Energy Management Strategies for Electric and Plug-in Hybrid Electric Vehicles

Conventional vehicles (CVs), which use petroleum as the only source of energy, represent a majority of the existing vehicles today. As shortage of petroleum is considered to be one of the most critical worldwide issues, costly fuel becomes a major challenge for CV users. Moreover, CVs emit greenhouse gases (GHG), thus making it harder to satisfy stringent environmental regulations.

Among all AFVs, electric vehicles (EVs) seems to be the future most viable vehicle option for reducing the rate of oil usage and improving air quality and less emission. Basically, driving an electric vehicle would feel very similar to drive a conventional car but it can be much quitter.

Through prior research results, it is well known that energy storage devices provide additional advantages to improve stability, power-quality, and reliability of the power-supply source. The major types of storage devices being considered nowadays, viz., batteries, ultracapacitors, and flywheel energy systems, will be presented in this chapter. It is empirical that precise storage device models are created and simulated for several applications, such as hybrid electric vehicles (HEV) and various power system applications.

Different types of alternate vehicles (AVs) exist, such as EVs, HEVs, and fuel cell vehicles (FCVs). However, HEVs are found to be the most practical and efficient substitutes for CVs in the near future. This is because the characteristics of an electric motor are found to be more favorable, compared to the characteristics of an internal combustion engine (ICE). Different combinations of energy sources exist, for example, electric and mechanical (fly-wheel) energy sources or electric and chemical (fuel cell) energy sources. However, the combination of fuel energy and electric energy sources is found to be the most acceptable, due to the combined usage of mature ICE techniques and well-established modern power electronics.

It is well-known today that batteries are indeed the main stumbling block to driving electric vehicles. In fact, the common issues related to lithium rechargeable cells can be summed up by one simple topic: cell equalization. Typically, a battery of a HEV consists of a long string of cells (typically 100 cells, providing a total of about 360 V), where each cell is not exactly equal to the others, in terms of capacity and internal resistance, because of normal dispersion during manufacturing. However, the most viable solution for this problem might not originate from mere changes in battery properties. The aim of this chapter is, first, to explain the role of power electronics based battery cell voltage equalizers and their role in improving cycle life, calendar life, power, and overall safety of EV/HEV battery energy storage systems.

A battery cell voltage equalizer is essentially a power electronic controller, which takes active measures to equalize the voltage in each cell. Furthermore, by few additional methods, such as measuring the actual capacity and internal resistance of each cell (followed by instantaneous SOC computation), it is capable of equalizing the SOC of each cell. As a result, each of the cells will have the same SOC during charging and discharging, even in conditions of high dispersion in capacity and internal resistance. If all the cells have the same SOC utilization, they will degrade equally, at the average degradation of the pack. If this condition is accomplished, then all the cells will have the same capacity during the whole lifetime of the battery pack, avoiding premature end of life (EOL), due to the EOL of only one cell.

One of the biggest challenges in electric transportation is storing electrical energy for use in desired times and desired amounts. Batteries are mostly considered because of their high energy density compared to their counterparts and also their ability to get charged providing regenerative braking capability. The electrochemical nature of batteries has a highly nonlinear behavior and dependent on many factors such as state of charge, state of health, runtime, temperature, aging, load profile, and charging algorithm. A very important concern is related to storage, because in order to have a given amount of energy for a reasonable All Electric Range (AER), tens or hundreds of cells should be connected in series and parallel for the desirable voltage and current ratings of the battery pack. This causes the nonlinear behavior of cells to be more prominent. Furthermore, there are some phenomena that are observed only in battery packs and not in single cells, such as thermal unbalance among the cells in pack.

When defining the technical goals for a distributed power converter system to be used in a PV-powered, grid-tied carport, suitable compromises must be made, in order to contain costs while providing acceptable performance. Essentially, the main design objective is dictated by the fact that the carport will be a public or semi-public structure. Hence, it is crucial that the system is robust, reliable, and offers high availability. It was already ascertained that both the PV resource and the power conversion system must be distributed, providing flexibility and redundancy, while choosing topologies that are characterized by low component count and stress levels, in order to ensure a high Mean Time Between Failures (MTBF). MTBF will broadly be referred to as “reliability” henceforth, in this thesis.

The challenge for the next few years is to reduce greenhouse gas (GHG) emissions from vehicles for global warming curtailment. GHG emissions are mainly due to internal combustion engines (ICE) used in transportation. To decrease this emission, a viable solution lies in using non-polluting electric vehicles.

A major portion of current research in the automotive industry involves the study of the overall efficiency of advanced vehicular power trains. The improvement of overall energy efficiency is one of the most important subjects for developing hybrid electric, fuel cell, and battery electric vehicle (HEV, FCV, and BEV) technologies. This paper aims at developing a basis for comparison of overall efficiencies of advanced vehicular topologies for the above-mentioned advanced vehicular systems.