Decarbonizing City Water Traffic: Case of Comparing Electric and Diesel-Powered Ferries

Abstract

The maritime sector aims to achieve carbon neutrality by 2050. Consequently, shipping companies are investigating efficient and optimal ways to minimize greenhouse gas emissions. One of these measures includes vessels that operate on alternative non-carbon fuels. In this study, we compared a diesel-fuelled catamaran’s greenhouse gas (GHG) emissions and its fully electric sister vessel, which operates on the same line. This study showed that the GHG emissions of the electric vessel were only 25% of those of its diesel-powered sister vessel. However, this figure highly depends on the source of electricity in the operating country. In this case, the energy cost of the fully electric vessel was 31% cheaper than the cost of diesel energy and the payback time without possible subsidy for replacing a diesel ferry with an electric one would be 17 years and 6 months. We also showed that the additional energy from solar panels sufficiently covers several application options for consumers even in winter, when there is low solar energy production. This study brings more insight into the academic literature on decreasing maritime CO₂ emissions from city water traffic. Regarding its managerial implications, our study findings can be used when shipping companies evaluate options for reducing their emissions. The results of this study show that using fully electric vessels has major benefits not only concerning carbon emissions but also financially.

1. Introduction

The International Maritime Organization (IMO) has established a goal to reach carbon neutrality by 2050 [1]. In addition, the European Union (EU) in its Fit for 55 Package has set a target for 2050 to reach carbon neutrality across Europe [2]. Both regulation packages are directed towards vessels with a gross tonnage of more than 5000 GT, even though the emissions trading system also includes vessels above 400 GT.

However, the rules enacted by the IMO and EU do not apply to several different kinds of ships, including yachts, fishing vessels, tugs, offshore ships, and common city waterway ferries. Armstrong [3] claims that small ships are responsible for 15–20% of the total greenhouse gas emissions. In other words, there is a significant amount of unregulated GHG emissions.

On the other hand, there is a constant trend towards decarbonizing smaller ferries, in particular, in cities that have strong political green strategies. Both the market and trends are in the process of shifting to low-carbon shipping, and numerous viable fuel alternatives [4,5,6,7,8,9,10,11] are either in the process of being developed or are already in use. Hence, the coastal ferry industry has both the desire and the initiative necessary to make progress towards increasing the number of electric ferries [12].

Nevertheless, there is no fit-for-all solution for minimizing GHG emissions due to ship design, operational envelopes, weather conditions, routes, and port infrastructure. This study compares actual energy consumption and GHG emissions between fully electric and marine-diesel-fuelled catamaran ferries. The vessels are almost identical, and they operate on the same line. This study brings more insight into the academic literature on decreasing maritime GHG emissions where ferries are compared in their regular operation. Both ferries regularly operate in the same environment (same ferry line/route for both ferries), providing this study with a unique perspective as it illustrates their actual CO₂ emissions.

In this study, we reveal the actual GHG emissions of the ferries and calculate the payback time of the vessels. We also conducted an operational energy cost analysis. In addition, we calculated the amount of energy the solar panels on the vessels produce. Regarding its managerial implications, shipping companies can use our study findings to evaluate options for reducing emissions. More background information and studies about the actual difference in GHG emissions will help shipowners decide on the power system and type of energy used for urban waterway or road ferry traffic in both the new and retro building sectors. The results of this study show that using fully electric vessels has major benefits not only concerning carbon emissions but also financially.

2. Literature Review

Ships that operate worldwide commonly have conventional marine propulsion systems supplied with marine fuels. Most of them use heavy fuel oil (HFO), marine diesel oil (MDO), marine gas oil (MGO), or liquid natural gas (LNG). These heavy fuel oils have been used due to their high energy density and low price. However, the use of fossil fuels is being gradually banned by IMO and the EU. However, carbon-free shipping has a long road ahead.

