Liquid hydrogen as a potential decarbonization solution for a large cruise ship

Abstract

An increase in customer demand for cruising is prompting major industry players to allocate considerable resources to the construction of new ships that are energy efficient and environmentally compliant. This study examines the energy transition possibilities by retrofitting a large cruise ship and develops a procedure to evaluate the suitability of liquid hydrogen as a potential decarbonization solution. The utilized approach in this paper includes technical and safety assessment of the possible energy system options with liquid hydrogen as fuel, to identify the optimal solution. The technical assessment evaluates the required volume and weight of each system option using vessel particulars and operational data. The energy system sizing is based on available technology data in the market, and the sizing of fuel storage system is based on a sample liquid hydrogen containment system and its adaptability onboard. Additionally, a safety assessment of hydrogen storage and each energy system option is conducted, analysing the applicability onboard, with a focus on the frequency of hazardous events. This approach can be transferred to other ship types and further developed to compare other alternative fuels. Furthermore, it can be improved by incorporating economic and regulatory assessments. The results of a sample case study with installed 40 MW power onboard, indicate that decarbonization of cruise ships using liquid hydrogen is possible. Among studied scenarios, a system with 40 MW PEMFCs and 10 MW batteries is suggested as an optimum solution. This system, despite requiring 1.87 more volume, represents only 35% of conventional system weight. Within the ship body three large liquid hydrogen tanks are installed to provide 4926 m³ fuel. Consequently, bunkering stops are required on a weekly basis.

Keywords

liquid hydrogen cruise ship decarbonization energy system storage tank

1 Introduction

Amid the increasing regulatory restrictions related to emissions from ships and the aims to reach net-zero emissions by 2050, it is necessary to identify technological pathways leading to environmental compliance while being technically feasible and economically viable in comparison to conventional fossil fuel-based solutions. Decarbonization goals can be achieved by optimizing several key areas, as operational strategies; hydrodynamic improvements; and machinery enhancements; suitable fuels.¹ Among these, the adoption of sustainable alternative fuels holds the greatest potential for long-term emission reductions. Alternative fuel options range from low-carbon fuels like liquefied natural gas (LNG) and methanol to carbon-free fuels like hydrogen and ammonia. Moreover, the energy converters and systems that can transform alternative fuels into power for the ship have to be considered. Energy converters transform the chemical energy of alternative fuels into mechanical or electrical power to propel the ship. The current mostly used power systems onboard are internal combustion engines (ICE). Fuel cells are increasingly being considered as alternative energy converters as they can eliminate pollutants, and exhibit low noise and high energy conversion efficiency.² The diversity of decarbonization pathways is coupled with the uncertainties related to future fuels availability, costs and integrability in ships. In such circumstances, providing an insight into relevant and suitable transition pathways for investment decisions into energy transition on ships is necessary to speed up the transition and prevent ship owners and operators from investing in stranded assets.

The rising consumer demands lead to higher interest of the industry in building newer ships. According to Syriopoulos et al.³ the orderbook for new-built cruise ships for the year 2024 and 2025 contains ship projects for a total capacity of 27,530 and 22,060 passengers, corresponding to 1.077 M GT and 0.940 M GT, respectively. In addition to increasing passenger capacity and enhancing customer experience, environmental concerns are considered to satisfy the growing restrictions on carbon emissions, air pollutants and comply with upcoming regulations and mandates enforced at regional and international levels. Energy-efficient ship designs combined with emission reduction technologies allow 5%–25% emission reduction.⁴ Liquid hydrogen (LH₂) emerges as one of the promising fuels for cruise ships, offering zero carbon emissions when obtained from green sources and combined with fuel cells for combustion-free energy production. Liquefied form at −253 °C allows for storing the highest energy density, with a gravimetric energy density of 33.33 kWh/kg and a volumetric energy density of 2.3 kWh/L.⁵ Despite the high gravimetric energy density of liquid hydrogen, a balance between the storage capacity and available volume on the ship is required. Boil-off losses constitute a challenge as well, as they lead to an increase in tank pressure, energy and economic losses when vented out.⁶

Different types of fuel cells exist, which can be classified based on operating temperature, power output, lifetime and fuel type. Among those, proton exchange membrane fuel cell (PEMFC) and solid oxide fuel cell (SOFC) systems are the most commonly used fuel cell types for maritime applications. PEMFCs are known for having relatively high power densities compared to other fuel cell types.⁷ PEMFCs operate only using pure hydrogen, for which currently no clear rules and regulations are available.⁸ On the other side, SOFCs are fuel flexible and can be powered by LNG or ammonia, making them suitable for the marine sector. Current gas engine designs, which have been operating on natural gas in the shipping sector for several years, are being further developed to utilize hydrogen and ammonia as fuels. This advancement remains an ongoing area of research and development among engine manufacturers.⁹

