Models of energy sources for EV and HEV: fuel cells, batteries, ultracapacitors, flywheels and engine-generators

Abstract

Resulting from a Ph.D. research a Vehicle Simulation Programme (VSP) is proposed and continuously developed. It allows simulating the behaviour of electric, hybrid, fuel cell and internal combustion vehicles while driving any reference cycle [Simulation software for comparison and design of electric, hybrid electric and internal combustion vehicles with respect to energy, emissions and performances, Ph.D. Thesis, Department Electrical Engineering, Vrije Universiteit Brussel, Belgium, April 2000]. The goal of the simulation programme is to study power flows in vehicle drive trains and the corresponding component losses, as well as to compare different drive train topologies. This comparison can be realised for energy consumption and emissions as well as for performances (acceleration, range, maximum slope, etc.).

The software package and its validation are described in [J. Automot. Eng., SAE IEE 215 (9) (2001) 1043L]. Different hybrid and electric drive trains are implemented in the software [Views on hybrid drive train power management strategies, in: Proceedings of the EVS-17, Montreal, Canada, October 2000]. The models used for the energy sources like fuel cells, batteries, ultracapacitors, flywheels and engine-generator units will be discussed in this paper in three stages: first their functionality and characteristics are described, next the way these characteristics can be implemented in a simulation model will be explained and finally some calculation results will illustrate the approach.

This paper is aimed to give an overview of simulation models of energy sources for battery, hybrid and fuel cell electric vehicles. Innovative is the extreme modularity and exchangeability of different components functioning as energy sources. The unique iteration algorithm of the simulation programme allows to accurately simulate drive train maximum performances as well as all kind of power management strategies in different types of hybrid drive trains [IEEE Trans. Veh. Technol., submitted for publication].

Introduction

The pollution caused by transport is a heavy burden, especially in urban areas [5]. The introduction of clean vehicles, like electrically driven vehicles, would be an interesting move in the direction of a significant reduction in harmful exhaust gases, with a view to a sustainable transport policy [6]. Since several years the family of the battery electric vehicles is extended with the hybrid electric and recently also with the fuel cell electric vehicles. These vehicle technologies use different energy sources, which will be discussed in this paper.

The energy sources that will be considered in this manuscript are mainly used in “series” hybrid vehicles. In series hybrid drives there are no mechanical connections between the internal combustion engine and the wheels [7]. All thermal energy is converted first into mechanical energy in a thermal engine and subsequently into electrical energy by a generator driven by the thermal engine. Additionally, there is an electric traction motor to drive the wheels. Hence a decoupling of energy source operation from the required traction power is possible. In most cases the energy source, also called auxiliary power unit (APU), will act as base power unit delivering power to the battery or directly to the electric traction motor. While driving the battery acts as peak power unit or energy buffer. The series hybrid has the advantage of operating a thermal engine in a selected optimal operating field, for instance, with low specific fuel consumption in the torque-speed operating area [8].

The fuel cell hybrid structure is a series structure in which the engine and the generator are replaced by a fuel cell system producing electric energy starting from stored hydrogen or from a fuel tank feeding a reformer to produce hydrogen [9]. The excess of electricity produced by the fuel cell can be stored in a buffer battery. When the battery is left out one has no longer a hybrid vehicle but a fuel cell electric vehicle. In this case, the fuel cell has to demonstrate enough dynamics to meet variable power demand.

Fig. 1 illustrates a simulation model of a series hybrid with peak power unit, where the generator group provides a constant average power, the battery works as an energy buffer and the flywheel (or ultracapacitor) caters for the brake and acceleration power peaks. In this configuration the battery can be smaller or it can possibly be omitted. The efficiency and reliability of the flywheel or ultracapacitor are an important factor for the development of these drive trains.

This model is part of a vehicle simulation programme (VSP) [2], [3], [4] whose front panel is illustrated by Fig. 2 and for which it is necessary to introduce models for all components or subsystems of each simulated vehicle and particularly of the energy and power sources. The basic modelling strategy used in VSP is the well-tried and trusted method of dividing the drive cycle into a number of time steps and calculating the characteristics of the vehicle at the end of each time interval, which is called the longitudinal dynamics simulation [10], [11].

This paper will focus on the description of different energy or power sources as well as on their simulation models and calculation results.