1. Introduction
The availability of cheap fuel and loose environmental regulations have led to shipping being an important backbone of global logistics and caused large growth in the recreational shipping sector [
1]. About 3% of the global anthropogenic carbon dioxide emissions come from the shipping sector [
2]. To comply with future regulations and to reduce emissions from ships, the maritime sector is looking for alternatives to internal combustion engine (ICE) propulsion systems, which currently power the majority of vessels [
3]. While less-emitting propulsion systems can be retrofitted and partially replace ICEs, ships can also be redesigned implementing fully electrified powertrains. Research into zero-emission drive systems for marine applications is focused, in particular, on fuel cells and lithium-ion batteries (LIB) [
4]. Ferries and other smaller vessels can already be powered entirely by batteries, as the energy density of LIBs is sufficient for these routes [
5]. However, the pure battery-powered propulsion of large ships, such as container ships or cruise ships, has so far only been the subject of concept studies and is only economically and technically feasible if the energy densities of LIBs continue to increase beyond 1000 Wh/L in the future [
6]. Reducing emissions of cruise ships is important as they frequently stop at busy ports where the emission of pollutants is an additional concern in these often densely populated areas. However, large vessels cannot solely be powered by state-of-the-art LIBs because of the low energy density in comparison to the energy content of large fuel storage tanks filled with conventional fuels such as heavy fuel oil or hydrogen. Hence, hybrid drivetrains are of interest to fulfill the power and energy requirements.
A solid oxide fuel cell (SOFC) is a viable and potentially sustainable alternative to replace conventional means of propulsion because it operates at high temperatures, enabling the utilization of process and excess heat and achieves high electrical efficiencies [
7]. In [
8], a lab-scale SOFC was operated for more than 100,000 h which proves the longevity of the component under laboratory conditions. SOFCs are particularly relevant for the maritime sector due to their compact design, high power density, and compatibility with various fuels like hydrogen, ammonia [
9], or methane [
10]. SOFCs operate at elevated temperatures between 600 and 1000 °C [
11]. A portion of the process heat and the heat generated from energy conversion within the cells can be utilized in the ship’s heating system, further increasing system efficiency. These features make SOFCs suitable for providing electrical and thermal power for vessels, enabling greater energy efficiency and reduced greenhouse gas emissions compared to conventional ICE-powered propulsion systems. SOFCs have a comparably slow responsiveness towards electrical load changes, which was investigated by Obara et al. in [
12]. Therefore SOFCs are typically supported by secondary power sources such as LIBs or supercapacitors to compensate for the low dynamic capabilities of the SOFC depending on the application [
13]. Zhang et al. [
14] have studied the utilization of SOFCs coupled with supercapacitors in microgrids and considered the slow dynamic responsiveness in their energy management development without utilizing additional data sources to forecast load events or applying machine learning to reduce stress factors on the SOFC. However, their work has not followed a generic approach for the implementation of the EMS and therefore cannot be easily adapted to other applications. Wu et al. [
15] studied an SOFC–engine hybrid power system, which could be used in marine applications due to the fuel flexibility of SOFCs. Their approach does not eliminate local emissions and the dynamic load components that cannot be provided by the SOFC might not be compensated by the motor.
While SOFCs appear to be a promising alternative to engine-powered powertrains, there are still challenges and considerations that need to be addressed before widespread market introduction is possible. Until now, there have only been a few drive concepts for vessels in which SOFCs have been used or investigated. A 150 kW SOFC using natural gas as fuel has been implemented in a cruise ship [
16]. However, a fuel cell of this size is at best suitable as a retrofit solution to replace ICEs. Within the German research project SchIBZ, a 50 kW diesel-powered SOFC for a marine application was investigated [
17]. Diesel-powered SOFC systems have a higher efficiency than conventional diesel engines but still produce exhaust gases and nitrogen oxides in the diesel fuel processing or reforming process. As part of the European NAUTILUS project, a 60 kW SOFC for different fuel types is being developed for a cruise ship [
18]. Consequently, fuels that do not emit any greenhouse gases are being investigated as part of this research work, which in the long term will enable the targeted emission-free shipping. However, SOFC technology has not yet been scaled up. This is also reflected in the costs incurred for the installation of an SOFC. While conventional powertrains cost between 250 and 300 EUR /kW, SOFCs require an investment of 2000 EUR /kW [
19]. Van Biert et al. [
20] also describe the low level of technological maturity and the mechanical vulnerability as additional factors that hinder the widespread adoption of this technology in the maritime sector. It is therefore essential to examine the service life of SOFCs more closely and to reduce stress factors that lead to premature degradation.
