Keywords

1 Introduction

Energy diversification is closely tied to the independence of nations around the world. Countries that source energy from a multitude of renewable energy sources will prove resilient for generations. Global economic volatility shows the unpredictability of oil sourcing and distribution. During global crises, supply chains are largely disrupted, especially when sanctions are imposed on specific countries. To deal with economic turmoil and trade disruptions, energy diversification is the solution. Traditionally speaking, the marine industry is dominated by internal combustion engines and steam/gas turbine propulsion systems. Although slightly different in mechanical power transmission, both methods derive power from a single source: petroleum-based products. Combustion propulsion systems have been fine-tuned and optimized over the last century but are still limited by the theoretical efficiencies of the Otto, Rankine, and Carnot cycles. Over the last 40 years, new ship construction has been centered largely on internal combustion engines that have the ability to burn various petroleum-based products. Countries walk themselves into a geopolitical and economic trap when sourcing energy outside of their own territory.

In 2020, the International Maritime Organization (IMO) updated the Regulations for the Prevention of Air Pollution from Ships via MARPOL 73/78 Annex VI. IMO changed the allowable limits of sulfur content of fuel to not exceed 0.5% globally and 0.1% in emission control areas. As a result, vessels are forced to use more expensive, cleaner fuel or install costly exhaust gas scrubber systems. This updated regulation incentivizes companies to look for alternative means of energy. Hybrid or fully electric drives are the solution for the marine shipping industry. Although fully electric shipping for large-scale containerships and freighters is currently not feasible, electrification solutions for smaller operations are realistic and lucrative. Based on recent progress, it is evident that the maritime industry is moving towards a cleaner and more sustainable future.

The benefits of electric propulsion systems are numerous and have been implemented in some large-scale applications. For example, the United States Naval Ship (USNS) T-AKE Class ships are powered by a diesel electric propulsion system that boasts high maneuverability and quick start/stop capabilities, which greatly out-performs direct-link slow-speed propulsion engines. These engines need to be fully stopped, reversed, and started again to change the rotation of the propulsion shaft. Although the USNS T-AKE vessels use electric propulsion motors to actuate the shaft, the primary source of energy still resides with diesel fuel. Current technology and infrastructure deem fully electric large-scale vessels infeasible. Because of this, the commercial maritime industry is very reluctant to move away from fossil fuels. For example, it is very difficult to match the energy density of diesel fuel (42,800 J/g) when using batteries as the sole fuel source.

Throughout the world, governments will need to encourage electric propulsion in the maritime industry while still maintaining necessary standards for safety. Direct current (DC) charging stations at mooring locations give hybrid and electric commercial river tenders, recreational boaters, and transportation vessels the opportunity to charge after each voyage. Growing electrical applications across transportation and power generation industries have spurred an energy revolution backed by versatility and efficiency. However, large scale marine installation of electric or plug-in-hybrid technology remains to be implemented. Such applications are suited for vessels with shorter operating periods, an ideal situation for vessels operating on inland waters. Several small-scale applications of hybridized technology have resulted in reduced maintenance costs, increased maneuverability, and reduced consumption of petroleum-based fuel. Large-scale implementation of marine DC charging stations will yield a cleaner, lower maintenance, propulsion solution to steer the industry away from petroleum-based fuels. However, specific practices for the prevention and suppression of large-scale lithium-ion battery fires have not been published or standardized. It is essential that governments and regulatory agencies are at the forefront of battery developmental research to ensure that maritime safety remains a priority.

2 Propulsion Systems for Vessels

During the design process for new ship construction, engineers and naval architects conduct an in-depth analysis of the operating characteristics of newly constructed vessels. Sizing a propulsion system for a vessel largely relies on the needs of the customer, which is typically outlined in the customer’s request for proposal (RFP) package. In the commercial maritime industry, sizing a propulsion plant for a standard vessel, like a containership or a bulk freighter, is relatively simple. The propulsion designer considers the expected tonnage and power requirements at maximum load parameters, while also meeting the speed requirement as specified by the RFP. In ideal situations, the vessel will be operating fully loaded with cargo for a majority of the time, which would maximize profits for the vessel owner. Internal combustion diesel engines are generally sized to operate at a high proportion of the maximum continuous rating (MCR), usually around 80%–90% MCR (Harrington 1992). That range of operation typically correlates to the best range for specific fuel consumption, meaning the engine operates most efficiently when fully loaded. Furthermore, smaller main engine load variations, while also maintaining the 80%–90% MCR, will lead to decreased fuel consumption. For operational characteristics of large vessels like containerships, diesel engines are functionally appropriate but retain high operational and maintenance costs.

