# Battery Swapping Implementation on Electric Sea Taxis; The Case of Ionian Islands

Faidra Kotarela¹, Nick Rigogiannis¹, Anastasios Kyritsis², Nick Papanikolaou¹

¹DEMOCRITUS UNIVERSITY OF THRACE
Department of Electrical and Computer Engineering
Xanthi, Greece

²IONIAN UNIVERSITY
Department of Environment
Zakynthos, Greece

E-Mail: fkotarel@ee.duth.gr, nrigogia@ee.duth.gr, kyritsis@ionio.gr, npapanik@ee.duth.gr

## Acknowledgements

This work was supported in part by the European Regional Development Fund of the European Union and in part by Greek National Funds through the Operational Program "ATTICA" 2014 – 2020, under the call “Research and Innovation Collaboration in the Region of Attica” (Project code: ATTP4-0359279 / MIS 5184666).

## Keywords

«battery swapping», «electric sea taxi», «life cycle cost», «life cycle assessment», «transportation electrification».

## Abstract

Waterborne transportation significantly impacts the environment, contributing to climate change. In light of the global shift towards electrification, the need for a transition of the maritime transport into a more sustainable paradigm becomes imperative. This study investigates the feasibility of implementing the battery swapping concept for small vessels operating short daily routes in the Ionian Islands, as an alternative to conventional battery charging. A comprehensive case study is presented, examining the proposed system from both techno-economic and environmental perspectives, through Life Cycle Cost and Life Cycle Assessment analyses. The findings highlight the sustainability benefits of the battery swapping concept, including reduced greenhouse gas emissions, along with valuable insights for advancing green maritime transport technologies.

## Introduction

The Transportation Sector plays a crucial role in today's economy and society, while it has a

significant impact on growth and employment, as it accounts for about 5% of the gross domestic product (GDP) [1]. As the global demand for transport services is expected to increase in future years, it is crucial to sustainably transform the Sector into a zero-carbon one, with the integration of renewables, providing highly efficient and green solutions [2], [3].

Motorized transport on land, air and water still depends highly on internal combustion engines (ICE) that generally run on carbon-based fossil fuels. Specifically, in 2022, the share of fossil fuels in transportations accounted for 30.95·10³ TWh (29.5x10³ TWh oil and 1.45x10³ TWh natural gas), while it is expected that by 2030 it will be reduced to 23.6 x 10³ TWh (22.5x10³ TWh oil and 1.1x10³ TWh natural gas) [4]. However, the Transportation Sector is expected to undergo accelerated rates of electrification in the next decades. At first, the electricity share in 1975 was about 13.8 TWh; then it increased to 458.3 TWh by 2022 and it is expected to reach 488.8 TWh in 2030. Moreover, an increase in hydrogen share is expected by 2030; around 8.33 TWh [4].

A combination of energy efficiency measures and low-carbon approaches are expected to reduce transport energy consumption from 3.36·10⁴ TWh in 2018 to roughly 2.61·10⁴ TWh by 2050 [2]. In parallel, electric vehicles (EV) would account for more than 80% of all road transport activity by 2050 [2]. This transition implies that close to a third of all light-duty vehicle sales and a fifth of truck sales should be electric, up to 2030. Even though the electrification of land transportation means has been widely investigated and adopted, this is not the case for waterborne vehicles, mainly due to techno-economic reasons (e.g., high costs and weight of the required batteries, in order to achieve an
acceptable range). However, marine transport is considered to be the backbone of the international trade and the global economy, as over 80% of the volume of international trade in goods is carried by sea [5]. In parallel, marine trade comprises 70% of global shipping emissions. In more detail, the share of shipping emissions in global emissions has reached 2.8% in 2018, whilst this figure is expected to be raised to 18% by 2050, if no actions will be taken [6]. Taking all the aforementioned into account, it is concluded that marine transport is one of the most challenging sectors to be decarbonized [2].

However, with the right strategic actions, the decarbonization of the international shipping sector is viable. In this context, EU has decided to reduce the volume of ship greenhouse gas (GHG) emissions by less than 80% by 2025 compared to 1990 [7]. As an interim target to achieve this, it is imperative to reduce GHGs by 40% up to 2030 and by 60% up to 2040 [8].

