Comparison of the topologies for a hybrid energy-storage system of electric vehicles via a novel optimization method

The combination of batteries and ultracapacitors has become an effective solution to satisfy the requirements of high power density and high energy density for the energy-storage system of electric vehicles. Three aspects of such combination efforts were considered for evaluating the four types of hybrid energy-storage system (HESS) topologies. First, a novel optimization framework was proposed and implemented to optimize the voltage level of a battery pack and an ultracapacitor pack for the four types of HESS topologies. During the optimization process, the dynamic programming (DP) algorithm was iteratively applied to determine the optimal control actions. The simulation results with DP were used to evaluate the energy efficiency of different HESS topologies at different voltage levels. Second, the optimized voltage level of the battery and ultracapacitor in each topology indicates that a higher voltage level usually results in a better system performance. The simulation results also illustrate that the optimized rated voltage level of the battery pack is approximately 499.5 V, while for the ultracapacitor pack, the optimized voltage level is at its maximum allowed value. Note that the constraint of the battery voltage is initialized at 200-600 V. Third, the control rules for different HESS topologies were obtained through the systematic analysis of the simulation results. In addition, advantages and disadvantages of the four topologies were summarized through evaluation of the efficiency and operating currents of the batteries and the ultracapacitor.