Shipboard distributed energy resources: Motivations,challenges and possible solutions in the cruise ship arena

Abstract

The International Maritime Organization has developed regulations intended to increase the ship’s efficiency both in design phase, through the introduction of an Energy Efficiency Design Index (EEDI), either in management phase, adopting the Ship Energy Efficiency Management Plan (SEEMP). Several approaches and technologies adopted in land-based engineering in the field of energy efficiency can also be advantageous for marine applications. This is the case of the Distributed Energy Resources (DER) solution applied in land-based microgrids, which increases both the system’s efficiency and reliability. In this work, a distributed energy system will be considered for a 140000+ GRT cruise ship, installed for a superior performance in terms of safety and energy efficiency. Number, typology, size and integration on board of the generation units will be defined in relation with aspects of zonal independence, electrical load, weights, volumes, fuel tanks, supply systems, auxiliaries, with the minimum possible impact on commercially valuable space and a special focus on the fuel cells technology. The critical issues in relation with the present safety rules and the whole ship design process will be addressed as a fundamental aspect.

Keywords

Cruise ship innovative energy generation shipboard power systems distributed energy resources LNG fuel cells microgas turbines

1. Introduction

Cruise vessels are one of the most challenging and technologically advanced engineering systems. In fact, their design process involves a wide range of knowledge: from engineering and physics to interior design, logistics and economy. Moreover, it should be noted that, in a cruise ship, passengers are provided with a very high standard of accommodation and leisure facilities. This results in a large superstructure as a prominent feature of the vessel and a high level of power required by the on-board users and the propulsion system as well. Nowadays, these ships present an integrated electric power system, where all the main users and especially those related to the propulsion, are powered by the shipboard power system. In this perspective, in the near future it would be possible to talk about the well-known all electric ship (AES), where all the users are powered by electricity. This continuous electrification of the on board systems is also due to the recent worldwide increase of attention on the environmental issue. In fact, the International Maritime Organization (IMO) has introduced new normative aimed to reduce the pollutant emissions of greenhouse gasses (GHG) from the ships [28,29]. This conversion to the electric power has already enhanced the ship’s flexibility, and reliability; also allowing the integration on board of alternative energy sources. In this perspective, aim of this paper is to present possible solutions in order to implement a distributed energy resource (DER) approach into the cruise ship design. This kind of approach has already been introduced for land-based applications such as the well-known microgrids, where both the electric and thermal energy sources are integrated in a more complex and efficient system [11,13]. Modern ships are considered as marine microgrids [23]. Therefore, a cross fertilization between terrestrial applications and marine ones seems possible, which should result in more efficient, flexible and reliable ships [25]. Nevertheless, several challenges and restrictions may be faced in this process. These are mainly due to the environment in which ships operate, to environmental normative and to safety requirements and to the limited spaces and volumes available on board [30]. Moreover when an international combustion engine (or set of combustion engines) with a total rated power of more than 375 kW is situated in a single room, this is considered regulations a Cat. A engine room, with all the relevant safety requirements for example in terms of fire fighting. In this paper such spaces are indicated as DER volumes or DER rooms. In the following paragraphs, the implementation of alternative fuels and innovative power sources are proposed for cruise ships together with an analysis on costs, volumes and efficiency of each solution, compared to the traditional design. In this context, a case study ship will be presented and considered in this work. Furthermore, the interaction between the electrical and thermal energy is studied and energy recovery solutions are proposed in order to enhance the whole efficiency of the ship.

2. Innovative solutions for shipboard energy systems and integration with ship design approach

Due to the ever-increasing amount of electrical power installed on board the new cruise vessels and due to the ever stringent environmental and safety normative, in this paragraph, several innovative power generation systems will be introduced and described. The main characteristics of these solutions will be analysed depending on the fuel adopted, their integration with the possible power system configurations, their costs, volumes and weights. In this perspective, the main challenges on the integration of these solutions on board a cruise ship are introduced, discussed and solutions are presented.

2.1. Distributed energy resources on board ship

Shipboard power systems can be properly identified as marine microgrids, where the loads and the generation units are geographically close. Moreover, these power systems usually operate in islanded mode, although these can be also connected to the shore grid. Therefore, it would be possible and useful to adopt some technologies and practice developed in the recent years for terrestrial microgrids. In this perspective, the distributed energy resources (DER) approach, which has been developed in land applications in order to integrate renewable sources into the grid, could be advantageously adopted also on board ships in order to increase both the efficiency and reliability of the whole system. In fact, a drastic improvement of the ship energy performances can be possible by adopting innovative configurations for the shipboard power systems combined with the integration of alternative power generation units such as fuel cells, micro gas turbines and energy storage systems [53].

However, one of the most challenging task on adopting these innovative generation units is the proper storage of alternative fuels such as the natural gas (NG) or hydrogen ( $H_{2}$ ). The working principle of the DER is the distributed installation of small generators in order to increase the availability of electric and thermal power sources close to the users; minimizing in this way the thermal losses, system oversizing, and pollutant emissions [44,53]. Therefore, this approach mainly consists in dividing and distributing on board the energy sources, which were previously concentrated in the engine room. Consequently, the traditional configuration of the power distribution system should be reviewed in order to maximize the benefits of the DER approach. In this context, a novel configuration of the grid can be selected instead of the traditional one. The DER approach is in principle characterized by a superior adaptability to the real load request, permitting at the same time a better optimization of the single energy modules. This is at the base of the possible improvement to the avoid system oversizing, mentioned above. In this paper the issue has not been specifically addressed and a further investigation could be carried out with reference to realistic thermal and electrical load profiles.