The sector's decarbonization strategy must involve a combination of enhanced energy efficiency solutions and renewable fuels (e.g., biofuels, biogas, methanol, ammonia, hydrogen) [9]. However, those renewable fuels are still considered to be immature technologies in the shipping sector, except for marine fuel oils and liquid natural gas (LNG), that are globally the most widely used, leading though to GHG emissions [9].

Keeping the above mentioned in mind and taking into account the crucial need for CO₂ emissions reduction, all-electric ships are considered as a promising technology to shape the future of green shipping, leading to a growing interest around the world [10]. Electric ships are powered by electricity stored in batteries, eliminating the need for fossil fuels and reducing GHG emissions. Those ships are not only environmentally friendly but also offer quieter and smoother operations, making them ideal for sensitive marine ecosystems and urban areas near ports [11].

The battery-powered electric ship technology is attracting more and more attention, because of the characteristic of almost zero-emission life cycle, compared with conventional diesel ships or alternative fuel ones. Apparently, in order to be eco-friendly and sustainable, this concept demands the use of renewable energy in battery charging [12]. In general, EV charging can create significant additional electricity demand, which can be successfully and cost effectively met with the use of renewables, including mainly solar and wind power fed into the grid. Such developments

offer a tempting prospect – particularly for ports – to decarbonize transport while also cutting air and noise pollution, reducing fossil fuel dependence and adopting new approaches to urban mobility [13]. Finally, the continuous cost reductions for renewable power generation make electricity an attractive low-cost energy source to supply the Transportation Sector.

## Electric ships and battery swapping stations

Currently, the majority of electric ships operate in typical battery charging mode. However, with the increased number of on-board batteries installed in the future, the increased battery charging time will reduce the ship operation readiness. In addition, due to the limited space and the high investment costs, the number of batteries installed on-board is limited and it cannot satisfy the demand for long-distance navigation; thus, energy supplement is inevitable. Moreover, the issue of electric ship battery charging is expected to increase the pressure on the main distribution network grid and affect power quality [14].

A promising solution to the abovementioned problems is the implementation of battery swapping operation [15]. In order to achieve longer distance navigation, battery swapping stations (BSS) could be developed in the inland ports along the ships' routes, where the ships could replace the depleted batteries with fully charged ones, so as to effectively continue their navigation [16]. The depleted batteries are being charged in the charging station of the BSS and stored for the next ship. The benefits of the battery swapping concept in marine electromobility are highly important, although this innovative technology faces many challenges that must be overcome. Firstly, the charging time is reduced, considering that the remaining time at the BSS port is reduced in total than in case of conventional charging [17]. Considering that the replacement of the batteries is being conducted via robotic systems or cranes, this procedure could take a few minutes, based on the number of batteries. Secondly, the battery swapping fee for the ship owners is lower than the charging fee, which depends on the current electricity price. Another benefit is that the batteries can be charged in peak off hours, which solves the problem of overloading caused due to increasing penetration of electric ships or EVs, in order to avoid problems that may cause grid instabilities [17]. Lastly, based on the scheduled charging, the
fully charged batteries remaining at the BSS could supply the stored energy to cover other loads of the BSS, to eliminate the dependence on the electricity grid, which provides high flexibility on both the charging power and the charging time [13]. The abovementioned, in conjunction with renewables' operation, could lead to significant benefits in the electricity grid stability and sustainability, as the batteries can be charged according to the renewable energy generation [17].

In this work, the conceptual design of BSSs for the Ionian Islands is proposed, in order to serve battery swapping in small electric vessels that manage short-distance voyage through the Ionian islands (sea taxi vessels). This work aims to investigate the installation of BSSs in those islands, to eliminate the use of conventional vessels for inland marine short-haul routes (i.e between mainland and island ports) and reduce the carbon footprint of sea transport. Finally, the sustainability of the proposed BSS is analyzed from both techno-economic and environmental aspect through the life cycle of the system.

The remainder of the paper is organized as follows; first, the proposed BSS system for the Ionian Islands is presented, describing the studied sea taxi vessel, the necessary BSS infrastructure and operation, as well as the navigation network of operation. Next, the life cycle cost (LCC) and the life cycle assessment (LCA) studies are presented for the life cycle span of the BSS. Finally, the conclusions of the proposed case study are summarized and discussed.

