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This paper deals with hybrid energy storage system (HESS) management strategies, optimized for urban road electric vehicle applications. These new control strategies aim to extend battery pack durability by reducing charging/discharging current peaks by means of supercapacitors. The optimization is carried out with reference to the case study of a HESS composed of a high power unit, i.e. supercapacitor module plus a high energy unit, i.e. a battery pack, based on nickel-chloride ZEBRA (ZEolite Battery Research Africa) technology. On-board integration of the two storage devices is obtained through a DC/DC bidirectional power converter, as this configuration is particularly convenient for many kinds of urban vehicle operations. The novelty of this work consists in an analytical methodology, based on non-linear programming and calculus of variations theory, to evaluate management strategies characterized by high effectiveness in reducing battery current transients. The identification and optimization of these strategies are initially performed on the basis of a vehicle model, built in the Matlab-Simulink simulation environment. To this purpose, the experimental characterization of the supercapacitor module is obtained with reference to three different models, whose selection depends on the required fitting performance and computational effort indexes, as evaluated in the paper. The energy management strategies identified show promising results in the simulation environment, followed by experimental activities carried out by means of a dedicated 1:1 scale laboratory test bench. The various experimental results presented in this manuscript highlight that the identified λ-control strategy presents effectiveness values up to 57%, close to the ideal results obtained in the simulation environment. In fact, the methodology proposed in this paper, validated by laboratory experiments, definitely reduces the negative consequences of power peaks on the HESS, indicating the real possibility of using these results in the design and control of urban road vehicles.

In this paper, an analytical approach that deals with the optimal sizing of energy storage systems in direct current networks is proposed. In modern power systems, the widespread use of power electronics, storage devices, and automation is driving power engineers to focus on the use of direct current networks. This new focus requires specific tools for the optimal planning and operation of these networks in order to increase energy efficiency and reduce operating costs. This paper is focused on the improvements in the efficiencies of direct current networks, which are characterized by the presence of loads, units for the generation of renewable power, and storage devices. Based on the calculus of variations, an original matrix formulation which starts with the nodal representation of the direct current network is proposed. Two attractive closed-form solutions are presented for minimizing power losses, i.e., (1) a solution based on the approximation of considering the voltage constant at all the network’s busses and (2) a solution based on the linear approximation of the load flow. In both cases, the goal is to minimize losses over a given time horizon (e.g., the daily cycle). The formulation of the problem allows an analytical solution to be obtained that represents a suitable tool for the purpose of designing storage. In addition, the proposed approach can be applied and extended to the optimal sizing of storage systems. The proposed sizing procedure, which uses an analytical approach, is formulated in a general manner that can be used for various storage technologies. The results of numerical applications clearly have demonstrated both the feasibility and accuracy of the methodology to be used in the proposed design. We also propose an interesting parametric study in order to determine the optimal technology and the optimal size of the storage device.

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.