The traditional radial configuration is characterized by a centralized generation system in medium voltage (MV), a MV primary distribution system from the main buses to the power transformers (e.g. which are installed in each accommodation and galley substation) and a secondary distribution in low voltage (LV) to fed the distributed loads on board the ships. Each substation is a single branch of the network, which is powered by the primary distribution system through the substation transformer, as proposed in Fig. 1. Being designed for a centralized power generation system, this configuration is well-suited for traditional applications.

Fig. 1.

Traditional radial distribution system configuration.

Fig. 2.

MV ring distribution system with substations connection in LV.

In the perspective of introducing a DER approach, another configuration is shown in Fig. 2, where a MV ring configuration is proposed. This can supply power to each single substation from two sides and, in case of failure, allows a complete reconfiguration of the network. Several power transformers have been eliminated and groups of substations connected to each other in LV (e.g. in red in Fig. 2). The distributed generation units have been positioned in each substation in order to cover their normal operating load; the LV connection between substations increases the flexibility of the power system and its reliability in case of fault of one unit.

The most interesting technologies for the distributed power generation are the micro-Gas Turbines (mGTs) and fuel cells. The firsts being characterized by a small size, reduced weight and very limited emissions (NOx, HC and SOx) [45] and the seconds by a very high efficiency, modularity and almost zero emissions in terms of noise, vibrations and air pollutants [42,44,51]. Fuel Cells can be fuelled with pure Hydrogen ( $H_{2}$ ) or Natural Gas (NG) properly processed, while mGTs are fuelled with NG or liquid fuel. This study considers hydrogen and liquefied natural gas as the most suitable fuels to comply with the environmental limitations. Due to its very low volumetric energy density, hydrogen must be stored in high-pressure tanks, from 35 up to 70 MPa, or liquefied (e.g. also known as cryogenic hydrogen) that requires under atmospheric pressure a temperature equal to −253°C. The use of liquefied $H_{2}$ guarantees smaller hydrogen volumes (e.g. energy density 2380 kWh/Nm³) and no high pressure tanks on board but requires almost the 30% of the stored energy for the cryogenic process and a complex and voluminous system (almost 3 times the stored hydrogen volume). The compressed $H_{2}$ is normally cheaper but the system is heavier and it occupies significantly volumes. On the other hand, NG is a petroleum subcategory and depending on the composition, its energy density is close to 47 MJ/kg. Natural gas can be stored as compressed gas up to 700 bar (e.g. energy density at 500 bar close to 3974 kWh/Nm³) or liquid and it is currently one of the most important source of both hydrogen and methanol. Therefore, if coupled with a reformer, it can be considered an economic solution for $H_{2}$ production on board, with easier fuel management.

2.2. Electrical power load analysis for cruise ships

In order to properly select the distributed power generation units, the electrical power load analysis (EPLA) for the ship under exam must be performed and analysed. In this context, typical results of an EPLA performed for cruise ships are proposed [3,5]. The DER approach proposed in this paper is aimed to cover the electrical load of each accommodation and galley substations. Therefore, the EPLA will be analysed for these groups of loads under several operating conditions. It should be noted that accommodation and galley loads all together account for a percentage of 10–20% the total power installed. Considering, for example, a total power installed equal to 60 MW, their overall load can be estimated between 6 and 12 MW, depending on the ship’s characteristics [5]. However, it is to be highlighted that loads such as those related to the central compressors for the heat ventilation and air conditioning system are not considered in these groups. Traditionally, due to safety reasons (e.g. prevent from fire and flood) cruise vessels are divided in main vertical zones (MVZ). In each MVZ, power generation units can be distributed and installed in order to cover their operating load, which can be in the order of 500–650 kW, considering for example the case of a ship with seven MVZ. Usually, two galley substations are installed on board these ships, with an operating load of 600 kW each. In this context, for example, nine distributed generation units rated 800 kW can be installed in each LV substation [22].

2.3. Innovative energy generation units for cruise ship applications, fuel cells, micro-gas turbines and energy storage systems

Fuel cells are devices that convert chemical energy directly into electrical one, without combustion. They offer advantages such as high efficiency and environmental benefits when compared to conventional energy conversion technologies. A wide range of cells is currently available. Between the most interesting technologies for maritime applications can be highlighted the Proton Exchange Membrane (PEMFCs), the Molten Carbonate (MOFCs) and the Solid Oxide (SOFCs). Their operative temperature increases from 70°C (PEMFC) up to 800°C–1000°C (SOFC) [21]. This paper is focused on PEMFC and SOFC, since these are considered the most suitable for marine implementation [33]. The costs of both these technologies can be estimated around 4000 $/kW with a life cycle of 5 years and a peak efficiency between 35–52% and 60% for the PEMFC and the SOFC, respectively [21,33,50]. Considering the SOFC technology, if the heat from high temperature exhausts is used to supply energy for a mGT, the overall efficiency can reach over 65% [9]. PEMFC, on the other hand, works at low temperatures allowing cold starts and fast load variation. However, it requires hydrogen with a high level of purity as fuel that can also be obtained by a LNG reformer, which cost can be estimated around 2500 $/kg of $H_{2}$ produced per day [37]. The SOFCs are highlighted for many advantages such as the fuel flexibility, high efficiency, high tolerance to impurities in the fuel and not requiring expensive noble metal catalysts. Micro-Gas turbines owes their origins to the military and aerospace industry and often present single-stage radial compressors and centrifugal expander with a pressure ratio around four, single-shaft and a high frequency alternate current generator. The micro gas turbine can either operate in a simple cycle configuration (e.g. where no heat is recovered from the exhaust for preheating of the combustion gases) or with a recuperated cycle. The system efficiency is around 33% (for recuperated cycle), and the whole costs are around 1200 $ per kW installed with a lifetime of 10 years [48]. The volumes and weights of this technology are smaller than those of other technologies. Depending on the type of ship, the generation technology and the power system configuration, energy storage systems (ESS) can be adopted for several purposes, from spinning reserve to peak shaving or dynamic support. Considering FCs as generation units, ESS are required in order to supply a dynamic support in case of fast load variability, due to the slow dynamics of the FCs systems. On the other hand, in the case of mGT as source of power, an ESS can be useful for load shaving, allowing the mGT to work closest to its optimal working point (i.e. between 70–85% of its rated power). However, in this case, also the losses due to the ESS should be considered in order to proper evaluate the benefits related to its adoption, selection and sizing (e.g. a 99% charge efficiency is a typical value for lithium-ion batteries). Therefore, for both these technologies, an ESS may further enhance the whole efficiency of the power generation system. Typical ESS for marine application are lithium-ion batteries, which present good characteristics for energy and power density [34].