## The case study of Zakynthos Island

The proposed BSS is installed in the island of Zakynthos, a Greek island located in the Ionian Sea, off the western coast of mainland Greece, which hosts a substantial number of tourists each year, particularly during the summer period [18]. The island of Zakynthos is selected for the BSS installation considering that it presents a high seasonal electric load during the summer touristic period. The aforementioned, along with the fact that the island's electricity grid is weak, leads to the conclusion that the operation of BSSs during peak loads could be beneficial. Furthermore, the high solar irradiation during summer months could enhance the installation of photovoltaic (PV) systems on the BSS to generate adequate energy for the battery charging, reducing the dependence on the electricity network and

providing flexibility to both BSS and electricity grid.

In the case of islands, sea taxis are frequently preferred for short-distance routes around the islands or between the island ports. Therefore, this work focuses on the electrification of those sea taxi vessels with the incorporation of battery stacks, in order to serve daily routes and operate in battery swapping mode. This case study is based on a specific type of electric vessels that operates short voyage routes through a specific network of ports in Ionian Islands. Specifically, the proposed electric sea taxis in Zakynthos Island are considered to operate daily 20-nautical mile routes from the main port of Zakynthos to Agios Nikolaos marine and to the port of Kyllini, once an hour. Thus, BSSs have to be installed in each of these ports.

## Description of the studied sea taxi vessel

In particular, the study focuses on a small 10-passenger sea taxi for leisure purposes, considering a conventional system (i.e., utilizing diesel ICEs). Next, an appropriate battery bank sizing is performed, via energy consumption calculations, leading to an alternative all-electric solution. The main features of the conventional vessel are summarized in Table I.

<table><caption>**Table I: Technical characteristics of the conventional sea taxi.**</caption><tr><td>Capacity</td><td>10 passengers</td></tr><tr><td>Range</td><td>20 nautical miles (n.m.)</td></tr><tr><td>Maximum speed</td><td>25 knots (n.m.)</td></tr><tr><td>Total weight</td><td>7 t</td></tr><tr><td>Net weight</td><td>3 t</td></tr><tr><td>ICE motor weight</td><td>1.2 t (approx.)</td></tr><tr><td>Fuel weight</td><td>1 t</td></tr><tr><td>Diesel consumption</td><td>15.9 L/n.m.</td></tr><tr><td>Waterline Length</td><td>12.13 m</td></tr><tr><td>Waterline Beam</td><td>3.43 m</td></tr><tr><td>Depth</td><td>1.75 m</td></tr></table>

As for the electrical specifications, Table II presents the technical features of the electric motor that the sea taxi vessel uses. The diesel consumption value is converted into thermal energy and thus into the delivered propulsion power, through the ICE thermodynamic operating cycle. Therefore, it is derived that the average propulsion power, considering an average speed of 20 knots, is $P_N = 350$ kW. For the electrical propulsion equivalent, an indicative (reference)
brushless electric motor is selected, i.e., the "Curtis 1239e-8521 HPEVS Dual AC-35 Brushless Motor Kit - 144 Volt" [19].

The required number of electric motors to meet the propulsion power demands is three, whereas an extra (auxiliary) motor is considered imperative, for redundancy purposes. Next, to achieve the required 20-nautical mile range, the required stored energy is calculated, being 280 kWh. Assuming a 20% minimum State-of-Charge (SoC) for the battery bank, the nominal stored energy is obtained, being 351 kWh.

<table><caption>Table II: Electric sea taxi specifications.</caption><tr><td>Motor type</td><td>Curtis 1239e-8521 HPEVS Dual AC-35 Brushless Motor</td></tr><tr><td>Nominal power</td><td>165 hp</td></tr><tr><td>Nominal voltage</td><td>144 V</td></tr><tr><td>Nominal speed</td><td>5,000 rpm</td></tr><tr><td>Weight</td><td>68 kg</td></tr><tr><td>Efficiency</td><td>88%</td></tr></table>

## Description of the battery stacks

In order to meet the selected motor nominal voltage (not exceeding the allowable limits) and respecting the electronic speed controller input voltage range, a 19S1P (72.2 V) lithium battery module is appropriately chosen.

A battery bank, comprising parallel connected battery strings, each one with two series-connected modules can be utilized, to support the propulsion system demands. The indicative (reference) battery module is the "GRP96C0240-38,000mAh" and it features the specifications presented in Table III [20].