Combinations of different alternative fuels and energy converters enable ship decarbonization. In order to find the most suitable solution for each ship type, a comparison has to be carried out based on different criteria. For this comparison, some papers address only a single ship or a single fuel option. A study focuses on a sample container ship and compares the alternative propulsion systems fuelled by ammonia, such as engines, gensets, PEMFCs and SOFCs. Then by considering economic and environmental perspectives, selects the most eco-friendly solution.¹⁰ Another study focuses on a sample cruise ship equipped with a SOFC fuelled by LNG and assisted by dual-fuel diesel/LNG generator sets.¹¹ Further recent publications investigate specific vessel types to recommend decarbonization solutions. One evaluates the application of fuel cells onboard coastal and inland waterway ships from a technical and economic point of view.¹² The other paper looks at near-term decarbonization solutions for deep-sea-going ships. It compares different fuel and technology options from fuel volume, emissions and cost aspects, considering the fuel savings and carbon tax.¹³ In a recent paper, a multi-criteria evaluation approach is developed for comparing different alternative fuels and energy systems. This method is presented in the form of a tool which rates the suitability of an alternative fuel solution, that is, fuel storage and energy converter, based on costs, volumes, weights and emissions for a given vessel type.¹⁴

This work explores the potential of retrofitting a ship by replacing a conventional propulsion system consisting of heavy fuel oil (HFO) and marine diesel oil (MDO) using ICE for propulsion and power generation with a liquid hydrogen-fuelled system composed of either fuel-cells and batteries, or hydrogen ICE. In this paper, an assessment approach is developed to examine energy transition possibilities with liquid hydrogen for large cruise ships and recommend the most suitable solution. The proposed methods can easily be extended to other ships and fuel types. The novelty lies in developing an approach, which compares several possible energy system combinations for liquid hydrogen instead of focusing on a single solution. Additionally, compared to the current literature, this work addresses the gap about detailed fuel tank size calculation and available space arrangement onboard. The existing literature mainly focus on developing a holistic approach to study the applicability of hydrogen-based fuels onboard, while a detailed space requirement of fuel containment system is not taken into account. Furthermore, this paper includes the safety aspect of each energy system option, where safety assessment has been somewhat disregarded in the existing literature. As alternative fuels have different specifications and safety requirements, it becomes an important indicator to assess the reliability of fuel storage and energy system performance. This approach is being further developed to incorporate economic and regulatory evaluations of potential energy transition options. This enhancement will enable comparative analyses of investment costs, operational expenses and prospective emission penalties. These factors are also of significant importance to the proposed solution.

The rest of this paper is structured, as follows: in Section 2, the assessment approach for technical and safety assessment is explained. In Section 3, detailed explanations of the sample vessel and different energy systems, fuel storage tank and the assumptions are presented. In Section 4, the results are reviewed and compared. Additionally, Section 5 provides the conclusion of study.

2 Holistic assessment approach

Figure 1 shows the assessment approach for the technical and safety assessment of alternative energy system solutions for use of hydrogen as fuel onboard a sample vessel. The technical evaluation takes ship input data and uses databases of the energy system components existing in the market today, to compare the volume and weight of the alternative scenarios with the current system onboard. The configurations which fit into the sample vessel from volume and weight perspective are thereby selected. The safety evaluation reviews the risks of using liquid hydrogen onboard in the feasible configurations and recommends the most reliable systems as suitable solutions.

Figure 1.

Flow chart of assessment approach.

2.1 Technical assessment

The technical evaluation compares the required volume and weight of the current energy system as well as fuel storage system with those of the liquid hydrogen-based alternative solutions onboard. The volume and weight calculation used in this work are shown in equations (1) and (2):

\begin{aligned} Volume = \sum_{i} V_{storage tank, i} + \sum_{j} V_{power supply system, j} \end{aligned}

(1)

\begin{aligned} Weight = \sum_{i} M_{storage tank, i} + \sum_{j} M_{power supply system, j} \end{aligned}

(2)

where V represents the volume and M the weight of the components.

For calculation of the required volume and weight of storage tank, basic design procedures are applied to assess the geometrical features. The other system components, for example the vaporizers and pressure change units are neglected here, due to their relatively lower volume and weight compared to the other equipment categories considered. The input parameters estimated for the technical assessment are summarized in Sections 3.2–3.6.

The selection of the required capacity of fuel cells, batteries and combustion engines in each alternative scenario is based on the maximum and minimum power demand onboard according to the ship operating profile. The required capacity of the alternative system is defined based on current energy system onboard and the total capacity of engines. Fuel cell solutions also consist of a battery to enable their operation under transient conditions. The energy capacity of the battery is defined so as to account for the difference between the allowable transient power delivery of fuel cells and the expected transient requirements of the onboard energy system. The volume and weight of the energy system is calculated based on the data collected from market-existing technologies.

