**1. Introduction**

Electric mobility represents a necessary transition to extricate from a mode of transport that depends on the use of harmful fossil fuels, of which emissions into the environment make living in urban centers increasingly hazardous. Electric mobility, being one of the branches of "smart mobility", is therefore a part of a pillar of "smart cities", which are a congregation of urban planning strategies and plans aimed at ensuring sustainable economic development and a high quality of life through a skillful managemen<sup>t</sup> of available resources (food, mining, energy, services, etc.) [1,2].

However, it would be reductive to limit "smart nobility" by merely switching from a type of transport with an internal combustion engine to one with an electric motor. The reality is that "smart mobility" is a much broader concept involving di fferent stakeholders (public administrations, private/public or mixed companies, end users, etc.) and technologies/services (car sharing, autonomous driving, trip planning apps, etc.). A comprehensive review is outside the scope of this study, which deals instead with the concept of vehicle-to-grid (V2G), as previously mentioned. Nowadays, in reality, the concept of the electric vehicle (EVs) is still of the "passive" type, of the grid-to-vehicle type (G2V), conceived to mere electric load. However, this paradigm could be destined to change in the immediate future. The idea of V2G fits into the more general concept of the "smart grid" (SG).

The current electrical system originated at the start of the 20th century and was conceived as an exclusively unidirectional system [3,4].

Going back in time, debates about the desirability of migrating to electric mobility commenced at the end of the 19th century. Even before problems associated with fossil fuel use in transportation began to manifest, a jury announced its introduction in 1898, following a contest in which several prototypes of cars (both electric and with an internal combustion engine) were presented.

Indeed, the early 1900s witnessed the evolution of this unique electricity-powered mode of transportation [5–7]. Electric cars seemed preferable to petrol cars, which were noisy, had annoying vibrations and smoky exhausts with attendant risks of fire and explosion. Furthermore, the manual gearbox and cranking made them di fficult to use. Although the first electric cars could only cover minimal distances of 80–90 km, this did not constitute a problem since the movements at the time were confined to domestic destinations, at least in the initial phase. However, that mild status seemed destined for change following the massive production of the famous T model of Tesla, a reduction in oil price, and improvements in road networks, which enabled greater distances to be covered. An alternative method of transport with an internal combustion engine became a necessity [8–10].

All these factors contributed to a change in direction towards fossil fuel mobility, which became predominant throughout the 20th century up to the present day. The oil crisis of the 1970s and the resultant economic crisis, however, highlighted the vulnerability of an economic system that depended heavily on fossil fuels, which, in addition to their obvious limitation of heavily polluting the environment, tended to be found predominantly in regions that were plagued with profound political instability. The crisis prompted world governments to radically modify their assets with the aim of reducing the West's dependency on Middle Eastern oil sources. This would be accomplished by reduced consumption, the reorganization of industrial production, and a search for alternative energy sources. The crisis clearly also involved the transport sector, and the migration to an electric transport mode—which, until then, had been confined to specific applications—began to attract considerable interest again [11,12].

As stated in the introduction to this paper, it is still premature to talk about an "electrical revolution". However, the subject of environmental protection is increasingly topical, and coupled with the reality that fossil fuels will likely become a rare resource, thus, the switch to an alternative in the form of electrical mobility has become a topic of considerable debate over the last ten years.

As stated in [1], regarding the current Europe panorama, the transportation sector occupies the fourth place as one of the largest emitters of greenhouse gases with a share of 14%. Increased use of electric mobility would certainly reduce this proportion and mitigate the critical problem of pollution in urban centers. Despite the initial problems and criticalities linked to the migration from thermal to electric mobility, a multigovernmental initiative known as the Electric Vehicles Initiative (EVI) was inaugurated in 2009 to accelerate and control the deployment of EVs worldwide [13]. Another recent initiative proposed by the EVI is the EV30@30 [2] Campaign, which was launched at the Eighth Clean Energy Ministerial in 2017 in Beijing (China). The aspirational goal of this campaign is to achieve 30% sales share for EVs by 2030. The main actions to attain this goal include the following: (i) provide support to the governments in need of policy and technical assistance; (ii) promote programs (such as the Global EV Pilot City Programme) to facilitate the exchange of experiences in the EV field, and propose the best practices for the promotion of EVs in cities; (iii) encourage public and private sector commitments for EV uptake in company and supplier fleets; (iv) support the deployment of EV chargers and track progress. In 2017, the global stock of electric passenger cars reached 3.1 million, which was a 57% increase on the number for 2016 and approximately two-thirds of the world's electric car fleet are battery-operated electric vehicles (BEVs).