<table><caption>Table III: Battery system specifications.</caption><tr><td>Battery type</td><td>GRP96C0240</td></tr><tr><td>Typical capacity</td><td>38,000 mAh</td></tr><tr><td>Nominal voltage</td><td>72.2 V</td></tr><tr><td>Charge current</td><td>10 A</td></tr><tr><td>Charge voltage</td><td>82.65 V</td></tr></table>

<table><tr><td>Maximum discharge current</td><td>50 A</td></tr><tr><td>Peak discharge current (≤ 3 sec)</td><td>190 A</td></tr><tr><td>Cut-off voltage</td><td>66.5 V</td></tr><tr><td>Charge temperature</td><td>10°C – 45°C</td></tr><tr><td>Discharge temperature</td><td>-20°C – 60°C</td></tr><tr><td>Height</td><td>261 mm</td></tr><tr><td>Length</td><td>203.5</td></tr><tr><td>Width</td><td>123.5</td></tr><tr><td>Weight</td><td>11,500 ± 150 g</td></tr></table>

The battery stacks system will be installed at the bottom of the lower deck of the vessel, under the passenger seats, as it is depicted in Fig. 1. It is noted that the specific position of the batteries has been appropriately selected, taking into account the maximum weight of the sea taxi, in order not to cause any stability and navigation issues.

To meet the calculated energy demand (351 kWh), 64 parallel connected battery strings are required, each one with two series-connected reference modules, leading to a total number of 128 battery modules. Each one of the battery stacks illustrated in Fig. 1 contains 16 modules, and thus the total amount of battery stacks is 8 of 43.9 kWh energy capacity each.

<figure><img src="image_1.png" /><figcaption>Fig. 1: The studied sea taxi layout, indicating the position of battery stack.</figcaption></figure>

Hence, the overall weight of the battery bank (excluding harness, wiring and other assembly equipment) is approximately 1.4 t, whereas its dimensions will be defined in more detail, according to the selected module arrangement. Finally, it is estimated that the overall battery
bank volume (excluding harness, wiring and other assembly equipment) is roughly 0.8 m³ or 800 L. The battery stacks are installed on a robotic railway on the bottom of the lower deck of the vessel. They are at the back side of the lower deck when the vessel is on sailing mode. When the vessel reaches the BSS quay, the batteries are being moved on the railway by a remote-controlled robot, in order to be placed in the front side. Then the BSS quay robot/crane draws the battery stacks through an existing opening (door) and operates the swapping procedure. The detailed description of the BSS to be installed on the island is presented in the next section.

## Description of the battery swapping station

The BSS design is based on an automatic battery swapping concept, in which robotic systems are utilized to swap the batteries in and out of the vessels. The BSS consists of the following systems:

* a robotic system that operates the battery swapping,

* the battery chargers' system,

* a supplement of batteries that are being stored for emergency,

* the renewable energy generation system.

The BSS infrastructure requires a suitable-constructed quay where the overall system will be installed. The sea taxis arriving at the port, reach the quay in order to make a short stop for battery swapping, without affecting the normal operation and others ships arrivals/departures to/from the port.

The robotic system is installed at the front side of the quay, so as to swap immediately the batteries from the vessels. In this work it is considered that the robotic system is an electric crane. A generic example of the BSS concept for electric sea taxis is presented in Fig 2.

<figure><img src="image_2.png" /><figcaption>Fig. 2: Port infrastructure for the battery swapping process of electric vessels.</figcaption></figure>

## Battery chargers

As regards the battery chargers, in this work, fast chargers are being installed in order to achieve lower charging times for the batteries arriving at the BSS. In particular, 3 fast chargers of 50 kW power are being installed at each of the main ports of the network (i.e., Zakynthos and Kyllini) and one 50 kW fast charger at the small ports (i.e., Agios Nikolaos dock in Zakynthos) in order to achieve the complete charge of 8 battery stacks within 1 hour.

When the vessel arrives at the port, the robotic crane removes the depleted batteries from the vessels for charging. Considering that each vessel incorporates 8 battery stacks, there must be an excess of batteries at every BSS, to assure that the BSS will not run out of batteries. In this work, it is assumed that each of the BSS features a supplement of 24 battery stacks that are being charged and swapped during the day. It is of paramount importance that at the end of the day the total number of the supplement batteries remaining at the BSS storage must be the same (balance of battery inputs and outputs).