3 A Closer Look, Inland Waterway Tugboat

3.1 Unique Operating Parameters of a Tugboat

Traditionally speaking, tugboat designers would size the rated power output of the engine according to the expected load at max towing capacity, which is usually around 80% of the maximum continuous rating (MCR). So far, this seems similar to the design profile of a containership, which is designed for optimal performance when operating at 80%–90% MCR.

To model the benefits of plug-in hybrid vessels, an in-depth review of an inland waterway tugboat will be examined. The vessel is a commercial tug with published operational data (Boyd and Macpherson 2014). See Table 1 and Table 2 below for the specific operating modes, conditions, and fuel specifications that will be referenced for the calculations and results.

Table 1. Engine and fuel data (Boyd and Macpherson 2014)
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Table 2. Tugboat operating conditions and data (Boyd and Macpherson 2014)
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Tugboats operate at 80% MCR only about 2% of the time underway. This is largely due to the small size of the vessel which requires high power output only when assisting larger vessels in maneuvering and mooring operations. This results in most tugboats operating the propulsion engines of the tug at an output much less than the MCR for 80%–90% of the time. Applying the above-mentioned procedure of sizing a diesel engine to a tugboat in order to maintain normal loading within 80%–90% of MCR creates a highly inefficient operating profile. The result is that most of the time the vessel is operating in a region of high inefficiency. For the purposes of this case study, a standard 8-h day was applied to the operating parameter percentages. Using the data from Table 2, the time bar chart was superimposed on the engine power output bar chart, as seen in Fig. 1.

Fig. 1.
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Operating mode showing time allocations with coinciding power requirements

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As seen above, for operational modes 4, 5, 8, and 9, a large amount of power is required for a short period of time. Conversely, for operational modes 1, 2, 3, 7, and 10, a much smaller amount of power is required for a significantly larger period of time. Considering both situations, a power threshold can be determined. The power threshold refers to the cut-off point where the vessel is operating at an abnormally low power output for a significant amount of time. The power threshold derived from this analysis is 360 kW. This means that modes that stay below 360 kW operate significantly outside of the range of peak efficiency for the rated 1800 kW engine, resulting in a high potential for fuel savings if alternative energy sources and power applications are used for these modes. Operating Mode 6 is an outlier because it requires moderate power for a moderate amount of time, approximately 518.4 kWh, with a peak power of 720 kW. In the specific case of this operating profile, the tug has a rated output power of 360 kW or less for 87% of the time underway.

3.2 Hybrid Propulsion

The sole focus of commercial enterprise in the maritime sector is centered on cost-saving measures with the goal of increasing profits. This includes maximizing fuel efficiency to decrease fuel costs, which account for a high percentage of the overall operating costs for commercial vessels. For vessels that have large fluctuations between high propulsion loads and low hotel loads, hybrid propulsion presents potential cost savings. Electric motors offer clear advantages in terms of versatility and maneuverability, namely, being able to operate at a larger range of loads while maintaining relatively high efficiency. While electric motors also tend to operate most efficiently at high loads, the efficiency taper at lower loads is much more gradual when compared to diesel engines (US Department of Energy 2014). This means the electric motors are able to retain a higher efficiency over the same operating conditions compared to diesel engines. Hybrid applications that boast large fuel savings are being implemented in various ways in industries that include ferry and tug vessels. One example of such benefits can be seen at Derecktor Shipyards of Mamaroneck, NY. Derecktor Shipyards has constructed a hybrid propulsion vessel with a 55% reduction in fuel usage by using two BAE Systems AC traction motors. Taking a closer look at Fig. 1 above reveals that electric propulsion from stored power in battery packs can be used to supplement the diesel engine for power conditions at 360 kW or less. Applying this analysis, Fig. 2 displays which operating modes should be powered by electricity.

