Decarbonising the Inland Waterways: A Review of Fuel-Agnostic Energy Provision and the Infrastructure Challenges

Abstract

Inland Waterway Transport (IWT) is widely recognised as an energy-efficient freight mode, yet its decarbonisation is increasingly constrained not by propulsion technology, but by the absence of infrastructure capable of delivering clean energy where and when it is needed. This paper presents a structured review of over a decade of academic, policy and technical literature, identifying systemic gaps in current decarbonisation strategies. The analysis shows that most pilot projects are vessel-specific, and poorly scalable, with infrastructure planning rarely based on vessel-level energy demand data, leaving energy provision as an afterthought. Current approaches overemphasise technology readiness while neglecting the complexity of aligning supply chains, operational diversity, and infrastructure deployment. This review reframes IWT decarbonisation as a problem of provision, not propulsion. It calls for demand-led, demand driven, fuel agnostic infrastructure models and proposes a roadmap that integrates technical, operational, and policy considerations. Without rethinking energy access as a core design challenge—on par with vessel systems and regulatory standards—the sector risks investing in stranded assets and missing climate and modal shift targets. Aligning vessel operations with dynamic, scalable energy delivery systems is essential to achieve a commercially viable, fully decarbonised IWT sector.

1. Introduction

Inland waterway transport (IWT) continues to be underutilised despite the proven advantages in energy efficiency and emissions reductions [1]. Historically, inland waterways (IWW) such as the Rhine, Danube, and Thames formed the backbone of regional trade. The expansion of road and rail infrastructure gradually reduced IWT’s dominance, but recent environmental and logistical challenges have prompted a renewed focus on its potential. When compared to other freight modes in like-for-like transportation, IWT produces significantly fewer greenhouse gas (GHG) emissions per tonne-kilometre (TKM), offering a viable method for freight movement that could support decarbonisation of the transport sector as a whole [1,2].

The urgency of decarbonisation in the transport sector is well-established. Transport accounts for more than 25% of the United Kingdom’s (UK) total GHG emissions and up to 30% globally which is discussed further in Section 3 in the case for decarbonising the IWWs [3,4]. As urbanisation increases and governments promote modal shift strategies to alleviate road congestion and lower road-based transport emissions, IWT is positioned as a key enabler of climate goals. However, realising that potential requires more than a modal substitution, it demands an infrastructure and operational ecosystem fit for zero-emission service. The European Union’s (EU) Green Deal and the NAIADES III Action Plan both explicitly endorse IWT as a low-emission alternative and support its integration into broader sustainable logistics frameworks [5,6].

However, there is a constant disconnect between strategic ambitions and real-world capability. Although some pilots have demonstrated feasibility to adopt low- and zero-emission solutions, deployment remains highly limited and tied to subsidised or short-distance routes. The underlying issue is not technological availability, but energy accessibility. Without a reliable, scalable, and flexible energy supply infrastructure, the potential for zero-emission IWT will remain unrealised [7,8]. This is explored in detail in Section 5 on infrastructure gaps.

Much of the literature reviewed as part of this study has focused on the development of new propulsion technologies, emission modelling, vessel optimisation, or lifecycle assessments of fuels and future energies [9,10]. This is not just because these were the active search terms, but because the academic focus is to address the symptoms and not the root cause. These are necessary contributions, but they often exist in silos and do not consider the holistic context of the complex environment of the IWWs. Such studies often assume that alternative energies are readily available along operational routes and do not consider the energy supply at all. In practice, energy infrastructure across Europe’s IWWs is fragmented, inconsistent, and underdeveloped [11,12]. Section 6 develops this further by showing how fleet diversity amplifies these shortcomings. Where electrification is deployed, it is restricted to major urban ports, and hydrogen refuelling remains experimental and geographically sparse. For operators with variable voyage patterns or extended duty cycles, fixed infrastructure will be insufficient in isolation; other solutions are needed.

This creates a bottleneck that undermines the wider decarbonisation agenda. While national and EU strategies promote modal shift, little attention has been paid to ensuring that the IWT sector can actually support zero-emission operations at scale [4,5]. Shifting freight from diesel trucks to diesel barges is simply emissions displacement. To realise the climate benefits of IWT, the sector needs to be supported by a coherent, route-adaptable energy delivery system that matches the flexibility and variability of vessel operations; the strategic implications of this requirement are synthesised in Section 7. The misalignment between propulsion technology and the energy provision required to support it is a recurring theme throughout the literature. However, it remains under-analysed and poorly addressed in current infrastructure and policy frameworks.

This paper reviews recent research across propulsion, infrastructure, operational realities, and policy to assess the systemic enablers and barriers to IWW decarbonisation. Beyond energy access, decarbonization is further constrained by slow infrastructure deployment, fragmented governance, and regulatory inconsistency [12,13,14].

The argument advanced in this paper is that current decarbonisation efforts in IWT are misaligned. While propulsion technologies have received considerable attention, the logistical realities of energy access remain underexplored. Without a robust and adaptive energy delivery system—capable of supporting real-world operations—the transition to zero-emission IWT cannot, realistically, be achieved. This review challenges the current direction of IWT research by arguing that further propulsion or optimisation studies, while valuable, are insufficient on their own as discussed further in Section 7. Instead, the contribution from this review is to consolidate existing knowledge, expose systemic limitations, and advance a new research and policy direction focused on provision rather than propulsion.

Recognising the limitations, this review sets out three objectives designed to realign IWT decarbonization research and policy with the realities of energy provision. Accordingly, these three main objectives are as follows:

• To synthesise recent academic and grey literature on IWT decarbonization, with a particular emphasis on the reciprocity between propulsion technologies, infrastructure readiness, and operational realities.
• To identify systemic gaps and misalignments that restrict the transition to zero-emission inland navigation, particularly those relating to energy demand and provision.
• To re-frame the decarbonization challenges as an energy provision problem rather than solely a propulsion technology issue, and to propose a research and policy agenda aligned with this perspective.

The paper’s structure reflects these aims. Section 3 establishes the case for the decarbonization of IWW’s in terms of emissions and modal shift potential. Section 4 reviews the technological landscape of fuels, storage, and propulsion. Section 5 examines the state of energy infrastructure and its barriers, while Section 6 analyses vessel heterogeneity and operational patterns to highlight how they shape energy demand. Section 7 synthesises these insights into a system-level discussion, and Section 8 concludes with a high-level strategic roadmap for research and policy. This progression, moving from context through technologies and infrastructure, to synthesis and recommendations, ensures that the analysis remains both comprehensive and logically connected to the papers overarching objectives.

2. Methodology

This review applies a structured synthesis of academic and grey literature to assess the state of decarbonisation efforts across IWT, with particular emphasis on energy provision and infrastructure readiness. The objective was not to evaluate propulsion technologies in isolation but to examine systemic constraints affecting the feasibility of zero-emission operations.

The initial literature pool was identified through Scopus, Web of Science, and Google Scholar, using combinations of terms such as “inland waterway transport,” “zero-emission vessels,” “alternative fuels,” “shore power,” “hydrogen bunkering,” and “electric propulsion.” Boolean operators and iterative queries were applied to these searches to broaden the results. Supplementary sources were identified using www.Litmaps.com, an AI literature researching tool, to trace citation networks and capture studies outside narrow keyword searches.

Eligibility criteria, as seen in

Table 1. Literature review research eligibility criteria for primary document capture and use.

Criteria Category	Eligibility Requirement
Publication date	Articles published from 2015 onwards to ensure relevant and recent technological, regulatory, and environmental developments.
Language	Articles published in English (or high-quality and translatable) to maintain consistency and allow thorough analysis.
Geographical Focus	Studies focusing primarily on European Inland Waterways, particularly the Thames, Danube, and Rhine, but also broader EU and international work, policies, and regulations such as China for example.
Sector Relevance	Research specific to Inland Waterways and Inland Waterway Transportation, vessel technology, energy systems, decarbonisation strategies, and maritime environmental impacts.
Sector Crossover	Research related to relevant technologies, energy systems, infrastructures, decarbonisation strategies, and environmental impacts, from alternative sectors that have relevance and crossover.
Type of Publication	Peer-reviewed journal articles, official reports (gov, industry, etc.), white papers, policy documents, and conference papers as well as high-quality data on respected websites.
Data and Evidence Quality	Studies providing empirical data, quantitative analysis, case studies or robust modelling related to IWW emissions, energy use or policy outcomes.
Thematic Focus	Articles addressing key themes such as energy efficiency, emissions reductions, vessel zero-emission technology, alternative fuels, economic feasibility, and regulatory frameworks.
Source Credibility	Publications from reputable academic journals, industry organisations, government bodies, and recognised research institutes.

, were applied across date, language, geographical focus, thematic scope, and source credibility. Both academic and grey literature, were included such as those from Central Commission for the Navigation of the Rhine (CCNR), the European Commission, the DNV, the Port of London Authority (PLA), and Zero Emission Services (ZES) where they provided empirical data, case studies, or insights into infrastructure deployment that were lacking in academic studies.

From an initial pool of ~290 documents, 111 were initially retained (see Figure 1), reducing further to 96 documents in final editing. This reduction reflects the refinement of sources as the manuscript developed, with only those contributing directly to the analysis included in the final bibliography. The literature was categorised thematically—emissions and climate impact, propulsion technologies, infrastructure availability, vessel operations, and governance frameworks—allowing overlaps to be cross-tagged for analysis. These categories also structure the review that follows as seen in Section 3, Section 4, Section 5 and Section 6.

Although systematic in its approach, this review does not follow formal systematic review protocols such as PRISMA. Given the heterogeneity of the field, a narrative synthesis was considered most appropriate to identify interconnections, gaps, and misalignments across technological, infrastructural, and policy domains—gaps which are later linked in the roadmap in Section 8.

3. The Case for Decarbonising Inland Waterway Transport

IWT is often portrayed as an environmentally efficient freight mode. As shown in

Table 2. GHG emissions across the major modes of freight transportation. Data taken from [16].

