New Technologies Replace Chemical Batteries
At NASA's Glenn Research Center (GRC), the Hybrid Power Management Program (HPM) is exploring the use of ultracapacitors in place of rechargeable batteries. The applications examined to date include everything from electrical power storage on the International Space Station to powering electric toothbrushes on Earth. Most recently, the HPM has focused on the control and regulation of hybrid propulsion systems.

A capacitor is a device that can store electrical energy. It consists of two conductors, or "plates," separated by an electrical insulator, or "dielectric." When the capacitor is subjected to an electrical current, a charge builds on the plates. Since the charge remains after the current is removed, energy can be stored in the electric field between the plates. The maximum charge that can be stored in a capacitor is a function of the size of the plates and how well the dielectric can insulate the charge. A better dielectric provides higher charge and results in greater capacity for energy storage.

Ultracapacitors can store significantly more charge than regular capacitors due to the use of highly effective materials. Although current ultracapacitor technologies have a lower charge density than electrochemical batteries, the technologies have several advantages. In particular, ultracapacitors can be recharged in a matter of seconds, compared with the hours required to recharge a standard battery. Ultracapacitors also have a much longer life - they can be recharged more than one million times, compared with a few hundred recharges for a battery. They are not susceptible to deterioration when exposed to cold temperatures, and their turnaround efficiencies (the percentage of charge energy that can be recovered) are more than 90 percent, compared with typical battery turnaround efficiencies of 50 percent. In addition, ultracapacitors are made of nonhazardous materials.

One of the early HPM projects involved design and development of the power control system for a hybrid electric transit bus (HETB), which was found to have improved fuel efficiency of more than 21 percent over the standard, diesel-powered RTA bus when regenerative braking was used.

The HPM now is evaluating a system that utilizes a combination of ultracapacitors and fuel cells as the primary power source. The system is mounted on a utility vehicle and includes two proton exchange membrane fuel cells powered by hydrogen. The hydrogen is stored at low pressure (200 psi) in a metal hydride canister.

Ultracapacitors are used for energy storage and to protect the fuel cell membranes from power transients (surges). This arrangement demonstrates how the optimized components of a system's architecture can complement each other. The fuel cells have excellent energy density, but not good power density; the ultracapacitors have excellent power density, but not very good energy density. The combination of the fuel cells and ultracapacitors, then, results in a power source with excellent power density and energy density.

The propulsion system has been demonstrated and in the future could serve as the basis for a planetary rover's propulsion system, likely extending operational life and performance characteristics. Furthermore, the total weight of the hybrid power source is comparable to the weight of an equivalent battery power source; however, the volume required is smaller. Thus, a hybrid power source utilizing ultracapacitors provides more space for other items than would be provided by the equivalent battery power source.

Editor's Note:
This news brief was previously published by the Federal Laboratory Consortium - Midwest Region. For more information, contact Laurel Stauber in the NASA Glenn Technology Transfer and Partnership Office, (216) 433-2820, laurel.j.stauber@nasa.gov.