**1. Introduction**

Shipping, which is a relatively energy-e fficient, environment-friendly and sustainable mode of mass transport of cargo [1], is the dominant and will remain the most important transport mode for world trade [2]. However, the shipping industry consumes more fuel in comparison with other transport modes [3] and shipping-related emissions contribute significantly to the global air pollution and long-term global warming [4,5]. Correlated with fuel consumption, shipping is responsible for approximately 3.1% of annual global CO2 and approximately 2.8% of annual GHGs (greenhouse gases) on a CO2e (CO2 equivalent) basis [6]. Approximately 15% and 13% of global human-made NOx and SOx emissions come from the shipping industry. It is projected that maritime CO2 emissions will increase significantly by 50% to 250% in the period up to 2050 [6]. Moreover, as fuel cost accounts for approximately 50% to 60% of the total operational cost of a ship [7], a significant fuel consumption reduction will contribute to a considerable save of a ship's operational cost. Consequently, the shipping industry is striving to reduce its fuel consumption and emissions due to the increasingly high fuel

price, social concerns on the environmental impact and the resulting mandatory and strict emission control regulations worldwide [8].

Ship mission profile during the voyage has a significant influence on the fuel consumption and exhaust emissions of ships [9]. Therefore, the ship mission profiles should be taken into consideration when evaluating ship transport performance [10]. However, one of the major drawbacks of the present IMO (International Maritime Organization)'s EEDI (Energy E fficiency Design Index) is that it only considers one operating point without taking the ship's representative mission profiles into account. On the contrary, the EEOI (energy e fficiency operational indicator), which is also developed by IMO, is calculated for a voyage or a number of voyage legs based on real operating conditions [11]. In [12], based on a case study of a handy size Chemical/Product Tanker of 38,000 DWT (Deadweight tonnage), Acomi, et al. investigate the voyage energy e fficiency by calculating the EEOI of the ship using both commercial software and onboard measures. In [13], in the case study of a RoPax vessel, Coraddu et al. estimate the ship operational performance of the ship voyage using the EEOI as the measure by real data statistics and numerical simulations. In [14], Hou et al. optimise the vessel speed of an ice zone ship to find a minimum EEOI in an ice zone. In [15], in the case study of a VLCC (Very Large Crude Carrier) tanker, Safaei et al. address the reduction in fuel consumption of the ship voyage using route optimisation considering ship profile and sea conditions. In [16], in the case study of a bulk carrier, Zaccone et al. develop a 3D dynamic programming optimisation method to select the optimal path and speed profile for the ship voyage aiming to minimise the voyage fuel consumption and taking also into account ship safety and comfort. However, most of the research focuses on route planning and ship speed profile when studying the ship voyage optimisation. During ship operation, propulsion control [17,18], power managemen<sup>t</sup> [19–21] and ship operational speeds [22,23] will significantly influence the fuel consumption and emissions performance of ships. However, quantitative and systematic investigations on the influence of various ship operations, including propulsion control, power managemen<sup>t</sup> and operational speeds, on the ship performance of the whole voyage are still limited.

Hybrid propulsion, which is a combination of mechanical and electrical propulsion, is a promising option to improve the economic, environmental and operational performance of ships [24,25]. In the basic form of the hybrid propulsion system, the propeller can be mechanically driven by an internal combustion engine and/or electrically driven by an electric motor, which may also be able to work as an electric generator. If the electric motor is powered by a hybrid power supply, such as diesel generator(s), natural gas generator(s), fuel cells and/or batteries, it will be a hybrid propulsion with a hybrid power supply system [26]. The operation modes of a hybrid propulsion system include power take o ff (PTO); slow power take in (PTI); boost power take in [27]. Among others, the benefits of a hybrid propulsion include reduced fuel consumption; reduced CO2 emissions and other pollutants; possibility to sail and operate with zero emission in coastal and port areas; greater redundancy; noise reduction; lower maintenance [24,28]. However, di fferent ship types can benefit di fferently from the hybrid propulsion due to their diverse operational profiles [28,29]. In [29], Jafarzadeh and Schjølberg study the operational profiles of eight di fferent ship types, including tankers, bulk carriers, general cargo ships, container ships, Ro-Ro ships, reefers, o ffshore ships and passenger ships, aiming to identify what ship types are able to benefit from hybrid propulsion. Hybrid ship propulsion is typically applied on naval vessels, towing vessels, o ffshore vessels and passenger ships including ferries. However, the current applications and research of hybrid propulsion are mainly limited on small ships, while the applications and research on large ocean-going vessels are rare.

