1.1 Background and Motivation

1.1.1 Economy Growth and the Demand for Maritime Transport

After achieving a 3.1% growth in 2017, the growth rate of global economy declines to 3.0% in 2018 and further declines to 2.3% in 2019 [1]. In 2020 and afterward, a range of downside risks may further intensify the economy growth, such as the tariff between US–China, the decision by the United Kingdom to leave the European Union (“Brexit”), and the global New Coronavirus spread. In this background, a new normal is about to take hold, reflecting a continuous moderate growth of the global economy. This trend will significantly influence all the attached subsystems or sectors in the maritime transportation system, including infrastructure requirements, ship carrying capacity needs, ship design and technology, port developments and performance, and so on.

The primary impact of the slowing-down economy puts on the demand of maritime transport. In 2017–2019, the international maritime trade shares similar moderate growths with the global economy. According to the “Review of Maritime Transport 2019” by UNCTAD [1], although the global maritime trade reaches a new milestone of 11 billion tons in 2019, the growth is only 2.7%, not only lower than 4.1% in 2017, but also lower than 3.0% average from 1970 to 2017 [1]. Figure 1.1 and Table 1.1 respectively show the total cargo volumes of specific types in ton-miles and tons.

Fig. 1.1
figure 1

Development of international maritime trade (Unit: billion ton-miles), reprinted from [1], open access

Table 1.1 Specific types of international maritime trade (Unit: million ton) reprinted from [1], open access

The moderate growth of maritime transport demand shall introduce more competition between different players, i.e., shipowners or port administrators and other stakeholders. This trend may re-shape the market structure since many less-efficient sectors in the maritime transportation system will fall into the brutal struggle between embracing the technology evolutions or being eliminated.

1.1.2 Ship Supply Capacity and Market Structure

The new trend of moderate growth also defines the recent supply-side development of the maritime transportation. In 2019, the world’s commercial fleet consists of 95,402 ships, with a combined tonnage of 1.97 billion dwt [1]. The share of each principal vessel type is shown in the following Table 1.2.

Table 1.2 World fleet by principal vessel type (Unit: dwt), reprinted from [1], open access

From the Table 1.2, the oversupply of ship capacity remains a prominent characteristic for most of shipping sectors. Among all sectors, the gas carriers experience the highest growth rate at 7.25%, which is driven by the significant expansion of the liquefied natural gas (LNG) trade [2]. Then the container fleet follows at 5% increment. On the contrary, the chemical-tanker and dry-bulk-carrier segments both only experience moderate growths, and the oil tanker segment even suffers a downward trend.

In summary, the oversupply of the ship capacity will further reduce the average freight fare and cut down the profits. Some new technologies are therefore motivated to integrate into the maritime transportation system to gain efficiency improvement and competitional advantage.

1.1.3 Shipping Services and Ports

One effect of the market re-shaping is to enlarge the average sizes of ships since mega-ships generally have cheaper transportation costs than smaller ships. This trend is suggested in Table 1.3 by the increasing of average vessel size in recent years.

Table 1.3 Vessel size distribution to service years (Unit: dwt), reprinted from [1], open access

The increasing trend of vessel size has great impacts on the port terminals, as well as the shipyards and the inland logistics. The resulted-in influences come from two aspects: (1) mega-ships generally have limited access to many ports since draft restrictions or berth-length requirements, which makes the mega-ships can only call for services in some ports; (2) larger ships normally call at fewer ports than smaller ships during one voyage, and less calls with greater cargo volumes will create greater pressure on the operation of ports.

From above, as ships become larger, the ports and terminals that can accommodate the service-calls become limited, which means the main ports around the world, such as Singapore, Shanghai, Istanbul, Houston, Genoa, Hamburg, and so on, will face more competitions and challenges in the future. New equipment and technologies are required for the future large ports to efficiently provide at least three types of services to the berthed-in ships.

  1. (1)

    Logistic services, including loading/unloading cargos from the onboard to the stacking areas, the restacking of cargos in the stackyard, the transportation to the inland logistic systems, and so on. This type of service is conventional but in current situations, large ports are required to further enhance the cargo handling efficiency and reduce the dwell-time in berth to strengthen their competitiveness.

  2. (2)

    Electrical services, namely the on-shore power supply, or cold-ironing technology. For the future efficient ports, cold-ironing technology is necessary since it can greatly reduce the gas emission of the berthed-in ships in the harbor territory. According to [3], cold-ironing technology will become a mandatory requirement for large ports in the future.