Fig. 2.
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Operating modes differentiated by diesel and electric power in proposed hybrid propulsion system

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The yellow-filled bars represent operating conditions to be powered by a fully electric system. The blue bars represent the operating conditions where the diesel engine will be turned on for high power output push-pull operations. Following this proposed power profile, the tugboat will use approximately 0.7 tons less fuel per 8-h working day when compared with a diesel-only vessel. Decreasing fuel usage by 0.7 tons per day equates to a 62% decrease in fuel consumption for this vessel. It should be noted that these calculations do not include the added weight of the electric propulsion system and storage batteries, which would require a capacity of just over 1320 kWh. Calculating new ship-specific power requirements and hull changes in order to implement hybrid technology are beyond the scope of this analysis. However, the estimated fuel savings shows the potential for hybridized electric propulsion in the tugboat industry and similar industries. Additionally, the 0.7 ton decrease in fuel consumption, the tugboat would only require 0.43 tons of fuel for an operational 8-h day, which would decrease the fuel storage tank capacity.

3.3 Battery Sizing

When designing a battery-electric or hybrid vessel, the primary limiting factor is the space required for battery storage. Even with modern advancements in battery technology, the volume required to store the needed watt-hours (W-HR) of energy needed for propulsion eventually limits the range and cargo capacity of the vessel. Balancing all three requires careful design of the battery storage areas.

Calculating the volume needed for battery storage first requires determining two design parameters: (1) the intended voltage (V) for the battery system, and (2) the total kilo-watt hours (kWh) needed for propulsion. With these two parameters determined, Eq. (1) can be used to calculate the Whr required for the system.

$$Watt-hours=Volt*Amp-hours=\left(kWh\right)*\left(\frac{1000\,W}{kW}\right)*\left(\frac{VA}{W}\right)$$
(1)

With the required kWh for the system calculated, the volumetric energy density (kWh/L) can be used to determine the Liters of volumetric space needed for batteries. The gravimetric energy density, in kilowatt-hours per kilogram (kWh/kg) can also be used to determine the mass of batteries needed for the system.

A predictive model was constructed to approximate the weight of battery packs per kWh using the Tesla 5.3 kWh Battery Module as a standard due to the high energy density of the battery. Figure 3 below shows the trend of the weight of the battery as the kWh increases for larger applications, such as vessels. Included in Fig. 3 is the calculated kWh for the tugboat with corresponding weight of 6,216 kg.

Fig. 3.
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Battery Pack Weight Approximation as kWh increases, based on Tesla 5.3 kWh Module

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Having batteries on board a vessel that replace liquid fuel serves several advantages. As batteries are used for power, their mass effectively does not change, only about 0.5 mg of mass for a 1.7 MW battery system. When petroleum is used, the vessel becomes increasingly lighter, posing a ballast issue (Schurke 2021). As a result, batteries become solid ballast for the vessel, without the dangerous addition of the free surface effect from liquid ballast or fuel tanks.

Once the volumetric and mass requirements for the batteries are determined, several other factors must be accounted for. For example, batteries generate heat as they charge and release energy, so a cooling system must be designed to account for this heat load. Additionally, the heat load can require spacing batteries to allow space for the cooling system, which results in increased volume and mass requirements for the overall system. Fire suppression systems also account for additional space in the battery storage area.

4 Inland Waterway Infrastructure, Charging Stations

4.1 Applicability

The benefits of hybridized technology for an inland waterway tugboat operating profile are numerous, providing benefits to the vessel owner as well as the surrounding environment. These solutions are undercut by waterway electric grid infrastructure, which would take significant time and money to upgrade. Improving grid capacity for highly trafficked ports, to include recreational marinas, will give consumers the ability to explore plug-in hybrid or even fully electric propulsion options for vessels. These options are appealing to sailors and commercial operators who operate for limited hours per day, allowing for the time to recharge the battery pack(s) at night or during periods of non-operation. Current electric vehicle charging stations can be referenced when looking to install charging stations at marinas and commercial ports. Table 3 below depicts current electric vehicle charging stations with operating characteristics.