For the volume and weight of storage tank, the maximum demanded fuel for each scenario is calculated based on the efficiency/fuel consumption data of each energy system component presented in Sections 3.2–3.6. Using the one-year operational data of the vessel, the maximum demanded fuel is calculated for the voyage with highest energy demand. So, the required amount of fuel to be stored onboard to supply energy for the longest voyage with highest power demand. The number of required tanks is thereafter defined accordingly.

2.2 Safety assessment

Safe and reliable onboard storage of hydrogen is necessary for utilization of liquid hydrogen as fuel for ships. The safety assessment objectively quantifies the frequency of occurrence of hazardous events in the powertrain, including fuel storage system, ICE, fuel cell and battery. This approach is applied using the tank design data and analyses the applicability of the storage technology onboard the vessel. The safety assessment described in this work focuses on the frequency of hazardous events, without explicitly considering their consequences.

In order to conduct a thorough evaluation, four hazardous events are considered: (i) pool fires, (ii) flash fires and (iii) vapour cloud explosions resulting from a fuel release, as well as (iv) battery fires. To obtain frequencies for the initial three events, an event tree methodology as presented in Figure 2¹⁵ is followed. The sequence of events commences with a release of fuel into the environment consequent to a loss of containment (LOC). LOCs assigned to each equipment are assumed based on technical literature. The frequencies thus obtained are intended to assess the credibility of LOC occurrence in the analysis and are based on leak frequency data for storage equipment established by the International Association of Oil & Gas Producers (IOGP).¹⁶ It should be noted that the IOGP data does not cover fuel cells or ICE. For the purpose of this study, data on fuel cells are drawn from the study by Gerbec et al.¹⁷ The frequency of fuel releases from an internal combustion engine is obtained from the Offshore and Onshore Reliability Data Handbook (OREDA).¹⁸ The frequencies of a battery fire are obtained without an event tree, utilizing data provided by DNV.¹⁹ The investigation encompasses three distinct leak sizes: 3 mm, 10 mm, and a full rupture (25 mm). Subsequently, the frequencies for each event and leak size are added to obtain a single value per system. The resulting values are then used to calculate a safety index, with lower values indicating a safer system.

Figure 2.

Event tree utilized in the safety assessment, based on Depken et al.¹⁵

3 Case study and alternative energy system options

The case study is based on a sample large cruise ship with general specifications, as presented in Section 3.1. For use of liquid hydrogen as fuel onboard this vessel, it can be coupled with different energy systems, such as ICEs, PEMFCs, SOFCs and Batteries. The specifications of these system components such as volumetric and gravimetric densities, as well as fuel consumption rates are provided in Sections 3.2–3.5. The attributes of liquid hydrogen storage system for this sample ship are presented in Section 3.6. The case scenarios for liquid hydrogen as fuel onboard this sample ship are presented and the energy system component sizes are compared in Section 3.7.

3.1 Sample ship

A large cruise ship, as shown in Figure 3, serves as a use case to evaluate the feasibility of a liquid hydrogen-fuelled system as a substitute for the HFO- and MDO-based conventional energy system. The reference ship operates in the North Sea region, calling ports at Hamburg, Rotterdam, Zeebrugge, le Havre, Southampton, Bergen and Stavanger. The duration of the tours varies between 5 days and 9 days, in which the ship accommodates up to 3200 passengers across around 1600 cabins. The main characteristics of this ship are listed in Table 1. The sample ship sets the boundaries of the system in terms of available space, energy demand, and costs.

Figure 3.

Sample large cruise vessel.

Table 1.

General specification of the sample ship.

Parameters	Unit	Value
Length over all	m	300
Breadth	m	37
Deadweight	Ton	9200
Passenger capacity	–	3200
Ship speed	knots	22.5
Total engine capacity	MW	40
Fuel types	–	HFO/MDO
Total fuel tank system volume	m³	3800

Figure 4 illustrates the distribution of the total power demand of the sample cruise ship over 1 year of operation. According to the operational data of the sample ship, a minimum power of 7 MW is continuously demanded during a whole year. Around 46% of the total operation occurs close to 10 MW, corresponding to the “hotel load”. The vessel demands a maximum power of 40 MW occasionally, which is hereafter referred to as “peak load”. Excluding the peak loads, the load profile represents power demand fluctuations between 10, 20 and 30 MW. The longest duration between bunker events of this sample ship is assumed as 14 days.

Figure 4.

Histogram and cumulative distribution of total power demand of the cruise ship over one year.

3.2 Hydrogen internal combustion engine (ICE)

Hydrogen ICE for marine application are not yet commercially available in the market due to their limited technology readiness levels. One of the few existing hydrogen engines in the market is developed in 2020 by BeHydro, in both spark-ignited mono fuel (100% hydrogen) and dual fuel versions, and a maximum power output of 2.67 MW. Larger scales of hydrogen engines are under development. The dual-fuel engine can operate on up to 85% H₂ with diesel as pilot fuel, but also fully with conventional fuels. This engine can reduce CO₂ emissions proportional to the hydrogen consumption, but still emits NO_x and particulate matter. Therefore, the engine may need an aftertreatment system to comply with the IMO tier III regulations.

