*2.2. Storage System*

As said, a train can send its braking energy on the catenary, only in the vicinity of other trains running, available to adsorb that energy. Therefore, when a stationary storage system is introduced, energy recovery is enhanced. In this way, also when no other trains are present, the storage system can adsorb the energy from the trains engaged in braking, and delivering it at a di fferent time, in the presence of enough load. A detailed description of the problem, together with evaluation of the energy saving from electrical feeding substations, is widely described in [6,8,9]. The storage is normally not directly linked to the grid, but it is interfaced through the use of a DC/DC converter, having di fferent functionalities: first of all, it is possible to control the energy flows; thus, preserving the storage system by the delivery or adsorption of high current peaks. Then, SOC drift can be avoided; therefore, maintaining the battery at an intermediate SOC value; thus, avoiding progressive charging or discharging, leaving it able to recover or deliver energy. Finally, to guarantee flexibility in the storage sizing, having the battery voltage not dependent from the operating pantograph voltage.

At least in theory, each electrical feeding substation (ESS) can be equipped with its dedicated storage or, to reduce costs, just a few storage systems can be installed, in correspondence to one or a few more substations. Certainly, when the number of storage systems equals the number of the feeding substations (ESSs), it is possible to reduce the sizing of the transformer, e.g., up to half of the original power, since the extra-power needed can be delivered by the storage systems.

This aspect, also by guaranteeing a reduction of the transformer *TOC*, as general rule, can therefore compensate, or at least cancel, the extra-costs needed for the storage systems. Additionally, extra-services can also be provided, by guaranteeing the train are at least able to reach the nearest train stop, in the case of failures of one or more ESSs. In this way, system reliability is enhanced, and adequate levels of redundancy are given.

Another aspect ofinterestis given by the technology used for the application. In fact, manylithium-based technologies are today available. One of the most common, based on utilization of lithium-iron-phosphate (LFP) cells [17], considered also in [8,9], which are typically considered in energy-oriented applications, and characterized by very low costs. In the last years, other technologies have been more and more

considered, muchmore orientedin delivering or adsorbing high powers for short time durations. In particular lithium-titanate (LTO) cells [18] are specifically power-oriented, and therefore aligned to the application requirements, in which high current peaks have to be adsorbed during the regenerative braking of the trains. In fact, according to authors experience and manufacturer indications [18], they are able to sustain high charging or discharging current peaks, in the order of ten times their nominal capacity, for several tens of thousands of cycles; thus, showing also a significant life resistance to number of cycles.
