Battery-Ultracapacitor based Hybrid Energy System for Standalone power supply and Hybrid Electric Vehicles - Part I: Simulation and Economic Analysis

Electric Vehicles (EVs) require energy storage system that can address its fast changing load requirements such as sudden acceleration or quick regenerative braking by being able to supply or absorb high power as demanded. Lead Acid Batteries, commonly used in cheaper EVs (like electric tempo in Nepal), can properly supply these variable loads and get recharged but this mode of operation drastically reduces the life cycle of the battery. A solution to improve the life cycle of the battery and enhance the system’s overall performance is to provide a secondary energy storage device, such as Ultracapacitor, having relatively high power density, to act as a buffer by supplying or absorbing the fast changes in load. This combination can reduce the power demand in battery and improves its life cycle. This paper proposes an approach to interface the Ultracapacitor with the battery using digital hysteresis current mode controlled bi-directional boost converter. This approach has the advantage of simplicity and lower cost compared to other topologies. The system was modeled in MATLAB® Simulink and was simulated for step changes in load and regenerative braking conditions. The results show that the fast changing load requirements are supplied by the Ultracapacitor and the lead acid battery supplies the slow changing demand and steady state demand. Considering the average life of batteries in vehicular system as 15 months, the storage system is found to be economically feasible if the battery life is extended to 18 months. Index Terms Boost Converter, Electric vehicles, Ultracapacitor.