2.4. Analysis of the power generation technologies

In order to analyse and compare the various technologies and the available fuels, a comparative study between 4 different possibilities is provided. The first one considers the use of PEMFC coupled with a steam reformer for hydrogen production. The second one studies SOFC directly fuelled with LNG [10,12]. Whereas the third and the fourth, on the other hand, account for PEMFC directly fuelled with Hydrogen stored in a cryogenic tank and mGT directly fuelled with LNG, respectively [1,6,7,27,35]. The comparison is developed in terms of efficiency, costs and volumes considering generation units rated at 800 kW and working from 100 to 200 hours per mission. From this analysis it can be highlighted that, although the PEMFC has an electrical efficiency of about 45%, the coupling with the reformer leads to a significant decrease of the overall performance up to a value of 28% (e.g. as proposed in Table 1). This fact would lead to prefer the solution with mGTs, since it couples a slightly higher system efficiency, with a considerable reduced footprint. In an “efficiency perspective”, it seems evident that the most interesting solution would be the direct use of $H_{2}$ with PEMFCs or the use of NG combined with SOFCs, since they do not require the use of fuel treatment and obviate the problem of the hydrogen storages on ships that has not yet been regulated by the IMO. Moreover, in Table 1, a comparison in terms of costs between the various technologies is presented, where also the fuel storage costs are taken into account (e.g. 0.168 $/kWh for LNG stored and around 3 $/kWh for Liquid $H_{2}$ stored [39]).

Table 1
Comparison table for the different configurations studied

Technology unit Fuel consumption t Fuel type Eff. % CAPEX $/kW Area m² Vol. m³ Weight t

PEMFC 9.6 H2 28 7000 165 460 151

LNG reformer 38.5 LNG

SOFC 5 LNG 55 4000 120 290 145

PEMFC 9.6 LH2 45 4500 130 380 180

mGT 33.1 LNG 33 1200 75 220 85

Technology unit	Fuel consumption t	Fuel type	Eff. %	CAPEX $/kW	Area m²	Vol. m³	Weight t
PEMFC	9.6	H2	28	7000	165	460	151
LNG reformer	38.5	LNG
SOFC	5	LNG	55	4000	120	290	145
PEMFC	9.6	LH2	45	4500	130	380	180
mGT	33.1	LNG	33	1200	75	220	85

Despite the fact that mGTs have the lower capital costs, it should be noted that the SOFCs fuelled with LNG have the lower variable costs. On the other hand, considering the solutions with PEMFC and reformer, it can be noticed that the high cost due to low overall efficiencies and large volumes leads to discard this choice for a marine application. The direct use of $H_{2}$ , stored in cryogenic tanks, on the other hand leads to sensible operational costs due to the high costs of liquid $H_{2}$ . From a space and volumes point of view, the mGT appears to be the most promising technology in terms of volume and weight reduction. On the contrary, the PEMFC combined with liquefied $H_{2}$ is the weightiest solutions, also requiring the largest spaces and volumes [30,32,39].

2.5. Innovative shipboard thermal energy recovery with DER

When referring to energy production for modern diesel electric cruise ships, the focus is usually addressed to the electrical power generation. Nevertheless, thermal energy production is also essential for a cruise ship, due to the huge number of users on board. Thermal energy is recovered form endothermic sources on board, mainly from exhaust gases from diesel generators. However, until now, the energy recovered is not sufficient to cover the total thermal energy demand in each ship’s operative condition. The distribution of energy sources in each MVZ could help the recovery of thermal power and the heat generation closer to users, which may increase the global efficiency avoiding the thermal losses through the longer pipelines.

2.6. New design approach

With DER the traditional system layout is overcome in favor of splitting the power production in a superior number of smaller generation units, to be located in each of the Main Vertical Zone (MVZ) already defined on-board that represent a suitable ship modularization for the purpose of the activity [30]. Engine room as traditionally conceived is modified, each MVZ has its own machinery space that is potentially independent from the others. Power generation distribution has strong positive attributes in terms of energy availability and delivery flexibility but it implied as well distribution of shortcomings like possible safety hazards and volume/area occupation on-board. This involves new guidelines to be considered and in the following a list of the design issues that have to be properly tackled with is given with brief comments [3,17,41].

Electrical power load analysis has to be developed for each MVZs that has consequently been provided with the necessary power generation. This choice allows for redundancy in each zones that in turn can rely on the suitable grid architecture for distribution adiacent zone. Each MVZs have their areas with different main purposes and characteristics, systems, load users and they have an influence on the electric and thermal local balance which can be much different among MVZs. A global balance is now less meaningful and useful.