Table 3. Typical battery charging stations (Palconit et al. 2018)
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For recreational sailors, installing lower-rated charging stations, like Level 1 or Level 2 chargers, would directly reduce the amount of fuel required at dockside refueling stations. Less frequent fueling operations directly correlate to a decrease in pollution incidents. For commercial operations that require significantly more power and larger capacity battery packs, like tugboats, Level 3 charging stations should be installed. If implemented, commercial enterprise and civilian sailors would have the opportunity to choose hybrid or fully electric boating options, further reducing the reliance on exclusively petroleum-based products.

4.2 Standardization

Bringing electric propulsion solutions to the maritime industry provides regulatory authorities and design engineers with an opportunity to learn from the previous implementation in the automotive transportation industry. Standardizing a grid approach is essential to streamline future upgrades to the system and accelerate widespread installation. Currently, there are three types of electric vehicle DC fast charging connections: CHAdeMO (developed by the Tokyo Electric Power Company for DC charging), SAE Combo (CCS), and Tesla Supercharger Plug. This is the result of regulatory authority lagging behind technological advancement. Simply put, private industry developed their own system and charger plugs, offering the public several options. Standardization was later implemented for Level 1 and Level 2 charging stations but remains to be updated as Level 3 charging stations become more abundant. The problem that this causes is inconvenience and inefficient charging station installment. This means, depending on the vehicle a customer purchases, only certain charging stations will be compatible. Learning from this, the marine industry has the opportunity to standardize two plugs for recreational boaters: regular charging and DC fast charging, resulting in base-line infrastructure widely available, regardless of manufacturer.

For commercial use, a higher rated standard should be developed based on the line voltage from the grid, to scale charging times for significantly larger battery packs. In the Port of Göteborg, medium voltage transformers provide 10 kV/6.6 kV 1250 kVA on the quay (Ericsson and Fazlagic 2008). Commercial ports have the available grid support to cater to larger battery packs used for propulsion, and a standard ship connection used for charging batteries, instead of supplying hotel loads, needs to be developed.

5 Solid-State Batteries

An in-depth discussion of the electrochemical properties of lithium-ion and solid-state batteries is beyond the scope of this paper. However, a brief overview and various benefits and drawbacks will be covered. Recent developments in solid-state lithium-ion battery production yield optimistic futures for recreational and commercial transportation industries. Limitations with liquid electrolytes in current lithium-ion batteries are extensive and have significant drawbacks in both performance and safety when compared to solid-state batteries. Therefore, further research in solid-state batteries may allow the maritime industry to decrease the added mass on a vessel when compared to traditional lithium-ion batteries.

The physical orientation of solid-state batteries allows for the cells to be arranged in series stacking and bipolar structures, thus allowing for less volume occupation on the vessel. As the total displaced volume of the battery decreases, ceteris paribus, the energy density increases. This is achieved by swapping the electrolyte material from an organic liquid to a solid electrolyte. This transition boasts higher electron transfer rates and further increases safety due to the inherently flammable nature of liquid organic electrolytes. Further, solid-state batteries are able to fully cycle thousands of times without losing capacity (Pistoia 2014).

Designing modular battery-pack systems in easily crane-accessible areas on the vessel would allow the battery storage pack to be upgraded over time, simply by swapping out the battery pack onto established battery mounts. Easily upgraded propulsion systems are the future of the marine industry.

6 Regulatory Oversight

6.1 Overview

Rapid advancements in technology require regulatory agencies to maintain safety compliance as a continuation of current standards, applying to shore-side infrastructure and waterborne transport. Electric propulsion systems and shore-side charging systems need to be designed and inspected to ensure tolerances for various load conditions that may arise from atypical operations. For example, systems need to be tested to withstand overloading conditions, under loading conditions, short circuit situations, and mechanical deformation of the battery packs, to name a few. The United States Coast Guard is working at the deck-plate level with manufacturers when constructing new systems, as well as reviewing plans for future builds to ensure safety compliance. Both levels of oversight are necessary to reduce the risk of failure at all system components.

