Gas-powered electronics

Nov. 3, 2005
Your next computer may run on gas — hydrogen gas, that is, as fuel-cell power comes to portable 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.

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.

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.

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.

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