Investigation of ship electric propulsion system performance from environmental and energy eciency perspective

The maritime industry faces many challenges regarding the adverse environmental impact, whether at the level of legislation set by the International Maritime Organization (IMO) or the economic crises that arising from the Covid-19 pandemic. IMO has found that the highest percentage of ship emission is mainly coming from the ship propulsion systems. Therefore, the current research proposes an electric propulsion system to drive the ship instead of the conventional one to reduce ship emissions and enhance energy eciency. As a case study, a passenger ship is investigated. The results showed that the proposed electric propulsion system has lower emission rates than the conventional one by 10%, 21%, and 88% for CO 2 , NOx, and SOx emission, respectively. From an energy eciency point of view, the diesel-electric propulsion system enhances the energy eciency and complies with the required IMO values as actual energy eciency is about 66%, 70%, 83%, and 95% of the required IMO values at baseline, phase 1, phase 2 and phase 3, respectively.


Introduction
Recent regulations set by the International Maritime Organization (IMO) and nancial challenges that arising from the Covid-19 pandemic made the shipping industry faces numerous di culties (Lee et al. 2014; Ammar and Seddiek 2020). Latest measurements from IMO shows that ships emitted 2.6% of the total worldwide Carbon dioxide (CO 2 ) emissions (El-Gohary 2012; Ammar and Seddiek 2021). Therefore, IMO has given a few enactments to lessen the unfriendly ecological effect (Halff et al. 2019). Mohseni et al (Mohseni et al. 2019) pointed that the most elevated level of ship out ow is fundamentally comes from the propulsion system. The solution is to propose a more e cient system than conventional propulsion system (Elgohary 2009;Geertsma et al. 2017). The most e cient solution is electric propulsion system as it can be adopted to different vessel types at different speeds (Nuchturee et al. 2020).
Lim et al (Lim et al. 2019) presented an application of electric system in a ship to reduce nitrogen oxides (NOx) to be accepted with the regulations released from IMO. Offshore supply vessels and Lique ed Natural gas ships can use the electric propulsion system incorporated with Azipod propeller as shown in (Bassam et al. 2016). Another research investigated the evaluation of power and other dynamic speci cations for electric propulsion system (Prempraneerach et al. 2009). Zahedi & Norum (Zahedi and Norum 2013) Validate the electric propulsion system components and its design through a simulation software. Moreover, Bassam et al (Bassam et al. 2017) investigated the hybrid system included electric system and found that an environmental and economic bene ts from the application. Therefore, the application of electric propulsion system onboard ship is very hot research issue because of its economic and environmental bene ts resulted from the previous literature survey.
The aim of the research is to propose an electric propulsion system to drive the ship instead of the conventional system to reduce ship emissions and enhance energy e ciency. The environmental assessment will be based on a comparative analysis between the proposed electric propulsion system and the conventional system through evaluation of greenhouse gas emissions. As a case study, a passenger ship will be investigated.

Case Study Description
The case study for the assessment process of energy e ciency and environmental impacts is selected to be a passenger ship. The ship is operated by Holland America line passenger vessels under name (MS Westerdam) with a capacity of 2366 passengers and 820 crew members. The ship was built in 2004 and sailing under the ag of the Netherlands. Principal speci cations of the ship are shown in Table 1 (Hollandamerica 2021). The ship is suggested to be operated by a diesel-electric propulsion system as shown in Fig. 1 with a total power of 51,840 kW covers both the electric propulsion and auxiliary/hotel power requirements for the ship. The propulsion system can be provided with the required electric power from 6 generators (G) through transformers. Cyclone convertor (C.C) regulates the frequency according to the required propulsion motor speed (P.M). The diesel generator will be operated by ultra-low sulfur heavy fuel oil (ULSHFO) with 0.1 sulfur and a speci c fuel consumption of 155.6 g/kWh with load factor of 90% and e ciency equals to 41.3 %.
The ship is sailing from Canada (Vancouver port) to Japan (Tokyo port) to attract more passengers to increase the nancial bene t of the ship. This sailing route is 10,560 nautical miles (NM) and takes 20 days in one trip the average number of trips per year is ve.

