*4.2. Fuel Cell Hybrid Electric Vehicles*

For a class 4 truck, the FCHEV is optimized to minimize its RCO. When we optimize components, the fuel cell power increases more than that of an FCREx. Because an FCHEV battery has less specific energy than that an FCREx, an FCHEV has a smaller capacity but its mass is higher. Therefore, an FCHEV needs more fuel cell power to reach the same performance as an FCREx. Figure 4 summarizes the optimal fuel tank and battery pack for an FCHEV and an FCREx. Specifically, optimizing battery size for FCHEV is important. Using the rule-based process in Section 2, the battery is sized for maximum regenerative braking. This helps improve overall vehicle fuel economy. Otherwise, optimization to minimize RCO shows that a 38% smaller battery is a better choice. This results in higher fuel consumption and higher operating cost. A smaller battery pack reduces fuel economy in the vehicle optimized for ownership cost. Table 5 shows how battery pack size affects fuel economy. Figure 6a shows battery power operating points and the optimized reduced operating range. Figure 6b,c are motor power and fuel cell power, respectively. The optimized battery reduces regenerative braking torque. Motor size remains unchanged due to performance requirements. As mentioned above, in the rule-based results, the battery is sized to maximize regenerative braking. In the optimized design, however, only the RCO value is considered for component sizing. By optimizing FCHEV components, the fuel mass and battery capacity decrease and the fuel cell power increases while satisfying vehicle performances. Although both processes ensure the same performance, the optimized design reduces RCO. Optimization strikes the right balance between higher initial cost and

higher operating cost. A large battery results in higher initial cost but increases fuel economy through additional regenerative braking. A small battery results in higher operating costs but reduces initial cost because the smaller battery is less expensive.


**Table 5.** Comparison of results for rule-based and optimized FCHEVs.

**Figure 6.** Simulation results for rule-based and optimized FCHEVs: (**a**) battery power operating points, (**b**) motor power, and (**c**) fuel cell power.

When the ownership period and hydrogen cost increase, the optimum solution also changes. Next we consider an ownership period of 10 years and a hydrogen cost of \$12 per gge. Table 6 summarizes the results for an FCHEV. Fuel economy increases in importance due to the higher cost of fuel, making a larger battery feasible. This gets closer to the battery size necessary to minimize fuel consumption. The optimization led to a 0.7% drop in fuel economy, reducing the RCO by about \$1,000. The optimum solution is sensitive to ownership periods and hydrogen cost assumptions. We compare both rule-based and optimized RCOs for class 4 trucks, as shown in Figure 7. Case #1 results when the ownership period is 5 years and the hydrogen cost is \$4/gge. Case #2 results when the ownership period is 10 years and the hydrogen cost is \$12/gge. All other assumptions remain the same. In this case, the optimized FCREx is cheaper than the optimized FCHEV by about \$800. When the hydrogen cost is \$4 over a 5-year ownership period, FCHEV and FCREx RCOs decrease by 1.8% and 24.2%, respectively. On the other hand, when the hydrogen cost is \$12 over a 10-year ownership period, FCHEV and FCREx RCOs decrease by 0.5% and 9.4%, respectively. Infrastructure cost is not considered in this analysis. There is no cost assigned to downtime associated with charging.


**Table 6.** Summary of results for class 4 fuel cell hybrid electric vehicle (FCHEV) with various assumptions.

**Figure 7.** Comparison of relevant cost of ownership (RCO) for class 4 delivery van.