Due to the limited number of existing large-scale hydrogen engines, for the scenario with an internal combustion engine, the mono fuel as well as the dual fuel hydrogen engines are considered. However, in reality this is not a possible case to install several small engines onboard. According to the datasheet, a 2.67 MW engine has the volume of 50.3 m³, weight of 21.75 ton. The hydrogen consumption of mono fuel engine at full load is 192 kg/hr and the dual fuel engine operates at full load with 163 kg/hr hydrogen and 120 kg/hr diesel as a pilot fuel.²⁰

3.3 PEMFCs

PEMFC stands out with its advantages of high efficiency, compact design, rapid response rate, and low performance degradation. Compared to the hydrogen ICEs, the market availability of PEMFCs for maritime applications is higher especially regarding different power scales. PEMFCs have been used for years in the transport sector and technology is well-developed and proven.

To estimate the realistic volume and weight of the PEMFC system for onboard application, some existing maritime-approved systems in the market are compared with each other as well as with literature-based volume and weight data. The minimum and maximum values are presented and the average value is considered for calculations of the sample vessel. Figure 5(a) presents the power-volume correlation and Figure 5(b) shows the power-weight correlation of different PEMFC stacks based on literature and market data. The market data are based on collected data from manufacturers such as Ballard, Nedstack, Genevos, Proton Motor, PowerCellution, Hydrogenetics, Loop Energy, Horizon, Nuvera⁸ and TECO2030.²¹ Table 2 summarizes the range of the gravimetric- and volumetric- power density of the PEMFC system.

Figure 5.

PEMFC stack (hydrogen as fuel) volume (a) and weight (b).

Table 2.

Minimum, maximum and average density of market existing PEMFC systems.

Range of density
Minimum	Maximum	Mean	Unit
50	651	271.3	kW/ton
27	473	146.4	kW/m³

The former fuel cell manufacturer TECO2030 developed PEM hydrogen fuel cell stacks and large-scale PEM hydrogen fuel cell modules, enabling ships and other heavy-duty applications to become emissions-free.²² According to the publicly available datasheet of the developed 6400 kW PEM fuel cell system (model FCC6400), the hydrogen consumption rate at 90% operating power is 375 kg/hr. The calculations for the required amount of fuel onboard for the scenario with PEMFC are based on this fuel consumption data. The start-up time of a PEMFC, defined as the time to reach 50% of the rated power²³ is assumed as 15 minutes.²⁴

3.4 SOFCs

SOFC technology compared to other power conversion technologies such as ICEs and PEMFCs, has higher energy efficiency and larger compatibility with different fuels without needing reforming apparatus. Therefore, SOFCs are seen as potential technology for medium- to long-distance shipping applications, especially when built within a hybrid energy system also consisting of ICEs, batteries, or other fuel cells. However, their limited development state, high volume and weight requirements and high costs have hindered their wide-spread adoption in shipping sector.²⁵

Literature and internet studies were conducted to find specifics of SOFC plants, particularly regarding efficiency, weight, volume, installed capacity and emission factors. A database was constructed considering commercial SOFC products by worldwide companies, such as Mitsubishi Hitachi Power Systems (Japan), BloomEnergy (USA), Convion (Finland), SunFire GmbH (Germany) and Elcogen (Estonia).⁸ These SOFC products are all fuelled by natural gas (NG).²⁶ Since the technology for running an SOFC system fuelled by hydrogen as fuel is not fully developed and market-ready, the system sizes of a natural gas-fuelled SOFC are considered for calculations. The distribution of volume data is shown in Figure 6(a) and weight data in Figure 6(b). Input parameters for SOFC system volume and weight based on market survey results in a range of volumetric and gravimetric densities shown in Table 3. SOFC technologies are not commercially mature, which results in a wide variation in their densities.

Figure 6.

SOFC plants (with natural gas as fuel) volume (a) and weight (b).

Table 3.

Minimum, maximum and average density market existing SOFC systems.

Range of density
Minimum	Maximum	Mean	Unit
9	61	24.6	kW/ton
4	36	13.1	kW/m³

The large-scale SOFC systems for maritime applications as well as the SOFC systems using hydrogen as fuel, are under development. The other point is that most of existing SOFCs at the moment are for terrestrial market and for this reason they are bulky. The maritime specific SOFC systems with lower volume requirements are also still being developed.

