*4.1. Increasing the E*ffi*ciency of Ships*

The most obvious way of limiting the environmental footprint is the reduction in fuel consumption, which means increasing the overall efficiency of ships. Thus, the shipbuilding industry is continuously working towards improving hull forms, engines, and propellers. Setting aside the hydrodynamic area, the reduction of the ship's pollutant emissions down to a level compliant with the above-depicted goals can be reached by means of three strategies [86]. These are: the switch to a different, cleaner, fuel (e.g., low sulphur fuel oil, natural gas, methanol, etc.); the installation on board of equipment for exhaust gas treatment (e.g., SCR, scrubbers, etc.); and the improvement of a ship's electrification. The use of other fuels can lead to different results, both on the pollutant emissions reduction and the overall ship's fuel consumption, depending on the specific fuel used. The second method is an application of technical solutions already used in other fields, like automotive and land power generation, with some specific modifications due to the particular marine environment. These two strategies have pros and cons, but are both aimed at improving the internal combustion engines (ICEs) performance in terms of pollutant emissions. Conversely, the electrification option implies substituting existing ICE with electrical motors, thus providing: an increase in power output controllability and power density; a reduction in noise, vibrations, heat, and maintenance complexity; more degrees of freedom for arranging the engines in the engine room. Thus, similarly to what is happening in other transportation modes, electrification is one of the most applied methods to improve ships' efficiency and reduce their environmental footprint [87]. Obviously, given the power required by a ship (as an example, the propulsion system of a cruise liner can reach a power of tens of MW [88]), the primary source of electric power is commonly a set of ICE powered generators. In particular, diesel generators are the most used, but generators powered by gas turbines can be also found, typically in naval applications. For these generators, the use of different fuels and the adoption of exhaust gas treatment are viable options to reduce further the ship environmental impact. Other methods to generate electric power are starting to be applied, like fuel cells. Regarding the latter, their use as main power sources is limited to small crafts, while there are some experimental installations on board large ships of fuel cells as auxiliary generators.

However, at present there is no viable option for totally powering a big ship without using fossil fuels. Indeed, the significant energy density of fossil fuels is one of the enabling factors that make big ships feasible, given their scope of work and their typical routes. To give an example, the very large container and bulk carriers that are used to ship goods all around the world require a range of up to 7500 nautical miles to be usable (e.g., the route from the port of Singapore to the port of Trieste is circa 7100 nm long, and must be sailed without intermediate stops to avoid lengthening transport times, and thus losing money). This means that such ships have up to 5000 m3 of fuel on board, which is usually replenished only at the departure and arrival ports. Conversely, ships for local transport (in a range of a thousand nautical miles) have nearly 1000 m3 of fuel on board, and the refuelling is done only at the home port due to economic reasons (there are bilateral contracts and national taxation differences among ports, making the refuel in other ports not economically convenient). Besides the energy density of fossil fuels, which is still not in the reach of present energy storage systems and other energy production systems, there is also the issue of the refuelling times. In fact, a ship's refuelling is usually done by means of tanker ships, that have tanks and pumps installed on board, dedicated to

such a task. The tankers can refuel a large ship at nearly 500 m3/h, while smaller ships (the local transport ones) are usually refuelled up to 250 m3/h, by using in both cases electric pumps that can reach 2–3 MW of power. These figures mean that a complete refuel can be achieved in 10 h for a large ship, and 4 h for a small one (excluding all the logistic and preparation times). By assuming a volumetric energy density of about 36 MJ/l for the fuel (that is nearly 10 MWh/m3), it is possible to calculate an equivalent refuelling power of 5 GW for a large ship, and 2.5 GW for a small one. Even assuming that it is possible to install on board an energy storage system capacity capable of delivering the required autonomy, the required recharging power level is so high it may be unsustainable for actual port power systems. For the tankers, these are refuelled in their port, with a speed that is very dependent on the specific port operation. Thus, at present, the focus on ships is given to the increase of the overall fuel efficiency of the ship, focusing on the different aspects stated above.

Considering only the propulsion system electrification, several different architectures can be applied. The first is the series configuration, where the propellers are powered by electric motors, and ICEs are used only to generate electric power (Figure 4). The second configuration is the parallel one, also called hybrid propulsion system [89], where electric motors and ICEs are both connected to the propeller (Figure 5).

