5.2.3. On-Board ESS

The main constraints of the on-board ESS sizing are the energy required by the cycle and the maximum power to be delivered. In the case of an electrochemical storage system, this must provide both the energy required by the cycle and the power during peaks related to accelerations. Instead, in the case of a H-ESS consisting of batteries and supercapacitors, the power profile required by the train is divided into two parts, obtaining a reference profile for the battery pack and another for the supercapacitor pack.

The high autonomy ESS implies an autonomy to go from Station A to Station B and recharge the storage system at Station A. According to this cycle, the energy to be supplied to the rolling stock is equal to 585 kWh. First, the use of high-power lithium-ion cells has been analyzed, whose characteristics are shown in Table 1. In this case, the sizing results in a 980 kWh (650 Ah) battery pack, of which only 586 kWh can be used, due to SOC limits. In Figure 17, PBATT represents the power associated with the battery pack, positive in the traction phase, and negative in the braking phase. The voltage of the battery pack is identified as VBATT and presents variations that re less than ±5%. The supplied current is identified as IBATT and is positive in the discharge phase (traction) and negative in the charging phase (regenerative braking). Since the capacity of the battery pack (C) is equal to 650 Ah, it is observed that both the maximum discharge and charge current are much lower than the maximum allowed by the battery pack (20C and 4C respectively, in the discharge phase and charge). Moreover, it is observed that the sizing is appropriate because the SOC does not drop too low, and over time (despite greater discharges), the SOC will not drop below a certain minimum (30%).

Afterwards, a high autonomy H-ESS has been analyzed. It is composed of the high-energy lithium-ion and supercapacitor cells, whose characteristics are shown in Tables 1 and 2. The subdivision of the original power profile is achieved through a low pass filter with a cut-off frequency of 1 Hz and an amplitude limiter which limits the maximum power supplied by the batteries to reference value equal to 2 MW, in order to reduce the maximum power peak that they must supply to about 50% of the original one. Instead, the regenerative braking power is limited in absolute value to about 5% of the maximum braking power (0.1 MW), in order to make the supercapacitors recover as much energy as possible during the braking phase. In this way two reference profiles are obtained to carry out the sizing: P\_REF\_B for the battery pack and P\_REF\_SC for the supercapacitors, as shown in Figure 18.

**Figure 17.** High autonomy ESS parameters-High-power lithium ion cells.

**Figure 18.** Power profiles H-ESS.

Carrying out the verification simulations of the H-ESS, it is observed in Figure 19a that the current required to the battery pack (in blue) tends to exceed the limits set by the manufacturer during discharge and is therefore limited to a lower current (in orange). Moreover, the supercapacitor SOC reaches too low values, as shown in Figure 19b. To avoid these situations, it is necessary to increase the quantity of parallel branches of high-energy lithium and supercapacitors cells, further increasing the total mass of the storage system. This hybrid solution is thus abandoned because it is too heavy.

**Figure 19.** H-ESS: (**a**) Current required to the battery pack; (**b**) Supercapacitor pack SOC.

The ESS with recharge station implies an autonomy to go from Station A to Station B, where the recharge needs to be performed. According to this cycle, the energy to be supplied to the rolling stock is equal to 383 kWh. It has been considered the use of high-power lithium-ion cells and the sizing procedure results in a 640 kWh (425 Ah) battery pack, which does not exceed the limits set by the manufacturer. The recharge at Station B can be performed from the 30% to the 50% of the SOC, since it allows arriving to Station A within the available SOC range. Therefore, the recharge must have a duration of 6 min, at 2C rate, implying a recharge station of at least 1.4 MW.

In Figure 20 it is shown the charging time for both high autonomy ESS and ESS with recharge station, to be performed at Station A (in the electrified section). This recharge takes the SOC from 30% to 90% and it is performed, at 2C, in 18 min (LFP cells). Table 4 presents a synthesis of the viable on-board ESS scenarios, including the ESS impact in the energy required by the rolling stock because of the weight increase.

**Figure 20.** On-board ESS charging time.

**Table 4.** On-board ESS scenarios.