The assumptions regarding fuel consumption are taken based on simulations done in the NAUTILUS project, which presents 6.4 kg/hr hydrogen consumption for a 100 kW stack.²⁷ The cold start-up time of SOFC takes several hours, corresponding to time needed for bringing the system from a cold state to operational temperature, ready for normal operation when the system accepts power delivery requests from the energy management system. Cold start-up leads to high thermomechanical stresses, and therefore, frequent start-up and shutdowns of SOFCs are avoided for economic reasons and a higher risk of failure.²⁸ The hot start-up or response time of SOFC is faster, since the power output of the stack increases gradually to the designed point. The total response time is estimated as 40 minutes.²⁹ For this sample vessel, 7 MW is continuously required, which can be covered by the continuously running SOFCs. In idle mode, the SOFCs provide 30% of their power capacity. Consequently, batteries would be used only for supporting the response time of the SOFCs.

3.5 Batteries

The market of maritime batteries has been developing fast over the last years and the types and number of available systems in the market are increasing. Lithium-ion batteries based on Nickel-Manganese-Cobalt (NMC), Lithium Iron Phosphate (LFP) and Lithium-Titanate-Oxide (LTO) chemistries are most suitable for ships.³⁰

NMC batteries have a high energy density but a lower lifetime; and are well-known and widely used in maritime applications. However, NMCs have higher safety perils related to fire risk. The LTO chemistry demonstrates the longest life span among the three chemistries and is the second-most widely used chemistry for maritime applications today.³¹ Upscaled LFP battery systems are evaluated to be slightly more promising due to a high energy density and small investment costs. On the other hand, when including production emissions, NMC-based battery cells are advantageous by a small margin.³² For this study, NMC batteries are considered, since they are currently the most commonly used marine batteries, having comparatively high specific energy capacities among other battery types. Figure 7(a) presents the volume and Figure 7(b) the weight of different NMC battery systems for marine application in the market, from manufacturers such as TESVOLT Ocean, Corvus Energy, Shift Clean Solutions, EST-Floattech and Leclanché.³³ Using the collected data, minimum, maximum and mean battery densities are extracted as in Table 4.

Figure 7.

Distribution of volume (a) and weight (b) versus power size of different NMC battery systems.

Table 4.

Density range of different market ready NMC batteries.

Range of density
Minimum	Maximum	Mean	Unit
54.2	329.6	111.4	Wh/kg
71.6	217.5	145.3	Wh/l

For calculation of battery capacity using the equation (3), it is considered that the battery has the maximum C-Rate (Discharge/Charge) of 3, which provides an estimated maximum battery power output of:

\begin{aligned} Battery size [kWh] \times C_{rate} = Power [kW] \end{aligned}

(3)

The C-rate of battery is estimated as 3, it means that each battery system fully discharges in roughly 20 minutes while providing full load power. This would be sufficient for covering the start-up of PEMFC. But for SOFC with a response time of 40 minutes, the number of installed batteries needs to be doubled compared to the case with PEMFC. Based on the load profile explained in 3.1, for the scenarios fully powered by fuel cells, the maximum required battery capacity is estimated as 10 MW, which is the load change step demanded from the fuel cells. This results in maximum required battery capacity of 3.5 MWh for the cases fully powered by PEMFCs and battery capacity of 7 MWh for the cases fully powered by SOFCs.

3.6 Fuel storage system

Liquid hydrogen storage offers the highest energy density among known forms of hydrogen storage. Liquid hydrogen storage onboard the ship requires the use of a containment system that maintains adequate storage conditions of temperature and pressure. Among other containment systems, type C tanks are pressurized tanks, independent from the ship structure thus not requiring a secondary barrier. Type C tanks have been used for LNG-fuelled cruise ships, making them a suitable choice for liquid hydrogen storage. Therefore, a modular tank system arrangement is used to accommodate the required amounts of liquid hydrogen within the available space, considering the clearance distance between the tanks and the ship side shells of at least 800 mm. It is a double-walled cylindrical tank with hemispherical end caps, multi-layered isolation (MLI) and vacuum insulation. Due to cryogenic conditions imposed by the liquid hydrogen and the need for corrosion-resistant materials, both the inner and outer tanks are made of stainless steel 316. Table 5 summarizes the attributes of the tank. The water volume of the inner tanks is 2027 m³, however, the maximum amount of LH₂ that can be loaded into the tank at 5 bars is 81% as per United States Coast Guard Regulations (USCG).³⁴ Maintaining the tank at a low temperature is crucial to avoid boil-off losses induced by the tank precooling, therefore, 5% is the minimum fill level allowed. Therefore, the amount of usable LH₂ is 76% of the tank water volume.

Table 5.
Liquid hydrogen tank main characteristics.

