Mobile Communications – Power

As laptops, organizers and cell phones assume ever more functions, power requirements are growing by leaps and bounds. In short, the coming multimedia revolution will need better power storage technologies and some exotic new batteries if it is to fulfill its promise.

Hours of talk time, standby for weeks—cell phone users know that ads promise a lot of things that often don't bear up to reality. The problem could soon become even more acute. On tap are third-generation mobile communication terminals with enough computing power to perform such feats as playing music from the Internet or transmitting images. But there's a hitch: more speed also means more power.

Today's GPRS-enabled cell phones already need about half an ampere when transmitting. 40 % of this electricity flows to the output stage that serves the antenna. An equal amount is required by the processor, and the rest is consumed for illumination. But a UMTS cell phone might easily devour three times as much power, much of which is going to the processor. "In UMTS phones, today's batteries would be flat after half an hour," warns Michael Kranawetter, who conducts research on cell phone batteries at Siemens in Ulm, Germany.

That's why engineers around the world are working feverishly to produce more efficient energy storage technologies. An even better idea is to build cell phones so that they conserve energy right from the start. According to Kranawetter, there's plenty of room for improvement in this field. The Siemens C45 cell phone, for example, is illuminated by ten light-emitting diodes (LEDs) that collectively consume 100 mA. New, high-efficiency LEDs, on the other hand, require only half as much power. Reducing the size of the chip structures in the microprocessor can also lower energy consumption by a few percent. Hardly any changes can be made to the output stage for the antenna, but the sophisticated software for UMTS cell phones should provide more development opportunities. Here, power consumption can be reduced through intelligent programming. The display is one of the features likely to gobble up the most energy. UMTS devices have larger color displays that require substantially more power than today's efficient monotone LCD displays. Color TFT displays, such as those used in the flat screens of notebooks, would be too heavy and consume too much energy, so hopes are being pinned on thin-film organic LEDs (OLEDs). As the image pixels themselves are luminous, these OLEDs require no backlighting, and thus conserve energy. The first generation of OLEDs is now ready for mass production (see article Presenting Multimedia: Maxi Displays and Mini Projectors).

Despite all of these energy-saving measures, batteries are likely to continue to be the toughest problem when it comes to turning UMTS devices into tomorrow's universal companions. One consolation, however, is that UMTS cell phones will be bigger than current devices, due to their larger screen size and additional functions. This in turn means there will be more space available for power supply units.

New lithium polymer batteries, consisting of successive layers of plastic film, usually separated by a gel-like electrolyte, hold great promise. These batteries do not require liquid electrolytes. They can thus be made in various forms, such as a thin film that can be stuck to the back of a display screen or stored in hollow spaces. In addition, polymer batteries weigh half as much as lithium ion batteries and should cost about the same within the next few months. The first Siemens cell phone with a polymer battery is the SL45, which is equipped with a battery from Sanyo. Another major step forward is expected to be taken in about three years, when lithium-sulfur technology hits the market with some 20 % more energy. This energy will go even further when today's 3.7-V battery output is replaced by 2.4 V, thus cutting some losses because the battery voltage will be closer to the 1.8 V required by chips.

Kranawetter regards these developments as mere temporary solutions, however. "The energy source of the future is the fuel cell," he says. Here, electricity is produced when the hydrogen in the fuel cell reacts with atmospheric oxygen. The process creates no exhaust emissions other than water vapor and heat. Moreover, when the tank is empty, it is simply replaced with a full one, ensuring a return to full power in a matter of seconds.

That, at least, is the theory. In practice, there are still hurdles to be overcome. The tank is the weak link in the chain when it comes to fuel cells requiring pure hydrogen. Not only must it safely enclose the volatile gas; it must also hold as much of it as possible. Metal hydride storage units can now pack some 1,000 Wh in one liter of volume (three times as much as lithium ion batteries), but they are heavy and expensive. The Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany, has already introduced a prototype that is only slightly thicker than a conventional battery and can be used with notebooks and camcorders.

The direct methanol fuel cell (DMFC) would be the most advantageous option for very small devices such as cell phones. Methanol is inexpensive and easy to store—4,770 Wh will fit into a liter (when viewed in terms of weight, the advantage becomes much clearer: methanol offers 6,030 Wh/kg as compared to the 120 Wh/kg offered by lithium ion batteries). Motorola has already introduced a DMFC prototype, which although only a few centimeters long, can power a palmtop computer. There are disadvantages, however. One is the emission of CO2, although this occurs only at low levels. Another more significant problem concerns the necessity of diluting the methanol, which reduces its high energy density. This occurs because the polymer membranes typically found in fuel cells, which should theoretically be permeable only to hydrogen ions (see article Gentle Revolution), only work in an aqueous solution and can also be partially penetrated by methanol. The methanol is thus not only lost to power generation; it also contaminates the electrodes.

Medis Technologies, a U.S.-Israeli company, has therefore developed a fuel cell that completely dispenses with the separating membrane, replacing it with a special methanol mixture and electrodes coated with polymers that conduct electricity. The fuel cell's methanol content and energy density can thus be increased three- to fivefold, according to the company. Numerous institutes and companies around the globe are now working on DMFC fuel cells.

Nevertheless, experts at ISE doubt that such miniature fuel cells will be able to supply all the power cell phones need for transmission. A second battery, such as a small polymer battery or "super capacitors" will thus be necessary. This additional battery can be charged by the fuel cell in standby mode and then contribute power during phone calls.

Researchers at Columbia University in New York have come up with much more exotic ideas. Using silicon, they have etched a 20-W gas turbine that is smaller than a button. The problem is the high frictional loss resulting from the unit's 2.4 million revolutions per minute. The turbine also has to be operated outside of the cell phone to dissipate heat. Another microsystems technology product, developed at the University of California at Berkeley, is a 2.5-W Wankel engine that is smaller than a sugar cube.

However, significant promotional work is required before fuel cells and miniature gas turbines or gas engines establish themselves on the market. For one thing, consumers used to clean, dry batteries, will have to get used to inserting a flammable, toxic liquid into their cell phones. In addition, fuel cells can heat up to 60 °C and give off steam. Ulf Groos, head of marketing at ISE, is unperturbed by the issue of market acceptance. "In principle, conventional batteries are poisonous and explosive too. And people are accustomed to filling gas tanks with an explosive liquid," he says. Ultimately, however, market success will depend on whether it will be possible to carry a cell phone in your jacket pocket or briefcase—and if such a device will be permitted on airplanes. After all, many airlines prohibit methanol in the passenger cabin.

But if all this talk of flammable liquids and explosive power sources has you worried, relax. There are two technologies that would have no acceptance problems whatsoever. ISE engineers have equipped the back of a Siemens cell phone with a highly efficient solar cell for test purposes. When the sun is shining, the solar cell provides 30 to 40 mA of charging current, theoretically enough to power the phone in standby mode indefinitely. A design model developed by ISE has a hinged cover with two large solar cells outside and a fuel cell inside. And ? Motorola's new "FreeCharge" functions on muscle power alone. A yellow egg-shaped unit with a crank dynamo is attached to the cell phone by a wire—one minute's cranking allows you to phone for five!