Presently, there is no ‘silver bullet’ to achieve decarbonizing targets in shipping as multiple means are needed to reduce vessel emissions. Means of reducing exhaust emissions from ships can be roughly grouped into the following: (i) fuel solutions; (ii) ship design and technological development, (iii) choices of ship operations, such as speed optimization [13].

Numerous literature reviews describe various ways to reduce the carbon emissions of shipping, see, e.g., [14,15,16,17]. There are multiple methods for reducing carbon emissions, such as slow steaming, main engine de-rating, waste heat recovery, and alterations in operational patterns. In the shipping industry, these techniques are not new; in fact, they were initially developed to reap benefits such as minimizing operational expenses and reducing fuel consumption. Various guidelines and rules are focused on improving energy efficiency; however, they might not have effective long-term results [13].

As one alternative, several shippers are finding biodiesel to be the best-suited measure [18,19] to fulfil the rules and requirements set by the IMO. In urban waterway ferries, replacements for diesel engines with fully electric ones can be identified in various countries and locations [20,21,22]. There is also evidence of using hydrogen [23] and methanol [24,25] as fuel alternatives. Hydrogen is an energy-dense fuel in weight but low on energy-density in volume. Hydrogen has great potential in different regions [26,27], and hydrogen use has already been considered and implemented in some application areas, including transportation. Nevertheless, the greatest challenges to using hydrogen are connected to its transportation, storage, and production price. There are also safety concerns derived from its flammability. Similarly, methanol has already been used as a shipping fuel alternative, and the trend is growing rapidly [24,28]. Methanol’s main drawbacks are connected to its low density and heating values; it also must be remembered that methanol is toxic and flammable [29]. In addition to the above-mentioned issues, it is widely dependent on price dynamics in different regions due to its production method. There have been different strategies [30] developed to assess the best-suited fuel alternatives; however, no uniform method can be used for all types of vessels and all navigational regions.

According to Hessevik [31], shipping companies join green shipping networks to learn about new technology. Knowledge of new technologies helps technical managers in decision-making; however, belonging to clusters is not the actual driver of fleet renewal. Retrofitting and new building are still a financial decision and having a green fleet of vessels <5000 GT has been an additional bonus in the sales or public relations aspect. After the European Parliament adopted the EU Climate Law in 2021 [32], member countries have more clear targets to achieve, and public authorities are encouraged to consider GHG emissions in their daily operations. Several cities and transportation authorities have already taken measures [33] to successfully implement the green shift in city transportation [34,35].

It should be stressed that to cover all the needs of future electricity resources (not only means of transportation), there must be strong collaboration between private enterprises, public initiatives, and governmental support [36]. Several low-emission fossil fuel alternatives depend on local resources. In some specific routes and countries, hydrogen is the most feasible alternative [37]. However, in many cases, hydrogen production still has several deficiencies [38]. Using fully electric power systems is suitable for inland waters, short distances, and a mild climate, offering great benefits under such conditions [39].

3. Methodology

In this study, we measured the emissions of two passenger catamaran ferries that navigate the same route in city traffic.

Both vessels operate on the river daily. The significance of this line is that the ferries sail on the same route (therefore, the change in weather conditions does not affect average consumption). The data collection time was 1 month in the winter period. Consumption data were collected directly from these vessels’ integrated alarm monitoring and control systems.

During the data collection period, the regular daily working hours of the diesel ferry were approximately 11 h 20 min, and the working hours of the fully electric ferry were approximately 17 h 25 min. The difference in working hours was calculated from the ferry schedules while considering rush hours and the actual passenger load. The electric ferry operates for more hours because it is cheaper. According to the financial reports of the fleet, navigating a fully electrical catamaran is 21% cheaper in energy unit cost than diesel. The consumption cost comparison shows that Belgium has long been one of the most expensive countries in Europe for electricity [40]. However, even in Belgium, electricity in this inland waterway environment is financially more viable.