To improve the safety and operability of ocean-going cargo ships and to reduce their global greenhouse gas emissions and the local pollutant emissions in coastal and port areas, few studies on the potential applications of hybrid propulsion and power supply system on the big ocean-going cargo ships can be found. In [30], based on a multi-physical domain model, a conceptual hybrid propulsion system for a very large crude oil carrier (VLCC) has been studied for the potential benefit of improving the ship's safety and operability in heavy sea conditions without reducing the system e fficiency. In [31]

and [32], the potential benefits of hybrid propulsion for large ocean-going cargo vessels to increase fuel efficiency and reduce greenhouse gas emissions and pollutant emissions are investigated as well. In [33], the impact of battery–hybrid propulsion on the fuel consumption and emissions of an ocean-going chemical tanker when sailing in coastal and port areas during port approaches has been investigated. However, it is concluded that the battery–hybrid propulsion for ocean-going cargo ships, even when only sailing at low ship speed in close-to-port areas for a short time, is still not a realistic option nowadays even though it can produce zero local emissions; the main reason is that the required battery capacity is very large and the weight of the battery becomes unacceptable.

Using LNG (liquefied natural gas) as the alternative marine fuel is another promising and attractive solution to reducing the local and regional environmental impact and operational costs of ships [34,35]. Compared to using conventional marine fuels, using LNG produces significantly less pollutant emissions, such as NOx, SOx and PM (particle matter), and CO2 emissions will also be reduced as well [24,36]. Another driver for using LNG as a marine fuel is the current favourable fuel price compared to the increasing price of conventional fuel oil [37]. However, one of the disadvantages in the use of LNG as marine fuel is that it may have a worse impact on climate change (global warming) than using conventional fuels, when taking the life-cycle emissions of methane (CH4), which is a worse greenhouse gas than CO2, into consideration [35,37]. Currently, a relatively small number of ships run on LNG and adopting LNG as a fuel is attractive for ships sailing on fixed routes and large ships sailing in short sea and coastal areas, especially in emission control areas (ECAs) [38–40]. With more stricter emissions regulations coming into force and more infrastructure of LNG fuel growing worldwide, larger ocean-going vessels are expected to select LNG as a fuel in the foreseeable future [39]. There are many publications indicating the potential benefits of using LNG as a marine fuel, however, quantitative investigations on the impact of using LNG as a fuel on the fuel consumption and emissions of ships over the whole voyage, taking the ship's operational profile into consideration, are limited.

This paper will therefore investigate the potential influence of the application of the hybrid ship propulsion and electric power generation system with different fuels as well as various propulsion control and power managemen<sup>t</sup> strategies on the ocean-going cargo ship in reducing the fuel consumption and emissions over the whole voyage.

The main goals and outline of this paper are:


In Section 7, the conclusions, limitations and uncertainties of the present paper and the recommendations for future work will be provided.

### **2. Hybridisation of the Benchmark Chemical Tanker**

The ocean-going 13,000 DWT chemical tanker introduced in [41] has been chosen to investigate the fuel consumption and emissions performance of the ship when transiting in open sea and manoeuvring in coastal and port areas. The propulsion system of the chemical tanker consists of a two-stroke diesel engine working as the main engine, a controllable pitch propeller (CPP) and the shafting system. The electric power generation system includes a shaft generator (power take <sup>o</sup>ff, PTO) driven by the main engine and three auxiliary generators. The ship particulars of the chemical tanker are given in Table 1 and the particulars of the main engine (two-stroke diesel engine) and auxiliary engines (four-stroke diesel engine) are given in Table 2.


