Owing to increasing fuel price and strict environmental legislation, the use of hybrid electric propulsion systems (HEPSs) has been increasing in recent years. Optimal design has been introduced to play a greater role of HEPS in fuel saving, emission reduction and economic improving. However, the optimization of fuel consumption may lead to degeneration in greenhouse gas (GHG) emission and net present value (NPV). A multi-objective optimization for the HEPS, taking account of fuel consumption, GHG emission and NPV, is discussed in this paper. GHG emission model and NPV model are established to make sure that the solution is environmental friendly and economically feasible. The effectiveness of the proposed method is demonstrated by a case of an anchor handling tug supply vessel (AHT). The HEPS designed by proposed method improves all of three objectives for more than 10% compared with the conventional propulsion system. In addition, the proposed method outperform the optimization of fuel consumption in GHG emission and NPV at the expense of slight reduction in fuel saving. Furthermore, the NPV sensitivity analysis suggests that the development trends in fuel price, electric price and battery industry will help the HEPS to make more economic sense in the future.
Oil price recovering and more rigorous environment legislations have motivated the development of hybrid electric propulsion system (HEPS) (Geertsma, Negenborn, Visser and Hopman, 2017). As shown in Fig. 1(a), diesel engines in HEPS are coupled with generators and power the propellers without mechanical connection, allowing constant-speed-operation under dynamic loads. Redundant energy from generator is stored in the energy storage system (ESS), which acts as a secondary energy source and enables peak shaving. Therefore, HEPS is born with the potential for fuel saving, greenhouse gas (GHG) emission reduction and profit promotion. In recent years, several ships have already been built or retrofitted with HEPS, most of which are offshore supply vessels and tugboats (Ovrum and Bergh, 2015).
It is worth noting that, the advantages of HEPS can not be fully exerted without comprehensive optimization considering fuel consumption, GHG emission and net present value (NPV), which has not been reported. Skinner, Parks and Palmer (2009) employed Genetic algorithm (GA) in the optimal design of submarine propulsion system targeting on maximum overall efficiency of the propulsion system. The comparison of three drive topologies suggested that the design with hybrid electric/mechanical drive architecture is the most efficient. Soleymani, Yoosofi and Kandi-D (2015) optimally designed a HEPS for a medium-size boat taking fuel consumption as objective. A fuzzy-thermostat strategy was introduced to manage the energy flow between the main components, which were optimal sized by particle swarm optimization (PSO) algorithm. Simulation results indicated that the HEPS leads to a 40 % reduction in fuel consumption comparing to the conventional propulsion system. However, in those researches, GHG emission and NPV were not considered which results in sub-optimal design. On one hand, the implementation of HEPS aims to improve the propulsion efficiency (fuel consumption) but also cut the GHG emission. On the other hand, NPV is an indicator of the profitability of the system taking cost and income into consideration, which are crucial for the possibility of widespread application of HEPS. Actually, the minimization of fuel reduction may result in an oversized ESS and the overuse of shore power, which are environmentally unfriendly and economical infeasible.