3.1. The Fleet

The vessels under study are owned by the Flemish government. The company has had a green fleet focus in action since 2009, the period before specific guidelines came into force. The diesel ferry analysed in the study was delivered in 2021 to substitute the old and less efficient river ferry. According to calculations conducted by the ship owner, the company has already saved 2.7 times in consumption costs with the new diesel catamaran compared to the previous old ferry and even more than the fully electric ferry.

In the future, this company is considering retrofitting diesel-fuelled propulsion systems using methanol. This method is supported by a recent study [41], which acknowledges methanol’s suitability for small working ship retrofits of marine diesel oil. Depending on the vessel’s specific purpose and the environment, there are other alternatives, e.g., ferries can be retrofitted to be fully electric.

3.2. Ship Particulars

The catamaran ferries under study have the same main dimensions (see

Table 1. Ship particulars.

	Vessel 1	Vessel 2
Type of vessel	Commuter ferry	Commuter ferry
Hull material	Aluminium	Aluminium
Superstructure material	Aluminium	Aluminium
Type of propulsion fuel	Diesel	Electricity
Maximum speed	18 km/h	18 km/h
Length overall	30.0 m	30.0 m
Breath moulded	9.5 m	9.5 m
Scantling draught	1.85 m	1.6 m
Gross tonnage	240 t	240 t
Crew	3	3
Passengers	200	150

).

Apart from the main dimensions, Vessel 1 also uses electricity to supply the vessel while not in operation at shore side. Vessel 2 uses minor amounts of marine diesel oil to supply the emergency generator. These consumptions were also considered in this study.

Both vessels have solar panels for supplying consumers. The systems consist of two sets with six 330 W panels each; therefore, in total, both vessels have 12 solar panels to produce additional energy from the sunlight. The study measurements were conducted in the winter period. Although the solar energy impact was minor, it reduced electrical energy consumption. Therefore, it was not included in the emissions calculation.

The ferries’ maximum passenger number difference is not due to technical limitations. According to the fleet owner, the passenger amounts never exceed 150 people. Therefore, lowering the passenger capacity saves from carrying unnecessary rescue equipment onboard and adds relevant storing space.

3.3. Operating Environment

The vessels operate on the same route daily (Figure 1). The operating distance for a one-way trip is very short, approximately 350 m. The harbour infrastructure has enough electrical reserves for battery charging, a significant benefit compared to other European city ports.

It is also worth noting that the ferry is not the only alternative for crossing the river. There is also a tunnel for pedestrians and bicycles. Nevertheless, the ferry line is used monthly by 30,989 pedestrians and 19,311 bicycles on average.

3.4. Assessing GHG Emissions

We assessed energy consumption and GHG emissions according to EN 16258:2012 [42] requirements using the well-to-wheel (WTW) method. The well-to-wheel method is used to evaluate an energy source’s efficiency and emissions considering its entire life cycle [43]. EN 16258 is the European standard for calculating and reporting GHG emissions from transportation. The standard contains general principles, definitions, system boundary descriptions, calculation methods, and data recommendations. The WTH method was used instead of TTW (tank-to-wheel) because according to the standard TTW emissions of electrical vehicles are equal to 0, actual operating emissions would always be in favour of fully electric vehicles.

3.4.1. Well-to-Wheels GHG Emissions for Marine Diesel Oil (G_wD)

$${\mathrm{G}}_{\mathrm{w}\mathrm{D}}\left(\mathrm{VOS}\right)=\mathrm{F}\left(\mathrm{VOS}\right)\times {\mathrm{g}}_{\mathrm{w}\mathrm{D}}$$

(1)

where,

• F(VOS) is the total fuel consumption used for the VOS (vehicle operation system);
• g_wD is the well-to-wheels GHG emission factor for the fuel used in marine diesel oil;
• g_wD = 3.53 kgCO₂e/L.

