Optimum sizing and optimum energy management of a hybrid energy storage system for lithium battery life improvement

In this paper, a formulation is developed for sizing of a Hybrid Energy Storage System (HESS) in different applications. Here, the HESS is a combination of Lithium battery and Ultra-Capacitor (UC), which is useful for many high energy and high power applications such as Hybrid Electric Vehicles (HEVs) and renewable energy. The sizing formulas are based on initial cost and 10-years battery replacement cost which is arranged as an optimization problem. For battery replacement cost, the Lithium battery capacity depletion formulas are studied for a LiFePO4 battery. As a case study, application of HESS in a Series Hybrid Electric Bus (SHEB) is considered. The results show by the addition of UC, the Lithium battery life is improved significantly. Furthermore, the optimum sizing of the HESS is dependent to the SHEB driving cycle. Therefore, considering the power profile of the HESS in its sizing process may reduce HESS cost. This effect is studied in three different cycles of the SHEB. In addition, the formulation is applied to cycle-based optimization of the Power Distribution Control Strategy (PDCS) of the HESS by dynamic programming. The results show the optimum PDCS has better LiFePO4 battery life in comparison with the conventional PDCS. (C) 2013 Elsevier B.V. All rights reserved.