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.
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.
Energy storage has always been a problem for electric vehicles, and even a greater problem for hybrid electric vehicles. In a purely electric vehicle, energy is stored, usually in batteries, and then used to power the vehicle until the energy is depleted. At that time energy is stored once more by recharging the batteries. In a hybrid electric vehicle, energy is constantly being stored and used; the repeated charging and discharging puts a tremendous strain on the batteries. This type of use reduces the lifetimes of presently available 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
Capacitors can also function at power densities greater than those of batteries. Therefore, very high power levels can be provided during acceleration and can be absorbed during charging. Capacitors have excellent low-temperature characteristics, do not require maintenance, and provide consistent perfor mance over time. In addition, capacitors promote safety in electric vehicles because of their relative ease of discharge.
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/tsp 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.