Mode of Transport	Average gCO2e/TKM
Air Freight	~280–500
Road Freight	~80–120
Rail Freight	~15–25
Inland Waterways	~20–40
Maritime (Ocean)	~5–15

, IWT emits significantly fewer GHG emissions per TKM than road freight, and are broadly comparable to rail [14,15]. This perception masks the sectors near-total reliance on fossil fuels for propulsion and all onboard power systems. The majority of vessels in operation today across the European IWWs are not only diesel-powered but rely on ageing engine technologies that pre-date modern emissions standards and lack basic after-treatment systems. These legacy systems are examined further in Section 6, where the age profile and heterogeneity of the fleet are shown to be a critical barrier for infrastructure compatibility.

As a result, the sector contributes significant levels of carbon dioxide (CO₂), nitrogen oxides (NO_x), and particulate matter (PM) to the maritime sector’s overall emissions, particularly in regions where IWWs intersect with urban environments [6,11,17,18]. Figure 2 illustrates this in context: while IWW navigation contributes only a small share of European transport emissions, it forms a substantial portion of maritime emissions. These proportional contributions clarify IWT’s relative efficiency compared to road freight but also its critical role within maritime decarbonisation strategies.

This underscores the need to decarbonise IWT on two fronts: the alignment with national and international climate obligations, and maintaining viability under tightening regulations [4,5,18,19]. A shift of freight to the IWWs cannot be credibly pursued as a low-emissions solution without a corresponding transition to low- and zero-emission propulsion technologies and energy infrastructure, even with its higher efficiency operations [19]. The environmental benefits commonly attributed to IWT are not essential to the mode, but conditional upon the decarbonisation of its operational base. This section reviews the case for action by examining the limits of current assumptions around environmental performance and the implications of maintaining the status quo.

3.1. The Underlying Problem: Technical and Structural Barriers

The IWW fleet in Europe and the UK is predominantly composed of vessels exceeding 30 years of operational life as discussed in more detail in Section 6. Many of these vessels were built before the introduction of modern emission regulations or any requirement for fuel efficiency optimisation. This legacy infrastructure creates a persistent barrier to decarbonisation, which will continue into the foreseeable future as the average lifetime of a vessel operating across European IWWs exceeds 40 years [21]. This entrenched longevity means most of today’s fleet will remain operational through near- and mid-term climate target periods creating a structural decarbonisation barrier. While the regulatory environment has begun to shift, for instance, with the introduction of Stage V emission standards for new inland engines—such measures do not apply retroactively, leaving the legacy fleet largely unaffected and unmonitored in terms of real-time emissions performance [11,20]. This is discussed further in Section 6, where the age profile of the fleet highlights the need for retrofit pathways and improved monitoring, and reinforced in the short-term roadmap in Section 8, which positions retrofits as an essential first step in bridging the gap between legacy vessels and future zero-emission operations.

Operationally, many vessels are overpowered relative to their typical duty cycles, especially during low-speed manoeuvring or during long idle periods at ports and locks. Without hybrid systems or shore power capabilities that can connect to a renewable energy source, this will result in unnecessary energy consumption and fuel use, particularly in ports without electrified infrastructure where hotel loads could, potentially, be supported from the grid. Furthermore, emission details in the existing literature are often based on steady-state engine test cycles, whereas real-world operations involve frequent transitions, variable load, and suboptimal flow conditions, none of which are captured in regulatory test cycles [22]. This disconnect undermines the accuracy of emission baselines which are essential for credible planning and evaluating retrofits or fuel-switching strategies.

3.2. Emission Efficiency vs. Operational Reality

Comparative emissions data underpin arguments for modal shift, with studies showing that IWT emits less CO₂/TKM than road freight and is comparable to rail freight under standardised conditions; see Figure 3 [1,14]. However, these figures are typically based on idealised conditions that do not account for route complexity, vessel condition or variable operating patterns. Therefore, the reliability of these comparisons diminishes considerably when applied to real-world operations. The assumptions used today in underpinning emissions factors used in policy and planning models are often drawn from laboratory or simulation-based conditions that overlook key variables present in day-to-day IWT.

Across the literature, there is a divergence between theoretical and observed performance. For example, the PLA notes that many vessels on the Thames demonstrate emissions and energy profiles far removed from the averages assumed in transport models [21,23,24]. These discrepancies stem from not only ageing propulsion systems, but also from operational behaviours, such as long idle times, engine over-sizing, and frequent manoeuvring in congested or tidal environments. Likewise, refs. [16,20] caution that aggregated emissions metrics obscure substantial variation across vessel types and duty cycles, raising doubts about the reliability of IWT carbon accounting in national inventories. Such inventories, often shaped by broader maritime frameworks like those form the International Maritime Organization (IMO), risk conflicting with the original regulatory structures that govern many IWW regions and may diverge from international strategies on GHG reduction [16,20,25].

This disconnect is further amplified by the lack of robust data on actual vessel performance. This is particularly problematic in assessing urban freight decarbonisation where small vessels often operate at lower efficiency and with higher emissions intensity. As the CCNR report shows, existing emissions reporting systems lack granularity and often exclude smaller vessels, which constitute a significant portion of the urban and regional IWW fleet [11]. As a consequence, claims of modal superiority often rest on partial data, modelled assumptions, and long-outdated baselines. In many cases, they reflect the theoretical efficiency of a fully loaded vessel under optimal conditions, rather than the average emissions intensity per journey or per operational hour. This is opposed to an ideal aggregated account across a relevant operational period such as a year. This would also capture seasonal and cyclical operational fluctuations. As such, an annualised, operationally weighted emissions profile would offer a far more reliable baseline for comparing modes and guiding investment.

Even where real-world data exists, the interpretation can be misleading. For example, ref. [26] identifies how simplified voyage assumptions—such as straight-line routing or constant speeds—can misrepresent the energy demands of actual IWW operations, particularly on narrower, shallower, or tidal waterways. These distortions not only affect emissions modelling but also undermine infrastructure planning, where refuelling and power demands may be under- or overestimated. This is especially true for infrastructure such as charging or refuelling points, where siting and sizing decisions must reflect peak and cumulative demand patterns rather than average assumptions [27]. These infrastructure mismatches are addressed in Section 5 which highlights the inadequacy of fixed, location-bound supply models.

Taken together, the literature reveals that while IWT holds significant efficiency potential, this potential is not automatically realised. Emissions performance is highly context-dependent and requires modernisation across both the vessel and infrastructure layers to align with the claims frequently made in support of a modal shift. Without this, there is a risk that policy reliance on IWT as a decarbonisation pathway will be based more on assumed benefits rather than verified outcomes.

3.3. Modal Shift Potential vs. Reality

Modal shift towards IWT is a recurring theme in national and regional transport decarbonisation strategies. Its strategic appeal lies in the assumption that existing infrastructure and underused capacity can be leveraged without requiring fundamental system changes. It is frequently promoted as a low-hanging fruit solution for reducing freight emissions, especially in urban centres and high-density corridors where river infrastructure already exists, such as London for example [20,28]. However, the literature presents a far more cautious and fragmented picture of the extent to which this shift can be delivered in practice, evident through the limited number of studies and the lack of solid empirical data assessing whether the shift would deliver the environmental benefits often assumed. Figure 4 shows how uneven IWT uptake remains. While the Netherlands, Romania, and Bulgaria show relatively high modal shares for IWT, most European states, including the UK, fall well below the EU average, with IWT rarely exceeding 10% total freight movement.

In [28], it is pointed out that while IWT is nominally efficient in terms of energy consumption and emissions per TKM, its competitiveness is often limited by infrastructural constraints, network fragmentation, and slow transit times, and that is aside from any additional time constraints when switching from one mode to another. These limitations confine IWT to specific geographies and cargo types. This also limits its ability to function as a widespread replacement for road freight particularly in countries where river access and navigable length are constrained. In the UK, the PLA highlights that although modal shift initiatives have delivered localised success, such as in construction logistics, these remain the exception rather than the rule [23]. Volumes of freight moved by water remain a fraction of total movement and are typically dependent on tailored, high-investment interventions that may not scale nationally. Even successful low-emission projects rely on bespoke regulation, stakeholder alignment, and dedicated infrastructure, which are difficult to replicate at scale.

From a systems perspective, the expectation that IWT will simply absorb road freight volumes overlooks critical constraints in IWW infrastructure. Critical bottlenecks include the lack of intermodal hubs that enable seamless transfer between truck, rail, and vessel, as well as inconsistent standards for energy infrastructure at ports. This includes energy infrastructure, vessel availability, and regulatory alignment and almost completely ignores the critical infrastructure required to facilitate simple freight mode transference. The European Commission acknowledges this across its Fit for 55 documentations, which recognises that significant investment will be required in alternative fuels, portside energy provision, and operational coordination before modal shift can yield consistent carbon reductions [29]. Without such interventions, shifting cargo to IWT will merely relocate emissions rather than reduce them, especially if the receiving fleet is fossil-fuelled and supported by a carbon-intensive infrastructure.

There is also a tendency in policy literature to treat IWT as a monolithic mode with uniform emissions performance, when in fact it spans a wide spectrum of vessel sizes, power systems, and operational behaviours. In [26,30], it is stressed that whilst some urban river services have demonstrated high energy efficiency with diesel propulsion systems, switching to alternative energy sources such as biofuels, including HVO, batteries, hydrogen, methanol, or ammonia can substantially improve emissions performance in some routes; other routes have limited benefit when upstream emissions and real-world inefficiencies are considered. Biofuels and HVO may deliver transitional benefits but face availability, cost, and lifecycle sustainability concerns, especially when scaled across an entire fleet. Considering this, modal shift, without the accompanying structural reform, risks becoming a symbolic gesture rather than a meaningful decarbonisation pathway.

The literature therefore makes clear that IWT’s decarbonisation potential is conditional, not inherent. Any assumption that modal shift will automatically translate to lower emissions risks misdirecting investment and delaying meaningful change. Modal shift will only deliver meaningful emissions reductions if paired with investment in fleet modernization, energy infrastructure, and digital systems, as treating in isolation risks overstating its impact and understating the scale of reform required. These conditions form the backbone of the phased roadmap in Section 8, where short-, medium-, and long-term interventions are structured to prevent exactly this kind of misalignment.