Power generation distribution is definitely more demanding in terms of ship areas and volume involved and it is more complicated to locate generation units on board. Each technical room for Power Generation Unit (PGU) to be installed must be wide enough to accommodate generation units and its auxiliaries, with due care about the safety issues requirements. New “machinery” volumes have to be distributed on board influencing the traditional arrangement.

The weights of PGU are not all located in a few compartments on lower decks anymore, but they are now distributed in several “machinery” rooms located also at higher decks for spaces reasons and general arrangements constraints. The new distribution of weights influences the ship attitude and stability. As described above it is likely that the Vertical Center of Gravity (VCG) moves up with negative impact on stability criteria.

Decks local strength must be assessed in all new machinery rooms. Each PGU is to be integrated with the main electric power distribution grid to supply all ship energy demand and/or the local power demand only. From this perspective, the MVZ can be self-contained or connected to adjacent zones. This aspect depends on the selected electrical grid model.

Fuel supply system is a very critical aspect. In the case of fuel cells as PGU, the new technologies use LNG, methanol or hydrogen as fuel that are intrinsically more dangerous than diesel. International rules are more stringent for these low ignition fuels requiring double walled piping for fuel supply with inert gas between layers or appropriate air change. Safety rules requirements for tanks location are defined as well 2 m higher from base line and over $B / 5$ distance from side for fuel storage. All these regulatory constraints limit the possible tanks locations on board: in particular the impossibility to install fuel tanks in the double bottom might imply an increment of ship center of gravity. New exhaust “fluid” systems have to be designed. Each MVZ needs an exhaust fluid system treatment and an engine casing. As an example, the reformer system, necessary for hydrogen generation from LNG, produces carbon dioxide as a waste product that needs to be expelled. It worth reminding that fuel cells produce water that can be expelled locally or reused on board for auxiliary services or co-generation.

The PGUs air supply system requires accurate design in order to minimize the loss of volumes to be destined for piping and casing.

Heat exchange and co-generation actions in general became even more important to improve PGU overall efficiency. Each power source must be provided with heat exchangers with the specific aim to provide co-generation at a local level.

The issue of noise & vibrations is to be discussed as well even though the selected technologies for PGUs are deemed to have a low impact in this perspective: fuel cells and energy storage by battery packs have no mechanical part and no vibrations or noise irradiation consequently. Small turbines or dual fuel engines can be simpler isolated than large generators now in use. Nevertheless an analysis about cost implications should be carrid out.

Safety is a critical aspect. Distributed engine rooms imply distributed hazards and safety measures to be replicated in every MVZ. Due to the kind of fuel (LNG and hydrogen) safety measures are even more severe. Just as examples of what just mentioned, each generation room must have its own fire insulation and fire systems; LNG tanks room must be protected by cofferdam and piping are double walled to prevent explosion or toxic leakage.

Continuity of service and reliability of the whole energy system is intrinsic in the innovative concept of a distributed system, and a single MVZ can help the adjacent zone in case of failure. The reliability and the need for maintenance of innovative technology like fuel cells in marine application are still to be supported by field data. The use of LNG or hydrogen has also refueling issues.

Nowadays few ports are equipped with LNG and/or $H_{2}$ bunkering facilities and, in any case, refueling actions must be carried out carefully.

A typical electric power load analysis on a modern cruise ship puts in evidence that about 70–80% of the total produced power is delivered to propulsion system, and the other 20–30% is used for accommodation services. From the analysis of the zonal load and the technology available at present, it appears that we have to focus our attention on hotel services only. With the current state of the art technology is not practically feasible to cover all the propulsion and hotel power demand in one solution.

The innovative approach integrates on-board new technologies, like dual fuel engines, gas turbines or fuel cells [19,49]. The latter are very promising for future developments in the maritime field and its highly modular nature is suitable to the new enhanced configuration in the next years.

3. Case study and system design applications

The reference vessel considered in this application is a typical 140000 GRT cruise ship. The 22 knots design speed is provided by two fixed pitch propellers supplied by two electric engines located in an astern compartment. In navigation the total power demand, for both propulsion and hotel services (for 5600 passengers), is little less than 50 MW supplied by four diesel engines Tier II compliant located in two different astern engine rooms. As already mentioned, the installation of a Distributed Energy Resource (DER) system on board needs volumes for LNG fuel tanks storage, PGUs, fuel supply systems, inlet and outlet piping, casings and appropriate electric connections [19,49]. In order to DEG installation, the reference ship needs preliminary deep transformations, such as:

installation of two symmetrical azimuth units for main propulsion, in order to avoid rudders, stern thrusters and the two electrical propulsion engines;

separation between hotel and propulsion electric power supply in order to reduce the main Gensets size and exploit them for propulsion supply only;

replacement of the original four diesel engines with smaller six dual fuel engines Tier III compliant. This type of engines uses both LNG as main fuel and Marine Diesel Oil (MDO) for ignition phase only [2];

installation of several LNG Type C tanks, as required for propulsion and for hotel services, in relation with the assumed range and endurance [31].

All these actions have the purpose to convert the reference ship in a LNG dual-fueled vessel [14,26,43], and to recover new precious volumes on board for the DER system elements. As previously described, PGUs installation is meant for hotel loads only. So, about 20–30% of the total energy amount needed on board needs to be covered by the innovative distributed energy system. The remaining 70–80% is loaded on the duel fuel engines. In the case studied, the hotel load is about 7 MW that is provided by the distributed system. The reference vessel is divided into 7 MVZs, numbered from bow to stern. In order to provide the zonal load through a homogeneous local size of the PGUs, it was considered to install a single 800 kW PGU in the first five MVZs and double 800 kW units in the last two MVZs, for a total of nine PGUs connected with the main electric power distribution grid. Due to the large amount of installed power generation, the physical proximity with the power loads, the island configuration, shipboard power systems can be defined as marine microgrids. In this context, it would be possible and useful to adopt some technologies and practices developed in recent years for terrestrial microgrids [4]. The main objective is to guarantee the continuity, flexibility, reliability and sustainability of the service for an optimal management of the ship. A smart design of the shipboard power system associated with the implementation of (DER) can represent a disruptive technology in the perspective of increasing the ship’s efficiency, reliability and decrease at the same time its environmental footprint. The requirements of a power generation system for on-board DER are mainly: Power Density (PD-kW/l); Specific Power (SP-kW/kg); Noise & Vibrations and Co-generation capabilities. Among the present available technologies, the ones that seems to better comply with the requirements are Fuel Cells (FC) and Micro Gas Turbine (mGT) [18].