Investigating the aging mechanisms of SOFCs is an important aspect of research to prepare the technology for commercial use in various applications. In long-term operation under static laboratory conditions, a lifetime of a short SOFC stack of up to 11.5 years of operation was achieved [
8]. As large ships have service lives of several decades [
21], a long service life is economically attractive for this type of application. If SOFCs are operated with dynamic power outputs, they undergo thermal cycling as the temperature in the stacks changes with power fluctuations which can increase the area surface resistance of the fuel cell [
22]. This thermal cycling can cause mechanical degradation within the cell, such as delamination and crack formation at the interfaces between the respective electrode and the electrolyte [
23]. Thus, it can be assumed that the steady-state operation of a SOFC is beneficial for its longevity. Hence, SOFCs are not suitable for load-dynamic applications and must be supplemented by another component such as a battery or supercapacitor as cycling stresses the materials of SOFC and leads to accelerated degradation.
The development of efficient EMSs for the power distribution between the propulsion components of hybrid propulsion systems is an essential element in the electrification of marine propulsion systems. For the EMS to achieve optimal overall efficiency of the powertrain, it is necessary to operate the individual power system components in efficient operating ranges. This can be achieved by operating the fuel cell in its most efficient operating range. Ma et al. [
24] have investigated a rule-based energy management method for a fuel cell–battery hybrid vessel, aiming to reduce stress factors on the battery to extend the longevity of the component. The size of each drive component should be in proportion to the savings that can be achieved by extending its service life. Dinh et al. [
25] studied an EMS based on equivalent fuel consumption minimization for a hybrid vessel drivetrain powered by an ICE and a battery, which aims to maximize the efficiency of the engine generators. However, only load sections in which dynamic positioning is carried out were examined. Bui et al. [
26] have developed a real-time capable EMS for a diesel generator and battery-powered hybrid propulsion system, enabling efficient dynamic positioning of ships and validating the control logic in a hardware-in-the-loop environment. Bassam et al. [
27] have studied various energy management methods for a hybrid fuel cell–hybrid ferry application focusing on the minimization of fuel consumption using characteristics of a real ship. However, no stress factors were investigated that could lead to accelerated degradation of the components, which could increase operating costs in addition to fuel costs. Kistner et al. [
28] have also considered the slow dynamic responsiveness of the SOFC in their optimization approach for the dimensioning of hybrid ship concepts. Characteristics and the dynamic behavior of the SOFC were considered, but no further data from ship operation were considered. In recent years, machine learning-based algorithms have become increasingly popular in energy management research [
29]. The rapid increase in computing power enables the development of complex methods for predicting load events. This enables the control of an energy management system (EMS) for a marine application using model predictive control [
30]. However, the resource-intensive training phases of machine learning algorithms or the high requirements on the computing power of the controller pose major obstacles to widespread use in energy management, burdening the achievement of real-time responsiveness necessary for efficient energy optimization.
Since ships are often cruising in densely trafficked waters, the surrounding area must always be monitored to avoid collisions. In addition to radar systems that enable a direct analysis of the surroundings, the automatic identification system (AIS) is an important instrument in modern shipping. Through AIS, ships continuously transmit information about their position, speed, direction, and destination via radio [
31]. These signals can be received by base stations and other vessels. AIS has been continuously enhanced and is also subject to research in the field of data mining for maritime research [
32]. Kim et al. [
33] have utilized AIS data to estimate the operational efficiency of ships to calculate fuel consumption without data from the drive train. However, no EMS has been developed that uses the AIS data to operate the vessels more efficiently.
To the best knowledge of the authors, no efforts have been made to develop an EMS for a large vessel that takes tracking and weather information into account to predict upcoming load events. Through the integration of the following main contributions, this work seeks to close the previously described research gap:
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Development of an energy management method for ship propulsion systems powered by SOFCs and batteries utilizing authentic cruise ship power profiles, tracking data, and weather information.