3.4. Summary of Structural Dependencies and Constraints

The literature consistently highlights a gap between policy ambitions and operational realities. Assumptions about IWTs efficiency are often based on idealised conditions, overlooking vessel diversity, variable duty cycles, and region-specific constraints. As a result, claims of environmental superiority risk exaggeration unless supported by modernised fleets, robust infrastructure, and data that reflects real-world performance.

The emissions profile of IWT is not a fixed but contingent on variables including propulsion system arrangements, vessel age, duty cycle, and infrastructure access. The continued use of ageing diesel-powered vessels, the lack of meaningful retrofitting solutions, and limited shore-side electrification and alternative fuel hubs, undermine the validity of emissions comparisons based on TKM metrics alone [11,23]. Even updated emission factors are rarely disaggregated by vessel type or operational mode, masking the variability and reinforcing a false sense of sector-wide performance. Furthermore, methodological inconsistencies in how emissions are measured, especially reliance on steady-state test cycles and manufacturer data, limit the ability to draw generalised conclusions about the sector’s climate performance, and can leave vessel operators without credible guidance on which technologies or fuels are more suitable for their operational conditions [23].

Policy frameworks often assume that modal shift to IWT will deliver net emissions reductions, yet this presupposes that IWT operations are already decarbonised or at the very least capable of a rapid transition. This disconnect is particularly problematic in urban and regional contexts where infrastructure adaptation timelines often lag behind policy ambitions. However, in practice, modal shift without propulsion and infrastructure reform simply shifts emissions to a different domain. As reported in [31], the substitution effect is often overestimated, while the structural barriers to rapid scaling—fragmented governance, limited public investment, and low digitalisation—are consistently under-acknowledged.

These findings point to a fundamental issue: decarbonising IWT is not simply a matter of fuel, propulsion or vessel choice. It involves confronting systemic challenges across energy provision, regulatory alignment, asset lifecycle management, and operational logistics. Without this broader systems view, there is a risk that IWT will remain trapped in a paradox—widely endorsed as a clean freight alternative, yet structurally unequipped to deliver those benefits at scale, and the longer delays persist, the more exposed IWT will become as other transport sectors demonstrate faster decarbonization progress. This disconnect is likely to deepen as other freight modes such as electric trucking or hydrogen rail, for example, continue to receive targeted decarbonisation support and outpace IWT in demonstratable emissions reductions [32]. Section 7 returns to this systemic perspective, arguing that only demand-led, provision-focused strategies can close these gaps.

4. Technological Landscape: Fuels, Storage and Propulsion

While decarbonisation of the IWW sector is frequently framed as a question of replacing diesel propulsion systems with cleaner alternatives, the literature reveals a fragmented and inconsistent evidence based on energy feasibility [33,34,35]. This compounded by the absence of harmonised terminology and comparative metrics makes it difficult to evaluate scalability between fuel types or propulsion strategies [36]. A range of propulsion and fuel technologies have been discussed, including battery–electric systems, hydrogen, and various low-carbon fuels, but each presents significant trade-offs related to energy density, vessel architecture, and operational compatibility [12,36,37].

Much of the literature concentrates on technological potential or isolated pilot deployments, with relatively few sources addressing full-system integration or the infrastructure dependencies required to support sustained, multi-vessel operations [20,28,38,39,40,41]. This mirrors the wider problem highlighted in Section 3, where efficiency claims rest on idealised conditions rather than real-world operability. Where integration is considered, it is often limited to specific case studies within controlled environments, which may not reflect the operational variability found across wider inland waterway networks. The following subsections examine the technological landscape as reflected in the existing literature, with specific attention to the physical, operational, and logistical characteristics of each option. Throughout, emphasis is placed on highlighting the extent to which these technologies are discussed in practical terms, rather than as abstract decarbonisation solutions.

4.1. Electrification and Battery Systems

Battery–electric propulsion is often positioned in the literature as one of the most promising options for decarbonising IWT, particularly for short-range operations on defined routes with frequent port access [33,35,38]. The primary advantage of electric propulsion lies in its ability to eliminate point-source emissions entirely, enabling zero-emission operations within urban areas and along sensitive environmental waterways. However, despite its technological maturity in other transport sectors, electrification of IWVs remains relatively limited in scope and application, constrained by both energy density limitations and infrastructure requirements [12,39,40].

The literature identifies several pilot projects where battery–electric propulsion has been trialled successfully, such as the Ampere ferry in Norway and the ZES model in the Netherlands, which employs a containerised battery swapping system specifically designed for IWW freight vessels [41]. These examples demonstrate technical feasibility and provide a basis for further development, particularly in geographies with high renewable energy to grid availability and centralised freight corridors such as those found along the Rhine, Albert canal or key logistics routes in the Netherlands [34,42]. However, they also highlight key limitations; most notably, such systems rely on fixed, high-capacity shore power infrastructure and standardised vessel–port interfaces. These conditions today are rarely met across the broader IWW network [7,41,43].

From a vessel design perspective, battery–electric systems introduce substantial trade-offs in displacement, range, energy-to-weight ratios, and available payload space. Battery energy density, while improving incrementally, remains significantly lower than that of diesel [44,45]. This constraint is particularly pronounced in retrofitting scenarios, where hull architecture and onboard systems are not optimised for modular battery installations in consideration of current operational profiles [6,15,20,24].

Battery systems must accommodate both average voyage energy use and infrequent peak loads [28,35,36]. For example, during upstream transits against current or prolonged manoeuvring, energy demand can exceed baseline consumption by 50% [22]. Without hybridisation or an auxiliary power system, premature depletion risks undermining a vessel’s operational reliability—an issue that Section 6 shows is particularly acute given the variability of vessel duty cycles. This is compounded by the absence of charging infrastructure across the vast majority of the UK and European IWW network, which limits the viability of pure electric solutions across multiple geographical contexts [20,45,46].

It is also noted that there is limited standardisation of battery technologies, connections, communication protocols, and vessel integration strategies across multiple jurisdictions [42,47,48,49]. Without harmonised technical and regulatory frameworks, the scalability of battery–electric propulsion remains constrained, even where vessel types and duty cycles are well-suited to electrification. This is particularly evident in urban logistics or passenger services where stop-start conditions are predictable and energy demand is more easily forecasted. This raises broader questions about the replicability of current pilot projects and the extent to which they represent scalable pathways for decarbonisation at a vessel fleet level.

4.2. Hydrogen Propulsion Systems

Hydrogen is widely seen as a viable zero-emission alternative for IWW applications, particularly where direct electrification is constrained by range or a vessel’s duty cycle [32,43,50,51]. In [31], it is suggested that hydrogen offers high gravimetric density but requires complex storage and handling, meaning that whilst hydrogen fuel cell systems can theoretically offer a longer range than battery–electric propulsion, they introduce significant technical and operational challenges [31,50,51]. The literature that focuses on hydrogen as a fuel of choice fails to consider the complexities of hydrogen storage and delivery across the grid nor does it consider the need for renewable energy integration to make it truly zero-carbon; it rather assumes it by calling on the use of green hydrogen [51]. The complexities facing shore-based energy infrastructure for viable hydrogen storage and delivery will pass to the maritime industry with increased complexities when moving the energy across the water. This means that the challenges of developing hydrogen storage and delivery structures for on-land applications, such as in the automotive industry, will not simply transfer to IWW applications; they will be compounded by additional maritime-specific constraints including dockside land and ownership limitations, safety requirements, and variable operational profiles.

In [27], hydrogen is identified as a technically promising fuel, yet one that remains in the early stages of adoption within IWT. Although several pilot project vessels have demonstrated feasibility—including the H₂ Barge project, a hydrogen-powered barge operating under controlled, short-range conditions—these remain exceptions rather than indicators of scalable deployment [36]. Report [27] goes on to note that practical applications of hydrogen are constrained by the absence of a mature bunkering infrastructure along the majority of European IWWs, and there continue to be unresolved questions concerning fuel availability, handling safety, and regulatory harmonisation [20,23,24,52].

From a vessel integration perspective, hydrogen propulsion is best suited to newly built vessels where storage tanks, fuel cells, and safety systems can be designed into the hull architecture from the outset [50,52,53]. Retrofitting is rarely viable except on larger vessels, echoing the retrofit barriers noted in Section 3 and later discussed in Section 7.5. This is due to space constraints and the need for extensive modification to meet safety and weight distribution requirements. Even where hydrogen is technically feasible, such projects are often considered economically unjustifiable given the remaining lifespan of legacy vessels and the uncertainty of long-term hydrogen pricing [24,52,54].

In [31], it is stressed that hydrogen’s comparative advantage must be evaluated not only in terms of operational emissions but also upstream production pathways. In contexts where hydrogen is derived from fossil fuels—via steam reforming of methane for example—the net carbon savings can be marginal unless carbon capture or renewable electrolysis is employed; however, the latter remains cost-prohibitive in many regions [55]. Consequently, the viability of hydrogen as a decarbonisation solution for IWT is tightly coupled to developments in the wider energy system and is dependent on the decarbonisation of hydrogen production itself [43,56].

Despite these constraints, hydrogen is identified across the literature as a key component of future fuel strategies—particularly for long-range, high-demand vessels operating outside the envelope of battery–electric feasibility. However, the consensus view across sources remains cautious, with hydrogen seen not as a universal solution but as a context-specific option requiring parallel infrastructure and regulatory development to become viable at scale.

4.3. Alternative Fuels: Ammonia Methanol and Biofuels

While most of the reviewed literature on IWW decarbonisation focuses on battery–electric and hydrogen-based propulsion, several alternative fuels are also spoken about—notably ammonia, methanol, and various biofuel derivatives [12,57,58,59]. These fuels are more widely considered across larger sectors in the maritime environment, and their application to inland contexts remains largely speculative, or at best underexplored in the academic and technical literature reviewed for this study.