Parameter Unit Value

Inner tank length m 43

Inner tank diameter m 8

Inner tank volume m³ 2027

Minimum filling level % 5

Maximum filling level % 81

LH₂ volume m³ 1641.8

Usable LH₂ volume 5%–81% m³ 1540.5

Design pressure bar 5

Boil-off rate %/day 0.15

Insulation thickness (type MLI + vacuum) m 0.5

Inner tank cylindrical part thickness mm 40

Inner tank end cap thickness mm 20

Outer tank volume m³ 2648

Outer tank cylindrical part thickness mm 45

Outer tank end cap thickness mm 25

Outer tank diameter m 9

Tank length m 44

Tank weight (empty) Ton 334.4

Parameter	Unit	Value
Inner tank length	m	43
Inner tank diameter	m	8
Inner tank volume	m³	2027
Minimum filling level	%	5
Maximum filling level	%	81
LH₂ volume	m³	1641.8
Usable LH₂ volume 5%–81%	m³	1540.5
Design pressure	bar	5
Boil-off rate	%/day	0.15
Insulation thickness (type MLI + vacuum)	m	0.5
Inner tank cylindrical part thickness	mm	40
Inner tank end cap thickness	mm	20
Outer tank volume	m³	2648
Outer tank cylindrical part thickness	mm	45
Outer tank end cap thickness	mm	25
Outer tank diameter	m	9
Tank length	m	44
Tank weight (empty)	Ton	334.4

The current diesel tanks onboard this sample vessel allow 14 days between two bunkering events. Equation (4) shows LH₂ volume calculation required, assuming the energy system operates at 28 MW (70% of the maximum capacity) during that time.

\begin{aligned} Required L H_{2} [m^{3}] = \frac{336 [hr] \times E n e r g y . S y s . F u e l . C o n s ._{@ 28 MW} [\frac{kg}{hr}] \times 1.2}{Densit y_{L H_{2}} [\frac{kg}{m^{3}}]} \end{aligned}

(4)

where 1.2 corresponds to 20% margin for safe return to port.

The Energy.Sys.Fuel.Cons._@28MW representing the fuel consumption rate of the energy system operating at 28 MW, is based on the fuel consumption rates presented in Sections 3.2–3.4. For the cases with ICE, the fuel consumption of engines operating at full load is considered. Since LH₂ has higher volume requirements than diesel, 14 days between two bunkering operations will not be feasible. Therefore, in order to make a realistic comparison with the current system as well the existing space onboard, the frequency of hydrogen bunkering is assumed as 7 days.

3.7 Case scenarios

For the sample vessel, five case scenarios are considered using different energy systems combined with liquid hydrogen as fuel:

Case 1a: 40 MW ICE

Case 1b: 40 MW dual fuel ICE (additionally using diesel)

Case 1c: 20 MW ICE, 20 MW PEMFC and 1.7 MWh battery

Case 2: 40 MW PEMFC and 3.5 MWh battery

Case 3: 40 MW SOFC and 7 MWh battery

Cases 1a and 1b use ICEs as the main power supply system onboard. Case 1c is a combination ICE, PEMFC and battery, in which the ICE and PEMFC have similar power capacities. The maximum required battery capacity in this case is estimated as 1.7 MWh which is half of the battery capacity in Case 2 covering the start-up time of half size PEMFC. Cases 2 and 3 are scenarios with PEMFC and SOFC as the main power supply with batteries as support. Further possible scenarios with different combinations of component capacities rather than those considered here exist. But this paper aims to develop a method for the general evaluation of most feasible solutions, including existing well-known hydrogen technologies.

Figures 8 and 9 provide an overview of the volume and weight requirements of the energy systems in each case scenario, using the minimum, maximum and mean values. It has to be noted that in this work only the component volume is considered and the other required space is not factored in. Comparing different scenarios, those with ICE as main power supplier have very similar power densities to the current system. PEMFC-Battery scenario, considering the mean values, has the lowest volume and weight compared to the other systems. The SOFC-Battery case has higher energy efficiency, especially since the produced heat can be used further onboard, but it requires more improvement in power density considering its high volume and weight. In the next section, for the technical evaluation, the mean values are considered.

Figure 8.

Volume comparison of alternative energy systems and the current system (bars represent minimum and maximum volumes).

Figure 9.

Weight comparison of alternative energy systems and the current system (bars represent minimum and maximum weights).

Table 6 shows the electrical efficiency of each alternative scenario, where PEMFC has the highest efficiency compared to the other energy systems. However, the SOFCs with their high temperature and heat efficiency, could reach higher total efficiency than the others.

Table 6.

Electrical efficiency of alternative case scenarios.

Case scenario	Electrical efficiency (%)
Case 1a: ICE-LH₂	42
Case 1b: ICE-LH₂-Diesel	39
Case 1c: ICE-PEMFC-Battery	46
Case 2: PEMFC-Battery	51
Case 3: SOFC-Battery	47

4 Results and discussion

The case scenarios are technically evaluated to assess the possibility of installation onboard from a volume and weight perspective. Afterwards, the most suitable scenarios are evaluated from a safety point of view.

