by Kenneth M. Wright At NASA's Glenn Research Center (GRC), the Hybrid Power Management Program (HPM) has been working on using 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 GRC 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 (called "plates") separated from each other by an electrical insulator (called a "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 present ultracapacitor technologies have a lower charge density than electrochemical batteries, the technologies do have several advantages. In particular, ultracapacitors can be recharged in a matter of seconds, compared to the hours required to recharge a standard battery. Ultracapacitors also have a much longer life (they can be recharged more than 1 million times, compared to a few hundred recharges for a battery), are not susceptible to deterioration when exposed to cold temperatures, and have turnaround efficiencies (the percentage of charge energy that can be recovered) of more than 90% compared to typical battery turnaround efficiencies of 50%. 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). This project was conducted in conjunction with the following partners:
The HETB utilized a bank of 30 ultracapacitors to store electrical energy. The ultracapacitor bank weighed 2,100 pounds and could store up to 1.6 MJ of energy. As tested, the HETB power system also included a compressed natural gas engine-generator and a variable-speed electric motor. A bank of 28 batteries was also mounted on the bus to provide a comparison between the use of ultracapacitors and batteries. The power system was set up in series so all energy (whether from the generator, the ultracapacitors, or the batteries) went to the motor and was then distributed to the wheels and other bus systems (e.g., lighting, heating, pneumatics, etc.). The HETB was—and remains—the largest vehicle to use an ultracapacitor energy storage system. During testing at the Transportation Research Center in East Liberty Ohio, the HETB was found to have improved fuel efficiency of more than 21% over the standard, diesel-powered RTA bus when regenerative braking was used. Project engineers estimated that the HETB would have a range of approximately 220 miles with regenerative braking. Without regenerative braking, the range was estimated to be approximately 180 miles. It was found that using ultracapacitors in the energy system allowed the HETB to come to a complete stop more quickly and in a shorter distance than an energy system incorporating batteries. The efficiency of regenerative braking when using batteries is dependent on the batteries' state of charge at the time of braking. If the batteries are close to fully charged, quickly accepting energy from a regenerative braking system can cause damage to the battery plates. The ultracapacitors can consistently accept large charges, regardless of their state of charge. Based on the HETB testing, regenerative braking with ultracapacitors is sufficient to stop the vehicle alone. A combination of regenerative and mechanical braking is required if the energy system uses batteries. The GRC HPM is presently 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 (PEM) 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 latter fact demonstrates the way in which 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 results in a power source with excellent power density and excellent energy density. The propulsion system is operational and is currently undergoing performance testing. In June 2007, the utility vehicle was driven for 1.5 hours continuously at maximum speed and had plenty of hydrogen remaining. This was the first demonstration of the system. In the future it could serve as the basis for a planetary rover's propulsion system. As described above, ultracapacitors have longer life, better low-temperature performance, and better turnaround efficiencies than conventional batteries. Given the likely environment that a planetary rover will encounter, these advantages translate to extended 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 the equivalent battery power source. In another series of ongoing projects, the GRC HPM is working with Rivas Technologies under a Space Act Agreement to replace the rechargeable lithium-ion batteries in several consumer electronic products with power strategies that incorporate ultracapacitors. Although lithium-ion batteries have excellent energy-to-weight ratios, they can lose capacity if they are repeatedly deeply discharged, which limits the useful life of the battery. In addition, if the batteries are not properly stored or if they are exposed to high temperatures, they can become unstable. Rivas is currently developing ultracapacitor-powered cell phones, digital cameras, and hearing aids that use HPM-based power system architectures.
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