  3. (3)

    Heating/cooling services. For specialized cargo such as refrigerated goods, large ports need providing reefer slots, and for future cruise ships, large ports may also provide on-shore heating/cooling services to the onboard passengers [3].

To efficiently and economically provide the above services, the future ports are need to be significantly upgraded in both infrastructure planning and management framework. It should be noted that the ports not only include the mainland ones, but also include those in islands, or “general ports” in various ocean platforms, i.e., ocean oilfields, ocean wind farms, or drilling platforms.

1.1.4 The Path to the Green Shipping

Besides the above motivations, the entry into force of several global environmental policies and the adoption of some voluntary standards also have some fundamental impacts on the maritime transportation system and set the following two main targets to achieve the future green shipping.

  1. (1)

    Relieving the heavy reliance on oil for propulsion.

Generally, more than 50% of the oil demand around the world is concentrated in the transport sector [4], and the global oil demand for maritime transportation is more than 300 million tonnes and accounts for 86% of the transport sector in 2012 [5]. According to [6], more than half of the fuel consumption increment in transportation is from maritime usage before 2040 if no further actions.

Additionally, the oil used for maritime transportation often has lower quality than other types of oil in the transport sector, i.e., denser and higher carbon-hydrogen ratio, as well as having more “polluted elements”. For example, the IFO380 is a frequently used oil type for large container ships, which has more than 3.8% sulfur, much higher than the light-oil used in land-based transportation.

As a result, the great consumption and low quality of maritime oil make the maritime transportation system emit diversified gas emissions, and the heavy reliance on oil for propulsion, therefore, becomes the main obstacle to limit the development of green shipping. The research on alternative fuels or energy sources should raise global concerns, such as hydrogen fuel and ammonia fuel, fuel cell technologies, and energy storage [4, 7,8,9].

  1. (2)

    Reducing greenhouse and polluting gas emissions.

The gas emission of maritime transportation usually has three types: carbon dioxide, sulfoxide, and nitrogen oxide. The carbon dioxide is regarded as the main culprit for the greenhouse effect and has been raised as a global concern since the subscription of the Kyoto Protocol in 1997 [10]. As for the sulfoxide and nitrogen oxide, they are viewed as two main types of polluted gas emissions, which are responsible for the acid rains and the ozone hole, respectively [11].

For global sustainable development, those three types of gases are all under strict surveillance, and for the future green shipping, multiple policies have been raised to address different types of gas emissions.

For the carbon dioxide, the International Maritime Organization (IMO) has announced an ambitious target to reduce 70% greenhouse gas (GHG) emission in 2050 compared with 2008 [12], shown as Fig. 1.2.

Fig. 1.2
figure 2

Reduction target of GHG emission from maritime transportation, reprinted from [12], open access

For better controlling the minimum required level of energy efficiency, the Energy Efficiency Design Index (EEDI) and Energy Efficiency Operating Index (EEOI) were established as the IMO’s strategies. Specifically, EEDI is a ship designing index proposed by the Marine Environment Protection Committee (MEPC 62) in 2011 [13]. Then in MEPC 63, four guidelines are amended in MARPOL Annex VI to further implement the EEDI as a mandatory regulation [14,15,16,17]. As for the EEOI, it is recommended by the Ship Energy Efficiency Management Plan (SEEMP) to manage the efficiency performance of ships and fleets over time using [18]. Both of EEDI and EEOI have been adopted by various ship companies.

For the sulfoxide, IMO has set certain limits since the year of 2000, shown as Fig. 1.3. From 1st, January 2020, the “ever strictest sulfur limit in history” has entered into force for the compulsory usage of low-sulfur fuel (0.5%), or the gas scrubber integration, or the alternative fuels.

Fig. 1.3
figure 3

Fuel sulfur limits in ECA and globe, reprinted from [12], open access

The first sulfur limit was introduced in the revised MARPOL Annex VI (Prevention of Air Pollution from Ships) and the concept of designated sulfur emission control area (SECA) was created correspondingly [12]. The Baltic Sea, North Sea, and North America have been designated as SECAs since 1997, 2005, and 2010, respectively. In 2011, the Caribbean Sea of United States has been designated as SECA. In October 2016, the regulation was confirmed at the MEPC 70, which dictates that from 2020 onward, the global limit of sulfur content will be 0.50% (outside SECAs), referring to the “ever strictest sulfur limit” [19].