Ammonia and methanol are often cited as long-term options due to their higher volumetric energy densities when compared with hydrogen and batteries [32,33,58]. Yet both present unresolved safety, handling, and emissions challenges. Ammonia’s toxicity and immature fuel cell technology limit its near-term IWT potential. Methanol, while easier to handle and already produced at scale, suffers from lower energy density than diesel and often relies on fossil feedstocks unless synthesised using renewable energy inputs—a process not yet commercially viable at scale [9,60]. Neither fuel is discussed in detail in the context of IWT applications in the reviewed studies, indicating a notable absence of peer-reviewed feasibility assessments or pilot projects specific to IWVs.

Similarly, biofuels are occasionally referenced in policy or modelling contexts as transitionary fuels for retrofitting legacy diesel engines [28,54,57]. However, the literature offers little consistency regarding the classification of biofuels, their lifecycle emissions under real operating conditions, or their compatibility with the wide variety of vessel types operating on the IWWs. Biofuels are noted as transitional options, but lifecycle performance and compatibility remain inconsistent [25,54,57].

One exception that is receiving increased attention in industry-led trials is HVO, a renewable drop-in fuel that can be used in existing diesel engines with minimal or no modifications. HVO could offer short-term benefits by reducing PM and NO_x emissions while avoiding the immediate need for vessel retrofitting, making it particularly attractive for operators with ageing fleets [61]. However, while HVO can lower lifecycle greenhouse gas emissions compared to conventional diesel, its performance is highly dependent on feedstock and supply chain conditions [62]. Questions remain about feedstock sustainability, indirect land use impacts, and long-term availability at scale—particularly as other sectors compete for the same supply. As such, HVO is best understood as a transitional solution rather than a zero-emission pathway, offering a pragmatic stopgap for certain vessels and regions, but unlikely to support deep decarbonisation without parallel progress in electrification or hydrogen systems.

The limited engagement with these alternative fuels underscores a broader trend in the literature: a disproportionate emphasis on technical viability in idealised conditions, with insufficient attention to operational realities, safety constraints, or infrastructure demands. This reinforces the systemic critique developed in Section 6 and directly informs the call in Section 7 for demand-led, not technology-led, pathways.

Table 3. GHG emissions across the major modes of freight transportation. Data taken from [12,17,29].

Energy Carrier	Energy Density	Maturity (IWT)	Advantages	Limitations	Infrastructure
Battery–electric	~0.25 MJ/L	Early pilots (e.g., Ampere)	Zero emissions at point of use; high efficiency.	Very low energy density; weight/space penalties; range limits	High-capacity charging; standardised interfaces
Hydrogen (compressed)	~4–8 MJ/L (350–700 bar)	Early pilots (e.g., ZES H2Barge)	High gravimetric density; suited to longer ranges	Low volumetric density; storage complexity; safety	Bunkering infrastructure; renewable production pathways
Methanol	~15.8 MJ/L	Limited trials (deep sea focus)	Easier storage/handling; existing engine retrofits	Fossil feedstock risk; lower energy density	Dedicated bunkering; dual-fuel engines
Ammonia	~11.5 MJ/L	Concept/Pilot phase (Thames projects)	No CO2 at point of use; scalable production potential	Toxicity; cracking tech immature; urban safety concerns	Specialised storage and handling; fuel cell/Engine development
HVO/Biofuels	~33 MJ/L	Already deployable (drop-in)	Retrofit ready; reduced PM/NOx	Lifecycle emissions uncertain; feedstock limits	Minimal (existing diesel infrastructure)

summarises the comparative characteristics of the principal alternative fuels discussed, highlighting the trade-offs in energy density, infrastructure requirements, and deployment readiness.

4.4. Energy Densities and Storage Trade-Offs

The choice of alternative fuel for IWT is not just a question of emissions reduction potential, but of physical feasibility and spatial compatibility [58,61,62]. A critical parameter in this context is energy density—the amount of energy stored per unit volume or mass—which fundamentally shapes the propulsion system design, storage integration, operational range, and the refuelling requirements of vessels operating in confined inland environments. For example, typical lithium-ion batteries store approximately 0.25 MJ/L compared to marine diesel’s ~35.8 MJ/L, and so it is easy to see the issue when framed in this way.

As noted above, the volumetric and gravimetric energy densities of batteries, hydrogen, ammonia, and methanol are all significantly lower than diesel. This creates a trade-off between ambition and practical operability. Section 6 will show how this interacts with vessel heterogeneity, reinforcing why fuel substitution alone cannot deliver fleet-wide decarbonisation. In [63], a comparative overview of marine fuel energy densities is presented, highlighting that compressed hydrogen at 350–700 bar, while promising zero-emission operation at point of use, delivers less than 5% of diesel’s energy density when compressed at 350 bar [64]. As a result, it imposes substantial spatial demands that may not be feasible for retrofitted vessels or those with limited underdeck storage capacity, such as ferries, pilot craft or workboats, or barge-based vessels such as dredgers.

Similarly, ref. [26] emphasises that while battery–electric systems have demonstrated high operational efficiency and zero ‘tank to wake’ emissions, their low energy density necessitates frequent charging or a reduced vessel range, or worse, still both, unless oversized battery banks are employed [26,36,43]. This introduces trade-offs not just in weight and displacement, but also in usable payload and vessel stability. Moreover, ref. [26] also warns that these trade-offs are route dependent. Vessels operating on short shuttle services, such as cross-river ferries, with known shore power access points may be well-suited to battery operation, but long-haul or irregular routes face significant limitations without dedicated frequent shore power charging stations or some kind of dynamic charging solution. The lack of such options will impose restrictions on voyage scheduling and disrupt turnaround time, which could impact operational viability.

Hydrogen-based solutions are often proposed as an intermediate between battery–electric systems and fossil fuels. However, as [58] notes, compressed hydrogen systems typically require external tank volumes up to four times greater than diesel for the equivalent range, alongside complex handling and safety systems—including high-pressure containment, ventilation, and fire suppression systems [65]. Methanol and ammonia offer slightly improved volumetric densities and simpler storage conditions compared to hydrogen, but their lower energy content in comparison to diesel still requires upsized storage tanks, and their emissions profiles are not entirely carbon-neutral unless derived from renewable energies [63,64].

Another key consideration highlighted by [31] is the system integration complexity introduced by alternative fuels. Unlike diesel, which benefits from mature bunkering, handling, and propulsion systems, fuels such as ammonia or hydrogen often require bespoke infrastructure, fuel cell systems, or dual-fuel engines. This not only raises capital and operational expenditure, but also introduces technical risk, especially for vessel operators unfamiliar with emerging fuel chemistries and without access to formal crew training pathways for these technologies [31,66].

These factors are acute in IWVs, where vessel diversity and operational flexibility limit the applicability of standardised retrofitting solutions. As noted in multiple studies across the reviewed literature, space constraints, stability requirements, and voyage profiles all restrict the replacement of diesel power trains with low-density alternatives without a parallel transformation in bunkering infrastructure and route planning [58,61,62,65].

Figure 5 summarises the relative technology readiness levels and applicability of different alternative fuels to IWT, highlighting both the diversity of technological maturity and the practical limitations of deployment across the inland fleet. The implications of these energy density trade-offs are twofold. First, they challenge the assumption that fuel substitution alone is a sufficient decarbonisation pathway for IWT. Second, they underline the need for system-level solutions, where energy infrastructure, propulsion technology, and operational models are co-designed to account for the unique limitations of the inland environment.

Innovation in propulsion and vessel optimisation is essential, but adoption of alternative fuels remains constrained by energy density trade-offs, upstream emissions, and infrastructure gaps. Demonstrator projects, though visible, remain context-specific and difficult to scale across the diverse inland fleet, underlining that without integrated infrastructure planning, as seen in Section 5, fuel substitution will remain insufficient.

5. Infrastructure Gap—Barriers to Scalable Energy Provision

A persistent limitation across the literature is the gap between zero-emission ambitions and the availability of infrastructure capable of delivering clean energy at the point of use [31,50,62,63,64]. This gap spans not only shore-side charging and bunkering facilities but also upstream enablers such as grid capacity, distribution infrastructure, and reliable renewable energy inputs.

While propulsion systems—particularly electric, hybrid, and hydrogen-based solutions—have advanced, the literature has not kept pace [6,44,63]. This mismatch echoes the earlier critique in Section 4: technological feasibility is discussed in isolation, while the enabling infrastructure remains underdeveloped. The deployment of fixed energy supply points, such as shoreside electrical charging or hydrogen bunkering facilities, remains fragmented and geographically limited. As summarised in

Table 4. Current availability of alternative energy infrastructure for inland vessels in Europe, showing limited geographic concentration and unresolved barriers. Data taken from [11,23,24,32,33,67].

Technology	Operational Sites (Approx.)	Geographic Concentration	Key Limitations
Shore power (low-capacity/leisure-focused)	>100’s (but mainly for leisure)	Widely dispersed, mostly small marinas in UK, NL, DE etc.	Low-voltage, not suitable for commercial freight; grid constraints; fragmented ownership
Shore power (high-capacity/commercial)	<20	Concentrated in major ports (e.g., Rotterdam, Duisburg, London, etc.)	Limited grid capacity; expensive retrofits; isolated pilots
Hydrogen bunkering/pilots	<7	NL (Rhine corridor) DE, some early-stage pilots on Thames	No mature bunkering network; regulatory uncertainty; high land/space demand
Battery-swap systems	2–4	NL (e.g., ZES, Alphenaar barge)	Context specific; no wider rollout; requires standardised containers
Other alternative fuels (HVO, methanol, etc.)	Limited ad hoc use	NL, DE, UK (trial projects only)	Supply chain sustainability concerns; no dedicated bunkering infrastructure

, current provision is sparse, concentrated around major ports or pilot initiatives, and rarely designed for scalable commercial operations.