4.1 Technical assessment results

Figures 10 and 11 show the space and weight requirements of the energy system and fuel storage tanks in the current system, as well as case scenarios. As assumed in Section 3.6, the tank size calculations for the case scenarios are based on weekly bunkering interval instead of once every fortnight for the current system. The results show that Case 2, followed by Case 1b has the lowest volumetric requirements; however, Case 1b with dual fuel ICE will have operational carbon dioxide emissions. The total volume required for these cases are 1.8 to 2 times higher than the current system respectively. Case 1a with mono fuel ICE requires 4 hydrogen tanks and, as explained, the tank arrangement is impossible to fit into the current ship design due to space limitation.

Figure 10.

Volume comparison of current system (bunkering every 14 days) and alternative hydrogen solutions (assumption of bunkering every 7 days).

Figure 11.

Weight comparison of current system (bunkering every 14 days) and alternative hydrogen solutions (assumption of bunkering every 7 days).

In Case 1c, the combination of PEMFC with mono fuel ICE reduced the required LH₂ volume to be stored for the longest bunkering duration of 7 days to 9% compared to Case 1a. Yet still, the required LH₂ volume to be stored onboard is 7% higher than the possible capacity of 3 tanks. Therefore, 4 tanks are required for this case.

In Case 3, the SOFC-Battery system has almost 4 times larger volume than the size of current system onboard. In this case also, the required LH₂ volume is 10% higher than the possible capacity of three tanks. Therefore, four tanks are needed, which makes it in total the largest option compared to the other cases. It has to be noted that the state-of-the-art solutions for SOFC do not exist and this is according to the theoretically based data.

From a weight perspective, Case 2 followed by Cases 1c, 1b and 1a represent 34%, 48% and 54% of the weight of the current system onboard. This is due to the lower density of liquid hydrogen compared to diesel. Case 3 represents 77% of the weight of the current system due to the lower gravimetric density of SOFC. Comparing the zero-emission scenarios, Case 2 with PEMFC-Battery followed by Case 1c with ICE-PEMFC-Battery have the lowest volume and weight.

For Cases 1b and 2, three liquid hydrogen storage tanks will suffice. For Cases 1a, 1c and 3, four liquid hydrogen tanks are required onboard. Within the available space inside the ship, only three tanks can be fitted, allowing the storage of 4926 m³ of liquid hydrogen even though the volume of the outer tanks combined is 7944 m³. This is illustrated in Figure 12, where the rectangular frame represents the boundaries of the ship from port to starboard and a height of 12 m at the lowest section of the ship. With such an arrangement, the quantities of liquid hydrogen are insufficient. Thus, to increase the amounts of liquid hydrogen stored, the number of tanks has to be increased, which requires an increase in the ship width. Alternatively, to remediate the low quantities of stored fuel, increasing the bunkering intervals guarantees the availability of liquid hydrogen onboard. However, bunkering infrastructure and supply chains need to be made available to meet the demand. The modification introduced to a ship energy system, both storage and energy converters, will result in significant changes on the structure as well as in the hydrostatic and hydrodynamic characteristics of the subject ship.

Figure 12.

Liquid hydrogen tank arrangement onboard sample ship.

4.2 Safety assessment results

For the safety assessment, the fuel storage system, the fuel cells and batteries are considered. The considered fuel storage system includes three cylindrical liquid hydrogen tanks, pumps, valves and heat exchanges with the arrangement shown in Figure 13. It is compared to the conventional fuel system based on HFO as shown in Figure 14.

Figure 13.

Liquid hydrogen storage tanks system.

Figure 14.

Heavy fuel oil and marine diesel oil (MDO) tank system onboard the sample ship.

The safety evaluation is performed for three different possible leak sizes of 3, 10 and 25 mm (full rupture). It is assumed that the system is operational 60% of the time (estimated according to the cruise ship propulsion system). Table 7 shows the frequency for the most promising system based on technical assessment (Section 4.1) and the conventional HFO ICE system.

Table 7.

Leak frequencies of most suitable scenarios compared to current system.

Energy system	Leak frequency
Current system: ICE – HFO – MDO	1.90 × 10⁻³
Case 2: LH₂ – PEMFC – Battery	5.62 × 10⁻⁴

It can be seen, that the PEMFC system has the smallest frequency of a hazardous event and can in this study be considered the safest system. But all systems are above a generally accepted individual risk per annum (IRPA), which is often considered to be 10⁻⁶. To lower the frequencies below the acceptable IRPA, additional safety measures need to be taken, but they are out of scope of this work, as only the safety of the initial system is considered.

It has to be noted, that this study only considers the frequency and not the possible consequences of a hazardous event. It can be expected, that HFO has lower consequences, due to its physical properties. The consequences are out of scope for this study and might be considered in later work.