For the nitrogen oxide, MARPOL has set tier I–III emission standards based on the speed of main engines and the ship-ages, shown as Table 1.4 [12]. Tier I is for the old ships built before 2000, and tier-II is the current NOx limit standard, and tier III is for the ships built after 2016 and sailing in nitrogen ECAs (NECAs). When outside the NECAs, the ships should follow tier II. It should be noted that the SECAs in the United States have been already set as NECAs. For the European SECAs (North and Baltic Sea), the NOx limits will be enforced from 2021.

Table 1.4 NOx emission limits (MARPOL Annex VI)

Accordingly, to fulfill the above ambitious targets on gas emission reduction, considerable investments should be going into the research and development for new technologies, such as better hydrodynamics in ships, more energy-efficient engines, efficient ships with new configurations, lower carbon or carbon-free fuels, the renewable integrations and more advanced energy management systems.

1.2 Promising Technologies

1.2.1 Overview

To achieve future green and efficient shipping, many technologies have been already or about to implement in maritime transportation. They are mainly classified as (1) the technical designs and (2) the alternative fuel or energy sources. The details are shown in the following Table 1.5.

Table 1.5 Promising technologies for the future green shipping

In the following context, the promising technologies related to the electrification of maritime transportation (in the bold context above) are illustrated in detail to show their usages.

1.2.2 Selected Technical Designs for Energy Efficiency Improvement All-Electric Ship (AES)

Long before gaining global concerns, the emergence of AES is quite early. In 1922, the first aircraft carrier of the United States named as “Langly (CV-1)” was converted from a coal carrier named as “Jupiter”, shown in Fig. 1.4a.

Fig. 1.4
figure 4

Main representatives of all-electric ships

This ship uses the configuration of “steamer-generator-electric machine” to drive the propeller and can be viewed as an embryo of AES. However, due to the technical limits at that time, the reliability of the power network in “Langly” was much lower than other similar mechanically-driven ships. Therefore, after the technical breakthrough in large-scale gearbox, the configuration of “Langly” has been hung on, and the mechanically-driven ships, which directly drive the propeller by the prime mover, have come to their golden age and dominate the configuration designs of ships until now and even the near future.

In the last decades, the advances in electrical engineering represented by the power electronic technologies have greatly improved the reliability of power systems, which progressively promote the development of microgrids. Nowadays, various types of microgrids have been utilized in different scenarios and applications. The bottleneck of AES has therefore been relieved.

With this great development, AES has gained the concerns from the shipping industry once again since higher energy efficiency and better controling ability. Currently, the configuration of AES has been already applied in warships, such as the Zumwalt-class destroyer and the America-class amphibious assault ship, which are shown in Fig. 1.4b and c, respectively.

The main advantage of AESs compared with conventional mechanically-driven ships is the usage of an “integrated power system” to dispatch energy, which can be shown as Fig. 1.5 [20].

Fig. 1.5
figure 5

Development of all-electric ships, reprinted from [20], with permission from IEEE

In Fig. 1.5a, the propellers of conventional ships are directly driven by the prime mover via a gearbox. This configuration limits the speed of prime mover and therefore limits the energy efficiency improvement. Additionally, another system of “prime mover-generator-service load” is necessary for the mechanically-driven ships to supply power to the onboard electrical equipment, which leads to great unnecessary redundancy.

In an AES (Fig. 1.5b), electricity is the only secondary energy form onboard. All the shipboard loads, including the propulsion load and various types of service loads, are supplied by the “integrated power system”. The energy flow can be precisely controlled to achieve the optimal energy efficiency, and the energy supply can be from multiple sources to improve the system reliability.

Due to the advantages above, AES has raised global concerns in recent years and has been viewed as the future direction of ship designs. Nowadays, this configuration begins to expand from the military applications to the commercial applications, such as the “ampere” ferry from Denmark [21], “puffer” cargo ship from China [22], and “Viking lady” off-shore support vessel (OSV) [23] and so on, which are shown in Fig. 1.6, respectively.

Fig. 1.6
figure 6

Some commercial all-electric ships Cold-Ironing Technology

The propulsion systems of most ships consist of the main engines and the auxiliary engines, and when berthed in a port, the main engines will be kept off and the auxiliary engines are used to support the onboard load demand, such as lighting, refrigeration, kitchen, entertainment and so on. The auxiliary engines burn fuel to generate electricity and emit various types of gas emissions in the harbor territory, such as CO2, SOx, and NOx, which brings a great amount of pollution.