In [42], it is emphasised that the absence of scalable, climate-resilient refuelling infrastructure is among one of the most significant barriers to inland vessel fleet decarbonisation, particularly for longer-haul routes and non-urban operations. Even in developed areas such as the UK marina network, provision is often underpowered, inconsistent, and poorly standardised [67]. Access remains geared towards leisure craft, not freight or passenger vessels, and fixed infrastructure is concentrated around a handful of major urban ports [7,27,46,68]. It is further highlighted in [6] that even where infrastructure exists, access is often constrained by grid limitations, funding shortfalls, and a misalignment between vessel operational profiles and the energy types on offer that might suit the vessel in these scenarios. In practical terms, this means that even zero-emission vessels capable of extended service may be forced to operate below capacity or revert to diesel operation due to poor energy access planning.

While hydrogen and ammonia are frequently cited as strategic priorities, as seen in Section 4, ref. [10] argues that refuelling logistics and specialised storage requirements remain unresolved in current infrastructure planning. Pilot projects like the modular battery-swap system demonstrated by [41] on the Alphenaar barge illustrate potential alternatives rather than scalable models. As [6,7] both conclude, the primary challenge is not propulsion readiness, but energy accessibility: zero-emission vessels cannot operate at scale without a reliable, route-flexible energy supply network.

5.1. Current Shore-Based Infrastructure and Its Limitations

The current shore-based provision is limited in both coverage and capability. The literature broadly recognises that the existing energy supply landscape is not fit for scaling zero-emission operations [42,67,69]. This is compounded by inconsistent policy incentives and fragmented regulatory authority, which vary not only by country but also by region within national IWW systems. In [70], it is noted that although port electrification is a key enabler of maritime decarbonisation, its deployment has been slow, fragmented, and largely confined to isolated pilot projects or capital city terminals in IWW contexts. Furthermore, the slow pace of electrification is often linked to the absence of dedicated funding mechanisms for IWT-specific energy infrastructure, unlike deep sea or urban transport initiatives which have clearer decarbonization pathways.

In some cases, marine charging infrastructure has been installed through leisure sector funding, with little consideration for commercial vessel power requirements. Grid constraints also feature prominently: even where charging points exist, limited upstream capacity restricts throughput [6,34,39]. As [7] observes, deploying infrastructure without grid reinforcement risks creating stranded assets with limited operational value. This misalignment is illustrated in Figure 6, which shows that most current provision remains geared towards leisure craft rather than freight or passenger vessels.

In Europe, the infrastructure situation is similarly unbalanced. Even along major corridors like the Rhine or Danube, most ports remain tied to diesel, with few alternative fuels or charging. In [26], in their assessment of IWT decarbonisation across the Danube, the author observes that despite support for alternative fuels across Europe, physical bunkering or recharging facilities for zero-emission vessels remains rare with diesel infrastructure dominating. While hydrogen and battery pilots exist, their numbers remain in single digits [11] and are dependent on EU innovation grants rather than sustained investment. These pilot projects are yet to demonstrate cost-effective scalability beyond their initial sites. On the Danube, ref. [69] has committed to infrastructure investment under the TEN-T initiative, but actual deployment is lagging and disproportionately focused on road and rail, with IWT receiving minimal share of infrastructure funding. This raises critical questions about the role of IWT in broader modal shift strategies, especially given its marginal position in current EU infrastructure budgeting frameworks.

Another recurring theme is the misalignment between infrastructure planning and vessel diversity. Fixed energy supply points tend to serve predictable, high-frequency services, but IWT vessels often operate on variable schedules and dispersed locations and they may lack the stationary downtime required to replenish energy storage from low-output connections, introducing inefficiencies and added costs [49].

5.2. Case Studies from Major Inland Waterways

The literature highlights a shared challenge across European IWWs: despite political support for decarbonisation and modal shift, energy infrastructure development remains fragmented, inconsistent, and often misaligned with operational realities. Case studies from the Thames, Rhine, and Danube systems offer important insights into the infrastructure gap and its implications for IWV decarbonisation [15,27,68,71,72]. These cases underline the tension between policy rhetoric and practical delivery, with infrastructure gaps reflecting deeper systemic issues such as fragmented funding pathways, national regulatory divergence, and limited commercial incentives.

5.2.1. The River Thames

The River Thames represents one of the more developed UK IWWs, with a mix of commercial and passenger vessel operations concentrated around London. Yet aspirations for zero-emission operations remain disconnected from infrastructure delivery. In [23], opportunities are identified to decarbonise operations on the Thames, particularly given the short, predictable routes of many urban passenger vessels [70]. Furthermore, stakeholder interviews conducted by the PLA highlight additional barriers, including unclear planning permissions, power availability at Riverside properties, and the difficulty in coordinating with multiple landowners across energy deployment zones.

Ambitions for greater shoreside electrification are outlined in [73] as part of the Electric Thames project, but deployment remains slow and confined to pilot-scale interventions that require significant funding alongside fixed infrastructure capacity challenges that are difficult to overcome. While the Electric Thames initiative is promising, its progress underscores the broader challenge of transitioning from demonstration to reliable, network-wide provision, a gap that risks undermining operator confidence. Even frequent-use operators such as Thames Clippers face limited suitability from existing infrastructure [24,70,73,74]. Without a coordinated infrastructure plan that accounts for the full range of vessel types and energy needs across the tidal and non-tidal Thames, efforts remain piecemeal and heavily constrained by port-specific limitations.

5.2.2. The Rhine

The Rhine is one of the busiest IWWs in Europe and is often cited as a testbed for zero-emission technologies [68,71,75,76]. However, infrastructure deployment lags behind policy ambition [77,78]. In [79], it is noted that the Rhine corridor has seen increased attention under the EU’s Green Deal and NAIADES programmes [4,5]. However, ref. [80] points out that across IWT, investment in energy infrastructure has been limited. Hydrogen and battery–electric pilots exist, but permanent infrastructure remains rare and reliant on grant funding [43,71,77]. The dependency on project-based funding models also means the infrastructure rollout is often driven by temporary grant cycles rather than sustained investment strategies.

In [11], it is reported that most ports still rely on conventional diesel fuelling, with minimal provision for future fuels. Even in larger ports like Duisburg, grid capacity and interoperability remain unresolved challenges. According to [80], attempts to scale infrastructure on the Rhine are further hindered by jurisdictional fragmentation, insufficient funding and financing mechanisms, and inconsistent regulatory support across national boundaries. Interoperability challenges and diverse national energy regulations will continue to constrain the rollout of zero-emission infrastructure. Smaller terminals remain excluded from most infrastructure plans [22]. Smaller vessel operators, especially those based in rural or low traffic terminals are particularly disadvantaged with no access to clean fuels reinforcing a reliance on diesel even when vessel technology advances. Furthermore, the Rhine’s highly trafficked nature introduces unique challenges around energy demand peaks, grid capacity constraints, and congestion at ports, which exacerbate the need for flexible, scalable alternatives to static infrastructure.

5.2.3. The Danube

The Danube presents a contrasting picture. While designated a TEN-T corridor, it suffers from chronic underinvestment and fragmented governance across 10+ countries [8,27,68,79]. Reports [5,6] both highlight that the Danube’s decarbonisation potential is severely constrained by limited port infrastructure, inadequate energy supply systems, and poor intermodal integration. The diversity of national regulations along its length compounds these issues, creating logistical and investment barriers [21,80,81].

In addition, vessel movements on the Danube are often seasonal, long-distance, and commercially marginal, which reduces private sector appetite for infrastructure investment [11,82]. Seasonal and marginal traffic reduces investment appetite, highlighting the need for differentiated models. As ref. [44] argues, this can create a cycle of inaction: without predictable energy demand, infrastructure investment is unlikely; without infrastructure, zero-emission operations remain infeasible. The Danube case then reinforces the critical role of coordinated, pan-European planning if IWT is to become a viable decarbonisation pathway [26,69]. An integrated investment and policy framework tailored to the Danube’s specific characteristics, zero-emission transition in this corridor is likely to remain aspirational rather than achievable. The contrasts and commonalities across these three waterways are summarised in

Table 5. Comparison of infrastructure readiness and barriers across three major European IWW’s. Data taken from [5,7,11,17,73].

Waterway	Current Infrastructure	Main Barriers	Progress Pilots
Thames (UK)	Limited fixed charging points (Central London located); minimal capacity for freight vessels.	Land ownership and planning conflicts; grid availability; fragmented stakeholder co-ordination.	Electric Thames Infrastructure project; small-scale pilot; some limited PLA-led electrification initiatives.
Rhine (DE, NL, CH, etc.)	A few hydrogen pilots and battery trials in major ports (e.g., Rotterdam, Duisburg, etc.).	Jurisdictional fragmentation; limited permanent bunkering/charging; funding dependant on temporary grants.	Hydrogen pilot barges; battery–electric demonstrators (ZES containers, etc.).
Danube (multi-country corridor)	Very sparse alternative energy provision; still dominated by diesel efficiency projects.	Underinvestment; fragmented governance across 10+ countries; uneven regulations; seasonal/marginal demand.	TEN-T commitments; early-stage planning; limited actual deployment of clean energy hubs.

, which highlights the limited infrastructure readiness and recurring systemic barriers that constrain scalable decarbonisation of IWT.

5.3. Structural Limitations of Fixed Infrastructure Models

Despite high-profile attention being paid to electrification and hydrogen refuelling, the literature consistently highlights a mismatch between fixed infrastructure models and the operational characteristics of IWT [11,15,27,64,67]. These limitations are not only logistical but systemically rooted in how vessels move, how ports operate, and how infrastructure planning is currently undertaken.

This problem is particularly evident in regions like the Thames, where infrastructure is planned to be concentrated at a limited number of high-traffic passenger terminals [73]. In [23], the limited scope of current electrification is highlighted, which fails to serve multiple passenger operators due to varying pier stopping locations between operators, as well as freight operators and service vessel operators who may not operate within reach of designated infrastructure [70]. Even where fixed systems do exist like the new Net Zero Marine Services (NZMS) high-capacity charging point project, they lack standardisation and remain underpowered for high-demand commercial applications [24,70,83]. It is also noted in [15] that the difficulty of achieving rapid turnaround charging, without degrading battery life or overloading peer infrastructure, further complicates widespread electrification. Single charging points also risk costly queuing, making them unfeasible for routine use in early deployment stages.

