# An Analysis of the Altitude Impact on Roots Compressor Operation for a Fuel Cell System
## 3 Results
### Altitude Impact  
Figure 6 analyzes the system-level performance as a function of the altitude level. Figure 6A shows the optimal pressure ratio for each altitude as a function of the balance-of-plant net power. Until approximately 100 kW, it can be seen that the optimal pressure ratio is approximately constant, with very little variation in terms of the altitude condition as well. From that point, the optimal pressure ratio increases rapidly when increasing the net power demand, which occurs more aggressively as the altitude increases, in order to compensate for the lower environmental pressure. For the most demanding conditions, the compressor can take up to 18% of the total electrical power produced by the fuel cell (Figure 6B). Looking at the extension of the data in the x-axis, it is noticeable that the maximum power also deteriorates with the altitude, with a maximum value of 135 kW (vs. the nominal 200 kW, a 32.5% deterioration) for the highest altitude of 5 km. Finally, Figure 6C provides the information on the balance-of-plant efficiency, where a deterioration between 3 and 7% can be observed when increasing the altitude at constant net power demand. This deterioration is more severe as the system approaches the rated power condition. It has to be noted that the balance-of-plant efficiency not only includes the effect of the compressor operation, but also two other power consumptions coming from the coolant and hydrogen recirculation pumps. However, these two contributions are nearly independent of the altitude since the hydrogen excess ratio and operational temperature are kept equal for all the simulations performed. Additionally, the pumps are simulated with a fixed efficiency. Instead, the altitude does have a direct impact on the compressor corrected mass flow, pressure ratio, and efficiency that drives the differences observed. Figure 7 analyzes, in further detail, the operation of the compressor in terms of the pressure ratio (Figure 7A) and efficiency (Figure 7B) evolutions against the corrected mass flow. As can be seen, the reduction in the ambient pressure as a function of the altitude tends to increase the corrected mass flow values, whose maximum moves from around 0.25 kg/s at sea-level conditions to 0.35 kg/s in the most extreme conditions. However, thanks to the characteristics of the roots compressor selected for this study, these conditions still fall into the high-efficiency region of the compressor map, allowing for the maintenance of efficiencies between 60 and 70% throughout the majority of the operating range, even at 5 km altitude. This result confirms the suitability of this compressor technology to minimize the negative impact of altitude on the working range and efficiency of fuel cell-powered vehicles compared to other boosting systems available. However, it has to be noted that for the highest altitude of 5 km, the highest working point is limited to a corrected mass flow of  
Figure 5: Balance-of-plant efficiency as a function of the pressure ratio at sea-level conditions. 0.3 kg/s and a pressure ratio of 1.8, which is well inside the compressor map. Therefore, the net power is not limited by the extension of the compressor map itself, but by the fact that after this point, the compressor efficiency starts deteriorating again. Thus, the expected increase in the fuel cell stack power induced by the higher pressure and air mass flow is also overcome by the higher compressor power consumption coming from the lower efficiency operation. Figure 6: Balance-of-plant efficiency and compressor as a function of the altitude: (**A**) optimal pressure ratio; (**B**) ratio of compressor power and net BOP power; and (**C**) BOP efficiency. Finally, Figure 8 shows the information about the fuel cell stack operating conditions as a function of altitude. Figure 8A depicts the actual pressure achieved in the cathode inlet stream. There is a clear decreasing trend in the working pressure with the altitude in the low-to-mid net power conditions.