demands over an 8 to 10-hr work day.
Most likely, companies will first adopt this new technology for
industrial applications, which can bear more cost than consumer
products.
Numerous hurdles still remain before handheld devices will
host hybrid fuel cell/batteries. Engineers must step up the low
0.7-V/cell output of the fuel cell to charge higher-voltage
batteries. The fuel cell must function at high and low power
levels and regulate energy flow to the battery. And it must stop
charging when the battery is full in a way that prevents battery
damage from overcharging. The fuelcell system must monitor the
state of charge of the battery pack and the level of fuel
remaining in its cartridge. Temperature is also monitored to
prevent overheating for safety and long life.
Mechanical challenges abound when developing these hybrid
systems for the handheld market. Host units are small and must
withstand extremes from –20 to 40°C and moisture levels from
dustbowl dry to dripping wet. Ruggedness is a must not only for
safety but also to continue operating after abuse such as
repeatedly dropping 4 to 6 m onto a concrete warehouse floor or
being run over by forklifts.
The sealing, latching, and protection of airways that allow
the fuel cell to breathe present significant design challenges.
Yet they pale compared to securing the fuel cartridge from
hydrogen leaks while maintaining accessibility and ease of
exchange for the user. The first models adopting this technology
may forego ruggedization in favor of operability.
Past actions have brought about significant progress in the
area of safety and regulatory requirements. Fuel cells and
hybrid systems must conform to multiple agency regulations and
standards. CSA America FC3 applies to portable fuel-cell power
systems operating at less than 60 V while ASME PTC50 applies to
all fuel-cell systems regardless of power output. A UN Committee
of Experts recently adopted a new shipping description for
methanol fuel cartridges containing flammable liquids (Class 3).
This lets them be transported as cargo both domestically and
internationally by ground, sea, and air. Work is progressing
towards changing regulations to let airline passengers carry and
use fuel cartridges.
In addition to design and manufacturing issues, logistical
challenges face developers integrating hybrid-fuel-cell/battery
systems into a host unit. Fuel cells need fuel cartridges. If a
user runs out of fuel, cartridges must be readily available and
easily purchased. Fuel-cell integrators must establish cartridge
distribution channels to make the product truly successful.
A major force driving developers to overcome these
hybrid-system obstacles is the military and homeland-defense
initiatives. The "land-warrior" soldier requires extreme energy
reserves at high power levels, a natural for the hybridfuelcell/battery
system. Other areas where long use and recharging abilities are
important include construction sites, oil fields, forestry
sites, marine science, and field and remote-site studies.
While military and industrial applications present a large
potential market for hybrid-power technology, greater
opportunities lie with consumer products. As cell phones, PDAs,
and cameras merge into one device, their energy requirements
will rival those of notebook computers. Only a
hybrid-fuel-cell/battery combination can provide enough power in
a small package for these devices.
Fuel-cell development is following the path blazed by other
products optimized for the needs of consumers. The Li-ion
battery was developed predominately to serve notebook computers.
Other markets soon reaped advantages of better Liion technology.
Li-ion batteries are now found in handheld scanners, heart
pumps, defibrillators, and GPS systems. All indications are
hybrid power systems will travel a similar path. |