Strategy Analytics has an interesting report about the possibility of ultracapacitors replacing batteries. Their report analyzed ultracapacitor potential, especially following the recent the Toyota Supra HV-R victory in the Tokachi 24-hour endurance race. The HV-R was equipped with ultracapacitors, instead of rechargeable batteries, for power storage.
The Toyota Supra HV-R was able to store large quantities of energy quickly from regenerative braking and apply this stored power quickly to its advantage. The fast charging and discharging of the ultracapacitors resulted in a first-ever hybrid car victory. BMW has also demonstrated this ability in its “syncap” concept whereby two-thirds of vehicle total torque was generated by the syncaps enabling heavy SUVs to accelerate more quickly than before, with improved fuel economy.
Batteries store electrical energy in chemical form, while capacitors utilize a pair of closely-spaced conductors to store energy in an electric field. Capacitors are much lighter than batteries, and do not require the use of toxic materials and are much lighter. In addition, ultracapacitors have a superior charging/discharging cycle lifetime compared to rechargeable batteries. Advanced materials such as carbon nanotubes are being investigated to further extend ultracapacitor abilities.