From an operational standpoint, the literature also makes clear that fixed infrastructure cannot adequately serve the needs of vessels operating over longer distances or through corridors with inconsistent port availability [14]. On the Danube, for example, vessels may travel hundreds of kilometres between viable energy supply points, making the idea of recharging or bunkering at static locations impractical [27,79]. The same challenge is echoed on the Rhine, where [7] highlights grid limitations and congestion as key reasons why fixed infrastructure cannot scale effectively today, especially when energy demand will be unevenly distributed across time and location.

There are also issues of redundancy and investment risk. In [44], it is argued that infrastructure designed for a specific propulsion type risks becoming obsolete if vessel technologies evolve or diverge. Without a high degree of certainty about energy demand profiles, operators are unlikely to commit to infrastructure investments, especially in low-traffic or marginal areas [78,84]. This standoff highlights the need for demand-led modular approaches; see Section 7.

Critically, fixed infrastructure models also fail to support dynamic decarbonisation strategies such as those that require agile, responsive energy delivery capable of meeting vessels where they operate, rather than requiring route adaptation around static supply points.

The literature shows a broad consensus: while infrastructure is necessary, no single fixed model can suit all vessel types and operations. The barriers are not only technical but structural, rooted in fragmented governance and unclear responsibilities. As illustrated in Figure 7, governance gaps and reliance on pilots emerge as the dominant themes limiting sector-wide adoption.

6. Operational Realities of the Inland Fleet

The feasibility of decarbonisation in IWT is shaped as much by operational realities as by propulsion or energy supply. Unlike road fleets, the inland fleet is not a homogenous group of vessels following uniform routes with predictable dwell times. Instead, as highlighted throughout the literature, it represents a diverse system, with each vessel class presenting different energy demands, operational behaviours, and infrastructural requirements [15,27].

Figure 8 illustrates the diversity in vessel class but also suggests the complexities of adding in the variances in powertrain, as well as operational realities and environmental conditions that can vary widely to create highly complex operational conditions across the sector.

These vessels vary in size and powertrain configuration, but they also vary in how and where they operate. This diversity directly influences the feasibility of adopting uniform infrastructure or energy provisioning models across the fleet. As [6,85] note, passenger vessels on urban routes often operate on dense timetables with little opportunity for layover or recharging, while freight vessels cover larger distances under varying load and environmental conditions. The energy demands between these classes are not only distinct in their requirements but are also sensitive to external factors [11,14,15]. This diversity is compounded by the age profile of the fleet, and the longevity of vessels in operation, many still in active service after several decades of operation. As shown in Figure 9, the Rhine and Danube fleets illustrate how legacy vessels dominate, shaping both retrofit and infrastructure planning as discussed in Section 3.

A core issue is variability in energy consumption, [22]. This is especially relevant for freight operations, where loading condition, direction of travel relative to the flow of the river, and the weather conditions can introduce fluctuations to energy demand. Upstream vs. downstream, load, and weather can make identical vessels on the same route consume vastly different amounts of energy. See Figure 10.

Whilst operational conditions, driven by multiple complexities, can also impact energy demand compounding the issues, and as Figure 11 illustrates, this variability can complicate the sizing of storage and infrastructure in any move towards low- or zero-emission [22,49,86].

These fluctuations are compounded by a lack of standardised energy reporting or interoperable monitoring systems. Both [87,88] identify this as a key barrier to infrastructure planning. Without empirical data on real operations, infrastructure planning remains a challenge. As [18,23] note in the UK context, the absence of high-resolution datasets undermines demand modelling and validation of simulation tools, reducing operator confidence.

Operational tempo adds another layer: urban passenger crafts (e.g., Thames Clippers, Amsterdam Ferries, etc.) follow dense timetables with minimal fixed intervals between stops, whereas Danube freight barges may undertake irregular, long-haul journeys mixed with extended periods of inactivity. This decoupled demand from simple time-based models makes fixed recharging schedules poorly aligned with actual needs [11,23,68]. As noted by [6], time-anchored recharging infrastructure is likely to be underutilised unless it can match the diversity of vessel activity.

Infrastructure compatibility is a further barrier. Many commercial vessels lack shore power interfaces compatible with high-voltage marine connections, limiting even hotel-load electrification, and reducing trust in an available energy source at the destination. As described in the Electric Thames Project, even where infrastructure exists, it is often configured to serve smaller leisure style vessels, not high-demand working crafts [73]. Without standardisation, infrastructure risks being present but unusable.

There is also a misalignment between vessel energy demand and port-based infrastructure logic. Infrastructure is typically sited where land-based access is convenient, not where vessel demand peaks [87,89]. This forces deviations, delays, and suboptimal routing [90]. Freight operations who prioritise continuous movement and minimal port calls are particularly penalised. Both [6,91] emphasise this point by showing that even pilot projects with high technical potential face operational uptake issues when they disrupt normal vessel schedules.

Most critically, the operational flexibility of IWT—once a strength—becomes a liability when infrastructure assumes uniformity. Unless design reflects asynchronous and unpredictable vessel movements, new systems risk underutilisation. Operational realities demand adaptive, demand-led provision, a principle developed further in Section 7.

7. Discussion—From Technology to Provision

The preceding section show that while technological solutions for vessel decarbonisation are advancing, deployment depends on infrastructure that matches IWT’s operational realities. Pilot projects illustrate potential but remain vessel-specific, costly, and non-scalable, as previously discussed in more detail in Section 6 [11,26]. This project tailoring limits interoperability and cross-fleet returns [16,20]. The literature consistently highlights promising zero-emission advances. However, a deeper reading reveals a critical gap: these solutions are rarely grounded in an accurate or detailed understanding of how energy is actually consumed in IWW contexts, discussed further in Section 7.2 [23,30]. Without this, the sector risks overestimating the readiness and impact of these technologies when extrapolated to fleet-wide scenarios that cannot be delivered.

This section reframes the decarbonisation challenge as an energy provision problem. It looks at the limitations of fixed infrastructure, the strategic risks of planning without real-world energy demand data, and the importance of embracing flexible, scalable, energy-agnostic approaches. The aim is not just to critique current strategies but to clarify what the literature is implicitly asking for: a fundamentally different approach to energy delivery that begins with vessel operations, not infrastructure assumptions. In doing so, the discussion shifts from hardware deployment to service provision, where energy becomes an operational resource delivered in alignment with the fluid and decentralised nature of IWT.

7.1. Reframing the Problem: Infrastructure, Not Just Propulsion

Much of the focus across the literature on decarbonising IWT has centred around technological innovation in vessel propulsion. Several papers, policy documents, and pilot projects have examined the feasibility of electric propulsion systems, hydrogen fuel cells, hybrid configurations, and combustion systems optimised for low- or zero-carbon fuels such as ammonia, methanol or biofuels [20,30,31,50]. The narrative suggests that once the appropriate propulsion technologies are selected, decarbonisation will naturally follow [16,26]. However, this narrative does not hold up under scrutiny. As shown in Section 3, demonstration projects foreground propulsion technology, while offering only cursory or project-specific solutions to energy supply. This suggests a fundamental oversight: propulsion efficiency gains are meaningless if the upstream logistics—supply, transfer, compatibility—cannot deliver fuel or electricity in operationally viable ways.

In reality, these propulsion technologies—no matter how advanced—are only as viable as the systems that supply them with energy. A hydrogen-fuel-cell vessel cannot function without high-purity hydrogen delivered at the required pressure and flow rate. Similarly, ammonia-fuelled pilots have encountered bunkering incompatibilities and safety regulation gaps that delay deployment [50]. This is evident in the hydrogen-powered Sea Change Vessel that operates out of San Francisco harbour and which struggles to run a full service in comparison to its hybrid electric competition the Red and White fleet, due to supply chain bottlenecks [53,71]. Electric ferries face similar vulnerabilities: without reliable high-power recharging, they become inoperable. Even methanol-optimised vessels or those capable of using drop-in fuels such as HVO, will not transition unless those fuels are affordable, readily available, and supported by compatible infrastructure [30,57]. These are not theoretical concerns; they are real-world operational constraints that define the operational boundary between technological potential and practical failure. As Section 7.4 discusses, treating propulsion systems in isolation from provision systems creates strategic risk and locks in failure before operations begin.

This points to a critical flaw in current approaches: propulsion systems are being developed in isolation from the infrastructure required to support them. Diesel has remained dominant, not just because of its cost or energy density, but because it can be delivered flexibly and rapidly and almost anywhere across the IWW network. This flexibility emerges repeatedly in the literature as a baseline requirement for decarbonisation [16,30]. Yet current infrastructure planning lags far behind. Rollouts are frequently delayed by planning constraints, stakeholder resistance, and regulatory barriers [20]. Even when deployed, infrastructure tends to follow land-based logic—favouring symbolic urban projects over network-wide utility [23,26,90]. Critical freight corridors and rural links are often left underserved. As highlighted in Section 6, this is compounded by the absence of standardised operational data, meaning infrastructure is often developed without a clear understanding of how, where, and when energy is actually needed. Without correcting this, the result will be a growing disconnect between what is technologically possible and what is operationally achievable—undermining modal shift, public confidence, and decarbonisation goals.

This is not a marginal concern. If energy cannot be delivered to a vessel when and where it is needed, then no amount of propulsion innovation will lead to meaningful emissions reduction. Worse still, current infrastructure strategies risk creating a two-tier fleet: well-supported demonstration vessels operating in showcase locations, and the majority of commercial vessels left locked into fuel dependency [23,26]. The vessels most in need of support—those operating intensively in urban corridors—are often the least served by existing plans [11,20,46]. Without a decisive framing—from propulsion innovation to energy provision logic—the sector will continue to invest in technologies that cannot deliver scalable, real-world impact. Projects may appear successful in isolation, but without addressing systemic energy provision, they do little to advance sector-wide decarbonization.

