Design and Real-Time Controller Implementation for a Battery-Ultracapacitor Hybrid Energy Storage System
In this work, two real-time energy management strategies have been investigated for optimal current split between batteries and ultracapacitors (UCs) in electric vehicle (EV) applications. In the first strategy, an optimization problem is formulated and solved using Karush-Kuhn-Tucker (KKT) conditions to obtain the real-time operation points of current split for the hybrid energy storage system (HESS). In the second strategy, a neural network based strategy is implemented as an intelligent controller for the proposed system. To evaluate the performance of these two real-time strategies, a performance metric based on the battery state-of-health (SoH) is developed to reveal the relative impact of instantaneous battery currents on the battery degradation. A 38V-385Wh battery and a 32V-4.12Wh UC HESS hardware prototype has been developed and a real-time experimental platform has been built for energy management controller validation, using xPC Target and National Instrument data acquisition system (DAQ). Both the simulation and real-time experiment results have successfully validated the real-time implementation feasibility and effectiveness of the two real-time controller designs. It is shown that under a high speed, high acceleration, aggressive drive cycle US06, the two real-time energy management strategies can greatly reduce the battery peak current and consequently decreases the battery SoH reduction by 31% and 38% in comparison to a battery-only energy storage system.
Electric vehicles (EVs) face significant energy storage related challenges, including the range anxiety, high cost, and battery degradation. Batteries, as the energy storage components in majority of current and upcoming EVs, deliver energy to the electric machine during propulsion and recover energy during regenerative braking. For urban drive cycles with frequent stop-and-go, the frequent high power exchange between the electric machine and the ESS results in accelerated battery aging. The battery aging decreases the battery capability of storing energy and providing power over the battery lifetime. One potential solution to this problem is to integrate high-energy (HE) density batteries with high-power (HP) density ultracapacitors (UCs) as hybrid energy storage systems (HESS). UCs has complementary features to batteries with fast charge-discharge, excellent power performance over broad temperature range, long lifetime and high reliability. UCs can protect batteries against fast charging/discharging, reduce high peak power and relieve the battery thermal burden; therefore, prolong the battery lifetime.
In this work, two real-time energy management strategies have been investigated for optimal current split between batteries and ultracapacitors (UCs) in electric vehicle (EV) applications. In this work, the semi-active HESS topology is considered. With this topology, the UC pack discharging/charging current Iuc can be controlled through the control of the DC-DC converter. In addition, as the UC pack is decoupled from the dc bus, its voltage can be lower than the dc bus voltage, and consequently the size and cost of UC can be reduced.
- Electric vehicles (EVs).