3.1. Power distribution system architecture

The power distribution system of the reference vessel can be defined as a radial configuration composed by a primary distribution in Medium Voltage (MV) at 11 kV and a secondary distribution in Low Voltage (LV), at 690 V and below, as shown in Fig. 1, respectively. The primary distribution system presents two main buses where propulsion motors, thrusters, compressors for heat ventilation and air conditioning system are connected.

On the other hand, the secondary distribution system in LV is composed by seven “accommodation substations” provided with 690 V, 230 V and 120 V sections and two “galley substations” provided with 440 V, 230 V and 120 V sections.

Finally, two “engine room substations” feed the auxiliary users in LV through four service sections, while the other two spare sections are connected to the other seven accommodation substations as back-up lines. Therefore, the total number of distribution transformers installed is equal to fifteen (i.e. excluding the transformers for the propulsion).

As more extensively described in Section 2.1, the power system configuration for DER presents a MV ring with groups of sub-stations connected in LV. The flexibility and reliability of the whole system has been increased by introducing the DER and implementing the proposed grid configuration. However, it is to be noted that these improvements would require more complex and sophisticated power and energy management systems.

3.2. Description and installation of micro-turbines system

The mGT technology considered for this application is a boxed system that incorporate the Turbine, the Compressors, the Combustion Chamber, the Heat Exchanger, the Current Conditioning System and all the smaller Auxiliary systems [15]. The mGTs power installation is composed by:

the mGT system,

the fuel system,

the control system

the high power electronics.

All the assessed systems are made by modular components composed by 100 or 200 kW turbines. The engine rooms where they are located are complain with the relevant rules, for example the IGF Code. The mGT system used is an 800 kW Capstone module, made of four 200 kW modules installed inside a compact enclosure of 65 m³ over a deck area of 22 m² [49]. mGTs can be directly fed by natural gas supplied by the LNG storage.

Kept in mind the issues exposed in Section 2, the micro-turbines based PGU system installation is described below. Micro-turbines, auxiliaries, voltage converters and inlet and outlet piping have been placed in a single A-60 insulated volume [16] i.e. in an engine room. Each micro-turbine has its own 60 m² engine room, mainly located at deck 8 to reduce the outlet piping extension (as shown in orange in Fig. 3). Three bilobed Type C tanks placed at deck 1 in MVZ 4 supply the all PGU systems: LNG is treated in a pre-tank room and it is distributed by double wall piping, exploiting the inside tank pressure itself [14,43]. Propulsion and hotel LNG tanks are independent from each other. For an endurance in terms of hours ranging between 100 and 200 hours about 600 m³ of LNG are provided for micro-turbines system only. Fresh air is taken from existing adjacent AC Stations while exhaust gas piping is routed through engine casing or near existing vertical large casing like stairs or lifts trunks [46]. Although the weight of each micro-turbine is about 20 tons, ship’s Vertical Center of Gravity (VCG) is not significantly influenced. Major effect is due to the capacity tanks modification i.e. three Type C tanks on deck 1 and the elimination of the HFO form the double bottom. The combination the above described actions increases the VCG for about 0.30 m.

Fig. 3.

Distributed microturbines system on board. Orange: PGU room; red: fuel supply system; yellow: outlet piping; blue: inlet piping.

3.3. Description and installation of fuel cells system (FC and reformer)

Proton Exchange Membrane Fuel Cells (PEMFC) have been found as the most suitable FC technology for the short-medium terms ships applications. PEMFC are characterized by good performances in terms of PD (0.24 kW/l), SP (0.29 kW/kg). A typical Fuel Cell Power Installation is composed by:

a Fuel Cell Power System,

the FC Auxiliary Systems,

the FC Control System,

the DC Power Conversion,

the FC Safety System.

The list is exhaustive in all the necessary element. For sake of completes it is worth mentioning that at present IMO rules focus on FC Power Installation addressing only the following aspects:

FC stack,

the Primary Fuel Process Unit,

the Air Process Unit.

Fig. 4.

Distributed fuel cells system on board. Orange: PGU room; red: fuel supply system; yellow: outlet piping; blue: water supply system.

