**5. Case Study 2: Optimizing a Class 8 Linehaul Truck**

Likewise, for a class 4 delivery van, we apply the optimum sizing process to FCREx and FCHEV class 8 linehaul trucks. The truck requires 483 km of range, less than 64 s of acceleration time, and 48 km/h of grade speed. For a class 8 linehaul truck, we modify this assumption: its VMT is 160,934 km per year and the fuel cell cost is \$200 per kW. A class 8 linehaul truck usually operates on the highway, so we simulate it on an EPA 65 driving cycle. Vehicle specifications appear in Table 2. Its objective is also to minimize RCO.

When components are optimized, an FCHEV is cheaper than an FCREx. The optimized FCREx relies primarily on onboard hydrogen storage. Table 7 summarizes the results for a class 8 linehaul truck. Figure 8 compares hydrogen mass and battery capacity sizes for an FCHEV and an FCREx. Battery size decreases from 770 kWh to 24 kWh, and the vehicle runs mostly in charge sustaining mode. Arbitrarily sizing the battery power to supply half the daily driving is not optimum. Fuel cell power and hydrogen storage compensates for the reduction in battery size. The fuel cell power and hydrogen storage increase by 50 kW and 22 kg, respectively. FCHEV sizing remains largely unchanged from the rule-based approach. The result chose a slightly smaller fuel cell and battery. This is likely because the optimization utilized the 2% tolerance allowed in grade speed and acceleration.



**Figure 8.** Summary of optimized hydrogen tank and battery pack for class 8 fuel cell vehicles.

The optimized FCREx has larger hydrogen tank than that of the optimized FCHEV. As the weight of the powertrain components increases, the optimization to satisfy the performances increases hydrogen tank as well as battery. The increased battery capacity increases the weight, and then the hydrogen tank also increases to cover longer distance.

The RCOs of class 8 linehaul trucks are compared in the rule-based technique and optimized as shown in Figure 9. A fuel cell-dominant hybrid is the most economical design choice for a class 8 linehaul fuel cell truck. The present value of costs is almost as high as the purchase price. This indicates that this design solution depends on VMT, energy cost, and duration of ownership. An FCHEV has the lower fuel cost in this case.

**Figure 9.** Comparison of relevant cost of ownership (RCO) for a class 8 linehaul truck.

For longer distance class 8 linehaul trucks, we changed the optimization objective to 644 km. We assume that the cost of hydrogen increases to \$12 per gge. Onboard hydrogen storage increases by optimizing components. Table 8 summarizes the results. Hydrogen storage increases by 20 kg for 161 km. It needs more fuel cell power to sustain both grade speed and acceleration performance. FCREx sizing increases fuel cell power by about 50 kW and FCHEV sizing also increases fuel cell power by 10 kW to compensate for the loss in battery power. EPA 65 may not offer many opportunities for regenerative braking either. The impact of battery size on fuel economy is not very prominent.


**Table 8.** Optimization results for a class 8 linehaul truck driving 643.7 km.

We add the RCO obtained by optimizing a class 8 linehaul truck for 644 km to Figures 7 and 8. In a longer-range case, a fuel cell–dominant hybrid is also the most economical design for a class 8 linehaul truck. Higher hydrogen costs result that the present value of total fuel costs is more than twice the vehicle purchase price.
