**3. Results Comparison**

This chapter compares the results of the three described energy storage systems installed on the same bus, with nominal specification described in Table 4. The comparison refers to those systems that offer the same transport service of 100 km per day (300 day per year). The bus powertrain remains the same, the traction motor requires 25 kW during peak request and 20 kW continuously.


**Table 4.** Bus nominal specifications.

Each prototype had a different weight, due to different technology installed, from batteries (SC if present), BMS, mechanical supplementary frame, pantograph, to additional electronics, etc. Weights are detailed in Table 5. Such differences of weight could affect performances. Hence, it was conducted tests of maximum speed, maximum acceleration and time from zero to maximum speed. The results indicate that there were no relevant differences, all buses reach 33 km/h in 60 s with a maximum acceleration of 0.6 m/s2.


**Table 5.** ESS weight in the projects.

The comparison shown in Figure 12 allows to assume the same average consumption, independently from technology. Due to minimum differences measured during tests, it could be different due to phenomena such as orography, payload, driving behavior, etc. Hence, the adoption of the same energy consumption value is a conservative choice. Figure 12 shows the average consumption versus the average speed trend, during different missions for the three projects P1, P2 and P3. The cloud is denser for P1, due to the large amount of data.

**Figure 12.** Trend of energy consumption related to average speed.

A slight difference in average consumption, e.g., of about 0.1 kWh/km (20% of average consumption), corresponds to consumption of 10 kWh per day and 3000 kWh per year.

The average energy cost depends on market factors and power requirements; in Italy, this value is between 0.1 and 0.3 Euro per kWh, leading to a cost of 300 to 900 Euro per year.

Hence, the economic value of energy is negligible in comparison with the battery itself.

Table 6 shows a comparison of parameters (partly proposed by [34]), costs and results.


**Table 6.** Comparison of cost parameters.

Table 6 is divided into main components: battery, supercap and charger; each component is detailed in the following.

Battery parameters are:


Supercap parameters are:


Charger parameters are:

• Cost due to one bus: some chargers can be used for many buses. The cost is the same paid for prototypes, but it can be reduced with a large-scale production. In other words, the P3 was equipped with flash charge and it used a charger only for forty seconds, then charger required five minutes more to restore its energy before charging another bus. A single charger costs up to twenty thousand Euros.

The SC adoption in P3 and P4 achieves two grea<sup>t</sup> benefits: it reduces the maximum current and the overall energy drained by the battery, during a daily transport service. Such benefits enlarge the number of battery cycles, while reduce the amount of energy stored on board (it decreases battery dimension, weight and cost).

Figure 13 shows trends of costs for each project, included the P4 with supercapacitor and lithium batteries (represented by black dots). Figure 13 highlights when hardware replacements will occur (as bus chassis, batteries, etc.) in a twenty-four-year timeframe.

**Figure 13.** Economic comparison.

All projects have the same incomes, due to the same transport service; only the costs change from one to another. Hence, a lower cost means a better NPV.

Worth of incomes depends on factors as transport policy, economic situation, social aspects, political choices, etc. Thus, calculating the incomes is not useful since they will equally affect all systems.

Table 7 shows results of the sum between the actualized costs for the four alternatives. This parameter is used in the net present value (NPV) evaluation [12], the cost of capital is set to 3%. P4 has lower actualized costs, closer to P3. These two projects have a higher purchase cost due to SC, but they have larger lifetime. However, starting from the seventh year, the sum of costs (as shown in Figure 13) is lower than in P1 and P2.

**Table 7.** Actualized costs.