Cold-ironing technology, or on-shore power supply, or shoreside power, is to supply the onboard hoteling load for the berthed-in ships by the port-side electricity, and the auxiliary engines onboard are all kept off, shown as Fig. 1.7 [24]. The electricity can be from the main grid, or port-side renewables and other clean fuels [24]. In the future, the cold-ironing technology will become a mandatory service from ports similar to the conventional logistic services.

Fig. 1.7
figure 7

Cold-ironing technology and conventional logistic services, reprinted from [24], with permission from IEEE

The first benefit of cold-ironing technology is the reduction of harbor gas emission. It is reported that the global harbor gas emission can have a 10% reduction by the integration of cold-ironing technology [25]. In UK ports, the cold-ironing technology can reduce 2% SOx emission. According to [26], cold-ironing technology reduces more than 57% of harbor gas emission in the Kaohsiung port in Taiwan. Secondly, cold-ironing technology may bring economic benefits to both the shipowners and the port authorities. Kenan et al. [27] shows in the regions of which the electricity price are lower than 0.19USD/kWh, the cold-ironing may reduce the operating cost of the berthed-in ships. In [25], the cold-ironing brings extra profits for the port with higher average handling time.

Cold-ironing technology is very suitable for the cruise ports, since when berthed-in, the cruise ships require a huge amount of power since many passengers staying on board [28, 29]. According to [30], an average of 29.3% of GHG emissions can be reduced in three different regional cruise ship cases when using cold-ironing. In other regions, the cruise ship ports can reduce 99.5% (Oslo, Norway), 85% (France) GHG emission by the cold-ironing technology, respectively.

Although the above outstanding advantages, the expansion of cold-ironing technology is still a challenging task. The main barriers include power quality [28], system stability [28], reliability and security [3], and synchronization problems [24]. Tsekouras and Kanellos [31] used a port-side reserved generator to improve the power quality of cold-ironing, and [32] proposed smart electrical interfaces to improve the performance of the cold-ironing facility. In [24], the synchronization problem of cold-ironing was investigated, and a control strategy is proposed to mitigate the voltage fluctuation when the ship plugged into the cold-ironing state, which is demonstrated by an OPAL-RT experiment. The Electrification of Ports

The ports are need to provide adequate logistic services to the berthed-in ships by many different types of equipment. The main equipment includes quay crane (QC), rail-mounted gantry crane (RMG), rubber-tire gantry crane (RTG), reach stacker (RC), straddle carrier (SC), and lift trunk (LT), which are shown in the following Fig. 1.8.

Fig. 1.8
figure 8

Main port-side logistic equipment

QC is used for loading/unloading cargo or containers from the ship-side. RMG and RTG are used to stack containers in the stackyard, and the main difference is that the RMG moves on the rail and the RTG moves on rubber tires. RS is used to reach a container in the stackyard. SC and LT are used to transport the containers within the stackyard. Conventionally, the above equipment are almost manually-driven, and in recent years, highly automated port equipment, such as automated RTG, RMG, LT, SC, begins in usage to improve the efficiency and reduce labor usage [33]. The energy sources of those equipment also become diversified. Table 1.6 gives the possible energy sources of the above equipment.

Table 1.6 Energy sources of different port-side equipment (data from [34])

From the Table 1.6, diesel and LNG are commonly used fuel types in port-side operation, which can power various port-side equipment. In addition from above, electricity is the most general energy source and can power all the main port-side equipment, and is also energy-efficient, easy to control, and convenient to fulfill automation, which makes the electrification of large ports as an irreversible trend in both shore-side operation and yard-side operation.

During the shore-side operation, the QCs can recover tremendous energy from the hoist-down movement [35]. In this way, the electrification and the integration of energy storage can shift the peak load of QCs and improve energy efficiency. In [35, 36], the peak load of QC can be reduced from 1211 to 330 kW with a supercapacitor integration. In [37], the peak load of QC is reduced from 1500 to 150 kW by the integration of energy storage. The shift of peak load not only represents higher energy efficiency but also can mitigate the influences on the port-side power system.

In yard-side operation, RMGs generally have higher energy efficiency than conventional RTGs since it is electrically-driven, but the advantage of RTG is the higher operating flexibility since its operation is not limited to the rails. In this sense, the electrification of RTG (E-RTG) can combine both advantages on energy efficiency and operating flexibility, which makes it a hot topic now and has reported gaining an 86.6% reduction in energy costs and 67% on GHG emission reduction [38]. The energy consumption comparison between RTG and E-RTG is shown in Table 1.7, and the results clearly show the energy saving ability of E-RTG. The energy cost of E-RTG is only 13% of RTG, and the GHG emission is only \( 1/3 \) of RTG. Similarly, for other yard-side equipment, such as RS, SC, and LT, the hybrid diesel-electric engine system has already been integrated. In literature, the hybrid SC has gained 27.1% fuel efficiency improvement, and the traveling motion, hoisting motion and lowering motion consume 52, 31 and 11% less energy [39]. As above, with the development of electrical engineering technologies, especially the energy storage technology, all-electric RS, SC, and LT will soon become reality and achieve the zero-emission target.

