Energy Distribution during Recharging for Constant Battery with Different Fuel Cell Powers 

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... first describes how additional fuel cell system power increases hybridization while battery size is kept constant — in other words, the trade-off between the power of a H2-ICE power and the fuel cell system. The second describes the effects of additional battery sizes under constant fuel cell power — in other words, the trade-off between battery power and H2-ICE power. The test matrix is shown in Appendix 2. Twenty-four vehicles were defined to represent all the possible combinations. All vehicles have been sized to achieve similar performance. As the battery power is maintained, the addition of fuel cell power leads to a decrease in H2-ICE power. On the basis of the assumption that the gradeability requirements have to be fulfilled by the H2-ICE only, several vehicles were not simulated because the H2-ICE power was too low. Figure 9 illustrates the fuel economy results for several battery sizes. Similar to the previous case, the fuel economy significantly improves when fuel cell power increases from 0 kW to 20 kW along with each constant battery power curve. Using the fuel cell as a primary source, rather than the H2-ICE and battery, leads to a significant increase in fuel economy because it is a highly efficient power source. Figure 10 explains the energy distribution during propulsion for the 29-kW battery with small (5 kW) and large (20 kW) fuel cell powers. The cumulative energy in kilowatt-hours (kWh) and the percentage of power dissipated by the component are shown. With a 5-kW fuel cell system, the H2-ICE plays an important role in propelling the vehicle (24%). Because the degree of hybridization is significant and the power requested during the drive cycles is low, the electric motor still provides most of the energy (76%). However, the low amount of fuel cell power allows the system to provide about half of the energy while the battery provides the remaining energy. Because the battery SOC has to remain constant between the beginning and the end of the drive cycle, the battery needs to be significantly recharged, which uses some energy from the H2-ICE, in addition to the regenerative braking. This process leads to overall powertrain inefficiencies. With a 20-kW fuel cell system, the H2-ICE is subjected to large power requests, but it provides only 5% of the requested energy. In the current research, the cumulative energy withdrawn from battery was reduced by half, and the fuel cell use almost tripled. The increase in fuel economy is a result of maximizing the use of highly efficient components while minimizing the energy from the H2-ICE used to recharge the battery Figure 11 shows energy distributions among the power sources during the recharging state. The middle chart represents the cumulative energies of an electric motor and the H2-ICE to recharge the battery. The left chart indicates how the fuel cell, regenerative braking, and engine supplied the power to increase the SOC level of the battery when the SOC is below its target. The right chart shows how much engine power goes either to the motor to recharge the battery or to the wheels to sustain the required power to follow the cycle during the recharging state. The chart helps one understand engine behavior during the recharging state. Although the H2-ICE was often used to recharge the battery when lower levels of fuel cell power were used, the engine energy used to charge the battery became negligible with a 20-kW fuel cell system. Rather than using a less-efficient component to recharge the battery (the H2-ICE provided 41% of battery charging energy for the 5-kW fuel cell case), most of the energy comes from the wheel in the 20-kW case. In fact, as the electric motor provides higher efficiency than the H2-ICE, even with a low level of fuel cell system power, the control strategy will favor the electrical components. As a consequence, the battery will be used to provide some energy for propulsion, although that energy should be returned to the battery to achieve constant SOC. With an appropriate level of fuel cell system power, the electric motor and the fuel cell can be used to provide energy for propulsion while the battery can be mostly dedicated to regenerative braking. Figure 12 shows the percentage of recoverable regenerative energy. As the size of the battery is maintained while the power of the fuel cell system is increased, the values are almost constant and vary only as a result of changes in vehicle characteristics (e.g., mass) and control strategies (e.g., SOC variation). Increased battery sizes allow increased recoverable regenerative braking energy and, consequently, higher fuel economy. An average improvement in fuel economy of 10 mpg was obtained as a result of increasing battery size from 18k W to 40 kW. Both the increase in fuel cell system power and the increase in battery power lead to an increase in the degree of hybridization. Figure 9 shows that an increase in hybridization leads to higher fuel economy. In addition, the introduction of a fuel cell system while maintaining battery size has a significant impact on fuel economy. The increase comes from not only using more efficient power sources but also from maintaining the maximum use of regenerative braking to sustain the SOC of the battery. A high degree of hybridization achieved by a trade-off between the H2-ICE and fuel cell or a trade-off between the H2-ICE and the battery improves the fuel economy significantly. Figures 13 and 14 show the overall effect of combining a H2-ICE HEV with a fuel cell system. The cases for constant hybridization (Cst Hyb.) and constant battery power (Cst Battery) are illustrated in the same graph. Figure 13 shows the fuel economy as a function of battery size. Each curve represents the vehicles with constant fuel cell system power. The results demonstrate that an HEV with a 40-kW battery and a 20- kW fuel cell provides the highest fuel economy. Overall, an increase in power from both the fuel cell system and battery will lead to higher fuel economy. However, this vehicle will be the most expensive to build. The fuel economy abruptly drops at low battery power with high fuel-cell-system power as a result of a loss in regenerative braking. Figure 14 shows the ratio between fuel economy and cost. This ratio indicates the fuel economy in miles-per- gallon, which takes into account the impact of the powertrain cost. Generally, the vehicles in the upper right-hand corner in Figure 14 can be considered as the most cost-effective configurations, which operate with low fuel-cell-system power and high battery ...