Future developments on international rules side are expected. The most voluminous and heavy component of the system is the Fuel Cell Power System made by Racks of Fuel Cell Stacks connected together to reach higher power. LNG supply has been considered for the FC system, even if PEMFCs require a pure hydrogen supply only. The main issue from the storage system is represented by the LNG reformer unit, indeed no marine system has been found on the market and an assessment on the performance of the main compact stationary applications Steam Reformer (SR) systems has been conducted. The high volume and weight required from the reformer is worsen by the large amount of fresh water required. A last consideration need to be done on the total efficiency. Even if the fuel cell efficiency is high, the total electrical efficiency results to be low (30%–36%). For this reason, co-generation is important in order to enhance the total system efficiency [8,42]. In this solution, on-board location of reformers is a big challenge. Each reformer occupies about 60 m² area, the same as an 800 kW FC module, and a CO₂ outlet piping must be provided [8,46]. The total weight amount of what above mentioned is about 70 tons, and we must consider that a single FC produces about 12 tons per day of water as an electrochemical conversion product. FC and its own reformer are placed in the same 200 m² A-60 insulated room, to minimize hazards due to hydrogen piping supply, reduce the global number of risk sources and reduce the volumes demand. As a consequence, all system elements have been located on the lower decks, mainly at deck 3 and 4 (as shown in Fig. 4). The driving criterion for location was the lower impact as possible on valuable areas form the commercial point of view on internal passenger cabins, crew cabins and technical areas have been modified. Fuel tanks are located in the same position described in the micro-turbines configuration, and its dimensions are pretty the same. LNG supply system is the same described in paragraph 2.3 and hydrogen piping is double walled anyway [4,14–16,18,26,43,46].

Each reformer for a PEMFC needs about 2 tons per hour water supply. Therefore, two daily water storage tanks are located in the double bottom and the main water flow comes from board fresh water-makers and a small part from FCs themselves. The reformer itself provide heat to start and supply the reforming reaction by taking a small percentage of LNG inlet to feed an internal burner. Co-generation could be done by heat generated in the FC. Future considerations on weights distribution and stability implications will be made.

4. Challenges and expectations from technology innovation

One of the challenges related to the introduction of DERs on board a cruise ship is their integration within the shipboard power system and the ship general arrangement. In particular, due to the high complexity of a cruise vessel and the high number of systems and services present on board, finding an adequate allocation for DERs is very demanding. However, in this application case, with proper modification on the original ship, additional volumes on board can be recovered by [5]:

introducing azipods for propulsion; in this way, the space dedicated for propulsion electric motor and bow thrusters can be almost entirely recovered, as well as space allocated to power shafts,

modifying the grid configuration, as proposed in Sections 2.1 and 3.1,

reducing the DGs rated power, since part of electrical power is generated with DERs. These new volumes can be used for LNG storage tanks and new DER engine rooms.

Nevertheless, even with these modifications, additional space shall be found out in order to provide all the required volumes. This in principle can be done with a smart integrated design of both the ship platform and the on-board systems, allowing the optimization of spaces and volumes since the early phases of a new ship design. In case of the modification of an existing design, the only solution would be reducing the number of internal cabins and other areas dedicated to hotel services (e.g. estimated in the order of 2.0–4.5% of the total), depending on the technology adopted. It is important also to remind that the distribution of volumes and the normative dedicated to fuel storage and treatment changes whether the main fuel is LNG, diesel oil or $H_{2}$ . Concerning the on board power system, the integration of DER should be analysed considering each technical aspect such as power quality, islanded analysis and power and energy management.

Fig. 5.

DERs integration within the shipboard design process.

In the context of fuel cells technology, the scenario is wide and in continuous development. In the future, Solid Oxide Fuel Cells (SOFC) technology is likely to capture the attention in the marine applications. SOFC are ceramic based fuel cells that works at high temperatures (800–1000°C) and are believed to become the most suitable fuel cell type for marine applications, due to a number of factors (as shown in Fig. 5):

High electric efficiency (up to 60% only fuel cell, up to 70% in hybrid GT configurations),

Good co-generation (hot water or vapor) with higher efficiency,

MW size systems,

Possibility to be directly fed with Natural Gas.

These positive characteristics are unfortunately in balance with some shortcomings i.e. fragility to vibrations (IBC ASCE class D, high seismic vulnerability), a present less mature technology and high cost also due to the limited number of system producers. Moreover, high temperatures and high thermal capacity require long start-ups and shut-down procedures that pose operations and safety issue. For these reasons, SOFC will probably find applications on-board ships in the medium-long period enabling the transition from medium power fuel cell systems based on PEMFC for auxiliary loads and small propulsion to high power SOFC able to propel the ship.

5. Conclusions

The main motivations to introduce innovative energy systems on board cruise vessels have been presented and discussed: the reduction of the ship environmental footprint is a strong leverage but also the possibility to drastically enhance the energy performance (overall systems efficiency, safety, reliability) adopting technologies and design approaches, already exploited in land-based applications represents a significant opportunity [20]. One of the most promising approach is to reinvent and redesign the shipboard power system with a DER configuration and adopt power generation technologies such as FCs, mGT and ESS.

Some limitations and challenges have been highlighted for the proposed solutions, which cover topics from the spaces and volumes to costs and normative gap. These limitations can be overcome or reduced with a smart design of the ship, which should consider the integration of these solutions already in the very early stage. Despite the challenges highlighted for the integration on board of these technological solutions, the benefits found by combining: new generation units, alternative fuel solutions, revised configurations of the electrical distribution system can justify future studies regarding their systematic use on board cruise vessels.

With specific reference to the application case about a large cruise passenger ship, some conclusions can be drawn in relation with the different technologies: micro-turbines and fuel cells technology implementation in fact have different impacts on the ship design. In the short-term time, gas micro-turbine technology appears as the more mature and results in a lower system complexity. Micro-turbines have limited impact in terms of areas on decks but they require considerable diameter of air intake piping to work properly and consequently extensive exhaust gas piping, with a not negligible impact on internal volumes distribution and vertical openings. The present PEMFC technology needs an external reformer to obtain hydrogen from LNG, therefore has a major impact in terms of areas on decks and weights, but piping impact is reduced. As a result of the application it has come out that FC solution requires about five times mGT solution area on board. The FCs solutions higher impact is due mainly to the reformer system. In the case of FCs, the major influence is on the spaces originally devoted to Crew Cabins and Hotel Services. This in turn can imply that a redistribution of spaces on-board is needed, in particular between passenger and crew cabins to maintain the original passengers/crew ratio.