3.4.2. Well-to-Wheels GHG Emissions for Electricity (G_wE)

The standard well-to-wheels energy factor is specified by the electricity supplier. GHG emissions energy factors and data from the European Commission Joint Research Centre (JRC) were used for assessment [44]. The GHG emissions were assessed accordingly:

$${\mathrm{G}}_{\mathrm{w}\mathrm{E}}\left(\mathrm{VOS}\right)=\mathrm{F}\left(\mathrm{VOS}\right)\times {\mathrm{g}}_{\mathrm{w}\mathrm{E}}$$

(2)

where,

• F(VOS) is the total energy consumption used for the VOS;
• g_wE is the well-to-wheels GHG emission factor for the average electricity emission factor used in EU countries;
• g_wE = 0.254 tCO₂e/MWh.

Well-to-wheel GHG emissions for electricity vary due to electricity production methods. It is worth noting that GHG emissions from electricity might even be higher than fossil fuels due to different policies and governmental decisions [45].

Another significant factor is that due to the European Green Deal policy, the carbon intensity of electricity is changing, and the general trend is to be lowered even more in the coming years [46], making fully electric vehicles even more favourable.

4. Results

During the one-month data collection period, consumption varied depending on operating schedules and weather conditions. Figure 2 shows the average fuel rate of Vessel 1. The energy consumption of Vessel 2 is shown in Figure 3. There is a much greater difference in the fuel consumption of Vessel 1 than in the energy consumption of Vessel 2.

4.1. CO₂ Emissions

The results of calculations based on monthly resources are shown in

Table 2. Total emissions from one month’s operations and emissions per hour.

	Vessel 1	Vessel 2
GHG total emissions	15,923.8 kgCO2	15,795.7 kgCO2
GHG total emissions per working hour	43.5 kgCO2/h	10.8 kgCO2/h

. The electric ferry produced only 25% of the emissions of its diesel-powered sister vessel. The monthly total emissions were the same, as the electric Vessel 2 was operating 35% more hours per day.

4.2. Emissions Comparison of Various Countries

We also calculated the potential difference in CO₂ emissions for these two ferries in different countries. The GHG emission calculation was based on 27 EU countries’ average emission factors [47]. It must be emphasized that with the WTW method, emission factors in EU member countries vary due to electricity suppliers’ different WTW energy factors. In other words, countries with higher amounts of renewable energy sources and lower energy consumption factors have lower GHG emissions.

Figure 4 shows that, if these vessels were operating in Estonia (which has a relatively high emission factor), the emission difference rate between the two vessels would be only 2.7 times. For vessels in Belgium (which has a relatively low emission factor), the difference would be 5.2 times, whereas for Sweden it would be 15.7 times. This comparison shows that using electricity for operating energy can have significantly less GHG emissions than a diesel-fuelled ferry in all European countries.

4.3. Solar Panel Production in Winter

In addition, we measured and analysed solar power during the period under study. This measurement was not included in the emission analysis of the previous chapter because of its minor impact on the total energy needed. According to the collected data, the total solar energy production for Vessel 1 and Vessel 2 in the measured month was 204.9 kWh and 160.4 kWh, respectively (see

Table 3. Solar panel energy production during the data collection period.

	Vessel 1	Vessel 2
Solar energy production set 1	104.4 kWh	78.7 kWh
Solar energy production set 2	100.5 kWh	81.7 kWh
Total	204.9 kWh	160.4 kWh

).

Figure 5 and Figure 6 show the actual measured daily power produced by the solar panels. The figures show that on some winter days, energy production was minor for both vessels. These results are generally from cloudy periods.

Figure 5 and Figure 6 show that even in winter, when there is low solar energy production, production can cover several application options and user areas during 11 h of navigation time (see

Table 4. Possible applications for solar energy.

