*3.1. Constant Load*

The fuel savings for each RTOk strategy compared to sFF strategy (Δ*FuelTk* = *FuelTk* − *FuelT*0, k = 1 ÷ 7) is listed in Tables 2–4 for *kfuel* = 0, *kfuel* = 25, and *kfuel* = 50 and different values of the constant load (*Pload* mentioned in first column of the Tables 2–4). The second column of the Tables 2–4 mentions for the sFF strategy the hydrogen consumption (*FuelT*0) during a load cycle.

ΔΔΔΔΔΔΔ

ΔΔΔΔΔΔΔ

The fuel economy (Δ*Fuel*) for all RTOk strategy (k = 1 ÷ 7) are presented in Figures 2–4 for *kfuel* = 0, *kfuel* = 25, and *kfuel* = 50.

**Figure 2.** Fuel economy for *kfuel* = 0 and different *Pload*.

**Figure 3.** Fuel economy for *kfuel* = 25 and different *Pload*.

**Figure 4.** Fuel economy for *kfuel* = 50 and different *Pload*.

ΔΔΔΔΔΔΔ

ΔΔΔΔΔΔΔ

In case of *kfuel* = 0 (see Figure 2), the optimization function will be *f*(*x*, *AirFr*, *FuelFr*, *PLoad*) = *PFCnet*, so the optimization objective is to increase the FC net power. Thus, the fuel savings will be higher in the case of *kfuel* 0 (see Tables 3 and 4 compared to Table 2) because the optimization function *f*(*x*, *AirFr*, *FuelFr*, *PLoad*) = *PFCnet* + *k f uel* · *Fueleff* takes in consideration the efficiency of the fuel consumption (*Fueleff*). It is worth mentioning that the fuel economy does not increase by increasing the value of the *kfuel* parameter (see Table 4 compared to Table 3). In fact, a sensitivity analysis of the fuel economy to values of the *kfuel* parameter in range 0 to 50 has been revealed the multimodal variation of the fuel economy for all RTO strategy. Based on this analysis performed in other work, the best economy can be obtained for a value of the *kfuel* parameter in range 20 to 30, but this is not the goal of the work shown here. As mentioned before, the goal is to identify which is the best RTO strategy in the full range of load demand or the best two RTO strategies that can be used: one in the range of high load demand and the other in the rest of range (of low load demand), respectively.

The analysis of Figures 2–4 highlights the best strategies for high load demand (*Pload* = 8 kW), rated load demand (*Pload* = 6 kW), and low load demand (*Pload* ≤ 5 kW).

In the case of *kfuel* = 25 (see Figure 3) the order of the RTO strategies (starting with that which gives the best fuel economy) is as follows:


It is worth mentioning this order in the case of *kfuel* = 0 and *kfuel* = 50.

In the case of *kfuel* = 0 (see Figure 2) the order of the RTO strategies (starting with that which gives the best fuel economy) is as follows:


In the case of *kfuel* = 50 (see Figure 4) the order of the RTO strategies (starting with that which gives the best fuel economy) is as follows:


Note that the same RTO strategies are in the first four positions for *Pload* = 8 kW and *Pload* = 6 kW when *kfuel* = 0, *kfuel* = 25, and *kfuel* = 50. The strategies RTO4 and RTO6 are between these four positions, but also are in the first two positions for *Pload* ≤ 5 kW. So, the strategies RTO4 and RTO6 can be used to optimize the FCHPS operating in the entire loading range. It is difficult to say which is the recommended strategy for variable load demand. So, more tests will be performed in the next section.

Also, it is worth mentioning that the strategies RTO5 and RTO3 cannot be used for *Pload* ≤ 5 kW due to low performance compared to other RTO strategies (such as the strategies RTO4 and RTO6, for example). So, it is clear that a switching (SW) strategy (which will use the strategies RTO4 and RTO6 for *Pload* ≤ 5 kW and the strategies RTO5 and RTO3 for *Pload* > 5 kW) can improve the obtained fuel economy, as compared to the case of one RTO strategy being used in the full range of load demand. The following SW strategies are identified based on the above results:


• SW4 strategy, which will use the strategies RTO4 and RTO3 for *Pload* ≤ 5 kW and *Pload*> 5 kW, respectively;

The strategies SW1, SW2, and SW3 request two GES controllers, due to the use of strategies RTO6 and RTO5, instead of one GES controller as for SW4 strategy.
