# Optimal Fleet Transition Modeling for Sustainable Inland Waterways Transport
## 1 Introduction  
The global transportation sector is a significant contributor to greenhouse gas emissions, accounting for approximately 23% of global energy-related CO\({}_{2}\) emissions in 2019 [1]. Among the various modes of transport, maritime transport was responsible for about 2.89% of global greenhouse gas emissions in 2018 [2]. In EU, maritime transport accounted for 13.5% of EU transport emissions in 2019 [3]. While these figures primarily represent oceanic shipping, inland water transport, including local lakes, also contributes to these emissions and thus cannot be overlooked in the transition to sustainable transport systems. In line with the global commitment to mitigate the effects of climate change, the European Union has embarked on an ambitious path towards sustainable development through its European Green Deal and the 2030 Climate Target Plan [4]. These strategic initiatives mandate an aggressive reduction of greenhouse gas (GHG) emissions, targeting at least a 55% reduction by 2030 compared to the 1990 levels, and strive for climate neutrality by 2050. All sectors of the economy, including maritime transport, are expected to contribute towards this ambitious environmental target [5]. To facilitate this objective within the maritime sector, the FuelEU Maritime initiative has been implemented [6]. As outlined in its provisions, this initiative aims to gradually decarbonize maritime transport by reducing the GHG intensity of the energy used onboard vessels. The transition towards greener and more sustainable energy sources for vessels is a critical step in this initiative, with the ultimate aim of completely decarbonizing the sector. By driving regulatory changes and promoting innovations in sustainable vessels, theFuelEU Maritime initiative forms a cornerstone of the European Union's approach towards achieving its climate goals. The decarbonization of waterborne transport presents numerous challenges, but also opens up a range of opportunities, especially with respect to technological advancements [5]. One such approach to reducing GHG emissions from maritime transport is the incorporation of zero-emission-fueled vessels into existing fleets. This transition to more sustainable energy sources is gaining increasing attention in scientific research and policy-making. The decarbonization of the transportation sector has been widely studied using energy system models. Several articles have examined the impacts of increased electrification on emissions and costs. For example, [PERSON] et al. [7] used EnergyPLAN [8] to analyze different levels of private vehicle electrification. They found electrification reduces costs and emissions, but grids may require capacity expansion. Similarly, [PERSON] et al. [9] showed that electrifying India's roads lowered fuel costs and supported climate change mitigation. However, [PERSON] et al. [10] demonstrated replacing all US personal vehicles with battery electric ones would only cut transportation emissions by about half due to electricity generation impacts. Sector-coupling between transport and electricity is very relevant and is a challenge for the energy system modeling research field [11]. These approaches have also been applied to vessels. [PERSON] et al. [12] developed marginal abatement cost curves for a small island where maritime transport dominated emissions. Electrification reduced emissions at reasonable costs. While prior works focus on roads and ocean-going vessels, local inland water transport has received less attention. Inland vessels, in particular, are being encouraged to consider a shift towards low-carbon energy sources. Such sources include, but are not limited to, lithium batteries [13], fuel cells [14], ammonia and methanol [15], and wind-assist and sail propulsion [16]. Several studies [13; 14; 15; 16] have confirmed that the use of these clean energy sources to progressively replace traditional diesel engines is a significant trend in the ongoing decarbonization process of vessel power technologies.