In addition to the 3.1 million passenger electric cars, there were nearly 250,000 electric light commercial vehicles (LCVs) on the road in 2017. The largest electric LCV fleet is in China (170,000 vehicles), followed by France (33,000 vehicles) and Germany (11,000 vehicles). Electric LCVs are often part of a company or governmen<sup>t</sup> fleet. For instance, the DHL Group, which is a major logistics company, operates with the largest EV fleet in Germany with 16,000 electric vans, bicycles and tricycles. The company has also undertaken in-house development and manufacturing of its

own electric vans, tricycles, and bicycles as part of this vision. Following its success in this venture, the company is now selling its EVs to third parties (mainly municipalities and other businesses).

The year 2017 recorded over one million sales of EVs worldwide, which was an increase of 50% in comparison to the previous year. Despite this, it is acknowledged that this is still a modest figure to refer to as an "electric mobility revolution". The two leading nations in this sector are Norway and China, with di fferent merits. China has the largest car market and nearly 580,000 electric cars were sold there in 2017. On the other hand, Norway can boast a greater market penetration of EVs, as 39% of vehicles sold in 2017 were electric.

In addition to e-mobility, it is necessary to give a brief overview of smart-mobility, which represents all the new possibilities of using the car and its interaction with the network [14]. On average, electric cars, due to their charging and autonomy characteristics, are used for their own movement and transport activities for very short periods, that is, they are in movement for just 5% of the time, while they remain stationary and therefore unused for the remaining 95% of time [15]. Given their flexible load when electric storage available, as well as the development of communication systems, it would be irrational to view them as just a regular load [16,17]. Their main function will remain in the transportation sector, but with a high potential in providing services, both to the grid and to the market [18].

The V2G mode of operation can be applied, exchanging energy in the system during critical conditions, and since electric vehicles are dispersed by nature, this increases their impact on grid regulation, facilitating the work of the network operator [19]. This application for EVs can reduce the overall costs of purchasing and maintaining storage units in the future, and reduce the need for grid upgrades, which would result in an economic benefit for the Distribution System Operator (DSO). Consumers providing this service through their vehicles, of course, should be properly compensated, with the possible creation of new markets [20,21]. However, the managemen<sup>t</sup> of this resource is extremely complex, with some elements requiring strong regulation. Furthermore, charging and discharging the battery for V2G increases the cycles carried out, causing greater degradation to the battery. The owner must also be paid for this reason, and must be informed about the amount of the remaining charge to be sure of being able to use the vehicle after having transferred energy to the network. The fee will depend on the market price and the time in which the vehicle remains available for the service [22,23].

Finally, vehicles must communicate with the electrical system via a special connection inside the plug. From this analysis was born the idea of a full time use of electric vehicles, using them not only for traditional activities but also as energy accumulators. In the energy managemen<sup>t</sup> process, the role played by storage systems is certainly complementary to renewable generation and contributes in giving stability to the network, both as a quick response to the required needs and as a reduction in power fluctuations related to renewable energy generation produced by other sources, such as photovoltaics and wind [24,25].

The latter, in fact, being dependent on climatic factors and therefore not programmable, cannot provide optimal and immediate solutions to sudden requests from the network, so an e fficient storage system is essential to manage the di fferences between production and use. By taking advantage of the charged batteries of electric cars connected to the network, it is possible to o ffer, in a very short period of time, an adequate response to a sudden peak in demand. If on one hand the di ffusion of electric vehicles could contribute to the stability of the network by regulating the frequency and reducing the power fluctuations linked to renewable generation, on the other hand, the uncontrolled growth of electric cars, not adequately supported by an expansion of the distribution network, could produce an opposite e ffect [26]. According to the simulations performed, the current network infrastructure has the potential to support a medium–low EV penetration (below 50%). In particular, considering the trend of the load in the tertiary sector, the e ffects of electric vehicles are not significant since the requests of recharge are concentrated in the morning and are not summed up to the typical afternoon peak.

In the residential area, however, the e ffect of electric vehicles a ffects the network more heavily, since the recharges are concentrated mainly in the evening, thus producing greater peaks. It is possible to make the vehicles interact with the grid at di fferent levels with several options, such as V2H (vehicle-to-home), V2B (vehicle-to-building) and V2G. In this way, the bidirectionality of the battery is exploited to allow an energy flow from the vehicle to the network. Vehicle batteries store a lot of energy, on average about 25 kWh (but those of the latest generation reach up to 90 kWh), which correspond to about two and a half days of energy required by normal users. It is therefore a big amount of energy that can be used. Taking advantage of this capacity is not easy, and it is convenient only when large quantities of energy are involved or when many vehicles are in the same place, located together as the only source such as in parking lots, near charging stations and in residential areas during the night. V2G can be used to provide power supply at peak demand for an intervention time varying between four and six hours every day, or for ancillary services in order to occasionally supply power to support the system in case of frequency variations due to loss of generation or problems on transmission lines. In this case, these are single interventions with a limited average duration of ten minutes. Finally, regulation can also be performed in order to exchange active and reactive power to guarantee the voltage regulation of the system. The amount of operations that can be performed is very high, but they can only be performed for short time intervals of a few minutes at a time.

