Abstract
Hybrid and electric vehicle batteries deteriorate from use due to irreversible internal chemical and mechanical changes, resulting in decreased capacity and efficiency of the energy storage system. This article investigates the modeling and control of a lithium-ion battery and ultracapacitor hybrid energy storage system for an electric vehicle for improved battery lifespan and energy consumption. By developing a control-oriented aging model for the energy storage components and integrating the aging models into an energy management system, the trade-off between battery degradation and energy consumption can be minimized. This article produces an optimal aging-aware energy management strategy that controls both battery and ultracapacitor aging and compares these results to strategies that control only battery aging, strategies that control battery aging factors but not aging itself, and non-optimal benchmark strategies. A case study on an electric bus with variously-sized hybrid energy storage systems shows that a strategy designed to control battery aging, ultracapacitor aging, and energy losses simultaneously can achieve a 28.2% increase to battery lifespan while requiring only a 7.0% decrease in fuel economy.
Highlights
Due to their low operating speeds and frequent stopping and starting, buses are a prime candidate for hybridization or electrification in the goal of reducing transportation sector emissions
For the cases that use a battery aging model, the lifespan of the Stochastic Dynamic Programming (SDP)-controlled battery is typically within 1% of the DDP result for a given MPGe, while the difference is greater for the controller that only limits battery power, especially near the peak
This paper develops controllable battery and ultracapacitor aging models for a Hybrid Energy Storage System (HESS)
Summary
Due to their low operating speeds and frequent stopping and starting, buses are a prime candidate for hybridization or electrification in the goal of reducing transportation sector emissions. Reference [18] considered a HESS that used lead-acid batteries rather than lithium ion, and developed an HEV energy management strategy that tuned for battery life extension They found that, for the HESS to be cost-effective, a 50% increase in battery cycle life was required. Reference [19] compared the aging benefits of an optimally-sized HESS to the theoretical maximum benefits—battery aging reductions with an infinitely large HESS These benefits were experimentally verified, with the developed approach decreasing battery power fade and temperature rise in lithium-ion batteries on a vehicle load profile. Studies on direct aging control for HEVs do exist, for instance [2,3,23,24], EVs pose a unique control problem due to the fewer controlled variables and different component sizes This research fills these gaps in knowledge: new energy management strategies to control battery aging and to jointly control battery aging, ultracapacitor aging, and energy losses are developed and compared to existing methods. These simulation results are analyzed, and conclusions are drawn regarding the benefits of optimal control and aging-aware control for vehicle energy management
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