Table 1.7 Energy consumption comparison between RTG and E-RMG (data from [34]) Multi-energy Management

In recent years, with the development of global cold-chain supply, the refrigeration power demand grows very fast. In various studies, the energy consumption of refrigeration energy is now between 20 and 45% of the total energy consumption of ports [38, 40]. This suggests the need to improve the energy efficiency of reefer areas, such as determining the number of reefers, locations, and power plans. Additionally, due to the large scale of heating/cooling power demand on board, future cruise ships may also require heating/cooling power from the port-side. In summary, the above refrigeration power demand and the onboard heating/cooling power demand are both supplied by the heating/cooling flow, and can be viewed as “temperature-controled power demand” [40].

With the integration of heating/cooling flows, there will be at least three energy flows coordinated to each other in maritime grids, i.e., fossil fuel, electricity, and heating/cooling power, which makes the future maritime grid as multi-energy systems (MESs), and proper multi-energy management is essential for this special MESs.

Multi-energy management is a newly proposed management framework to coordinate multiple energy flows, shown in Fig. 1.9 [41,42,43], which shows different energy forms can convert to each other in MES to shift the peak load to fill up the valley, thus gaining higher energy efficiency compared with the single-energy system, such as the conventional power system. This management framework has been used in many land-based applications. In Jiangsu and Guangdong provinces of China, there are already system-scale projects of MESs.

Fig. 1.9
figure 9

Energy flows in a conventional multi-energy system

In an MES, the main power sources are the upper electric network (UEN) and the upper gas network (UGN). The main power demands are the electricity demand, gas demand, heating demand, and cooling demand, which are supplied by the electrical bus, gas bus, heating bus, and cooling bus, respectively. Among different energy forms, the gas turbine can generate electricity by burning gas. The by-produced heat can supply the heat load or the cooling load after the absorption chiller. The electricity also can be converted to gas by the power to gas equipment (P2G). In the future, the municipal water supply may also implement into MES since the expansion of the electric water pump.

However, the ships and ports have quite different operating scenarios compared with conventional land-based applications, such as extra electrical and logistic constraints, which makes current multi-energy management methods cannot be directly used, and further research efforts should be put into this field. Gas Capture Systems

In Sect. 1.4, the main targets of the gas emission control have been discussed. The alternative fuel and electrification technologies are generally viewed as promising routes to resolve this energy efficiency problem. However, before the maturity of above technologies, the integration of gas capture system can be viewed as an effective transitional approach. With its integration, the gas emission can be captured and stored in a location and permanently away from the atmosphere, thus the energy efficiency (gas emission per unit task) can be improved with continuously using the conventional fossil fuel. Nowadays, the capture systems of CO2, SOx, NOx are all mature technologies and ready to integrate into maritime grids.

Generally, the gas capture systems have three main working frameworks, which are shown in Fig. 1.10a–c, i.e., the pre-combustion, oxygen-fuel and post-combustion methods [44].

Fig. 1.10
figure 10

Main gas capture working frameworks

Among the above three working frameworks, the post-combustion method is the most frequently used (Fig. 1.10c) since a relatively simpler process, and the gas capture systems manufactured by the Wasilla and Mann are mostly using the framework of post-combustion.

In recent years, driven by the “ever strictest sulfur limit” planned to enter into force from 1st, January 2020 [19], many shipowners have planned to invest the shipboard gas capture systems to act as the transitional approach. With the gas capture system installed, the ships can continue to sail on the heavy oil (IFO380, 3.8% sulfur) with a lower price (cheaper than MGO, 0.5% sulfur) meanwhile meeting the environmental requirements. The initial cost-benefit analysis shows this investment can be refunded in 4–5 years under current oil prices [45, 46].

A typical illustration of gas capture system into ships is shown in Fig. 1.11 [47]. The emitted gas from the main and auxiliary engines are first absorbed and then stored in a storage. With sufficient energy supply, the gas capture system can reduce more than 70% gas emission [44].