Specifically, when a vessel reaches the BSS, it provides 8 discharged battery stacks at the ports and therefore, it is supplied with 8 fully charged battery stacks. The batteries remaining at the port for charging are being charged every one hour, within a time interval from 8:00 am to 20:00 pm. Based on this schedule, the batteries are mainly charged by PV-generated energy during the day. The batteries that have to be charged during the night will be charged at times when no peak electricity loads are present, to avoid the high electricity price and to not affect the electricity grid operation (i.e., between 1:00 am to 7:00 am). After charging, the fully charged batteries are being stored and prepared for the next vessel to reach the BSS, in order to be swapped.

Based on the 3 battery chargers of 50 kW and considering that one vessel arrives at the port per hour, the total BSS energy demand during the day (8:00 am to 20:00 pm) is equal to 1,950 kWh and 208 kWh during the night hours at the main ports (Zakynthos and Kyllini), where only one of the three battery chargers operate, to charge the remaining battery stacks within 5 hours. As for the smaller ports (Agios Nikolaos), where the arrivals/departures of the vessels may feature lower frequency, only one 50 kW charger operates, under similar schedule. In the case of one sea taxi arrival at the smaller port, the energy
consumptions between the main port and the smaller one are similar.

## Renewable energy generation

One of the main objectives of the discussed BSS implementation concept is clean energy generation for the battery charging procedure. Hence, PV systems are installed, in order to charge the batteries and cover the BSS loads during the day. As for the night hours, the batteries are charged through the electricity grid. Considering the total energy consumption of a BSS during the day, the overall installed PV capacity (peak) that each port requires is 0.5 MWp. The annual BSS energy consumption is 711.7 MWh/year while the PV system of 0.5 MWp generates 797.2 MWh/year. Thus, the energy transactions between the PV and the corresponding BSS are equal to 85.5 MWh/year (supplement energy production).

In order not to affect the normal operation of the island electricity grid, this particular amount of energy could be used to cover other loads, such as EV charging, public building loads etc. Finally, this amount of energy could be stored and used to provide ancillary services to the island electricity grid, as a supplement energy supply, or to supply loads when the electricity network is on emergency conditions. The economic and environmental analysis of the proposed BSS is presented in the following section.

## Techno-economic and environmental impact analysis

### Life cycle cost

The economic analysis of the discussed case study is conducted with a robust and rigorous methodology, designed to evaluate the economic viability and valuation of BSSs dedicated to electric sea taxis. A 25-year timeline has been selected to fully capture the comprehensive lifecycle from the project commencement to its ultimate decommissioning, while also considering site-specific factors such as operating and maintenance costs, energy prices and the impact of the regulatory framework.

The economic assessment begins with a precise definition of the project scope, followed by a systematic classification of all relevant economic costs into six major categories, i.e., capital, operating and maintenance, battery, end-of-life, financing, and insurance costs. Capital costs

include all initial expenditures required for the project setup, such as land, construction, and infrastructure for the BSS and PV power facilities. Next, operating and maintenance costs cover daily operational expenses, regular maintenance and repairs essential for the smooth and efficient BSS operation. Battery costs account for the initial purchase and periodic replacement of batteries, acknowledging their degradation and technological advancements. In addition, end-of-life costs encompass the financial obligations at the project conclusion, ensuring environmentally sound decommissioning and disposal practices. Financing costs reflect the expenses incurred in securing capital, including interest and other related financial service fees. Finally, insurance costs provide financial protection against operational risks and potential losses, ensuring the project's financial stability.

The financial plan of this study incorporates a wide set of financial variables, from upfront investment to ongoing operational costs, leading to a calculation of the Net Present Value (NPV). NPV is a parameter that represents the difference between the present value of cash inflows and outflows, incorporating the time value of money – a principle asserting that the immediate availability of money is more valuable than the same amount in the future, because of its potential earning capacity [21].

Considering the abovementioned parameters and costs, the initial cost of the three BSSs investment is equal to 9.09 million €. This accounts for the installation of BSSs in three ports, i.e., the main port of Zakynthos, the port of Agios Nikolaos (in Zakynthos Island) and the port of Kyllini (in mainland). As for the battery chargers, 3 chargers will be installed at the main ports of Zakynthos and Kyllini, while one charger will be installed at the port of Agios Nikolaos. Considering that each port must have a supplement of battery stacks in order not to get drained due to the expected ships arrivals, 72 battery stacks must be produced for the normal operation of those three BSSs.