Consumer Group	Vessel 1	Vessel 2
Lighting	204.9 kWh	217.8 kWh
Signalling system	180.5 kWh	180.5 kWh
Fire system	19.8 kWh	24.8 kWh
Bilge water system	130.0 kWh	66 kWh
Entertainment system	84.5 kWh	34.9 kWh

); e.g., lighting the vessel during its operation could be alternatively covered by solar energy supply. The interior lighting analysis was conducted with all lighting switched on. In real life, the hull compartments’ lighting is mostly switched off and used for only a few hours a month. Switching off the interior lighting in the passenger area in daylight saves even more electrical energy, allowing another consumer or consumer group to use the energy from solar panels.

4.4. Cost Comparison

The diesel and electric commuter ferries are newly built ships (delivered in autumn 2021 and 2022). The purchase price of the electric ferry was EUR 5,500,000 while the diesel ferry was EUR 4,300,000. The purchase price of the electric ferry was 27.9% higher than the construction costs of the diesel ferry [48,49]. It is worth noting that the electric ferry’s higher price was caused not only by higher equipment costs, technical innovations, and inflation but also by the unstable market situation initiated by the COVID-19 pandemic during construction time.

We calculated the difference in cost between electric and diesel vessels. In other words, which alternative is more economical.

In the analysis, we assumed that both ferries operate equally 10 h a day. With this assumption, the annual fuel and electricity costs for Vessel 1 in Belgium would have been EUR 22,100. For vessel 2 this operational cost would be EUR 15,200.

The analysis was conducted with average electricity and marine fuel oil prices from Q4 2022 [50,51] in Belgium. The operational cost was calculated from average energy consumption (marine diesel oil and electricity only). This study did not include any technical crew costs, port fees, maintenance, consumer costs, or other relevant and significant expenses of normal operation.

The payback time of Vessel 2 in relation to Vessel 1 (more expensive by purchase cost) was calculated using the following formula:

$${\mathrm{t}}_{\mathbf{p}}=(\frac{{\mathrm{C}}_{\mathrm{p}\mathrm{p}}}{{\mathrm{C}}_{\mathrm{c}\mathrm{c}}})$$

(3)

where,

• t_p is the payback time [years];
• C_pp is the purchase price difference [€];
• C_cc is the yearly consumption cost difference [€].

Due to the heavy purchase price difference, the payback time of replacing a diesel ferry with an electric one resulted in 17 years and 6 months.

Thereafter, we calculated the subsidy needed to reduce the payback time of this investment to <10 years. The target of <10 years considers the 20–30-year service life of both ferries. The calculation was determined using the following formula:

$${\mathrm{t}}_{\mathrm{p}\mathrm{s}}=(\frac{{\mathrm{C}}_{\mathrm{p}\mathrm{p}}-{\mathrm{S}}_{\mathrm{s}\mathrm{u}\mathrm{b}}}{{\mathrm{C}}_{\mathrm{c}\mathrm{c}}})$$

(4)

where,

• t_ps is the payback time with subsidy [years];
• C_pp is the purchase price difference [€];
• S_sub is the subsidy amount [€];
• C_cc is the yearly consumption cost difference [€].

The calculation showed that with a EUR 500,000 subsidy for purchasing the vessel, the payback time of investment in the vessel would be reduced to 10 years and 2 months.

Thereafter, we compared the operational energy cost differences in countries with different electricity prices. We found that operational costs would be 39.6% lower for Vessel 2 in Estonia. See differences by country in Figure 7 below.

Finally, we analysed the approximate payback time in various European countries. The European average would be 12 years and 1 month but reduced to 7 years and 1 month with a subsidy of EUR 500,000. See Figure 8 and Figure 9 below.

5. Discussion

Many countries have begun using alternative green fuels in city traffic. Hydrogen is one of the most promising fuels. However, the vessel’s lifetime plays a significant role in building hydrogen-fuelled ferries [52,53]. Currently, the infrastructure in most cities and ports lacks opportunities to supply hydrogen. In addition, hybrid solutions using other energy combinations have been used [24,27], but these are not as advantageous for urban waterways. Moreover, hybrid solutions are often heavier than one-type energy solutions and increase fuel consumption.