Fig. 1.11
figure 11

Gas capture system into ships, reprinted from [47], with permission from IEEE

However, most of the ships are not designed with the gas capture system, thus the onboard engines may not have enough capacities to supply the power demand after the installation, and this is one of the main obstacle to limit the gas capture system integration in the views of energy management. In [44], an extra generator is invested to supply the power demand of gas capture system, and in [47], the energy storage and onboard generators are coordinated to meet the power demand of the gas capture system, and the capturing rate is more than 90%.

1.2.3 Selected Alternative Fuels or Energy Sources Renewable Power Generation

Generally, the renewable power generation integration into ships and ports is the fundamental approach to resolve the energy efficiency problem of maritime grids, i.e., when the penetration rate of renewable power generation increases, the usage of conventional fossil fuel will reduce. Until now, the integration of renewable power generation into maritime grids already has many practical cases, shown as Fig. 1.12a–c.

Fig. 1.12
figure 12

Practical renewable integrated ships

Figure 1.12a shows the “Zhongyuan Tengfei” photovoltaic (PV) integration project in 2016. The total PV module has 143.1 kW capacity and can provide power for lighting in 12 decks [48]. Figure 1.12b shows a conceptual hybrid renewable energy ship proposed by Sauter Carbon Offset company, Germany [49] in 2010. This ship uses wind and PV energy to sustain 16 knot speed with zero-emission. Figure 1.13c is the “Shangde Guosheng” ferry in Shanghai Expo, 2010, which has a length of 31.85 m, a breath of 9.8 meters, and a height of 7 m. Now, this ship serves as a tourist ship in Huangpu River, Shanghai [50].

Fig. 1.13
figure 13

Classification of energy storage

However, since the low energy density and limited installment area, renewable energy integration into ships can only supply a small part of total energy demand now, and due to the uncertainties, the shipboard energy management system needs to be upgraded to mitigate their influences [51].

On the port-side, the capacity of renewable energy is much higher since larger installment area. In the Jurong port (Singapore), the installed PV can generate more than 12 million kWh electricity per year [3]. In Houston port (US), the Spilman’s island is planned for PV modules, and the potential PV capacity can be more than 4 MW [52]. In Hamburg port (Germany), the installed wind turbines scale up to 25.4 MW [53]. The above cases have demonstrated that the renewable power generation has a very promising future in the port-side applications. Energy Storage and Fuel Cell Integration

Generally, electrical energy can be converted to many different forms for storage, which are shown as following Fig. 1.13 [54]. Among all the technologies in Fig. 1.13, pumped hydro storage, compressed air storage and flow battery are not suitable for maritime applications since the limits in locations and operating conditions. The superconductivity storage has very limited energy capacity, which is also not practical nowadays. The most promising energy storage technologies currently in maritime applications include flywheel, battery, ultra-capacitor, and thermal tank. Strictly speaking, fuel cell is a power source technology rather than a type of energy storage technology. But it has similar characteristics and operating conditions with conventional energy storage systems, i.e., no combustion process, small installment area, directly outputting electricity, no spinning components. In this section, the fuel cell is discussed together with the other conventional energy storage systems (ESSs).

In maritime applications, ESSs are used for (1) peak load shifting by the high energy density ESS; and (2) resolving power quality issues by the high power density ESS. In long-term timescale, the peak load shifting can mitigate the burdens of main power sources (generators) and the energy efficiency can be improved [55,56,57,58,59,60,61,62]. Then the high power density ESS can respond to the load fluctuations in short-term timescale to resolve the power quality problems [63,64,65].

Since no combustion process, fuel cells have higher generation efficiency and smaller unit-sizes than the traditional internal combustion engines, which is the promising power source technology for maritime applications in the future. At present, the hydrogen fuel cell based on proton exchange membradune technology is a relatively mature technology and has been used in the energy supply of submarine [66], but the production and storage of hydrogen are still expensive, which limits the further commercial applications of hydrogen fuel cells. On the other side, liquefied natural gas (LNG) and Maritime Diesel Oil (MDO) are currently the main fuel types in commercial ships, and the corresponding Molten Carbonate Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC) on these two types of fuels, therefore, have higher commercial values.

Currently, fuel cell acts as an auxiliary power source in ships, an illustration in all-electric ships can be shown as Fig. 1.14 [67].

Fig. 1.14
figure 14

Integration of fuel cell into all-electric ships

In the Fig. 1.14, fuel cell is installed at one bus to supply the low-voltage hotel load and the propulsion load. Besides, there are two cases in Fig. 1.14, and the first case is the “Viking lady” offshore supporting vessel (OSV) has already installed a 330 kW fuel cell compared with the total generation system of 8040 kW [23]. The other case is, in 2019, the 712 ship institute of China has invented a 500 kW shipboard fuel cell.