Thus, considering a typical discount and inflation rate of 0.25% and 2.9%, respectively, the total NPV life cycle cost for the 25-year period analysis is presented in Figure 3, compared to the life cycle cost for a diesel sea taxi. As for the diesel sea taxi, the price of the Marine Diesel Oil (MDO) is 0.766 €/lt. It is also noted that the initial cost of the investment includes the total NPV replacement cost of 360 battery stacks that are going to be replaced within 25 years.
Moreover, considering the typical route fee per passenger in the Ionian islands (i.e., 60 € for a route of 20 nautical miles), as well as the previously mentioned discount and inflation rates, the total NPV income of the electric sea taxi is equal to 98.83 million €; therefore, the payback period of the proposed investment can be achieved within 4 years, as presented in Figure 4.

<figure><img src="image_3.png" /><figcaption>Fig. 3: Calculated LCC of the three BSSs compared to the diesel sea taxi LCC.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 4: Calculated LCC of the three BSSs compared to the annual income.</figcaption></figure>

## Life cycle assessment

The methodology for the life cycle assessment is designed to deliver a meticulous and robust evaluation of the BSS environmental footprint. This framework involves a clear definition of life cycle assessment goal and objective, as well as a detailed analysis of components and quantities of the battery part. The LCA is a methodology used to assess the environmental impacts of products. The ISO 14040:2021 and the ISO 14044:2021 international standards define it as the compilation and evaluation of the inputs, outputs, and the potential environmental impacts of a products system [22], [23].

The goal of this work is to measure – from a life cycle perspective – the environmental performance of the considered lithium-based batteries, used for battery swapping in electric sea taxis. For the purpose of this analysis and assuming that the battery packs are being replaced with new ones every 5 years, a cradle-to-grave approach is adopted. Thus, considering the various stages over life of batteries, the product system includes:

* extraction of raw materials,

* manufacture of the battery and its components,

* installation on site,

* use phase,

* production and delivery of the stored electricity,

* the end-of-life treatments.

For the considered lithium-polymer battery stack, utilized by the electric vessels, the battery stack energy capacity is 43.9 kWh, the stack weight is 409.6 kg and the cycle life is 912 times over the 5 years. Finally, the energy density of the battery is 107 Wh/kg at the battery stack level. Last, the present LCA is conducted for the three ports mentioned above, i.e., the main port of Zakynthos, the port of Agios Nikolaos (in Zakynthos Island) and the port of Kyllini (in mainland) and the corresponding equipment (which has been already described).

## Battery production phase

Considering the required components and quantities to produce 1 kWh battery pack, 23.2 kg of material will be used [24]. Thus, in order to produce a 43.9 kWh battery stack 1,018.6 kg of materials are used; according to the sea taxi specifications, each vessel comprises 8 battery stacks (and thus the total battery system capacity is 351 kWh). As for the specific lithium-polymer batteries considered in this work, the total weight of one battery stack that contains 16 battery modules of 11.5 kg each (considering the packaging and the battery management system weight), is equal to 409.6 kg [24].

Therefore, in the first stage (battery material production) the energy consumption is 35.42 kWh per kg of battery produced [25]. This number refers to the exploitation of raw materials from the earth or from a recycling stream and their refinement into usable basic (commodity) materials, purchased by a manufacturer to produce battery components. The second stage is
the battery manufacturing, which represents all processes needed to convert those basic materials into battery components, such as anodes, metallic foams, cathodes and electrolytes and to assemble them into a purchase-ready product. In this project, the energy consumption for the manufacturing of lithium-polymer batteries is equal to 25.6 kWh per kg of battery produced [25] and the energy consumption during the materials' production is 35.42 kWh per kg of battery produced [25]. Thus, the overall energy consumption for the cradle-to-gate phase (from the extraction of the materials up to the phase of the transportation to the consumers) of the 43.9 kWh battery stack system is equal to 25 MWh; and 200 MWh for the production of 8 battery stacks (351 kWh capacity for one sea taxi vessel). Hence, for the production of 72 battery stacks the total amount of energy consumption during the cradle-to-gate phase is 1,800 MWh. However, considering that the 72 battery stacks are being replaced every five years, the total amount of energy consumption during the cradle-to-gate phase of 360 battery stacks is 8,966 MWh

