The Palouse Piezoelectric Power (P3) engine is three millimeters wide, three millimeters long, and 100 microns thick, making it the world’s smallest engine. Just over 6,447 engines placed side by side would cover a page of this magazine, and each engine would be no thicker than the page on which it rested. The Washington State University researchers who created it believe the P3 has the potential to one day replace the batteries that power electric devices.
To operate, the P3 needs only an external heat source, such as a burning fuel, the sun, a wood stove, waste heat from electronics, or even body heat. The P3 engine consists of a fluid and bubble sealed between two membranes. It converts thermal power to mechanical power when the bubble expands in response to pulses of heat that sometimes near 300 times a second. As the bubble enlarges, so does the entire engine. Between heat pulses, the engine contracts as its heat dissipates. The P3 expands and contracts so quickly it actually vibrates. One of the engine’s membranes is made of silicon; the other is a thin-film piezoelectric generator as wide as a white blood cell (two to three microns). As heat moves in and out of the engine, the expanding and contracting bubble puts pressure on the piezoelectric membrane, which turns the mechanical power created by the engine into electric power.
The P3 engine is the work of three associate professors in the WSU School of Mechanical and Materials Engineering: David Bahr, Bob Richards, and Cecilia Richards. The researchers first detected voltage from a P3 engine in December 2000 and are currently in the fifth year of the project. They have made exponential progress over the last two years with P3 engines producing 1,000 times more voltage than they did originally-four volts versus four millivolts. Current prototypes produce electrical power measured at one-thousandth of a watt and have powered a blinking LED and a Power Puff Girl’s watch in lab tests.
The researchers’ progress has attracted the attention of the U.S. military, which has been searching for an alternative to batteries. In March, the School of Mechanical and Materials Engineering and the Center for Materials Research received a contract worth more than $7 million from the Army Space & Missile Defense Command (SMDC) and the Defense Advanced Research Projects Agency (DARPA). The funds allow continued research on the P3 engine for the next four years in order to produce a portable micro-power generation system that can replace the batteries currently used in military applications.
Batteries are heavy, says Bahr. It is not unusual for a soldier to carry an 80-pound backpack onto the battlefield with batteries accounting for 10 to 20 pounds of that pack. In addition, batteries lose power over time. The P3, however, needs only a source of heat to continuously produce electric power. “Even in Kabul, you can go out and buy gasoline to heat our engines,” says Bob Richards, “but it’s hard to find a place that sells rechargeable batteries.”
Industry has a growing need for micro-power supplies. Companies are making devices smaller and smaller, but power supplies aren’t shrinking with them. “Once people have tiny power supplies, you’re going to see tiny airplanes, tiny robots, tiny electronic sensors,” says Bob Richards.
Other universities have had limited success developing micro engines. The WSU researchers attribute their success to the simplicity of the P3 engine. “UC Berkley, MIT, University of Illinois, Cal Tech, Stanford all have amazing facilities and resources we don’t have,” says Bob Richards. “Consequently, the engines they build are incredibly complex, so complex that they can’t even build them. We’re beating them because we don’t have the resources to get complex. We were forced to work with what we had, and it worked out well for us.”
Power supplies made up of different numbers of P3 engines could produce one-thousandth of a watt or hundreds of watts. The engines can be put together in various configurations: in sheets, in stacks, or in cubes. This allows for flexibility in tailoring P3 power supplies to electronic devices with differing shapes. “With the P3 you don’t need to build a device around the limitations of a power supply,” says Bahr. “If a device needs eight microwatts, we can give them eight microwatts. If it needs eight megawatts, we can do that too.”
The P3 is fabricated from a silicon wafer using techniques developed for making integrated circuits. Utilizing these techniques, thousands of identical engines can be produced from a single batch of material for pennies apiece. “The engine is very small, very cheap, and made in a way that no other engine has ever been made,” says Bob Richards.