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The implementation and enhancement of data-based load prediction algorithms within the energy management to increase the efficiency of the propulsion system.
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Improvement of the load distribution to better conform with the dynamic behavior of the SOFC to extend its lifetime and increase the resource efficiency of the propulsion system.
Furthermore, this work extends the existing research with the following secondary contributions:
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Introduction of a method using physical relationships between the characteristics of the ship and its flow properties.
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Verification of the functionality of the proposed data-based energy management method based on real data from cruises.
5. Discussion
This work aimed to investigate the extent to which an EMS for a hybrid ship propulsion system can benefit from the use of additional data sources. We quantitatively investigated external influences on a ship to be able to predict the future load demand. Predicting the future load demand of a ship seems to be a reasonable method to maintain a steady-state operation for power sources with low dynamic capabilities. The rigid route planning in shipping, which makes reliable forecasting over longer periods possible in the first place, is particularly helpful here. Unlike in automobile traffic, ships rarely have to react to spontaneous events on long sections of the route, so that a planned route can usually be executed. Most of the environmental influences acting on the ship can be suitably modeled and predicted to a certain extent considering the hydrodynamic properties of the investigated vessel so that an EMS strategy that adjusts its behavior based on predictions of future load demand seems to be a reasonable method which can be realized without high computational burden.
After modeling the physical relationships and selecting appropriate data sets to determine the model parameters, an estimator for the load demand of a cruise ship was developed, and the accuracy of the prediction was compared against real measurement data. This yielded an accuracy of 7.68%, which would still represent powers in the multi-MW-range for such a large application. Since the upper limits of the battery power capability could be exceeded in this case, this could present a technical issue for the hybrid drive system. The error could be decreased to 2.68% by analyzing and correcting the systemic error. This is an acceptable prediction given that the average deviation in this case is approximately 1.25 MW for the maximum power output of the investigated ship application. This load can be supplied by the battery assumed in the investigated configuration. From this, an EMS has been developed that utilizes load prediction to improve the dynamic operation of the SOFC. As the SOFC is the main propulsion component, good dynamic behavior, which reduces the stress factors on the component, is essential for the efficient operation of the ship’s propulsion system. The functionality of the EMS was verified with the help of simulations over different authentic cruise profiles.
The data-based energy management method that has been developed predicts load curves with a high degree of accuracy based on tracking and weather data and can therefore process very large amounts of data to implement a resource-efficient power distribution between the drive components. Predictions are realized without the implementation of neuronal networks or regression models which can be resource-intensive to set up, train, and tune. This algorithm is therefore able to run on conventional ship control systems and is therefore a retrofit option. In addition, the use of this EMS is relevant for an economic operation, as the procurement and commissioning of SOFCs has comparatively high costs [
42]. By reducing stress factors, a longer service life of the SOFC can be achieved, which could significantly reduce the operating costs of this ship application.
However, despite the interesting results, the explanations of this work have a few shortcomings, which could be overcome in future work. For example, due to a lack of data, the simulation could only be performed for a few periods. For a more comprehensive validation, it would be interesting to study the functionality over a longer period. This could reveal further potential for improvement. Also, a higher sampling rate of the load profiles underlying the calculations and simulations would have been helpful to investigate the dynamic performance of the hybrid propulsion system in a higher level of detail.
Outlook and Future Work
The estimation function presented for the load demand does not yet represent all environmental influences. Particularly wind and waves, which have a significant influence on the power demand of a ship, could not be considered. By additional physical modeling of these effects and the use of further data, the estimation of the future load consumption of a ship could be further improved.
Besides a mapping of further environmental influences, it could also be investigated whether the estimation procedures in general can be improved. In this work, a balancing calculation was performed using polynomial function approaches and the least squares method. This could involve the use of other methods to establish a functional relationship between the measured values. A mapping with Gaussian processes or the training of suitable neural networks could improve the results in the case of large data sets.
Furthermore, the functionality of the algorithms could be validated in a hardware-in-the-loop test bench environment. Such a measurement series would allow for a precise examination of the propulsion components concerning their dynamic behavior or technical suitability for the designated application. In addition, the real-time capability of the algorithms on controllers could be verified within the test bench.