Access Type

Open Access Embargo

Date of Award

January 2020

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Electrical and Computer Engineering

First Advisor

Caisheng Wang

Abstract

With the market blooming world wide of electrified vehicles, or xEV, the battery production and vehicle installation blooms as well. As the energy storage system and the synergy buffer between vehicle and grid, batteries are widely used in electric vehicles (EV), hybrid / plug-in hybrid electric vehicles (PHEV) and fuel cell electric vehicles (FCEV). However, due to inevitable manufacture errors and inconsistence condition during operation, battery imbalances take place on modules and cells of a pack. The battery imbalances could result in several damaging effects including over-charge and over-discharge. To deal with the imbalance issue of battery packs, various battery balancing topologies and control algorithms have been researched and developed, including passive balancing, active balancing and other schemes. Because of cost, complexity, among other factors, passive balancing is still the most popular solution for electrified vehicles, though it brings power loss and is not applied during discharging. To provide a better solution for battery balancing, especially module balancing for electrified vehicles, in this thesis, an onboard microgrid with solar panel (PV) for battery module balancing and V2G application is proposed and researched. With the proposed microgrid, the vehicle battery modules can be balanced with roof PV or the auxiliary power module (APM) when solar power is not available. When the battery is fully charged or there is specific request from the user, the proposed system is able to deliver energy from PV with or without the battery to the grid or off-grid loads.

In this thesis, various of battery balancing criterions for the proposed system have been studied as well, including conventional voltage/SOC, DOD in capacity and SOC-SOH for solar balancing. Based on the results of the model-based analysis, it is concluded that for APM balancing, the SOC is the best equalization criterion for extending battery life and suppressing SOH imbalance at EOL. While when solar power is available, the proposed SOC-SOH balancing algorithm is suggested. With the SOC-SOH balancing, the PV charges the battery module with lowest SOH when the SOCs are well balanced during discharge. When the vehicle is being charged in the sunshine, only SOC balancing will be applied.

In this thesis, the benefits of the proposed system on electricity grid and derivative counterparts for grid applications are researched as well. Based on the effects of vehicle aggregation, the concept of virtual solar farm (VSF) has been proposed. Simulations have been carried out for a VSF formed by 10,000 vehicles equipped with proposed system and verified that this size of VSF reaches utility-level of solar farms. On utility user side, a study on converting a backup battery of a communication station to general energy storage system (ESS) for time-of-use (TOU) bill reduction is carried out. Real-data based simulation verifies that the solar-assisted balancing scheme is remarkably helpful to balance the battery SOCs from daily charge/discharge cycles and keep more redundant capacity for continuous discharging when power outage happens.

Available for download on Thursday, January 27, 2022

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