Effective Optimization of Power Management for Fuel Cell Hybrid Vehicles Based on Pontryagins Minimum Principle

Abstract

A considerable amount of research on power management strategies of hybrid vehicles has been conducted during last few decades in order to improve fuel economy and performance of hybrid vehicles. This dissertation introduces a Pontryagins Minimum Principle (PMP)-based power management strategy for fuel cell hybrid vehicles (FCHVs) and extends this strategy mathematically for considering three important factors in FCHVs. These factors include limitations on the battery state of charge (SOC) usage, the fuel cell system (FCS) lifetime, and the effects of battery thermal management on the fuel economy. The PMP-based power management strategy is implemented in a computer simulation for each case. The limitation problem on the battery SOC usage is solved by introducing a new cost function other than the fuel consumption rate to the PMP-based optimal control problem. The limitation requirements on the battery SOC are satisfied while minimizing the fuel consumption by this solution. In order to take into account the lifetime of an FCS while considering its fuel consumption minimization, a second cost function is defined and added to the PMP-based optimal control problem. The second cost function is related to the power changing rate of the FCS. Simulation results show that the lifetime of the FCS can be prolonged by the reformulation of the PMP-based optimal control problem. However, there is a tradeoff between the FCS lifetime and the fuel consumption because of the added cost function. The effect of battery thermal management on the total fuel consumption is considered by designating the battery temperature as an extra state variable other than the battery SOC in the PMP-based optimal control problem. The relationship among the final battery SOC, the final battery temperature, and the total fuel consumption is illustrated by simulation results. This relationship can be expressed by a surface, which is composed of two intersecting half-planes with similar gradients. This surface is defined as an optimal surface in this dissertation, which indicates the optimal solutions, as it is derived from the PMP-based power management strategy. Fuel economy potential gains attributed to the battery thermal management are determined using the optimal surface. The battery thermal management can improve the fuel economy of an FCHV up to 4.77% depending on the driving cycle. A discussion on the combined case is carried out to consider the three factors together. For the three extended cases, global optimality of the PMP-based power management strategies is discussed. Simulation results of the PMP-based strategy are also compared to those of Dynamic Programming (DP) approach for the three cases. The PMP-based power management strategy saves much time compared to DP approach while it guarantees global optimality under battery assumptions. The time-saving effect of the PMP-based strategy is outstanding especially when there are more than two state variables.