7.2. The Critical Blind Spot: Energy Demand Understanding

The single most significant gap in the reviewed literature is the lack of vessel-specific, empirical data on energy demand. Even where data exists, it lacks depth, is limited to individual subsystems, or confined operational contexts. Studies rely on assumed duty cycles, manufacturer specifications, or extrapolated averages—rather than actual vessel logs—failing to capture the variability and complexity of inland vessel behaviour [26]. This disconnect is particularly acute in environments such as tidal, shallow, or constrained waterways, where simplified routing or constant-speed assumptions can understate peak demands and distort infrastructure requirements [26]. As noted in Section 6, this leads to models that cannot anticipate steady-state operations, high-drag scenarios, or energy-intensive manoeuvring such as in tugboat use. [16,20,21]. In short, the sector is attempting to plan energy infrastructure without a reliable understanding of what that energy demand actually looks like.

This is not a methodological detail; it is a foundational problem. Energy demand determines not just how much energy is required, but when, where, how it should be delivered, and with what level of redundancy. Without accurate data at this stage, infrastructure systems risk being overbuilt, underused or rapidly obsolete [47]. In the absence of vessel-specific demand profiles, infrastructure planners are effectively unable to determine where infrastructure should be located, how much capacity is required, or whether it can support diverse vessel types. The result is an infrastructure landscape that is shaped by land-based logic or political opportunity, not by the operational need.

The problem is compounded by the lack of standardisation in how energy consumption is measured or reported across the sector. Most IWVs are not equipped with integrated monitoring systems capable of logging energy use in real time. Even where such systems exist, the data is often held privately, inconsistently reported, or aggregated in ways that obscure operational detail. This mirrors early barriers in other transport sectors, where proprietary data silos delayed standardisation and slowed sector-wide coordination [47,92,93,94]. This lack of transparency not only inhibits research but prevents coordinated infrastructure planning and weakens the evidence base for policy.

Furthermore, energy demand in IWT is not static; it varies dramatically based on a range of operational factors. Load condition, flow direction, vessel draft, environmental conditions (e.g., wind, current, temperature), and auxiliary systems (e.g., HVAC, refrigeration, navigation), all influence total energy consumption [8,22]. As discussed in Section 6, two identical vessels can display dramatically different energy datapoints across their operation. Whilst these instances are small in the scheme of a complex duty cycle, if they happen frequently enough, the result is a serious shortfall in energy required. Generic models fail to capture these nuances and ultimately produce misleading results that infrastructure requirements may then be built upon [87].

This absence of detailed energy demand profiling has serious downstream effects. It results in infrastructure that is either overbuilt and underutilised or undersized and overwhelmed [7,49]. Charging or bunkering points are often cited for land access convenience, not vessel behaviour [40]. It limits the ability to forecast future demand growth, manage load balancing, or coordinate multi-vessel energy distribution [90]. In short, it renders infrastructure planning reactive, inefficient, and misaligned. The absence of robust demand data will force planners into a reactive posture that continually chases problems rather than anticipates them.

A fundamental shift is needed—from top-down infrastructure strategies based on static models to bottom-up planning informed by dynamic, empirical data. Only then can the energy provision systems of IWT be aligned with the operational behaviours they are meant to serve. This shift would also enable scenario testing for different propulsion and fuel pathways, ensuring that infrastructure remains adaptable as technology choices evolve.

7.3. System Inflexibility vs. Operational Diversity

One of the main characteristics of the inland fleet is its diversity. Some vessels operate on fixed schedules in populated urban areas, whereas others operate irregular freight routes across international waterways. Some vessels are used seasonally, whilst others operate 24/7. Some vessels are modern and digitally integrated, whilst others are legacy vessels with no onboard instrumentation or standard interfaces. This lack of uniformity extends to maintenance practises and crew training, further complicating any attempt to implement a ’one-size-fits-all’ infrastructure solution [8]. As highlighted in Section 5, this diversity reflects a layered operational reality that must be engaged with directly, not designed around.

This diversity is not an obstacle to decarbonisation; it is a reality that must be accommodated. The vessel fleet will not be replaced overnight. A large share of the European inland fleet is over 30 years old [11,20,21]. This means that it is more likely that the majority of the current fleet will be retrofit with a zero-emission solution rather than being replaced with a newly designed vessel that might stand a better chance of optimised integration [20,21]. Whilst a new vessel will be optimised for energy carrying, to limit the impact on load for operational purposes, a retrofit vessel will need to consider the current dimensions and space [37,38,65]. Consequently, retrofitted vessels will tend to externalise energy capacity to the network, increasing dependency on frequent, high-rate replenishment. This reliance also increases sensitivity to even minor infrastructure outages, where a delay or a fault at a single location can cascade into service disruptions across an entire route; this raises strategic concerns, as explored in Section 7.4, where reliance on fixed assets compounds risk [39,52]. However, current infrastructure models are ill-equipped to support this diversity.

Currently, vessels rely heavily on fixed assets such as port-side bunkering terminals, and grid-connected facilities that assume vessels will come to them. These assets are typically deployed based on land-based planning logic: ease of construction, available utilities, regulatory permissions, or proximity to urban centres. Vessel behaviour is often an afterthought, despite being a crucial element in the supply to demand methodology [49,70].

There is now a growing mismatch between infrastructure availability and vessel need. Many high-energy-utilisation vessels do not operate on routes that intersect often with current fixed infrastructure [20]. This is especially true for vessels serving rural or freight-dense corridors, where infrastructure rollouts remain limited, as shown in Section 7.1. Others cannot afford the downtime required to reach energy charging points that are not on their optimal path. In cases where vessels will need to modify their operations to access infrastructure, it will lead to commercial inefficiencies which could have a serious impact on the operational effectiveness of the vessel. Over time, these operational compromises may incentivise operators to revert to fossil fuel assets, undermining modal shift efforts and eroding investor confidence [21,49].

This rigidity is compounded by the technical limitations of many infrastructure components. Fixed chargers are typically standardised around voltage and connector types, making them incompatible with a wide range of vessel systems [38]. Hydrogen bunkering facilities, where they exist, are often sized for pilot projects, and cannot support the volume or pressure requirements of wider commercial operations [65]. Similarly, shore-power facilities optimised for smaller craft can bottleneck larger vessels, forcing them to accept sub optimal charging rates that extend turnaround times beyond commercially viable limits [39]. These issues are not just logistical; they are indicators of a system designed without sufficient regard for wider operational diversity. As discussed in Section 7.6, this also raises questions about the role of standards and regulatory support in ensuring future compatibility.

What emerges is a pattern of infrastructure failure, not because of poor technology, but because of poor alignment with the operational landscape. Unless this alignment is corrected, no amount of infrastructure investment will yield meaningful decarbonisation outcomes. Instead, the sector must adopt a fundamentally different model, one that embraces mobility, modularity, and fuel-agnosticism, enabling energy delivery systems to adapt to the vessels they are meant to serve [31,37]. This approach would not only close the gap between supply and demand but also build resilience into this network, allowing infrastructure to evolve alongside changes in vessel technology and traffic patterns.

7.4. Why Planning Without Demand Profiles Is a Strategic Risk

The implications of infrastructure misalignment extend beyond operational inefficiency; it will introduce significant strategic and financial risk. Across Europe and the UK, public and private funds are already being invested in decarbonisation infrastructure for IWT. These investments are often made on the basis of decarbonisation roadmaps, regional development strategies, or pilot project success stories. However, in the absence of vessel-specific energy demand data, such investments risk becoming stranded assets—technologically sound but operationally irrelevant. As shown in Section 7.2, infrastructure developed without reference to real energy demand patterns is prone to over- or under-utilisation, regardless of its technical quality [20,21]. This risk is amplified in waterways with mixed-use traffic, where diverse vessel categories may have entirely different energy profiles, yet are often grouped together in feasibility studies.

Consider the consequences: if a charging facility is installed at a location with minimal vessel traffic, limited energy choices or incompatible connector standards, for example, its utilisation rate will remain low. If a hydrogen supply chain is developed without understanding which vessels are willing and able to transition to hydrogen, the infrastructure will sit idle [6,95]. If fuel bunkering is designed around assumed operating schedules that do not reflect commercial realities, vessel operators may revert to fossil fuels out of necessity. If energy replenishment hubs are positioned in locations that suit land-based challenges, then it is plausible that many operators will see increased inefficiencies in their operations to take on energy which may prevent them making that switch to begin with [17]. These concerns have already been observed in numerous pilot programmes across the sector. Post project launch data is often limited, with performance data from high-profile pilot projects rarely shared openly, breeding uncertainty amongst operators [20,21]. This lack of transparent post-implementation data also inhibits the replication of successful approaches, and planners are left guessing which design elements contributed to the operational successes, if any.

The financial cost of such misalignment is considerable. Infrastructure development will inevitably require major grid reinforcements, land acquisition—particularly complex and time-consuming in dense urban contexts such as the Thames—and provision for hazardous materials storage, all of which make projects highly capital-intensive. Public confidence in IWT decarbonisation remains fragile, partly because emissions savings are often misunderstood and difficult to verify in practice [11,20,22]. Worse still, such projects may delay meaningful decarbonisation by tying up scarce funds in infrastructure that cannot adapt to evolving vessels behaviours. Once established, fixed assets are path-dependent and costly to reconfigure, creating a risk of stranded investments that embed sub optimal energy delivery patterns for years to come [94].

Strategic resilience requires a planning model that is not static but adaptive, and one that begins with understanding how vessels operate and designs energy delivery systems to meet that demand [20]. This is not just more efficient; it is essential for making IWT work for the future and to aid in a modal shift to help overall transportation decarbonisation efforts. Without this shift, the sector risks becoming an outlier in wider transport decarbonisation progress, eroding its competitive position and undermining modal shift objectives [79,96]. There is really one chance to get this right, the sector cannot afford to waste another decade building infrastructure that vessels cannot or will not use. The literature may not always say this explicitly, but the evidence points clearly in this direction. Infrastructure separated from operational insight is a serious strategic liability [11].

