## 5 Electric Bus Specification and Energy Storage Unit Sizing  
In this research, the authors intend to design a HESS for a pure electric bus for an urban public transportation application. The very first step for this design is to specify the bus requirements, including the electric motor power, energy storage capacity, passenger capacity, and requested range. The bus specifications are presented in Table 2, which is based on a commercial electric bus (Solaris Urbino 12 Electric) produced in Poland.  
In order to calculate the peak current demand and the requested energy to travel for 240 km with one charge, the Millbrook London Transport Bus (MLTB) driving cycle is selected. As shown in Figure 12, the driving cycle is composed of Inner and Outer London to have a more realistic load profile.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Length & 12 m \\ Net Weight plus passengers & 13.6 t + 6.4 t \\ Electric motor power & 120 kW (two motors of 60 kW) \\ Range & 240 km \\ Battery capacity & 240 kWh \\ \hline \hline \end{tabular}
\end{table}
Table 2: Electric bus specification.  
Figure 11: The LiB and LiC current component for the proposed electric bus based on the London driving cycle.  
Table 3 gives a summary of this driving cycle.  
The speed load profile should be converted to the power and current profile for further calculation. Based on the simple physics equations we have:  
\[a\ =\ \frac{V_{2}-V_{1}}{t_{2}-t_{1}} \tag{11}\]  
where \(V_{2}\)\(\left(\frac{\mathrm{m}}{\mathrm{s}}\right)\) and \(V_{1}\)\(\left(\frac{\mathrm{m}}{\mathrm{s}}\right)\) are the speed at \(t_{2}\)\(\left(\mathrm{s}\right)\) and \(t_{1}\)\(\left(\mathrm{s}\right)\) time sequences, respectively, and a \(\left(\frac{\mathrm{m}}{\mathrm{s}^{2}}\right)\) is the acceleration. The required force \(F\)\(\left(\mathrm{N}\right)\) to reach this acceleration is calculated based on the total mass \(m\)\(\left(\mathrm{kg}\right)\) of the vehicle as given in the Equation (12):  
\[F\ =m\ \times a \tag{12}\]  
Having the average speed in each time interval, the average power can be calculated based on the Equation (13):  
\[P_{ave}\ =V_{ave}\times F \tag{13}\]  
where \(V_{ave}\) is the average speed in one sampling period. Then the power transformation efficiency from mechanical form to the electrical form and from AC mode to the DC mode, and vice versa, is considered to calculate the required power profile. In this study, the transformation efficiency is calculated as 80%. Figure 13 shows the power profile of the MLTB driving cycle.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Parameter** & **Unit** & **Ph1 Outer London** & **Ph2 Inner London** & **Overall** \\ \hline Total time & s & 1381 & 901 & 2282 \\ Time at idle & s & 1381 & 901 & 2282 \\ Distance & km & 6.46 & 2.50 & 8.96 \\ Average speed & km/h & 22 & 15.9 & 20.5 \\ Max speed & km/h & 47 & 34.4 & 48.7 \\ Max acceleration & m/s\({}^{2}\) & 5 & 1.5 & 1.5 \\ Max deceleration & m/s\({}^{2}\) & \(-\)2.1 & \(-\)2.2 & \(-\)2.2 \\ No of stops \(\geq\)15 s & & 11 & 8 & 19 \\ \hline \end{tabular}
\end{table}
Table 3: Summary of the MLTB driving cycle.  
Figure 12: Speed profile of the MLTB driving cycle.  
As it can be seen in Figure 13, there are positive and negative power peaks. Negative power peaks mean that the power returns to the HESS. To have a more efficient electric bus and to improve the energy density of the vehicle, the negative power should be restored in the ESS.  
As mentioned before, in this study the NMC 20 Ah cell is used as a HE storage unit. The specifications are given in [13]. The discharge and charge current for this cell are limited to the 5 C and 0.5 C, respectively. Specific measures should be taken in these cases to avoid extra current extraction and insertion which can negatively affect the lifetime of the cell.