Implementation and Evaluation of an Ultracapacitor-Based Auxiliary Energy System for Electric Vehicles

Searching for a better efficiency, an auxiliary energy system for electric vehicles was designed, implemented and tested. The system, composed of an ultracapacitor bank and a buckboost converter, was installed in an electric vehicle, which is powered by a lead-acid battery pack and a 54 kW brushless DC motor. Two control strategies where developed: one based on heuristics and the other based on an optimization model using neural networks. These strategies were translated to algorithms, implemented in a DSP and their performance was evaluated in urban driving. The results were incorporated to an economic evaluation of the system, showing that the reduction in costs would only justify the inclusion of this type of system to a lead-acid battery powered vehicle if the battery life is extended in 50% or more, which is unlikely. The same results where extrapolated to a case in which the lead-acid batteries are replaced by a fuel cell. In this case, costs of different power support systems where evaluated, such as ultracapacitors and high specific power lithium based batteries. The results showed a significant cost reduction when auxiliary energy systems configurations are included, in contrast to a system powered by fuel-cells only. Also, the cost reduction was higher when using ultracapacitors for this purpose.