4.3 Discussion

Various combination possibilities of different alternative fuel and energy system options exist. Similar to the paper by Rivarolo et al., this study develops an approach for comparing different alternative solutions. With the main difference, that this paper studies several possible technologies in combination with each fuel type. The other existing literature, such as studies by Micoli et al. and by Kim et al., also examine specific fuel types or energy systems. The selection of most reasonable combination possibility of these technologies depends on exact vessel's load profile and sailing data. The reviewed scenarios in this approach are limited to the database of market existing technologies for marine applications, as well as the limited input data regarding vessel's operation. In the procedure developed by Rivarolo et al., the tank size calculation is limited to commercial tanks from manufactures. But in this study, it is carried out case specific in order to size the final volume. Additionally, the arrangement of tanks is done for each vessel based on its dimensions to better estimating the space requirements and fitting of storage system onboard. Safety is as well a critical aspect, in particular when considering passenger and cruise ships. Evaluation of the frequency of hazardous events in each scenario allows to indicate the safer and more reliable system. Including safety criteria in this approach, distinguishes it from other existing approaches, and makes it capable of holistic scenario comparison. This is important, especially when it comes to comparison of various fuel types with their different characteristics.

Other than current literature, such as studies by Wang et al. and Farrukh et al., investigating decarbonization solutions for single vessels or specific ship types, this approach is capable to be further used for different alternative fuel and energy system solutions for several maritime vessel types. This approach can be extended to different alternative fuel and energy system solutions for different vessel types. The result is a recommendation of technologies or technical solutions suitable for implementation in the ship design. This can be used as a generic feasibility study, before advancing into naval architectural analysis. Overall, the case study analysed the applicability of the approach and the usefulness of the results, providing a holistic assessment of the technological and safety performances of alternative green fuels and technologies for decarbonization of ships.

By examining five combinations of energy converter arrangement fuelled by liquid hydrogen to identify the suitable solution from a weight and volume perspective. This study found that the use of PEMFC-Battery arrangement is a comparably suitable solution. However, for a more specific recommendation, all relevant assessment criteria and a biased benchmark have to be considered. In this recommended solution, despite the volume limitation and lower energy stored onboard, a considerable reduction in the system weight has been achieved. Then a safety assessment is applied to the most promising solutions from the technical perspective and compared with the current installed system onboard. From safety aspect, the PEMFC-Battery arrangement is safer than ICE. Where ICEs are prone to more leakages and failures, leading to fire or explosions. This highlights that liquid hydrogen with a PEMFC-Battery system may represent a viable alternative solution for a cruise ship.

The tank arrangement is proposed to fit within the subject ship, enabling the storage of the highest amount of liquid hydrogen possible. Accordingly, the dimensioning of the tanks is carried out using simplified assumptions to determine an estimated overall volume and weight for the tanks. The proposed assumptions are suitable for the early stages of decision making; however, accurate sizing of the tank's shell thickness, stiffening structure and fittings is required at the final stages of the design. Type A membrane tanks and independent type B tanks need to be investigated as they can offer a higher storage capacity within the rectangular volume available inside the ship.

5 Conclusion

Liquid hydrogen is a viable alternative green fuel for shipping; however, several challenges need to be addressed to prove the feasibility of its storage and consumption onboard. One main drawback is the large volume required to store relatively low amounts of energy. Increasing the bunkering frequencies will partially remediate the low energy storage on board. In this study, the bunkering frequency is weekly, based on the vessel's sailing routes and the future availability of liquid hydrogen at specific ports. Another limitation to integrating liquid hydrogen as fuel in ships is the absence of a mature regulatory framework at the international level to govern, facilitate and regulate the use of hydrogen as a fuel onboard ship. Finally, the ramp-up of green hydrogen production and the infrastructure required for transport and storage are crucial to fulfil the future demand.

The developed holistic assessment approach compares a traditional fuel storage and power generation set-up against several liquid hydrogen-fuelled energy converts for onboard power generation. It is based on a collection of existing market solutions correlating their volumes, weights and fuel consumption. The ship-specific tank size calculation makes this technical assessment more specific, particularly for cryogenic fuels like liquid hydrogen. On top of that, the safety assessment improves this by considering the frequency and probability of hazards for fuel storage tanks and different energy system options. Improving the approach for an accurate and detailed evaluation requires considering more system components, which in this study are not considered because of their comparatively smaller volume and weight requirement, plus the lack of detailed information about them. Also, a detailed and accurate layout and structural design of each alternative fuel system onboard is required. The approach can be further developed to include economic and environmental perspectives, providing a more holistic evaluation. This approach can also be transferred to other fuel types and vessel types.

Footnotes

ORCID iDs

Shaghayegh Kazemi Esfeh

Cherif Ait Aider

Jorgen Depken

Dheeraj Gosala

Lars Baetcke

Sören Ehlers

Author contributions

Shaghayegh Kazemi Esfeh: conceptualization, formal analysis, methodology, investigation, visualization, project administration and writing–review and editing. Cherif Ait Aider: investigation and writing–review and editing. Jorgen Depken: investigation and writing–review and editing. Dheeraj Gosala: review and editing. Lars Baetcke: supervision. Sören Ehlers: supervision.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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