# Multiport Energy Management System Design for a 150 kW Range-Extended Towing Vessel
###### Abstract  
This paper proposes a multiport energy management system (EMS) and its rule-based expert control strategy for a 150 kW range-extended towing vessel (RETV). The system integrates a diesel generator system, a permanent magnet synchronous motor, a lithium battery, and supercapacitors. To verify its feasibility and effectiveness, the proposed multiport EMS was modelled and tested through MATLAB/Simulink. Simulation results demonstrate that the designed multiport EMS works efficiently under the five typical operating conditions of the 150 kW RETV. In addition, two case studies were conducted and compared to investigate the impact of the battery's initial state of charge (SoC) on the system's energy efficiency. It was found that an overall 85% energy efficiency can be achieved for the RETV when the initial SoC is either 75% or 15%. The battery consistently operates within the optimal SoC range of 20% to 80%, and the supercapacitors effectively meet the instantaneous high-power demand.  
energy management system; energy efficiency; lowing vessel; battery; expert control strategy +
Footnote †: journal: _sciences_  
control strategy is the most effective method for commercial range-extended systems [7]. However, the design of the rules requires rich engineering experience, mathematical models, and extensive data [21]. Despite the wide application of rule-based energy management strategies in range-extended EMS, there are still opportunities for fuel and energy efficiency enhancements [22; 23].  
In addition to energy efficiency, several other challenges exist for developing an efficient EMS for (range-extended) hybrid electric vessels. First, the peak discharge current can significantly reduce lithium batteries' lifespan. Second, the ESS with lithium batteries only struggles to meet the high transient power demand of the modern motors in vessels [24; 25; 26]. Although integrating supercapacitors can resolve these issues, this solution imposes higher requirements on the EMS control strategy and the ESS size [27]. The lithium battery should maintain within the optimal state of charge (SoC) range during operation. Insufficient SoC level will significantly reduce lithium battery efficiency and life span [28; 29]. Predicted operational conditions might differ from actual conditions in SoC levels, potentially leading to SoC imbalance risks [30].  
In this study, a multiport EMS was designed and modelled for a 150 kW range-extended towing vessel (RETV) by using MATLAB/Simulink. The multiport EMS included a diesel generator system, a drive system with a permanent magnet synchronous motor (PMSM) and its controller, a lithium battery, and supercapacitors. A typical operation cycle with five operation conditions was proposed for the investigated 150 kW RETV in terms of power demand. Then, a rule-based expert control strategy was proposed to control the EMS to improve the overall energy efficiency of the RETV. Experimental scenarios were designed to verify whether the proposed multiport EMS enables the RETV to balance fuel economy and battery life, and reduce the high current impact. The main contributions of this work are as follows.  
* Define five typical operating conditions for a 150 kW RETV regarding the energy flow and power demand.
* Propose a rule-based expert control strategy, enabling the EMS to support various power demands.
* Construct the EMS simulation platform for the 150 kW RETV by incorporating the proposed rule-based expert control strategy.  
The remainder of this paper is organized as follows. Section 2 introduces the components and their models for the investigated 150 kW RETV. Section 3 details the RETV operating conditions for an operation cycle and the proposed rule-based expert control strategy. Section 4 details the simulation of the multiport EMS to verify the proposed expert control strategy, followed by the conclusion.