V2G energy transfer has been extensively investigated in various nations to mitigate fluctuating demands and supply availability changes. For instance, Perujo and Ciu ffo [27] assessed the introduction of electric vehicles in the private fleet with regard to potential impact on the electricity supply and on the environment for a case study for Milan, Italy. Ekman [28] investigated the interaction between large EV fleets and high wind penetration in Denmark. Hartman et al. [29] assessed the influence of various exploitation scenarios of EVs on the German grid in 2030. Drude et al. [30] examined photovoltaics and V2G approaches for peak demand reduction in Brazilian urban regions in a smart grid environment. Similar studies were performed for other areas in the world [31–34].

Moreover, as disclosed in a literature review, several researchers have dealt with V2G and renewable energy source integration [35–39]; smart grid operation considering large-scale integration of EVs enabling V2G systems [40,41]; peak shaving and valley filling of power consumption using an EV parking lot [42,43]; optimization of integration of EVs in SG and EV charging stations [44–49].

As per the authors' understanding, it appears that no V2G feasibility study or alternative form of work has been accomplished on connecting the car park of Ferrara railway station in the region of Emilia-Romagna, northern Italy, to the SG.

Therefore, considering the V2G concerns and challenges in the context of the SG described above, this study aims to evaluate the impact of energy on the distribution network at the point of connection of an electric plant of a railway car park in which some charging points for EVs were installed. Case studies are presented after an overview of peak shaving and load levelling, descriptions of the various types of EVs, the charging infrastructure and the possible applications in SGs. The objective is to identify a possible load curve of the simulated car park and, based on the principle of V2G technology, an appropriate algorithm will be developed. This algorithm will explore the possibility of a two-way energy flow between the connected vehicles and the electricity grid, and perform a peak shaving of the load curve of the plant under examination in order to avoid absorption peaks which are normally di fficult to manage by the distribution system operator (DSO). The work also presents the coupling with a photovoltaic system designed specifically for the car park.

### **2. Overview of Peak Shaving and Load Levelling**

The managemen<sup>t</sup> of the electricity grid is very complex, since the power consumption by the users is characterized by very marked fluctuations [14,15]. Peak production by Renewable Energy Sources (RES) such as solar panels does not correspond with the peak loads of the system, so storing that energy in the electrical vehicles would be suitable for guaranteeing system adequacy, reducing the need of investments in the generation sector. In addition, the stability of the system might be

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endangered due to a high percentage of uncontrollable loads. The use of an electrical energy storage system according to the load levelling logic consists in the storage of energy during the low load hours of the electrical system and its subsequent supply during periods of high demand from users, as simulated in the QDSL battery model [16,17]. This operating criterion entails, above all, technical benefits, in particular by optimizing the operation of the thermal plants and the managemen<sup>t</sup> of the electricity network, in addition to the peak shaving operation, which is useful for levelling the peaks of maximum electricity demand. Furthermore, the intervention of a storage system can satisfy the peak demand while maintaining a more homogeneous generation profile. Thermal plants, in particular those that exploit low-cost primary sources, are not designed to operate at a partial load with good efficiencies. By accumulating the surplus of electricity that they generate during periods of low demand, it is possible to allow these systems to always operate near nominal load and e fficiency. The stored energy can be subsequently supplied during the hours of maximum demand by reducing the load of expensive peaking systems powered by natural gas. Higher penetration of EVs means higher electrical energy requirements, as well as bigger stress on the electrical grid. The storage systems can intervene, as previously described, to level the load curve and manage the periods of maximum demand by postponing investments on new network infrastructures and on increasing installed power (Figure 1). This operating strategy also favors the practice of energy price arbitrage. It takes advantage of the price di fference of electricity that occurs between two times of the day or week to make an economic profit for the storage system. Electricity is purchased and stored when it costs little due to the low demand from users, and then it is resold during the hours when prices are higher. From an economic point of view, it is possible to a ffirm that energy storage systems favor the reduction and stabilization of prices in the electricity market because they release electricity production from speculation and the volatility of prices linked to fossil fuels.

**Figure1.**Peakshavingand loadlevellingexample.
