## 3 The Impact of Size and Dynamic Limitations
### Impact on the Performance of Fuel Cell Hybrid Vehicles  
The previous analysis discussed the impact of size and dynamic limitations on the performance output and degradation rate of fuel cell systems. However, in practical applications, it is more important to consider the changes in overall vehicle power performance and economy. The overall energy consumption of a fuel cell hybrid electric vehicle is ultimately provided by hydrogen gas. From the perspective of energy conversion, electrical energy consumption can be equivalently translated into the mass of hydrogen consumed by the fuel cell. In this case, the hydrogen consumption of the vehicle can be calculated using the following formula:  
\[m_{veh}=m_{fc}+\frac{\Delta SOC\cdot Q_{bat}}{\eta H_{L}} \tag{17}\]  
In this section, hydrogen consumption was normalized in order to visually compare its relationship with power. Figure 9 illustrates the impact of the current variation rate and maximum power on the overall vehicle hydrogen consumption. As mentioned earlier, a higher-rated power of the fuel cell leads to lower current density and higher efficiency when operating at the same power. The hydrogen consumption presented here is the actual hydrogen consumption of the fuel cell, converted and compared with the variation in the power battery's state of charge (SOC). Due to the limitations in the fuel cell's current density variation rate, there are cases where the power battery's SOC at the end of the operation does not return to its initial state. To account for this, this portion is converted into equivalent hydrogen consumption for comparison. From Figure 9, it can be observed that the equivalent hydrogen consumption for a fuel cell hybrid vehicle is the lowest at 160 kW. Increasing or decreasing the fuel cell's rated power will affect the hydrogen consumption economy of the entire vehicle. This is because when the fuel cell's size is reduced, it tends to operate in the high current density region to meet the vehicle's energy demands, resulting in decreased efficiency. Compared to a fuel cell with a power rating of 160 kW, a 100 kW fuel cell exhibits an increase in hydrogen consumption ranging from 6% to 33%. On the other hand, increasing the power of the fuel cell does not proportionally increase hydrogen consumption, even though the fuel cell operates more efficiently. Increasing the power of the fuel cell stack necessitates higher power consumption by accessories such as the compressor, leading to a higher increase in the overall hydrogen consumption of the fuel cell vehicle compared to the increase in power. As a result, a fuel cell with a rated power of 200 kW exhibits a hydrogen consumption that is 1% to 7% higher than that of a 160 kW fuel cell. Regarding the current density variation rate of the fuel cell, an unrestricted state (1 A/(cm\({}^{2}\cdot\)s)) does not correspond to the lowest hydrogen consumption. By limiting the current density variation rate to 0.01 A/(cm\({}^{2}\cdot\)s), the hydrogen consumption of the 160 kW fuel cell can be further reduced by 1%. Combined with the analysis in Figure 8c, it can be observed that when the current density variation rate is limited, the fuel cell tends to operate in a steady-state region, achieving a balance between economy and power under conditions with relatively minimal changes. This also ensures the maintenance of charge capability. However, when the current density variation rate is limited to 0.001 A/(cm\({}^{2}\cdot\)s), the fuel cell's power output is insufficient, resulting in more energy being supplied by the power battery. At the end of the operation, the power battery's charge maintenance capability is insufficient, resulting in a maximum difference of 29.8%. The lack of energy from the power battery needs to be converted into equivalent hydrogen consumption. This conversion leads to an increase of 32.6% in hydrogen consumption for a vehicle equipped with a 100 kW fuel cell. In vehicles equipped with a 160 kW and 200 kW fuel cell, hydrogen consumption increases by 7% and 10%, respectively.