All around the world, the fuel cell applications in ships are shown in Table 1.8, which includes both military and commercial applications. With the development of fuel cells, the capacity of the fuel cell will further increase to replace the current onboard spinning prime mover. However, there is still a long way to go before the fully replacement of internal combustion engine to fuel cell. Many obstacles, such as the energy management problems, the lifetime management problems, are still pending.

Table 1.8 Projects of some selected fuel cell based ships Low-Carbon and Carbon-Free Fuel

Current maritime transportation highly relies on fossil oil for propulsion, especially the usage of heavy oil (IFO380). The replacement of heavy oil to low-carbon and even carbon-free fuel is the fundamental approach to resolve the energy efficiency problem of maritime transportation. Generally, alternative fuels are classified as low-carbon fuels, i.e., LNG, methanol, ethanol, and carbon-free fuels, i.e., hydrogen, ammonia.

Among the low-carbon fuels, LNG is the alternative fuel for heavy oil which gained the highest concerns [4]. It is estimated that the shipping sector can reduce more than 30% GHG emission by using LNG [4]. In 2018, the LNG transport ships have expanded by 7.25%, which represents the prosperous development of this sector [1]. Among the carbon-free fuels, hydrogen has been regarded as the future green fuel for a long time, since the only product after burning is water. The ship using hydrogen for propulsion can achieve zero gas emission [4]. Ammonia is a newly proposed carbon-free fuel in recent years, with the products of nitrogen and water [7].

Despite of the above great achievements, there are still many limits on the application of low-carbon and carbon-free fuels. The gaps come from the following four aspects, and a promising and practical alternative fuel should resolve all the below problems before it can replace the heavy oil for propulsion.

  1. (1)

    Mass production

To become a practical alternative fuel, the primary problem is the mass production problem. Although Hydrogen, Methanol, and Ethanol are all vital raw materials in chemical industry, the global production of those fuels is still hard to sustain global shipping [7]. Among current fuels, LNG and ammonia are only two fuels with enough production capacity to sustain global shipping. LNG is for its great amount of natural resource reserves, and the ammonia is for its great production ability as a widely used chemical fertilizer [7].

  1. (2)

    Commercial power source technologies

Current engines in ships are mostly designed for heavy oil. When changing to other fuels, the burning chambers should be reformulated or re-designed to keep the burning stability of fuels. However, except for the LNG, there still lacks large-scale commercial power source technologies for other alternative fuels to sustain long-distance of navigation [7]. Furthermore, some alternative fuels, like ammonia, are very hard to burn in conventional conditions. In this way, the fuel cell can be a very promising way for maritime applications since it has no burning process.

  1. (3)

    Mass storage

The heavy oil usually has much higher energy density and less volatileness than the low-carbon and carbon-free fuels. When changing to alternative fuels, the storage conditions need to be adjusted, i.e., larger volume to sustain the navigation distance, proper sealing conditions to reduce the volatilization of fuels, and so on. For example, the volatilization loss of LNG transport in a week is about 5% by current technology. In this way, the mass storage technology of LNG, such as re-liquefaction, is needed urgently in this field.

  1. (4)

    Global supply chain

Since the shipping fuel is used all around the world, the transport cost will greatly influence the expansions and usages of alternative fuel, which means the candidate alternative fuels should have a mature and complete global supply chain to reduce its transport cost. In fact, LNG and ammonia are the only two fuels with a complete and mature global supply chain [7].

1.3 Next-Generation Maritime Grids

From the above illustrations, the main characteristic of the next-generation maritime grids is the trend of electrification and the involvement of multiple energy flows, and current green shipping technologies are convenient to implement into the maritime grids. With various new technologies integrated, the next-generation maritime grids are defined as those local energy networks (combined with electrical, fossil fuel and heating/cooling energy networks) installed in harbor ports, ships, ferries, or vessels, which consists of generation, storage and critical loads, and can operate either in grid-connected or in islanded modes and operate under both the constraints of power system and maritime transportation system. In the following context, two main representatives of the next-generation maritime grids, i.e., shipboard microgrid, seaport microgrid, are illustrated and then the coordination between them is shown.

1.3.1 Shipboard Microgrid

With full electrification, the integrated power system of ships formulates a microgrid, and the ship is referred as “AES”, which is illustrated in Fig. 1.15.