Performance Evaluation Methodology
The environmental performance can be assessed by evaluating the exhaust emissions from ships. The energy e ciency can be assessed by using the procedure recommended from IMO by using Energy E ciency Design Index (EEDI) procedure (Elkafas et al. 2021b).
Firstly, the total emissions during ship cruise can be evaluated by using Eq. (1) which depending also on the type of engine (Eng) like main engine and auxiliary engine.

Results And Discussions
The environmental performance can be assessed by evaluating the exhaust emission rates per trip. The examined emission types are NOx, SOx, and CO 2 as these types are related with IMO regulations. The assessment process depends on the comparative study between the proposed diesel electric propulsion system operated with ULSHFO (0.1% S) and the conventional one operated with MDO (1%S), therefore, the different emissions rates can be compared in Fig. 2. The emissions rates are in ton/trip as discussed in Eq. (1). As shown in Fig. 2, the conventional diesel engine emitted more NOx emission rates than the diesel electric engine as NOx emission have a solid relation with the combustion temperature inside the engine and the combustion of MDO (1%S) will produce higher NOx rates than ULSHFO (Mrzljak and Mrakovčić 2016). The conventional diesel engine will produce higher SOx emission than the diesel electric as SOx emission depends on the sulfur content of the combustion fuel. On the other hand, the CO2 emission rates in two options are very closely as it depends on the carbon content of the fuel, but the diesel electric will produce fewer CO 2 emission than the conventional one.
NOx and SOx emission rates have been compared with the IMO 2016 and 2020 emission-limit rates, respectively. The IMO 2020 SOx and tier III 2016 NOx limits are 1.555 kg/min and 2.008 kg/min, respectively. Figure 3 shows a comparative diagram between IMO limit and the SOx and NOx emission rates for diesel electric propulsion system. It can be noticed that SOx emissions rates for the diesel electric engine comply with the IMO 2020 limits because of it use ULSHFO with a little amount of sulfur.
On the other hand, it can be noticed that NOx emissions from diesel electric engine isn't comply with IMO 2016 limits, therefore, it is recommended to use selective catalytic reduction (SCR) technique which can reduce NOx emission rate from the proposed diesel electric engine by up to 90% (Ammar and Seddiek 2020).
Finally, the energy e ciency can be assessed by the calculation of EEDI for electric propulsion system as recommended from IMO. By conducting the procedure in Sect. 3 to the case study, it is shown that the reference EEDI and its value in the three phases can be calculated based on the gross tonnage of passenger ship as investigated in Fig. 4.
This reference value will be compared with the actual attained EEDI which can be calculated by using Eq.
(3) based on 22 knots service speed, 3.114 ton-CO 2 /ton-fuel conversion factor of fuel to CO 2 and 82,897 gross tonnages. The attained EEDI will be 10.03 g CO 2 /GT-NM. The relative attained EEDI value to the reference value at different phases for the proposed diesel electric propulsion system can be described in Fig. 5.
It is shown that, the proposed diesel electric propulsion system will comply with the required IMO phases now and in the future as the attained EEDI is about 66%, 70%, 83%, and 95% of the reference EEDI value at baseline, phase 1, phase 2 and phase 3, respectively.

Conclusions
Page 6/10 The present research discusses the electric propulsion system for a passenger ship from environmental and energy e ciency point of view. The diesel electric propulsion system operated by ULSHFO (0.1% S) is proposed to propel the ship instead of the conventional diesel engine which operated by MDO (1% S). The main conclusions from the current paper are: From environmental point of view, diesel electric engine operated by ULSHFO (0.1%S) comply with IMO SOx and not comply with NO x emission. Be comparing the exhaust emissions rates between diesel electric and conventional engine, it is shown that diesel electric has lower emission rates than the conventional one by 10%, 21% and 88% for CO 2 , NOx, and SOx emission, respectively.
From energy e ciency point of view, the diesel electric propulsion system enhances the attained EEDI and complies with the required IMO values as it is about 66%, 70%, 83%, and 95% of the reference EEDI values at baseline, phase 1, phase 2 and phase 3, respectively.

Consent to Participate
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Con ict of interest
The authors declare that they have no con ict of interest.

Ethics approval
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Consent for publication
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Data availability
The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Funding
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Authors' contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Ahmed G. Elkafas. The rst draft of the manuscript was written by Ahmed G.   Reference EEDI values for passenger ship Figure 5 Relative attained to the reference EEDI and the average EEOI values for the case study