Capacitors are superior to batteries with respect to energy density, longevity, and performance.

A government/ industry/ academic cooperative has developed a hybrid electric transit bus (HETB). The goals of the development program, which continues, include doubling the fuel economy of city transit buses currently in service, and reducing emissions to one-tenth of the levels allowed by Environmental Protection Agency (EPA) standards. A unique aspect of the power system of the HETB is the use of capacitors in its the energy-storage subsystem. At a gross weight of more than 17,000 kg, this is the largest known vehicle to use capacitors to store energy.

Figure 1. The HETB Power System includes a dedicated power-management controller and an energy-storage subsystem that utilizes capacitors instead of batteries.

Ultracapacitors that are now available eliminate many of the problems of batteries for hybrid electric vehicles. The ultracapacitors used in the HETB are electrochemical capacitors, which have extremely high volumetric capacitances because of large electrode surface areas and very small electrode separations. The cycle lives of capacitors can be extremely long relative to those of batteries. Thus, it may never be necessary to replace the energy-storage medium in the HETB; consequently, the reliability of the HETB energy system is greater than it would be if batteries were used, the life-of-system cost is reduced, and adverse environmental effects are diminished.

Figure 2. Charging and Discharging Currents were measured in two tests, in each of which the HETB was driven through two cycles at speeds between 0 and 15 mi/h (0 and 24 km/h). In one test, capacitors were used to store energy; in the other test, batteries were used.

Figure 1 is a block diagram of the HETB power system. A dedicated power-management controller has been developed to coordinate the operation of all of the various vehicle components. The auxiliary power unit (APU) is set to provide the normal average power level required by the vehicle. Power surges such as those needed for acceleration and climbing hills are provided by a combination of the APU and the ultracapacitors.

Regenerative braking is also provided on the vehicle. Regenerative braking takes advantage of energy available from the traction drive system during braking to charge the energy-storage system. Because of their higher power-density limits and their greater efficiency in capturing energy, capacitors can take much greater advantage of regenerative braking than do batteries. The plots in Figure 2 show the superiority of capacitors over batteries as sources of current for the traction motor during acceleration and as acceptors of braking current during deceleration.

This work was done by Jeffrey C. Brown, Dennis J. Eichenberg, William K. Thompson, and Larry A. Viterna of Glenn Research Center and Richard F. Soltis of Cortez III. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components and Systems category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4 —8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16876.

This Brief includes a Technical Support Package (TSP).

Ulacapacitors Store Energy in A Hybrid Electric Vehicle (reference LEW-16876) is currently available for download from the TSP library.

Login first to download.

Username

Password

Remember me


Lost Password? Register