Transitioning the energy source to fully electric in all transportation modes also has natural resource challenges. According to a recent study [54], the demand for battery metals has grown, and annual volumes in 2050 are estimated to be 4–10 times higher than today. This demand implies a significant increase in mining metals such as lithium, nickel, cobalt, and manganese. The analysis also emphasizes that smaller batteries are key to reducing the demand for raw materials. This perspective benefits the shipping industry and ferry operation, especially coastal and inland ferries.

It is also essential to consider the operating area of the vessels. Important factors such as ice conditions and lack of infrastructure currently influence fully electric vessel applications; therefore, retrofitting the ferries to fully electric might not be sufficiently justified.

Despite focusing on the type of ship, other factors also benefit from fully electric ships [39]. Fully electric ferries have less noise and vibration than diesel ferries. This perspective is relevant not only to urban waterways but also to marine biologists and tourism. A study [55] involving whale watching by small ships was conducted in Iceland that, among other results, supports our findings on the importance of using fully electric ships from a financial perspective. Our research results are also supported by findings from other means of transport, where e-fuels and fully electric vehicles have been seen as potentially beneficial [56].

For this study, it is worth noting that there are also proposals to change the assessment of WTW emissions. Among these proposals is connecting carbon emissions to the country’s electricity production, as the current methodology might not provide the necessary steps and actions [45]. Additionally, trade and political situations also heavily affect the electricity market [46].

According to the study, even though renewable energy resources such as solar panels may not be able to cover all energy resources for operating needs, there are several alternatives for specific application areas or consumers. For example, solar panel energy is an excellent choice for supplying ferries’ interior lighting systems.

In our cost analysis, we found that in most countries, Vessel 2 had significantly lower costs of total energy used. It is worth noting that both the electrical energy and fuel oil markets are volatile and vulnerable to global trends and port infrastructure. According to these results, operation of Vessel 1 would have been economically similar or more viable in three countries (Ireland, Greece, and Italy) in Q4 2022. This finding shows no unique fuel alternative for all European countries. Although fully electric vessels might be low in GHG emissions, they have higher financial costs. These expenses must be obtained from external resources if the targets of the EU Fit for 55 package are to be reached.

The EU has set several supportive measures for helping member countries reach the goals targeted in the Fit for 55 package [57]. One of these measures is a fund to support the most affected citizens and businesses [58], which is an example of how affected companies and industries can find additional funds to support their efforts to become carbon neutral and obtain subsidies. As our study results show, using fully electric vessels has major benefits financially and for carbon emissions. Therefore, the EU, alongside states and local municipalities, should find supportive measures. Supportive measures cover subsidies and financial support for new building/retrofitting as well as finding partnerships, building a supportive infrastructure, and discovering means of supplying such vessels.

6. Conclusions

This study focused on the GHG emissions assessment of two commuter ferries operating in city traffic on the same line. A fully electric and a diesel-fuelled catamaran alternated the same route daily. Total energy and fuel consumption were measured for one month of regular operation for both ferries. Based on these measurements, the well-to-wheel impact of GHG emissions was calculated.

Our results showed that the electric ferry produced only 25% of CO₂ emissions compared to the diesel engine. Depending on the energy sources of various EU countries, GHG emissions for a working hour can be up to 15.7 times lower for fully electric ferries than for diesel engines.

The study also analysed solar panel energy production in winter periods and concluded that using renewable energy sources such as solar panels is justified for reaching carbon-free targets. Solar panel energy production can fully cover the energy needs of modern systems such as internal lighting or other important consumers.

With the assumed operation time, the payback time compared to the purchase price and operational energy costs in Belgium is 17 years and 6 months. However, with a subsidy of EUR 500,000, this length of time is reduced to 10 years and 1 month. Since electrical energy and diesel prices are volatile and vary between countries, these results depend on the country of operation. According to our study results, the EU corresponding average would be 12 years and 1 month, reduced to 7 years and 1 month with a subsidy of EUR 500,000.