**1. Introduction**

The International Energy Agency and the International Union of Railways have stated that the 23.1% of worldwide CO2 emissions from fuel combustion are attributable to the transport sector (8258 million tons of CO2). The breakdown of the emissions in this sector is attributable as follows: 73.2% to road transport, 10.4% to maritime transport, 10.5% to air transport and 3.6% to railways [1]. Regarding the European situation, the amount relating to the transport sector is around 30.4% (71.1% road transport, 13.9% maritime transport, 12.7% air transport and 1.5% railways). In the United States, the transport sector accounts for 34.4% of emissions from fuel combustion, in Japan for 20.6%, in Russia for 17.5%, in India 12.7% and in China for 9.6% [2,3]. According to the Statistical Compendium on Transport published annually by the Commission, net of emissions indirectly attributable to rail transport (which are accounted in electricity generation phase), therefore 44% of total emissions are attributable to road passenger transport, 28% for road freight, while air and maritime transport accounted for 13.0 and 13.4%, respectively. Overall, urban mobility would account for 25% of greenhouse gas emissions from transport, interurban for 54% and intercontinental for 22%) [1,4].

From the point of view of railway line electrification, Europe has 55% of the total length of the railway line electrified. Italy has 72% of electrified lines and 28% with diesel-powered trains [5].

E-mobility will be widespread thanks to the European Union funding programs and capital investments from the automotive industry. However, several technical, financial and social challenges need to be overcome. Although electric vehicles present higher purchase and infrastructure costs, they present lower operating costs than a conventional car [6–8]. Due to the limited autonomy and required charging times, battery electric vehicles are indicated for consumers with a limited daily range, but the technology, especially for storage systems, is constantly improving. In the long term, options like plug-in hybrid vehicles are expected to have a relatively big market share [9].

The worldwide diffusion of electric vehicles is still very limited. One of the main reasons of different trends in the sales of electric cars is the presence in some parts if the world of incentive mechanisms such as purchase incentives and free access to controlled priority circulation zones. Another relevant factor is the presence of proper charging infrastructures, which requires the companies involved to adapt their business models [10].

One of the main target areas of the climate goals is road freight transport, since about 25% of the CO2 emissions from road transport in the European Union are produced by heavy-duty vehicles. Although shifting from road freight transport to electrified railways is a possible solution, several studies indicate that the potential for this is limited. New technologies such as electrified highways (eHighways) are proposed, in which an overhead line supply energy to trucks provided with an integrated pantograph. A two-pole catenary system has been developed, ensuring a stable current transmission at speeds up to 100 km/h [11]. This technology is already being used in several projects in countries such as Sweden and United States [12], and several such projects are in the planning phase.

The design of electrified mass transit systems for urban railway traffic, such as trams, trolleybuses and metros, requires taking into consideration several elements: safety, efficiency, cost and visual impact. Traditional electrified transit systems are based on electrical contact wires, such as active rails or catenary. Catenary-free systems have advantages of as low electromagnetic radiation and good visual design [13] In the absence of fixed traction systems, the rolling stock has on board the energy storage system (e.g., batteries, hydrogen, diesel, ultracapacitors, flywheels). Moreover, catenary-free systems are being consolidated in public transportation, especially to restrict the electrical infrastructure, for example in city historical centers, limiting the environmental impact and reducing costs of electrical installations for traction [14].

In the current state some form of on-board storage system is often used for heavy traction (passenger and freight trains), but generally only to recover braking energy and not to operate in catenary-free mode. Battery-powered locomotives were one of the first technologies to be adopted in the early 1900s but due to the limited storage capacity available at that time, it was decided to focus on electrified solutions. Use thus became limited to mining locomotives and shunting locomotives [15].

Nowadays several studies have focused on the possibility to design new trains to operate autonomously because of the advances in energy storage technologies [16]. Trains with on-board storage systems could be adopted for expansion of already electrified railways, point-to-point connections or commuter transport systems [17–19]. It will be important in the near future to considerate the need for prospective analysis of the marketing of rolling stock approved for bimodal operation (i.e., with and without contact lines supplying different voltages). The diffusion of a new system will strongly depend on the cost and the development of new rolling stock with on board storage technology for traction [20–22].

It is well known that for heavy traction, the electrified solution via catenary represents a consolidated and highly reliable approach that is economically sustainable for high traffic lines [23,24]. Moreover, considering the contribution of renewable sources it is possible to increase the energy savings from the primary network. For this reason, several studies and projects have been carried out highlighting the trend of replacing diesel locomotives with electric locomotives to cover long distances, powered by AC or DC electrification systems [25].