7.5. A New Model: Dynamic, Data-Led, Fuel-Agnostic Energy Provision

The core message that emerges from the reviewed literature is not that the technologies are lacking; it is that the supporting systems are being poorly conceived. The tools for vessel decarbonisation seem to exist, but they are being deployed with a fundamental misunderstanding of how IWT actually works. To bridge this gap, a new model for energy provision is needed, one that reflects operational reality, supports fuel diversity, and is responsive to real-time demand.

Firstly, this model begins with empirical data collection ensuring planning reflects actual vessel operations [87,88,90]. This includes trip distances, journey times, load conditions, speed profiles, hydrodynamic drag, dwell times, and auxiliary loads, etc. When aggregated across regions and vessel types, such data can generate dynamic energy demand maps that will then evolve over time, becoming more accurate for energy supply. The key here is to understand this in the current state, not to switch to cleaner energies and then try to make the infrastructure fit, but to simply understand what energy is currently being used across the regions, vessel types, and operations. Capturing this baseline before significant fleet transition occurs ensures that planning is based on a representative picture of operational energy use, rather than distorted post-transition conditions [6,49]. These energy demand maps can then inform infrastructure placement, capacity planning, and scheduling logic as well as provide vessel operators with their best options for propulsion and energy choice for the future and use it to plan ahead to get on that pathway to zero-emission operations.

Secondly, the model must be fuel-agnostic. It is still too early to predict which alternative fuel or energy carrier will dominate in IWT, and potentially none will dominate like diesel does today. Electricity, hydrogen, ammonia, methanol, and biofuels such as HVO, all have different advantages and limitations, and their uptake will vary across regions, vessel classes, and regulatory environments [31,50,63]. Committing to a single energy vector prematurely risks obsolescence. Provision systems must therefore be modular and fuel agnostic, able to accommodate diverse or emergent fuels without major reinvestment [61,65].

Third, and perhaps most critically, the model must embrace mobility and modularity. Static infrastructure is too inflexible to support the full spectrum of vessel operations. Instead, energy should be delivered dynamically—through a number of solutions which could include mobile energy barges, battery-swapping vessels, floating fuel stations, or hybrid support platforms [36,65]. Mobile units can be deployed to meet vessels in-transit; extending reach and improving utilisation rates also enables target deployment to support seasonal peaks, emergency operations, or unexpected surges in demand.

This approach is not just more aligned with the realities of IWT; it is more resilient. It allows infrastructure to evolve over time, adapt to shifts in technology, and respond to changes in traffic patterns which also supports the modal shift [6,61]. It supports equitable access to clean energy for vessels operating outside of major hubs. It avoids sunk-cost infrastructure that may become incompatible with future systems [6]. And it places energy access at the centre of decarbonisation strategy, rather than treating it as a downstream concern. By embedding energy provision into the core of infrastructure and policy planning, the sector can ensure that clean propulsion technology has the operational environment it needs to succeed.

In doing this, it will answer the most important question posed by the literature: how can the industry move from a series of successful pilot projects to a functioning, scalable decarbonisation system [31,49]? The answer is by rethinking not just how vessels are powered—but how energy itself is provided. Figure 12 summarises this proposed model, illustrating the transition from fixed, siloed infrastructure to a dynamic, data-led, fuel agnostic approach.

7.6. Research and Policy Implications

Reframing IWT decarbonisation as an energy provision challenge, rather than a propulsion technology problem, has deep implications for both research and the policy landscape. It shifts from test-bed innovation and demonstration pilot projects to system-level design, data acquisition, and network planning. If followed, this reframing would realign the priorities of funding bodies, regulatory agencies, and academic institutions. It also redefines what constitutes ‘success’, shifting evaluation metrics from individual project outputs to sector-wide operational impact and scalability.

There is a need for interdisciplinary work, bringing together transport modelling, renewable energy systems, systems engineering, and operational behaviours. Understanding the energy needs of vessels is not just a technical exercise, it also involves commercial constraints, policy incentives, scheduling preferences, and user behaviour. Developing robust, replicable models that can inform policy and investment requires drawing on multiple fields and methodologies. This breadth of expertise is essential to bridge the persistent gap between theoretical feasibility and practical implementation.

For researchers, the priority is to move beyond comparative propulsion studies and instead develop tools for robust, vessel-specific energy demand modelling. For policy makers, the key task is to integrate energy provision explicitly into decarbonization strategies, ensuring investment decisions are grounded in operational demand.

Table 6. Research and policy priorities for aligning IWT decarbonisation with energy provision needs.

Research Priorities	Policy Priorities
Develop standardised data collection frameworks for vessel energy use.	Integrate energy provision explicitly into decarbonisation strategies.
Link operational vessel data with AIS and hydrographic information.	Require demand assessments for infrastructure investment decisions.
Validate simulation tools with real-world operator feedback.	Encourage modular, interoperable, and redundant energy systems.
Advance multi-disciplinary models (engineering, commercial, behavioural).	Align regulation across transport, energy, and environment ministries.
Build dynamic energy demand maps to guide infrastructure.	Treat infrastructure as a strategic, adaptive asset, not static facilities.

summarises these dual priorities, highlighting the distinct but complimentary actions required from research and policy to enable scalable decarbonization.

7.7. Summary: A Missed Opportunity Unless the System Is Rethought

The review of existing literature on IWW decarbonisation reveals a contradiction: there are the propulsion and energy technologies, and in many cases, the policy ambition. Yet without the systematic thinking required to make them viable at scale, their potential remains unrealized. The bottleneck is not innovation; it is provision. This misalignment between capability and implementation is what turns technological potential into operational stagnation.

The absence of reliable, vessel-specific energy demand data has meant that infrastructure planning is frequently based on incomplete or insufficiently evidenced assumptions [7,9]. As a result, new facilities are often deployed in advance of robust demand assessments, raising questions about their long-term utilisation and relevance [61]. For vessel operators, this creates uncertainty while pilot projects provide access to new infrastructure; they are typically tied to the scope of the individual project and may not guarantee continuity beyond it. Governments, meanwhile, are investing in decarbonisation infrastructure without consistent visibility of utilisation or return on investment. A challenge is also identified in recent assessments of IWW development strategies [20]. The outcome is a fragmented energy landscape where knowledge and responsibility remain concentrated within individual projects rather than shared across the sector.

This trajectory is not sustainable. The reviewed literature highlights the availability of relevant technological tools, but it also reveals the limitations inherent in the current strategic framing of IWW decarbonisation. A continued emphasis on the vessel in isolation risks overlooking the systemic requirements of the transition. Treating infrastructure as a secondary consideration similarly undermines the likelihood of success. Furthermore, investing in infrastructure without a clear understanding of energy demand creates a high probability of failure. Such failures would not only carry financial consequences but also strategic ones, jeopardising modal shift, delaying emissions reduction, and eroding confidence within the industry.

This is the “so what” of the literature. It is not just a collection of insights about vessels, fuels, and infrastructure. It is a call to action to redesign the system from the waterline up. Figure 13 illustrates this contrast, highlighting the missed opportunity of current approaches versus the potential of a demand-led provision model.

8. Conclusions and Next Steps

The literature on IWT decarbonisation has expanded rapidly, but much of it continues to frame the challenge as one of propulsion technology rather than energy provision. Across the studies reviewed, little attention has been given to whether the necessary infrastructure exists, how energy should be delivered, or how vessel-level demand profiles should inform infrastructure design. This gap has created a misleading picture: vessels are imagined as technically capable of zero-emission operations, yet the energy systems required to support them remain fragmented, underdeveloped, and misaligned with real-world needs.

The critical bottleneck is clear: decarbonisation is no longer a propulsion problem but a systems problem, and above all, a provision problem. The success of any technology pathway depends on whether clean energy can be made reliably available, at scale, and at the point of need. Planning based on static, top-down assumptions is insufficient. What is required is a shift to demand-led, data-driven infrastructure planning that recognises the dynamic realities of vessel operation and the long lifecycles of inland fleets.

8.1. A Strategic Roadmap

Figure 14 outlines a high-level phased roadmap for the decarbonisation of the IWWs, highlighting how short-, mid-, and long-term actions must align technology, infrastructure, and policy.

• Short-term (0–5 years): Stabilise emissions and begin transition using drop-in fuels and low-cost retrofits that extend vessel life while lowering carbon intensity. Pilot projects in charging, hydrogen bunkering, and mobile delivery should be prioritised in high-traffic corridors. Supportive but realistic policy measures are needed to build momentum without penalising smaller operators.
• Mid-term (5–15 years): Scale and integrate zero-emission systems. Infrastructure must expand from pilots to corridors, designed with interoperability, modularity, and adaptability. Hybrid and modular vessel solutions will become mainstream, while system-level optimisation—linking IWT with other transport modes—should embed IWT firmly within decarbonised logistics chains.
• Long-term (15+ years): Complete the transition to net-zero through widespread deployment of renewable electricity, green hydrogen, or other zero-carbon carriers. Fixed and mobile infrastructure should be integrated into flexible delivery networks, supported by predictive analytics and harmonised cross-border regulation.

8.2. A Research and Collaboration Priorities in Next Steps

The next phase of work must move decisively from theory to practice. The first priority is the capture of vessel-level operational data across a diverse range of contexts in order to model energy demand with high fidelity. This empirical foundation should then inform the development of dynamic simulation environments capable of testing alternative pathways, quantifying risks, and refining strategies over time. Equally important is the creation of modular, fuel-agnostic delivery systems that integrate fixed, mobile, and on-demand infrastructure in ways that remain adaptable to technological change and market volatility. Finally, the success of this transition depends on collaboration that extends well beyond the academic and engineering domains. Policymakers, operators, designers, regulators, and energy providers must work together through structured and ongoing mechanisms to ensure continuity beyond isolated projects or short-term funding cycles.

8.3. Final Remarks

Decarbonisation of IWWs will not succeed through propulsion and vessel innovations alone. It will only become viable when energy provision is treated as a central design challenge—planned, modelled, and delivered with the same rigour once reserved for diesel. If this shift occurs, IWT can deliver not only a credible zero-emission pathway, but also a compelling model for sustainable transport more broadly.