Pareto-Based Design Optimisation of Hybrid Energy Storage Systems for Full-Electric Vessels

Full electric vessels can benefit from hybrid energy storage systems (HESS) that combine two storage technologies of different characteristics in terms of power and energy density. The optimal design of a HESS for a vessel is generally a rather complex multivariable optimization with several degrees of freedom and constraints. Indeed, the optimization should account for the operational characteristics of the storage units, including their progressive aging. Moreover, the sizing of the storage units is tightly linked to the strategy implemented in the energy management system (EMS) for allocating the power needed by the load to the storage units. This paper presents a two-stage Pareto-based design optimization procedure for HESS intended for a full-electric vessel. The methodology first identifies a Pareto front as the set of all the optimal configurations in terms of capacity of the two storage units that fulfil the operational constraints within a large discrete configuration space. These constraints account for capacity degradation and limitations in power and energy. The degrees of freedom in the EMS are included in the configuration space. A second stage identifies the optimal configuration on the Pareto front based on a defined cost function. The approach decouples the analysis of the solutions that can fulfil the operational constraints from the optimization and can be very effective in exploring the effect of several alternative cost functions on the optimal solution. Moreover, the shape of the Pareto front can offer a visual clue to the benefits offered by a hybrid storage compared to a single technology solution and on the optimization margins. The procedure is illustrated with a case of a full electric tugboat highlighting when a HESS can be beneficial and how the optimal design can be facilitated.