**Figure 4.** Electric propulsion, series configuration.

**Figure 5.** Electric propulsion, parallel configuration.

For ships with ship services loads requiring a significant electric power (e.g., cruise liners), the integrated power system (IPS) configuration is a viable option (Figure 6) [88]. In such a case, the same set of ICE generators is used to power both the propulsion and the other loads through a single power system [90]. Such a configuration allows obtaining a reduction in the total size of on-board generators in respect to the use of separated sets of generators, one for each subsystem. Moreover, the IPS architecture can exploit either the series or the parallel configuration for the propulsion system, depending on the ship operative requirements. Nowadays, the evolution in power electronics is

pushing forward the performance levels achievable from the electric drives, leading to the pervasive presence of power electronics converters on board ships. These can be used to control electric motors, manage power flows on the power system, and interface energy storage systems, leading to the so called integrated power and energy system (IPES), shown in Figure 7 [91,92].

**Figure 6.** Integrated power system architecture.

**Figure 7.** Integrated power and energy System architecture.

The integration of energy storage systems is useful for further increasing the overall ship efficiency. In fact, small ships can sail short routes on stored power only, while big ships can use energy storage systems as a transitional source of power to be used in emergency, avoiding running additional generators in some operative conditions. Moreover, storage systems can be used to perform peak shaving, allowing installing on board smaller generators, with a consequent reduction of weights, volumes, and overall fuel consumption [93].

In addition to the propulsion system electrification, also the replacement of mechanically driven equipment (e.g., pumps, cranes, etc.) with electrically driven ones is a viable option to increase the overall ship efficiency. The coupling of these new electric loads with power electronic converters enables further improvement in efficiency, reliability, and performance, by removing mechanical regulation equipment. Obviously, the introduction of these additional loads requires an increase in the on-board generators' power, further motivating the shift towards IPS or IPES configurations for ships.

### *4.2. The Issue of Designing E*ffi*cient Ships*

The design of a ship is a complex process, because it is necessary to integrate different subsystems in a reduced space with several constraints. In fact, each subsystem is needed for the correct operation of the ship, but at the same time, it is in competition with the others for space and weight allocation. Ship designers have to consider both naval architecture issues (e.g., stability, hull form, manoeuvrability, structure, etc.), ship's operative requirements, costs, efficiency, and so on. The ship designers must in fact design a system of systems [94], where the optimal design solution for the overall ship is never the sum of the optimal solutions for each subsystem design. In fact, in the shipbuilding industry the most significant proof of concept is not related to the demonstration of a working technology. Instead,

it is given by the demonstration of achievable results in improving ships' key performance indicators (KPI—such as space, weight, safety, and efficiency). Thus, a promising technology which is able to increase the efficiency of a single subsystem may prove to be irrelevant for increasing efficiency in a ship, or may worsen it either due to its effect on other subsystems or due to the ship's specific operative requirements. As an example, electric propulsion allows a significant reduction in fuel consumption compared to mechanical propulsion mechanisms in ships with several different operative speeds, thanks to the small loss of efficiency of electric drives at variable loads. Conversely, ships sailing for long times at constant speed (e.g., tankers) achieve the lowest fuel consumption by using mechanical propulsion, due to the high efficiency of low speed 2-stroke diesel engines when operated at their optimal load point.

In such a context, integrating new technologies dedicated to the efficiency increase into an existing ship is an engineering challenge, possibly being unfeasible or requiring an amount of modifications so high as to make the obtainable gains not justifiable. Likewise, the design of a new ship able to exploit new subsystems to achieve a more efficient operation is a complex task too. Ship designers have to take into account the integration of the new technologies since the first stages of the ship design (i.e., the early stage design), to assure their correct on board exploitation [95–97]. Such an approach is required, since the most impacting decisions about ship design are taken during the first stages of the design process, and cannot be changed on later stages without deeply affecting costs and times. For common ships' designs, the previous knowledge base is, in general, sufficient for making a correct guess about viable design solutions. Conversely, technologies with a significant impact on the ship's KPI may require proceeding with several tests before reaching a feasible design [98].

To overcome the need of making design choices based on uncertain data, tools are being created. In fact, the advancements in information technology led to the creation of software tools, aimed at easing the designers' work [92,99,100]. These tools allow inferring the effect of the design choices overall ship, already during the early stage design. Consequently, designers can compare different solutions in terms of the ship's KPI, possibly leading to the choice of the best overall design. Moreover, such tools provide a means to assure the correct on board integration of innovative technologies, thus ensuring the achievement of the expected efficiency increase, as well as other advantages [101].

