Electronic Design

MEMS: A New Power Source For Portables

Revelations in miniature fiel cells and power generators for portable electronics highlight the IEEE 2005 Conference.

Fuel cells and power generators are the newest and hottest applications for microelectromechanical-systems (MEMS) technology. Their aim is to run future portable electronic devices like CD players, digital cameras, and PDAs. The point was driven home at last month's IEEE 2005 Conference in Miami Beach, Fla., where power developments stood out among the more than 200 technical presentations.

MEMS continues its progress on all fronts, refusing to rest on its steady stream of commercial market successes. Refining and fine-tuning of the technology continues with advances in software design tools, packaging, processing, and manufacturing. As a result, it's gaining a greater foothold on many existing applications—RF, biomedical, optical, motors, gas chromatography, microfluidics, sensing, and high-temperature arenas—as well as opening up new areas.

A fuel cell is an electrochemical device similar to a battery. Unlike batteries, though, it's designed for continuous replenishment of the reactants consumed. In other words, it produces electricity from an external fuel supply as opposed to the battery's limited internal energy storage capacity.

Fuel cells have the potential to offer higher energy densities than batteries, and they don't require any time for recharging. They may work as a replacement for various types, such as lithium-ion (Li-ion) batteries, which haven't kept pace with consumer electronic appliance demands.

Work on reducing the size of fuel cells operating at room temperature will likely get a boost from MEMS and nanotechnologies. Results obtained so far from a development out of the R&D Center of Toshiba Corp. support this claim.

The Toshiba team produced a MEMS-based direct methanol fuel cell with a 140-cc capacity and 1 W of output power for 20 hours of operation. A micropump was developed to pump gases and liquids and to keep power consumption and size within reasonable limits. Though no specific dimensions were provided during the presentation, pictures reveal it's about as big as a medium-size cell phone. Toshiba uses a polymer membrane electrolyte assembly with a cathode and an anode in this fuel cell. Each electrode has a catalyst layer and a gas-diffusion layer (Fig. 1).

A PERMANENT-MAGNET MOTOR
One joint development that drew lots of interest was the multiwatt electric power generators from the Massachusetts Institute of Technology (MIT) and the Georgia Institute of Technology. This technology is based on a microfabricated permanent-magnet MEMS structure.

The generators are three-phase, axial-flux, synchronous machines. Each consists of a multipole surfacewound stator and a permanent-magnet rotor (Fig. 2). The microfabricated windings, with small interconductor gaps and a variable-width geometry, are the key enablers for high power density.

At a rotational speed of 120,000 rpm, one generator demonstrated 2.6 W of mechanical-to-electrical power conversion. Coupled to a transformer and rectifier, it delivered 1.1 W of dc electrical power to a resistive load. For an active machine volume of 110 mm3 (9.5-mm outer diameter, 5.5-mm inner diameter, and 2.3 mm thick), this corresponds to a power density of 10 MW/m3.

Previously, the group had investigated magnetic induction machines as potential electric generators. These machines were tested as tethered motors (but not in the generating mode), and they demonstrated a peak performance of 2.5 µN-M.

Permanent-magnet motors for given small sizes offer some advantages over induction machines. The independent source of rotor flux is much larger than what induction machines of the same size can produce. They also offer higher efficiency because they have no eddy currents.

The researchers believe that MEMS-based generators such as these can be made to produce 10 to 100 W of power. They also feel that this watt-scale electrical power generation demonstrates the viability of scaled permanent-magnet machines for practical applications. According to the researchers, suitable electric generators could be powered by a variety of prime movers, including liquid flow, pressurized gas, or small combustion engines, such as a microscale gas turbine.

A joint team of researchers at MIT and the school's Lincoln Laboratory created a MEMS electroquasistatic induction turbine generator for electricity. A maximum power of 192 mW was achieved under self-resonating excitation. The researchers believe this is the first machine of its type anywhere. No external excitation is required by this self-exciting generator.

The generator consists of five silicon layers, fusion-bonded together at 700°C (Fig. 3). The stator is a platinum-oxide electrode structure formed on a 20-±m thick recessed oxide island. The rotor is a thin (10-±m) film of lightly doped polysilicon that also resides on an oxide island.

Power generation is limited by the generator's internal and external capacitances. Consequently, it's necessary to minimize those effects through modeling to achieve higher levels of power. Nevertheless, the researchers feel their development holds lots of promise.

POWER FROM LIQUID DROPLETS
In a novel approach to producing tiny power-generation systems, researchers at the California Institute of Technology reported on using MEMS arrays of liquid rotor electret power generators (LEPGs) to produce power.

These LEPGs are essentially fixed-charge, Teflon-coated capacitors with air-filled gaps and liquid droplets that move by vibration. As the liquid moves into and out of the gaps, a net voltage generates across the capacitor while image charges on the electrode redistribute themselves according to the position of the droplets.

Researchers studied series and parallel arrays to increase power output. They noticed that the power output from parallel arrays scales linearly, as expected, with the number of devices used. This should lead to outputs of up to 10 ±W. Researchers also noted that series arrays require further study, as they involve more complicated impedance-matching issues.

MEMS also has shown promise for making tools that will assist in creating miniature fuel cells and catalytic chemical microreactors. One such tool is a passive micro gas regulator for hydrogen flow control in miniature fuel cells. This joint development from Canon and the University of Tokyo is claimed to be the first of its kind.

According to the microregulator's developers, unlike previously reported active microregulators, this passive device makes it possible to achieve miniaturization. Its total size of just 8 by 8 by 1 mm suits it well for portable appliances.

Another important tool is a MEMS-based high-temperature-compatible nickel-silicide thermometer and heater for catalytic chemical microreactors. Developed at the Technical University of Denmark, it can operate at up to 800°C, a temperature limited only by the integrity of the Pyrex lid it uses.

What new applications will MEMS scientists think of next? Heard around the conference corridors was one comment from a Swiss MEMS researcher: "We're ready to replace all those steel precision-made gears in Swiss watches using MEMS gears." Switzerland is world-renowned for making millions if not billions of steel watch gears for the last two centuries. It remains to be seen if MEMS gears can replace them cost-effectively.

NEED MORE INFORMATION?
California Institute of Technology
www.caltech.edu

Canon Inc.
www.canon.com

Georgia Institute of Technology
www.gatech.edu

Massachusetts Institute of Technology
www.mit.edu

MIT's Lincoln Laboratory
www.ll.mit.edu

Technical University of Denmark
www.dtu.dk

Toshiba
www.toshiba.com

University of Tokyo
www.u-tokyo.ac.jp.ac

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