In terms of energy management, micro-turbines have a better dynamic response to load increase and its optimal work point, with high efficiency, is on high load. FC system allows a better energy setting and splitting because of its efficiency is independent from power system dimensions. Their dynamic response is less performing and in case of variable load, batteries system is needed. Also in terms of thermal energy recovery, at a local level within the very same MVZ, the micro-turbines solution appear to be more convenient, even though the final outcome should be supported by an evaluation based on plausible local thermal balance values. In term of noise & vibrations FCs are an outstanding solution, with positive influences the passenger comfort, instead the micro-turbines acoustic emissions are about 65 dB.

It is worthwhile to stress again that a more complex energy system on-board calls for a higher attention to the ship energy management during ship operation. Therefore, a close collaboration between the shipyard and the owner is more and more advisable during the design phase, in the perspective of a ship superior energy efficiency deployed along the ship life.

Footnotes

Acknowledgements

This study is part of Project “Leadership Tecnologica”, a Research and Innovation project coordinated by Fincantieri S.p.A. with the participation of the National Research Council (CNR) and the University of Genoa; the project receives grants from the Italian Ministry of Infrastructures and Transport (MIT).

Conflict of interest

None to report.

References

BloomEnergy Corporation, Solid Oxide Fuel Cell UPM-571 Product Datasheet, https://www.bloomenergy.com/.

E.K.

Boulougouris and

L.E.

Chrysinas, LNG Fueled Vessel Design Training, University of Strathclyde, 2015.

Boveri,

Gualeni,

Neroni and

Silvestro, Stochastic approach for power generation optimal design and scheduling on ships, in: IEEE PES Innovative Smart Grid Technologies Conference Europe (ISGT-Europe), 2017, pp. 1–6.

Boveri,

Maggioncalda,

Rattazzi,

L.M.P.

Gualeni,

Silvestro and

Pietra, Innovative energy systems: Motivations, challenges and possible solutions in the cruise ship arena, in: NAV 2018 – 19th Iternational Conference on Ship & Maritime Research.

A.P.

Boveri,

Gualeni and

Silvestro, Stochastic electrical plan load analysis for increasing flexibility in electrical ship system, in: RINA Smart Ship Technology Conference, 2017.

Capstone, Micro Gas Turbine C800 Product Specification, https://www.capstoneturbine.com/.

Capstone Turbine Corporation, Capstone Low Emissions MicroTurbine Technology, 2000.

C.H.

Choi,

Yu,

I.-S.

Han,

B.-K.

Kho,

D.-G.

Kang,

H.Y.

Lee,

M.-S.

Seo,

J.-W.

Kong,

Kim,

J.-W.

Ahn,

S.-K.

Park,

D.-W.

Jang,

J.H.

Lee and

Kim, Development and demonstration of PEM fuel-cell-battery hybrid system for propulsion of tourist boat, International Journal of Hydrogen Energy 41(5) (2016), 3591–3599, ISSN 0360-3199, http://www.sciencedirect.com/science/article/pii/S0360319915028438. doi:10.1016/j.ijhydene.2015.12.186.

Costamagna,

Magistri and

A.F.

Massardo, Design and part load performance of a hybrid system based on a solid oxide fuel cell reactor and a micro gas turbine, Journal of Power Sources (2001).

10.

Cuneo,

Zaccaria,

Tucker and

Sorce, Gas turbine size optimization in a hybrid system considering SOFC degradation, Applied Energy 230 (2018), 855–864. doi:10.1016/j.apenergy.2018.09.027.

11.

Dall’Anese,

Mancarella and

Monti, Unlocking flexibility: Integrated optimization and control of multienergy systems, IEEE Power and Energy Magazine 45(1) (2017), 43–52. doi:10.1109/MPE.2016.2625218.

12.

U.M.

Damo,

M.L.

Ferrari,

Turan and

A.F.

Massardo, Test rig for hybrid system emulation: New real-time transient model validated in a wide operative range, Fuel Cells 15 (2015), 7–14. doi:10.1002/fuce.201400046.

13.

Deboever and

Grijalva, Modeling and optimal scheduling of integrated thermal and electrical energy microgrid, in: North American Power Symposium (NAPS), 2016, pp. 1–6.

14.

DNV GL, Rules for Classification of ships, Part 6, Additional Class Notation, 2016.

15.

DNV GL, Rules for Classification of ships, Part 4, Ch. 1 – Machinery System, General, https://rules.dnvgl.com/docs/pdf/DNVGL/RU-SHIP/2018-01/DNVGL-RU-SHIP-Pt4Ch1.pdf.

16.

DNV GL, Rules for Classification of ships, Part 4, Ch. 6 – Piping Systems, https://rules.dnvgl.com/docs/pdf/dnvgl/ru-ship/2017-01/DNVGL-RU-SHIP-Pt4Ch6.pdf.

17.

DNVGL, LNG as Ship Fuel, The Future Today, 2014.

18.

M.A.R.

do Nascimento,

de Oliveira Rodrigues,

E.C.

dos Santos,

E.E.B.

Gomes,

F.L.G.

Dias,

E.I.G.

Velásques and

R.A.M.

Carrillo, Micro gas turbine engine: A review, in: Progress in Gas Turbine Performance,

Benini, ed., IntechOpen, Rijeka, 2013, Chapter 5. doi:10.5772/54444.

19.

Dual Fuel Engines, http://www.wartsila.com.

20.

M.L.

Ferrari,

Traverso and

A.F.

Massardo, Smart polygeneration grids: Experimental performance curves of different prime movers, Applied energy 162 (2016), 622–630. doi:10.1016/j.apenergy.2015.10.144.

