Optimal design and energy management of hybrid storage systems for marine propulsion applications

This paper discusses the themes of optimal design and management strategies of hybrid energy storage system (HESS) for marine applications. This design and related strategy are aimed to improve battery pack durability, ensuring a smooth profile of the required current, through the complementary action of super-capacitors. A DC/DC bidirectional power converter allows for the integration of these devices and control of on-board power fluxes. The proposed methodology is based upon the solution of a constrained optimization problem. In order to overcome computational issues, proper formulation of the mathematical problem is based upon the Ritz method, which is one of the direct methods for solving problems related to calculus of variations. The solution of the optimization procedure, which is the key finding in this paper, permits to obtain battery voltage and current profile, which enables the designer to determine battery and super-capacitor sizes and also provides battery/super-capacitor reference current profiles to be tracked by the DC/DC bidirectional power converter. The procedure is evaluated with reference to the case study of a water-bus operating in the Venetian lagoon, whose operative cycle has been acquired during proper measurement campaigns. Then, the performance of energy management strategies are verified by means of laboratory experimentations on the hybrid energy storage system, which have been carried out by scaling operative cycles at single storage cell level. The obtained results show the potential advantages of proper design and energy management obtained through this new methodology, in terms of investment and maintenance costs for sustainable maritime transport.