Can We Pull Energy from the Wings of a Drone?September 2017
Topics: Nanoelectronics, Technological Innovations, Unmanned Aircraft Systems, Aviation and Aeronautics, Military Operations (General), Systems Engineering, Electronics Manufacturing
We all know the Energizer Bunny keeps going and going, thanks to the single battery tucked into his fuzzy leg. But what if the Energizer Bunny, instead of being built from pink fur, was instead built completely of batteries? How long and far could he go then?
A MITRE research team is investigating the use of multifunctional energy systems—basically, structural materials that also store energy—to build UAVs that can remain in the air for a much greater length of time.
"Our calculations show that by making its wings into a structural battery, we can increase the flight time of a UAV by 140 percent," says MITRE's Nick Hudak, one of the principal investigators on the project.
Solving the Energy System Dilemma
MITRE's military sponsors are fielding systems—from drones to sensors to communications equipment—that require more and more power. At the same time, they need to make these systems smaller and more mobile. Traditionally, systems that require more power require bigger, heavier batteries to store that power. At the same time, portable systems require smaller, lighter batteries. Finding the right trade-off can be frustrating.
MITRE has worked on this problem for years in our "SWaP" research. "Size, weight, and power are what every energy system designer wrestles with," says Dr. Kurt Eisenbeiser. He and Hudak, two engineers from MITRE's Emerging Technologies Department, started their research on structural batteries for UVSs to explore new solutions that involve two emerging technologies—structural batteries and nanotechnology. This research is being funded by MITRE’s Technology Futures research program.
They were aware of recent breakthroughs in multifunctional energy systems. For example, Volvo has developed the S80 prototype car, which incorporates structural batteries made from carbon fiber into its body panels. British defense firm BAE Systems is creating small drones from structural batteries composed of nickel-based chemicals.
However, while using carbon fiber can lighten the load, it doesn't have a very large energy storage capacity. And structural batteries that rely on the same chemistry as traditional batteries require frequent recharging.
Adding a Jolt with Nanotubes
Hudak and Eisenbeiser looked to take structural energy storage devices in a new direction by putting MITRE's advanced nanotechnology resources to work. They theorized that mixing carbon nanotubes into a UAV’s carbon fiber material might unlock the full energy storage potential of the material. Carbon nanotubes are cylindrical molecules with unusual properties; one of those properties is the ability to carry 1,000 times more current than copper.
As demonstrated by Hudak and Eisenbeiser in MITRE’s Biotechnology-Nanotechnology Laboratory, the addition of carbon nanotubes to a UAV's carbon fiber material provides 28 times more energy storage capability while only increasing its weight by 15 percent—a valuable SWaP swap. The group submitted a patent application based on this work and published the research in the Journal of the Electrochemical Society.
This increase in energy storage assumes that the nanotube-infused materials in the UAV structure would be used in a supercapacitor, a system for providing short-term power pulses. For longer-term operation, small UAVs must primarily be powered by batteries with higher amounts of stored energy.
"Switching from supercapacitors to batteries is our next stage of research," Eisenbeiser says. Using the fabrication skills acquired in the supercapacitor work, the group recently developed a structural battery prototype based on lithium-ion battery chemistry. The amount of energy stored per unit weight of this prototype was more than 500 times higher than that of the structural supercapacitor.
The Project Takes Flight
In addition to expanding from carbon-fiber supercapacitors to batteries, Hudak and Eisenbeiser spend their time in MITRE's Biotechnology-Nanotechnology Laboratory examining the ways in which carbon fibers can be used as active materials in lithium-ion batteries.
"The high-strength carbon fiber composite can be used as a host for lithium ions, the charge carriers in the battery," Hudak says. "Thus, the fibers themselves are multifunctional, providing strength and energy to the system."
Soon, the team will put its idea to the test, Hudak says. "We’re going to incorporate our batteries into the structure of a small UAV and measure the effect on flight endurance."
The researchers also point out that there exist many other potential uses for multifunctional energy systems. "For example, the military protects a lot of its battery-operated equipment with carbon-fiber casings," Eisenbeiser says. "And the warfighters using that equipment are protected by carbide body armor. By constructing those protective casings and that body armor with structural battery material, we could greatly extend the operational life of the systems our warfighters depend on to complete their missions and get back safely."
—by Christopher Lockheardt