21.

Flore,

Neroni,

Lamberti,

Gualeni,

Magistri and

Silvestro, Distributed energy resources on-board cruise ships: Integration into the ship design process, in: NAV 2018 – 19th Iternational Conference on Ship & Maritime Research, ATENA, Trieste, 2018.

22.

Gualeni,

A.P.

Boveri,

Silvestro and

Margarita, Decision support system for power generation management for an 110000+ GRT cruise ship, RINA Transaction on the International Journal of Maritime Engineering (IJME) (2015).

23.

J.M.

Guerrero, Shipboard microgrids: Maritime islanded power systems technologies, international exhibition and conference for power electronics, intelligent motion, Renewable Energy and Energy Management (2016), 1–8.

24.

Hanke and

Knees, A phase-field damage model based on evolving microstructure, Asymptotic Analysis 101 (2017), 149–180. doi:10.3233/ASY-161396.

25.

R.E.

Hebner,

F.M.

Uriart and

Kwasinski, Journal of Modern Power Systems and Clean Energy (2016).

26.

R.E.

Hebner,

F.M.

Uriarte,

Kwasinski et al., J. Mod. Power Syst. Clean Energy.

27.

Hydrogenics, PEM Fuel Cell HyPM Product Specification, https://www.hydrogenics.com/hydrogen-products-solutions/fuel-cell-power-systems/.

28.

IMO, Guidelines for voluntary use of the ship EEOI, Resolution MEPC.1/Circ.684.

29.

IMO, Guidelines for the development of a ship energy efficiency management plan (SEEMP), Resolution MEPC.213(63).

30.

IMO, International Convention for the Safety of Life At Sea, 1 November 1974.

31.

IMO, International Code of Safety for Ships Using Gasses or Other Low-Flashpoint Fuels (IGF Code), Resolution MSC.391(95).

32.

Karlsson and

Sonzio, Enabling the safe storage of gas onboard ships with the Wartsila LNGPac.

33.

Korner, Technology Roadmap Hydrogen and Fuel Cells, 2015.

34.

Lan,

Wen,

Y.-Y.

Hong,

D.C.

Yu and

Zhang, Optimal sizing of hybrid PV/diesel/battery in ship power system, Applied Energy 158 (2015), 26–34. doi:10.1016/j.apenergy.2015.08.031.

35.

Law,

Rosenfeld,

Han,

Chan,

Chiang and

Leonard, U.S. Department of Energy Hydrogen Storage Cost Analysis Final Public Report, 2013. doi:10.2172/1082754.

36.

Lefever, A hybrid approach to domain-independent taxonomy learning, Applied Ontology 11(3) (2016), 255–278. doi:10.3233/AO-160170.

37.

Melaina and

Penev, Hydrogen Station Cost Estimates Comparing Hydrogen Station Cost Calculator Results with Other Recent Estimates, National Renewable Energy Laboratory, 2015.

38.

P.S.

Meltzer,

Kallioniemi and

J.M.

Trent, Chromosome alterations in human solid tumors, in: The Genetic Basis of Human Cancer,

Vogelstein and

K.W.

Kinzler, eds, McGraw-Hill, New York, 2002, pp. 93–113.

39.

Michel,

Fieseler and

Alliederes, Liquid Hydrogen Technologies for Mobile Use, 2006.

40.

P.R.

Murray,

K.S.

Rosenthal,

G.S.

Kobayashi and

M.A.

Pfaller, Medical Microbiology, 4th edn, Mosby, St. Louis, 2002.

41.

Papanikolaou, Ship Design: Methodologies of Preliminary Design, Springer, St. Louis, 2014.

42.

P.A.

Pilavachi, Mini- and micro-gas turbines for combined heat and power, Applied Thermal Engineering 22 (2002), 2003–2014. doi:10.1016/S1359-4311(02)00132-1.

43.

44.

Rivarolo,

Cuneo,

Traverso and

A.F.

Massardo, Design optimization of smart poly-generation energy districts through a model based approach, Applied Thermal Engineering 99 (2016), 291–301. doi:10.1016/j.applthermaleng.2015.12.108.

45.

Rivarolo,

Rattazzi and

Magistri, Best operative strategy for energy management of a cruise ship employing different distributed generation technologies, International journal of hydrogen energy 43 (2018), 23500–23510. doi:10.1016/j.ijhydene.2018.10.217.

46.

Ronchetti, Celle ad elettrolita polimerico, in: Celle a Combustibile, Stato della Tecnologia e Prospettive della Tecnologia,

Enea, ed., Ministero per lo Sviluppo Economico, Italy, 2002, pp. 33–92.

47.

RT-LAB and ARTEMiS, http://www.opal-rt.com/software-downloads-documentation.

48.

Tanner, Microturbines: A disruptive technology, Applications Marketing Capstone Turbine Corporation.

49.

Tronstad,

H.H.

Astrand,

G.P.

Haugom and

Langfeld, Study on the use of fuel cells in shipping, EMSA, 2016.

50.

Tronstad,

H.H.

Astrand,

G.P.

Haugom and

Langfeldt, Study on the Use of Fuel Cells in Shipping, DNV, 2015.

51.

van Biert,

Godjevac,

Visser and

P.V.

Aravind, A review of fuel cell systems for maritime applications, Journal of Power Sources 327 (2016), 345–364. doi:10.1016/j.jpowsour.2016.07.007.

52.

Wilson, Active vibration analysis of thin-walled beams, PhD thesis, University of Virginia, 1991.

53.

Zhang,

Chen,

Xu,

Li,

He,

Guo and

Huang, Distributed generation with energy storage systems: A case study, Applied Energy 204 (2017), 1251–1263.