For solutions involving on-board energy storage systems (ESS), several technologies can be used, as shown in Figure 1. Since each one of them presents different characteristics, it is important to choose the most suitable technology for the particular case under study. Currently, there are different types of electrochemical accumulators. Lead-acid accumulators, widely used in the automotive sector, represent an economical and reliable solution. However, they have a very low specific energy. Those with nickel cadmium (Ni-Cd), initially used as substitutes for the aforementioned lead-acid accumulators, have been nowadays abandoned due to the toxicity of the cadmium present in them, and the focus has shifted to nickel-metal hydride accumulators (NiMH), which have the disadvantage of presenting memory effects. Finally, lithium batteries (Li-ion) are characterized by high energy and specific power.

Technologies such as flywheels offer a high specific power and a large number of charge and discharge cycles but they present low specific energy. The use of fuel cells in railway transport systems is currently in the early development state and they present many advantages such as zero emissions fuel and high specific energy [26]. Though fuel cell sources can be coupled with high-power density storage elements [27], the combination of Li-ion batteries with supercapacitors (SC) is currently found to be the most economical solution for ESS [26].

Supercapacitors, also known as ultracapacitors, are an innovative electrical EES that has considerably higher capacity values than those of ordinary capacitors and high specific power. The combination of Li-ion batteries and SC in a hybrid energy storage system (H-ESS) allows one to reduce battery stress and aging effects, depending on the charge and discharge cycles. The sizing of the ESS is strictly linked to the control strategy used for the managemen<sup>t</sup> of energy resources and is limited by the maximum weight and volume allowed by the rolling stock [28,29].

**Figure 1.** Ragone chart [30].

Several studies have been carried out on the applications of storage systems that use lithium-ion cells with high specific power (power-oriented) or high specific energy (energy-oriented). Usually, energy-oriented lithium-ion cells are accompanied by supercapacitors, which deliver the stored energy during power peaks in traction and absorb energy during regenerative braking [31,32]. It is possible to find different applications on electric vehicles [33,34], electric buses [35], small boats [36], water buses [37], and even machines for lifting and displacement of loads [38].

In [39], a battery pack of about 250 kWh and about 2 tons of mass is used on a freight train, in order to solve the problem of the absence of the catenary in the connections of the railway lines to the freight terminals and production plants, avoiding the use of diesel-powered vehicles. In [40], prototypes of battery packs of 200 kWh and about 2 tons are reported for a railway locomotive and of 500 kWh and 5 tons of mass for powering a mining truck. In [41], the authors present a battery pack of 2.22 MWh and 11 tons of mass to power a rolling stock with a total mass of 276 tons that must travel a route of 212 km.

Among the few solutions commercially available for trains equipped with energy storage systems there is a Spanish tramway in Seville, which uses a hybrid storage system consisting of supercapacitors and batteries [42]. In addition, the same train constructor has developed a battery-powered regional train, proposing it as a solution for non-electrified railway lines. The roof-mounted traction batteries provide the power and energy required to propel the train for distances up to 100 km [43,44]. It is important highlight that the adoption of storage system on board trams and trolleybuses is economically convenient for short route sections. There are already design solutions and rolling stock equipped with on-board storage systems available on market.

In addition to the technical and infrastructural characteristics of a given traction system, it is important to carry out a cost assessment, especially when comparing di fferent achievable solutions. In the technical and scientific literature, several studies have been published using the annual cost of energy (ACOE) in order to evaluate the e ffectiveness in terms of costs of di fferent types of energy resources [45]. In this study, the ACOE is also used to compare the di fferent technical solutions proposed.

This paper presents the evaluation taken into consideration to perform a comparison between direct current electrification and the use of trains with on-board energy storage systems. Numerical simulations have been performed on a real railway line to carry out a preliminary techno-economic comparison showing the models and procedure used. The case study presents an electrified 3 kV DC section and a non-electrified section, currently being covered by diesel-powered trains. For the non-electrified section, the following scenarios have been analyzed: 3 kV DC electrification, since it is the feeding system currently used in the preceding section; high autonomy ESS and ESS with recharge station. For the ESS it has been considered the use of power-oriented Li-ion battery cells and, in the case of hybrid ESS, energy-oriented Li-ion battery cells accompanied by supercapacitors. For the sake of simplicity, only recharging the ESS has been considered, the possibility of swapping the ESS is outside the scope of this work, as it would have an impact on capital costs. It is highlighted that, in cost assessment is not taken into account the cost associated with the purchase of new trains. In this paper, it is assumed that the cost of the on board ESS is the di fference cost between traditional electric trains and new trains equipped with on board ESS.

The paper is structured as follows: Section 2 describes the electric models of the vehicle, energy storage systems and DC feeder system, as well as the economic model. Section 3 presents the on-board ESS sizing procedure. Section 4 introduces the simulation software and procedures while Section 5 presents the case study and the numerical results of the simulations. Section 6 concludes the paper.

### **2. Modeling of the Railway System**

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The proposed model is obtained by using three di fferent sub-models: the railway vehicle and its kinematics, the DC feeder system and the on-board ESS. An economic model is also introduced for calculating the annual energy cost of the proposed solutions.