Finally, it has to be highlighted that the pursuit of an increase in ships' efficiency may have a significant impact also on other applications. The most obvious case is the need to connect the ship to the port electrical power system (i.e., shore connection [102,103]), for recharging on-board installed energy storage systems or at least shutting off on-board generators. Such a practice can reduce the pollutant emissions in the ports (which are commonly placed inside cities), as well as increase the overall system efficiency (ship's on board generators have a higher CO2 footprint in respect to the land power grid). However, in order to use a shore connection a specific set of equipment is to be installed on the berth, and the port power system may need to be refitted to support the additional load (e.g., a cruise ship requires up to a MW when at berth). Another example of the global impact of efficiency increase methods can be made referring to the use of cleaner fuels. Indeed, besides the modifications to ship's engines and fuel treatment systems, it is required to create a dedicated supply chain for these new fuels.

### **5. Conclusions**

The paper includes a comprehensive review on efficiency issues related to three important sectors of the transportation systems: railways, electrical vehicles and marine. The measures recently investigated and implemented, for improving three transportation systems from the point of view of efficiency, are reported and analyzed. Many actions deal with the application of devices, apparatus and systems that can improve the efficiency acting on the infrastructure, but other several actions deal with the suitable management techniques for existing systems, also with the aim of integrating them in a wider system. The analysis pointed out that a common way to compare the increased efficiency of the transportation system in the three analyzed sectors is to assess the percentage reduction of the

CO2 emissions, thanks to the implementation of different electrification solutions of various means. For the rail transport sector, these solutions are related to changes to an already existing electrical infrastructure, that can be integrated with innovative devices (e.g., storage) and management systems (e.g., traffic control), differently from the on road and maritime sectors, where the improvement of the efficiency can be achieved through electrification actions of the vehicles and of the propulsion systems.

However, there are several different factors that make a direct comparison among the different transportation systems a very complex work. First of all, railway, road, and marine sectors can be either integrated or mutually exclusive transportation means. Indeed, for goods transportation at present there is a good integration among them, which implies using ships for long-range transportation, then railways for medium range transportation, and finally road vehicles for short range delivery. This means that a complete assessment of the efficiency of the goods transportation framework includes all of the above described systems, with ratios that depend on the specific start and finish point. Conversely, the people transportation is caused by a different set of needs and aims, depending on the transportation system. Road and railway transportation can be partially overlapped in this regard (excluding locations that are not reachable by train only), while ships are used either for marine routes that cannot be achieved by other means, or for leisure trips. This makes an efficiency comparison among the three systems fully dependent on the specific type of application and route. A second critical point in such a comparison is the strict dependency among the CO2 footprint of the recharging energy, for grid connected vehicles. As depicted in this paper, different countries have different energy mixes for producing the electrical energy, making it possible to compare these figures for short range transportation only. As an example, the CO2 emissions of an electric truck that delivers a given set of goods going through different countries is a composition of the amounts of emissions given by the energy mixes used in the places where it stops for recharging. Moreover, given a single starting point and a single arrival point, different routes can be taken, implying different energy consumption and different recharging energy mixes. Thus, a single figure, able to provide a full assessment and comparison of the efficiency among the presented transportation systems cannot be provided.

So, independently from the common metric for the assessment of efficiency and its improvement (in relation to different solutions adopted in the three transport sectors), the main result of the review reported in this paper has been collecting all the possible measures that can be applied to the three transport sectors for improving the efficiency, in function, of their peculiarities. It contributes to the idea of what has been already done within this field and what could be the margin of development to be explored in the future.

**Author Contributions:** Conceptualization, M.B., M.C.F. and G.S.; methodology, M.B., M.C.F. and G.S.; validation, M.B., M.C.F. and G.S.; investigation, V.B., F.F., A.R., A.V.; resources, V.B., F.F., A.R., A.V.; data curation, V.B., F.F., A.R., A.V.; writing—original draft preparation, V.B., F.F., A.R., A.V.; writing—review and editing, M.B., M.C.F. and G.S.; supervision, M.B., M.C.F. and G.S., M.C.F. and A.R. mainly contributed on Section 2; M.B. and F.F. mainly contributed on Section 3; V.B., G.S., and A.V. mainly contributed on Section 4. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare that there is no conflict of interest regarding the publication of this paper.

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