6.2 Fire Prevention

Prevention of fire becomes a life or death scenario when applied to waterborne transport. There are several difficulties that present themselves when extinguishing a lithium-ion battery that will be covered in Sect. 6.3, but preventing fires in the first place is a priority. Lithium-ion batteries are at risk of thermal runaways. Thermal runaways occur when an exothermic reaction is caused by exposure to high temperatures (130 \(^\circ{\rm C} \)–150 \(^\circ{\rm C} \)) or when the battery is short-circuited. When this occurs, temperatures rise and gasses build up, which create an environment prone to fire or explosion. To prevent the occurrence of a fire or explosion, several safety measures must be installed and regulated. These measures include preventing thermal runaway using separators, preventing fire using flame retardant additives, and preventing the buildup of gas and pressure by cell venting.

Separators are components within the structure of the battery made of a semi-porous polymer in most cases. Placed in-between the positive and negative electrodes inside the battery, the separator prevents direct contact between the electrodes, which prevents an electrical short from occurring. Furthermore, as the temperature in the battery increases during a thermal runaway, the polymer material approaches its melting point, thus closing the pores and preventing the electrochemical reactions caused by the pathway between the electrodes. When this occurs, it is called a separator shutdown and is an inherently safe component of the battery structure.

Using flame retardants as an additive to the electrolyte or directly in the separator makes the batteries much safer. As the temperature in the battery rises during a thermal runaway, the electrolyte will not flash or combust when the flame retardant additive is used.

Safety vents are installed to release gasses from the battery in the event of a buildup. If the separator shutdown fails, thermal runaway will continue causing the internal venting mechanism to activate. This mechanism vents the battery abruptly, purging the gas buildup and drastically reducing the pressure to prevent a rupture or explosion. This will include toxic gas sensors and ventilation systems to remove toxic gasses created by batteries while charging/discharging. Items such as ductwork for ventilation and discharge piping and nozzles for the fire extinguishing system all must be accounted for when sizing a battery room for a vessel (Kong et al. 2018).

6.3 Fire Suppression

Subsequent systems for new technology must also be installed, and further research needs to be conducted to learn more about the thermal properties of lithium-ion batteries. For the safety of vessel operators, a fire detection and extinguishing system is necessary. When lithium batteries catch fire, they cause fires that range in classification. Additionally, thermal runaways make the extinguishing process slightly more complex. Instead of simply putting out the flame, extensive cooling is necessary to prevent a re-flash from occurring. Various agents can be used to extinguish lithium-ion batteries, including dry chemical, carbon dioxide, water, and halons to name a few. However, regulatory standards published by the Institute of Electrical and Electronics Engineers (IEEE) only focus on abuse and threshold testing of the batteries. Published data indicates that large-scale battery storage spaces require extended water hose stream application in order to fully extinguish the fire (Long and Misera 2019). There is a need to develop a system that adequately extinguishes lithium-ion batteries, cools the batteries sufficiently to prevent them from reigniting, and does not put the vessel in danger in terms of stability. There is no specific standard for extinguishing lithium-ion batteries, which leaves a major gap in the regulatory realm of safety compliance.

7 Conclusions

Electricity provides an opportunity to create a maritime industry more resilient to future energy changes. Rather than relying solely on petroleum-based energy, tapping into a diverse energy grid to source power from the growing number of renewable resources provides a flexible and secure maritime economy. There is an opportunity to start implementing electric or hybrid propulsion with smaller vessels that operate for relatively short periods of time while standardizing practices like charging station plugs. In addition to the energy diversification benefits, significant fuel savings are observed when compared to traditional internal combustion engines. It is common for vessels with large engines to operate under-loaded. These situations provide an opportunity to implement electric propulsion. A case study was conducted for an inland waterway tugboat with 10 distinct operating modes. The study yielded a 62% reduction in fuel consumption with hybridized technology. Applying this technology to vessels with similar operating modes has the potential to provide energy savings industry-wide. Progressing the grid in a more sustainable direction will take significant time and resources. As new battery technologies and applications continue to enter the maritime industry, governments need to be at the forefront of research and development in order to enforce new safety regulations that include fire prevention and suppression of lithium-ion battery fires. In order to get there, maritime authorities need to incentivize the installation of charging stations while maintaining regulatory oversight on emerging technologies to ensure safety compliance.