Fig. 1.15
figure 15

Typical topology of future all-electric ships, reprinted from [24], with permission from IEEE

From above, the shipboard microgrid consists of both an energy network (blue lines and arrows) and a communication network (green lines and arrows). The generators and battery deliver power via the energy network to meet the propulsion and service loads. The propulsion load is used to drive the ship. The service load supplies electricity to various onboard equipment, including the onboard radar, navigation system, air conditioning, as well as the gas capture system in Sect. In the future, fuel cells may further replace the generators to act as the main power sources. To further improve the energy efficiency of AES, renewable energy can be integrated, like the photovoltaic modules in Fig. 1.15. As for the communication network, the shipboard energy management system (EMS) can optimally calculate the generators and battery outputs and then send the dispatch signals to each component by it.

A special case is the cruise ship since the large scale of thermal load demand onboard. In fact, the shipboard microgrid of cruise ship can be viewed as an MES [77], shown as Fig. 1.16.

Fig. 1.16
figure 16

Topology of a multi-energy cruise ship, reprinted from [77], with permission from IEEE

The main differences between Figs. 1.15 and 1.16 are the involvement of heat flow. In a cruise ship, the combined cooling/heating power generator (CCHP) and power to cooling/heating (PTC) equipment are installed to act as the heating sources, and the electrical flow and heating flow should be coordinated to achieve a better economic and environmental behaviors.

1.3.2 Seaport Microgrid

Seaport microgrid is a newly proposed concept for seaport management, which is depicted by [24]. The incentive of the seaport microgrid is to make it as an energy district to improve renewable energy penetration and enhance the grid storage capacity by selling the electricity to the market through the main grid. There are already many practical cases around the world of seaport microgrid. In [78], the author advocated the harbor area as a unique territory that should have new business models with its energy plan. In [79], two practical projects of seaport microgrids in Hamburg (German) and Genoa (Italy), are manifested in detail, and the operating data proves the validity of seaport microgrid.

A typical seaport microgrid is illustrated in Fig. 1.17. Generally, the seaport is connected with the main grid and various renewable energy are integrated, i.e., seaport wind farms and PV farms. All the port-side equipment, including the quay cranes, gantry cranes, transferring trunks, are electrical-driven.

Fig. 1.17
figure 17

Typical topology of future port microgrid

The seaport provides four types of services to the berthed-in ships: (1) logistic service. The berth allocation and quay crane scheduling for loading/unloading cargo; (2) fuel transportation. Unloading or refilling fuel for the berthed-in ships; (3) cold-ironing. Providing electricity to the berthed-in ships; and (4) refrigeration reefer for the cold-chain supply. With the above multiple energy flows involved, the future seaport microgrid is a “maritime multi-energy system” [24], and the port central control should give both the energy and logistic control signals to each sub-system in the seaport [24].

1.3.3 Coordination Between Shipboard and Seaport Microgrids

In the future, the connection between the ship and the port is no longer limited in the logistic-side, and will be also expanded to the electrical-side. Figure 1.18 shows the coordination between the ships and ports. When the ship berthed in, the seaport will allocate a berth position and some corresponding port cranes for loading/unloading onboard cargos. In the electric-side, the berthed-in ship is directly connected to the seaport by the AC/DC converters, and all the load demands are met by the on-shore side. The seaport becomes a coordinated electric-logistic multi-microgrid system.

Fig. 1.18
figure 18

Coordination between the seaport microgrid and shipboard microgrid, reprinted from [24], with permission from IEEE

1.4 Summary

In this chapter, we have concluded that the maritime grids are those local energy networks installed in harbor ports, ships, ferries, or vessels, which consists of generation, storage and critical loads, and are able to operate either in grid-connected or in islanded modes and operate under both the constraints of power system and maritime transportation system. After the illustration of various promising technologies which are about to integrate, the implementation of next-generation maritime grids is suggested to be a promising approach to resolve the energy efficiency problem of the maritime transportation system, and may have the ability to re-shape the future relationship between the ocean and inland.

Using full-electrification as the backbone, the future maritime grids, i.e., all-electric ships, seaport microgrids, and various electrified ocean platforms, become the “maritime multi-energy system”, which requires an advanced energy management system to achieve the economic and environmental targets. From the aspect of electrical engineering, the future maritime grids are a special type of power systems. In land-based applications, the optimization-based power system operation has been extensively studied and should be expanded to the maritime grids for future usages. This is also the main goal of this book, i.e., the optimization-based energy management for the next-generation maritime grids.