Optimal Sizing of Battery Energy Storage Systems for Hybrid Marine Power Systems Rene Barrera-Cardenas, Olve Mo and Giuseppe Guidi Energy Systems, Sintef Energy Research, Trondheim, Norway Rene.Barrera-Cardenas@sintef.no, olve.mo@sintef.no, giuseppe.guidi@sintef.no Abstract—This paper introduces an analytical design methodology for Battery Energy Storage Systems (BESS) in hybrid marine vessels. Models for performance evaluation of a BESS composed by a battery array, an AC/DC power converter and a transformer are introduced and used to evaluate the potential fuel savings, projected lifetime and cost-benefit of BESS installation on a considered hybrid marine power system. By parametric sweep of free design parameters, the trade-off between different performance indices is analyzed and the optimal sizing (energy capacity and rated power) of the BESS can be obtained. Keywords— BESS sizing, BESS design, Hybrid Marine Power Systems, Hybrid Electric Vessel. I. INTRODUCTION Stricter regulations for marine vessels are undergoing like the introduction of maximum Energy Efficiency Design Index for new ships [1, 2] and the planned expansion of the current Emission Control Area in Europe [1], so there is a need for reducing fuel consumption and cut emissions in future Marine Power Systems (MPS). Normally, the vessel operation is restricted by rules, regulations and procedures for safety, so it is often not possible or allowed to operate diesel/gas generators at best efficiency [3]. Introducing an Energy Storage System (ESS) in MPS introduces an extra degree of freedom for the Energy Management System (EMS), which can be used to improve ship overall energy efficiency under different operation conditions, making the Hybrid-MPS (HMPS) an attractive solution for future marine vessels. The choice of ESS technology is related to the ESS- technology's performance and functional requirements that best fits the area of application. An obvious application of ESS in HMPS is to serve as backup power source, which requires an ESS technology with high energy density and rate of discharge [4]. Many suitable ESS technologies are offered in the market Today. ESS based on battery technology have been successfully applied in road transportation [5, 6], resulting in major improvements of battery technology regarding key performance parameters like energy density, lifetime and safety. Volume production has also contributed to significant cost reduction [7, 6]. Therefore, a Battery Energy Storage System (BESS) can be considered as good alternative for Hybrid Marine Power Systems. This paper introduces a design methodology for optimal sizing of BESS in HMPS. The methodology explores the trade-off between different performance indices (potential fuel savings, projected BESS lifetime and cost-benefit index) by parametric sweep of free design parameters. The methodology is applied on a HMPS with onboard Diesel Generators (DGs) and BESS without charging from shore. However, the proposed methodology can be adapted to others HMPS, like 'plug-in' HMPS. As the use of BESS can contribute to increase HPMS performance in several different ways [8, 9], then two specific cases are considered to illustrate the proposed methodology: Strategic Loading (SL) and Spinning Reserve (SR) [3]. When BESS is used for SL, the operation point of DGs is shifted by cycling the BESS to minimize fuel consumption. In the SR case, the BESS is considered as part of the backup source in case of contingencies, so the number of running DGs could be reduced for optimal operation. First, the models and parameters for evaluation of power losses, cost and lifetime of a battery energy storage system based on Li-ion battery technology are presented in section II. Then, the BESS design methodology is introduced in section III, where the BESS models are used to evaluate the HMPS performance. The results for BESS design solutions from a parametric sweep of considered free design parameters, are presented in section IV. Finally, main conclusions are summarized in section V. II. MODELLING OF BATTERY ENERGY STORAGE SYSTEMS A. BESS Topology The considered BESS topology is shown in Fig. 1, and it is composed by a battery array, an AC/DC bidirectional power converter and a line transformer. The battery array consists of n p parallel connected strings, each of them with n s battery cell/modules series connected to fulfill the desired DC voltage (V DC ). The AC/DC power converter is assumed to be a two-level Voltage Source Converter (VSC), which is a Cell/ Module Cell/ Module Cell/ Module Cell/ Module Cell/ Module Cell/ Module Cell/ Module Cell/ Module Cell/ Module String 1 String 2 String np 1 2 ns I BM I B + V DC ‐ Main Choke & Filter AC breaker on‐board ac bus Power converter Battery Array Transformer Fig. 1 Considered Battery Energy Storage System TABLE I. CHARACTERITICS FOR THE CONSIDERED BATTERY MODULE Property Unit Value Reference Seanergy 48P from Saft Nominal Voltage ( ௕௘ )  46.2 Nominal Energy at 0.2 C-rate ℎ 2600 Nominal Capacity at 0.2 C-rate ℎ 60 Maximum continuos currrent ( ௕஼.௠௫ ,  ௕஽.௠௫ )  240 (Charge) 240 (Discharge) Voltage Window  37.8 to 53.2 Equivalent Resistance ( ௕௘ ) mΩ 16 (Charge) 12 (Discharge) Nominal Energy Density Wh/l 76 This is the accepted version of a paper published in 2019 IEEE Electric Ship Technologies Symposium - ESTS 2019 DOI: 10.1109/ESTS.2019.8847932