The increasing environmental concerns due to emissions from the shipping industry have accelerated the interest in developing sustainable energy sources and alternatives to traditional hydrocarbon fuel sources to reduce carbon emissions. Predominantly, a hybrid power system is used via a combination of alternative energy sources with hydrocarbon fuel due to the relatively small energy efficiency of the former as compared to the latter. For such a hybrid system to operate efficiently, the power management on the multiple power sources has to be optimised and the power requirements for different vessel types with varying loading operation profiles have to be understood. This can be achieved by using energy management systems (EMS) or power management systems (PMS) and control methods for hybrid marine power systems. This review paper focuses on the different EMSs and control strategies adopted to optimise power management as well as reduce fuel consumption and thus the carbon emission for hybrid vessel systems. This paper first presents the different commonly used hybrid propulsion systems, i.e., diesel–mechanical, diesel–electric, fully electric and other hybrid systems. Then, a comprehensive review of the different EMSs and control method strategies is carried out, followed by a comparison of the alternative energy sources to diesel power. Finally, the gaps, challenges and future works for hybrid systems are discussed.

The use of hybrid energy systems in ships has increased in recent years due to environmental concerns and rising fuel prices. This paper focuses on the development and study of a hybrid energy system using liquefied natural gas (LNG) and batteries for a tugboat. The hybrid system model is created in MATLAB/Simulink^® and uses fuel data obtained from an operational diesel-powered tugboat. The LNG–hybrid system is then subjected to testing in four distinct configurations: fixed speed, variable speed, and with and without a battery. The different configurations are compared by computing the daily fuel cost, CO₂ emissions, energy efficiency operation indicator ($EEOI$) and carbon intensity indicator ($CII$) ratings in three distinct operation cases. The analysis reveals that the use of an LNG–battery hybrid tugboat results in an average reduction of 67.2% in CO₂ emissions and an average decrease of 64.0% in daily fuel cost compared to a diesel system. An energy management system using rule-based (RB) control is incorporated to compare the daily cost and CO₂ emissions for one of the case studies. The rule-based control that requires the battery to be used and the LNG engine to be switched off at the lowest allowable minimum power based on the specific gas consumption produces the most cost-effective control strategy out of all the different control strategies tested. The result demonstrates that an additional reduction of CO₂ and daily fuel cost for LNG–battery hybrid tugboats by 23.8% and 22.3%, respectively, could be achieved with the implementation of the cost-effective strategy as compared to not having a control strategy.

This paper reviews the application of Life Cycle Assessment (LCA), Life Cycle Cost Assessment (LCCA), and ship design optimisation methods and examines the extent to which these approaches have been integrated to support sustainable electric ship design. The main goal is to understand how these methods have been applied and where the key gaps remain. The review analyses current practices in environmental and cost assessments of ship systems and design, along with ship design optimisation methods aimed at improving energy efficiency and reducing emissions. Overall, the reviewed literature shows that LCA, LCCA, and ship design optimisation are generally applied as separate processes rather than parts of an integrated framework. Key gaps include the lack of models that account for time-varying costs, changes in technology and energy systems. Additionally, this paper highlights the need for integrated and adaptive frameworks, supported by open and standardised data and digital tools, to better connect environmental and economic considerations across a vessel’s life cycle, for more practical and sustainable ship design.