**Table 1.** Ship particulars of the benchmark chemical tanker.



In the benchmark ship, originally, the shaft generator can only work in PTO mode. In order to investigate the potential fuel consumption and emissions performance of a hybrid ocean-going cargo ship especially when sailing in coastal and harbour areas, in this paper, the original propulsion system and electric power generation system of the benchmark chemical tanker have been conceptually hybridised. In the conceptual hybrid ship propulsion and electric power generation system (Figure 1), the shaft generator can also work as a shaft motor in PTI (power take in) mode. To investigate the influence of sailing on different fuels on the fuel consumption and emissions of the ship, both the main engine and auxiliary engines have been assumed (conceptually updated) so that they can also use LNG (liquefied natural gas) as their fuels, without considering the details of how the engines will be updated and how different fuels will be stored and managed onboard the ship, which are out of the scope of this paper. So, after the updates, the benchmark ocean-going chemical tanker will have a hybrid ship propulsion and electric generation system.

**Figure 1.** Layout of the updated chemical tanker propulsion system and electric generating system.

### **3. Mean Value Indicators of Fuel Consumption and Emissions**

The mean value indicators of fuel consumption and emissions presented and used in this paper are defined based on the theories introduced in [10,33,41]. When taking the ship mission profile into account and in order to express the ship performance as a single value, an operational average value of energy e ffectiveness and energy (fuel) index has been introduced in [10].

The mean energy conversion e ffectiveness ε*EC* over voyage, which is the weighted average value over the mission profile of the ship that will be defined later in the following chapter, is defined in Equation (1).

$$\overline{\varepsilon}\_{\rm EC} = \frac{\sum\_{\bar{i}} W\_{D,\bar{i}} \cdot V\_{\bar{i}} \cdot \Delta t\_{\bar{i}}}{\sum\_{\bar{i}} \left(\Phi\_{\rm FE,main,\bar{i}} + \Phi\_{\rm FE,aux,\bar{i}}\right) \cdot \Delta t\_{\bar{i}}} \tag{1}$$

where *WD*,*i*, *Vi*, <sup>Φ</sup>*FE*,*main*,*i*, <sup>Φ</sup>*FE*,*aux*,*<sup>i</sup>* and Δ*ti* are the ship dead weight (N), ship speed (m/s), energy flow into main engine (J/s), energy flow into auxiliary engines (J/s) and time of duration in each part of the voyage (h).

Same as for the definition of the mean energy conversion e ffectiveness, the mean fuel index *FI* (g/(ton·mile)) and mean emission index *EI* (g/(ton·mile)) averaged over the whole voyage of the ship are defined by Equations (2) and (3), respectively [33].

$$\overline{FI} = \frac{\sum\_{i} \left(\Phi\_{\text{Fuel}, \text{main}, i} + \Phi\_{\text{Fuel}, \text{aux}, i}\right) \cdot \Delta t\_i}{\sum\_{i} M\_{D,i} \cdot V\_i \cdot \Delta t\_i} \tag{2}$$

$$\overline{EI} = \frac{\sum\_{i} \left(\Phi\_{Emission, main,i} + \Phi\_{Emission,aux,i}\right) \cdot \Delta t\_i}{\sum\_{i} \mathcal{M}\_{D,i} \cdot V\_i \cdot \Delta t\_i} \tag{3}$$

where <sup>Φ</sup>*Fuel*,*main*,*i*, <sup>Φ</sup>*Fuel*,*aux*,*i*, <sup>Φ</sup>*Emission*,*main*,*i*, <sup>Φ</sup>*Emission*,*aux*,*<sup>i</sup>* and *MD*,*<sup>i</sup>* are the fuel mass flow into the main engine (g/h), fuel mass flow into auxiliary engines (g/h), emission mass flow generated by the main engine (g/h), emission mass flow generated by auxiliary engines (g/h) and dead weight tonnage of the ship (t) in each part of the voyage, respectively.