## Use phase

The use phase of the battery system is based on the service life that it is designed for and the number of cycles that performs over its lifetime. Regarding the service life of the studied battery stacks, considering that each of them will be replaced with a new one every five years, and that each stack will operate 2 full cycles per day, they will conduct 912 cycles within the 5-year period. As for the battery autonomy of each sea taxi (battery capacity of 351 kWh), the range of each vessel is equal to 19.8 nautical miles (36.63 km) considering an average speed of 20 knots. Thus, each battery stack features a total service life of 16,712.4 km. Considering an energy consumption of 9.58 kWh/km, the total amount of energy that each battery stack will deliver over its use life is 160.2 MWh. Moreover, during service life, the energy required during charging is both renewable energy from PVs production during day hours and energy from electricity grid during night hours, as already referred. Considering the BSS load demand during night hours that already referred, the carbon footprint of the BSSs is 186 tn CO2./y.

## Disposal and recycling – End-of-life phase

Currently, the main recycling technologies for waste lithium-based batteries include physical

dismantling, pyrometallurgy and hydrometal- lurgy. Under optimal conditions, the recovery efficiency of 81% for lithium could be obtained [26].

As for the end-of-life phase of the lithium batteries, the impact of recycling has a positive effect on the environment. In particular, recycling of lithium-based batteries can reduce the CO2 emissions by 4.79 kg CO2eq per kWh of battery recycled [27]. Thus, the recycling of the batteries being replaced every five years could lead to a significant reduction of CO2 emissions. In more detail, a reduction of 1,682 kg of CO2 emissions every 5 years could be achieved, while the recycling of the 72 battery stacks could lead to 75.73 tn CO2 reduction within the 25 years.

## Economic input-output phase

An economic input-output life-cycle assessment (EIO) involves the use of aggregate sector-level data to quantify: (a) the amount of environmental impact that can be directly attributed to each sector of the economy and (b) how much each sector purchases from other sectors in producing its output [28].

EIO systems contribute a relatively small portion (of around 10%) on the life cycle GHG emissions in lithium-polymer batteries. However, the EIO systems play a key role in evaluating the life cycle environmental impacts of lithium batteries, especially in life cycle energy consumption. Specifically, for the life cycle energy consumption, EIO systems become the most significant part, which contributes 47% of the life cycle energy consumption for lithium-polymer batteries [28].

<figure><img src="image_5.png" /><figcaption>Fig. 5: Total GHG emissions through the use phase of the electric sea taxi and the diesel sea taxi over the 25-year period.</figcaption></figure>
The LCA results are translated into relevant environmental impacts. In more detail, Fig. 5 presents the total amount of CO₂eq emissions calculated for the use phase of the 360 battery stacks that have been produced for the BSSs' operation, compared to the carbon footprint of diesel sea taxi vessels. In more specific, regarding the use of electricity for charging the batteries during night hours, the carbon footprint of the electric sea taxi remains lower than the diesel one, leading to the reduction of 85.87 tn CO₂ over the 25 years.

In parallel, Fig. 6 illustrates the total amount of CO₂ emissions during the various stages of batteries overlife, while Fig. 7 presents the total amount of energy required for each life cycle stage for the production, use, recycling and EIO of 360 battery stacks.

<figure><img src="image_6.png" /><figcaption>Fig. 6: Total CO₂ emissions during the various phases over the life cycle of the 360 battery stacks to be produced.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 7: Total energy consumption for the various phases over the life cycle of the 360 battery stacks to be produced.</figcaption></figure>

## Conclusion

This work focuses on marine transportation electrification and highlights the need for BSSs installation in islands, in order to enhance vessel electrification and reduce GHG emissions. The

proposed BSS installation in Zakynthos Island offers several benefits to both sea taxis and the electricity grid. First of all, the adaptation of the battery swapping mode in small sea taxi vessels is proved to be advantageous from both economic and time-management aspect, extending their distance navigation too. In addition, the operation of BSSs benefits the island's distribution network as it prevents voltage stability issues. Finally, by the aid of the insightful LCC and LCA analyses the sustainability of the proposed case study is validated, making the battery swapping concept a promising solution for further waterborne transportation electrification in the future.

## References

[1] Joint Research Centre European Commission, 2020. [Online]. Available: https://joint-research-centre.ec.europa.eu/scientific-activities-z/transport-sector-economic-analysis_en.

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